| Download | Main Page |
|
|
GENERALIZATION OF SITE EFFECTS FOR EARTHQUAKE SCENARIO
APPLICATIONS THE COASTAL PLAIN AREA
FINAL REPORT December, 2002 GII Report 595/274 /02 Dr. Yuli Zaslavsky, Dr. Avi Shapira, Marina Gorstein, Michael Kalmanovich, Nahum Perelman, Vadim Giller, Ion Livshits, Dagmara Giller, Ilana Dan, Tatyana Aksienko and Galina Ataev Principal Investigator: Dr. Y. Zaslavsky Prepared for The Steering Committee for National Earthquake Preparedness and Mitigation |
 |
| ABSTRACT |
About 2 million inhabitants in Israel, almost one third of total population of the country, live in the band of seacoast between the towns of Ashqelon and Haifa. Due to the population density here, this region may be considered a high seismic risk zone. The objective of this study is to derive the ground shaking characteristics, resonance frequency and amplification factor for different site conditions. The quantitative assessment of seismic response is made from the horizontal-to-vertical spectral ratios of ambient noise. In order to characterize the seismic response of the seacoast area (130x10 km2), the ambient noise survey was carried out at 190 sites. The measurement points were selected to provide good coverage of the area, considering different surface sedimentary deposits, thickness of sediments and the shear-wave velocity contrast between sediments and bedrock. Results indicate site amplifications ranging from 1.0 to 8.0 within the frequency band 1.0-6.0 Hz. We present two maps that reflect the fundamental characteristics of site effects in the area: dominant frequency and maximum relative amplification. The observed resonance frequencies and their amplifications were correlated with analytical functions that correspond to 1-D subsurface model. The key parameters of the model are shear wave velocity, thickness and density of materials for each layer. Collection of available geological, geotechnical and geophysical data relevant to local geology and combining theoretical and experimental response functions provided reliable estimations of analytical site effects. We used the adequate analytical transfer functions and Stochastic Evaluation of Earthquake Hazard (Shapira and van Eck, 1993) to predict the site dependent Seismic Ground-Motion Hazard Maps in terms of ground motion parameters used for engineering purposes: Peak Ground Acceleration, Spectral Acceleration at different frequencies and Maximum Spectra acceleration for a probability of 10% during an exposure time of 50 years and a damping ratio of 5%. The databases created for the investigated area allowed preparation of a number of realistic Earthquake Hazard scenarios. In order to facilitate estimations of earthquake loss scenarios, we divided the Coastal Plain area into six geographical zones, each one characterized by fundamental frequency and amplification factor. For each zone we adjusted the overall soil-column model, which allows calculation of the response spectrum throughout the whole Zone. |
| INTRODUCTION |
In order to mitigate earthquake risk and initiate preparedness plans we must be able to estimate the possible consequences of strong earthquakes, i.e., implement our accumulated experience of past earthquakes to present a scenario of an eventual earthquake (Shapira et al., 2001). This scenario must be supported by the ability to provide a realistic assessment of damage and losses due to strong earthquakes. Lessons of past large earthquakes have shown that, within 24 hours after events, the number of fatalities under building debris may reach 48% of total number of those trapped under debris unless rescue operations are started in time (Shakhramanjyan et. al., 2000). The strong influence of near-surface geological conditions has been apparent in damage distribution of many destructive earthquakes. Two “famous” examples of such effects are San Francisco and Mexico City. In the San Francisco earthquake, local amplifications over unconsolidated sediments have been shown to be responsible for intensity variations as large as two degrees (MM scale) during both the 1906 “big” San Francisco earthquake and the more recent 1989 Loma Prieta event. In Mexico City, there are very soft lacustrine clay deposits underneath the downtown area of the city. These lead to very large amplifications, which, in turn, caused a high death toll and considerable economic losses during the distant Guerrero Michoacan earthquake of 1985. Nearly all recent destructive earthquakes (Spitak, Armenia 1988, Northridge 1994, Kobe 1995, Armenia, Columbia 1999, Turkey 1999 and many more) have provided additional evidence of the dramatic influence of site effects. The first step in forecasting the type of losses, is to map the amplification of seismic waves by soft near-surface deposits. One alternative approach in predicting site effects involves determining the physical properties of local soils setting by conducting borehole and seismic profile studies. Measured parameters can then be used in theoretical models to predict site response functions. The main disadvantage of this method is the relatively high cost of conducting the necessary geotechnical or geophysical studies that, in many cases, prove to be problematic. Although theoretical approaches are instructive, conducting the necessary sensitivity tests (with respect to different locations and sizes of possible events) and incorporating the inherent uncertainty (with respect to our limited knowledge of the Earth’s structure), is usually impractical. For example, hundreds of thousands of dollars were spent on geotechnical site-characterization at Turkey Flat, located near the Parkfield section of the San Andreas Fault. According to Field and Jacob (1993) “Turkey Flat will be the most extensively studied sediments field in the word”. However, these authors reached the conclusion that “the average spectral ratios of earthquake recording provide a better estimate of the weak motion site response at Turkey Flat than do theoretical prediction”. The superiority of empirical observations was also suggested by Seed et al., (1988) in stating, “…it is desirable to refine direct measurements of shear wave velocities with data that may be obtained from actual earthquake records”. The site response functions are, by definition, equivalent to the spectral ratio with respect to a reference site, located on rock, when the data are strong ground motions (Rogers et al., 1984; Singh et al., 1988; Jarpe et al., 1988; Darragh and Shakal, 1991; Borcherdt and Glassmoyer, 1992; Gutierrez and Singh, 1992; Satoh, et al., 1995; Aguirre and Irikura, 1997; Su, et al., 1998; Beresnev, et al., 1998; Hartzell, 1998; Reinoso and Ordaz, 1999; Zaslavsky and Shapira, 2000). In regions where the seismic activity is relatively low, as in Israel, this type of analysis is usually impractical. Many investigators evaluated site response functions from moderate to weak motions of earthquakes (Tucker and King; 1984; King and Tucker; 1984; McGarr et al., 1989; Field et al., 1992; Field, 1996; Liu et al., 1992; Jongmans and Campillo, 1993; Carver and Hartzell, 1996; Hartzell et al., 1996; Steidl et al., 1996; Toshinawa at al., 1997; Zaslavsky et al., 2000). The main deficiency of the reference site technique is the choice of reference motion represents the true input motion to the soil site. According to Steidl (1993), when it is possible, the reference ground motion should be calculated by averaging several rock sites. Steidl et al., (1966) and Zaslavsky et al., (2002) show that the assumption of a flat response rock site is often false, mainly due to weathering. Use of these surface-rock sites with flat response as reference sites often leads to underestimation of the amplification by factor of 2 to 4 in the frequency range 2 to 7 Hz. Lermo and Chavez-Garcia (1993) drew significant results from a non-reference technique, the receiver function technique, i.e., using the horizontal-to-vertical spectral ratios of shear-wave. Many studies report that the frequency dependence of site response can thus be obtained from measurements made at only one station at the analyzed site (Lermo and Chavez-Garcia 1994; Theodulidis et al., 1996; Seekins, et al., 1996; Malagnini et al., 1996, Zaslavsky et al., 1995 and many others). The implementation of this approach, however, still requires the rather frequent occurrence of earthquakes. The idea of evaluating site characteristics from microtremor records originated from the pioneering work by Kanai and Tanaka (1961). They pointed out that predominant frequency of horizontal spectra of microtremors is related to shallow, local geological conditions. Since then it has been reported that this technique has proved effective in estimating fundamental frequency (Tanaka et al., 1966; Tanaka et al., 1968; Katz, 1976; Katz and Bellon, 1978; Ohta et al., 1978; Kagami et al., 1982; Zaslavsky, 1984, 1987). However, in most cases, due to the influence of artificial sources from dense population, high traffic and various industries, the resonance frequency cannot be directly identified in the microtremor spectra (Zaslavsky et al., 2001b, 2002). Kagami et al., (1982) proposed that the ratio of the horizontal components of the velocity spectra at the sediment site to those at the rock site could be used as a measure of microseism ground motion amplification. This technique is widely used for site response estimating (Rovelli et al., 1991; Field et al., 1990, 1992; Hough et al., 1992; Malagnini et al., 1996; Gutierrez and Singh, 1992; Dravinski et al 1995, Gaul et al., 1995; Zaslavsky et al., 1995, 2000; Shapira et al., 2001). Our experiments (Zaslavsky at al., 2000) show that bedrock ground motion can be considered a good reference site when distances as smaller as 0.5-1.0 km from the soil site. Nakamura (1989) hypothesized that site response could be estimated by simply evaluating spectral ratio of horizontal versus vertical component of noise observed at the same site. Most studies show that the H/V ratio obtained from microtremors coincides with response functions of near surface structures to incident shear wave (Ohmachi et al., 1991; Lermo and Chavez-Garcia, 1994; Zaslavsky et al., 1995; Seekins et al., 1996; Gitterman et al., 1996; Konno and Ohmachi 1998; Mucciarelli and Monachesi, 1998; Chavez-Garcia and Cuenca, 1998; Toshinava et. al., 1997; Shapira et al., 2001). There is another conclusion regarding microtremors horizontal-to-vertical spectral ratios. Recently, Field and Jacob, 1995; Bonilla et al., 1997; Horike et al., 2001; Satoh et al., 2001 contended that estimates of the frequency of the predominant peak are similar to those obtained from traditional sediment-to-bedrock spectral ratio of earthquake records (Borcherdt, 1970), however the absolute level of site amplification does not correlate with the amplification obtained from this method. The objective of this study included performing quantitative estimates of site effects in the seacoast area between Ashquelon and Haifa using ground motion from ambient noise (microtremor). The horizontal-to-vertical spectral ratios obtained with the Nakamura method (H/V ratio or QTS, Quasi-Transfer Spectra) were compared with numerical simulations. Available data relevant to local geology, subsurface layering and soil geotechnical characteristics were compiled from borehole data, velocity-depth maps of the shallow part of the geological section of the Mediterranean Sea Coastal Plain, and from local S-wave refraction surveys in different parts of this area. Considering the subsurface structural data, we calculated the trial 1D transfer functions. For 1-D modeling we used the Joyner (1977) program that requires as input the S-wave velocity model, density and thickness of the each layer. The mechanical characteristics at our sites are not well known, therefore the numerical techniques were applied starting from trial models. The 1-D response functions were compared with the empirical determinations. By means of trial and error, we obtained physical models established for 90 points in the investigated area consistent with our experimental observations. We used the adequate analytical transfer functions and Stochastic Evaluation of Earthquake Hazard (Shapira and van Eck, 1993) for predictive site dependent Seismic Ground-Motion Hazard Maps in terms of ground motion parameters used for engineering purposes: Peak Ground Acceleration, Spectral Acceleration at different frequencies and Maximum Spectra acceleration for a probability of 10% during an exposure time of 50 years and a damping ratio of 5%. The databases created for the investigated area enabled us to prepare a number of realistic Earthquake Hazard Scenarios. |
| GEOLOGICAL SETTINGS |
The investigated area is situated along the Coastal Plain extending from Ashquelon to Haifa, about 140 km long and almost 8-10 km wide. The basic data of the region were collected from Gvirtzman ( 1965, 1969, 1983) and the new version of the geological map of Israel to a scale of 1:200,000 (Sneh et al., 1998). The Quaternary sediments outcropping in the investigated area are represented by alluvium, dune sands, hamra and kurkar. Sand dunes of Holocene age: These sediments are exposed along the coastline as a strip, up to 5km wide in some places, covering 15-20% of this area. Sand dunes usually overlay some sandstone and sometimes alluvium or loam. The thickness of these sediments varies from 5 to 20m. Alluvium sediments of Holocene age: These are composed of sand, soil, gravel, clay and loess. These sediments are developed along river beds with thickness varies from 5m to 30m. Alluvium and “Hamra” (red sandy clay) sediments occupy about 80% of the area and usually overlay calcareous sandstone. The thickness of these sediments could reach 50m along riverbeds. Kurkar Group of Pleistocene age consists of marine and eolian calcareous sandstones named “kurkar”, some reddish silty-clayey “hamra”, silts, clays, loose sands, loam and conglomerates. These rocks outcrop along the coastline as a narrow strip up to 5 km wide, covering the Coastal Plain and the Israel Continental Shelf representing 5-7% of the area investigated. The Kurkar Group is characterized by two different representative provinces. The western province is represented by calcareous sandstones (“kurkar”) with intercalation of clayey-silty sandstones (mostly “hamra”) and finer grained sediments. This unit covers the Coastal Plain and Continental Shelf. The eastern province is subdivided into three sub-units including the Rehovot Fm. (hamras, sands eolianes and some shales); Ahuzam Fm. (conglomerates); and Pleshet Fm. (marine calcareous sandstones). The thickness of the Kurkar Group varies from 180-200m near the shoreline to 100-120m at a distance of 5-7 km from the coastline. The Kurkar Group unconformable overlays the clays of the Yafo Fm. of Pliocene age. In the area from Binyamina to Haifa (The Hof- Ha-Carmel area) the geological structure is considerably different. Here, the Kurkar Group, with a thickness of approximately 50 m near the coastline, wedges out eastward overlying the Turonian-Cenomanian carbonates. In Table 1 we present the basic geological and lithological characteristics of the sedimentary rocks exposed in the investigated area. |
| SITE-RESPONSE ESTIMATION: NAKAMURA’S TECHNIQUE | ||||||||||||
|
For estimating dynamic characteristics of surface layers in our investigations in the Coastal Plain we used Nakamura’s technique. By Nakamura, vertical component of ambient noise maintains the characteristics of source to sediments surface ground, is relatively influenced by Rayleigh wave on the sediments, and can, therefore, be used to remove both the source and the Rayleigh wave effects from the horizontal component and identify fundamental resonance frequency and amplification factor of sediments. Comparing H/V ratio of microtremors with H/V of Rayleigh wave, some theoretical studies have concluded that the peak H/V could be explained with the fundamental mode of Rayleigh wave (for example Lachet and Bard, 1994). Nakamura demonstrates that the H/V peak of a microtremor is explained with multiple refracted vertical incident SH waves, while amplitudes of horizontal and vertical components of Rayleigh waves around the peak frequency of H/V are very small, almost zero. As for the maximum energy of Rayleigh waves, it is high in the trough of the H/V ratio. Microtremors, by Nakamura, can be divided into two parts since they contain Rayleigh and other waves. Then, horizontal and vertical spectra on the surface of the sedimentary basin can be written as follows: Hf = Ah*Hb + Hs; Vf = Av*Vb + Vs Where - Ah and Av are amplification factors of horizontal and vertical motions of vertically incident body waves; - Hb and Vb are spectra of horizontal and vertical motion in the basement of the basin; - Hs and Vs are spectra of horizontal and vertical directions of Rayleigh waves. If there is no effect of Rayleigh waves, Vf ≈ Vb. If Vf >Vb, it is considered as the effect of surface waves. Estimating the effect of Rayleigh waves as Vf / Vb, horizontal amplification can be written as,
where, |
| Microtremor Records and Data Processing |
|
Our observation of
microtremors was carried out from November 2001 to May 2002.
Ambient noise measurements were carried out at 190 points
in different lithological units in the coastal area between
Ashqelon and Haifa. The distribution of the Points for
microtremor recording was based on the surface geological
formation map
(see Fig. 1).
We planned 22 measurement
lines from Ashqelon (N: 121049) to Haifa (N: 246600) so that
18 of them would be perpendicular the coastline and intersect
the different geological units that may amplify seismic waves
and three lines along the coastline. The distance between
lines was approximately 5km. Most of the stations are located
close to boreholes. The number of stations distributed along
each line and the distance between them depended on the
spatial distributions of the geological units and the
availability of borehole data. Distribution of investigated
sites is shown in
Figure 1.
The acquisition equipment includes: 12-channel amplifier with a band pass filter 0.2‑25 Hz, GPS (for timing) and a laptop computer with analog-to-digital conversion card. Digital recordings were made with a sampling rate of 100 samples per second. The recorder has 16-bit data word. Digital seismic data acquisition system designed for site response field investigations (Shapira and Avirav, 1995). The seismometers used are sensitive velocity transducers with a natural frequency of 1.0 Hz and damping at 70% of critical. Each of the stations is equipped with one vertical and two horizontal seismometers (oriented north-south and east-west). The seismometers were installed on leveled metal ground plates and connected to the data acquisition system via cables. At each site microtremors were recorded continuously for 1-1.5 hour, creating data files of 3 minutes each of microtremor data. All the equipment – sensors, power supply, amplifier, personal computer and connectors - were installed in the vehicle, which also served as a recording center. Information on the superficial geology of the sites where the stations were installed and recoding times are listed in Table A1. In Figure 2 we present pictures of the locations of the seismic stations during the site investigations. The empirical methods that we implemented to evaluate the site response are based on the H/V spectral ratios of selected time windows of recorded ambient noise. The selected time windows were Fourier transformed, using cosine-tapering (1 sec at each end) before transformation and then smoothed with a triangular moving Hanning window (0.5 Hz). In the analysis the selected time windows is 30 sec., however, we experimented with a range of values in order to find the window length that gave satisfactory resolution of the peak being studied but was still stable in frequency. After several tests, we concluded that a window 30 sec in length provides stable results. The selected time window was free from recordings of passing vehicles, noticeable harmonic noise from nearby machinery, spiky data and other transient signals. The horizontal-to-vertical spectral ratio [AH/V(f)] was obtained by dividing the individual spectrum of each of the horizontal components [SNS(f) and SEW(f)] by the spectrum of the vertical component [(SV(f)]. To obtain systematic and reliable results from the spectra of microtremors, we used several time windows that yield a number of spectral rations that, in turn, were averaged. We also experimented with computing the average of the spectral ratios and found the differences to be negligible. The average of the two horizontal-to-vertical ratios is defined as the site amplification function:
|
| RESULTS |
|
The stability results
obtained from microtremors must be confirmed before
interpretation of microtremor data. We studied (Zaslavsky and
Shapira, 1998, Zaslavsky et al., 2001) the evolution of the
horizontal-to-vertical spectral ratios during the day taking
different observation intervals, different days and months and
concluded that the dominant frequency and its amplitude are
stable. Examples of the spectral ratios obtained at Point 4
from microtremors recorded in October 2001 and
January 2002 are shown in
Figure 3. The shape of all
average curves is similar. The dominant feature of the
spectral ratios is a maximum in the frequency range
3.5-3.8 Hz with an amplification factor of about 4.5. In
spite of the difference in the location of the fundamental
frequencies, the general shape of spectral ratios for NS and
EW components do not differ significantly. For that reason, we
base our discussion of the average spectral ratio computed for
two horizontal components.
Our observations revealed consistent and clear peaks of spectral ratios, which we could trace from site to site. We would like to demonstrate two cases that may produce them. Figure 4 illustrates the character average spectra of microtremors recorded at Point 65-3 and its horizontal-to-vertical spectral ratios. An increase in the spectral levels of the horizontal components is clear in frequency range from 2.5 to 3.0 Hz, while the spectrum of vertical component is flat. Therefore, spectral ratios show a prominent peak at about 3.0 Hz with an amplification of about factor 3. In the second case (Figure 4), if we compare the average spectra horizontal and vertical motions at Point 51, we can see that in the vertical spectrum there is a narrow-bandwidth trough at frequency near 1.5 Hz. Hence, the general character of the spectral ratios is clear amplification at a frequency of about 1.5 Hz. Consequently, the high levels of amplification obtained from H/V spectral ratios are controlled not only by peaks in the spectra of the horizontal components but also by narrow-bandwidth “holes” (troughs) in the spectra of the vertical components. On the other hand, for a few sites the spectral ratios does not exhibit a clear peak and the predominant frequency (the frequency at which maximum amplification is reached) is very difficult to identify. Figure 5 shows examples of the average spectra for the three components and their H/V spectral ratios. The general character of these spectra is that spectral levels for horizontal components exceed the levels for the vertical component within broadband frequencies. The large “amplifications” that we can see in spectral ratios in the frequency range from 4.0 to 8.0 Hz is difficult to explain. One possibility is that a cultural feature of some sort may have affected the signal of horizontal motion at this frequency range. In Figure 6, we plotted the spectral ratios for the three points located on kurkar. These figures demonstrate that the scatter between individual curves is high but the variations in the averaged functions are small. From all average functions of the spectral ratios it may be concluded that in the frequency range 0.6 Hz to 10 Hz transfer functions are flat with unit amplification, i.e., there is no site effect. Microtremor H/V spectral ratios for two sites (Points 49 and 50) composed by unconsolidated sediments with thickness varying from 2 to 6m to bedrock are shown in Figure 7. The bedrock consists of kurkar. These figures demonstrate similarity among the individual functions. The average spectral ratios (response functions) are flat in the frequency range 0.6 to 10 Hz. Consequently these observations show that there are no site effects at Points 49 and 57. Examples of the individual and average H/V spectral ratios for two neighboring points are displayed in Figure 8. We can see that parameters of site effects are remarkably robust: differences in the location of the fundamental frequencies and amplification level are small. These findings significantly increase the reliability of the information obtained and emphasize the importance of a dense grid of observation points in microzoning studies. Horizontal/vertical spectral ratios at Points 4, 27 and 134 are shown in Figure 9. At these sites, the soil profile is very simple, namely “basement rock”, consisting of kurkar underlying the soft layer represented by clay, sand and silt (hamra). As shown in Figure 5, H/V ratio at Point 4 shows a very clear peak at frequency 3.3 Hz. On the other hand, at Point 134 the H/V does not show a well- defined peak at about 2 Hz. It should be noted, that the S-wave velocity for kurkar is about 1100 m/sec, 900 m/sec and 700 m/sec at Points 4, 27 and 134, respectively. Using these examples we could illustrate that we obtain much clearer and more stable characteristics of H/V spectral ratios at points where a good S-wave velocity values contrast between the surface soft soil layer and the basement layer is observed. It is understood that microtremor H/V spectral ratio techniques do not clearly define the fundamental resonant frequency of sedimentary deposits if the shear-wave velocity of a deposit layer with respect to the shear-wave velocity of the half space is less than 0.5. Figure 10 illustrates the site response at four sites (Points 47, 84-1, 90 and 127) where the contrast between rock and soil is low (<2.5). Examining the shapes of the amplifications, it can be realized that response is almost flat over the entire frequency range and does not show a predominant frequency. In contradiction, in Figure 11 H/V spectral ratios for four sites (Points 72, 89, 69 and 65-B) with low contrast are also shown. Well defined peaks with lower amplifications (values 2 between 0.9 and 1.9 Hz) are quite clear in this figure. The observed amplitude variation is small. The average Fourier spectra for three components and individual and average (NS and EW components) H/V spectral ratios at Point 82 are shown in Figure 12. This site has a soft soil profile in the uppermost surface layer and a very clear contrast between soft soil layers and basement soil layers. Figure 12 reveals that the peak we found stable within H/V ratio is localized by a narrow-bandwidth trough at frequency near 2.0 Hz in the vertical spectra as well as high in the horizontal spectra near 2 Hz. In Figure 13 we present average Fourier spectra for three components motion of microtremors recorded at Points 85, 93 and 99. In all horizontal spectra there is a dominant frequency that may be associated with the resonance frequency of the sediments layers. The important point here is that the spectral levels for vertical components are smaller than spectral levels for the horizontal components within the frequency range 1 to 10 Hz. Therefore, the horizontal-to-vertical spectral ratios in Figure 13 overestimate the spectral ratio evaluated by a frequency-independence factor of about 1.5-1.7. Figure 14 shows observed horizontal-to-vertical spectral ratio obtained from ambient noise recorded at Points 85-1 and 85, which are only 150 m apart. These sites demonstrate the great variability in site response possible over very short distances. Spectral ratios for Site 85 are flat with no amplification while average spectral ratio of Site 85-1 shows a prominent peak at 1.2 Hz with amplification factor up to 8. For preparation of earthquake damage scenarios the necessary is information about modification of ground motion by site condition. The ground shaking characteristics, resonance frequencies and amplification factors, obtained from the microtremor observations across the investigated area are summarized in Table 2. From this data set we are able to map the Coastal Plain area: the map of distribution of fundamental frequency of resonance (Figure 15) and map of distribution of maximum relative to the rock amplifications (Figure 16) in the region of interest. Clearly, the main geological structure is reflected in the measurements result:In the southern part of the area (from Ashqelon to Binyamina) dominant frequencies reach 3 Hz on the sand along the beach and decrease to the west up to 1.0 Hz correlating with the thickness of the sedimentary deposits. In the northern part of the area (from Binyamina to Haifa) the sediments present a predominant frequency of 2-6 Hz suggesting that the sedimentary cover is rather thin. Amplification map of the Coastal Plain reveals two geographical zones differentiated by amplification values. In the southern part sand dunes and alluvium sediments of Holocene age over the Kurkar Group do not show a strong contrast. Therefore, most of examined sites are associated with amplification factors between 2-3 and only some sites reveal amplifications between factors 3-6. In the northern part silts, clays, loose sands, and loam of the Kurkar Group over the Turonian-Cenomanian carbonates show a strong contrast and cause amplification factor up to 7. |
| One-Dimensional Modeling |
The assessment of site amplification is made from horizontal-to-vertical spectral ratios. It is commonly agreed that H/V ratios obtained from microtremors reveal the fundamental mode of vibrations. In order to obtain a more general representation of the site response function we computed one-dimensional models. The mechanical characteristics at our sites are not very well known so the numerical techniques were applied starting from a trial model. For the theoretical transfer functions calculations we used Joyner’s program for non-linear site response determination (Joyner, 1977). During the initial phase of compilation and summation of geothechnical information we gathered detailed surface geology (Sneh A. et al., 1998), borehole information (Atlas of Geological cross-sections, 1999; Lithological Borehole Data, 2001), as well results of interpretation of seismic refraction surveys carried out in the Sea Coast and Hashephela areas (Stivelman, 1994, 1998; Ronen Amit, 2001; Bek A., Kravzov. A., 1998 and others). Our attempts to use information from the map of sediment thickness at the Coastal Plain (Kravtsov at al., 1997) did not yield satisfactory results. Thus, for example, at Point 130 situated at the Or Yehuda 29/5 borehole, the refractor is found at a depth of 17m, which coincides with the water level within the homogeneous layer of clay (see lithological cross- section in Figure 17a). However, the subject of our investigation, i.e. shear-wave velocity contrast, is not related to water level. According to our measurements a half-space is revealed at a depth of 20m lower than that obtained by the refraction survey. We observed a similar situation at Point 69, located at the Nordia 45/3 borehole. There the refractor was detected at a depth of approximately 40m within the layer of loamy sand, while from our measurements the refractor top is defined approximately 35-40m deeper (see Fig. 17b). Information on S-wave velocities used for trial numerical models of subsurface was obtained from refraction surveys data and summarized in Table 3 In order to collect additional information about geotechnical parameters in the Coastal plain area, more than 230 oil and gas boreholes have been inspected. Owing to lack of core data we could not use them in our study. Many of the inspected boring logs contain just a few data of interest. For example, we found only one borehole (Suzie 12), where density logging above 100m depth is available. Natural radioactivity, sonic, neutron log data are available in three boreholes in the upper part of the section. To obtain density and P-wave velocity of rocks three boreholes were involved in the processing of log data. Density could be calculated by porosity, which in turn is determined by sonic and radioactivity logging. P-wave velocity is calculated by sonic log. It should be noted that S-wave velocity values were estimated on the basis of P-wave velocities using Vp/Vs ratios obtained from the seismic refraction method data (see Table 3). The results of the logging data interpretation at borehole Ashdod-1 are presented in Figure 18 and Table 4. We can see that velocity values derived by logging are consistent with velocities obtained from refraction serveys presented above in Table 3. Development of an analytical site response function is an important component in the evaluation of seismic hazard in terms of Uniform Hazard Site-Specific Acceleration Spectra. Based on the site response measurements, surface geology information and borehole data, considering velocity and depth maps and seismic survey refraction data, we compiled numerical models of the subsurface. Information about lithology and depth of layers at sites situated close to or at boreholes were taken from “ATLAS of cross-sections”, 1999 and Lithological Borehole Data, 2001. The shear-wave structures for different layers were deduced by trial-and-error fitting of observed and theoretical transfer function. Figure 19a shows the comparison of analytical and empirical response functions at Point 123 (Yehoshua well) together with the corresponding lithological cross-section. As The initial range of S-wave velocity values for model development were taken the results of refraction surveys (see Table 3). In Table 5 we present a three-layers model for Point 123 derived by trial-and-error procedure of adjusting the analytical response functions to empirical one. Suggested S-wave velocities yield satisfactory agreement between fundamental resonant frequency and maximum amplification of the model and experimental function. The fitting procedure was repeated retaining velocity values derived from modeling at the previous Point 123 for some other sites situated close the borehole. Figure 19b and Table 6 display such an example for Point 58. S-wave velocities for loamy sand, loam and calcerous sandstone are taken from model described above, while for the layer of sand we fitted a value of 200 m/s. Another type of model, for which the bedrock is represented by limestones with S-wave velocity ranging, depending on its location, from 900 to 1100 m/s, we propose for Points 144, 147, 20-22 (Ashdod), Delek (Tel-Aviv) and for Points 64-70 situated in the Netanya area. Suitable models in comparison with experimental ratios and their parameters are shown in the Figures 20a,b and in Table 7. Our conclusions regarding higher Vs velocity of kurkar in the Tel-Aviv area are in agreement with the refractor velocity obtained from the refraction survey carried out at the Tel-Aviv Power Station, that is 1170 m/s. For Point 147 situated in the Ashdod area, only assuming as bedrock the 40m thick layer of limestone bedding at a depth of 115m, we obtained modeled frequency and amplification close to the measured ones (see Table 6 and Fig. 21a,b). It is interesting to note that using clay underlaying the limestone at a depth of 155m as half-space, we also obtain a good agreement (see Figure 21b). This result confirms our assumption that just a layer of limestone 40m thick provides the shear-wave reflection . In Figure 22a we demonstrate an example of a model in which the 18m thick layer of kurkar cannot be considered as a halfspace, while the clay of the Yaffo Fm. gives a good similarity between analytical and experimental functions (see Table 6 for parameters). Conditions where clay appears as a half-space were observed in the eastern part of the investigated area where the kurkar is thinning. For the Carmel coastal area, where the loose sediments overlaying the kurkar bed on the Turonian-Cenomanian carbonates and distinguished owing to the high level of amplifications and we present the following two examples: At Point 82c sediments and kurkar rest on dolomites, while in the case of Point 90-1 the bedrock is chalk. Geotechnical parameters used for model construction are shown in Table 5, average spectral ratio and analytical model are displayed in Figures 22b, c. The shear-wave structures for different sediments deduced by trial-and error fitting of observed and theoretical transfer functions helped to adjust the thickness of the sedimentary layers at locations where no borehole information is available. The geotechnical parameters, which, in turn, were used to compute the Uniform Site-specific acceleration spectra, are summarized in Table A2 (see Appendix A). |
| Prediction of Horizontal Response Spectra |
Site Response Function determination is an important stage in the overall process of seismic hazard assessment despite the fact that the function itself has no direct engineering application. In order to estimate the ability of buildings at a certain site to withstand seismic activity, we need to obtain the site-specific acceleration spectrum. This design acceleration spectrum is essentially a representation of the maximum acceleration amplitudes, for a prescribed probability of occurrence, developed on a set of one degree of freedom oscillators with a given damping ratio. Since the seismic activity in areas like Israel is low, local acceleration data from strong earthquakes is insufficient to estimate directly the design acceleration spectrum, therefore, we must resort to the use of synthetic data. For this purpose Shapira and van Eck (1993) developed the SEEH method (Stochastic Estimation of the Earthquake Hazard) which is based on the generation of synthetic seismic events using events assembled in simulated earthquake lists and local seismological characteristics such as mechanism and strength of the event, epicenter location, mechanical and dynamic characteristics of the propagation paths etc. (Shapira and Hofstetter, 1993; Shapira and Shamir, 1994; Hofstetter, 1996;Shamir et al 2001). So, the regional information is used to synthesizing many accelerograms for the surface of the underlying bedrock and then they are convolved with the response function of the site under investigation, to yield the expected accelerations on the free surface of that site. The final result of the application of the SEEH method is the uniform hazard, site specific acceleration spectrum for the investigated site in which the same probability of occurrence applies to each point. This design acceleration spectrum is calculated for a different probability of occurrence over different period of years with different damping ratio. The basic input parameters of optimal analytical models used in stochastic simulations are presented in Table A2. As previously mentioned, based on the analysis of the observation site response, it is suggested that the coastal plain area be divided into two regions, each of them characterized by an amplification factor. The Uniform hazard site specific acceleration spectra for the two regions was computed for a probability of an exceedence of 10% during an exposure time of 50 years and a damping ratio of 5%. We used 70 and 35 analytical response functions for southern and northern zones, respectively. The results of our computations are shown in Figures 23 and 24 . We can see that the scatter between acceleration spectra calculated for different sites is very high. Comparison of the acceleration spectra curves shows that the maximum of spectral acceleration changes from 0.12g to 0.42g for the southern zone and from 0.16g to 0.6g for the northern zone of area. On the other hand, the scatter of peak ground acceleration (PGA) is small. Therefore, buildings built on soft soil may be subjected to seismic forces several times larger than similar buildings built on rock, if the peak ground acceleration is the same in both cases. In the corresponding National Earthquake Hazard Reduction Program (NEHRP) recommended for seismic regulations for new buildings and other structures (USA Seismic Code, 1997) a new design response spectrum curve will be developed based on two fundamental parameters: SDS – the design spectral response acceleration at short period (T0=0.2 sec) and SD1 – the design spectral response acceleration at 1 second. Therefore, in Figures 25 and 26 spectral acceleration maps with 10% probability of exceedence in 50 years (damping ratio 5%) calculated for 0.2 sec and 1 sec are plotted. |
| Distribution of the Soil-Column Models |
|
In order to facilitate earthquake hazard assessment in terms of the Uniform Hazard Site-specific Acceleration Spectra for the Coastal Plain, we divided the investigated area into six zones based on the distribution maps of fundamental frequency and amplification factor (see Fig. 27). We constructed soil-column model, based on microtremor recordings, geological, geotechnical and borehole information, for each zone. The model was obtained using cluster analysis and robust statistical approach. The parameters obtained are presented in Table 7. For verification of suggested generalized models we compared Spectral accelerations computed using averaged models and models calculated for some individual measurement points located within each zone with different geographical coordinates. The Uniform Hazard Site-specific Acceleration Spectra were computed for a probability of exceedence of 10% during an exposure time of 50 years and a damping ratio of 5%. For Zone 1 the site-specific acceleration response was calculated under the assumption that there are no site effects. The results of the comparison are plotted in Figure 28. Locations of selected for comparison points are shown on Figure 27. We can see that in the majority of cases the difference between spectral acceleration computed using generalized model and acceleration computed for individual sites does not exceed 15%. Thus we concluded that the proposed generalized models, which encompass the gross features of the geology, incorporate borehole data and geophysical information, can be useful for making regional hazard decisions. |
| Discussion |
Local site condition maps are major factors in preparing earthquake damage scenarios. In-situ measurements with controlled-source seismic methods allow one to image the subsurface structure. However, it is, in general, not possible to carry out such measurements in regions where the seismic activity is relatively low, as in Israel. We therefore focused on microtremor measurements. We computed horizontal-to-vertical spectral ratios (Nakamura’s ratio) over 190 sites across the Coastal Plain between Ashqelon and Haifa. The estimation of ground motion amplification at resonance frequency using different techniques is very controversial, with some authors achieving good results while others find no correlation between the different methods. Based on measurements of explosions, earthquake and ambient noise by reference and non-reference techniques, Malagnini et al., (1996) showed that the use of microtremors (Nakamura technique) failed to identify both the resonance frequency and its amplification. Nevertheless, Seekins et al., 1996, showed that horizontal-to-vertical spectral ratios obtained from microtremors agree better with sediment-to-bedrock spectral ratio from S-wave than the microtremor ratio with respect to the reference site. Moreover, Meneroud et al., (2000) show that H/V ratio from microtremor measurements is very successful and gives the same results than more expensive and time consuming methods. During the last decade, more than 50 sites in Israel have been investigated in an attempt to estimate the possible amplification of seismic ground motion (Shapira et al., 2001; Zaslavsky et al. 2002a; Zaslavsky et al., 2002b). We used various empirical methods to study the site response, including reference and non-reference techniques as well as different sources of excitation – earthquakes, explosions and ambient noise. Our results obtained from many case studies show that the Nakamura’s method predicts amplifications, in addition to the fundamental frequency, that are similar to those that estimated from earthquake data and modeling techniques. We should emphasize that there were cases in which the Nakamura’s technique failed to yield conclusive results. This often happens when the ratio of the shear-wave velocity of the soil to the shear wave velocity of the underlying half space (bedrock) is higher than 0.5-0.6 (amplification up to a factor of ~2) or when we are dealing with a complicated 3D structure of the underlying geology. Other examples are associated with poor excitation of the soil column due to weakness or remoteness of the microtremor sources. Thus, in many cases, this poor behavior of the Nakamura method could be foreseen and other methods should have been used. In other cases, when the situation is better suited to the feasibility of the method, the results showed great similarity to the results obtained by other techniques and, thus, provide useful feedback to improve the reliability of the experimental results. In rare cases, the Nakamura technique even provided estimations of higher harmonics of the resonating soil column. Analysis of the site response of 190 sites located in the Coastal plan area reveals two geographical zones differentiated by amplification levels. The first zone is the Ashqelon-Binyamina zone (the central Coastal Plain) and the second zone is the Carmel zone (northern Coastal Plain). Using results of microtremor analysis we established that in the central region of the area investigated (Ashqelon-Binyamina) the reflector (half space) is calcareous sandstone of the Kurkar Group, but not clay of the Top Yaffo Fm. as was assumed anteriorly according to geological information. Loose sediments overlaying calcareous sandstone form impedance contrast in this region. Fig. 29 presents a geological cross-section showing the subsurface structure along Line 6 situated in the Ashdod area. The dominant frequency along this profile decreases from 2.7 Hz (sites 20 and 21) to 1.7 Hz (Site 147) related to the thickness of the loose sediments represented by sand dunes and loam. At Points 22 and 147 we observe a relatively high amplification factor. This observation could be associated with the thickness of alluvium sediments accumulated along the Lakhish paleo riverbed and can reach 50m. To the east along the profile, amplification decreases to factor 1 (no amplification) that could be connected with outcropping of Kurkar and/or facies substitution of calcareous sandstone by loose sand and sandy loam (Rehovot Fm.) with no impedance contrast between the surficial and underlying beds. Based on our noise investigation we corrected the relief of the bedrock near Points 21 and 22 as shown on Fig. 29 by the red line. On the geological cross-section located north of the Tel-Aviv area (see Fig. 30) we observe the increase in sedimentary thickness from 5-10m at the shore line up to 50-70m to the east. This increasing thickness is detected by the measurements, namely, by a dominant frequency decreasing from 3.4 Hz at Point 58 up to 1.6 Hz at Point 60. Higher amplifications up to factor 4-5 were observed at the Yarkon riverbed and could be explained by a consolidation of sandstone along the riverbed and its facies substitution by limestone. The Carmel coastal zone is distinguished on the amplification map owing to its higher level of amplification values (up to 7). As seen in the geological cross-section along the Carmel coast (Fig. 31), in the southern part of the profile (sites 131 and 132) loose sediments overly the calcareous sandstone, i.e. we have the geological conditions described above in the central region. Close to Binyamina a deep-seated transverse fault was mapped. To the north of this fault, according to our observations, the reflector (half space) in this region is correlated to the hard carbonates of Top Judea Group. The calcareous sandstone exposed along the coastline yields amplification less than factor 2. Higher levels of amplification from factor 4 up to factor 7 were observed in the Carmel Coastal Plain where 15-35 m thick silt together with calcareous sandstone overlays dense dolomites and chalks. The prevalent frequency in Carmel Coastal Plain area is 2-3 Hz. The increase in dominant frequency to the north is related to the thinning of the sedimentary cover (site 86) and/or the lithological composition of sediments. Generally, theoretical site response requires input local geology data of spatial distribution of soft materials above the hard bedrock in terms of densities and shear velocities or equivalent parameters. In many cases the complexity of actual conditions and the uncertainties associated with interpreting indirect measurements, limit the availability, quality and reliability of these data (see also Boor and Brown, 1998); hence, different possible 1D models of the subsurface based on the same geological and geophysical observations, may yield analytical functions that are very different from those obtained empirically. As demonstrated in Figure 32, there is a difference between the theoretical transfer function derived on the basis of available information about subsurface structure at Point 59 and the response function obtained from measurement data. It is strongly recommended that prediction of earthquake ground motions be obtained by combining different empirical approaches, supplemented with geophysical and geological information. If a model is required for estimating nonlinear strong motion effects, the empirical results can be utilized to select a plausible geological model that yields agreement with the empirical determination at weak motion levels. Local-scale uniform seismic hazard site-specific acceleration spectra maps are an important component of loss estimation, because they provide information on possible site effects. In recent studies (Boore and Joyner, 1997; Boore, 2000) a new definition of site classes and empirical amplification factor characterized by average shear-wave velocity in the upper 30m is given. The site characterization is now dependent only on the top 30m of soil disregarding the soil properties below 30m and that of the rock underlying the soil. Therefore, the site characterization is dependent on single parameters. Figure 33 shows examples of Site-specific acceleration spectra for two sites obtained by Stochastic Estimation of the Earthquake Hazard method (SEEH) developed Shapira and van Eck (1993) superimposed on pseudo-acceleration response spectra for the random horizontal component (Boore and Joyner, 1997). The computations were made for M=6.5, distance 70km and shear-wave velocity V30=250 m/sec. The shapes of the spectra obtained are significantly different from the ones prescribed by the average shear-wave velocity in the shallowest 30m and SEEH method based on convolution of the synthetic acceleration with the response function of the site. Note, however, that the PGA values are very similar. |
| Conclusions |
|
The experiment discussed in the present study had the following goals: - in situ site effect estimation in the Coastal Plain area between Ashqelon and Haifa using the microtremor measurements of ground motions; - improving theoretical site response determinations by comparing the empirical and the analytical assessments, selecting parameters of the soil column models for satisfactorily predicting the transfer function by multi-layer 1-D models when linear behavior of the soil is assumed; - evaluating site-dependent seismic hazard in terms of ground motion parameters used for engineering applications. The conclusions may be summarized as follows: 1. The selection of an appropriate ensemble of windows of microtremors for horizontal-to-vertical spectral ratios facilitates successful removal of time-variant source effects. In the studied area the ground motion amplifications are of factor 2-7 over the frequency range from 1 to 6 Hz. The amplification obtained by H/V ratios may be explained not only by peaks in the spectra of the horizontal components but also by trough in the spectra of the vertical component. 2. The soil profiles at the investigated sites are very different. Some sites have simple profiles in the uppermost surface layer and very clear contrast between the soft soil layer and half-space rock layer. Other sites have complicated surface soil layers structures, less distinct contrast between the surface soils and underlying Vs reflector. In many cases our attempts to estimate depth to the half-space from borehole data failed. Only when the distribution of the predominant frequency and maximum amplifications maps were constructed, were strong correlation between features of geological structure and measurement results revealed: a) The 1.2-3.5 Hz ratio pattern and amplification factor 2-3 matched loose deposits lying on kurkar of 10-50m thick. The bedrock represented by limestone with shear-wave velocities of 900-1110 m/s can increase amplification up to factor 6 in the frequency range 1.2-3.5 Hz. b) In the Carmel coast loose sediments and kurkar with a total thickness of 15-35m overlaying carbonates of the Top Judea Gr. can yield amplification factor up to 8 at frequencies of 2-3 Hz. c) Thinning sedimentary thickness causes increase in the dominant frequency up to 6 Hz. Such frequencies are observed in the northern part of the Carmel coastal area. d) We observed that the site response is almost flat over the entire frequency range and does not show a predominant frequency in cases where the kurkar is resting on clays of the Yafo Fm., or loose deposits a few meters thick overlay kurkar. e) Locally enhanced amplification (factor 3-6) along river beds could be interpreted as an indication of consolidation of sandstone and its facies substitution by limestone. 3. Numerical methods for estimating site effects required the characterization of the geotechnical properties of the area. The specific parameters used for such analysis are shear-wave velocity, density, damping and thickness of each layer. The main disadvantage of these methods is the relatively high cost in conducting the necessary geotechnical or geophysical studies and the inherent uncertainty of the data. The joint application of analytical and empirical techniques for assessing soil response functions can provide useful feedback to improve the reliability of the obtained results. A detailed comparison of the analytical and empirical values constitutes a low-cost, efficient and fast procedure in order to establish the spatial dependence of both suitability and reliability of the method, improvement of models assumed and delimitation of those areas for which in-depth surveys are needed for proper assessment of soil response. 4. It is very important to derive the ground shaking characteristics, resonance frequency and amplification factor in order to estimate the dynamical behavior of structure for seismic resistant design and for the damage assessment due to future predicted earthquake. Microtremors studies with horizontal-to-vertical spectral ratio can yield relevant information to the field of earthquake hazard assessment and microzonation. This is especially true given the lack of alternative economic and time-saving method available for characterizing site response in regions with low levels of seismicity. 5. The hazard maps presented here in terms of Uniform Site Specific Acceleration Response Spectra may be useful for land use planning or for making regional hazard mitigation decisions. The use of these maps in loss estimation can help state and the authorities to set priorities in managing enforcing building codes, conducting seismic strengthening programs for existing structures, and planning for emergency response and long term recovery. |
| Acknowledgment |
|
Our cordial thanks for the financial support of the Steering Committee for the National Earthquake Preparedness and Mitigation. We are most thankful to Dr. A. Hofshtetter and L. Fleisher for fruitful discussion. Thank are also due to I. Chelinski, D. Artzi and Y. Menahem for their assistance in preparing this report. |
| References |
Aguirre, J. and Irikura, K., 1997. Nonlinearity, liquefaction, and velocity variation of soft soil layers in Port Island, Kobe, during the Hyogo-ken Nanbu earthquake, Bull. Seismo. Soc. Am., 87:1244-1258. Beresnev, I.A., Field, E.H., Koen, V.D., Abeele, K., and Johnson, P.A., 1998. Magnitude of nonlinear sediment response in Los Angeles basin during the 1994 Northridge, California, Earthquake, Bull. Seismol. Soc. Am., 88:1079-1084. Bonilla, L.F., Steidl, J.H., Lindley, G.T., Tumarkin, A.G., and Archuleta, J., 1997. Site amplification in the San Fernando Valley, California: variability of site effect estimation using S-wave, coda and H/V method, Bull. Seism. Soc. Am., 87:710-730. Boore, D. M., and Joyner, W.B., 1997. Site amplification for generic rock sites, Bull. Seism. Soc. Am., 87:327-341. Boore, D.M., 2000. SMSIM – Fortran programs for simulating ground motions from earthquakes: Version 2.0 – a revision of OFR 96-8—A, U.S. Geological Survey Open-File Report OF 00-59, 55pp. Boore, D.M., and Brown, L.T, 1998. Comparing shear-wave velocity profiles from inversion of surface-waves phase velocities with downhole measurements: systematic differences between the CXW method and down-hole measurements at six USC strong motion sites, Seismological Research Letters, 69, No. 3, 222-229. Borcherdt, R. and Glassmoyer, G., 1992. On the characteristics of local geology and their influence on the ground motions generated by the Loma Prieta earthquake in the San Francisco Bay region, California, Bull. Seismol. Soc. Am., 82:603-641. Borcherdt, R.D., 1970. Effects of local geology on ground motion near San Francisco bay, Bull. Seism. Soc. Am., 60:29-61. Carver, D. and Hartzell, S.H., 1996. Earthquake site response in Santa Cruz, California, Bull. Seismol. Soc. Am., 86:55-65. Chavez-Garcia, F.J. and Cuenca, J., 1998. Site effects and microzonation in Acapulco, Earthquake Spectra, 14, No. 1, 75-93. Darragh, R.B., Shakal, A.F., 1991. The site response of two rock and soil station pairs to strong and weak motion, Bull. Seismol. Soc. Am., 81:1885-1889. Ecker, A., 1999. Selected geological cross-sections and subsurface maps in the coastal aquifer of Israel, Atlas, Jerusalem. Ezersky, M., Shtivelman, V., 1999. Seismic refraction surveys for updating the microzonation map of Israel, GII Report No. 802/43/99,12 pp. Field, E.H., Jacob, K.H., Hough, S.E., 1992. Earthquake site response estimation: a weak-motion case study, Bull. Seism. Soc. Am., 82:2283-2306. Field, E.H., and Jacob, K.H., 1993. Monte-Carlo simulation of the theoretical site response variability at Turkey Flat, California, given the uncertainty in the geotechnically derived input parameters, Earthquake Spectra, V 5, No. 9, 669-701. Field, E.H., Jacob, K.H., 1995. A comparison and test of various site-response estimation techniques, including three that are not reference-site dependent, Bull. Seismol. Soc. Am., 85:1127-1143. Field, E.N., 1996. Spectral amplification in sediment-filled valley exhibiting clear basin-edge-induced waves, Bull. Seismol. Soc. Am., 86:991-1005. Fleischer, L., Gelberman, E. and Wolff, O., 1993. A geological-geophysical reassessment of the Judea Group, IPRG Report No. 244/147/92. Gitterman, Y., Zaslavsky, Y., Shapira, A., Shtivelman, V., 1996. Empirical site response evaluations: case studies in Israel, Soil Dynamics Earthquake Engineering, 15:447-463. Gutierrez, C., Singh, S.K., 1992. A site effect study in Acapulco, Guerrero, Mexico: comparison of results from strong-motion and microtremor data, Bull. Seismol. Soc. Am., 82:642-659. Gvirtzman, G. , Shachnai, E., Bakler, N. and Ilani, S., 1984. Stratigrahy of the Kurkar group (Quaternary) of the Coastal Plain of Israel, GSI Current Research 1983-1984, 70-82. Gvirtzman, G., 1970. The Saqiye Group (Late Eocene to Early Pleistocene) in the Coastal Plain and Hashephela regions, Israel, Ph D. Thesis, Hebrew University Jerusalem. Gvirtzman, G., 1969. Subsurface data on the Saqiye Group in the Coastal Plain and Hashelphela regions, Israel, Geological Survey of Israel, OD/1/69. Gvirtzman, G. and Reiss, Z., 1965. Stratigraphic nomenclature in the Coastal Plain and Hashefela regions, GSI Report OD/1/65. Hofstetter, A., T., van Eck, A. Shapira, 1996. Seismic activity along fault branches of the Dead Sea-Jordan transform system: The Carmel-Tirtza fault system, Tectonophysics, 267: 317-330. Hartzell, S., 1998. Variability in nonlinear sediments response during the 1994 Northridge, California, earthquake, Bull. Seismol. Soc. Am., 88:1426-1437. Hartzell, S.H., Leeds, A., Frankel, A., Michael, J., 1996. Site response for urban Los Angeles using aftershocks of the Northridge earthquake, Bull. Seismol. Soc. Am., 86:S168-192. Horike, M., Zhao, B., Kawase, H., 2001. Comparison of site response characteristics inferred from microtremors and earthquake shear wave, Bull. Seism. Soc. Am., 91:1526-1536. Jarpe, S.P., Cramer, C.H., Tucker, B.E., Shakal, A.F., 1988. A comparison of observations of ground response to weak and strong motion at Coalinga, California, Bull, Seismol. Soc. Am., 78:421-435. Jarpe, S.P., Cramer, C.H., Tucker, B.E., Shakal, A.F., 1988. A comparison of observations of ground response to weak and strong motion at Coalinga, California, Bull, Seismol. Soc. Am., 78:421-435. Jongmans, D., Campillo, M., 1993. The response of the Ubaye valley (France) for incident SH and SV waves: comparison between measurements and modeling, Bull. Seismol. Soc. Am., 83:907-924. Joyner, W.B., 1977. A Fortran program for calculating nonlinear seismic response, U.S. Geological Survey, Open File Report 77-671. Joyner, W.B., Boore, D.M., 1981. Peak horizontal acceleration and velocity from strong-motion records including records from the 1979 Imperial Valley, California, earthquake, Bull. Seism. Soc. Am., 71:2011-2038. King, J.L., Tucker, B.E., 1984. Observed variations of earthquake motion across a sediment-filled valley, Bull. Seismol. Soc. Am., 74:137-151. Konno, K., Ohmachi, T., 1998. Ground-motion characteristics estimated from spectral ratio between horizontal and vertical components of microtremors, Bull. Seismol. Soc. Am., 88:228-241. Lachet C., and Bard, P., 1994. Numerical and theoretical investigations on the possibilities and limitations of the Nakamura’s technique, J. Phys. Earth, 42:77-397. Lermo, J., Chavez-Garcia, F.J., 1993. Site effect evaluation using spectral ratios with only one station, Bull. Seismol. Soc. Am., 83:1574-1594. Lermo, J., Chavez-Garcia, F.J., 1994. Are microtremors useful in site response evaluation? Bull. Seismol. Soc. Am., 84:1350-1364. Liu, H.-P., Warrick, R.E., Westerlund, R.E., Sembera, E.D., Wennerberg, L., 1992. Observation of local site effects at a downhole-and-surface station in the marina district of San Francisco, Bull. Seismol. Soc. Am., 82:1563-1591. Malagnini, L., Tricarico, P., Rovelli, A., Herrmann, R.B., Opice, S., Biella, G., de Franco, R., 1996. Explosion, earthquake, and ambient noise recording in a Pliocene sediment-filled valley: inferences on seismic response properties by reference and non-reference-site techniques, Bull. Seismol. Soc. Am., 86:670-682. McGarr, A., Celebi, M., Sembera, E., Noce, T., Mueller, C., 1991. Ground motion at the San Francisco international airport from the Loma Prieta earthquake sequence, Bull. Seismol. Soc. Am., 81:1923-1944. Meneroud, J-P., Duval, A-M., and Bard, P-Y., 2000. Methodological study on seismic risk assessment, 12th World Conference on Earthquake Engineering. Auckland, New Zealand, Proceedings, Article No. 1874. Mucciarelli, M., Monachesi, G., 1998. A quick survey of local amplifications and their correlation with damage observed during the Umbro-Marchesan (Italy) earthquake of September 26, 1997, Journal of Earthquake Engineering, Vol. 2., No 2, 325-337. Nakamura, Y., 1989. A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, QR of RTRI, Vol. 30, No.1, 25-33. Nakamura, Y., 2000. Clear identification of fundamental idea of the Nakamura’s technique and its applications, 12th World Conference of Earthquake Engineering, Auckland, New Zeeland, Proceedings, No.0084, pp8. Ohmachi, T., Nakamura, Y., Toshinawa, T., 1991. Ground motion characteristics of the San Francisco bay area detected by microtremor measurements, 2nd International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. St. Louis, Missouri, pp. 1643-1648. Reinoso, E., Ordaz, M., 1999. Spectral amplification for Mexico City from free-field recordings, Earthquake Spectra, Vol. 15, No. 2, 273-295. Rogers, A.M., Borcherdt, R.D., Covington, P.A., Perkins, D.M., 1984. A comparative ground response study near Los Angeles using recordings of Nevada nuclear tests and the 1971 San Fernando earthquake, Bull. Seismol. Soc. Am., 74:1925-1949. Safak, E., 1997. Models and methods to characterize site amplification from a pair of records, Earthquake Spectra, 13:97-129. Satoh, T., Kawase, H., Matsushima, S., 2001. Difference between site characteristics obtained from microtremors, S-wave, P-waves and codas. Bull. Seism. Soc. Am., 91:313-334. Satoh, T., Sato, T., Kawase, H., 1995. Nonlinear behavior of soil sediments identified by using borehole records observed at the Ashigara valley, Japan, Bull. Seismol. Soc. Am., 85:1821-1834. Schnabel, P.B., Lysmer, J., Seed, H.B., 1972. SHAKE – A computer program for response analysis of horizontally layered sites, Report No. EERC 72-12, Univ. of California at Berkeley. Seed, H.B., Romo, M.R., Sun, J.I., Jaime, A., and Lysmer, L., 1988. The earthquake of September 19, 1985 – relationship between soil conditions and earthquake ground motion, Earthquake Spectra, 4: 687-729. Seekins, L.C., Wennerberg, L., Margheriti, L., Liu, H-P., 1996. Site amplification at five locations in San Francisco, California: a comparison of S waves, codas and microtremors, Bull. Seismol. Soc. Am., 86:627-635. Shamir, G., Bartov, Y., Sneh, A., Fleischer, L., Arad., A., Rosenshaft, M, 2001. Preliminary seismic Zonation in Israel, GSI Report No.GSI12/01, GII Report No. 550/95/01(1), pp 10. Shapira, A., Avirav, V., 1995. PS-SDA Operation Manual, IPRG Report Z1/567/79, 24pp. Shapira, A., Feldman, L., Zaslavsky, Y., Malitzky, A., 2001. Application of a stochastic method for the development of earthquake damage scenarios: Eilat, Israel test case, Computational Seismology, 32:58-73. Singh, S.K., Mena, E., Castro, R., 1988. Some aspects of source characteristics of the 19 September 1985 Michoacan earthquakes and ground motion amplification in and near Mexico City from strong motion data, Bull. Seismol. Soc. Am., 78:451-477. Sneh, A., Bartov, Y. and Rosensaft, M., 1998.Geological Map of Israel 1: 200,000. Sheet 2, Geological Survey of Israel. Steidl, J.H., Tumarkin, A.G., Archuleta, R.J., 1996. What is a reference site? Bull. Seismol. Soc. Am., 86:1733-1748. Shtivelman, V., 1999. Using surface waves for estimating shear wave velocities in the shallow subsurface onshore and offshore Israel, European Journal of Environmental and Engineering Geophysics, 4:15-35. Stivelman, V., 1998. Geophysical site investigation in the Haifa port extension project area using shallow water seismic surveys, GII Report 838/49/48. Stivelman, V., 1998. P- and S-wave refraction surveys near the power stations in Haifa, Ashqelon and Hadera., GII Report No. 592/86/97. Stivelman, V., 1998. Seismic refraction serveys near the fuel storage devices DELEK-1 and DELEK-4 in the Reading power station area., GII Report No. 516/22/98. Su, F., Anderson, J.G., Zeng, Y., 1998. Study of week and strong ground motion including nonlinearity from the Northridge, California, Earthquake Sequence, Bull. Seismol. Soc. Am., 88:1411-1425. Theodulidis, N., Bard, P.Y., Archuleta, R., Bouchon, M., 1996. Horizontal-to-vertical spectral ratio and geological conditions: the case of Garner valley downhole in Southern California, Bull. Seismol. Soc. Am., 68:767-779. Toshinawa, T., Taber, J.J., Berrill, J.B., 1997. Distribution of ground motion intensity inferred from questionnaire survey, earthquake recordings, and microtremor measurements - a case study in Christchurch, New Zealand, during 1994 Arthur’s Pass earthquake, Bull. Seismol. Soc. Am., 87:356-369. Tucker, B.E., King, J.L., 1984. Dependence of sediment-filled valley response on input amplitude and valley properties, Bull. Seismol. Soc. Am., 74:153-165. Zaslavsky, Y., Gitterman, Y., Shapira, A., 1995. Site response estimations in Israel using weak motion measurements. 5th International Conference on Seismic Zonation. Nice, France, Proceedings, 1713-1722. Zaslavsky, Y., Shapira, A., 2000. Questioning nonlinear effects in Eilat during MW=7.1 Gulf of Aqaba Earthquake, XXVII General Assembly of the European Seismological Commission (ESC), Lisbon, Portugal, Proceedings,343-347. Zaslavsky, Y., Shapira, A., Arzi, A.A., 2000. Amplification effects from earthquakes and ambient noise in Dead Sea Rift (Israel), Soil Dynamics and Earthquake Engineering, 20/1-4, 187-207. Zaslavsky, Y. et al., 2001. Microzoning of the earthquake hazard in Israel. Seismic microzoning of Lod and Ranmla, GII Report 569/143/01. Zaslavsky, Y., Shapira, A., Arzi, A.A., 2002a. Earthquake site response on hard rock – empirical study, 5th International Conference on Analysis of Discontinuous Deformation, ICADD. Beer-Sheva, Israel, Proceedings (in print). Zaslavsky, Y., Shapira, A., Kenigsberg, M., 2002b. Earthquake site response study for designed bridges in Israel. 12th Conference on Earthquake Engineering. London, UK, Proceedings (in print). á÷, à., ÷øáöåá, à., 1998. ñ÷ø âéåôéñé áùéèú äøôø÷öéä äñééñîéú ìîéôåé úú ä÷ø÷ò áàéæåø çåó ãåø, ãå"ç îñ' 217/65/97 øåðï, ò., 2001. ñ÷ø øôø÷õéä ñééñîéú òí âìé ìçéöä åâìé âæéøä áàúø úçðú äëåç "àùëåì" , àùãåã., ãå"ç îñ' 287/116/01. æñìáñ÷é, é., ôìã, à., 1998. äòøëú ñô÷èøåí äúàåöåú áúëðú äëåç "øéãéðâ" , ú"à, ãå"ç îñ' 516/22/98. ùèéáìîï , å., 1994. ñ÷ø øôø÷õéä áðúðéä,42/109/93 ùèéáìîï, å., ùôéøà, à., æñìáñ÷é, é., 1996. ç÷ø àúø áòæøú ñ÷øéí âéåôéñééí áàéæåø øçåá çéøéí, çéôä, ãå"ç îñ' 819/129/96.äöâú ðúåðéí ìéúåìåâééí á÷éãåçéí , îùøã úùúéåú; äîëåï äâéàåìåâé; äùøåú ääéãøåìåâé ìàùøàì. 2001 |
| LIST OF FIGURES |
|
Figure 1 Geological map and locations of observation points Figure 2 Locations of the seismometers during various sets of site investigations Figure 3 H/V spectral ratio from microtremors observed at Point 4 at different times Figure 4 Examples of average Fourier spectra and H/V spectral ratios of microtremor ground motion in different sites providing true evaluation of site effects:(a) Point 65-3 and (b) Point 51 Figure 5 Examples of average Fourier spectra and H/V spectral ratios of microtremor ground motion in different sites providing false evaluation of site effects:(a) Point 87; (b) Point 135 and (c) Point 140 Figure 6 Examples of individual and average H/V spectral ratios for three points located on the kurkar outcrop Figure 7 Individual and average H/V spectral ratios for two sites located on thin unconsolidated sediments (thickness varying from 2 to 6 m to bedrock). Figure 8 Individual and average H/V spectral ratios for two neighboring points Figure 9 Examples of individual and average H/V spectral ratios for three points located on identical soil profile overlying the kurkar with different S velocities:(a) VS=1100 m/sec; (b) VS=900 m/sec; (c) VS=700 m/sec Figure 10 Examples of individual and average H/V spectral ratios for four points with low contrast (VSrock/VSsoil<1.4) Figure 11 Examples of individual and average H/V spectral ratios for four points with medium contrast (VSrock/VSsoil=2) Figure 12 Example of localization peak in H/V spectral ratios: narrow trough in vertical spectra and high in horizontal spectra Figure 13 Examples of average Fourier spectra and H/V spectral ratios of microtremor ground motions at different sites facilitating spectral ratio estimates using the frequency-independents factor Figure 14 Example of significant variation of site effects over very short distances:(a) Point 85; (b) Point 85-1. Figure 15 Distribution of the maximum amplification factor estimated from the H/V spectral ratios of microtremors for the Coastal Plain area Figure 16 Distribution of the fundamental frequency estimated from H/V spectral ratios of microtremors for the Coastal Plain area. Figure 17 Comparison between refractor depth obtained by mapping the sediment refractor and well section Figure 18 Example of logging interpretation at Ashdod-1 borehole Figure 19 Comparison between experimental and analytical response functions:(a) – for Point 123; (b) – for Point 58. Figure 20 Comparison of analytical and experimental functions:(a) for Point 144; (b) – for Point 68. Figure 21 Example of two possible models for Point 147 with different half-space:(a) – limestone; (b) – clay Figure 22 Theoretical response functions compared with experimental spectral ratios:(a) - for Point 149; (b) – for Point 82C and (c) – for Point p90-1 (Carmel Coast) Figure 23 Uniform hazard site-specific acceleration spectra for different sites in the central Coastal Plain. Figure 24 Uniform hazard site-specific acceleration spectra for different sites along the Carmel coast Figure 25 Distribution of spectral acceleration at 0.2 sec for 10% probability of exceedence in 50 years and damping ratio 5% in Coastal Plain area Figure 26 Distribution of spectral acceleration at 1 sec for 10% probability of exceedence in 50 years and damping ratio 5% in Coastal Plain area Figure 27 Map showing zones division in the Coastal Plain Figure 28 Comparison between site-specific acceleration spectra computed on the basis of generalized models and models calculated for individual sites Figure 29 Geological cross section along Line 6 (Ashdod area) Figure 30 Geological cross section along Line 12 (Tel-Aviv area) Figure 31 Geological cross section along Carmel coast Figure 32 Difference between analytical response functions for Point 59 calculated by geological data and on the basis of measurements Figure 33 Comparison of 5%-damped site-specific acceleration spectra for two sites obtained by Shapira and van Eck method (1993) and pseudo-acceleration response spectra for the random horizontal component proposed by Boor et al., (1997). The computations were made for M=6.5, distance 70 km and V30=250 m/sec |
| LIST OF TABLES |
|
Table 1 Stratigraphic nomenclature of sedimentary rocks for the Coastal Plain of Israel Table 2 Dominant frequency and maximum amplification factor at Coastal Plain Table 3 Velocity structure of the Coastal Plain obtained from seismic surveys at different sites. Table 4 Mechanical properties of the different materials in the geotechnical model obtained by logging at Ashdod-1 borehole Table 5 Geotechnical model used in theoretical site response estimation and comparison between fundamental parameters of experimental and calculated response function for Yehoshua well (Point 123) Table 6 Optimal geotechnical models used in the theoretical site response estimation and comparison between fundamental parameters of experimental and calculated response functions Table 7 Parameters of the generalized soil-column models for the Zones at the Coastal Plain |
| APPENDIX A |
|
Table A1. General information on seismic station locations. Table A2. Basic input parameters of analytical models used in stochastic simulations. |
