EXPERIMENTAL STUDY OF TOPOGRAPHIC AMPLIFICATION USING THE ISRAEL SEISMIC NETWORK

Earthquakes and microtremor records are used for estimating the site response of hard rock sites comprising four three-component seismic stations which operate as part of the Israel Seismic Network. The response functions are determined by implementing the horizontal-to-vertical component spectral ratio of earthquake shear-waves (receiver function estimates) and microtremors (Nakamura's estimate) observed simultaneously at the site. The sites of seismic stations ATZ (Mt. Atzmon), MBH (Mt. Berech) and MRNI (Mt. Meron) exhibit amplification attributed to topography effects. At ATZ, within the 1.3–2.0 Hz range, the amplification is in the order of factor 4. At MBH amplification levels of 3.0–3.5 are observed in the frequency range 1.5–4.0 Hz. Station MRNI exhibits a relatively strong amplification effect (up to 4) in the frequency range of about 2.5 to 3.5 Hz. Slight amplification around 5 Hz is observed at ATR (the proposed site for a nuclear power plant). These effects were correlated with the thickness of the weathered layer above unweathered chalk. A comparison between the amplification factor observed during earthquakes and those inferred from microtremors shows that these are, in general, in agreement. However, details of the spectral ratios from different microtremor recordings are not exactly the same. Differences appear mainly in the frequency at which the maximum amplification occurs. These observations demonstrate the usefulness of non-reference technique in estimating the topographical effects of ground shaking. These methods may be used in the process of seismic hazard assessment for ridges and mountain tops, common sites for settlements, communication relay stations, bridges, rope-drive and power transmission towers.     

EMPIRICAL EVALUATION OF SITE EFFECTS BY MEANS OF H/V SPECTRAL RATIOS AT THE LOCATIONS OF STRONG MOTION ACCELEROMETERS IN ISRAEL

Acceleration data from local and regional earthquakes is of prime importance in evaluating the seismic hazard. Consequently, strong motion accelerometers are currently installed at more than 60 locations in Israel. We have explored the possibility of site amplification effects at 10 sites where local earthquakes triggered strong motion accelerometers by integrating empirical and analytical estimations. Implementing H/V spectral ratio techniques using 15 accelerograms from nine earthquakes, 105 seismograms shear-wave records of 35 local and regional earthquakes and seismograms of microtremors were used in the empirical evaluations. The subsurface models were constructed by integrating available geological and geophysical information at the analysed site with empirically evaluated site response functions. Amplification effects of factor 3-6 are observed at various frequencies in the 0.8-6.0 Hz band. Through the analysis process it became evident that the instant availability of many useful time windows of microtremors provides systematic estimations of the fundamental resonance frequency of each site and their associated amplification levels, which are similar to those obtained from H/V spectral ratios of seismograms and accelerograms and to those inferred from the subsurface geology. Analytical transfer functions should be reviewed with respect to empirical site response evaluations. Estimations that are based on only one approach may be totally misleading.     

MICROZONATION OF THE EXPECTED SEISMIC SITE EFFECTS ACROSS PORT VILA,VANUATU

The city of Port Vila, Vanuatu, is located in one of the most active seismic regions on earth. Earthquakes are felt frequently and, due to very rapid plate convergence rates, return period of large earthquakes (M>6) in the New Hebrides Benioff zone can be less than 10 years. Even though Port Vila does not lie on an identified seismic fault zone, strong motions by nearby earthquakes have to be expected due to the city's geographical location close to the plate boundary of New Hebrides convergence zone. An accurate estimation of the seismic ground motion across the city is of prime importance for urban developments and mitigation of earthquake risk. Following many examples of monitored strong earthquakes in the current century, it is evident that the local site effects may have a dominant contribution to the intensity of damage and destruction. In this study we focussed on the first stage of associating site effects and seismic hazard by preparing a microzonation map for Port Vila. The seismic microzonation of the city has been carried out to provide a detailed map of the zones that exhibit site effects in terms of resonance frequencies and approximated amplification of the ground shaking. Having in mind that these data will be used in improving building design to sustain strong ground motions, our analysis is limited to the frequency band of 1–10 Hz, corresponding to the expected resonance of different types of buildings in Port Vila. The Nakamura technique has been used to estimate site amplification effects from single station noise recordings. Interestingly, excluding one site located on an old dump zone, the amplification factors at about the 100 sites surveyed in Port Vila remain below 3 with an average well below 2 in the 1 to 10 Hz frequency band. These results suggest that there is no significant Vs velocity change in consequently layered material and that the uppermost sedimentary layers in the surveyed down town area are relatively thin. These observations are in agreement with the mapping of limestone terraces throughout Port Vila area. However, both the surface geology and results from seismic zonation indicate a thicker (up to several tens of meters) sedimentary cover around the Bauerfield airport and in the Mele terrace zone. Low resonance frequencies (around and below 1 Hz) and amplification factor of the order of 5 were observed over this large area, immediately outside Port Vila. Any building development in this area should take these results into account.