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.