Site Response from Ambient Vibrations in the Towns of Lod and Ramle (Israel) and Earthquake Hazard Assessment

Abstract

Throughout their long history, the towns of Lod and Ramle have been severely affected by strong earthquakes. The last destructive earthquake occurred on July 11, 1927 and caused the destruction of large parts of these cities, reaching a seismic intensity of VIII-IX on the MSK scale. Such a high intensity from a relatively distant earthquake (about 70 km) of magnitude 6.2 is likely to be the result of local site effects of the sedimentary layers that may have significantly enhanced earthquake ground motions.This study is focused on estimating the seismic hazard to Lod and Ramle by implementing a three-step process: (1) detailed mapping of the characteristics of the H/V spectral ratios from ambient noise, (2) incorporating geological information and well data to construct subsurface models for different sites within the investigated area and (3) estimating the seismic hazard in terms of uniform hazard site-specific accelerations.The horizontal-to-vertical spectral ratios of ambient noise were used to approximate the fundamental resonance frequencies of the subsurface and their associated amplitudes. About 360 sites in Lod and Ramle were instrumented for varying periods. The soil sites exhibits H/V peak amplitudes ranging from 4 to 6 in the frequency range 0.5-2.5 Hz. These data were used to constrain 1-D subsurface models that were developed using geological data and borehole information. H/V spectral ratio observations were checked against theoretical subsurface transfer functions at locations where borehole information is available farther constraint the range of possible V s velocities of the different layers and thus, by means of trial an error it was possible to conclude a systematic spatial distribution of the V s velocity and thickness in the substrata that are also consistent with the spatial distribution of the fundamental resonance frequencies of the soft sediments obtained by means of the H/V spectral ratios, and other geological and geophysical information available at different locations in the study area.The evaluated subsurface models are introduced using the SEEH procedure of Shapira and van Eck [(1993) Natural Hazards 8, 201-205] to assess Uniform Hazard Site-Specific Acceleration Spectra for different zones within the towns of Lod and Ramle. These evaluations are very important for realistic assessment of the vulnerabilities of all types of existing and newly designed structures and for urban and land use planning. 356Y. ZASLAVSKY ET AL.

Combination of HVSR and MASW Methods to Obtain Shear Wave Velocity Model of Subsurface in Israel

“…It was demonstrated through many studies (Zaslavsky et al 2005(Zaslavsky et al , 2008(Zaslavsky et al , 2009) that, when noise measurements are made near boreholes and/or near refraction surveys, the fundamental frequency and its corresponding H/V amplitude are practically the same as the fundamental frequency and its corresponding amplification level derived from the computed transfer function of SH-waves at low strains propagating through a relatively simple 1-D model of the site, as known from geotechnical and geophysical surveying. Computer code SHAKE (Schnabel et al 1972) is used to analytically evaluate the site response function.…”

“…The HVSR technique has become the primary tool of choice in many of the ambient noise related studies and it has been successful in seismology to estimate the local transfer function in the site response problem in Israel and worldwide (Lerma and Chávez-García 1994, Malischewsky et al 2010, Mucciarelli and Gallipoli 2004, Seekins et al 1996, Zaslavsky et al 2005, 2008). Nakamura's method is based on the assumption that micro-tremors consist of body waves.…”

“…Moreover, predicting site effect parameters based on models inferred from geological and geophysical information only, may differ significantly from experimental estimation (Zaslavsky et al 2005(Zaslavsky et al , 2008(Zaslavsky et al , and 2009). …”

Modeling warning times for the Israel’s earthquake early warning system

“…Even with only a few seconds after the alarm, one may undertake mitigating actions: hiding under a desk/table or moving inside a special shelter or, if on the groundfloor, leaving a building. Important argument in favor of the decision to build the REEWS in Israel was the fact that strong earthquakes, such as the 1927 magnitude M=6.2 event, can cause great destruction and lost of life at distances of up to 60-70 km from the epicenter, as it did in Ramla and Lod, enhanced by strong site effects (Zaslavsky et al 2005) and, possibly, directivity. As it will be shown below, at such a distance, the REEEWS should be able to provide a warning time of about 10-12 s, which may be enough to perform elementary life-saving actions for trained people or automatic systems (stopping elevators, shutting gas lines, closing bridges, tunnels, etc.…”

Questioning the applicability of soil amplification factors as defined by NEHRP (USA) in the Israel building standards

“…Hence, the site classification system should include adequate dynamic parameters of the soil during earthquakes. Over the years, the Seismology Division of the Geophysical Institute of Israel conducted site investigations in several thousands of sites across Israel in more than 30 towns and many surrounding villages [6][7][8][9]. Our studies cover areas of more than 900 km 2 .…”