In this study we examined the applicability of the NEHRP soil classification which is based on the V_S,30 parameter and the corresponding F_a and F_v factors to correct acceleration response spectra for local site classification. We calculated acceleration response spectra (10% in 50 years) for more than 1900 sites across Israel. Computations were made for hard rock conditions and for the actual site conditions, while considering nonlinear response of the soils. Based on synthetic acceleration response spectra of real sites (using stochastic procedures), it is concluded that generalizing site classification and consequently, amplification, by means of a single parameter V_{S, 30} is not recommended. The geology of Israel is complex and may vary significantly over short distances. Sites of similar class, show different soil effects both in shape, amplitude and in frequency. In many cases there is no correlation between amplification and V_S, and in other cases, the scatter is unacceptably high

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.

La amplificación del movimiento del terreno, como resultado de la presencia de suelos blandos, es un fenómeno común en áreas urbanas y bien identificado como un factor que incrementa el daño y el número de pérdidas humanas. Por otro lado, para el análisis de la amenaza sísmica, el estudio de la elipticidad de las ondas de Rayleigh se ha hecho más popular en el contexto del uso de registros de vibraciones ambientales, además, los resultados pueden ser usados en la inversión de la estructura de velocidades. Los efectos de sitio normalmente pueden ser modelados a partir de un perfil de velocidades simple, dado el alto contraste de impedancias en la estructura somera del subsuelo. Por lo tanto, el análisis y el entendimiento de las implicaciones de un modelo tan simple como una capa sobre un semiespacio (LOH, por sus siglas en inglés) son de suma importancia, no sólo teórica sino también práctica. Adicionalmente, para registros de vibraciones ambientales todavía no se cuenta con un modelo teórico que explique de manera satisfactoria los resultados de los cocientes espectrales H/V; un punto de inicio sobre la elipticidad de las ondas Rayleigh es la fórmula exacta propuesta por Malischewsky y Scherbaum (2004). Es possible mostrar que un modelo tan simple como LOH puede producir una gran variedad de curvas H/V-versus-frecuencia y mostramos como ejemplo las curvas H/V con más de un máximo para los casos de Israel y de México. Este fenómeno se atribuye a la contribución de capas adicionales, esto es, que el primer máximo se asocia con la frecuencia de resonancia de la primera capa y los máximos secundarios se asocian con las frecuencias de resonancia de capas más profundas. Demostramos que con un modelo LOH obtenemos dos máximos, para ciertos valores del módulo de Poisson. Sin embargo, este modelo simple no puede explicar las curvas experimentales consideradas, para las que posiblemente se requieran perfiles de velocidades más complejos y modos de propagación superiores. Estas consideraciones pueden implicar restricciones para los valores del módulo de Poisson, que normalmente no se toman en consideración. En conclusión, estas investigaciones analíticas y semi-analíticas son indispensables para un mejor entendimiento del comportamiento de la elipticidad de las ondas de Rayleigh en su uso para estudios de efectos de sitio.

Estimating possible site effect is an integral part of evaluation of the seismic hazard and reduction of earthquake damages. In regions with low or moderate seismicity as in Israel, the site response should be determined by analytical tools. These computations require knowledge of the subsurface geological structure in terms of shear-wave velocity (Vs) profile down to seismic bedrock. Conventionally, this problem is resolved by joint implementation of Horizontal-to-Vertical Spectral Ratios (HVSR or Nakamura's) technique, which is based on ambient noise measurements and seismic methods such as S-wave refraction or Multichannel Analysis of Surface Waves (MASW) method. The first one does not allow deep penetration of seismic waves because of its weak source. The MASW method using 4.5 Hz geophones is restricted in penetration depth of surface waves because of frequency (wavelength) limitations. In this study, we have applied 2.5 Hz geophones and special data processing to provide constructing Vs section to a depth of 100 m and deeper. In combination with HVSR measurements, MASW enables constructing reliable subsurface model, which could be integrated into the seismic hazard assessment. Testing of this combined methodology was carried out at a number of sites with differing geological structures in Israel.