EMPIRICAL EVALUATION OF THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO TECHNIQUE: RESULTS FROM THE “SESAME” PROJECT

In order to empirically evaluate the horizontal-to-vertical (H/V) spectral ratio technique, ambient noise measurements performed in about two hundred sites mainly in Europe where weak or/and strong motion data was recorded. Standard Information Sheets (SIS) and earthquake information data were included in the SESAME [Site EffectS assessment using AMbient Excitations] project database, specially designed to facilitate data selection. All noise recordings were processed with JSESAME software to calculate (H/V) spectral ratio, whereas weak and strong motion earthquake recordings were processed with a similarly standardized procedure. For the latter, (H/V) receiver function for all sites were calculated. Experimental site transfer functions obtained from earthquake recordings were compared with the (H/V) spectral ratios from noise recordings in terms of fundamental frequency, amplification bandwidth and amplification level. Similarities and differences between (H/V) spectral ratio of noise and earthquake recordings are presented and discussed. In addition, a dense grid of noise measurements were performed within urban environment of cities affected by strong earthquake (Greece: Thessaloniki, Kalamata, Italy: Palermo). It seems that the (H/V) spectral ratio may satisfactorily indicate areas favorable to the occurrence of higher damage in urban environment. However, quantitative correlation between (H/V) spectral ratio properties and damage distribution (macroseismic intensity, damage grades) in some cases, is difficult to be established given the complexity of parameters involved

Sesame project - Deliverable D23.12 - WP12 - GUIDELINES FOR THE IMPLEMENTATION OF THE H/V SPECTRAL RATIO TECHNIQUE ON AMBIENT VIBRATIONS MEASUREMENTS, PROCESSING AND INTERPRETATION

A significant part of damage observed in destructive earthquakes around the world is associated with seismic wave amplification due to local site effects. Site response analysis is therefore a fundamental part of assessing seismic hazard in earthquake prone areas. A number of experiments are required to evaluate local site effects. Among the empirical methods the H/V spectral ratios on ambient vibrations is probably one of the most common approaches. The method, also called the „Nakamura technique“ (Nakamura, 1989), was first introduced by Nogoshi and Igarashi (1971) based on the initial studies of Kanai and Tanaka (1961). Since then, many investigators in different parts of the world have conducted a large number of applications. An important requirement for the implementation of the H/V method is a good knowledge of engineering seismology combined with background information on local geological conditions supported by geophysical and geotechnical data. The method is typically applied in microzonation studies and in the investigation of the local response of specific sites. In the present document, the application of the H/V technique in assessing local site effects due to dynamic earthquake excitations, is the main focus, whereas other applications regarding the static aspects are not considered. In the framework of the European research project SESAME (Site Effects Assessment Using Ambient Excitations: Contract No. EVG1-CT-2000-00026), the use of ambient vibrations in understanding local site effects has been studied in detail. The present guidelines on the H/V spectral ratio technique are the result of comprehensive and detailed analyses performed by the SESAME participants during the last three years. In this respect, the guidelines represent the state-of-the-art of the present knowledge of this method and its applications, and are based on the consensus reached by a large group of participants. It reflects the synthesis of a considerable amount of data collection and subsequent analysis and interpretations. In general, due to the experimental character of the H/V method, the absolute values obtained for a given site require careful examination. In this respect visual inspection of the data both during data collection and processing is necessary. Especially during the interpretation of the results there should be frequent interaction with regard to the choices of the parameters for processing. The guidelines presented here outline the recommendations that should be taken into account in studies of local site effects using the H/V technique on ambient vibrations. The recommendations given apply basically for the case where the method is used alone in assessing the natural frequency of sites of interest and are therefore based on a rather strict set of criteria. The recommended use of the H/V method is however, to combine several other geophysical and geotechnical approaches with sufficient understanding of the local geological conditions. In such a case, the interpretation of the H/V results can be improved significantly in the light of the complementary data. The guidelines are organised in two separate parts; the quick field reference and interpretation guidelines (Part I) and detailed technical guidelines (Part II). Part I aims to summarise the most critical factors that influence the data collection, analysis and interpretation and provides schematic recommendations on the interpretation of results. Part II includes a detailed description of the technical requirements, standard data processing and the interpretation of results. Several examples of the criteria described in Part I and II are given in Appendix A. In addition, some physical explanations of the results based on theoretical considerations are given in Appendix B. In Part II, section 1, the results of the experiments performed within the framework of the SESAME project are given in smaller fonts to separate these from the recommendations and the explanations given in the guidelines. The word „soil“ should be considered as a generic term used throughout the text to refer to all kinds of deposits overlying bedrock without taking into account their specific origin. The processing software J-SESAME developed specifically for using in H/V technique, is explained (provided on a separate CD accompanying the guidelines) in Part II. However, the recommendations given in the guidelines are meant for general application of the method with any other similar software. J-SESAME is provided as a tool for the easy implementation of the recommendations outlined in this document. Regarding the processing of the data, several options can be chosen, but the recommended processing options are provided as defaults by the J-SESAME software

Can broad-band earthquake site responses be predicted by the ambient noise spectral ratio? Insight from observations at two sedimentary basins

Site-effect assessments performed through earthquake-based approaches, such as the standard spectral ratio (SSR), require good quality records of numerous earthquakes. In contrast, the use of ambient noise appears to be an attractive solution for ease and rapid computation of site responses with sufficient spatial resolution (microzonation), especially in low seismicity areas. Two main approaches are tested here: the horizontal-to-vertical spectral ratio (HVSR) and the noise-based SSR (SSRn). The HVSR uses the relative amplitude of the horizontal and vertical components of the ambient noise. Instead, the SSRn defines the spectral ratio between the seismic noise recorded simultaneously at a site and at a rock reference station, similar to earthquake-based SSR. While the HVSR is currently used in hundreds of site-specific studies, the SSRn approach has been gradually abandoned since the 1990s. In this study, we compare the results obtain from these two approaches with those of earthquake-based SSR. This comparison is carried out for two sedimentary basins, in Provence (southeastern France) and in Argostoli (western Greece). In agreement with the literature, the HVSR does not provide more than the fundamental resonance frequency of the site (f0). The SSRn leads to overestimation of the SSR amplification factors for frequencies higher than the minimal f0 of the basin (f0min). This discrepancy between SSRn and SSR is discussed, and appears to be mainly dependent on the local geological configuration. We thus introduce the hybrid standard spectral ratio (SSRh) approach, which aims to improve upon the SSRn by adding an intermediate station inside the basin for which the SSR is known. This station is used in turn as a local reference inside the basin for the SSRn computation. The SSRh provides site transfer functions very similar to those of the SSR, in a broad frequency range. Based on these results, the SSRn (or SSRh) should be further tested and should receive renewed attention for microzonation inside sedimentary basins.ISSN:0956-540XISSN:1365-246