Determination of shallow earthquake geotechnical profile using microtremor array method with semi-circular layout

摘要

Shear wave velocity profile of subsoil is of great importance in earthquake geotechnical engineering. This parameter plays main rule in assessing strong ground motion and seismic response of soil. The specification of site effects frequently relies on the determination of the soil profile at a site, followed by the forward computation of the local transfer function. The reliability of the result depends to a large extent on the reliability of the determined soil profile. The more reliable techniques are usually too expensive (exploration using boreholes) or unsuitable (e.g., explosion seismology is out of the question in an urban environment) to be of general application. For these reasons, alternative methods have been devised. Currently, the most popular technique involves the measurement of microtremor waves using several seismometers simultaneously. The aim of this paper is to define material parameters of sediments in the western part of Tonkabon in north of Iran useful in the prediction of strong ground motion. Many geological studies, as well as geophysical surveys and geotechnical boreholes, contribute to our understanding of the general sedimentary structure of the studied region. These studies showed that subsoil of the site mainly composed of saturated poorly graded sand and the region mainly consisted of young quaternary sediments. In the studied site, four boreholes in different depth and maximum depth of 70 meters have been drilled; thus, the site layering has been carefully identified directly. In microtremor survey, 7 different array layouts have been used with minimum 5 to maximum 15 receivers. Field work related to an ambient vibration array method included a three-component microtremor records in 21 consecutive hours using semi-circular arrays. Data processing for each hour was performed using Geosphy software with conventional methods SPAC and F-K, and by back analysis, the shear wave velocity profiles are derived. The results showed that both F-K and SPAC methods had good capability in determination of subsoil layering and accurately detected depth of each layer. The differences in the results of each array in SPAC method were considerably less than F-K. Finally, it can be inferred from the results that SPAC method predicted shear wave velocity of each layer more precisely than F-K method in semi-circular array shape.