Abstract
The Manjil-Rudbar earthquake of June 1990 caused widespread damage to buildings in the city of Rasht located 60 km from the epicenter. Seismic surveys, including refraction P-wave, S-wave and downhole tests, were carried out to study subsurface geology and classify materials in the city of Rasht. Rasht is built on Quaternary sediments consisting of old marine (Q1m), deltaic (Q2d), undivided deltaic sediments with gravel (Qdg) and young marine (Q2m) deposits. We used the variations of Vp in different materials to separate sedimentary boundaries. The National Earthquake Hazard Reduction Program (NEHRP) scheme was used for site classification. Average S-wave velocity to a depth of 30 m was used to develop site categories, based on measured Vs values in 35 refraction seismic profiles and 4 downhole tests. For each geological unit histograms of S-wave velocity were calculated. This study reveals that the Vs(30 of most of the city falls into categories D and C of NEHRP site classification. Average horizontal spectral amplification (AHSA) in Rasht was calculated using Vs(30 . The AHSA map clearly indicates that the amplification factor east and north of the city are higher than those of south and central parts. The results show that the lateral changes and heterogeneities in Q1m sediments are significant and most damaged buildings in 1990 Manjil earthquake were located in this unit.
Highlights
It is well known that local site conditions could give rise to significant local amplification of ground motion during earthquakes
This paper presents the shear-wave velocity (Vs) characteristics for the various sedimentary units throughout Rasht and the boundaries of different geological units and their lateral heterogeneity
The Rasht plain was further divided into four units
Summary
It is well known that local site conditions could give rise to significant local amplification of ground motion during earthquakes. NEHRP uses Vs(30) to rank sites into five broad classes (Building Seismic Safety Council – BSSC, 2001), as shown in table I. Due to the lack of suitable data base, a through site classification of Rasht was not accomplished. In 2006, as part of national geotechnical and seismological survey program, investigation for strengthening the water supply system of Rasht was conducted which lasted for a year. In this project, 12 boreholes, 26 P and 26 S refraction seismic tests. Considering characteristics of geologic units of Rasht, this paper describes the effort to define the shear-wave velocity classification for Rasht, based on NEHRP site classes
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