Shallow subsurface geology and Vs characteristics of sedimentary units throughout Rasht City, Iran

Abstract

The Manjil-Rudbar earthquake of June 1990 caused widespread damage to buildings in the city of Rasht located<br />60 km from the epicenter. Seismic surveys, including refraction P-wave, S-wave and downhole tests, were<br />carried out to study subsurface geology and classify materials in the city of Rasht. Rasht is built on Quaternary<br />sediments consisting of old marine (Q1m), deltaic (Q2d), undivided deltaic sediments with gravel (Qdg) and<br />young marine (Q2m) deposits. We used the variations of Vp in different materials to separate sedimentary<br />boundaries. The National Earthquake Hazard Reduction Program (NEHRP) scheme was used for site classification.<br />Average S-wave velocity to a depth of 30 m was used to develop site categories, based on measured Vs values<br />in 35 refraction seismic profiles and 4 downhole tests. For each geological unit histograms of S-wave velocity<br />were calculated. This study reveals that the Vs(30 of most of the city falls into categories D and C of NEHRP<br />site classification. Average horizontal spectral amplification (AHSA) in Rasht was calculated using Vs(30 . The<br />AHSA map clearly indicates that the amplification factor east and north of the city are higher than those of south<br />and central parts. The results show that the lateral changes and heterogeneities in Q1m sediments are significant<br />and most damaged buildings in 1990 Manjil earthquake were located in this unit.

Keywords

• Shear wave velocity – Vs(30 – NEHRP site classification – Rasht – Iran