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

Latif Samadi, Behzad Mehrabi


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.


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

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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X