Shear-wave velocity characteristics of geological units throughout Tehran City, Iran

One simple way of accounting for site conditions in calculating seismic hazards is to use the shear-wave velocity (Vs) in the shallow subsurface to classify materials. We used the average shear-wave velocity to a depth of 30 m (V¯s(30)) to develop site categories for Tehran. This parameter can further be used for modifying a calculated ground motion to account for site conditions. The National Earthquake Hazard Reduction Program (NEHRP) classification was used to group sites into different classes. Our classification was based on V¯s(30) values measured in 188 seismic profiles and on the assumption of similar values for similar geological materials. For each geological unit, characteristics of shear-wave velocity distribution including histograms of shear-wave velocity and variation of Vs versus depth were calculated and shown. This study reveals that the V¯s(30) of the mountainous zones fall into categories A and B of NEHRP site classification. But, in the Tehran Plain, the northern alluvium tends to have V¯s(30) values which fall into range of category C of the NEHRP site classification, whereas, those in the southern alluvium would be categorized as NEHRP D.