Extracting the shear wave velocity structure of deep alluviums of “Qom” Basin (Iran) employing HVSR inversion of microtremor recordings

Abstract

In recent decades, numerous observations of the connection between sedimentary basins structure and severity and variability of damages during earthquake events have sparked a series of studies for characterization of such subsurface structures. However, these investigations were always accompanied with cost concerns and executive restrictions. Among non-destructive and cost effective approaches which have been developed in seismic response evaluations, usage of microtremor has attracted considerable attentions. In the current study, we have focused on inversion of microtremor acquired in the Qom basin, Iran. As the area is known to be seismically active, our main contribution is the construction of a three-dimensional model of the sedimentary basin in terms of shear wave velocity, which extends and integrates the previous knowledge (limited to the Vs30) down to the bedrock. Additionally, we provided further evidence of several possible faults in the area, one of which has been introduced earlier but poorly investigated so far: the “Qomrud” Fault. The results of a series of previous studies on the area, comprising down-hole surveys, electrical resistivity surveys, surface seismic refractions and surficial bore-hole excavations have been used, either to form a more precise initial model for inversion process or to evaluate the outputs. Additionally, we investigated possibility of obtaining the distribution of the Vs30 in the area directly from inversion of HVSR curves. It revealed that, although for relatively shallow bedrocks (80–100 m) the differences of achieved velocities is lower than 10–15%, in deep sediments, the Vs30 from microtremor is largely overestimated. Furthermore, we discussed the applicability of empirical relationships for estimating bed-rock depth in the investigated basin.