Impact of source characterization, structure, and building type on the shape of macroseismic region of 1990 7.3 Mw Rudbar, Iran earthquake

Abstract

The 1990 Mw 7.3 Rudbar, Iran earthquake occurred along an 80 km long, nearly vertical left-lateral strike-slip fault, yet produced a macroseismic region strongly asymmetrical along and across the fault. Preliminary macroseismic map of the Rudbar earthquake reveals a larger macroseismic region east of the fault compared to the west, with a narrower macroseismic zone north of the NW-SE trending fault. This study performs a forensic analysis of this event, using detailed historical data to investigate the physical processes, namely source effects, seismic wave propagation, and building typology, that controlled the observed damage patterns. Understanding these factors provides crucial insights into earthquake physics, which are fundamental for improving the physical basis of future seismic hazard and risk assessments. To investigate the asymmetries, we used the detailed statistics of destroyed and damaged buildings and human fatalities to produce maps of the percentage of destroyed and damaged buildings and fatality rate. The maps are generally in agreement with the preliminary macroseismic map and show a clear correlation between the region with damaged buildings and the topography. The relocated Rudbar seismic cluster shows a mostly SE-trending directivity. The directivity combined with a larger release of seismic moment in the eastern segment of the Rudbar fault caused the along-fault strike asymmetry of the macroseismic and building damage maps. The correlation between building damage and topography may result from higher attenuation of high-frequency seismic waves in the plains with thick, soft sedimentary cover surrounding the Alborz Mountains. Building type also contributed to the across-fault asymmetry, with lighter roofing in the northern hills of Alborz reducing susceptibility to shaking. The extension of damage along the Sefidrud River may reflect high-frequency seismic wave amplification at the valley edges, where the soft sedimentary cover is thin.