A case study for seismic zonation in municipal areas

Abstract

We determined the seismic model of the soil column on a district basis within the Municipality of Izmit, Turkey. Specifically, we conducted refraction seismic survey at 16 locations and estimated the P‐ and S‐wave velocity‐depth profiles down to a depth of 30 m. We then combined the seismic velocities with the geotechnical borehole information about the pedology and lithology of the soil column and determined the geotechnical earnthquake engineering parameters for each district. Specifically, we computed the soil amplification and its effective depth range, design spectrum periods TA‐TB, and liquefaction probability and depth range. We diagnosed that the cause of the severe damage by the August 1999 earthquake with 7.4 magnitude within the municipal area of Izmit is soil amplification and localized liquefaction. In addition to geotechnical characterization of the soil column at each district, we also conducted shallow reflection seismic surveys at 10 locations within the municipal area along line traverses with an average length of 450 m primarily in the EW direction and derived seismic images down to a depth of 100 m. From the interpretation of the seismic sections, we delineated several faults most of which reach the surface and cause significant lateral velocity variations within the nearsurface as verified by the first‐arrival tomography solution for P‐wave velocity‐depth models along the line traverses. These are most likely the active auxiliary faults associated with and oblique to the North Anatolian right‐lateral strike‐slip fault which traverses the muncipal area in the EW direction. We conclude that the neotectonic model of the area is represented by pull‐apart tectonism rather than pure strike‐slip fault mechanism.