ACTIVE TECTONIC AND SLIP RATE OF BAM EARTHQUAKE FAULT, CENTRAL IRAN

On 26 December 2003, an Mw 6.6 earthquake struck the city of Bam in the Kerman province of southeast Iran. The intense shaking in the city caused the complete collapse of over 50% of buildings in the central parts of the city and killed at least 25,000 people out of a population of more than 100,000. Bam is located on the eastern edge of the Central Iran. A relatively rigid block of Central Iran moves northward relative to Eurasia and hence relative to Afghanistan which is attached to the Eurasian plate. This motion causes right-lateral shear in eastern Iran with rate, estimated from recent GPS measurements to be 12–14 mm/yr [Vernant et al., 2004]. The resulting right lateral shear in eastern Iran is taken up on two north-south right-lateral strike-slip fault systems located on each side of the aseismic Lut desert.

The Bam earthquake was due to reactivation of hidden faults within the western of the strike slip fault systems bordering the Lut desert. This system has been especially active in recent years, with four major earthquakes between 1981 and 1998 [Berberian et al.,1984; Berberian and Qorashi, 1994; Berberian et al., 2001]. Earlier study of the coseismic deformation revealed that coseismic slip occurred on two sub parallel faults with eighty percent of moment release occurring on a previously unknown strike-slip fault running into the center of Bam (Fielding et al. 2004, Jackson et al., 2006). The remainder occurred on a southward extension of the previously mapped Bam reverse Fault, 5 km to the east.

Unfortunately limited information is available about the slip rates of individual faults in this region. Estimating slip rates of two earthquake faults (the N-S trending strike-slip and blind reverse fault) is the subject of this study. We report optically stimulated luminescence (OSL) ages of samples taken from uplifted plain deposits near the south end of the Bam reverse fault. The OSL ages indicate deposition of alluvial gravels between ~9 and ~19 ka which have been uplifted by about 2.5 m relative to the present river level. These results indicate that the fault-associated-fold is growing in the vertical direction with a rate of about 0.5 mmy-1.

Considering geometric relation between the Bam reverse fault and the south Bam strike-slip fault, we estimate a slip rate of about 2 mm/y for the south Bam earthquake fault. We therefore conclude that return period of large earthquakes on the south Bam fault will be about 1000 years.