2003 Bam Earthquake in Southeastern Iran: an Unexpected Event Associated With a Segmented Fault

The Bam fault is an active Quaternary fault that runs over 50 km. The segments north and south of the Bam scarp are high-angle faults with small east-side-up component. There are possible offset streams that indicate predominant right-lateral strike-slip offset. The Bam scarp represents an east dipping flexure or monocline on the east wing of an asymmetric anticline probably above a west dipping reverse fault. During the December 26, 2003 Bam earthquake, continuous ruptures with consistent right-lateral strike-slip occurred in the north of Posht-e-Rud. West strand extends about 5 km with small amount of extension. The west strand coincides with a trace of a Quaternary fault, namely, northern Bam fault. East strand extend about 2 km with small amount of compression. There is no evidence of precedent faulting along this east strand. These ruptures were probably triggered by the intense ground shaking of the 2003 earthquake. There is no systematic change in the amount of offset. The central part of the Bam scarp might have grown during the earthquake. The swarm of tension cracks extends mostly 165 degree with opening of 1 to 10 cm. There is a E-W trending normal fault away from the scarp. The N-S extension may be correlated to the E-W compression. In south of the Bam scarp, there was no systematic surface effect. The topography and geology suggest there is a southern extension of the Bam fault over 30 km. Geological and geomorphological evidence clearly indicate that the Bam fault has repeatedly ruptured over a 50 km long strand. The repeated faulting has created the Bam scarp and other surface features. However, the 2003 Bam earthquake did not create any significant fault topography on the surface. The absence of tectonic displacement at the surface is concordant with the small magnitude. This means the 2003 earthquake was not the largest earthquake expected from geologic evidence. Only a small portion of the Bam fault plane was ruptured at depth, or another adjacent blind fault plane was ruptured in 2003. There is no way to forecast such small blind earthquake either by seismological research or by geological research. The most devastating event for Bam has not occurred for past 700 years or more and did not occur in 2003, but will occur in the future judging from the characteristics of the Bam fault. It is now possible to foresee the large, probably M 7.5 or larger, event but there is no historic and geologic data to quantify the risks. In order to reasonably reconstruct the city of Bam, we need much information about an earthquake cycle of the Bam fault.