TEMPORAL CLUSTERING OF LARGE-MAGNITUDE EARTHQUAKES AND INTERACTION OF ACTIVE FAULTS ON THE IRANIAN PLATEAU

The diffuse plate boundary of the Iranian Plateau, with interactive earthquake sequences along active strike-slip and reverse faults, is analogous to other intra-continental active regions like southern California, New Zealand, Venezuela, and Argentina. However, unlike those places, the 2000-year-long historical seismic data for Iran show temporal and spatial clustering of intense seismic activity of large-magnitude earthquakes within very short time periods along: (i) individual faults, (ii) densely populated interacting sub-parallel faults, and (iii) tectonically-related unstable interlocked faults with interaction between left- and right-lateral strike-slip and reverse faults. Thirteen clustered earthquakes of M_w 6.0-7.1 took place within 61 years in an area of 160 x 110 km (approximately the size of the Los Angeles basin). The Qohestān sequence triggered a chain of earthquakes between nine interlocked faults with right-lateral, left-lateral, and reverse mechanisms oriented N-S, E-W, and NW-SE, respectively. During a short time period of 62 years, six clustered earthquakes of M_w 6.3-7.2 occurred along a dense network of five closely-related strike-slip and thrust faults in the Eastern Kopeh Dāgh Shear Zone (analogous to the Eastern California Shear Zone, or the Marlborough, New Zealand). Four large-magnitude earthquakes struck the Neyshābur-Binālud range-front, closely spaced, reverse fault system (7-10 km wide zone) within a period of 196 years (a pattern similar to the Murchison-Inangahua sequence in New Zealand; San Juan-Caucete sequence in the Sierras Pampeanas, Argentina; or the Sylmar-Northridge sequence in the San Fernando Valley, California). The historic seismic record and coseismic surface rupture pattern provide useful case histories for earthquake hazard assessment and active tectonics in other parts of the world including the Los Angeles and Ventura basins, New Zealand, Venezuela, and Argentina.