The 2004 May 28 Baladeh earthquake ( M w 6.2) in the Alborz, Iran: overthrusting the South Caspian Basin margin, partitioning of oblique convergence and the seismic hazard of Tehran

SUMMARY We use teleseismic waveform analysis and locally recorded aftershock data to investigate the source processes of the 2004 Baladeh earthquake, which is the only substantial earthquake to have occurred in the central Alborz mountains of Iran in the modern instrumental era. The earthquake involved slip at 10–30 km depth, with a south‐dipping aftershock zone also restricted to the range 10–30 km, which is unusually deep for Iran. These observations are consistent with co‐seismic slip on a south‐dipping thrust that projects to the surface at the sharp topographic front on the north side of the Alborz. This line is often called the Khazar Fault, and is assumed to be a south‐dipping thrust which bounds the north side of the Alborz range and the south side of the South Caspian Basin, though its actual structure and significance are not well understood. The lack of shallower aftershocks may be due to the thick pile of saturated, overpressured sediments in the South Caspian basin that are being overthrust by the Alborz. A well‐determined earthquake slip vector, in a direction different from the overall shortening direction across the range determined by GPS, confirms a spatial separation (‘partitioning’) of left‐lateral strike‐slip and thrust faulting in the Alborz. These strike‐slip and thrust fault systems do not intersect within the seismogenic layer on the north side, though they may do so on the south. The earthquake affected the capital, Tehran, and reveals a seismic threat posed by earthquakes north of the Alborz, located on south‐dipping thrusts, as well as by earthquakes on the south side of the range, closer to the city.