Understanding the August 11 2012, East Azerbaijan – Iran earthquake

Last update: August 14, 2012 at 11:20 pm by By

Earthquake overview : 2 consecutive M6+ earthquakes in a matter of 10 minutes and 10 km apart from each other have changed the lives of thousands of people near Ahar, Iran.  The earthquake came as a surprise because they occurred within a tectonic plate (intraplate).

Read also : our main in-depth report of this earthquake describing the events from minutes after it happened

The August 11, 2012 M 6.4 and M 6.3 earthquakes in northwestern Iran occurred as a result of oblique strike-slip faulting in the shallow crust of the Eurasia plate, approximately 300 km east of the plate boundary between the Eurasia and Arabia plates. The two earthquakes are separated by just 10 km in an east-west direction.
Focal mechanisms, describing the style of faulting for the earthquakes, suggest slip on either fault planes striking roughly east-west, or those striking roughly north-south. Because these earthquakes are intraplate events, away from the main plate boundary structures in the region, precise identification of the causative fault(s) is difficult at this time, though their offset suggest they may be associated with an east-west striking structure.

Seismic Hazard map of the area

On a broad scale, the seismotectonics of this region are controlled by the collision of the Arabia and Eurasia plates; at the latitude of the earthquakes, the Arabia plate moves almost due north with respect to the Eurasia plate at a rate of approximately 26 mm/yrThe collision between these plates deforms an area of ~ 3,000,000 km2 of continental crust.To the south of today’s earthquakes, towards Iraq and the Persian Gulf, tectonics are dominated by the Zagros fold and thrust belt. To the west, in Turkey, tectonics are dominated by strike-slip faulting on the East (in southern Turkey) and North (in northern Turkey) Anatolian fault zones, accommodating the westward motion of the Anatolian block as it is being squeezed by the converging Arabian and Eurasian plates. The August 11, 2012 earthquakes occurred in the broad, elevated Turkish-Iranian Plateau region between these regimes and the Alborz Mountains further east. The events are consistent with the distributed, dominantly strike-slip mechanisms of historic earthquakes nearby, and with the orientation of mapped faults in the region.

Left the tremors of the Van 2011 (Turkey earthquake) - Right epicenter of the Iranian earthquake

Over the past 40 years, seven earthquakes of M 6 or greater have occurred within 300 km of today’s events.
The closest was a M 6.1 earthquake in February of 1997, approximately 100 km to the east, which caused 1100 fatalities.
In October of 2011, a M 7.1 earthquake struck the region of Van in eastern Turkey, near the Iranian border 300 km to the west of the August 11, 2012 earthquakes, resulting in over 500 fatalities.

Shaking map of the M6.4 12:23 UYC earthquake.

Text an images : USGS

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