Aftershock Observation of the 22 June 2002 Changoureh-Avaj Earthquake (Mw 6.5), NW Iran

Iran is located in Alps-Himalayan belt with a high seismicity. On 22 June 2002, a shallow earthquake (Mw6.5) occurred in northwest of Iran, latitude 35.67N and longitude 48.93E (NEIC), for about 225 km west of Tehran, the capital of Iran. This earthquake caused a lot of damages, 1466 wounded and 230 killed persons in the villages with adobe constructions. It is very important to study this earthquake not only for seismological interests but also for knowing fault activity around Tehran with about 7 million population. Taking instruments from Japan one month after the main shock, we installed seismographs in four temporal stations around the damaged area in order to observe aftershocks of this earthquake. The four stations were laid out as a triangle pattern with sides long about 20 km for getting the accurate hypocenters of aftershocks. We decided to put the central station of the triangle near surface fissures caused by the main shock because of no accurate hypocentral data of the mainshock. In each station we installed a high sensitive seismograph (Lennartz-LE3Dlite) with three components. And in one station we installed an acceleration seismograph (Akashi-JEP6A3) with three components. Seismic wave data were recorded in 100Hz sampling by using 16 bit digital recorders (Datamark- LSH8000SH-HD) with 2Gbyte hard disk card and GPS. We observed continuously from July 24 to 29 and succeeded getting good continuous data of all stations during about three days. We started to pick the P and S phase of each event observed in all stations and calculated hypocenter and magnitude of each event by using WIN software in Unix system. By processing of 113 aftershocks, we suggest that most of those epicenters distributed in a square with 25km side in the damaged area. By using NS vertical projection of the hypocenters we can imagine the reverse fault surface with 35 degrees dip to the south from 0 to 15 km depth. Our result is comparable with focal mechanism solutions of USGS and ERI of the University of Tokyo, and preliminary report of BHRC (Building and Housing Research Center of Iran). This fault could reach to the surface between Abdarreh and Changoureh villages where most of houses were destroyed. More investigation and data processing will be need to have a better imagine of fault and sub-fault structures of this earthquake.