Magnitude 7.6 TURKEY
1999 August 17 00:01:39 UTC
The earthquake likely occurred on a branch of the North Anatolian fault. Although this is the largest earthquake in the epicentral region in this century, the region of the earthquake has a long history of destructive earthquakes. In 1967, a magnitude 7.1 earthquake caused extensive damage along the North Anatolian fault just east of the current shock.
The 900 kilometer-long North Anatolian fault has many characteristics similar to California's San Andreas fault. These two faults are right-lateral, strike-slip faults having similar lengths and similar long-term rates of movement. If a person is looking across a right-lateral, strike-slip fault during such an earthquake, that person would see the opposite side move to the right.
The North Anatolian fault has produced seven large (MS >= 7.0) earthquakes in the period from 1939 through 1999. These earthquakes have ruptured the fault progressively from east to west. The seismic gap on the western part of the North Anatolian fault led Turkish and American seismologists to specify, in published papers, that the zone ruptured by the August 17, 1999, earthquake was a zone of special concern.
Following are data for the seven large earthquakes that have progressively ruptured the North Anatolian fault:
|1939/12/26||7.9 - 8.0 MS||about 360 km||30,000 deaths.
Initiated the westward migration of significant earthquakes on the North Anatolian fault.
(Termed the 1939 Erzincan earthquake.)
|1942/12/20||7.1 MS||about 50 km||(Termed the 1942 Erbaa earthquake)|
|1943/11/26||7.6 MS||about 280 km||(Termed the 1943 Tosya earthquake)|
|1944/02/01||7.3 MS||about 165 km||(Termed the 1944 Bolu-Gerede earthquake)|
|1957/05/26||about 7. MS||about 30 km||(Termed the 1957 Abant earthquake)|
|1967/07/22||7.1 MS||about 80 km||(Termed the 1967 Mudurnu Valley earthquake)|
Earthquakes on the North Anatolian fault are caused by the northwards motion of the Arabian plate against the Eurasian plate, squeezing the small Turkish microplate westwards. Also, compression in this region is due to the northwards motion of the African plate, which produces subduction at the Cyprus and Hellenic arcs. The small Turkish microplate is bounded on the east by the East Anatolian fault zone (EAFZ), on the north by the North Anatolian fault zone (NAFZ), on the west by a diffuse zone of deformation surrounding the greater Aegean region, and on the south by the Hellenic and Cyprus arcs.
Figure from Rob Westaway, "Present-day kinematics of the Middle East and eastern Mediterranean," Journal of Geophysical Research, Volume 99, pages 12,071 - 12,090, 1994.