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Aftershock Activity of the 1999 zmit, Turkey, Earthquake Revealed from Microearthquake Observations

Akihiko Ito, Balamir Üçer, erif Bari, Ayako Nakamura, Yoshimori Honkura, Toshio Kono, Shuichiro Hori, Akira Hasegawa, Riza Pekta and Ahmet Mete Iikara

Faculty of Education
Utsunomiya University
Utsunomiya 321-8505
Japan
ito{at}cc.utsunomiya-u.ac.jp
(A.I.)
Geophysical Department
Kandilli Observatory and Earthquake Research Institute
Boaziçi University
Çengelköy
Istanbul 81220
Turkey
(B.Ü., S.B., R.P., A.M.I.)
Research Center for Prediction of Earthquakes and Volcanic Eruptions
Graduate School of Science
Tohoku University
Sendai 980-8578
Japan
(A.N., A.H., T.K., S.H.)
Department of Earth and Planetary Sciences
Tokyo Institute of Technology
Tokyo 152-8551
Japan
(Y.H.)

Manuscript received 7 October 2000.

An accurate aftershock distribution of the 1999 zmit, Turkey, earthquake was obtained by using the data from a local seismic network, IZINET, and 10 temporary seismic stations. More than 2000 aftershocks were relocated for the period of about 2 months following the mainshock. From this aftershock distribution we obtained several pieces of information on the characteristics of the mainshock. First, the mainshock initiated fault rupture from a place adjacent to an active swarm area where many microearthquakes had been occurring for more than 20 yr prior to the mainshock. Second, the aftershock region extended in the east-west direction along the North Anatolian Fault Zone (NAFZ). This confirms that the mainshock was caused by a slip on the NAFZ. Third, the western end of the rupture caused by the mainshock is likely to have reached up to about 29.2° E in the zmit Bay, and hence the total length of the fault rupture caused by the mainshock amounts to about 150 km, as long as the estimate of the fault rupture length is based on the aftershock distribution. This information is important for the discussion on the possibility of future large earthquakes in the west of the source region of the zmit earthquake. We also found a clear tendency that aftershocks occur in clusters, which implies strong heterogeneity in both the rupture process and the medium along the fault zone.

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