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Seismoelectromagnetic Effect Associated with the zmit Earthquake and Its Aftershocks

M. Matsushima, Y. Honkura, N. Oshiman, . Bari, M. K. Tunçer, S. B. Tank, C. Çelik, F. Takahashi, M. Nakanishi, R. Yoshimura, R. Pekta, T. Komut, E. Tolak, A. Ito, Y. Iio and A. M. Iikara

Department of Earth and Planetary Sciences
Tokyo Institute of Technology
Tokyo 152-8551
Japan
mmatsush{at}geo.titech.ac.jp
(M.M., Y.H., F.T., M.N.)
Disaster Prevention Research Institute
Kyoto University
Kyoto 611-0011
Japan
(N.O., R.Y.)
Geophysical Department
Kandilli Observatory and Earthquake Research Institute
Bogaziçi University
Çengelköy
Istanbul 81220
Turkey
(A.M.I., S.B., M.K.T., S.B.T., C.Ç., R.P., T.K., E.T.)
Faculty of Education
Utsunomiya University
Utsunomiya 321-8505
Japan
(A.I.)
Earthquake Research Institute
University of Tokyo
Tokyo 113-0032
Japan
(Y.I.)

Manuscript received 31 July 2000.

In order to investigate the resistivity structure beneath the northern and the southern branches of the western part of the North Anatolian Fault Zone (NAFZ), which has been known as a seismic gap area, we started wideband magnetotelluric (MT) measurements on 27 July 1999, along a north-south profile crossing the northern branch near zmit and the southern branch near Iznik. When the zmit (Kocaeli) earthquake (M_w 7.4) occurred on 17 August 1999, MT fields were being measured at five sites, four of which happened to be located near the surface rupture zone associated with the mainshock, and large variations in the MT fields were observed in association with seismic waves. We propose motional electromagnetic induction in the electrically conducting crust, which vibrates under the Earth's magnetic field, as one of the possible generation mechanisms for such MT field variations. For more detailed studies of such a mechanism, we installed a wideband MT instrument and a three-component seismometer at two of the four sites on 16 September 1999 and made simultaneous measurements during several aftershocks. The estimate of the order of the magnitude of motional electromagnetic response, derived from the MT fields and ground motion observed during a large aftershock (M 4.5), supports our claim that the seismo-dynamo effect is a plausible mechanism for variations in the MT fields associated with the seismic waves.