STUDY OF THE EVALUATION OF EARTHQUAKE OCCURRENCE POTENTIAL IN THE WESTERN PART OF THE NORTH ANATOLIAN FAULT ZONE

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In each year of this research project, we intensified seismic observations and advanced the analysis system, resulting in the establishment of an earthquake monitoring system in the Istanbul area, western Turkey. At present, seismic activity can be monitored on the web and also some data on major earthquakes are shown on the web, so that the data can be used for studies of earthquake generation mechanism. We have used the data for our own researches on seismic tomography and found that seismic activity becomes shallower in the earthquake source region of a big earthquake anticipated to the west of the Izmit earthquake source area. We also accumulated information on earthquake generation mechanism from waveform inversion for middle-size earthquakes, and increased our understanding of the distribution of crustal stress. As for seismicity itself, seismic activity is no very active in the earthquake source area for a big earthquake anticipated to the south of Istanbul, but it is rather act … More ive in its vicinity. Yearly data of hypocenters suggest an image of east-west active faults beneath the Marmara Sea and an image of the anticipated big earthquake has emerged. In electromagnetic studies, we made wide-band, long-period and short-period MT observations, and clarified the resistivity structure in a wide region including the Izmit earthquake source area. As a result, we found that earthquake source areas are resistive in general and highly resistive areas correspond to asperities. We also found a partial melting zone and its extension, as revealed by low resistivity zones widely existing in the lower crust and upper mantle. As for the seismic dynamo effect which we found during the Izmit earthquake, we made further studies for aftershocks associated with some major earthquakes, such as the Niigata Prefecture earthquake, which occurred recently in Japan. We could confirm the effect for earthquakes larger than magnitude 4.0, but could not detect electromagnetic signals anticipated theoretically before the arrival of seismic wave, simply because of low S/N for small earthquakes. Less