PRECURSORY SEISMIC QUIESCENCE IN THE MUDURNU VALLEY, NORTH ANATOLIAN FAULT ZONE, TURKEY

The seismicity rate in the Mudurnu Valley of Turkey was studied using an earthquake catalogue that reports events homogeneously down to magn itude 2.3 for the years 1985-1989, and covers the area between latitud es 40.2 degrees and 41.0 degrees N, and longitudes 30.0 degrees and 31 .5 degrees E. During this period the only two main shocks, M = 4.0 and M = 4.3, occurred on 1988 September 6 and 1988 December 9 within abou t 30 km of each other. A highly significant seismic quiescence is evid ent in the area surrounding these main shocks, while the seismicity ra te in the rest of the area covered by the catalogue remains constant. The quiescence becomes more pronounced the smaller the area around the main shocks that is studied. The smallest areas that can be studied c ontain about 60 earthquakes and have dimensions of approximately 25 km on each side. The decreases in seismicity rates are 50-80 per cent de pending on the volume and period used for defining the quiescence. The quiescence started in 1988 January and lasted about seven months, wit h approximately 4.5 months of normal activity separating it from the m ain shock of December. The precursor time of 12 months for an M = 4.3 main shock is similar to those observed in California. It is concluded that it is possible to resolve precursory quiescence before moderate and large earthquakes in the Mudurnu area with the existing seismograp h network.