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Abstract

Summary: The seismicity rate in the Mudurnu Valley of Turkey was studied using an earthquake catalogue that reports events homogeneously down to magnitude 2.3 for the years 1985-1989, and covers the area between latitudes 40.2 degr. and 41.0 degr. N, and longitudes 30.0 degr. and 31.5 degr. 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 about 30 km of each other. A highly significant seismic quiescence is evident in the area surrounding these main shocks, while the seismicity rate 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 contain about 60 earthquakes and have dimensions of approximately 25 km on each side. The decreases in seismicity rates are 50-80 per cent depending on the volume and period used for definiting the quiescence. The quiescence started in 1988 January and lasted about seven months, with approximately 4.5 months of normal activity separating it from the main 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 seismograph network.