Earth Planets Space, Vol. 56 (No. 8), pp. 725-740, 2004
Max Wyss1, Gennady Sobolev2, and James D. Clippard3
1World Agency of Planetary Monitoring and Earthquake Risk Reduction, Geneva
2Russian Academy of Sciences, Moscow, Russia
3Shell International Exploration and Production B.V., Rijswijk
(Received September 11, 2003; Revised March 11, 2004; Accepted March 15, 2004)
Two large earthquakes occurred during the last decade on Sakhalin Island, the Mw7.6 Neftegorskoe earthquake of 27 May 1995 and the Mw6.8 Uglegorskoe earthquake of 4 August 2000, in the north and south of the island, respectively. Only about five seismograph stations record earthquakes along the 1000 km, mostly strike-slip plate boundary that transects the island from north to south. In spite of that, it was possible to investigate seismicity patterns of the last two to three decades quantitatively. We found that in, and surrounding, their source volumes, both of these main shocks were preceded by periods of pronounced seismic quiescence, which lasted 2.5 ± 0.5 years. The distances to which the production of earthquakes was reduced reached several hundred kilometers. The probability that these periods of anomalously low seismicity occurred by chance is estimated to be about 1% to 2%. These conclusions were reached independently by the application of two methods, which are based on different approaches. The RTL-algorithm measures the level of seismic activity in moving time windows by counting the number of earthquakes, weighted by their size, and inversely weighted by their distance, in time and space from the point of observation. The Z-mapping approach measures the difference of the seismicity rate, within moving time windows, to the background rate by the standard deviate Z. This generates an array of comparisons that cover all of the available time and space, and that can be searched for all anomalous departures from the normal seismicity rate. The RTL-analysis was based on the original catalog with K-classes measuring the earthquake sizes; the Z-mapping was based on the catalog with K transformed into magnitudes. The RTL-analysis started with data from 1980, the Z-mapping technique used the data from 1974 on. In both methods, cylindrical volumes, centered at the respective epicenters, were sampled. The Z-mapping technique additionally investigated the seismicity in about 1000 volumes centered at the nodes of a randomly placed regular grid with node spacing of 20 km. The fact that the two methods yield almost identical results strongly suggests that the observed precursory quiescence anomalies are robust and real. If the seismicity on Sakhalin Island is monitored at a completeness-level an order of magnitude below the present one, then it may be possible to detect future episodes of quiescence in real time.
Key words: Earthquake prediction, seismic quiescence, seismicity patterns.