Earth Planets Space, Vol. 56 (No. 8), pp. 813-821, 2004
Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan
(Received December 22, 2003; Revised July 13, 2004; Accepted July 13, 2004)
In the past decade, nine silent earthquakes were documented along the Nankai and the Sagami Troughs in Japan, which form the northwestern margin of the Philippine Sea plate. They occurred in the stable-unstable transition zone at depths of around 30 km on the subduction interface and were segregated from major asperities of the 1923 Kanto, the 1944 Tonankai and the 1946 Nankai earthquakes. Their equivalent magnitudes were less than 7 and overall slips were less than 0.2 m, one-order smaller than those of the major asperities of ordinary great earthquakes. Moment release rates of the silent earthquakes were less than 1014 Nm/s, five-orders smaller than 1019 Nm/s of the great earthquakes. Two methodologies are attempted to obtain order of magnitude estimates of the roughness and friction parameter of source areas of some of the silent earthquakes. One method compares observed waveforms to synthetics with an empirical source time function based on laboratory experiment. The other relates sizes of silent earthquakes to the friction parameter a-b.
Key words: Silent earthquake, fault roughness, stable-unstable transition zone, Nankai Trough, Sagami Trough.