Characteristic silent earthquakes in the eastern part of the Boso peninsula, Central Japan

Analysis of Global Positioning System (GPS) data shows transient crustal deformation in the Boso peninsula, central Japan, for about fifty days from October 2002. The southeastward horizontal displacements detected suggest the occurrence of interplate aseismic slip between the Philippine Sea plate and the North American plate, six years after a similar event in 1996. We estimate an area of aseismic slip with moment magnitude (Mw) 6.6 centered off the Boso peninsula, adjacent to associated seismic activity. Slip evolution of the 2002 aeismic slip started on October 4 and migrated from north to south for about ten days, followed by gradual subsidence and restarting of slip which lasted until ∼December 2, propagating more southward. Similarities in focal area and slip process between the 1996 and the 2002 events suggest the Boso aseismic slips as characteristic silent earthquakes, together with the repetitive occurrence of seismic swarm in a similar region every six to seven years. A more tightly coupled southern area that did not release enough energy in the 1996 event may have caused the differences in magnitude, central slip area, and duration between the 1996 and the 2002 events, if the hypothesized recurrence interval for strain accumulation is true, based on swarm seismicity.