Characteristics and Distribution of Small Asperities on the Plate Boundary in the NE Japan Subduction Zone Estimated From Repeating Earthquakes

A diversity of seismic events including large interplate earthquakes, tsunami earthquakes and intraplate earthquakes, have occurred in the northeastern Japan subduction zone. However the precise activity and nature of small events under the Pacific Ocean has not been clear except in special cases (e.g. Hino et al., 2000; Gamage et al., present meeting). Repeating earthquakes, which have been found in California and NE Japan subduction zone, are significant because they indicate recurrent rupture of small asperities on the plate boundary. We investigated the distribution and characteristics of these small asperities. We selected repeating earthquakes based on the waveform similarity, near by location and size (M>=2.5). The criterion for similarity was such that the cross-correlation coefficients must be larger than 0.95 for a 40s window for 1-4 Hz band-pass filtered waveform. This strict criterion for similarity ensures the events share the same asperity or are adjacent to each other for M3-4 events. The frequency distribution of the lifetime of the repeaters had two peaks. The burst type (duration < 3years) repeating earthquakes have a relatively low cross correlation coefficients compared to the continual type (duration >= 3 years). It should be noted that successive rupture of nearby asperities sometimes involves burst-type repeating groups. The focal mechanisms of most repeating earthquakes involve low-angle thrust faulting. This suggests the repeating earthquakes occur on the plate boundary. The spatial distribution of repeating earthquakes are complementary with large asperities (Yamanaka and Kikuchi, 2004). These asperities occur in area of high coupling coefficient as determined from GPS data analysis (Suwa et al., 2003). Repeating earthquakes near the Japan Trench tend to have relatively low stress drop (Yamashita et al., 2004). These features of repeating earthquakes probably not only show the characteristics of small asperities but also the nature of the active plate boundary.