Characteristic small-earthquake sequence off Sanriku, northeastern Honshu, Japan

We found eight similar earthquakes of M4.8 regularly occuring from 1957 to 1995 on the plate boundary off Sanriku, northeastern Japan. The mean reccurrence interval of the events is 5.35 years and its standard deviation is only 0.53 years. These events have the same focal mechanisms of low-angle thrust fault type and their source sizes are estimated to be around 1 km. The slip of each event is comparable to the cumulative amount of relative plate motion in the inter-seismic period. Thus, these events are probably caused by repeating slip at a small but strong asperity surrounded by creeping zone on the plate boundary. The next event was expected to occur by the end of November 2001 with 99% probability and actually M4.7 event occured on November 13, 2001.