Estimation of crustal structure in Horonobe area, Hokkaido, Japan, using multiplet-clustering analysis

Hypocenter locations of shallow earthquakes in Horonobe area, northern Hokkaido, Japan, which is near the convergent boundary between Okhotsk and Amuria plates, were determined to reveal the subsurface structure and the mechanisms of earthquake occurrence. The absolute source locations of 211 earthquakes, which occurred in the period from December 2002 to September 2005, were determined; then, those earthquakes with similar waveforms were identified, and the source locations of 26 multiplet groups were relocated by using cross-spectrum and clustering analyses. The relocated hypocenters allowed two seismically active areas to be identified, at 10-20 km and 25-30 km depth. The earthquake locations indicate structures trending nearly N-S direction, and the structures causing repeated stick-slips at asperities, thus generating similar earthquakes. The relationships between magnitude and the distance to the next event, and between magnitude and the time interval of event occurrences were investigated. The relationships between them suggest that earthquake swarms would be induced by continuous strain accumulation at asperities along faults and its subsequent release as a result of plate tectonic movements. A cutoff line could be also seen in the relationship between magnitude and the distance to the next event, suggesting that the distance between the source locations depends on the event magnitude. These results have given us the knowledge that the asperities on the delineated faults intermittently release strain energy as similar earthquakes with magnitudes of less than about 3.0. Keywords: Subsurface measurement, Multiplet-clustering analysis, Repeating similar earthquakes