Aseismic slip during the 1996 earthquake swarm in and around the Onikobe geothermal area, NE Japan

In August 1996, an earthquake swarm including 4 earthquakes of magnitudes greater than 5 occurred in and around the Onikobe geothermal area, northeast Japan. While earlier studies detected ground displacements by L-band JERS radar interferograms, there remained certain puzzling discrepancies between the observations and the predictions from a seismologically inferred model, and no satisfactory models have yet been proposed. Here, by identifying that a steep gradient in the radar line-of-sight changes exists in seismicity gaps to the east and west of the Torage area (just to the north of the Onikobe caldera), we propose two aseismic reverse faults, which turn out to contribute to a local topographic growth as well as explaining the pre-existing discrepancies. Another significant signal is detected in a region in which no large earthquakes occurred; we account for this signal by another aseismic fault. This fault adds to another evidence for the recent hypothesis that aseismic fault motion drives earthquake swarm.