New detection of tremor triggered in Hokkaido, northern Japan by the 2004 Sumatra–Andaman Earthquake
The transient stress change caused by the passage of surface waves resulting from the 26 December 2004 Sumatra–Andaman earthquake triggered seismic tremor in two regions in Hokkaido, in the northernmost part of the Japanese islands, where tectonic tremor associated with the subducting plate has not previously been detected. The amplitude pattern of the tremor envelope in both regions is characterized by a periodic enhancement at an interval of about 20 s, which correlates with the surface wave. One tremor that was triggered in central Hokkaido at a depth of around 10–20 km coincides with active seismicity linked to previously known, deep low-frequency microearthquakes related to volcanic activity. Another tremor occurred in northernmost Hokkaido, where there are no known active faults, volcanoes, or microearthquake seismicity. If the source of the northern tremor is located near the ground surface, it would be possible that the tremor is related to fluid pressure change, because the periodic enhancement of tremor amplitude is in phase with the largest compressional strain caused by surface waves.