Triggered tremor in inland region in Japan (Invited)
Abstract
Episodic activity of nonvolcanic ambient tremor accompanied by short-term slow slip event observed in Nankai and Cascadia subduction zones reflects stick slip at the transition zone along the plate interface as one of the subduction process. On the other hand, tremor is sometimes activated temporally by surface wave from teleseismic large event. The triggered tremor has the same properties in frequency content and source location as those of ambient tremor. Therefore the detection of triggered tremor suggests the existence of ambient tremor, which might reflect slow slip event. Ambient tremor activity has been detected not only in the subduction zone but also along the strike slip fault system like as San Andreas Fault. Therefore, even in Japan, there is a possibility to detect tremor associated with the active fault system in inland region. Here we tried to search the triggered tremor during propagation of large amplitude surface wave from M8 class teleseismic large events. All Hi-net stations operated by the National Research Institute for Earth Science and Disaster Prevention were used in this analysis. We compared the envelope trace for the bandpass filtered seismogram (2-8 Hz) with the long-period surface wave with the passband between 0.02 and 0.05 Hz. The envelope pattern characterized by a periodic enhancement at an interval of about a few 10 s correlating to the surface wave is recognized as triggered tremor. So far, we newly detected triggered tremor in some inland regions in Japan in addition to subduction tremor. In central Hokkaido, the tremor at a depth of around 10-20 km coincides with active seismicity linked to previously known, deep low-frequency microearthquakes related to volcanic activity. In northernmost Hokkaido, where there are no known active faults, volcanoes, or microearthquake seismicity, the triggered tremor is located near the ground surface. It would be possible that the tremor is related to fluid pressure change within a limestone cave, because the periodic enhancement of tremor amplitude is in phase with the largest compressional strain caused by surface waves. In the southeast off the coast of Kyushu, the location of triggered tremor is coincident with sources of shallow very low-frequency earthquakes with predominant frequency of 0.1 s (Obara and Ito, 2005). On the other hand, in western Kyushu, triggered tremor is located along a strike slip fault with vertical plane. The large amplitude tremor signals correspond to peaks of Coulomb failure function calculated from displacement field at the ground surface recorded by Hi-net. In northwest Kanto, triggered tremor is also located along the extension of the active fault. As a result, there exist triggered tremor in some inland regions including active faults. The existence of triggered tremor along the fault might suggest ongoing slip. We expect that many fault systems around the world might be associated with tremor. Therefore, continuous seismic monitoring near the fault system is very important to evaluate the seismic potential at the fault.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S42B..04O
- Keywords:
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- 7215 SEISMOLOGY Earthquake source observations;
- 7230 SEISMOLOGY Seismicity and tectonics;
- 8118 TECTONOPHYSICS Dynamics and mechanics of faulting