Earthquake cluster activity beneath the Tanzawa Mountains region, Japan: Migration of hypocenters and low stress drop
Abstract
An earthquake cluster activity was observed beneath the Tanzawa Mountains region, Japan with a depth of 20 km in the end of January, 2012. Japan Meteorological Agency (JMA) determined hypocenters of 76 earthquakes with M > 2 in the area within 50 hours. Five of them had magnitudes greater than 4 and the largest one was 5.4. Four out of the five earthquakes had the reverse-type focal mechanisms with the P axis in the NW-SE direction. First we relocated hypocenters of the activity following the method of Yukutake et al. (2012). We estimated relative arrival times of P and S waves by calculating the coefficients of the cross correlation and relocated hypocenters with the double-difference relocation method (Waldhauser and Ellsworth, 2000). We found that the cluster activity showed a migration from the first earthquake of the activity. The parabolic migration speed was consistent with the migration speed of the deep tremor sources (Ide et al., 2010) for which the fluid activity would play an important role. We then analyzed stress drops of 17 earthquakes with M > 3.5 that occurred from January, 2000 to June, 2012 in the area of the cluster activity. We calculated empirical Green's functions from waveforms of earthquakes with magnitudes of 3.0 to 3.2 and estimated stress drops of the earthquakes assuming that the source spectra can be expressed as the omega-squared model. We found that earthquakes of the cluster activity had smaller stress drops by an order of magnitude than the values of earthquakes that occurred in the same area before the cluster activity. These results suggest that the fluid played an important role for the earthquake cluster activity. That is, the fluid increased the pore pressure, decreased the effective normal stress and triggered the cluster activity. The difference of the rupture speed and the change of the rigidity might also be candidates that account for our results. They, however, can hardly explain the results quantitatively. Fig.(a) Elapsed time and distance from the first earthquake. Parabolic line shows constant diffusivity. (b) Temporal change in values of stress drop. Horizontal broken lines indicate the value of 60 MPa. Earthquakes of the cluster activity had smaller values of stress drop by an order of magnitude.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S11B2375Y
- Keywords:
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- 7215 SEISMOLOGY Earthquake source observations;
- 7230 SEISMOLOGY Seismicity and tectonics;
- 7240 SEISMOLOGY Subduction zones;
- 7218 SEISMOLOGY Lithosphere