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[SSS30-16] Earthquake cluster activity beneath Tanzawa Mountains in 2012: Migration with a small stress drop
First we relocated hypocenters by using the double difference method and found that earthquakes of the cluster activity migrated away from the first earthquake of the activity. The migration was consistent with the fluid diffusion and could be characterized as following two patterns. Earthquakes that occurred within an hour of the first earthquake had a migration speed similar to that of non-volcanic tremors. On the other hand, those occurred between an hour and 50 hours from the first earthquake of the activity showed a migration with a similar speed to the activity of induced earthquakes due to water-injection experiments. These results suggest that the cluster activity would be triggered by a slow slip and fluid diffusion. We confirmed that this migration would not be an apparent one by numerical simulations.
We then analyzed stress drops of 16 earthquakes with M ≥ 3.5 that occurred from July, 2003 to June, 2012 in the area of the activity. Earthquakes that occurred before and after the cluster activity had stable values of stress drop with 30 MPa estimated by the equation of Madariaga (1976), or 5 MPa by Brune (1970). On the other hand, earthquakes of the cluster activity included ones with significantly small stress drops. A hypothesis that the cluster activity was associated with fluid explains both the migration of hypocenters and small stress drops of the cluster activity. This is because the shear strength on a fault can be decreased due to the pore pressure of the fluid. This hypothesis is also supported by the fact that earthquakes before and after the cluster activity had similar values of stress drop and that structural studies indicated the existence of little fluid in the region, suggesting that the activity was triggered by a different mechanism from the other earthquakes in the same region. The most plausible explanation is that there is a little fluid in a closed system beneath the Tanzawa Mountains which is undetectable by structural observations.
Acknowledgments: We used waveforms at stations of Hi-net (NIED), Hot Spring Research Institute of Kanagawa Prefecture, Univ. of Tokyo, and JMA, as well as the seismograph network called the MeSO-net, which has been developed under the “Special Project for Earthquake Disaster Mitigation in the Tokyo Metropolitan Area” since 2007. We also used arrival times of P and S waves determined by JMA. Figures were created using Genetic Mapping Tool.