[SSS17-P07] Reestimation of pore fluid pressure fields in the region with intensive swarm activity around Mt. Ontake volcano
Keywords:pore fluid pressures, earthquake, stress field
In this study we reevaluated the 3-D pore fluid pressure field in the frank of Mt. Ontake in Terakawa et al. (2013). The previous study applied the FMT method to microseismic activity around Mt. Ontake, and estimated overpressureized fluid reservoirs with a peak of 100-150 MPa (with estimation errors of 20 MPa) at depths between 5 and 12 km in the southeast and east franks of the mountain, assuming a tectonic stress field with 10-20 km resolution inferred from events with M > 3 (Terakawa and Matsu’ura, 2010). In this study we analyzed the same data set as that in Terakawa et al. (2013), assuming a regional stress field with 5 km resolution inferred from smaller events with M > 1 (Terakawa et al., 2016). The pore pressure field obtained in this study is consistent with the former one in the north frank of Mt. Ontake, but discrepancy is large in the southeast and east franks. The peak pore fluid pressure in this study is by > 30 MPa smaller than the former one. In the southeast and east franks difference of the two stress pattens assumed in the two analyses is the largest, although in the two stress patterns the maximum compressive principal stress axes are commonly in the direction of the northwest-southeast. The estimation errors in pore fluid pressures are attributed to both accuracy of the stress pattern and focal mechanism solutions. The level of the pore fluid pressures in the previous study may be overestimated. We reconsider the estimation errors of the stress patterns, and estimate appropriate pore pressure triggering swarm activity.