Abstract
Microseismic swarm activity has taken place since 1976 around Mt. Ontake, the second highest stratovolcano in Japan. This activity is thought to be linked to high pore-fluid pressure in the vicinity of the volcano. We analyzed well-constrained focal mechanism solutions of microseismicity to re-estimate the 3-D pore-fluid pressure field driving vigorous swarm activity around Mt. Ontake. Pore-fluid pressures were measured by mapping earthquake focal mechanisms on the 3-D Mohr diagram for the regional stress field with high resolutions of 2–5 km. The assumption of the reference stress pattern can cause modeling errors in measurements of pore-fluid pressure. To remove the effect, we statistically evaluated the estimation errors of the regional stress field and included these errors in the analysis. We detected an overpressurized fluid reservoir with a peak of about 10–30 MPa in the east flank of Mt. Ontake, where microseismic swarm activity has been vigorous for the last two decades. The level of pore-fluid pressure was maintained for at least 5 years after 2009. This finding indicates that there are some interactions between the intensive swarm activity and overpressurized fluids: the swarm activity has been driven by overpressurized fluids, whereas pore-fluid pressures have been suppressed by the swarm activity.Graphical abstract.
Highlights
Overpressurized fluids in the Earth’s crust are thought to play an important role in earthquake generation (e.g., Nur and Booker 1972; Yamashita 1999; Miller et al 2004; Sibson 2007)
Comparing the pore-fluid pressure fields in the present study with those obtained in Terakawa et al (2013b), we examine the effects of the assumed stress patterns on estimates of pore-fluid pressures
Ontake we investigated the dependence of porefluid pressures on the assumed stress pattern and reevaluated the 3-D pore-fluid pressure field around Mt
Summary
Overpressurized fluids in the Earth’s crust are thought to play an important role in earthquake generation (e.g., Nur and Booker 1972; Yamashita 1999; Miller et al 2004; Sibson 2007). Using the higher-resolution results for the regional stress field, in the present study, we apply the FMT method to the same dataset as Terakawa et al (2013b) to re-estimate and re-evaluate the 3-D pore-fluid pressure field around Mt. Ontake. Using the regional stress field obtained by Terakawa et al (2016), a standard friction coefficient of 0.6, and a rock density value of 2.7 × 103 kg/m3, we applied the FMT method to the final dataset of focal mechanism solutions (“Data for the FMT analysis” section). 2 and 5, overpressurized fluid reservoirs are visible in the north flank of Mt. Ontake (the region B1, longitude: 137.45°–137.55°, Estimation errors of the regional stress field around Mt. Ontake The CMT data inversion can yield the best estimates and the variance–covariance matrix of the model parameters for the stress field (Terakawa and Matsu’ura 2008). The overpressurized fluid reservoir in the region B3, which is located at the western end of the source region of the 1984 Western Nagano prefecture earthquake, has a peak value of 35 ± 20 MPa (Fig. 5f ); the existence of the reservoir appears to be uncertain within the 95% confidence region
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