Toshiko Terakawa

Continuous long-term array analysis of seismic records observed during the 2011 Shinmoedake eruption activity of Kirishima volcano, southwest Japan

We deployed a seismic array at a site 5 km east of Shinmoedake volcano, in the Kirishima volcanic complex of southwest Japan, five days after the sub-Plinian eruption on 26 January, 2011. The array record between February and September 2011 included explosion earthquakes and episodes of weak continuous tremor during eruption periods. We estimated slownesses and back azimuths of seismic waves on a sliding 1-min window using the semblance method. The slownesses of the weak continuous tremor clustered within the range 0.2–0.8 s/km, consistent with a mix of body and surface waves. A probabilistic approach based on a grid search was used to estimate the source locations of the explosion earthquakes and weak continuous tremor. The sources of the explosion earthquakes were beneath the crater at depths of −0.5–1 km above sea level, while the source of the weak continuous tremor was beneath the northern part of Shinmoedake at depths between 1 km below sea level and 1 km above sea level. This latter region corresponds to a shallow low-resistivity layer, suggesting that hydrothermal processes are more plausible than magmatic processes as the generating mechanism of the weak continuous tremor.

Monitoring eruption activity using temporal stress changes at Mount Ontake volcano

Volcanic activity is often accompanied by many small earthquakes. Earthquake focal mechanisms represent the fault orientation and slip direction, which are influenced by the stress field. Focal mechanisms of volcano-tectonic earthquakes provide information on the state of volcanoes via stresses. Here we demonstrate that quantitative evaluation of temporal stress changes beneath Mt. Ontake, Japan, using the misfit angles of focal mechanism solutions to the regional stress field, is effective for eruption monitoring. The moving average of misfit angles indicates that during the precursory period the local stress field beneath Mt. Ontake was deviated from the regional stress field, presumably by stress perturbations caused by the inflation of magmatic/hydrothermal fluids, which was removed immediately after the expulsion of volcanic ejecta. The deviation of the local stress field can be an indicator of increases in volcanic activity. The proposed method may contribute to the mitigation of volcanic hazards.

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three-dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre-condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low-pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations. Plain Language Summary The three-dimensional prestress field around the 2016 Kumamoto earthquake controlled fault behavior of the earthquake. The estimated heterogeneous state of stress on the fault forced the fault to slip in the direction predicted. The stress field also exposed the precondition of pore fluid pressure on the fault. Large coseismic slip occurred not only at the low-pressure part of the fault but also highly pressured part. It indicates that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to upgrading seismic hazard evaluation.