Monitoring eruption activity using temporal stress changes at Mount Ontake volcano

Abstract

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.

Modeling the dynamics of a phreatic eruption based on a tilt observation: Barrier breakage leading to the 2014 eruption of Mount Ontake, Japan

“…Figure summarizes our interpretation for the precursors of the 2014 eruption and comparisons with the results of several previous studies. Hypocenters of VT earthquakes showed an NNW‐SSE trend [ Kato et al , ], which is subparallel to one of the maximum shear directions in the regional stress field [ Terakawa et al , ] (Figure b). According to Terakawa et al [], focal mechanisms of the VT earthquakes changed to an ENE‐WSW tension before the eruption.…”

“…Hypocenters of VT earthquakes showed an NNW‐SSE trend [ Kato et al , ], which is subparallel to one of the maximum shear directions in the regional stress field [ Terakawa et al , ] (Figure b). According to Terakawa et al [], focal mechanisms of the VT earthquakes changed to an ENE‐WSW tension before the eruption. These results were explained by the injection of fluids into preexisting seismic faults along the NNW‐SSE trend.…”

“…The 2014 eruption was preceded by precursory signals with two time scales. One was a swarm of volcano‐tectonic (VT) earthquakes that preceded the eruption for approximately 1 month [ Kato et al , ; Terakawa et al , ]. Although a few long‐period (LP) events also took place during this period [ Kato et al , ], the LP activity was considerably lower than that observed before the 2007 eruption [ Nakamichi et al , ].…”

Special issue “The phreatic eruption of Mt. Ontake volcano in 2014”

“…Ogiso et al (2015) track the source location of tremor beneath the crater. Finally, Terakawa et al (2016) reveal stress changes associated with the eruption, strongly implying fluid-pressure changes. Kato et al (2015) perform a high-resolution analysis of seismicity associated with the 2014 eruption using a matched-filter detection technique.…”

“…They find that it was located beneath the crater and descended 2 km over the course of several minutes following the onset of the eruption; this appears to be a robust solution. Terakawa et al (2016) (Nature Communications) analyze temporal change in the focal mechanism of earthquakes beneath the craters to document the stress variation associated with the phreatic eruption. Substantial deviation from the average stress field is observed in the focal mechanism in the period before the phreatic eruption.…”