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.
Highlights
Volcanic activity is often accompanied by many small earthquakes
The local stress field around volcanoes represents the superposition of the regional stress field and stress perturbations related to volcanic activity[1,2,3,4]
We assumed that the source was double couple (DC) and considered three velocity models (Supplementary Fig. 1) to account for possible errors in the hypocentres and take-off angles
Summary
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. Examining the focal mechanism solutions of volcano-tectonic (VT) earthquakes provides detailed information on the state of a volcano via the local stress field This has the potential to contribute to prediction of eruptions over the short and medium term (weeks to months). Application of a matched-filter technique to continuous waveforms (23 August to 30 September 2014) resulted in detection of thousands of microseismic events, indicating that an increase in VT events was followed by an increase in the number of long-period events 5 days later[6] This detailed analysis revealed that VT events beneath the craters migrated upwards as well as laterally in the NNW–SSE direction for the final 10 min preceding the eruption[6]. The various observations of precursor processes make eruption forecasting difficult, changes in volcanic seismicity have been successfully used empirically for predicting eruptions[10,11]
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