Abstract
Autocorrelation functions (ACFs) for ambient seismic noise are considered to be useful tools for estimating temporal changes in the subsurface structure. Velocity changes at Hakone volcano in central Japan, where remarkable swarm activity has often been observed, were investigated in this study. Significant velocity changes were detected during two seismic activities in 2011 and 2013. The 2011 activity began immediately after the 2011 Tohoku-oki earthquake, suggesting remote triggering by the dynamic stress changes resulting from the earthquake. During the 2013 activity, which exhibited swarm-like features, crustal deformations were detected by Global Navigation Satellite System (GNSS) stations and tiltmeters, suggesting a pressure increment of a Mogi point source at a depth of 7 km and two shallow open cracks. Waveforms that were bandpass-filtered between 1 and 3 Hz were used to calculate ACFs using a one-bit correlation technique. Fluctuations in the velocity structure were obtained using the stretching method. A gradual decrease in the velocity structure was observed prior to the 2013 activity at the KOM station near the central cone of the caldera, which started after the onset of crustal expansion observed by the GNSS stations. Additionally, a sudden significant velocity decrease was observed at the OWD station near a fumarolic area just after the onset of the 2013 activity and the tilt changes. The changes in the stress and strain caused by the deformation sources were likely the main contributors to these decreases in velocity. The precursory velocity reduction at the KOM station likely resulted from the inflation of the deep Mogi source, whereas the sudden velocity decrease at the OWD station may reflect changes in the strain caused by the shallow open-crack source. Rapid velocity decreases were also detected at many stations in and around the volcano after the 2011 Tohoku-oki earthquake. The velocity changes may reflect the redistribution of hydrothermal fluid in response to the large stress perturbation caused by the 2011 Tohoku-oki earthquake.
Highlights
To obtain information about Earth’s interior, it is important to monitor temporal changes associated with the subsurface structure because seismic velocity is sensitive to stress states and rock properties (e.g., Grêt et al 2006)
Through the analysis of Autocorrelation function (ACF) of ambient noise, two distinct temporal changes in the subsurface structure in the Hakone volcanic and geothermal region were clearly detected during the 2013 swarm activity and after the 2011 Tohoku-oki earthquake
During the 2013 swarm activity, a sudden velocity decrease was observed at the OWD station, which is located near an open crack source
Summary
To obtain information about Earth’s interior, it is important to monitor temporal changes associated with the subsurface structure because seismic velocity is sensitive to stress states and rock properties (e.g., Grêt et al 2006). Comparing coda waves from identical sources is a useful method of estimating temporal changes in seismic velocity (e.g., Snieder et al 2002). Seismic velocity changes caused by a large earthquake or volcanic activity have been detected using the cross-correlation of seismograms of repeating earthquakes (e.g., Poupinet et al 1984; Rubinstein and Beroza 2004; Yamawaki et al 2004) and active control sources (e.g., Nishimura et al 2000; Wegler et al 2006). To estimate highly resolved temporal changes in the velocity structure, the theory of seismic interferometry was applied to the auto- and crosscorrelation functions (ACFs and CCFs) for continuous ambient noise records using passive image interferometry (PII) (Sens-Schönfelder and Wegler 2006; Wegler and Sens-Schönfelder 2007)
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