Determination of temporal changes in seismic velocity caused by volcanic activity in and around Hakone volcano, central Japan, using ambient seismic noise records
© The Author(s). 2016
Received: 29 March 2016
Accepted: 1 September 2016
Published: 27 September 2016
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.
KeywordsSeismic velocity changes Ambient noise Passive image interferometry Autocorrelation function Hakone volcano Earthquake swarms Volcanic activity
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). However, because repeating earthquakes do not occur frequently and artificial explosions are expensive, these methods often yield velocity changes with poor temporal resolution. Recently, seismic interferometry methods have been developed to obtain the Green’s function between two seismic stations by using the correlation function of coda waves or ambient noise (e.g., Campillo and Paul 2003; Shapiro et al. 2005). To estimate highly resolved temporal changes in the velocity structure, the theory of seismic interferometry was applied to the auto- and cross-correlation functions (ACFs and CCFs) for continuous ambient noise records using passive image interferometry (PII) (Sens-Schönfelder and Wegler 2006; Wegler and Sens-Schönfelder 2007).
Several studies have reported temporal changes in the velocity structure during large earthquakes obtained using PII (e.g., Wegler et al. 2009; Minato et al. 2012; Ohmi et al. 2008). Two major candidates have been proposed as mechanisms for the velocity changes before and after a large earthquake. One is a nonlinear site effect in the shallow part of the crust due to the strong ground motion occurring during the earthquake. By using the ACFs for ambient noise records, Minato et al. (2012) found a velocity decrease related to a shallow nonlinear site effect caused by the 2011 Tohoku-oki earthquake and its large aftershocks. The site effect was also demonstrated by applying a coda deconvolution method to small earthquake data recorded on the ground surface and at a borehole station (Sawazaki et al. 2009) and by investigating repeating earthquakes (Rubinstein and Beroza 2004). The other mechanism causing the temporal velocity change is the co- and post-seismic deformation in the deeper part of the crust caused by dislocation on the fault plane. Wegler and Sens-Schönfelder (2007) and Ohmi et al. (2008) reported sudden velocity decreases after the occurrence of large intraplate earthquakes based on the ACFs for the ambient noise. These studies deduced that the velocity reduction was related to coseismic stress changes in the source region of the earthquake.
In a volcanic or geothermal region, temporal changes in the subsurface structure caused by volcanic activity have also been estimated using PII. Brenguier et al. (2008) reported velocity reduction prior to the eruptions at Piton de la Fournaise volcano on La Réunion Island. Ueno et al. (2012) also observed an abrupt decrease in velocity during swarm activity in the eastern Izu Peninsula, central Japan, and the subsequent gradual recovery process. Both studies concluded that the velocity changes were related to crustal deformation caused by the intrusion of a magma body into the shallow depth region. Conversely, Maeda et al. (2010) found a localized velocity decrease during swarm activity in a geothermal region in Kyushu, Japan, and suggested that the intrusion of magmatic fluid into a deep part of the source region was the primary contributor to the localized velocity reduction. Temporal changes in the subsurface velocity structure during volcanic activity are likely controlled by several factors, such as crustal deformation and the migration of magmatic or hydrothermal fluids.
The objective of this study is to detect temporal changes in the velocity structure of Hakone volcano, where intense earthquake swarms have often occurred. Because various monitoring systems have been installed in and around the volcano, including a dense seismic observation network, geodetic observation networks such as the Global Navigation Satellite System (GNSS) operated by the Geospatial Information Authority of Japan (GSI), and tiltmeters, detailed information about earthquake swarms and deformation sources can be obtained from the dense seismic and geodetic data provided by these networks. In this study, the relationships between the temporal changes in the velocity structure and other observations, such as seismicity, crustal deformation, and strong ground motions accompanying the volcanic activity, were investigated in detail, and the main factor contributing to the velocity changes is discussed in this paper.
Volcanic activity in Hakone volcano
The activity in 2011 began immediately after the passage of a surface wave from the 2011 Tohoku-oki earthquake (Mw 9.0) (Yukutake et al. 2011). The 2011 activity exhibited an Omori law-like decay rather than swarm-like features (Harada et al. 2012). The focal area of this seismic activity expanded from the northern and southern parts of the caldera (Fig. 2b). Yukutake et al. (2013) suggested that the seismic activity was triggered by the dynamic and static stress changes generated by the 2011 Tohoku-oki earthquake.
These results suggest that remarkable changes in stress, strain, or the supply of materials such as hydrothermal fluids derived from a deep magma source, occurred in Hakone volcano, accompanying the intense activity. Therefore, it is important to examine whether temporal changes in the subsurface structure associated with the volcanic activity occurred.
where T is the inverse of the frequency bandwidth; t 1 and t 2 are the lower and upper limits of the processing time window in the ACF lag time, respectively; ω c is the central frequency; and CC is the cross-correlation coefficient between the reference and individual ACFs.
Average changes in dv/v and their standard deviations at each station for the 2011 Tohoku-oki earthquake and 2013 activity
2013 swarm activity
Velocity changes related to 2013 swarm activity
Significant decreases in dv/v related to the 2013 swarm activity were detected at the KOM and OWD stations (Figs. 5 and 6 and Table 1). The linear relationship between the time shift and lag time (Fig. 7d, e) suggests that the velocity changes were uniform in space. Several past studies reported a relationship between the decrease in dv/v and the volumetric strain changes caused by large earthquakes (e.g., Wegler et al. 2009; Ohmi et al. 2008) or magma intrusion (Ueno et al. 2012). Before and during the 2013 swarm activity, the crustal deformations associated with the volcanic activity were detected by the GNSS stations and tiltmeters (Fig. 3b, c). Therefore, the strain changes due to the crustal deformation sources are likely attributable to the velocity decreases.
A sudden decrease in dv/v at the OWD station (Fig. 5) was detected after the onset of the tilt change observed at the KZY station, which is located 2 km northeast of the OWD station (Fig. 8a). The tilt change at the KZY station was observed starting on 10 January 2013 and reflects the opening of the shallow crack near the OWD station (Figs. 3c and 8a). The sudden velocity decrease (Fig. 5) may have resulted from the volumetric strain change produced by the opening of the shallow crack (Fig. 10a). Honda et al. (2014) reported a noticeable decrease in anisotropic intensity at Hakone volcano based on S-wave splitting analysis during the 2001 swarm activity, and concluded that the decrease in the anisotropic intensity resulted from changes in the stress caused by the opening of the shallow cracks. The results imply that changes in the strain and stress caused by the deformation sources could be a major factor influencing the changes in the subsurface structure. However, the opening of the shallow cracks identified based on the tilt change may have allowed the intrusion of hydrothermal fluid into the shallow region. This hydrothermal fluid may have changed the seismic velocity at shallow depths. However, the precursory velocity decrease observed at the KOM station cannot be explained by fluid migration, because no evidence indicating fluid migration near the KOM station, such as shallow seismic activity or changes in tilt, was observed before the 2013 swarm activity.
The temporal changes in dv/v can also be explained by rainfall (Sens-Schönfelder and Wegler 2006). As shown in Fig. 3c, heavy rainfall was not observed in the Hakone region during the 2013 swarm activity. From May to July 2012, which is the rainy season in this area, rainfall exceeding 200 mm/day was observed several times (Fig. 3c). Even during this rainy season, changes in the velocity at the KOM and OWD stations were not observed to be coincident with the rainfall (Fig. 5a). This result indicates that the ACFs in this frequency range are not sensitive to fractional velocity changes near the surface caused by precipitation.
In addition to the effect of rainfall, a nonlinear site effect on the velocity change must be considered. A velocity decrease exceeding the standard deviation was initially detected on 2 February 2013 (Fig. 5b). A peak ground velocity of 2.8 cm/s was observed during two earthquakes on 28 January 2013 (M 1.4) and 10 February 2013 (M 2.3), which occurred just beneath the OWD station (Fig. 5b). No significant velocity reduction was detected after the M 2.3 earthquake, whereas the velocity decrease continued after the M 1.4 earthquake on 28 January 2013 (Fig. 5b). Moreover, no significant relationship was obtained between the maximum peak ground velocity and the average change in dv/v during the 2013 swarm activity (Fig. 9b). These results suggest that it is difficult to interpret the sudden velocity reduction at the OWD station as being a result of a nonlinear site effect caused by an earthquake.
A sudden decrease in dv/v at the OWD station was also detected at the end of January 2012 (Fig. 5), corresponding to the occurrence time of a moderate-sized earthquake (Mw 5.4) (e.g., Yamada et al. 2015) that occurred in the eastern part of Yamanashi Prefecture at a depth of 18 km and an epicentral distance of approximately 30 km from Hakone volcano (Fig. 1a). During this earthquake, a peak ground velocity of 4.1 cm/s, which is larger than those observed during the 2013 swarm activity (Fig. 3a), was observed at the OWD station. No activation of seismicity or crustal deformation was observed at Hakone volcano. Although comparable peak ground velocities were also observed at the stations near the OWD station (3.1 and 3.6 cm/s at the KIN and KZR stations, respectively) (Fig. 1b), a significant velocity change was not detected at these stations (Fig. 5). This result may imply that the subsurface velocity structure close to the OWD station is sensitive to the changes in the dynamic stress. Because the OWD station is located near the Owakidani geothermal region, an active fumarolic area, hydrothermal fluid associated with geothermal activity likely exists near the station at shallow depths. The presence of highly pressurized fluid may be related to the high sensitivity at the OWD station. An offset in the dv/v values was also observed at the OWD station at the end of June 2011. Because the traces of ACFs changed at this time, especially after a lag time of 5 s, the sudden increase in dv/v may have been caused by a change in the noise source around the station.
Velocity decrease after 2011 Tohoku-oki earthquake
Sudden decreases in velocity were observed at most of the stations immediately following the 2011 Tohoku-oki earthquake (Figs. 5a and 6a). The static strain changes caused by the Tohoku-oki earthquake were less than 10−6 at Hakone volcano (Harada et al. 2012). Conversely, the dynamic strain changes caused by the large-amplitude surface waves exceeded 10−5 (Yukutake et al. 2013). These large dynamic strain changes likely affected the velocity structure at Hakone volcano, given the discussion of the 2013 swarm activity. Brenguier et al. (2014) reported a similar velocity decrease after the 2011 Tohoku-oki earthquake and found that the area in which the velocity was reduced was concentrated near the active volcanoes in the eastern part of Honshu, including Hakone volcano, whereas the large strain changes acted in a broad area spanning the Japanese archipelago. They determined that the velocity structure in the volcanic and geothermal regions is sensitive to stress perturbations because of the presence of hydrothermal and magmatic fluids. Hydrothermal fluid and a magma body were found to be present at depths of 3–10 km and greater than approximately 10 km, respectively, under Hakone volcano using seismic tomography (Yukutake et al. 2015). Yukutake et al. (2013) suggested that the redistribution of hydrothermal fluid by large dynamic strain changes may have contributed to the initiation of the seismic activity in 2011. The velocity reductions at many stations occurred rapidly in comparison with those during the 2013 activity. A velocity reduction of up to −1.06 % was observed within 10 days after the occurrence of the Tohoku-oki earthquake. The sudden velocity reductions may reflect a sudden redistribution of fluid, corresponding to the large stress perturbation from the Tohoku-oki earthquake (Fig. 10b). The phase delays after a lag time of 7 s at the KOM station (Fig. 7c) imply that the velocity change possibly occurred at a depth of approximately 12 km if the constituents of the ACFs are assumed to be backscattered S-waves (e.g., Maeda et al. 2010). The velocity changes might have occurred mainly around the deep magma source of the volcano.
Through the analysis of ACFs 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. These two events showed different characteristics in their velocity reductions, implying different mechanisms. During the 2013 swarm activity, a sudden velocity decrease was observed at the OWD station, which is located near an open crack source. The velocity reduction started immediately after the onset of the tilt change and the swarm activity. Conversely, the velocity was found to gradually decrease at the KOM station, which is located above a Mogi point source positioned at a depth of 7 km. The velocity decrease started before the 2013 swarm activity. The changes in the stress and strain caused by the deformation sources accompanying the volcanic activity likely contributed to the velocity changes. 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 can be explained by the strain change caused by the shallow crack source that opened 3 months after the inflation of the deep Mogi source. We eliminated the possibility that these velocity reductions were caused by precipitation or a nonlinear site effect resulting from strong ground motion. Furthermore, many stations detected significant velocity reductions in and around the caldera rim of Hakone volcano immediately after the 2011 Tohoku-oki earthquake. This result indicates that the velocity reduction occurred in a wide area in and around the Hakone caldera. It was concluded that the velocity changes reflect the fluid redistribution caused by changes in the dynamic stress due to the seismic waves associated with the 2011 Tohoku-oki earthquake. The different types of velocity changes are important observations to discuss the response system for changes in the strain and stress in volcanic and geothermal regions. The results of this study also indicate that PII can be used to detect small changes in the stress and strain states caused by volcanic activity with high temporal resolution. Therefore, the application of this technique to monitor fractional changes in the velocity structure would be useful in volcanic hazard assessments.
Global Navigation Satellite System
Geospatial Information Authority of Japan
Hot Springs Research Institute
Japan Meteorological Agency
National Research Institute for Earth Science and Disaster Prevention
Passive image interferometry
We used seismic waveform data observed at the NIED Hi-net and JMA stations and GNSS data obtained by GSI stations. We greatly appreciate the helpful comments given by Dr. Takuo Maeda and Dr. Tatsuhiko Saito. We are thankful to the three anonymous reviewers and Editor Hitoshi Kawakatsu, who greatly helped us improve this manuscript. The authors would like to thank F. Waldhauser for providing the hypoDD code. JMA provided precipitation data at Hakone volcano.
This work was partly supported by a MEXT Grant-in-Aid for Scientific Research (C) 25400450 (MM) and JSPS KAKENHI Grant Number 15K17755.
YY analyzed the data and wrote the manuscript. TU assisted with the interpretation. MK estimated the source model for the 2013 swarm activity using geodetic data. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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