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.
The observed tilt vectors were used to obtain the source parameters for the deformation sources. The variation patterns for the tilt vectors observed at borehole stations during the 2013 swarm activity (Fig. 8a) were similar to those observed during the 2001 swarm activity. This result implies that similar deformation sources were displaced during each swarm activity. Using the crustal deformation data for the 2001 swarm activity, Daita et al. (2009) modeled a Mogi point source at a depth of 7 km based on the GNSS data and the presence of two open cracks near the surface at Owakidani and Mt. Komagatake based on the tilt vectors. The location of the Mogi point source and the locations, sizes (length and width), and orientations (azimuth and dip) of the two open cracks in 2013 were assumed to be the same as in the deformation source model for the 2001 swarm activity by Daita et al. (2009). A grid search was performed to find the optimal volume change at the Mogi point source and the two open cracks. Synthetic tilt vectors were calculated using the formula by Okada (1992). As a result, the optimal total volume change at each source was approximately a quarter of that in the 2001 source model. The variations and orientations of the observed tilt vectors at each borehole station can be mostly explained by the optimal source model (Fig. 8a).
The volumetric strain changes caused by the source model were estimated using the formula of Okada (1992). Because the delay in the ACFs was observed at an early lag time (Fig. 7d, e), the relationship between the changes in the velocity and strain at a depth of 0 km (sea level) beneath each station was investigated. Figure 8b indicates the spatial pattern of the volumetric strain changes at sea level. All of the stations used in this study are located within an area in which the volumetric strain changes were positive (dilatational). Figure 9a shows the relationship between the volumetric strain change at each station at sea level and the corresponding changes in dv/v for the 2013 swarm activity. The strain changes during the 2013 swarm activity exceeded 10−6 at the OWD and KOM stations. Conversely, other stations at which no remarkable decrease in dv/v was detected are located in areas where the dilatational strain changes were less than 10−6. It is likely that the large dilatational strain changes (exceeding 10−6) contributed to the decrease in dv/v in this region. This result is consistent with the estimations obtained in other volcanic areas, such as in the eastern part of the Izu Peninsula (Ueno et al. 2012).
However, the temporal variations in dv/v at the KOM and OWD stations are clearly different. The gradual decrease in dv/v at the KOM station started in the beginning of December 2012, after the onset of crustal expansion (Fig. 3b), whereas the sudden decrease in dv/v at the OWD station was observed after the onset of the 2013 swarm activity. The temporal change in the stacked baseline length (Fig. 3b) implies that the stretching of the baseline length across Hakone volcano started in the beginning of October 2012. This crustal expansion was mainly caused by the Mogi point source at a depth of 7 km beneath the KOM station (Fig. 8a). At the beginning of December 2012, when the velocity began to gradually decrease, the baseline seemed to have stretched by approximately 25 % of its total deformation (Fig. 3b). If it is assumed that 25 % of the total inflation at the Mogi point source had already occurred at that time, the area in which the volumetric strain change exceeded 10−6 is distributed only in the shallow region (depth of approximately 1.5 km) beneath the KOM station. The gradual velocity decrease at the KOM station may reflect the strain changes caused by the inflation of the Mogi point source that began prior to the swarm activity (Fig. 10a).
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.