In the present study, we analyze the seismic signals from a continuous volcanic tremor that occurred during a small phreatic eruption of the Hakone volcano, in the Owakudani geothermal region of central Japan, on June 29, 2015. The signals were detected for 2 days, from June 29 to July 1, at stations near the vents. The frequency component of the volcanic tremors showed a broad peak within 1-6 Hz. The characteristics of the frequency component did not vary with time and were independent of the amplitude of the tremor. The largest amplitude was observed at the end of the tremor activity, 2 days after the onset of the eruption. We estimated the location of the source using a cross-correlation analysis of waveform envelopes. The locations of volcanic tremors are determined near the vents of eruption and the surface, with the area of the upper extent of an open crack estimated using changes in the tilt. The durationamplitude distribution of the volcanic tremor was consistent with the exponential scaling law rather than the power law, suggesting a scale-bound source process. This result suggests that the volcanic tremor originated from a similar physical process occurring practically in the same place. The increment of the tremor amplitude was coincident with the occurrence of impulsive infrasonic waves and vent formations. High-amplitude seismic phases were observed prior to the infrasonic onsets. The time difference between the seismic and infrasonic onsets can be explained assuming a common source located at the vent. This result suggests that both seismic and infrasonic waves are generated when a gas slug bursts at that location. The frequency components of the seismic phases observed just before the infrasonic onset were generally consistent with those of the tremor signals without infrasonic waves. The burst of a gas slug at the surface vent may be a reasonable model for the generation mechanism of the volcanic tremor and the occurrence of impulsive infrasonic signals. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Deep low‐frequency earthquakes (DLFEs) are ubiquitous seismic activities in the deep parts of volcanoes. Owing to the low signal‐to‐noise ratio, the seismic activities of DLFEs have not been characterized in detail; particularly, the linkage between DLFEs and shallow volcanic activity has not been understood sufficiently. In this study, numerous DLFEs have been successfully detected beneath the Hakone volcano, central Japan, by cross‐correlating a template to the continuous seismic signals. The resulting seismic catalog reveals that DLFEs are activated prior to notable earthquake swarms in the shallow part of a volcano and to the crustal expansion caused by a pressure source at a depth of 7 km. Results indicate that the activation of DLFEs reflects the feeding of magmatic fluid from depth. The subsequent increment in the magmatic‐fluid pressure triggers shallow volcanic activities.
Global navigation satellite system data from Hakone volcano, central Japan, together with GEONET data from the Geospatial Information Authority of Japan, were used to investigate the processes associated with the volcanic activity in 2015, which culminated in a small phreatic eruption in late June 2015. Three deep and shallow sources, namely spherical, open crack, and sill, were employed to elucidate the volcanic processes using the observed GNSS displacements, and the MaGCAP-V software was used to estimate the volumetric changes of these sources. Our detailed analysis shows that a deep inflation source at 6.5 km below sea level started to inflate in late March 2015 at a rate of ~ 9.3 × 10 4 m 3 /day until mid-June. The inflation rate then slowed to ~ 2.1 × 10 4 m 3 /day and ceased at the end of August 2015. A shallow open crack at 0.8 km above sea level started to inflate in May 2015 at a rate of 1.7 × 10 3 m 3 / day. There was no significant volumetric change in the shallow sill source during the volcanic unrest, which is evident from interferometric synthetic aperture radar analysis. The inflation of the deep source continued even after the eruption without a significant slowdown in inflation rate. The inflation stopped in August 2015, approximately 1 month after the eruption ceased. This observation implies that the transportation of magmatic fluid to a deep inflation source (6.5 km) triggered the 2015 unrest. The magmatic fluid may have then migrated from the deep source to the shallow open crack. The phreatic eruption was then caused by the formation of a crack that extended to the surface. However, steam emissions from the vent area during and after the eruption were apparently insufficient to mitigate the internal pressure of the shallow open crack. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
S U M M A R YIn order to investigate the strain accumulation process around the Niigata-Kobe Tectonic Zone in central Japan, we estimate a precise GPS velocity field around a representative fault area, the Atotsugawa fault system. The velocity profiles obtained from two dense GPS arrays crossing the Atotsugawa fault demonstrate the existence of a laterally heterogeneous deformation pattern along the fault strike. The West Array suggests the possibility of surface fault creep, which disagrees with inferences gleaned from previous EDM measurements. High shear strain rate, which is calculated from wider velocity field, distributes around not only the Atotsugawa fault system, but also the Takayama-Oppara fault zone, which is located 50 km south from the Atotsugawa fault. In order to consider the effect of interaction between those active faults, we conduct a block-fault model analysis. Our block model consists of nine blocks and 33 fault segments. Relative motion between two blocks at the northern and southern ends of the fault system is calculated to be 9.9 mm yr −1 . The intervening region between these two blocks corresponds to the Niigata-Kobe Tectonic Zone with a strain rate of 0.2 ppm yr −1 . Slip deficit rates at the Atotsugawa, Ushikubi, and Takayama-Oppara fault zones are estimated to be 2.2, 3.9 and 2.3 mm yr −1 , respectively. Although the ratio of slip deficit rate (2.2 : 3.9 : 2.3 ≈ 1 : 2 : 1) of these three faults is similar to that of geological long-term slip rates (1 : 2 : 0.7), the magnitude of geodetic slip deficits is 30 per cent to 50 per cent larger than long-term slip rates. This systematic difference may be due to inelastic deformation of the crust in this region. Since the total slip deficit rate between the Ushikubi and the Takayama-Oppara fault zone is 8.4 mm yr −1 , about 85 per cent of the relative block motion, the deformation is mostly accommodated by elastic strain accumulation on these three faults.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.