Seismic swarms in volcanic regions are commonly attributed to fluid processes; however, the precise mechanisms linking fluid migration to earthquake generation remain poorly understood due to limited near-field geodetic observations. We investigated the deformation processes associated with the 2017 M5.6 earthquake at Mount Ontake to understand the role of fluid upwelling in earthquake generation. We conducted precise leveling surveys at multiple time periods from 2016 to 2023 and applied tensile crack and fault modeling to explain the observed deformation patterns. Significant uplift of approximately 35 mm was detected in the focal area during April 2017–April 2018, followed by subsidence of approximately 7 mm during April 2018–May 2023. A fault with a tensile crack model best explained the uplift, with a tensile crack opening of 238 mm and a fault slip of 689 mm. The tensile crack was located approximately 2 km southeast of the M5.6 earthquake, with the fault’s downdip extension adjacent to the shallow edge of the tensile crack. The results suggest that upwelling crustal fluids expanded tensile cracks and weakened fault strength through increased pore-fluid pressure, thereby triggering the M5.6 earthquake. These results demonstrate that precise leveling can provide critical insights into the fluid–earthquake relationships in volcanic seismic swarm areas.

During the 2000 eruption of the Miyakejima volcano in Japan, step- like tilt changes (TC) generally accompanied by very long period (VLP) seismic signals with a pulse-like shape and widths of similar to 50 s were repeatedly observed (TC/VLP events). Kinematic GPS time series for Miyakejima were investigated in order to detect displacements associated with these events. We found that the kinematic GPS time series could be interpreted as the superposition of the following features: (1) displacement associated with the TC/VLP events, the source of which possibly corresponded to a shallow magma chamber represented by an almost vertical ellipsoidal cavity elongated NE- SW at a depth of 2-3 km; (2) displacement following TC/VLP events that may have been caused by exponential-type volume decreases of the same magma chamber with a decay constant of approximately half a day; and (3) displacements that may be caused by continuous volume decreases of the same magma chamber. These features broadly support the piston model of VLP seismic signals, in which a vertical piston of solid conduit material intermittently sinks into a magma chamber located a few kilometer beneath the edifice following deflation caused by continuous outflux of magma. The volume increase of the magma chamber associated with the TC/VLP events was found to be much smaller than that of the collapsed caldera, suggesting that most of the mass in the conduit sank into the magma chamber without generating VLP seismic waves or step- like TC.

We examine the initial phases of explosion earthquakes accompanying Vulcanian eruptions at Lokon-Empung volcano in Indonesia to reveal the triggering process of explosive eruptions. In 2012–2013, 56 Vulcanian eruptions at Lokon-Empung were observed by our temporary observation network being comprised of four broadband seismometers and two infrasound microphones at 1.6–6.8km from the active vent. The seismic records of each explosion earthquake share almost the same waveform characteristics of initial phases, consisting of a small compressional onset (P phase) and a subsequent large dilatational phase (D phase). Particle orbits of both phases show straight motion from beneath the active vent, which suggests that these phases are composed of a longitudinal body wave. For each explosion, the origin times of the P phase precedes 0.8–2.5s before the occurrence of an explosion at the vent that are detected by infrasound data. Since the signal-to-noise ratio of the P phase is insufficient for a quantitative analysis, we analyze the D phase dominating the initial phases. Our analysis for the signals of 0.2–1.0Hz shows the D phase are well explained by a cylindrical contraction source with a half-cosine shaped time function located at 1.0–1.3km depth beneath the active vent. We also recognize that some explosions are followed by a prominent tremor that coincides with continuous ash emission (ET). The seismic amplitudes and intensity of the D phase of events in ET are larger than the explosions without accompanying tremor (EX). The frequency distribution of the time interval from the previous eruption is also different in the events in ET and EX. The implosion source in the initial phases of explosion earthquakes at several km beneath the active vent has been reported at Sakurajima volcano. Since our result shows considerable agreement with the previous works at Sakurajima, both Lokon-Empung and Sakurajima may share similar initial processes of Vulcanian eruptions.

Following the eruption of the Miyakejima Volcano in the Izu Islands, Japan, in the year 2000, a continuous GPS network observed the ongoing contracting crustal deformation. Subsequently, a slight inflation of the island was detected from around 2006, and we initiated a campaign of dense GPS observations around the volcano from 2011. Precise crustal deformation studies indicated inflation in the southern part of the island and deflation around the center of the crater. Using these observations, we estimated that three magma sources (a shallow deflation sill under the crater, a southern inflation dyke, and a deep inflation spherical source) were activated during 2011–2013. In particular, the presence of an inflation dyke at an intermediate depth had not been inferred by previous studies. Accordingly, we posit that the supply of magma from a deep spherical source to the new dyke source has been initiated only recently.

Active Volcanism Convener:*Yosuke Aoki(Earthquake Research Institute, University of Tokyo), Mie Ichihara(Earthquake Research Institute, University of Tokyo), Chair:Mare Yamamoto(Department of Geophysics, Graduate School of Science, Tohoku University), Takahito Kazama(Graduate School of Science, Kyoto University) Thu. May 1, 2014 4:15 PM 5:30 PM 416 (4F) This session discusses various phenomena associated with active volcanisms including, but not limited to, geophysical and geochemical observations, geology, historical eruptions, and development of modern instruments.

Miyakejima Island is an active volcanic Island located about 175 km south from Tokyo, Japan. Miyakejima volcano has had at least 15 historical eruptions and erupted about every 20 years in the past 100 years. The latest eruptive activities began in 2000. These activities included forming a caldera for the first time in 2500 years and gigantic volcanic gas emission that forced islander to evacuate over four and half years. This style was different from the style of the last 100 years. A dense GPS observation campaign had begun at Miyakejima volcano in cooperation with the University of Tokyo, Kyushu University, and Nagoya University in 1995. At the eruption in 2000, the state of the magma intrusion was captured in detail from the observed displacement. However, this campaign observation had stopped from 2002 to 2010 because of the landing restrictions to the island due to the large amount of volcanic gas emission. We rebuilt the dense GPS network and restarted the campaign observation from 2011. In this study, we examined the magma-supplying system under Miyakejima volcano by means of GPS observations to get insights about the future activity of Miyakejima volcano. We used the data of our campaign observation of 2011 and 2012 recorded by 45 stations, and the data of four GEONET sites of Geospatial Information Authority of Japan (GSI) in this analysis. The observation data were analyzed by RTKLIB (Takasu et al., 2007) using GPS precise ephemeris from IGS. We estimated the crustal deformation of Miyakejima from 2011 to 2012 from the obtained coordinate values, and calculated the position and volume of spherical source using the software named Magnetic and Geodetic data Computer Analysis Program for Volcano (MaGCAP-V). The result showed there was the small inflation source at a depth of about 3 km beneath Mt. Oyama, which is the central cone of Miyakejima volcano. From this result, we can say Miyakejima have the possibility that the magma supply to the magma chamber leading to the next eruption has begun. We will carry out the observation this year and examine whether the expansion trend continues or not.

Using GPS data, we evaluate the volume change of the magma reservoir associated with the eruption of Kirishima Shinmoe-dake volcano, southern Kyushu, Japan, in 2011. Because ground deformation around Shinmoe-dake volcano is strongly affected not only by regional tectonic movement but also by inflation of Sakurajima volcano located approximately 30–40 km to the southwest, we first eliminate these unwanted contributions from the observed data to extract the signals from Shinmoe-dake volcano. Then, we estimate the source locations and volume change before, during, and after the highest eruptive activity occurring between January 26 and 31. Our model shows that the magma began to accumulate about one year prior to the sub-Plinian eruption, with approximately 65% of the accumulated magma being discharged during the peak of the eruptive activity, and that magma accumulation continued until the end of November 2011. An error analysis shows that the sources during the three periods indicated above are located in almost the same position: 5 km to the northwest of the summit at a depth of 8 km. The 95% confidence interval of the estimated source depth is from 7.5 to 13.7 km.