研究者詳細 - 前田　裕太

論文 - 前田　裕太

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An approach to source characterization of tremor signals associated with eruptions and lahars 査読有り国際共著

Hiroyuki Kumagai, Patricia Mothes, Mario Ruiz, Yuta Maeda

Earth, Planets and Space 67 巻頁： 178 2015年11月

Preparatory and precursory processes leading up to the 2014 phreatic eruption of Mount Ontake, Japan 査読有り

Aitaro Kato, Toshiko Terakawa, Yoshiko Yamanaka, Yuta Maeda, Shinichiro Horikawa, Kenjiro Matsuhiro, Takashi Okuda

Earth, Planets and Space 67 巻頁： 111 2015年7月

A subsurface structure change associated with the eruptive activity at Sakurajima Volcano, Japan, inferred from an accurately controlled source 査読有り

Yuta Maeda, Koshun Yamaoka, Hiroki Miyamachi, Toshiki Watanabe, Takahiro Kunitomo, Ryoya Ikuta, Hiroshi Yakiwara, Masato Iguchi

Geophysical Research Letters 42 巻 ( 13 ) 頁： 5179 - 5186 2015年7月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Temporal variations of Green functions associated with the eruptive activity at Sakurajima Volcano, Japan, were estimated using an accurately controlled routinely operated signal system (ACROSS). We deconvolved 400 s waveforms of the ACROSS signal at nearby stations by a known source time function and stacked the results based on the time relative to individual eruptions and the eruption intervals; the quantities obtained by this procedure are Green functions corresponding to various stages of the eruptive activity. We found an energy decrease in the later phase of the Green functions in active eruptive periods. This energy decrease, localized in the 2-6 s window of the Green functions, is difficult to explain by contamination from volcanic earthquakes and tremors. The decrease could be more reasonably attributed to a subsurface structure change caused by the volcanic activity.

DOI： 10.1002/2015GL064351

Changes to the volcanic outgassing mechanism and very-long-period seismicity from 2007 to 2011 at Mt. Asama, Japan 査読有り

Ryunosuke Kazahaya, Yuta Maeda, Toshiya Mori, Hiroshi Shinohara, Minoru Takeo

Earth and Planetary Science Letters 418 巻頁： 1 - 10 2015年3月

Development and operation of a regional moment tensor analysis system in the Philippines: Contributions to the understanding of recent damaging earthquakes 査読有り国際共著

Baby Jane T. Punongbayan, Hiroyuki Kumagai, Nelson Pulido, Jun D. Bonita, Masaru Nakano, Tadashi Yamashina, Yuta Maeda, Hiroshi Inoue, Arnaldo A. Melosantos, Melquiades F. Figueroa, Ponczh Colleen M. Alcones, Karl Vincent C. Soriano, Ishmael C. Narag, Renato U. Solidum Jr.

Journal of Disaster Research 10 巻 ( 1 ) 頁： 25 - 34 2015年2月

記述言語：英語掲載種別：研究論文（学術雑誌）

A network of 10 satellite-telemetered broadband stations was established under a cooperative project between Japan and the Philippines, and a source analysis system based on waveform inversion of regional seismograms was adapted to operationalize a regional moment tensor analysis of Philippine earthquakes. This study presents the source information generated by the system for recent damaging earthquakes: the Mw6.7 Negros and Mw7.6 offshore Samar in 2012, and the Mw7.2 in Bohol in 2013. Results show that the Negros event was generated by shallow NE-SW thrust faulting with a small strike-slip component, and that the centroid was located slightly offshore. The Samar event occurred in relation to an outer-trench thrust fault within the Philippine Sea Plate, adjacent to a part of the Philippine Trench that has relatively low seismicity. Our centroid moment tensor (CMT) solutions show that the Samar event triggered distinct clusters of outer-rise normal and thrust aftershocks, which we explain as being consistent with a Coulomb stress change in the area. Finally, we infer that the previously unidentified fault zone that generated the Bohol earthquake has a length of ~ 100 km, is oriented ENE-WSW, transects parts of Bohol, and extends offshore towards Cebu. These examples show how recent improvements in Philippine earthquake monitoring could contribute to the characterization of earthquake sources and in the understanding of the seismotectonics of the area.

A phreatic explosion model inferred from a very long period seismic event at Mayon Volcano, Philippines 査読有り国際共著

Yuta Maeda, Hiroyuki Kumagai, Rudy Lacson Jr., Melquiades S. Figueroa II, Tadashi Yamashina, Takahiro Ohkura, and Alejo V. Baloloy

Journal of Geophysical Research: Solid Earth 120 巻 ( 1 ) 頁： 226 - 242 2015年1月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

During a phreatic explosion at Mayon Volcano, Philippines, on 7 May 2013, a very long period seismic event with a peak frequency of 0.4 Hz was observed. Our frequency-domain waveform inversion solution of the event in the frequency range 0.1-0.6 Hz is consistent with a subhorizontal tensile crack and a vertical single force at a shallow location beneath the summit crater. The source time functions obtained by the waveform inversion are band-passed forms. We estimated the deconvolved forms of the source time functions (DSTFs), which are source time functions corrected for the effects of the band-pass filter. The DSTF of the crack can be approximated by an impulse-type function composed of inflation followed by deflation, whereas the DSTF of the single force can be approximated by a downward impulse. The inflation of the crack may be attributed to boiling of underground water and its deflation can be attributed to discharge of water vapor, whereas the downward force may be understood as the counterforce of the explosion. Our results suggest that only a portion of the crack wall was destroyed by the explosion. We could not find clear precursors in seismic, thermography, geothermal, geodetic, and meteorological data. We present a model of repeated explosions in which an explosion can occur once the fragmented portion of the crack is sealed by precipitation of clay minerals or hydrothermal secondary deposits. This model may explain the absence of clear precursory signals before the 2013 explosion.

DOI： 10.1002/2014JB011440

Shallow S wave attenuation and actively degassing magma beneath Taal Volcano, Philippines 査読有り国際共著

Hiroyuki Kumagai, Rudy Lacson Jr., Yuta Maeda, Melquiades S. Figueroa II, Tadashi Yamashina

Geophysical Research Letters 41 巻 ( 19 ) 頁： 6681 - 6688 2014年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

Taal Volcano, Philippines, is one of the world's most dangerous volcanoes given its history of explosive eruptions and its close proximity to populated areas. A real-time broadband seismic network was recently deployed and has detected volcano-tectonic events beneath Taal. Our source location analysis of these volcano-tectonic events, using onset arrival times and high-frequency seismic amplitudes, points to the existence of a region of strong attenuation near the ground surface beneath the east flank of Volcano Island in Taal Lake. This region is beneath the active fumarolic area and above sources of pressure contributing inflation and deflation, and it coincides with a region of high electrical conductivity. The high-attenuation region matches that inferred from an active-seismic survey conducted at Taal in 1993. These features strongly suggest that the high-attenuation region represents an actively degassing magma body near the surface that has existed for more than 20 years.

DOI： 10.1002/2014GL061193

Seismic moment and volume change of a spherical source 査読有り

Hiroyuki Kumagai, Yuta Maeda, Mie Ichihara, Nobuki Kame, and Tetsuya Kusakabe

Earth, Planets and Space 66 巻頁： 7 2014年4月

記述言語：英語掲載種別：研究論文（学術雑誌）

A spherical source, one of the simplest seismic sources, has been represented in various ways in the literature. These representations include a spherical source with outward radial expansion (S1), a spherical crack source with outward and inward crack wall motions along the spherical surface (S2), an isotropic source represented by three mutually perpendicular vector dipoles (S3) or three mutually perpendicular tensile cracks (S4), and a spherical source undergoing a transformational expansion (S5). We systematically examined these sources and their static displacement fields to clarify how these representations are mutually related. We also considered the sources in a bimaterial medium, in which the source material is different from the surrounding medium, as a model of a magma or hydrothermal reservoir. Our examinations show that the source volume change of a spherical source (S1) (actual volume) can be uniquely determined from the seismic moment of an isotropic source (S3) regardless of our assumption of the source medium and that the actual volume of S1 is related to the seismic moment of S3 through the equivalence of the displacement fields due to these two sources. The seismic moment of S3 is also related through another equation to the source volume change of three tensile cracks (S4), which is equal to the source volume changes defined in S2 and S5. This relation has different forms, depending on the source medium and source process. This study provides a unified view for quantifying a spherical source using the seismic moment of an isotropic source determined from waveform inversion analysis.

DOI： 10.1186/1880-5981-66-7

An analytical formula for the longitudinal resonance frequencies of a fluid-filled crack 査読有り

Yuta Maeda, Hiroyuki Kumagai

Geophysical Research Letters 40 巻 ( 19 ) 頁： 5108 - 5112 2013年10月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

The fluid-filled crack model has been widely used to interpret the peak frequencies of long-period (LP) seismic events at volcanoes. Up to now, numerical methods have been used to compute resonant frequencies in the model. We propose a simple analytical formula for the longitudinal resonance frequencies of a fluid-filled crack. We evaluated the formula by comparing its results with the resonant frequencies computed by finite difference method code. The comparison revealed that the formula well describes the resonant frequencies of both 2-D and 3-D cracks. The formula enables simple and rapid estimates of the fluid properties and geometries of LP source cracks.

DOI： 10.1002/grl.51002

Source process of long-period seismic events at Taal volcano, Philippines: Vapor transportation and condensation in a shallow hydrothermal fissure 査読有り国際共著

Yuta Maeda, Hiroyuki Kumagai, Rudy Lacson Jr., Melquiades S. Figueroa II, Tadashi Yamashina

Journal of Geophysical Research 118 巻 ( 6 ) 頁： 2832 - 2846 2013年6月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

We analyzed observations of a swarm of more than 40,000 long-period (LP) seismic events at Taal volcano, Philippines, in 2010-2011. The event waveforms are strongly correlated to each other, consistent with a fixed source location, and begin with a dilatational first motion. They have a peak frequency around 0.8 Hz and a quality factor Q of 6. Waveform inversion of the events pointed to a tensile crack source dipping 30^o-60^o at a shallow (100-200 m) depth. A simulation using a fluid-filled crack model indicated that the complex frequencies of the waveforms are explained by the fundamental longitudinal mode resonance of a vapor-filled crack 188 m long. A satellite thermal infrared image acquired during the swarm period suggests that the LP events were not accompanied by surface gas releases. We considered a vapor transportation model in which vapor exsolved from magma and rose in a fissure extending to the LP source. This model yielded estimates that 10⁵-10⁷ m³ of magma was involved in the LP swarm and that the temperature of vapor in the LP source crack was around 600 K. We modeled a triggering mechanism of the crack resonance based on sudden condensation of vapor at the crack tip in a cold aquifer. This model explained observed characteristics of the events including the dilatational first motion, the total volumetric change, and the fixed source location.

DOI： 10.1002/jgrb.50205

Source amplitudes of volcano-seismic signals determined by the amplitude source location method as a quantitative measure of event size 査読有り国際共著

Hiroyuki Kumagai, Rudy Lacson Jr., Yuta Maeda, Melquiades S. Figueroa II, Tadashi Yamashina, Mario Ruiz, Pablo Palacios, Hugo Ortiz, Hugo Yepes

Journal of Volcanology and Geothermal Research 257 巻頁： 57 - 71 2013年5月

記述言語：英語掲載種別：研究論文（学術雑誌）

The amplitude source location (ASL) method, which uses high-frequency amplitudes under the assumption of isotropic S-wave radiation, has been shown to be useful for locating the sources of various types of volcano-seismic signals. We tested the ASL method by using synthetic seismograms and examined the source amplitudes determined by this method for various types of volcano-seismic signals observed at different volcanoes. Our synthetic tests indicated that, although ASL results are not strongly influenced by velocity structure and noise, they do depend on site amplification factors at individual stations. We first applied the ASL method to volcano-tectonic (VT) earthquakes at Taal volcano, Philippines. Our ASL results for the largest VT earthquake showed that a frequency range of 7 -12 Hz and a Q value of 50 were appropriate for the source location determination. Using these values, we systematically estimated source locations and amplitudes of VT earthquakes at Taal. We next applied the ASL method to long-period events at Cotopaxi volcano and to explosions at Tungurahua volcano in Ecuador. We proposed a practical approach to minimize the effects of site amplifications among different volcano seismic networks, and compared the source amplitudes of these various volcano-seismic events with their seismic magnitudes. We found a proportional relation between seismic magnitude and the logarithm of the source amplitude. The ASL method can be used to determine source locations of small events for which onset measurements are difficult, and thus can estimate the sizes of events over a wider range of sizes compared with conventional hypocenter determination approaches. Previously, there has been no parameter widely used to quantify the sources of volcano-seismic signals. This study showed that the source amplitude determined by the ASL method may be a useful quantitative measure of volcano-seismic event size.

DOI： 10.1016/j.jvolgeores.2013.03.002

Effects of water domains on seismic wavefields: A simulation case study at Taal volcano, Philippines 査読有り

Yuta Maeda, and Hiroyuki Kumagai

Earth, Planets and Space 65 巻 ( 2 ) 頁： 85 - 96 2013年3月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

We examined the effects of water domains on seismic wavefields at the Taal volcano, Philippines, where there is a summit crater lake on an active volcanic island surrounded by a caldera lake. We conducted forward waveform simulation and synthetic waveform inversion using the 3D finite-difference method for various models with and without the water lakes. Our study demonstrated the existence of both near- and far-field effects of the water domains. The near-field effect is related to source excitation affected by a water domain, and the far-field effect is caused by dispersion of Rayleigh waves propagating through a water domain. Our study also showed that the replacement of water by a vacuum rather than by a solid medium provides a better approximation of a water domain. A water domain has considerable effect on seismic waveforms and must be properly treated when the waveform period is shorter than 2 s, even if recording stations lie in the near field. If the waveform period is longer than 5 s, a water domain has a minor effect on seismic waveforms and may be replaced by a vacuum.

DOI： 10.5047/eps.2012.07.004

Relation between single very-long-period pulses and volcanic gas emissions at Mt. Asama, Japan 査読有り

Ryunosuke Kazahaya, Toshiya Mori, Minoru Takeo, Takao Ohminato, Taku Urabe, Yuta Maeda

Geophysical Research Letters 38 巻 ( 11 ) 頁： L11307 2011年6月

記述言語：英語掲載種別：研究論文（学術雑誌）

Multiple volcanic observations conducted at Mt. Asama, Japan, provide evidence of a link between single very-long-period (VLP) seismic pulses and volcanic gas emissions. SO₂ flux measurements were conducted on 2 June 2009, when Mt. Asama was producing ash-free eruptions with VLP pulses. Gas bursts from a vent at the crater bottom following the VLP pulses provided an excellent opportunity to examine the relation directly. The SO₂ emission for each eruption was calculated by integrating high temporal SO₂ flux data obtained by the SO₂ imaging system and subtracting the contribution from quiescent degassing from fumaroles around the crater bottom. A seismic moment of VLP pulse was estimated by the waveform inversion. We observed seven eruptions and obtained the proportional relation between VLP pulse moment and SO₂ emission. The relation determined is consistent with the VLP source model; these observational results are the first report of a quantitative comparison between single VLP pulse moment and volcanic gas emission.

DOI： 10.1029/2011GL047555

Very-long-period pulses at Asama volcano, central Japan, inferred from dense seismic observations 査読有り

Yuta Maeda, Minoru Takeo

Geophysical Journal International 185 巻 ( 1 ) 頁： 265 - 282 2011年4月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

We observed seismic events consisting of several one-sided velocity pulses with durations of 5-30 s (hereafter called very-long-period or VLP pulses) using up to 14 broad-band seismometers installed near the summit area of Asama volcano, Japan. Particle motions do not point to a single source location despite their rectilinearities, suggesting a non-isotropic source mechanism, together with path effect. We conducted moment-tensor inversion analyses for these events using the Green's functions of not only translational motions but also of tilt motions. The best-fitting source locations are located north of the crater centre, at depths of 100-150 m below the crater bottom. The principal value ratios of moment-tensor solutions can be approximated as 5:3:2, which is indicative of a combination of a tensile crack and a cylinder, with the strike and dip angle of the crack both being nearly 70^o, and symmetry axis of the cylinder being inclined by nearly 30^o from the vertical. The time histories of moment-tensors are indicative of an initial sudden pressurization and subsequent gradual depressurization at the source region. We sought to model VLP pulses at Asama volcano by considering the inflow and outflow of gas at the source cavity, compensated by boiling in another deeper reservoir.

DOI： 10.1111/j.1365-246X.2011.04938.x

A waveform inversion including tilt: method and simple tests 査読有り

Yuta Maeda, Minoru Takeo, Takao Ohminato

Geophysical Journal International 184 巻 ( 2 ) 頁： 907 - 918 2011年2月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A new waveform inversion method is proposed to deal with horizontal seismograms strongly contaminated by tilt signals, without assumptions regarding tilt motions. Instead of decomposing the horizontal seismograms into translational and tilt contributions, we realize this task by calculating the Green's functions consisting of not only the seismometer's response to the synthetic translational motions but also to the synthetic tilt motions. A finite difference method (FDM), which uses a staggered grid and a perfect matched layer (PML) boundary condition, is used to calculate the Green's functions. This method enables calculation of wavefields which have wavelengths of up to 10 times larger than the dimension of the computational grid. Although reflected waves from the numerical boundaries are negligible, comparisons with analytical solutions evidence errors of up to several per cent. These occur either because the source-to-receiver distance is small once compared to the grid size, or as a consequence of the half-cell difference between the actual free surface and the station location. In addition, in the case that the free surface is not flat, an error of up to 60 per cent occurs in calculating the tilt motion, due to the stair-like approximation of topography in the FDM. This latter error can be reduced to 10 per cent by averaging the tilts at five adjacent cells aligned in the direction of the tilt component, centred on the target grid cell. By applying this algorithm, a waveform inversion can be used to successfully reconstruct the vertical and horizontal seismograms of very-long-period (10-30 s) pulses at Asama Volcano, central Japan, despite the fact that the horizontal seismograms are strongly contaminated by tilt signals.

DOI： 10.1111/j.1365-246X.2010.04892.x

浅間山における人工地震探査: 探査の概要と初動の走時について査読有り

青木陽介, 武尾実, 辻浩, 小山悦郎, 青山裕, 藤松淳, 松本聡, 宮町宏樹, 中道治久, 大倉敬宏, 大湊隆雄, 及川純, 棚田理絵, 筒井智樹, 山本圭吾, 山本希, 山里平, 山脇輝夫, 市原美恵, 井本良子, 風間卓仁, 小山崇夫, 前田裕太, 前野深, 森田裕一, 中田節也, 中村祥, 長田昇, 渡辺秀文, P.K.B. Alanis, T. Anggono, 藤原善明, 福山由朗, 萩原慎太郎, 橋本武志, 平野舟一郎, 堀口桂香, 飯島聖, 石原吉明, 石川渓太, 石坂和之, 北脇裕太, 黒木英州, 草野富二雄, 前川徳光, 増田与志郎, 松村智之, 中元真美, 西村太志, 野上健治, 奥田隆, 坂井孝之, 佐藤正良, 鈴木敦生, 丹下豪, 植木貞人, 渡邊篤志, 八木原寛, 山崎友也, 吉川慎

東京大学地震研究所彙報 83 巻 ( 1 ) 頁： 1-26 2008年

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