Papers by Tomokazu Kobayashi
Zisin (Journal of the Seismological Society of Japan. 2nd ser.), 2012
Additional file 4: Fig. S3. Variation of fault parameter estimates obtained from a bootstrap appr... more Additional file 4: Fig. S3. Variation of fault parameter estimates obtained from a bootstrap approach. Longitude and latitude are center position of fault plane, while depth is fault top position.
Japan Geoscience Union, 2015
Preface: Persistent scatterer interferometry (PSI) makes use of points with temporally coherent p... more Preface: Persistent scatterer interferometry (PSI) makes use of points with temporally coherent phase to achieve high accuracy measurements. However, in non-urban environments, the PS density is generally very low and distributed scatterers (DS) are dominant. Thus, it is difficult to monitor crustal deformation in a mountainous area where many active faults/volcanoes are located. To improve the spatial density of the measurement points, the use of DS, in which the phase quality should be comparable to PS, is indispensable.
Japan Geoscience Union, 2016
Coseismic deformation derived from the 2016 Kumamoto Earthquake was observed by GNSS stations of ... more Coseismic deformation derived from the 2016 Kumamoto Earthquake was observed by GNSS stations of the permanent GPS Earth Observation Network system (GEONET) and ALOS-2/PALSAR-2 interferometric SAR. Clear coseismic displacements due to the Kumamoto earthquake were observed by GEONET. NE displacement of 75 cm and subsidence of 20 cm and SW displacement of 97 cm and uplift of 28 cm were detected at sites 0465 and 0701, which are located near the Futagawa fault zone, respectively. We have also successfully detected distributed ground displacements for the Kumamoto Earthquake by applying a SAR interferometry analysis of Advanced Land Observing Satellite 2 (ALOS-2) L-band data. The interferograms suggest that fault motion of the main shock has right-lateral motion on the Futagawa fault and the Hinagu fault.
Journal of Volcanology and Geothermal Research, 2018
The linear Maxwell viscoelastic response to the inflation of a sill is examined. • Inflation-indu... more The linear Maxwell viscoelastic response to the inflation of a sill is examined. • Inflation-induced surface uplift is reduced by viscoelastic relaxation. • Subsidence occurs after magmatic inflation stops. • The subsidence in the Kutcharo caldera is explained by viscoelastic relaxation. • A crustal viscosity of ~4 × 10 17 Pa s is estimated beneath the caldera.
Zisin (Journal of the Seismological Society of Japan. 2nd ser.), 2012
Journal of Geophysical Research: Solid Earth, 2016
During the 2000 eruption of the Miyakejima volcano in Japan, step‐like tilt changes (TC) generall... more 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 ∼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 ...
Journal of the Japan society of photogrammetry and remote sensing, 2016
Earth, Planets and Space, 2017
Journal of Geophysical Research, 2010
Joint inversion study using tsunami waveforms and InSAR data provides a new way to understand the... more Joint inversion study using tsunami waveforms and InSAR data provides a new way to understand the magnitude and spatial extent of subduction zone earthquakes. A great earthquake (M w 8.5) occurred on 12 September 2007 off the west coast of Bengkulu, Indonesia. The tsunami generated by the event was recorded by tide gauge stations around the Indian Ocean and by two bottom-pressure sensors in the deep sea. The ground surface displacements produced by the earthquake on Pagai Islands and on Sumatra Island were observed by Interferometric Synthetic Aperture Radar (InSAR). We estimated the slip distribution of the earthquake by joint inversion incorporating a spatial smoothness constraint, using tsunami waveforms and InSAR data. The total released seismic moment calculated from the slip distribution is 6.7 × 10 21 N m (M w 8.5), consistent with the seismic moment of the Global CMT solution, 6.71 × 10 21 N m. The maximum observed tsunami heights along the coast of Bengkulu agree with those computed from the slip distribution. The slip amount of the 2007 earthquake is smaller than the amount of potential slip that has been accumulated since after the 1797 and 1833 events. The premise is that, averaged over long periods of time, the entire fault must slip equal amounts. Therefore the 2007 great earthquake could be followed by several great earthquakes that will rupture the plate interface until the potential slip that has been accumulated is completely released.
Earth, Planets and Space, 2017
By applying conventional cross-track InSAR and multiple-aperture InSAR (MAI) techniques with ALOS... more By applying conventional cross-track InSAR and multiple-aperture InSAR (MAI) techniques with ALOS-2 SAR data to foreshocks of the 2016 Kumamoto earthquake, ground displacement fields in range (line-of-sight) and azimuth components have been successfully mapped. The most concentrated crustal deformation with ground displacement exceeding 15 cm is located on the western side of the Hinagu fault zone. A locally distributed displacement which appears along the strike of the Futagawa fault can be identified in and around Mashiki town, suggesting that a different local fault slip also contributed toward foreshocks. Inverting InSAR, MAI, and GNSS data, distributed slip models are obtained that show almost pure right-lateral fault motion on a plane dipping west by 80° for the Hinagu fault and almost pure normal fault motion on a plane dipping south by 70° for the local fault beneath Mashiki town. The slip on the Hinagu fault reaches around the junction of the Hinagu and Futagawa faults. The slip in the north significantly extends down to around 10 km depth, while in the south the slip is concentrated near the ground surface, perhaps corresponding to the M j 6.5 and the M j 6.4 events, respectively. The focal mechanism of the distributed slip model for the Hinagu fault alone shows pure right-lateral motion, which is inconsistent with the seismically estimated mechanism that includes a significant non-double couple component. On the other hand, when taking the contribution of normal fault motion into account, the focal mechanism appears similar to that of the seismic analysis. This result may suggest that local fault motion occurred just beneath Mashiki town, simultaneously with the M j 6.5 event, thereby increasing the degree of damage to the town.
Japan Geoscience Union, Mar 14, 2019
<p>Reservoirs accumulate and evolve magma during decades to centuries under... more <p>Reservoirs accumulate and evolve magma during decades to centuries under Canary Islands volcanoes. Finally, magma migrates towards the surface before eruptions. However, little is known about the pathways and mechanisms controlling this migration. Past low eruption recurrence rate and the fact that the most recent 2011-2012 El Hierro eruption was off-shore hampered us to fully understand the magma(s) migration process. During the 2021 Cumbre Vieja eruption eruptible magmas showed remarkable mobility during the preceding 8 days before the eruption on the 13th of September 2021. This magma migration was reflected as surface ground deformation and seismicity. We used satellite radar interferometry to track 1) the geometry of the active magmatic reservoirs, and 2) the dynamics of magma emplacement and migration. To further, speculate about the reasons for that geometry and dynamics. Hence, the 2021 Cumbre Vieja eruption represents a unique opportunity to learn more about the mechanisms that facilitate magma migration beneath these volcanoes, and compare it with similar basaltic volcanoes. Our work aims to contribute knowledge that will help hazard assessment and volcanic risk reduction. </p>
Japan Geoscience Union, 2018
This session discusses various aspects of active volcanisms including, but not limited to, recent... more This session discusses various aspects of active volcanisms including, but not limited to, recent and historical eruptions, various phenomena associated with the volcanic activities, underground structures of the volcanoes, and developments of new instruments based on geophysical, geochemical, geological, and multidiscipline approaches. We also welcome studies on understanding and predicting the transitions of the eruptive activities from observational, theoretical, and experimental approaches.
Japan Geoscience Union, 2018
Japan Geoscience Union, 2016
Japan Geoscience Union, 2016
A devastating earthquake with a moment magnitude (Mw) of 7.8 (USGS) struck central Nepal on April... more A devastating earthquake with a moment magnitude (Mw) of 7.8 (USGS) struck central Nepal on April 25, 2015, with its hypocenter located in the Gorkha region. A Mw 7.3 aftershock occurred approximately 150 km east of the hypocenter of the main shock on 12 May 2015, which is the largest aftershock as of this writing. In this presentation, we report the detailed crustal deformation associated with these earthquakes obtained by InSAR analyses and the InSAR-inferred distributed slip model. We employed a new Japanese L-band synthetic aperture radar satellite launched in 2014, called Advanced Land Observing Satellite-2 (ALOS-2), to measure the ground displacement. ALOS-2 possesses a ScanSAR mode which has an ability to observe over broad area with a swath width of 350 km in one action. The ScanSAR-based InSAR is indeed suitable for mapping the spatially comprehensive and detailed crustal deformation of the 2015 Gorkha earthquake. We have successfully detected widely distributed ground disp...
Geophysical Journal International, 2020
SUMMARYAn elastic layer plays an important role in deformation of the crust. At active volcanoes,... more SUMMARYAn elastic layer plays an important role in deformation of the crust. At active volcanoes, its thickness would be effectively thinned by a higher geothermal gradient, particularly in a region beneath which magmatic activity is relatively high. This study examines the influence of elastic thickness non-uniformity on viscoelastic crustal deformation by magma emplacement. A 3-D linear Maxwell viscoelastic model is employed, in which an elastic layer underlain by a viscoelastic layer with a spatially uniform viscosity is thinned to be hi in the volcano centre, compared with hi + Δh in the peripheral regions, and a sill-like magma emplacement occurs in the upper layer beneath the centre. It is found that the post-emplacement viscoelastic subsidence is diminished or enhanced by the elastic thickness non-uniformity, depending on whether or not the horizontal width of the magma emplacement (ωs) is greater than the horizontal width (ωe) over which the elastic layer is thinner. The ava...
Earth and Planetary Science Letters, 2018
Geodetically observed volcano deformation has been analysed in terms of either elastic or viscoel... more Geodetically observed volcano deformation has been analysed in terms of either elastic or viscoelastic crustal rheology, for which the presence of magma has usually been considered as a deformation source. From a rheological viewpoint, however, a magma and the peripheral rocks it influences are presumably low viscosity materials, which allows detection of the presence of magma as a zone that has rheologically less strength. In this study, using a 3-dimensional finite element model that introduces a rectangular low viscosity zone (LVZ) into relatively high viscosity crust, the response of the linear Maxwell viscoelastic crust to the emplacement of a spheroidal sill is adopted for interferometric synthetic aperture radar (InSAR) data of the Kutcharo caldera, eastern Hokkaido, Japan, to image a spatial crustal viscosity variation. The observed temporal displacement at the surface point, where the line-of-sight (LOS) displacement is largest in the deformation field, requires the LVZ to have a spatially uniform viscosity of ∼4-5 × 10 17 Pa s. The LVZ is also required to be present immediately below a 5-km elastic layer down to mid-crustal depths or deeper, with a horizontal extent of at least 10 km. Models with acceptable minimum sizes of LVZ are found to satisfy the overall temporal displacement field with root mean square misfits of less than ∼2 cm. The geodetic data reasonably accept the presence of an LVZ beneath the Kutcharo caldera that is consistent with the magnetotelluric image of the accumulated magma, providing the mechanical aspect of the low resistivity domain. The LVZ model behaviour suggests an important implication that the accumulation of magma in the middle to lower crust is detectable using geodetic data after magma emplacement into the upper crust, where the deformation rate represents the degree of magmatic influence on the crustal viscosity.
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Papers by Tomokazu Kobayashi