Papers by Youichiro Takada
Japan Geoscience Union, 2017
The 2011 Tohoku-Oki earthquake (Mw9.0) provides us the first opportunity to examine the responses... more The 2011 Tohoku-Oki earthquake (Mw9.0) provides us the first opportunity to examine the responses of strain accumulation zones and active faults to megathrust earthquakes with dense permanent GPS network. In this presentation, we report the differences and/or similarities between pre, co, and post seismic crustal deformation of the Tohoku-Oki earthquake using GPS data in and around the Atotsugawa fault, located at the central part of Niigata-Kobe Tectonic Zone (NKTZ, Sagiya et al., 2000). We used daily coordinates obtained from the GPS stations operated by university group in addition to GEONET by Geospatial Information Authority (GSI). For the pre-seismic period, we estimated the velocity field by removing annual and semi-annual components from the daily coordinates. For the post-seismic deformation, we extracted the period from 25 November 2014 to 2 July 2016, and estimated the velocity field in the same manner. For the co-seismic displacement, we calculated the average coordinate...
Japan Geoscience Union, 2015
I introduce recent two extensive surveys conducted in and around the Atotsugawa fault system. Fir... more I introduce recent two extensive surveys conducted in and around the Atotsugawa fault system. First is the joint seismic observation by university group during 2004 to 2008. With this seismic data, I have estimated the focal mechanisms of small earthquakes and tectonic stress field by using stress inversion methods. Second is to estimate the inter-seismic crustal deformation with very high spatial resolution using GNSS and InSAR time series analysis. The estimated stress rotation can be explained by a viscoelastic dislocation model assuming cumulative slip deficit relative to surrounding part up to several tens of meters. On the other hand, the geodetic data indicate strain concentration near the fault trace, which may require a minor change of the fault model. However, the velocity fields still include systematic error coming from atmospheric and/or tropospheric disturbances. Farther noise reduction is required to constrain the physical model.
Japan Geoscience Union, 2014
Various types of seismic studies, including EEW systems, require automated programs for the accur... more Various types of seismic studies, including EEW systems, require automated programs for the accurate picking of P and S wave arrival times. The study on automatic picking was started in the late 1970 decade; however, even now, the accuracy is low and not suitable for detailed studies. We developed a new method of automatic picking, which is similar with the software used in computer chess games. The method defines an initial model of evaluation equation, which can select actual P and S wave arrival times among candidates by using values that show the characteristics of waveforms in time periods between candidates and about 100 unknown coefficients. By using a large number of waveform data together with manually picked P and S wave data, the unknown coefficients are determined such that the square of arrival time differences between manually picking and by the evaluation equation is minimized. It takes only 0.1 sec in the calculation of the evaluation equation for 10,000 events; very...
Japan Geoscience Union, 2018
Synthetic aperture radar Interferometry (InSAR) is a versatile tool for the crustal deformation m... more Synthetic aperture radar Interferometry (InSAR) is a versatile tool for the crustal deformation monitoring with all-weather capability without any ground-based instrumentation in the target area. For a heavily vegetated region L-band SAR is advantageous to get good coherence. As a matter of fact, the L band SAR sensors installed in JERS, ALOS, and ALOS2, which are series of Japanese satellites operating on L band, have been providing many important results for both scientific and disaster mitigation purposes. Space-borne InSAR is thus already a standard method for volcanic crustal deformation monitoring. On the other hand, airborne InSAR still remains in a premature stage because of the difficulty of achieving effective method to mitigate problems caused by imperfect repeatability of the aircraft flight trajectory. To demonstrate the capability of L-band airborne SAR interferometery for volcanic crustal deformation monitoring, we are in the process of developing a standardized proce...
Journal of Geography (Chigaku Zasshi), 2020
Journal of Geography (Chigaku Zasshi), 2020
Journal of Geography (Chigaku Zasshi), 2019
Journal of Geography (Chigaku Zasshi), 2019
Earth, Planets and Space, 2017
A right-lateral shear zone in the San-in region, southwest Japan, has been proposed by previous g... more A right-lateral shear zone in the San-in region, southwest Japan, has been proposed by previous geological and seismological studies. It locates 350 km north of the Nankai Trough, that is, the main plate boundary between the subducting Philippine Sea and overriding Amurian plates and presumably accommodates a part of the relative plate motion. We present a geodetic evidence of the proposed shear zone using GNSS velocity data. Distinct shear deformation is identified only between ~132.5°E and ~135°E along a coastline which is a part of the proposed shear zone, and we propose to call the geodetically identified shear zone as the San-in shear zone (SSZ). The SSZ is a concentrated deformation zone with a width of ~50 km and can be modeled by a deep creep on a vertical strike slip fault with a creep rate of ~5 mm/year. There are some active faults parallel and oblique to the overall trend of the SSZ, but no single active fault coincides with the SSZ. Lineaments of microseismicity and source faults of large earthquakes are almost oriented in NNW-SSE in the SSZ and oblique to the overall trend of the SSZ. They are interpreted as conjugate Riedel shears. Based on these geodetic, seismological, and geomorphological observations, we suggest that the SSZ is a developing and young shear zone in a geological time scale.
Tectonophysics, 2016
Abstract We estimated 275 focal mechanisms from P-wave first-motion polarities of small earthquak... more Abstract We estimated 275 focal mechanisms from P-wave first-motion polarities of small earthquakes obtained in an extensive seismic survey during 2004–2008 in and around the Atotsugawa fault, central Japan, where ongoing dextral shear strain concentration has been observed. Along the fault trace, the azimuth direction of P-axes is oriented WNW–ESE, which agrees well with previous studies. The regional stress disturbance is detected by stress inversion analysis. The azimuth of the maximum principal stress axis systematically rotates counterclockwise as the distance from the fault trace decreases. The regional stress disturbance is explained by a cumulative slip deficit in the shallower portion of the Atotsugawa fault relative to the surrounding fault surface (i.e., the eastern, western, and deeper extensions of the fault plane).
Himalayan Journal of Sciences, 2008
Journal of Disaster Research, 2014
Studies using spaceborne interferometric synthetic aperture radar (InSAR) analysis showed that tw... more Studies using spaceborne interferometric synthetic aperture radar (InSAR) analysis showed that two megathrust earthquakes – the 2011Mw9.0 Tohoku-oki earthquake in Japan and the 2010Mw8.8Maule earthquake in Chile – triggered unprecedented subsidence in multiple volcanoes. There are strong similarities in the characteristics of the surface deformation in Japan and Chile: (1) Maximum subsidence is about 15 cm. (2) Areas of subsidence are elliptically elongated in a north-south direction perpendicular to the principal axis of the extensional stress change. (3) Most of this subsidence is coseismic. These similarities imply that volcanic subsidence triggered by the megathrust earthquakes is a ubiquitous phenomenon. Nonetheless, the mechanism of subsidence is yet to be investigated. Two main hypotheses have been proposed thus far: 1) The localized deformation of hot and weak plutonic bodies. 2) Water release from large hydrothermal reservoirs beneath the volcanoes.
電子情報通信学会技術研究報告 Sane 宇宙 航行エレクトロニクス, Oct 3, 2012
Tectonophysics, 2004
The present-day convergence rate between the Indian and the Eurasian plates has been estimated to... more The present-day convergence rate between the Indian and the Eurasian plates has been estimated to be about 50 mm/year. At the collision boundary extending along the Himalayas, about 40% of the total convergence is consumed by the subduction of the Indian plate beneath the Eurasian plate. The rest of about 60% is consumed by the internal deformation of the Eurasian plate. The present crustal movement in this region is characterized by rapid uplift along the high Himalayas and large-scale horizontal deformation in and around Tibet. The fundamental causes of these two different types of crustal movement are the same: interaction between the Indian and the Eurasian plates. In this study we represent the plate interaction by steady increase in tangential displacement discontinuity (dislocation) across the interface that divides a surface layer overlying a viscoelastic half-space into the Indian and the Eurasian plates. First, given a steady slip of 20 mm/year at the plate interface with a ramp-shaped undulation below the high Himalayas, we computed the profile of surface uplift rates along a line perpendicular to the Himalayan arc. The result accords with observed free-air gravity anomalies and the intermediate-and short-term uplift rates estimated, respectively, from the present heights of river terraces and levelling data. This means that the rapid uplift of the high Himalayas is due to the steady slip along the ramp-shaped plate interface below it. Second, given the steady slips of 20 mm/year along the 2000-km-long collision boundary and 50 mm/year along the remaining portions of the India-Eurasia plate boundary, we computed the increase rates of horizontal deformation in and around Tibet. The result accords with the observed strikes and slip rates of major Quaternary active faults. This means that the horizontal deformation in and around Tibet is due to the slip deficits of 30 mm/year at the collision boundary. From these two results we can conclude that the present rapid uplift of the high Himalayas and large-scale horizontal deformation in and around Tibet are consistently explained by a single plate interaction model based on elastic and viscoelastic dislocation theory. This study sheds new light on the driving mechanism of the crustal deformation in the India-Eurasia collision zone as follows. The plate interaction on single plate interface consists of coexisting two different physical mechanisms: (1) slip along the ramp undulation (spatial changes in slip direction), (2) slip deficit at the collision boundary (spatial changes in slip
Journal of Asian Earth Sciences, 2007
Crustal deformation due to fault slip depends strongly on fault geometry, and fault geometry is c... more Crustal deformation due to fault slip depends strongly on fault geometry, and fault geometry is changed by the deformation of the crust. This feedback mechanism causes the geometrical evolution of the fault system. We have studied the progress of the geometrical evolution of a plate interface-branch fault system through numerical simulation, based on elastic-viscoelastic dislocation theory. If the plate interface is smooth, no significant change occurs in fault geometry. If the plate interface has a ramp, we observe the gradual horizontal motion of the ramp toward the hanging-wall side of the interface at half the plate convergence rate. The offset of the ramp decreases with time. The dip-angle of thrust faults branching from the plate interface increases more rapidly as the dip of the fault increases. We have applied these results to the plate interface-branch fault system at the India-Eurasia collision boundary and obtained a scenario for the tectonic development of the Himalayas for the last 30 Myr.
Geophysical Journal International, 2008
In the framework of elasticity theory any indigenous source can be represented by a moment tensor... more In the framework of elasticity theory any indigenous source can be represented by a moment tensor. We have succeeded in obtaining general expressions for internal deformation due to a moment tensor in an elastic/viscoelastic multilayered half-space under gravity. First, starting from Stokes' classical solution, we obtained the expressions for static displacement fields due to a moment tensor in an infinite elastic medium. Then, performing the Hankel transformation of the static solution in Cartesian coordinates, we derived static displacement potentials for a moment tensor in cylindrical coordinates. Second, representing internal deformation fields by the superposition of a particular solution calculated from the displacement potentials and the general solution for an elastic multilayered half-space without sources, and using the generalized propagator matrix method, we obtained exact expressions for internal elastic deformation fields due to a moment tensor. Finally, applying the correspondence principle of linear viscoelasticity to the elastic solution, we obtained general expressions for quasi-static internal deformation fields due to a moment tensor in an elastic/viscoelastic multilayered half-space. The moment tensor can be generally decomposed into the three independent force systems corresponding to isotropic expansion, crack opening and shear faulting, and so the general expressions include internal deformation fields for these force systems as special cases. As numerical examples we computed the quasi-static internal displacement fields associated with dyke intrusion, episodic segmental ridge opening and steady plate divergence in an elastic-viscoelastic two-layered half-space. We also demonstrated the usefulness of the source representation with moment tensor through the numerical simulation of deformation cycles associated with the periodic occurrence of interplate earthquakes in a ridge-transform fault system.
Bulletin of the Seismological Society of America, 2010
Bulletin of the Seismological Society of America, 2013
After the 11 March 2011 M w 9.0 Tohoku earthquake, Japan, a swarm of shallow normal-faulting eart... more After the 11 March 2011 M w 9.0 Tohoku earthquake, Japan, a swarm of shallow normal-faulting earthquakes was triggered in a localized region beneath the border between Fukushima and Ibaraki prefectures in eastern Honshu. Here we examine the coseismic displacement field and a fault model of the largest event of the swarm, the 11 April 2011 M w 6.6 Iwaki earthquake. Radar interferometry applied to data of the Japanese ALOS satellite reveals complex ruptures associated with the Iwaki earthquake. In particular, the interferogram clearly shows multiple surface ruptures along each of the subparallel Yunodake and Itozawa faults. The slip distributions on these faults, as estimated by inverting the interferogram, suggest that the faults dip 60°-70°westward and that slip on the Itozawa fault extended to a splay fault at depths shallower than about 5 km. Our results indicate that the highland area west of the Itozawa fault was downthrown by up to 2.4 m during the Iwaki earthquake.
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Papers by Youichiro Takada