Afterslip and viscoelastic relaxation following the 2011 Tohoku-oki earthquake (Mw9.0) inferred from inland GPS and seafloor GPS/Acoustic data

We simultaneously estimate 2.5 years of afterslip and viscoelastic relaxation, as well as coseismic slip, for the 2011 Tohoku-oki earthquake. Displacements at inland GPS and seafloor GPS/Acoustic stations are inverted using viscoelastic Green's functions for a model with an upper elastic layer and lower viscoelastic substrate. The result shows that afterslip is isolated from the rupture area and possibly asperities of historical earthquakes and has almost decayed by 10 September 2013, 2.5 years after the main shock. The inversion result also suggests that observed landward postseismic displacements at the seafloor GPS/Acoustic stations are caused by the viscoelastic relaxation, whereas trenchward displacements at inland stations are mainly an elastic response to afterslip

Numerical Verification Criteria for Coseismic and Postseismic Crustal Deformation Analysis with Large-scale High-fidelity Model

Numerical verification of postseismic crustal deformation analysis, computed using a large-scale finite element simulation, was carried out, by proposing new criteria that consider the characteristics of the target phenomenon. Specifically, pointwise displacement was used in the verification. In addition, the accuracy of the numerical solution was explicitly shown by considering the observation error of the data used for validation. The computational resource required for each analysis implies that high- performance computing techniques are necessary to obtain a verified numerical solution of crustal de- formation analysis for the Japanese Islands. Such verification in crustal deformation simulations should take on greater importance in the future, since continuous improvement in the quality and quantity of crustal deformation data is expected

Interseismic pore compaction suppresses earthquake occurrence and causes faster apparent fault loading

Physical and chemical processes operating on faults during interseismic periods are important for earthquake generation. In this study, we focus on pore compaction within fault zones driven by chemical kinetic effects. One established model of compaction causes an increase of pore fluid pressure almost linearly with time, if fluid diffusion is neglected. We introduced it into a simple numerical model for earthquake cycles and found that this effect can drastically change the recurrence of earthquakes: (1) It gradually stabilizes the fault and eventually suppresses earthquake occurrence (2) It surprisingly causes faster "apparent" fault loading. Furthermore, we developed an expression for the "apparent" loading velocity and checked that it is consistent with the results of numerical calculations. We also extended the expression for the "apparent" loading velocity to include the effect of pore dilatancy. This "apparent" loading would not only appear in a simple elastic system with a single degree of freedom, but also in complicated systems that involve simulations of realistic earthquake cycles

ニホンレットウカノサンジゲンソクドコウゾウ : シズミコムタイヘイヨウオヨビフィリピンカイプレート

京都大学0048新制・課程博士理学博士甲第2491号理博第661号新制||理||354(附属図書館)6899UT51-56-G4京都大学大学院理学研究科地球物理学専攻(主査)教授 三雲 健, 教授 岸本 兆方, 教授 三木 晴男学位規則第5条第1項該当Kyoto UniversityDA

Interseismic pore compaction suppresses earthquake occurrence and causes faster apparent fault loading

Physical and chemical processes operating on faults during interseismic periods are important for earthquake generation. In this study, we focus on pore compaction within fault zones driven by chemical kinetic effects. One established model of compaction causes an increase of pore fluid pressure almost linearly with time, if fluid diffusion is neglected. We introduced it into a simple numerical model for earthquake cycles and found that this effect can drastically change the recurrence of earthquakes: (1) It gradually stabilizes the fault and eventually suppresses earthquake occurrence (2) It surprisingly causes faster "apparent" fault loading. Furthermore, we developed an expression for the "apparent" loading velocity and checked that it is consistent with the results of numerical calculations. We also extended the expression for the "apparent" loading velocity to include the effect of pore dilatancy. This "apparent" loading would not only appear in a simple elastic system with a single degree of freedom, but also in complicated systems that involve simulations of realistic earthquake cycles

Megaquake cycle at the Tohoku subduction zone with thermal fluid pressurization near the surface

For the 2011 Tohoku earthquake, we propose a mechanical model to explain rare giant (M9-class) and frequent large (M7-class) earthquakes on a thrust fault in the subduction zone. Observations implied, in the M9 Tohoku earthquake, that extremely large slip on the order of tens of meters occurs in a shallower part to release a slip deficit, as well as substantial slip about ten meters or so in a deeper part including the source area of the M7-class earthquakes. Here, we present a model in which the extremely large slip is caused by hydrothermal weakening (dynamic thermal pressurization of pore fluid) on the fault plane, not by contrast of frictional properties in terms of rate- and state-dependent friction. The model explains that the Tohoku earthquake followed a M7-class earthquake in two days, but M7-class earthquakes are not always followed by a giant earthquake. In a giant event, large coseismic slip can occur over an area where quasistatic slip, namely, afterslip of M7-class earthquakes or spontaneous slow slip events, takes place. Slight differences of stress state in the shallow part can result in drastically different coseismic slips. We further perform numerical experiments varying hydraulic parameters and the length of effective hydrothermal weakening area. The experiments imply that observations for monitoring the effective hydrothermal weakening area need spatial resolution on the order of 10 km or finer

Source mechanisms of subcrustal and upper mantle earthquakes around the northeastern Kyushu region, southwestern Japan, and their tectonic implications

The source mechanisms of subcrustal and upper mantle earthquakes with magnitudes from 6.0 to 6.8 that occurred around the northeastern Kyushu region have been closely investigated to clarify its tectonic features in relation to the subducting Philippine Sea plate with a laterally bending configuration. Two subcrustal earthquakes that occurred in Suonada and the Bungo channel, which are located close to the leading edge of the subducting Philippine Sea plate, show the mechanism of normal faulting type. These events may have been generated by lateral bending of the oceanic plate. The Kunisaki peninsula earthquake of August 26, 1983 (M=6.8, h=116 km), which was the largest upper-mantle earthquake in this region, shows a reverse fault type mechanism. Comparisons between the observed and synthetic seismograms suggest that the southeastward dipping nodal plane may be the fault plane, and that the rupture propagated northwest-upwards. The seismic moment was estimated to be MO=-1.13×1026 dyn·cm. The focal mechanism of three upper mantle earthquakes inland of Kyushu, including the Kunisaki peninsula earthquake, suggests that they may have been caused under the stress regime of down-dip extension. The tensional stress working southwest-downwards at intermediate depths in the Suonada and inland Kyushu regions may be explained by the gravitational pull acting on the Philippine Sea plate subducting deeper southwestwards