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Japan Marine Science and Technology Center Institute for Frontier Research on Earth Evolution (IFREE) evolution after shear failure. The permeability of the basalt is estimated to range from - to - m under the environmental conditions corresponding to the seismogenic zone. The permeability showed a strong reduction with increasing effective confining pressure and temperature. Following shear failure of the basalt, rapid sealing at elevated temperatures was observed during hold experiments: a three orders of magnitude decrease in permeability after about hours holding. This result indicates that the permeability of the subduction megathrust fault would rapidly reduce due to the precipitation of clay-like minerals and other minerals, and shows the potential of high fluid pressure in the fault zone. Further, the maximum slip-weakening rate of the basalt during the shear failure process has nearly the same value as that of granite in the brittle regime, which suggests the possibility that unstable slip occurred along the fault. () Shallow portion of a splay fault The Chi-Chi, Taiwan, earthquake (Mw.) Shear Resistance (MPa) 0.15 0.1 0.05 Normal Stress: 0.15 MPa Peak Slip Velocity: 2 m/s Porosity = 47 % 0.15 0.05 0 0 0 1 2 3 4 5 6 7 Time (sec) Shear Resistance Pore Pressure Fig.23 Result of high-velocity ring shear experiment on simulated fault gouge showing fluidization during slipping. 0.1 Pore Pressure (MPa) produced a spectacular surface rupture along the east dipping Chelungpu thrust fault and provided new important near-field strong motion data. The analysis of the possible rupture zone and a high-velocity ring shear experiment using simulated material were performed to clarify what dynamic processes in the shallow portion of the splay fault control the large slip and slip velocity with a low level of high-frequency seismic radiation. The results clearly indicate that fluidization occurred during co-seismic slip (Fig.). When the undrained condition was maintained during the earthquake rupture process, the fault composed of loosely packed, granular material can lose frictional resistance due to fluidization and enhance rupture propagation even in a stable frictional slip regime. () Deep portion of subduction zone In order to understand fluid flow processes at the transition between the seismogenic zone and the creep zone, sealed cracks developed in the past plate boundary rocks of the Shimanto accretionary prism and Sambagawa metamorphic rocks were studied in detail. Three types of sealed cracks were classified for the Sambagawa metamorphic rocks in terms of geometry, distribution patterns, and spacio-temporal relationships between the host rocks. Mineral composition and microstructures suggests that each type corresponds with the tectonic stages of subduction, underplating and exhumation. Regional differences that are probably due to the opening interval and frequency or fluid flux were inferred from the relationship between the length and width of the same sealed crack types in the Kanto mountain and Central Shikoku. Sealed cracks in the Shimanto accretionary prism revealed that two opening directions, trench-parallel and trench-vertical, are common for the plate boundary cracks. Stress fields suggested by the structural relationships show vertical maximum stress and a horizontal conversion between minimum and intermediate stress. A precise inverse method was used to evaluate the temperature-pressure-fluid flux path. Results show that P-T conditions during subduction are nearly equal to that of exhumation, and that each path is composed of substatic a low-P segment and a rapid high-P segment. High fluid flux and deformation correspond with the latter segment. 103
JAMSTEC 2002 Annual Report Institute for Frontier Research on Earth Evolution (IFREE) 4. Plate Dynamics Modeling Research Group 4.1. Outline The objective of our group is to develop models for subduction zone structure, deformation, failure, and flow mechanisms. Numerical modeling is done with crustal structure data obtained from the structure research group. Results from rock experiments and field surveys of the material property research group also aid in numerical fault zone simulations. The Earth Simulator (ES) supercomputer provides the computational power for large scale -D numerical models to obtain an integrated model of subduction dynamics. Several simulation codes for the ES are being developed. 4.2. Results () Rupture Characteristics of Plate Boundary Earthquake More details of the source process of the Tonankai earthquake were estimated using tsunami wave analysis. To obtain more reliable and finer slip distribution, especially near the trough axis, the plate geometry and the tsunami initial model were improved. The distribution of slip is similar the distribution of splay faults found east of the Kii peninsula by reflection surveys (Fig.). This may support the possibility of slip on such splay faults. 36N () Thermal Modeling of a Subduction Zone To investigate the relationship between the source process of the Nemuro-oki earthquake and the crustal structure there, a thermal model was constructed based on the results of a seismic structure survey crossing the source region. The obtained thermal structure indicates that the locked area of the plate boundary includes the source region and goes deeper. The relocated hypocenters of small earthquakes around the source region concentrate in the deeper part of the locked area. () Estimation of Stress Field using Shear-wave Splitting Method To estimate the stress field in the subduction zone, shear-wave splitting analysis was applied to the seismic wave data observed using ocean bottom seismometers off Cape Muroto. The results revealed for a b Vmax c d Hx Landward flank τ = 0 Hy>Hx (regional E-W compression) OBS98 shear stress distribution V+ τ V Hx small τ Hy>Hx 0 (and local recovery of the regional stress) 25 V+ τ MPa Hx>Hy large τ 50 Seaward flank Hy V x x y z y OBS1 Kii peninsula Splay faults Omaezaki 0 1 2 3 4 5 slip (m) 35N 34N KTG OBS98 S02 S04 S06 S09 OBS1 134˚ 00' 135˚ 00' 136E 137E 138E 139E 34˚ 00' 33˚ 00' 32˚ 00' Fig.24 Slip distribution of the 1944 Tonankai earthquake: The rectangles represent subfaults on the fault plane, and the slip amounts are shown in the red scale. The triangles shows tide gauges. The up-dip limit of the rupture area is consistent with the upper edge of the splay faults which had been detected by seismic surveys. Fig.25 Direction of maximum principal stress axes estimated from S-wave splitting analysis using OBS data and an interpretation: (a) stress field caused by seamount subduction, (b) and (c) expected stress change around a subducted seamount, (d) estimated maximum principal stress axes. Large and small circles show the positions of subducted seamounts. 104
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