Name : YAMASHITA,  Teruo
            Position : Professor
            Division/Center : Division of Monitoring and Computational Geoscience
            Research Area : Seismology
 

Research:

       My current research activity is roughly classified into the following two categories: (1) Geometrical complexity of earthquake faults
       and its effect on earthquake rupture growth, and (2) Earthquake occurrence due to fluid migration in a fault zone.

       Earthquake rupture complexity has most often been ascribed to heterogeneous distribution of strength, stress drop on a planar fault.
       Evidently, the source of rupture complexity must be much more complicated, possibly involving fault bends and bifurcations, transfer of
       slip onto a different fault and so on. Our aim in the first category is to understand the effects of non-planar fault geometry on the
       dynamics of earthquake faulting. We have derived a set of rigorous boundary integral equations for the analysis of arbitrarily shaped
       2D cracks. Using this formulation, we analyzed dynamic growth of a hackly crack. Our investigation has been extended to the analysis
       of a spontaneous growth of in-plane shear crack with no constraints on crack geometry.

       It is now widely believed that fluids exert significant mechanical effects on earthquake faulting. As a typical effect, we can mention the
       reduction of the effective confining stress by zones of high pore fluid pressure, which facilitates frictional slip at low fault shear stress.
       It is well known that hydrological properties of fault zones change with fault slip. In the second category of my research activity, we
       carry out numerical simulations of earthquake occurrence due to fluid migration taking account of such hydrological changes of fault
       zones. We found that rupture activity shows a high complexity due to the interactions between fluid flow and fault slip when the
       fracture strength is inhomogeneously distributed on the fault. However, a statistical regularity is also observed in the frequency
       distribution of magnitudes, which agrees well with the GR relation;the b value ranges from 1.0 to 2.0. It is also found that rupture
       sequences with features of earthquake swarms can be simulated when the rate of pore creation is relatively large. Such sequences
       generally start and end gradually with no single event dominating in the sequence. In addition, the b values are shown to be unusually
       large.

Publications:

Yamashita, T., Pore creation due to fault slip in a fluid-permeated fault zone and its effect on seismicity, Pure Appl. Geophys., 155, 1, 625--647, 1999.

Kame, N. and T. Yamashita, A new light on arresting mechanism of dynamic earthquake faulting, Geophys. Res. Lett., 26, 13, 1997--2000, 1999.

Kame, N. and T. Yamashita, Simulation of spontaneous growth of dynamic crack without constraints on the crack tip path, Geophys. J. Int., 139, 2, 345--358, 1999.

Teisseyre, R. and T. Yamashita, Splitting stress motion equation into seismic wave and fault-related fields, 地震2, 47, 2, 135--148, 1999.

Teisseyre, R. and T. Yamashita, Stress and dislocation field evolution and prediction problems of numerically simulated events, Acta Geophys. Pol., 48, 1, 43--56, 2000.

Yamashita, T., Generation of microcracks by dynamic shear rupture and its effects on the rupture growth and elastic wave radiation, Geophys. J. Int., 143, 2, 395--406, 2000.