13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Dynamic Fracture Associated with Shallow Dip-Slip Seismic Faulting Koji Uenishi1,*, Keisho Yamagami1, Fukutaro Ishida1, Koji Fujimoto1 1 Department of Aeronautics and Astronautics, The University of Tokyo, Tokyo 113-8656 Japan * Corresponding author: uenishi@mat.t.u-tokyo.ac.jp Abstract Unlike strike-slip earthquakes, the physical properties of shallow dip-slip quakes remain unexplored because of the insufficient number of near-field seismological recordings and analytical difficulties at the tip of a surface-breaking fault. Here, based on the finite difference technique and dynamic photoelasticity in conjunction with high speed digital cinematography, we numerically and experimentally simulate the source dynamics of dip-slip faulting. Our two-dimensional model may contain a flat fault plane (interface) dipping either vertically or at some angle in a monolithic linear elastic medium. We record the evolution of wave field related to the crack-like rupture along this fault. The observations suggest if the fault rupture, initiated at some depth, arrests just below or reaches the free surface, four Rayleigh-type waves are generated: two propagating along the free surface into the opposite directions to the far field, the other two moving back along the fractured interface downwards into depth. These downward interface waves may largely control the stopping phase of the dynamic rupture. In the case of an inclined fault plane, the interface and Rayleigh waves interact with each other and a shear wave carrying concentrated kinematic energy (corner wave) is induced to generate enormously strong particle motions in the hanging wall. Keywords Earthquake dynamics, Earthquake ground motions, Fracture dynamics, Experimental mechanics, Computational seismology 1. Introduction After the reasonable agreement between the theoretical and observational near-field seismograms related to the 1966 strike-slip Parkfield, California, earthquake [1], the techniques to evaluate the near-field generated by strike-slip faulting have become remarkably refined. At present it is common to invert seismological recordings for estimating fault slip distribution, rupture history and their effects for large, shallow strike-slip earthquakes [1−4]. However, the situation is different for shallow dip-slip earthquakes since only few seismic events of this type have been well recorded in the near-field and the mechanical characteristics have not been fully clarified owing to analytical difficulties at the tip of a surface-breaking fault [4−15]. Hazard risk due to such dip-slip earthquakes may be higher in the tectonically compressive regions like Los Angeles, Japan and Taiwan, Central and South America, and in extensional regimes such as the Mediterranean and the Great Basin of Nevada, Utah, and Idaho [5], and an extensive effort has been made to model dip-slip events and the strong motion (particle motion) generated by them [4−15]. Earlier study of shallow dip-slip faulting is based on kinematical models, using the Cagniard-de Hoop [12] or a numerical spectral method [13]. But the analysis becomes very sophisticated and often it does not work correctly, because of analytical singularities, when the fault rupture reaches the free surface. Nevertheless, the previous works posed questions regarding the effect of the free surface near the shallow dipping fault, which require careful study [4]. One noteworthy observation in shallow dip-slip earthquakes is the asymmetric ground motion in the vicinity of the fault: Generally, strong motion is much larger in the hanging wall than in the footwall. For example, the 1971 San Fernando and the 1994 Northridge earthquakes caused systematically severer damage or larger ground motion on the hanging wall [5]. The more recent earthquakes, the 1999 Chi-Chi in Taiwan [6, 7] and the 2004 Niigata-ken Chuetsu [16] and 2008 Iwate-Miyagi Inland [17] in Japan, seem to support this viewpoint. This observed effect is considered to be caused by the strong disturbance in the proximity of the propagating rupture front (rupture front wave) [4], the trapped wave in the hanging wall [5], or the asymmetric mass distributions on each wall and the
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