Fracture Dynamics of Shallow Dip-Slip Earthquakes

While the fracture (rupture) dynamics of strike-slip earthquakes has been clarified to a practically acceptable level, the mechanical characteristics of shallow dip-slip seismic events remain unexplained owing to the shortage of the near-field seismological observations and the analytical difficulties at the rupture tip of an interface (fault plane) in the proximity of a free surface. In this contribution, utilizing the techniques of finite difference modeling and dynamic photoelasticity, the fracture dynamics of a dip-slip fault plane located near a free surface is studied numerically as well as experimentally. Each two-dimensional fracture model may contain a flat fault plane (initially welded interface) that dips either vertically or at an angle (e.g. 45 degrees) in a monolithic linear elastic medium (representing rocks). The time-dependent development of wave field associated with the crack-like rupture along every fault plane is recorded. Both numerical and experimental observations indicate when the fault rupture that is initiated at some depth approaches the free surface, four Rayleigh-type waves are produced. Two of them move along the free surface as Rayleigh surface waves into the opposite directions (in the hanging wall or footwall) to the far field, and the other two propagate back downwards along the fractured interface into depth. These downward interface waves may considerably govern the stopping phase of the dynamic fracture, and according to the seismological recordings, they seem to have existed during the rupture process of the 2011 off the Pacific coast of Tohoku, Japan, earthquake. In the case of an inclined fault plane, the interface and Rayleigh waves interact with each other and a shear wave possessing concentrated energy (corner wave) is generated and causes stronger disturbances in the hanging wall. The existence of the downward interface and corner waves, first numerically predicted in 2005 by Uenishi and Madariaga, seems to have been confirmed by this series of laboratory fracture experiments.