Shallow Dip-Slip Fault Rupture: Possible Origin of Asymmetric Seismic Motion

Abstract:

As observed again on the occasion of the November 2014 Nagano-ken Hokubu (Kamishiro Fault), Japan, earthquake, a shallow dip-slip earthquake usually gives asymmetric ground motion in the vicinity of the rupturing fault plane. In the Nagano event, although the dip angle of the associated fault plane is basically relatively large (40-70 degrees) and the plane is nearly vertical near the hypocenter, the peak ground acceleration (PGA) recorded on the hanging wall is some 2.5 times as large as that on the footwall at almost the same epicentral distance. However, the mechanical origin of this asymmetry has not been completely understood so far. Therefore, in this contribution, we investigate the rupture mechanics of a model dip-slip fault plane situated in a two-dimensional, monolithic (scenario I) or stratified (scenario II) linear elastic medium. In the numerical simulations, we employ our finite difference simulator developed on a PC basis, and in the laboratory fracture experiments, we utilize the technique of dynamic photoelasticity in conjunction with a high speed digital video camera system. Both numerically and experimentally, we observe the time-dependent dynamic wave field produced by the crack-like primary rupture along a flat fault plane, and notice that in the scenario I, as numerically predicted by Uenishi and Madariaga (Eos, 2005), geometrical asymmetry is required to have asymmetric seismic motion due to the propagation of the "corner waves." The corner waves are generated by the dynamic interaction of Rayleigh-type waves that are produced by the primary fault rupture and the wave reflection at the free surface. In the scenario II, contrarily, asymmetric geometry is not needed: The (anti-)symmetry of mode-II (in-plane shear) motion in a geometrically symmetric model can be simply broken as soon as secondary fracture occurs at an interface between layers, and like in the 2014 earthquake in Nagano, rupture of a fault plane dipping at a relatively large angle can really produce larger dynamic stresses in the hanging wall than in the footwall.