UC San Diego

Growth of major populated cities near active faults (e.g., Los Angeles and San Francisco in USA, Tokyo and Osaka in Japan) has significantly elevated the seismic hazards. Understanding complex paradigm of near-fault ground motions is crucial in order to mitigate seismic hazards. Since the 1994 Mw 6.7 Northridge earthquake, there has been much discussion about the adequacy of building code and a term of “pulse”. The engineering effects of near-fault pulse-like ground motions were strikingly exhibited in the 1994 Northridge earthquake in which great seismic damage was attributed to the large impulsive ground shaking of this type. Such near-fault pulselike ground motions with high intensity and damage potentials are hypothetically associated to either pulse-like rupture on fault or the rupture directivity. These mechanisms will be introduced and studied.

In Chapter 2, we study far-field effects of a self-healing pulse-like rupture mode with dynamic weakening. Pulse-like rupture leads to development of a second corner frequency, and the intermediate spectral slope is approximately 2 in most cases. The focal-sphere-averaged lower P and S wave corner frequencies are systematically higher for pulse-like models than crack models of comparable rupture velocity. The slip-weighted stress drop exceeds the moment-based stress drop for pulse-like ruptures, with the ratio ranging from about 1.3 to 1.65, while they are equal for the crack-like case. The transition from arresting- to growing-pulse rupture is accompanied by a large (factor of 1.6) increase in the radiation ratio. Thus, variations in rupture mode may account for the portion of the scatter in observational spectral estimates of

source parameters.

In Chapter 3, we confirm the pulse-like ground motion in the 2015 Nepal Gorkha earthquake is related to the causing fault geometry of the Main Himalayan Thrust (MHT). Our dynamic rupture simulations in an elastoplastic medium yield earthquake parameters comparable to those deduced from kinematic inversions, including seismic moment and rupture velocity. The simulations reproduce pulse-like behavior predicting pulse widths in agreement with those kinematic studies and supporting an interpretation in which the pulse-like time dependence of slip is principally controlled by rupture geometry and it is supported by near-field high-rate GPS recording at station KKN4.

In Chapter 4, we will discuss the directivity-induced pulse-like ground motions and assess the extent to which plastic yielding, which is absent in standard kinematic models, may systematically affect the amplitude, frequency content, and distance scaling of directivity pulse. We perform some simple 2D kinematic and 3D spontaneous dynamic ruptures with and without plastic yielding on a planar and rough fault, and find that each of the four 3D models (flat and rough faults, with and without off-fault yielding), scaled to approximately magnitude 7, predicts a fault-normal pulse with characteristic behavior of observed pulses. Plastic yielding systematically reduces pulse amplitude and increases its dominant period, relative to models that neglect off-fault yielding. Yielding saturates near-fault peak ground velocity (PGV) with greater stress drops, alternatively interpreting observed magnitude saturation of PGV near a magnitude of 7, and provides physical supports of period-dependent distance taper and along-strike saturation of directivity-induced amplification, undermining the commonly used wedge-shaped directivity amplification.