Seismic Source Spectral Properties of Crack-like and Pulse-like Modes of Dynamic Rupture

Abstract:

Earthquake source properties such as seismic moment and rupture extent are routinely estimated from far-field body-wave amplitude spectra. Low-frequency spectral level, corner frequency, and the high-frequency spectral slope are often measured and combined to make inferences about physical parameters such as stress drop and radiated energy. Based on theoretical models (e.g., Brune, 1970), some quantitative, but model-dependent, relations have been established between far-field spectra and such physical parameters. Numerical simulations of dynamic rupture, of either artificial fixed rupture speed (Madariaga,1976, Kaneko and Shearer, 2014) or physically spontaneous models, have extended the scope of available models.

Rupture processes based upon rate and state frictional laws with dynamic weakening have been found from simulations to exhibit two different modes: expanding crack and self-healing rupture, which result in distinctly different shapes for the slip velocity function. In the expanding crack mode, a fault keeps slipping everywhere within the rupture zone until arrested by a strong stopping phase coming from the unbroken edges of the fault. Strong high-frequency energy can be radiated from such a stopping phase. However in the self-healing mode, rupture occurs as a slip pulse controlled by the local frictional stress rather than by rupture cessation signals.

Models with spontaneous propagation and stopping of rupture, both in expanding crack or self-healing modes, may improve our understanding of the relationship between spectral parameters and the physical parameters of rupture. We use a simplified model of rupture propagating and stopping spontaneously, in expanding crack and self-healing modes, to investigate the relationship of spectral slope and corner frequency to rupture properties. We find that the self-healing rupture mode may induce double corner frequency spectral shapes and increase the spherically averaged corner frequency.