Seismic source spectral properties of crack-like and pulse-like modes of dynamic rupture†

Earthquake source properties such as seismic moment and stress drop are routinely estimated from far-field body wave amplitude spectra. Some quantitative but model-dependent relations have been established between seismic spectra and source parameters. However, large variability is seen in the parameter estimates, and it is uncertain how the variability is partitioned among real variability in the source parameters, observational error, and modeling error due to complexity of earthquake behaviors. Earthquake models with dynamic weakening have been found to exhibit two different modes of rupture: expanding-crack and self-healing pulse modes. Four representative models are generated to model the transition from crack-like to pulse-like. 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 sigma(E) exceeds the moment-based stress drop sigma(M) 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 variations in rupture mode introduce variability of the order of a factor of 2 in standard (i.e., crack model-based) spectral estimates of stress drop. The transition from arresting- to growing-pulse rupture is accompanied by a large (factor of approximate to 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.