[1] One of the challenging tasks in predicting near-source ground motion for future earthquakes is to anticipate the spatiotemporal evolution of the rupture process. The final size of an event but also its temporal properties (propagation velocity, slip velocity) depend on the distribution of shear stress on the fault plane. Though these incipient stresses are not known for future earthquakes, they might be sufficiently well characterized in a stochastic sense. We examine the evolution of dynamic rupture in numerical models of a fault subjected to heterogeneous stress fields with varying statistical properties. By exploring the parameter space of the stochastic stress characterization for a large number of random realizations we relate generalized properties of the resulting events to the stochastic stress parameters. The nucleation zone of the simulated earthquake ruptures in general has a complex shape, but its average size is found to be independent of the stress field parameterization and is determined only by the material parameters and the friction law. Furthermore, we observe a sharp transition in event size from small to system-wide events, governed mainly by the standard deviation of the stress field. A simplified model based on fracture mechanics is able to explain this transition. Finally, we find that the macroscopic rupture parameters (e.g., moment, moment rate, seismic energy) of our catalog of model quakes are generally consistent with observational data.Citation: Ripperger, J., J.-P. Ampuero, P. M. Mai, and D. Giardini (2007), Earthquake source characteristics from dynamic rupture with constrained stochastic fault stress,

This study investigates near-field ground-motion variability due to dynamic rupture models with heterogeneity in the initial shear stress. Ground velocity seismograms are synthesized by convolving the time histories of slip velocity obtained from spontaneous dynamic rupture models with Green's functions of the medium calculated with a discrete wavenumber/finite-element method. Peak ground velocity (PGV) estimated on the synthetics generally matches well with an empirically derived attenuation relation, whereas spectral acceleration (SA) shows only an acceptable match at periods longer than 1 sec. Using the geometric mean to average the two orthogonal components leads to a systematic bias for the synthetics, in particular at the stations closest to the fault. This bias is avoided by using measures of ground motion that are independent of the sensor orientation.The contribution from stress heterogeneity to the overall ground-motion variability is found to be strongest close to the fault and in the backward directivity region of unilaterally propagating ruptures. In general, the intraevent variability originating from the radiation pattern and the effect of directivity is on the same order or larger than the interevent variability. The interevent ground-motion variability itself is dominated by the hypocenter-station configuration and is influenced only to a lesser extent by the differences in the dynamic rupture process due to the stress heterogeneity. In our modeling approach the hypocenter location is not picked arbitrarily but is determined to be mechanically consistent with the stress heterogeneity through a procedure emulating tectonic stress loading of the fault and nucleation. Compared to the peak ground motion recorded during the 2004 Parkfield, California, earthquake our simulated seismograms show enhanced spatial correlation that may be attributed to the simplicity of the assumed crustal model or to an incomplete representation of the spatial heterogeneity of dynamic rupture parameters. Nevertheless, the intraevent PGV variability in the near-fault region determined for the Parkfield dataset is of the same order of magnitude as for our simulations.

Earthquake rupture is a notoriously complex process, at all observable scales. We introduce a simplified semi-dynamic crack model to investigate the connection between the statistical properties of stress and those of macroscopic source parameters such as rupture size, seismic moment, apparent stress drop and radiated energy. Rupture initiation is treated consistently with nucleation on a linear slipweakening fault, whereas rupture propagation and arrest are treated according to the Griffith criterion. The available stress drop is prescribed as a spatially correlated random field and is shown to potentially sustain a broad range of magnitudes. By decreasing the amplitude of the stress heterogeneities or increasing their correlation length the distribution of earthquake sizes presents a transition from GutenbergRichter to characteristic earthquake behavior. This transition is studied through a mean-field analysis. The bifurcation to characteristic earthquake behavior is sharp, reminiscent of a first-order phase transition. A lower roll-off magnitude observed in the Gutenberg-Richter regime is shown to depend on the correlation length of the available stress drop, rather than being a direct signature of the nucleation process. More generally, we highlight the possible role of the stress correlation length scale on deviations from earthquake source self-similarity. The present reduced model is a building block towards understanding the effect of structural and dynamic fault heterogeneities on the scaling of source parameters and on basic properties of seismicity.

[1] Computing the distribution of static stress changes on the fault plane of an earthquake, given the distribution of static displacements, is of great importance in earthquake dynamics. This study extends the approach developed by Andrews [1980], and compares it against existing analytical formulations. We present calculations for slip maps of past earthquakes and find that the stress-change results are accurate to about 1 -2% of the maximum absolute stress change, while the computation time is greatly reduced. Our method therefore provides a reliable and fast alternative to other methods. In particular, its speed will make computation of large suites of models feasible, thus facilitating the construction of physically consistent source characterizations for strong motion simulations.