[1] We perform systematic simulations of slip using a quasi-dynamic continuum model of a two-dimensional (2-D) strike-slip fault governed by rate-and state-dependent friction. The depth dependence of the a À b and L frictional parameters are treated in an innovative way that is consistent with available laboratory data and multidisciplinary field observations. Various realizations of heterogeneous L distributions are used to study effects of structural variations of fault zones on spatiotemporal evolution of slip. We demonstrate that such realizations can produce within the continuum class of models realistic features of seismicity and slip distributions on a fault. We explore effects of three types of variable L distributions: (1) a depth-dependent L profile accounting for the variable width of fault zones with depth, (2) uncorrelated 2-D random distributions of L with different degrees of heterogeneity, and (3) a hybrid distribution combining the depth-dependent L profile with the 2-D random L distributions. The first type of L distribution, with relatively small L over the depth range corresponding to the seismogenic zone and larger L elsewhere, generates stick-slip events in the seismogenic zone and ongoing creep above and below that region. The 2-D heterogeneous parameterizations generate frequency-size statistics with event sizes spanning 4 orders of magnitude. Our results indicate that different degrees of heterogeneity of L distributions control (1) the number of simulated events and (2) the overall stress level and fluctuations. Other observable trends are (3) the dependency of hypocenter location on L and (4) different nucleation phases for small and large events in heterogeneous distributions.Citation: Hillers, G., Y. Ben-Zion, and P. M. Mai (2006), Seismicity on a fault controlled by rate-and state-dependent friction with spatial variations of the critical slip distance,

[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] We quantify the correlation between spatial patterns of aftershock hypocenter locations and the distribution of coseismic slip and stress drop on a main shock fault plane using two nonstandard statistical tests. Test T 1 evaluates if aftershock hypocenters are located in low-slip regions (hypothesis H 1 ), test T 2 evaluates if aftershock hypocenters occur in regions of increased shear stress (hypothesis H 2 ). In the tests, we seek to reject the null hypotheses H 0 : Aftershock hypocenters are not correlated with (1) low-slip regions or (2) regions of increased shear stress, respectively. We tested the hypotheses on four strike-slip events for which multiple earthquake catalogs and multiple finite fault source models of varying accuracy exist. Because we want to retain earthquake clustering as the fundamental feature of aftershock seismicity, we generate slip distributions using a random spatial field model and derive the stress drop distributions instead of generating seismicity catalogs. We account for uncertainties in the aftershock locations by simulating them within their location error bounds. Our findings imply that aftershocks are preferentially located in regions of low-slip (u 1 3 u max ) and of increased shear stress (Ds < 0). In particular, the correlation is more significant for relocated than for general network aftershock catalogs. However, the results show that stress drop patterns provide less information content on aftershock locations. This implies that static shear stress change of the main shock may not be the governing process for aftershock genesis.