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[1] The extent to which interseismic coupling controls the slip distribution of large megathrust earthquakes is unclear, with some authors proposing that it is the primary control and others suggesting that stress changes from previous earthquakes are of first-order importance. Here, we develop a detailed stress history of the Sunda megathrust, modified by coupling, and compare the correlation between slip and stress with that of slip versus coupling. We find that the slip distributions of recent earthquakes are more consistent with the stress field than with the coupling distributions but observe that in places, the stress pattern is strongly dependent on poorly constrained values of slip in historical earthquakes. We also find that of the 13 earthquakes in our study for which we have hypocentral locations, only two appear to have nucleated in areas of negative stress, and these locations correspond to large uncertainties in the slip distribution of pre-instrumental events. Citation: Nalbant, S., J. McCloskey, S. Steacy, M. NicBhloscaidh, and S. Murphy (2013), Interseismic coupling, stress evolution, and earthquake slip on the Sunda megathrust, Geophys. Res. Lett., 40, 4204–4208

Scale invariance, either in space or in time, has been shown in many papers to characterize earthquake distributions. Unfortunately, little work has been dedicated to looking at the general space-time scaling invariance of seismicity systems, even though a better understanding of how the two domains (spatial and temporal) link together could help the development of the stochastic dynamical modeling of earthquake populations. In this paper we report the observation of diffusion processes of temporally correlated seismic activity for three different data sets: a mine (Creighton Mine, Canada), the Long Valley Caldera in eastern California, and a 7-year period of recorded seismic activity in southern California. The observed subdiffusion processes are indicative of the general space-time scaling of the system, taking the form of a slow power law growth R(t) similar to t(H) of the mean distance R(t) between the main event arid the temporally correlated afterevents occuring after a delay t. H is found on average to be small (0.1 for Creighton Mine, 0.22 for the Long Valley Caldera, and 0.22 for the southern California main events with magnitude greater than or equal to 1.5) but fluctuates significantly from one main event to the other: the diffusion is found to be intermittent (non-Gaussian) and multiscaling, and except for the Long Valley Caldera, a systematic correlation between the sizes of the main event and subsequent afterevents and the growth exponent H is observed. While classical viscous relaxation models (e.g., elastic listhosphere-plastic asthenosphere coupling, or fluid flow triggered by sudden changes in pore pressure) have been proposed to characterize this relaxation by homogeneous (i.e., nonintermittent) normal (H = 0.5) diffusion processes, the direct implication of the reported results is that seismicity systems, at spatial scales from meters to hundreds of kilometers and small (microearthquakes in a mine) to intermediate magnitudes, relax spatiotemporally in a nonelastic way, revealing the stochastic space-time scale-invariant nature of such systems. Since these diffusion processes correspond to a loss of information with time on the location of the main event, they can be used to investigate the limits of predictability, at all spatial scales, of seismicity systems in terms of the spatiotemporal clustering of temporally correlated earthquakes.

[1] There is an on-going debate in the seismological community as to whether stress drop is independent of earthquake size and this has important implications for earthquake physics. Here we investigate this question in a simple 2D cellular automaton that includes heterogeneity. We find that when the range of heterogeneity is low, the scaling approaches that of constant stress drop. However, clear deviations from the constant stress drop model are observed when the range of heterogeneity is large. Further, fractal distributions of strength show more significant departures from constant scaling than do random ones. Additionally, sub-sampling the data over limited magnitude ranges can give the appearance of constant stress drop even when the entire data set does not support this. Our results suggest that deviations from constant earthquake scaling are real and reflect the heterogeneity of natural fault zones, but may not provide much information about the physics of earthquakes.

[1] In this introduction, we review much of the recent work related to stress transfer, earthquake triggering, and time-dependent seismic hazard in order to provide context for the special section on these subjects. Considerable advances have been made in the past decade, and we focus on our understanding of stress transfer at various temporal and spatial scales, review recent studies of the role of fluids in earthquake triggering, describe evidence for the connection between volcanism and earthquake triggering, examine observational evidence for triggering at all scales, and finally discuss the link between earthquake triggering and time-dependent seismic hazard. We conclude by speculating on future areas of research in the next decade.

[1] The apparent strong correlation between Coulomb stress changes and the spatial distribution of aftershocks suggests the possibility of making near-real-time estimations of areas at risk of experiencing off-fault aftershocks. In order to do this in practice a number of issues must first be addressed, including the extent to which the main shock slip must be known in detail before a meaningful stress map can be constructed. Here we investigate this issue by constructing a time-ordered sequence of slip solutions for the Landers earthquake, computing Coulomb stress changes for each solution, and quantitatively comparing the stress field with the observed aftershocks by (1) resolving the Coulomb stress change onto the aftershock nodal planes and calculating the percentage of events consistent with triggering and (2) constructing a two-dimensional map of Coulomb stress and computing the correlation coefficient between the positive and negative areas and the locations of the aftershocks. We find that slip solutions based on empirical relations and either focal mechanism or moment tensor data produce stress fields inconsistent with the observed spatial distribution of aftershocks, whereas slip solutions incorporating the correct rupture geometry but greatly simplified slip produce stress fields consistent with the aftershock distribution when very near-fault events are excluded. We further find that resolving stress perturbations onto earthquake nodal planes and computing the percentage of events experiencing positive Coulomb stress provides a poor measure of success because of the limited range of structures on which events occur and the compatibility of the main shock stress field with these structures. Our results support the hypothesis that Coulomb stress changes affect the spatial distribution of aftershocks and suggest that meaningful calculations of Coulomb stress can be made as soon as an earthquake's rupture geometry is well constrained.

[1] Calculations of static stress changes due to large earthquakes have shown that the spatial distribution of aftershocks is predictable to first order, with aftershocks primarily occurring in areas experiencing positive stress changes. Delineation of these areas relies on resolving the stress perturbation onto planes with known orientations; common practice is to use poorly constrained regional stress information to compute optimally oriented failure planes, assuming that they exist everywhere. Here we show that this assumption is not supported by observation but rather that aftershock failure planes are controlled by geological structure. We argue that useful aftershock hazard estimates are better made by replacing information on regional stress with statistical measures of structural orientations.