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Fault models are quickly produced and iteratively improved over weeks to years following a major earthquake, to characterise the dynamics of rupture, evaluate the role of stress transfer, and contribute to earthquake forecasting. We model Coulomb stress transfer (ΔCFS) between the largest foreshock (Mw 5.4; 1 year prior to first mainshock) and three Mw 6.1 to 6.5 earthquakes that occurred in a 12-hour period on January 22, 1988 in central Australia (Tennant Creek earthquake sequence) to investigate the role of static stress transfer in earthquake triggering relative to progressive source model development. The effects of fault model variance are studied using ΔCFS modelling of five different fault source model sequences (27 total models) using different inputs from seismic and geospatial data. Some initial models do not yield positive ΔCFS changes proximal to hypocentres but in all models, preceding earthquakes generate positive ΔCFS (≥0.1 bar) on ≥10 to 30% of the forthcoming receiver fault rupture areas. The most refined and data-integrative model reveals ΔCFS ≥ +0.7 to +13 bars within 2 km of impending hypocentres and large (≥30 to 99%) areas of positive ΔCFS. When compared to global compilations of threshold ΔCFS prior to impending ruptures (average = 3.71 bar, median = 1 bar), this suggests that Coulomb stress change theory adequately explains the Tennant Creek rupture sequence. In the most-refined model, earthquake inter-event times decrease as ΔCFS increases, suggesting that higher stress magnitudes may have more rapidly (within hours) triggered successive events, thus accounting for some temporal aspects of this sequence. ΔCFS analyses provide a useful framework for understanding the spatiotemporal aspects of some intraplate earthquakes. The progressive refinement of source models using emergent data may reduce epistemic uncertainties in the role of stress transfer that result from different model inputs, approaches, and results.

We investigate the postseismic viscoelastic flow in the lower crust and upper mantle due to the 19th and 20th century large earthquakes in eastern Turkey. Three possible rheological models are used in the viscoelastic postseismic deformation analysis to assess the extent to which these events influence the velocity fields at GPS sites in the region. Our models show that the postseismic signal currently contributes to the observed deformation in the eastern part of the North Anatolian fault and northern and middle parts of the East Anatolian Fault Zone, primarily due to the long-lasting effect of the Ms 7.9 1939 earthquake. None of the postseismic displacement generated by the Ms 7.5 1822 earthquake, which is the earliest and the second largest event in the calculations, exceeds observed error range at the GPS stations. Our results demonstrate that a postseismic signal can be identified in the region and could contribute up to 3–25% of the observed GPS measurements.

The static coulomb stress hypothesis is a widely known physical mechanism for earthquake triggering and thus a prime candidate for physics-based operational earthquake forecasting (OEF). However, the forecast skill of coulomb-based seismicity models remains controversial, especially compared with empirical statistical models. A previous evaluation by the Collaboratory for the Study of Earthquake Predictability (CSEP) concluded that a suite of coulomb-based seismicity models were less informative than empirical models during the aftershock sequence of the 1992 Mw 7.3 Landers, California, earthquake. Recently, a new generation of coulomb-based and coulomb/statistical hybrid models were developed that account better for uncertainties and secondary stress sources. Here, we report on the performance of this new suite of models compared with empirical epidemic-type aftershock sequence (ETAS) models during the 2010-2012 Canterbury, New Zealand, earthquake sequence. Comprising the 2010 M 7.1 Darfield earthquake and three subsequent M = 5:9 shocks (including the February 2011 Christchurch earthquake), this sequence provides a wealth of data (394 M = 3:95 shocks). We assessed models over multiple forecast horizons (1 day, 1 month, and 1 yr, updated after M = 5:9 shocks). The results demonstrate substantial improvements in the coulomb-based models. Purely physics-based models have a performance comparable to the ETAS model, and the two coulomb/statistical hybrids perform better or similar to the corresponding statistical model. On the other hand, an ETAS model with anisotropic (fault-based) aftershock zones is just as informative. These results provide encouraging evidence for the predictive power of coulomb-based models. To assist with model development, we identify discrepancies between forecasts and observations. © 2018 Seismological Society of America. All rights reserved.

Forecasting the spatial and temporal distribution of aftershocks is of great importance to earthquake scientists, civil protection authorities and the general public as these events cause disproportionate damage and consternation relative to their size. At present, there are two main approaches to such forecasts—purely statistical methods based on observations of the initial portions of aftershock sequences and a physics-based approach based on Coulomb stress changes caused by the main shock. Here we develop a new method which combines the spatial constraints from the Coulomb model with the statistical power of the STEP (short-term earthquake probability) approach. We test this pseudo prospectively and retrospectively on the Canterbury sequence against the STEP model and a Coulomb rate–state method, using data from the first 10 d following each main event to forecast the rate of M ≥ 4 events in the following 100 d. We find that in retrospective tests the new model outperforms STEP for two events in the sequence but this is not the case for pseudo-prospective tests. Further, the Coulomb rate–state approach never performs better than STEP. Our results suggest that incorporating the physical constraints from Coulomb stress changes can increase the forecasting power of statistical models and clearly show the importance of good data quality if prospective forecasts are to be implemented in practice.

The Canterbury earthquake sequence, which includes the devastating Christchurch event of 2011 February, has to date led to losses of around 40 billion NZ dollars. The location and severity of the earthquakes was a surprise to most inhabitants as the seismic hazard model was dominated by an expected Mw > 8 earthquake on the Alpine fault and an Mw 7.5 earthquake on the Porters Pass fault, 150 and 80 km to the west of Christchurch. The sequence to date has included an Mw = 7.1 earthquake and 3 Mw ≥ 5.9 events which migrated from west to east. Here we investigate whether the later events are consistent with stress triggering and whether a simple stress map produced shortly after the first earthquake would have accurately indicated the regions where the subsequent activity occurred. We find that 100 per cent of M > 5.5 earthquakes occurred in positive stress areas computed using a slip model for the first event that was available within 10 d of its occurrence. We further find that the stress changes at the starting points of major slip patches of post-Darfield main events are consistent with triggering although this is not always true at the hypocentral locations. Our results suggest that Coulomb stress changes contributed to the evolution of the Canterbury sequence and we note additional areas of increased stress in the Christchurch region and on the Porters Pass fault.

[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

Several independent indicators imply a high probability of a great ( M > 8) earthquake rupture of the subduction megathrust under the Mentawai Islands of West Sumatra. The human consequences of such an event depend crucially on its tsunamigenic potential, which in turn depends on unpredictable details of slip distribution on the megathrust and how resulting seafloor movements and the propagating tsunami waves interact with bathymetry. Here we address the forward problem by modelling about 1000 possible complex earthquake ruptures and calculating the seafloor displacements and tsunami wave height distributions that would result from the most likely 100 or so, as judged by reference to paleogeodetic data. Additionally we carry out a systematic study of the importance of the location of maximum slip with respect to the morphology of the fore-arc complex. Our results indicate a generally smaller regional tsunami hazard than was realised in Aceh during the December 2004 event, though more than 20% of simulations result in tsunami wave heights of more than 5 m for the southern Sumatran cities of Padang and Bengkulu. The extreme events in these simulations produce results which are consistent with recent deterministic studies. The study confirms the sensitivity of predicted wave heights to the distribution of slip even for events with similar moment and reproduces Plafker's rule of thumb. Additionally we show that the maximum wave height observed at a single location scales with the magnitude though data for all magnitudes exhibit extreme variability. Finally, we show that for any coastal location in the near field of the earthquake, despite the complexity of the earthquake rupture simulations and the large range of magnitudes modelled, the timing of inundation is constant to first order and the maximum height of the modelled waves is directly proportional to the vertical coseismic displacement experienced at that point. These results may assist in developing tsunami preparedness strategies around the Indian Ocean and in particular along the coasts of western Sumatra.

Summary We explore the possible stress triggering relationship of the M≥ 6.4 earthquakes that occurred in Kerman Province, southern Iran since 1981. We calculated stress changes due to both coseismic sudden movement in the upper crust and the time-dependent viscous relaxation of the lower crust and/or upper mantle following the event. Four events of M≥ 6.4 occurred between 1981 and 2005, on and close to the Gowk fault, show a clear Coulomb stress load to failure relationship. The 2003 M= 6.5 Bam earthquake, however, which occurred approximately 95 km SW of the closest Gowk event, shows a very weak stress relation to preceding earthquakes. The coseismic static stress change at the hypocentre of the Bam earthquake is quite small (∼0.006 bars). The time-dependent post-seismic stress change could be 26 times larger or 7 times lower than that of coseismic static stress alone depending on the choice of viscoelastic crustal model and the effective coefficient of friction. Given the uncertainties in the viscoelastic earth models and the effective coefficient of friction, we cannot confidently conclude that the 2003 Bam event was brought closer to failure through coseismic or post-seismic stress loading. Interestingly, the southern Gowk segment with a similar strike to that of the Bam fault, experienced a stress load of up to 8.3 bars between 1981 and 2003, and is yet to have a damaging earthquake.

The 2003 Bingol earthquake (Mw=6.4) occurred very close to a region along the east Anatolian fault zone which was identified in 2002 as posing a particularly high seismic risk. This damaging earthquake occurred on a conjugate right-lateral blind fault that was inconsistent with the stress-change field calculated for preceding large earthquakes in the region. In this paper, four reasons which might be responsible for this stress discrepancy are identified and investigated individually. Firstly, co-seismic stress changes are considered. The time frame of the previous stress calculations is extended to include the large earthquakes in the 1780s which were not included in the earlier study. A sensitivity analysis is then conducted on the more recent events since 1822 to examine the effect of errors in their location and sizes. The possibility of the occurrence of a small magnitude event close to the 2003 epicentre is considered. We argue that, barring the occurrence of a low-probability, unmodelled local event, the Bingol earthquake was unlikely to have been triggered by co-seismic stress transfer from any known sequence of previous earthquakes. Finally we examine and modify the secular loading model used in the 2002 study and show that loading which is properly constrained by regional GPS data produces a positive stress change on the 2003 rupture. As a result of our examination of the stressing history of the Bingol hypocentre we argue that it is through a combination of historical seismology, guided and constrained by structural geology, directed paleoseismology in which the locations and extent of historical events are confirmed, and stress modelling which has been informed by detailed GPS data, that an integrated seismic hazard program might have the best chance of success. D 2005 Elsevier B.V. All rights reserved.

The 1999 November 12 Duzce earthquake (M (w) = 7.1) was apparently the eastward extension of the August 17, Izmit earthquake (M (w) = 7.4). The Duzce event caused heavy damage and fatalities in the cities of Duzce and Bolu. Here a finite-fault inversion method with five discrete time windows is applied to derive the co-seismic slip distribution of the Duzce earthquake. The fault plane is best modelled as a 40 x 20 km(2) plane, with a strike of 262degrees and a dip of 65degrees to the north, and that the majority of slip occurred in two distinct patches on either side of the hypocentre, implying bilateral rupture. The possible triggering of this event by the Izmit earthquake is investigated using Coulomb stress modelling of all large events since 1943 with the inclusion of secular loading. The results show that although the Duzce rupture plane was in a stress shadow prior to the Izmit earthquake, that event caused a significant Coulomb stress load, taking the Duzce fault out of the stress shadow, which probably precipitated failure. A comparison of the mapped Coulomb stress change with the inferred slip shows no correlation between the two. Finally, the stress modelling indicates that the northern branch of the North Anatolian fault zone, beneath the Sea of Marmara towards the city of Istanbul, is presently the most highly loaded segment of the North Anatolian Fault Zone.

Unlike the North Anatolian fault zone, which has produced 11 large earthquakes since 1939, the East Anatolian fault zone (EAFZ) has been relatively quiescent in the last century when compared to historical records and has therefore accumulated significant stresses along its length. Determination of the location and likely magnitude of a future probable earthquake along the EAFZ is of interest both because of this history of large earthquakes, (M≈8), and the density of population in the area. Here we calculate stress evolution along the fault zone due to both seismic and tectonic loading since 1822. A sequence of 10 well constrained historical earthquakes is selected and the resulting stresses are calculated, summed with tectonic loading stresses and resolved onto the mapped active faults. We identify two areas of particular seismic risk, one of which might be expected to yield a large event. Our results are sensitive to the previous history of large earthquakes in the region and indicate a need for detailed investigations to constrain the exact rupture geometries of previous earthquakes on these segments.

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.

We investigate controls on tsunami generation and propagation in the near-field of great megathrust earthquakes using a series of numerical simulations of subduction and tsunamigenesis on the Sumatran forearc. The Sunda megathrust here is advanced in its seismic cycle and may be ready for another great earthquake. We calculate the seafloor displacements and tsunami wave heights for about 100 complex earthquake ruptures whose synthesis was informed by reference to geodetic and stress accumulation studies. Remarkably, results show that, for any near-field location: (1) the timing of tsunami inundation is independent of slipdistribution on the earthquake or even of its magnitude, and (2) the maximum wave height is directly proportional to the vertical coseismic displacement experienced at that location. Both observations are explained by the dominance of long wavelength crustal flexure in near-field tsunamigenesis. The results show, for the first time, that a single estimate of vertical coseismic displacement might provide a reliable short-term forecast of the maximum height of tsunami waves. Published version

On 28 March 2005 the Sunda megathrust in Indonesia ruptured again, producing another great earthquake three months after the previous one. The rupture was contiguous with that of the December 2004 Sumatra-Andaman earthquake, and is likely to have been sparked by local stress, although the triggering stresses at its hypocentre were very small - of the order of just 0.1 bar. Calculations show that stresses imposed by the second rupture have brought closer to failure the megathrust immediately to the south, under the Batu and Mentawai islands, and have expanded the area of increased stress on the Sumatra fault. Palaeoseismologic studies show that the Mentawai segment of the Sunda megathrust is well advanced in its seismic cycle and is therefore a good candidate for triggered failure.

Coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation. It is often assumed that aftershock fault planes are "optimally oriented"; in other words, the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations. This practice assumes that faults capable of sustaining aftershocks exist at all orientations, an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends. Here we systematically investigate the best planes onto which stress should be resolved for the Landers, Hector Mine, Loma Prieta, and Northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes (two- and three-dimensional), main shock orientation and regional structural trend. We find that the best model differs between different tectonic regions but that in all cases, models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions, although not all such models do so equally well. Our results suggest that when the regional stress field is poorly defined, or in structurally complex areas, the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary. Copyright 2005 by the American Geophysical Union.

Following the massive loss of life caused by the Sumatra-Andaman earthquake in Indonesia and its tsunami, the possibility of a triggered earthquake on the contiguous Sunda trench subduction zone is a real concern. We have calculated the distributions of co-seismic stress on this zone, as well as on the neighbouring, vertical strike-slip Sumatra fault, and find an increase in stress on both structures that significantly boosts the already considerable earthquake hazard posed by them. In particular, the increased potential for a large subduction-zone event in this region, with the concomitant risk of another tsunami, makes the need for a tsunami warning system in the Indian Ocean all the more urgent.

[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.

On 28 March 2005 the Sunda megathrust in Indonesia ruptured again, producing another great earthquake three months after the previous one. The rupture was contiguous with that of the December 2004 Sumatra–Andaman earthquake, and is likely to have been sparked by local stress, although the triggering stresses at its hypocentre were very small — of the order of just 0.1 bar. Calculations show that stresses imposed by the second rupture have brought closer to failure the megathrust immediately to the south, under the Batu and Mentawai islands, and have expanded the area of increased stress on the Sumatra fault. Palaeoseismologic studies show that the Mentawai segment of the Sunda megathrust is well advanced in its seismic cycle and is therefore a good candidate for triggered failure.

Understanding the geometry and evolution of fracture systems in the crust is a fundamental problem in geomechanics. Although the basic elements of brittle fracture, such as the nucleation, growth, and interaction of microcracks from preexisting flaws, have been studied extensively in the laboratory and are reasonably well understood theoretically, patterns of such fractures that develop in zones of crustal deformation is a topic of current research. The reason is that crustal fracture networks are developed over a wide range of scales, and the interaction between structures at different scales is not well understood. Most laboratory experiments involve deformation on only one scale before the effects of sample boundaries become important.

Last year's Indonesian earthquake has increased seismic hazard in the region. The scale of death and destruction caused by the recent Sumatra–Andaman Islands earthquake and its tsunami has raised concerns that another earthquake might be triggered on the contiguous Sunda trench subduction zone. A new calculation of stress distribution here and on the neighbouring Sumatra fault reveals stress build-up on both structures, severe enough to increase the already high earthquake hazard. This finding makes the need for an Indian Ocean tsunami warning system all the more pressing. Following the massive loss of life caused by the Sumatra–Andaman earthquake in Indonesia and its tsunami, the possibility of a triggered earthquake on the contiguous Sunda trench subduction zone is a real concern. We have calculated the distributions of co-seismic stress on this zone, as well as on the neighbouring, vertical strike–slip Sumatra fault, and find an increase in stress on both structures that significantly boosts the already considerable earthquake hazard posed by them. In particular, the increased potential for a large subduction-zone event in this region, with the concomitant risk of another tsunami, makes the need for a tsunami warning system in the Indian Ocean all the more urgent.