Coulomb Stress Increase Research Articles

A Study of the Impact of Static Coulomb Stress Changes of Megathrust Earthquakes Along Subduction Zone on the Following Aftershocks

AbstractResearch on static earthquake triggering has been carried out widely in the world, and achieved remarkable results. But it is still unclear whether this model is effective to all large earthquakes. In this paper, we investigated the Coulomb stress changes of 3 megathrust earthquakes along subduction zones (the 2011 Mw9.1 Tohoku earthquake, the 2010 Mw8.8 Chile earthquake, and the 2004 Mw9.0 Sumatra‐Andaman earthquake) to test the triggering effects by examining the correlation between Coulomb stress increases and spatial distribution of the following aftershocks. The calculated results suggest that there is no obvious evidence that the Coulomb stress changes caused by the 3 megathrust earthquakes promoted the occurrence of the aftershocks. There are only 47% of the encouraged aftershocks following the Tohoku earthquake. And there are 47.6% and 49.8% for the Sumatra‐Andaman and the Chile earthquake, respectively. We also calculated the Coulomb stress changes with different focal models and parameters. It is still less than 60% of the promoted aftershocks in the optimal case. However, the static triggering model is good for the Wenchuan earthquake and the Chi‐Chi earthquake which have enhanced more than 85% of the subsequent aftershocks. This model may be not reasonable for large subduction earthquakes. Therefore, other model should be introduced in studying earthquake triggering in subduction zone and further studies will be performed.

Fault geometry, coseismic-slip distribution and Coulomb stress change associated with the 2009 April 6, Mw 6.3, L’Aquila earthquake from inversion of GPS displacements

The 2009 April 6, Mw= 6.3 L’Aquila earthquake occurred within a complex system of NW-SE trending normal faults in the Abruzzi Central Apennines (Italy). We analyse the coseismic deformation as measured by >70 global positioning system (GPS) stations, both from continuous and survey-mode networks, providing unprecedented details for a moderate normal faulting earthquake in Italy from GPS measurements. We use rectangular, uniform-slip, dislocations embedded in an elastic, homogeneous and isotropic half-space and a constrained, non-linear optimization algorithm, to solve for the best-fitting rectangular dislocation geometry and coseismic-slip distribution. We use a bootstrap approach to investigate uncertainties in the model parameters and define confidence bounds for all the inverted parameters. The rupture occurred on a N129°E striking and 50° southwestward dipping normal fault, in agreement with geological observations of surface breaks along the Paganica fault. Our distributed slip model exhibits a zone of relatively higher slip (>60 cm) between ~1.5 and ~11 km depth, along a roughly downdip, NW-SE elongated patch, confined within the fault plane inverted assuming uniform-slip. The highest slip, of the order of ~1 m, occurred on a ~16 km2 area located at ~5 km depth, SE of the mainshock epicentre. The analysis of model resolution suggests that slip at depth below ~5 km can be resolved only at a spatial scale larger than 2 km, so a finer discretization of different asperities within the main patch of coseismic-slip is not allowed by GPS data. We compute the coseismic Coulomb stress changes in the crustal volume affected by the major aftershocks, and compare the results obtained from the uniform-slip and the heterogeneous-slip models. We find that most of the large aftershocks occurred in areas of Coulomb stress increase of 0.2-13 bar and that a deepening of the slip distribution down to a depth greater than 6 km in the SE part of the fault plane, in agreement with the inverted slip model, can explain the deepest, April 7, Mw 5.3 aftershock.

Tectonic implications and seismicity triggering during the 2008 Baluchistan, Pakistan earthquake sequence

A damaging and widely felt moderate earthquake (Mw 6.4) hit the rural, mountainous region of southwestern Pakistan on October 28, 2008. The main shock was followed by another earthquake of identical magnitude (Mw 6.4) on the next day. The spatial distribution of aftershocks and focal mechanism revealed a NW–SE striking rupture with right-lateral strike-slip motion which is sympathetic to the NNW–SSE striking active mapped Urghargai Fault. The occurrence of strike-slip earthquakes suggests that along with the thrust faults, strike slip faults too are present beneath the fold-and-thrust belt of Sulaiman–Kirthar ranges and accommodates some of the relative motion of the Indian and Eurasian plates. To assess the characteristics of this sequence, the statistical parameters like aftershocks temporal decay, b-value of G–R relationship, partitioning of radiated seismic energy due to aftershocks, and spatial fractal dimension (D-value) have been examined. The b-value is estimated as 1.03 ± 0.42 and suggests the tectonic genesis of the sequence and crustal heterogeneity within rock mass. The low p-value of 0.89 ± 0.07 implies slow decay of aftershocks activity which is probably an evidence for low surface heat flow. A value of spatial fractal dimension of 2.08 ± 0.02 indicates random spatial distribution and that the source is a two-dimensional plane filled-up by fractures. The static coseismic Coulomb stress changes due to the foreshock (Mw 5.3) were found to increase stress by more than 0.04 bars at the hypocenter of the main shock, thus promoting the failure. The cumulative coseismic Coulomb stress changes due to the foreshock and mainshocks suggest that most of the aftershocks occurred in the region of increased Coulomb stress, and to the SE to the mainshock rupture.

Rate/state Coulomb stress transfer model for the CSEP Japan seismicity forecast

Numerous studies retrospectively found that seismicity rate jumps (drops) by coseismic Coulomb stress increase (decrease). The Collaboratory for the Study of Earthquake Prediction (CSEP) instead provides us an opportunity for prospective testing of the Coulomb hypothesis. Here we adapt our stress transfer model incorporating rate and state dependent friction law to the CSEP Japan seismicity forecast. We demonstrate how to compute the forecast rates of large shocks in 2009 using the large earthquakes during the past 120 years. The time dependent impact of the coseismic stress perturbations explains qualitatively well the occurrence of the recent moderate size shocks. Such ability is partly similar to that of statistical earthquake clustering models. However, our model differs from them as follows: the off-fault aftershock zones can be simulated using finite fault sources; the regional areal patterns of triggered seismicity are modified by the dominant mechanisms of the potential sources; the imparted stresses due to large earthquakes produce stress shadows that lead to a reduction of the forecasted number of earthquakes. Although the model relies on several unknown parameters, it is the first physics based model submitted to the CSEP Japan test center and has the potential to be tuned for short-term earthquake forecasts.

Relationship of compaction bands in Utah to Laramide fault-related folding

Multiple sets of compaction bands of southwestern Utah occur in Navajo Sandstone near a bend in the adjacent East Kaibab monocline. Forward mechanical models of blind-thrust fault deformation are used in this paper to investigate the spatial relationship between compaction bands and fault-related folding. Results are consistent with the East Kaibab monocline being underlain by a west-dipping blind high-angle reverse fault between ~ 1.5 and 5 km depth below the top of the Navajo Sandstone. Reverse slip along this reactivated fault during the Laramide orogeny is inferred to have produced values of compactional normal strain that were largest in the footwall near the concave map-view bend in the monocline. The calculated compactional normal strains and Coulomb stress increases are consistent with the field occurrence of pure and shear-enhanced compaction bands in this area. Bends, relays, stepovers, or other geometric complexities above blind reverse faults are thus inferred to be potential sites for compaction band localization in appropriate rock types.

Critical sensitivity of load/unload response ratio and stress accumulation before large earthquakes: example of the 2008 Mw7.9 Wenchuan earthquake

The Load/Unload Response Ratio (LURR) method is often defined as the ratio between Benioff strains released during the time periods of loading and unloading, corresponding to earth tide induced Coulomb Failure Stress change on optimally oriented faults. According to the method, anomalous increase in the time series of LURR usually occurs prior to occurrence of a large earthquake. Previous studies have indicated that the stress field that existed before a large earthquake has strong influence on the evaluation of LURR. In order to augment the sensitivity of LURR in measuring the criticality of stress accumulation before an earthquake, we replace the circular region usually adopted in LURR practice with an area within which the tectonic stress change would mostly affect the Coulomb stress on a potential seismogenic fault of a future event. Coulomb stress change before the hypothetical earthquake is calculated based on a simple back-slip dislocation model of the event. Retrospective test of this new algorithm on the 2008 Mw7.9 Wenchuan earthquake shows remarkable enhancement of the LURR precursory anomaly. To illustrate the variation of LURR time series associated with our choice of identified areas with increased Coulomb stress before the earthquake, we calculate the spatial distributions of LURR within a circular region of 700 km radius centered at epicenter of the event. Comparing the spatial LURR distributions of different periods, the change of LURR within the Coulomb stress increase areas looks more prominent than the others: it remains at a low level for most of the time and markedly increases few years before the quake. This result further shows the validity of the Coulomb stress algorithm. Unlike circular regions, areas of increased Coulomb stress with anomalously increased LURR values before a large earthquake could provide a relatively more precise estimation of the criticality of the ensuing event.

Stress triggering and stress memory observed from acoustic emission records in a salt mine

SUMMARY Some of the most interesting questions in geosciences are whether results from laboratory experiments can be applied to processes in the earth crust and whether in situ studies with high spatio-temporal resolution can bridge the gap between laboratory work and seismology. In this study, acoustic emission (AE) activity caused by stress changes due to the backfilling of a cavity in an abandoned salt mine is studied to answer questions regarding (1) the dependence of AE event rates, event distribution and b-value on the stress state, (2) the stress memory effect of rock (Kaiser effect), (3) the possibility to detect significant changes in the system like the initiation of macrocracks and (4) the possibility to estimate future activity from previous AE records. The large number of events studied (>3 × 10 5 ) allows a spatial resolution of the order of 1 m and a temporal one on the order of 1 hr. Stress changes are created due to the thermal expansion and contraction of the rock mass in response to the temperature changes caused by the backfilling. A roughly 20 × 50 × 50 m section of the mining complex just above the backfilled cavity is well covered by a network of 24 piezo-electric receivers and poses an optimal volume for the study. Results of a 2-D finite element thermoelastic stress model are in agreement with the spatio-temporal AE event distribution. In addition to the initial upward migration of the AE event front, which correlates with the calculated stress field, the rock salt exhibits a pronounced Kaiser effect for the first few thermal loading cycles throughout the whole study region. The deviation from the Kaiser effect during later loading cycles seems to be caused by the initiation of a planar macroscopic crack, which is subsequently reactivated. AE activity tends to concentrate along this macrocrack. Calculated b-values decrease before and increase after the supposed initiation of the macrocrack supporting this explanation. In intact rock volumes not subjected to macrocracking a linear relation between the maximum event rate and the calculated absolute Coulomb stress increase is observed. This indicates that future maximum AE event rates can be estimated from expected loading. AE activity during stress loading cycles is most prominent in regions with Coulomb stress maxima indicating possible shear cracking that has the potential to create macrocracks. Strong bursts of AE activity observed during thermal unloading phases are concentrated in regions for which the minimum principal stress becomes tensile. These regions exhibit significantly higher b-values than those active during thermal loading. We interpret these weak events with tensile microcracking.

Influence of surface load variations on eruption likelihood: application to two Icelandic subglacial volcanoes, Grímsvötn and Katla

SUMMARY We investigate how surface load variations around volcanoes act on shallow magma chambers. Numerical calculations are carried out in axisymmetric geometry for an elliptical chamber embedded in an elastic medium. Magma compressibility is taken into account. For variable chamber shape, size and depth, we quantify how unloading events induce magmatic pressure change as well as variation of the threshold pressure required for dyke initiation at the chamber wall. We evaluate the triggering effect of these surface events on onset of eruptions and find it depends strongly on the surface load location and the magma chamber shape. We apply this model to two active Icelandic subglacial volcanoes: Gr´ omsv¨ otn and Katla. The 2004 eruption of Gr´ omsv¨ otn was immediately preceded by a j¨ okulhlaup, a glacial outburst flood of 0.5 km 3 . We show that this event may have triggered the eruption only if the system was very close to failure conditions. Katla volcano is covered by the M´ yrdalsj¨ okull ice cap. An annual cycle, with up to 6 m change in snow thickness, occurs from winter to summer. As the seasonal snow load is reduced, a pressure decrease of the same order of magnitude as the load is induced within the magma storage zone. Our model predicts that, in the case of a spherical or horizontally elongated magma chamber, eruptions are more likely when the snow cover is smallest, which appears consistent with the fact that all the last nine major historical eruptions at Katla occurred during the summer period. The model predicts an increase in Coulomb stress around the caldera, up to 7 km from its centre, during unloading periods, enough to trigger earthquakes. Stress due to snow load variations, with focusing of it in weak zones near the caldera boundary, is considered a contributing factor to seasonal seismicity observed beneath M´ yrdalsj¨ okull.

Aftershock Sequences Modeled with 3-D Stress Heterogeneity and Rate-State Seismicity Equations: Implications for Crustal Stress Estimation

In this paper, we present a model for studying aftershock sequences that integrates Coulomb static stress change analysis, seismicity equations based on rate-state friction nucleation of earthquakes, slip of geometrically complex faults, and fractal-like, spatially heterogeneous models of crustal stress. In addition to modeling instantaneous aftershock seismicity rate patterns with initial clustering on the Coulomb stress increase areas and an approximately 1/t diffusion back to the pre-mainshock background seismicity, the simulations capture previously unmodeled effects. These include production of a significant number of aftershocks in the traditional Coulomb stress shadow zones and temporal changes in aftershock focal mechanism statistics. The occurrence of aftershock stress shadow zones arises from two sources. The first source is spatially heterogeneous initial crustal stress, and the second is slip on geometrically rough faults, which produces localized positive Coulomb stress changes within the traditional stress shadow zones. Temporal changes in simulated aftershock focal mechanisms result in inferred stress rotations that greatly exceed the true stress rotations due to the main shock, even for a moderately strong crust (mean stress 50 MPa) when stress is spatially heterogeneous. This arises from biased sampling of the crustal stress by the synthetic aftershocks due to the non-linear dependence of seismicity rates on stress changes. The model indicates that one cannot use focal mechanism inversion rotations to conclusively demonstrate low crustal strength (≤10 MPa); therefore, studies of crustal strength following a stress perturbation may significantly underestimate the mean crustal stress state for regions with spatially heterogeneous stress.

Seismic cycle stress change in western Taiwan over the last 270 years

The island of Taiwan is affected by intense seismic activity, which includes large events as the disastrous 1999 Chi‐Chi earthquake. To improve seismic hazard assessment in this area, we estimate the effect of both interseismic loading and major events since 1736 on the state of stress of major active faults. We focus our approach on western Taiwan, which is the most densely populated part of Taiwan. We pay a specific attention to faults geometry and to both interseismic and coseismic slip distributions. Our results suggest that both earthquakes and interseismic loading before 1999 increase the Coulomb stress in the north‐western part of the Chelungpu fault, a region which experienced the highest coseismic slip during the Chi‐Chi earthquake. More importantly our results reveal a Coulomb stress increase in the southern part of the Changhua thrust fault, below a densely populated area.

The 1998 and 2004 Bovec-Krn (Slovenia) seismic sequences: aftershock pattern, focal mechanisms and static stress changes

SUMMARY We investigate the main seismotectonic aspects of the Bovec-Krn sequences (northwestern Slovenia), caused by the 1998 April 12 MD 5.6 and the 12 July 2004 MD 5.1 earthquakes. The seismicity occurred along an extended right-lateral strike-slip fault zone, trending NW–SE. The analysis involves the aftershock pattern, the focal mechanism of the largest events and the stress changes induced by the 1998 main shock. The study is performed with high-quality earthquake locations obtained with tomographic techniques that provided also velocity images of the fault zone, related to the mechanical properties of the medium and to the geometry of the fault. The tomographic model shows in the fault zone sharp lateral P-wave velocity contrasts and marked lateral Vp/Vs variations and images an extended high rigidity body along the fault, where the 1998 and 2004 main shocks are located. The aftershock activity is intense along highly heterogeneous zones characterized by sharp variation of Vp and Vp/Vs values. The spatial distribution of the 1998 aftershocks defines sharp vertical planar features consistent with the orientation of the main strike-slip fault while the 2004 aftershock pattern is quite diffuse. The tomographic images and the spatial distribution of the aftershocks indicate a fault zone characterized by branches. The fault plane solutions of the 1998 and 2004 main shocks show evidence of dextral strike-slip motion concordant with the kinematics of the Bovec-Krn fault. The largest aftershocks of both sequences are characterized by different types of focal mechanisms and variable plane orientation. The different focal mechanisms in the fault zone suggest that the mode of fracture of the largest aftershocks can be influenced by the fault strength heterogeneities imaged by the tomographic inversion. The Coulomb stress changes caused by the 1998 main shock was calculated to explore the stress transfer mechanism and the earthquake interactions. A positive correlation of the Coulomb stress increase is found with the largest aftershock (MD 4.6) and with a part of the aftershocks. Our modelling shows also that the Coulomb stress changes caused by the 1998 main shock and its largest aftershock were not sufficient to trigger the 2004 main shock. The locations of the 1998 and 2004 sequences correlate well with areas of increased Coulomb stress changes when the regional tectonic loading is considered.

Geodetically Inferred Coseismic and Postseismic Slip due to the M 5.4 31 October 2007 Alum Rock Earthquake

On 31 October 2007 the M 5.4 Alum Rock earthquake occurred near the junction between the Hayward and Calaveras faults in the San Francisco Bay Area, producing coseismic and postseismic displacements recorded by 10 continuously operating Global Positioning System (GPS) instruments. The cumulative postseismic displacements over the four months following the earthquake are linearly related to the cumulative number of aftershocks and are comparable in magnitude to the coseismic displacements. The postseismic signal suggests that, in addition to afterslip at seismogenic depths, localized right-lateral/reverse slip occurred on dipping shallow fault surfaces southwest of the Calaveras. The spatial distribution of slip inferred by inverting the GPS data is compatible with a model in which moderate Calaveras fault earthquakes rupture locked patches surrounded by areas of creep, afterslip, and microseismicity (Oppenheimer et al. , 1990). If this model and existing Calaveras fault slip rate estimates are correct, a slip deficit remains on the 2007 Alum Rock rupture patch that may be made up by aseismic slip or slip in larger earthquakes. Recent studies (e.g., Manaker et al. , 2005) suggest that at depth the Hayward and central Calaveras faults connect via a simple continuous surface illuminated by the Mission Seismic Trend (MST), implying that a damaging earthquake rupture could involve both faults (Graymer et al. , 2008). If this geometry is correct, the combined coseismic and postseismic slip we infer for the 2007 Alum Rock event predicts static Coulomb stress increases of ∼0.6 bar on the MST surface and on the northern Calaveras fault ∼5 km northwest of the Alum Rock hypocenter.

Kilauea slow slip events: Identification, source inversions, and relation to seismicity

Several slow slip events beneath the south flank of Kilauea Volcano, Hawaii, have been inferred from transient displacements in daily GPS positions. To search for smaller events that may be close to the noise level in the GPS time series, we compare displacement fields on Kilauea's south flank with displacement patterns in previously identified slow slip events. Matching displacement patterns are found for several new candidate events, although displacements are much smaller than previously identified events. One of the candidates, 29 May 2000, is coincident with a microearthquake swarm, as are all of the previously identified slow slip events. The microearthquakes follow the onset of slow slip, implying that they are triggered by stress changes during slip. The new slow slip event brings the total number of events on Kilauea, between 1997 and 2007, to eight, the smallest having MW = 5.3, and the largest having MW = 6.0. While the recurrence time between the four largest events is 2.11 ± 0.01 years, the repeat time for all eight events is 0.9 ± 0.6 years. We invert for the fault geometry and distribution of slip during the slow slip events. The optimal source depths of 5 km, assuming uniform slip dislocations in an elastic half‐space, are considerably shallower than the accompanying swarm earthquakes (6.5–8.5 km), which would place the earthquakes in a zone of decreased Coulomb stress. Inversions including the effects of topography and layered elastic structure in the forward models favor depths comparable to microearthquake depths, such that the earthquakes are located in a region of increased Coulomb stress. We also invert for time‐dependent fault slip directly from the 30 s GPS phase observations, constraining the source to the optimal uniform slip geometry. On the basis of these inversions, the larger events last between 1.5–2.2 days. The data are unable to resolve migration of slip along the fault. The temporal pattern of accompanying microearthquakes is consistent with the fault slip history assuming a seismicity rate theory based on rate and state‐friction, making the swarm earthquakes coshocks and aftershocks of the slow slip events.

Stress evolution following the 1999 Chi-Chi, Taiwan, earthquake: consequences for afterslip, relaxation, aftershocks and departures from Omori decay

SUMMARY We explore how Coulomb stress transfer and viscoelastic relaxation control afterslip and aftershocks in a continental thrust fault system. The 1999 September 21 M w = 7.6 Chi- Chi shock is typical of continental ramp-dsystems throughout the world, and so inferences drawn from this uniquely well-recorded event may be widely applicable. First, we find that the spatial and depth distribution of aftershocks and their focal mechanisms are consistent with the calculated Coulomb stress changes imparted by the coseismic rupture. Some 61 per cent of the M ≥ 2 aftershocks and 83 per cent of the M ≥ 4 aftershocks lie in regions for which the Coulomb stress increased by ≥0.1 bars, and there is a 11-12 per cent gain in the percentage of aftershocks nodal planes on which the shear stress increased over the pre- Chi Chi control period. Second, we find that afterslip occurred where the calculated coseismic stress increased on the fault ramp and dsubject to the condition that friction is high on the ramp and low on the d´ ecollement. Third, viscoelastic relaxation is evident from the fit of the post-seismic GPS data on the footwall. Fourth, we find that the rate of seismicity began to increase during the post-seismic period in an annulus extending east of the main rupture. The spatial extent of the seismicity annulus resembles the calculated ≥0.05-bar Coulomb stress increase caused by viscoelastic relaxation and afterslip, and we find a 9- 12 per cent gain in the percentage of focal mechanisms with >0.01-bar shear stress increases imparted by the post-seismic afterslip and relaxation in comparison to the control period. Thus, we argue that post-seismic stress changes can for the first time be shown to alter the production of aftershocks, as judged by their rate, spatial distribution, and focal mechanisms.

Detecting seismogenic stress evolution and constraining fault zone rheology in the San Andreas Fault following the 2004 Parkfield earthquake

We investigate temporal changes in seismic scatterer properties at seismogenic depth attributed to the 2004 M 6 Parkfield earthquake, making use of the San Andreas Fault Observatory at Depth repeating‐earthquake target sequences, as well as nearby similar‐earthquake aftershock clusters. We use a two‐step process: (1) observing temporal variations in the decorrelation index, D(t), reflecting changes in the scattered wavefield of repeating‐earthquake sequences and (2) estimating the spatial distribution of time‐dependent scatterers by using a larger‐aperture source array. We focus on three scatterers exhibiting clear time dependence, using pairs of earthquakes that span or follow the 2004 Parkfield earthquake. They are found to be located on the fault at the northernmost extent of coseismic rupture, beneath Middle Mountain, with a depth range of 11 to 17 km. The shallowest and most prominent scatterer is located near a region of increased Coulomb stress, as well as significant postseismic slip following the 2004 Parkfield earthquake, and a large M = 5 aftershock. The other two deeper ones are also in regions of increased Coulomb stress. We show that D(t)1/2 is expected to be proportional to the level of stress in the fault zone, and then we constrain the form of fault zone rheology by comparing the time dependence of D(t)1/2 with geodetic or seismic measures of strain rate, assuming a power law rheology between stress and strain rate characterized by exponent n. Such a comparison yields n ranging from 1.6 through 3.3, a value that is more consistent with ductile behavior, rather than frictional sliding, at the base of the seismogenic zone.

Increasing critical sensitivity of the Load/Unload Response Ratio before large earthquakes with identified stress accumulation pattern

The Load/Unload Response Ratio (LURR) method is proposed for short-to-intermediate-term earthquake prediction [Yin, X.C., Chen, X.Z., Song, Z.P., Yin, C., 1995. A New Approach to Earthquake Prediction — The Load/Unload Response Ratio (LURR) Theory, Pure Appl. Geophys., 145, 701–715]. This method is based on measuring the ratio between Benioff strains released during the time periods of loading and unloading, corresponding to the Coulomb Failure Stress change induced by Earth tides on optimally oriented faults. According to the method, the LURR time series usually climb to an anomalously high peak prior to occurrence of a large earthquake. Previous studies have indicated that the size of critical seismogenic region selected for LURR measurements has great influence on the evaluation of LURR. In this study, we replace the circular region usually adopted in LURR practice with an area within which the tectonic stress change would mostly affect the Coulomb stress on a potential seismogenic fault of a future event. The Coulomb stress change before a hypothetical earthquake is calculated based on a simple back-slip dislocation model of the event. This new algorithm, by combining the LURR method with our choice of identified area with increased Coulomb stress, is devised to improve the sensitivity of LURR to measure criticality of stress accumulation before a large earthquake. Retrospective tests of this algorithm on four large earthquakes occurred in California over the last two decades show remarkable enhancement of the LURR precursory anomalies. For some strong events of lesser magnitudes occurred in the same neighborhoods and during the same time periods, significant anomalies are found if circular areas are used, and are not found if increased Coulomb stress areas are used for LURR data selection. The unique feature of this algorithm may provide stronger constraints on forecasts of the size and location of future large events.

Rupture parameters of the 1999 Chamoli earthquake in Garhwal Himalaya: Constraints from aftershocks and change in failure stress

A positive correlation between areas of increased coulomb stress changes, induced by an earthquake, and the spatial distribution in the occurrence of aftershocks have been reported in recent years. We consider such a correlation between the aftershocks of March 29, 1999 Chamoli earthquake (Ms 6.6) that occurred in the Garhwal Himalaya, India, and the mainshock induced coulomb stresses, to constrain some of the rupture parameters of the mainshock. Most of the aftershocks occurred above and near the updip edge of the rupture implying reactivation of pre-existing thrust fault of the Himalayan wedge. The 15 × 12 km 2 rupture appears to be lying above the detachment, the plate boundary fault between the underthrusting Indian shield rocks and the Himalayan wedge rocks, in the Basement Thrust Front (BTF) zone.

Response of seismicity to Coulomb stress triggers and shadows of the 1999 Mw = 7.6 Chi‐Chi, Taiwan, earthquake

The correlation between static Coulomb stress increases and aftershocks has thus far provided the strongest evidence that stress changes promote seismicity, a correlation that the Chi‐Chi earthquake well exhibits. Several studies have deepened the argument by resolving stress changes on aftershock focal mechanisms, which removes the assumption that the aftershocks are optimally oriented for failure. Here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand. For Chi‐Chi we find a 28% increase for thrust and an 18% increase for strike‐slip mechanisms, commensurate with increases reported for other large main shocks. However, perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows, or sites of Coulomb stress decrease. Detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate, conditions that are met at Chi‐Chi. We find four lobes with statistically significant seismicity rate declines of 40–90% for 50 months, and they coincide with the stress shadows calculated for strike‐slip faults, the dominant faulting mechanism. The rate drops are evident in uniform cell calculations, 100‐month time series, and by visual inspection of the M ≥ 3 seismicity. An additional reason why detection of such declines has proven so rare emerges from this study: there is a widespread increase in seismicity rate during the first 3 months after Chi‐Chi, and perhaps many other main shocks, that might be associated with a different mechanism.

Stress interaction between subduction earthquakes and forearc strike‐slip faults: Modeling and application to the northern Caribbean plate boundary

Strike‐slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers. We explore the interaction between thrust events on the subduction interface and strike‐slip faults within the forearc region using three‐dimensional models of static Coulomb stress change. Model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the Coulomb stress on strike‐slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region. In contrast, subduction events with slip vectors perpendicular to the trench axis enhance the Coulomb stress on strike‐slip faults farther back in the forearc, while reducing the stress adjacent to the trench. A significant contribution to Coulomb stress increase on strike‐slip faults in the back region of the forearc comes from “unclamping” of the fault, i.e., reduction in normal stress due to thrust motion on the subduction interface. We argue that although Coulomb stress changes from individual subduction earthquakes are ephemeral, their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant. We use the Coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the Caribbean subduction zone. Subduction earthquakes with slip vectors nearly perpendicular to the Caribbean trench axis is dominant in the Hispaniola segment, where the strike‐slip faults are more than 60 km inland from the trench. In contrast, subduction slip motion is nearly parallel to the Caribbean trench axis along the Puerto Rico segment, where the strike‐slip fault is less than 15 km from the trench. This observed jump from a strike‐slip fault close to the trench axis in the Puerto Rico segment to the inland faults in Hispaniola is explained by different distributions of Coulomb stress in the forearc region of the two segments, as a result of the change from the nearly trench parallel slip on the Puerto Rico subduction interface to the more perpendicular subduction slip beneath Hispaniola. The observations and modeling suggest that subduction‐induced strike‐slip seismic hazard to Puerto Rico may be smaller than previously assumed but the hazard to Hispaniola remains high.

Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike‐slip faults

We argue that key features of thrust earthquake triggering, inhibition, and clustering can be explained by Coulomb stress changes, which we illustrate by a suite of representative models and by detailed examples. Whereas slip on surface‐cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress on some nearby zones, particularly above the source fault. Blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks. Short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults. We calculate that during a progressive thrust sequence in central California the 1983 Mw = 6.7 Coalinga earthquake brought the subsequent 1983 Mw = 6.0 Nuñez and 1985 Mw = 6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to Coulomb failure. The idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events, in agreement with findings of prior investigations. Subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust‐faulting events on the periphery of the seismic rupture and its downdip extension. These features are evident in aftershocks of the 1957 Mw = 9.1 Aleutian and other large subduction earthquakes. We further examine stress changes on the rupture surface imparted by the 1960 Mw = 9.5 and 1995 Mw = 8.1 Chile earthquakes, for which detailed slip models are available. Calculated Coulomb stress increases of 2–20 bars correspond closely to sites of aftershocks and postseismic slip, whereas aftershocks are absent where the stress drops by more than 10 bars. We also argue that slip on major strike‐slip systems modulates the stress acting on nearby thrust and strike‐slip faults. We calculate that the 1857 Mw = 7.9 Fort Tejon earthquake on the San Andreas fault and subsequent interseismic slip brought the Coalinga fault ∼1 bar closer to failure but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 Mw = 7.3 Kern County shock but inhibited slip on the left‐lateral Garlock fault, which has not ruptured since 1857. We thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales.