Systematic assessment of the static stress triggering hypothesis using interearthquake time statistics

A systematic analysis is made of static Coulomb stress changes and earthquake occurrence in the area of the North Aegean Sea, Greece, in order to assess the prospect of using static stress changes to construct a regional earthquake likelihood model. The earthquake data set comprises all events of magnitude M ≥ 5.2 which have occurred since 1964. This is compared to the evolving stress field due to constant tectonic loading and perturbations due to coseismic slip associated with major earthquakes (M ≥ 6.4) over the same period. The stress was resolved for sixteen fault orientation classes, covering the observed focal mechanisms of all earthquakes in the region. Analysis using error diagrams shows that earthquake occurrence is better correlated with the constant tectonic loading component of the stress field than with the total stress field changes since 1964, and that little, if any, information on earthquake occurrence is lost if only the maximum of the tectonic loading over the fault orientation classes is considered. Moreover, the information on earthquake occurrence is actually increased by taking the maximum of the evolving stress field since 1964, and of its coseismic-slip component, over the fault orientation classes. The maximum, over fault orientation classes, of linear combinations of the tectonic loading and the evolving stress field is insignificantly better correlated with earthquake occurrence than the maximum of the tectonic loading by itself. A composite stress-change variable is constructed from ordering of the maximum tectonic loading component and the maximum coseismic-slip component, in order to optimize the correlation with earthquake occurrence. The results indicate that it would be difficult to construct a time-varying earthquake likelihood model from the evolving stress field that is more informative than a time-invariant model based on the constant tectonic loading.

The role of static versus dynamic stresses in earthquake clusters is unclear. Analysis of earthquakes triggered by a dyke intrusion at an Icelandic volcano unambiguously demonstrates that static stresses are important for earthquake clustering. Large earthquakes can generate small changes in static stress: increases that trigger aftershock swarms, or reductions that create a region of reduced seismicity—a stress shadow1,2. However, seismic waves from large earthquakes also cause transient dynamic stresses that may trigger seismicity3,4. This makes it difficult to separate the relative influence of static and dynamic stress changes on aftershocks. Dyke intrusions do not generate dynamic stresses, so provide an unambiguous test of the stress shadow hypothesis. Here we use GPS and seismic data to reconstruct the intrusion of an igneous dyke that is 46 km long and 5 m wide beneath Barðarbunga Volcano, central Iceland, in August 2014. We find that during dyke emplacement, bursts of seismicity at a distance of 5 to 15 km were first triggered and then abruptly switched off as the dyke tip propagated away from the volcano. We calculate the evolving static stress changes during dyke propagation and show that the stressing rate controls both the triggering and then suppression of earthquake rates in three separate areas adjacent to the dyke. Our results imply that static stress changes help control earthquake clustering. Similar small static stress changes may be important for triggering seismicity near geothermal areas, regions being hydrofractured and deflating oil and gas fields.

The 1992 M7.3 Landers earthquake may have played a role in triggering the 1999 M7.1 Hector Mine earthquake as suggested by their close spatial (∼20 km) proximity. Current investigations of triggering by static stress changes produce differing conclusions when small variations in parameter values are employed. Here I test the hypothesis that large‐amplitude dynamic stress changes, induced by the Landers rupture, acted to promote the Hector Mine earthquake. I use a flat layer reflectivity method to model the Landers earthquake displacement seismograms. By requiring agreement between the model seismograms and data, I can constrain the Landers main shock parameters and velocity model. A similar reflectivity method is used to compute the evolution of stress changes. I find a strong positive correlation between the Hector Mine hypocenter and regions of large (>4 MPa) dynamic Coulomb stress changes (peak Δσf(t)) induced by the Landers main shock. A positive correlation is also found with large dynamic normal and shear stress changes. Uncertainties in peak Δσf(t) (1.3 MPa) are only 28% of the median value (4.6 MPa) determined from an extensive set (160) of model parameters. Therefore the correlation with dynamic stresses is robust to a range of Hector Mine main shock parameters, as well as to variations in the friction and Skempton's coefficients used in the calculations. These results imply dynamic stress changes may be an important part of earthquake trigging, such that large‐amplitude stress changes alter the properties of an existing fault in a way that promotes fault failure.

The 2023 Mw 7.8 and Mw 7.6 Kahramanmaraş earthquake doublet produced complex deformation patterns across southeastern Turkey, offering a rare opportunity to investigate the response of non-main fault structures to large strike-slip earthquakes. Using time-series Interferometric Synthetic Aperture Radar (TS-InSAR) analysis, we quantify both coseismic and postseismic deformation in the epicentral region, with a particular focus on off-fault structures that were previously considered inactive. Our results reveal that, beyond the primary rupture zones, several off-faults exhibit significant postseismic deformation characterized by increased deformation rates, indicating fault reactivation rather than residual coseismic effects. To explore the driving mechanisms of this off-fault activation, we compare the observed deformation patterns with modeled static Coulomb stress changes and dynamic stress perturbations associated with the earthquake doublet. The spatial distribution of activated off-faults shows a strong correlation with areas experiencing positive Coulomb stress changes and regions affected by strong dynamic shaking. Temporally, the deformation signals display heterogeneous behavior, ranging from rapid early postseismic transients to sustained deformation persisting for months to years after the mainshocks. These observations suggest that the combined effects of static and dynamic stress transfer played a key role in triggering off-fault deformation following the Kahramanmaraş earthquakes. Our study highlights the importance of off-fault structures in accommodating postseismic strain and emphasizes their potential contribution to regional seismic hazard, which is commonly underestimated in traditional fault-based assessments.

[1] Increases in seismicity have been widely observed at varying distances from the source area following large earthquakes. The increased number of earthquakes are usually called aftershocks if the area is within a rupture length of the mainshock, and called remotely triggered events if they are well beyond that distance. These earthquakes can be explained as being induced by static and/or dynamic stress changes due to the mainshock. However, clear observations of dynamic triggering have been inadequate to differentiate between the two mechanisms. This study shows that early post-seismic events triggered by the 2011 Mw 9.0 Tohoku-Oki earthquake systematically propagated over Japan in a southwestern direction, associated with the strong seismic waves from the source. The propagation front was consistent with the arrivals of large amplitude surface waves traveling at 3.1 to 3.3 km/s and extending to a distance of 1,350 km. There were no observations of triggered earthquakes in the northern direction. Dynamic stress changes toward the north were comparable to or smaller than those necessary for triggering in the southwestern direction. Static stress changes were one to two orders smaller than dynamic stress changes at remote distance, indicating that static stress was not the main mechanism of the triggering. Furthermore, the dynamic stress/strain changes play an important role for remote triggering if the value is more than ∼500 kPa in stress or ∼10−6 in strain.

The South Iceland Seismic Zone experienced the largest earthquakes for 88 years in June 2000, with a MS = 6.6 event on June 17, followed by another MS = 6.6 earthquake on June 21. These events occurred on two parallel N‐S striking, right‐lateral strike slip faults, separated by about 17 km. We calculate the static Coulomb stress change for the June 17 and 21 earthquakes using a distributed slip model derived from joint inversion of InSAR and GPS data. We find that the static stress change caused by the June 17 event is about 0.1 MPa at the location of the June 21 hypocenter, promoting failure on the second fault. Locations of aftershocks agree well with areas of increased Coulomb failure stress. Our calculations indicate that positive stress changes due to the two earthquakes make the area west of the June 21 rupture the most likely site of the next large earthquake in South Iceland.

For decades there has been a debate about the relative effects of dynamic versus static stress triggering of aftershocks. According to the static Coulomb stress change hypothesis, aftershocks should not occur in stress shadows—regions where static Coulomb stress has been reduced. We show that static stress shadows substantially influence aftershock occurrence following three M ≥ 7 California mainshocks. Within the modeled static Coulomb stress shadows, the aftershock rate is an order of magnitude lower than in the modeled increase regions. However, the earthquake rate in the stress shadows does not decrease below the background rate, as predicted by Coulomb stress change models. Aftershocks in the stress shadows exhibit different spatial–temporal characteristics from aftershocks in the stress increase regions. The aftershock rate in the stress shadows decays as a power law with distance from the mainshock, consistent with a simple model of dynamic stress triggering. These aftershocks begin with a burst of activity during the first few days after the mainshock, also consistent with dynamic stress triggering. Our interpretation is that aftershock sequences are the combined result of static and dynamic stress triggering, with an estimated ∼34% of aftershocks due to dynamic triggering and ∼66% due to static triggering.

Induced seismic events with moment magnitude larger than 2.0 occurred during and after a hydraulic stimulation for creation of reservoir of Engineered Geothermal System (EGS) at Basel, Switzerland in 2006. The large events considerably damaged the constructions and resulted in shutdown of EGS project in Basel after risk analyses for further large events. The authors have analyzed the large events and investigated fundamental characteristics of the large events, including time series of occurrence, epicenter location, waveform similarity to the other events, Fault Plane Solutions (FPSs), and source parameters. In this study, we investigated relation between the occurrence of the large seismic events and some possible trigger mechanisms for shear slip of the large seismic events. One of the possible trigger mechanisms is an increase in pore pressure. We estimated the critical pore pressure from the Coulomb failure criterion using the information of the stress state at Basel and the orientation of the fault planes. We observed that the large events occurred just after the final stage of the stimulation at relatively low critical pore pressure. We also estimated diffusion pressure based on diffusion model and the results showed diffusion pressure disturbed even after two months of the stimulation. The other possible trigger mechanism is the static stress re-distribution caused by the preceding events. The static stress changes were calculated from orientation of the fault planes and seismic source parameters as input. The result showed that the stress state on the fault plane became stable for most of large events and that the magnitude of the stress change was negligibly smaller than change in pore pressure.

In this research the Coulomb stress changes due to earthquake sequence that began in the 1936 to 1997 earthquakes in Eastern Iran and their triggering effect on the rupturing of adjacent faults were investigated. Obtained results revealed a well triggering relation between main shocks and later events on the Dasht-e-Bayaz and Abiz faults, respectively. The calculated stress maps indicate that positive and negative zones of Coulomb stress changes with increasing lobes of 0.1‒1 bar are found in the off-faults tips, located at rupture terminates while the decreasing Coulomb stress lobes lie in the high angle relative to the rupture plane or relieve stress along the ruptures. Cross sections drawn respect to the source and received faults confirm the results. Also, the stress perturbations resulted from successive earthquakes and the spatial patterns of the following earthquake distribution have a good consistency. This correlation shows that the population of secondary faults and subsequent earthquakes can be encouraged in the future by coseismic Coulomb stress changes due to mainshock and background loading.

The Hojedk area in the southeastern Iran experienced three earthquakes during 12 days in December 2017. These events occurred in proximity to a region that has experienced several moderate and large events in the past. In this paper, the role of Coulomb stress changes in occurrence of the Hojedk cluster was investigated, and also the produced stress changes due to these events on the surrounding and optimally oriented faults was calculated. Previous earthquakes (events in the Golbaf–Sirch region, the Shahdad slip, and the Dahuiyeh–Zarand earthquake) imparted positive stress changes on the fault planes of the Hojedk earthquakes, especially on the ruptured plane of the first main shock. The Hojedk first main shock triggered the second one by imparting maximum stress changes of about 11.7 MPa on the fault plane of this event, and these two events brought the third main shock to failure by producing more than 6.0 MPa stress changes on its ruptured plane. Among the major active faults in the surrounding area, the middle part of the Nayband fault, northern part of the Golbaf–Sirch fault, and southern part of the Kuhbanan fault received positive stress changes and southen parts of the Ravar and all parts of the Chatrud faults received negative stress changes due to the Hojedk earthquakes. Furthermore, our results for correlation between Coulomb stress changes and seismicity distribution showed that following the Dahuiyeh–Zarand earthquake, the majority of the seismicity located on the positive stress area and Coulomb stress changes have a controlling role on the spatial distribution of seismicity even after a decade. The occurrence of the Hojedk first main shock affects the spatial distribution of aftershocks until the occurrence of the next main hocks. By triggering the next one, the stress patterns change in the area, and seismicity follows from the stress patterns of the last event.

Hillslope materials and processes. M.J. Selby, 1993. Oxford University Press. xv + 451 pages. Price: £48.00. ISBN 0-19-874165-0

Seismicity rate changes in the Hellenic subduction zone (southern Aegean Sea) were studied by applying the Dieterich (1994) rate/state‐dependent friction model combined with static Coulomb stress changes (ΔCFF). The coseismic slip of the strongest earthquakes (with moment magnitude, M w≥6.0) was considered to contribute to the stress field evolution along with the continuous tectonic loading. Stress changes were calculated just after each strong event, and their influence was examined in connection with the smaller magnitude earthquake occurrence rates. Qualitative and quantitative comparison between the observed seismicity rates (smoothed by the means of a probability density function) and the expected ones, as they were forecasted by the rate/state model, were investigated for the interseismic periods (study periods) between subsequent strong earthquakes. The calculations aim to identify areas of expected increased seismicity rates as candidates to accommodate enhanced seismic activity. Results strongly depend on the determination (smoothing) of the unperturbed (reference) seismicity rates and data adequacy. Seismicity rate results were filtered by certain criteria and constraints, in an attempt to overcome model uncertainties (epicentral errors, rupture models, parameter values) and to provide reliable results for specific areas of major interest, that is, in areas with increased positive Coulomb stress changes values. The modeling approach resulted in satisfactory correlation between observed and synthetic seismicity rates and, in particular, the two strong ( M w≥6) earthquakes that occurred in 2013 are located in areas of increased expected seismicity rates.

Coulomb stress changes and seismicity in central Taiwan due to the Nantou blind-thrust earthquakes in 2013

It has been globally documented over different tectonic environments that Coulomb static stress changes caused by a mainshock can promote or demote stresses along the neighboring faults and thus triggers or delays following seismicity. In the present study Coulomb stress changes of the earthquakes in the Lake Van area are calculated using available data and the likely source faults. The calculated stress change maps demonstrate that the large earthquakes in the Lake Area are mostly stressed by the preceding earthquakes, suggesting earthquake rupture interactions. It is further suggested that Coulomb stress maps could be used for constraining the likely locations of the future large earthquakes and in the earthquake hazard mitigation studies.

The Kaikoura earthquake on November 14, 2016 is one of the largest and most complex earthquakes in New Zealand since 1947. Despite the fact that it has ruptured about 12 separate faults, triggered 2132 aftershocks within one week of the mainshock and induced considerable stress changes, few studies have been conducted to comprensively investigate the characteristics. The current study examines the horizontal and vertical displacements as well as the stress and gravity changes, aftershock distributions and also find out whether these changes affect the surrounding regions along the complex fault systems. The study covers the entire area affected by the Kaikoura event, which includes the northern part of the South Island and the southern part of the North Island. The dislocation theory was employed to evaluate the coseismic slip model on the multiple faults. The displacement results revealed that the maximum horizontal displacement is about 6 m and the vertical about 2 m, which are reasonably consistent with earlier study findings. Besides, the stress and gravity changes are quite complicated and inhomogeneous as evidenced by our coseismic model, demonstrating the complexity of the Kaikoura earthquake as well. Almost all the aftershocks are distributed in places where the stress and gravity change are found to be significant. In order to investigate the stability of our stress change models, we applied different friction coefficients and receiver fault parameters. The results justify the friction coefficient (μ=0.4) and the receiver fault parameters (230°, 70°, 150°) are suitable to define good stress change estimates. According to the stress change results at 15 km depth, the northern parts of the mainshock region, Hundalee fault, Humps fault and Jordan thrust areas together with the Wellington area are closer to failure and situated in a seismic risk zone. The multidimensional analysis adopted in this paper is helpful for making decisions and applications of stress and gravity change models in assessing seismic hazards.