Coulomb Static Stress Changes Research Articles

Pore Fluid Pressure in St. Gallen Geothermal Field (Switzerland) Based on Earthquake Focal Mechanisms

AbstractInduced seismicity represents a negative drawback during subsurface exploitation for geothermal energy production. Understanding the triggering mechanisms of induced earthquakes can help implement effective seismic hazard mitigation actions. Among the triggering mechanisms, the pore fluid pressure is of primary importance. Here we provide a static picture of the excess pore fluid pressure at the hypocenters of a seismic sequence induced at the deep geothermal field in St. Gallen, Switzerland, in July 2013. We find that in addition to the Coulomb static stress change, fluids play a key role in promoting the sequence. The estimated excess pore fluid pressure for approximately half of the earthquakes is higher than the injection pressure necessary during the well control phase to fight the unexpected gas kick, that accidently occurred during field operations when a trap of overpressured gas was broken.

Evaluating the coulomb static stress change and fault interaction in an extensional intra-volcanic arc: 1000 years of earthquake history in the Acambay Graben, Trans-Mexican Volcanic Belt

One of the main challenges in slow slip-rate intra-continental fault systems is evaluating which fault could generate the next large earthquake. Therefore, it is crucial to characterize the spatiotemporal distribution of the seismicity and to understand the influence of moderate to large earthquakes on successive events. The Acambay Graben, in the central part of the Trans-Mexican Volcanic Belt, is an intra-continental normal fault system and the source of the 1912 Acambay earthquake (mb = 6.9) that ruptured mainly the Acambay-Tixmadejé fault as well as other fault segments of the graben. Paleoseismological studies have identified earlier events significant events produced by other graben faults, such as the Venta de Bravo Fault event occurred between 1026 and 1700 CE. In this study, we model the static Coulomb stress transfer related to the 1026–1700 and the 1912 CE events showing that the older event may have set the conditions to induce or accelerate the historical 1912 Acambay earthquake. In the same way, this last event could have triggered the Maravatío earthquake (February 22, 1979, Mw = 5.3). Furthermore, the cumulative variation of the Coulomb stress (Ev1 = 0.5–10 bar, Ev2 = 5–30 bar, Ev3 = 0.5–1 bar) corresponding to the three events reveals a spatial correlation between the areas that have experienced an increase in static stress with the regional distribution of the instrumental seismicity. The obtained results suggest that some active faults of the graben, which have accumulated static stress during the last 1000 years, are potential sources for the next fault rupture, i.e., faults that would need to be better studied and characterized. This information is of great importance to improve the seismic hazard assessment and risk estimations in the heavily populated Central Mexico region with more than 24 million people and better understand the fault interaction in slow slip-rate intra-continental fault systems.

Usefulness of Coulomb Static Stress Changes in Earthquake Hazard Studies: An Example from the Lake Van Area, Eastern Turkey

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 northern Thessaly strong earthquakes of March 3 and 4, 2021, and their neotectonic setting

A sequence of earthquakes occurred on March 3rd and 4th in Northern Thessaly, northern Greece, associated with previously unknown, blind normal faults within the crystalline Palaeozoic basement of the Pelagonian geotectonic zone. Surficial ground deformation, such as liquefaction phenomena in fluvial plains, as well as soil fissures and rock falls, have been mapped. Geological indications of the unmapped seismic fault, i.e., reactivated shear zones, open cracks, etc., have been identified within the bedrock. Based on geological indications, the main fault projection to the surface could be considered a 15 km NW-SE trending structure and average dip of 45o to the NE. The seismic source of the main shock was modelled, and the Coulomb static stress changes calculated for receiver faults similar to the source. The determination of the active tectonic regime of the region by geodetic data and the well-known faults of NE Thessaly plain are also presented, as well as the revised historical and instrumental seismicity. This earthquake raises new concerns and challenges, revising some established views, such as the status of main stress orientations, the orientation of active tectonic structures, the occurrence of a seismogenic fault in a mountainous massif of crystalline rocks without typical geomorphological expression and the role of blind faults in Seismic Hazard Assessment.

Volcano‐Tectonic Interactions at Sabancaya Volcano, Peru: Eruptions, Magmatic Inflation, Moderate Earthquakes, and Fault Creep

AbstractWe present evidence of volcano‐tectonic interactions at Sabancaya volcano that we relate to episodic magma injection and high regional fluid pore pressures. We present a surface deformation time series at Sabancaya including observations from ERS‐1/2, Envisat, Sentinel‐1, COSMO‐SkyMed, and TerraSAR‐X that spans June 1992 to February 2019. These data show deep‐seated inflation northwest of Sabancaya from 1992–1997 and 2013–2019, as well as creep and rupture on multiple faults. Afterslip on the Mojopampa fault following a 2013 MW 5.9 earthquake is anomalously long lived, continuing for at least 6 years. The best fit fault plane for the afterslip is right‐lateral motion on an EW striking fault at 1 km depth. We also model surface deformation from two 2017 earthquakes (MW 4.4 and MW 5.2) on unnamed faults, for which the best fit models are NW striking normal faults at 1–2 km depth. Our best fit model for a magmatic inflation source (13 km depth, volume change of 0.04 to 0.05 km3 yr−1) induces positive Coulomb static stress changes on these modeled fault planes. Comparing these deformation results with evidence from satellite thermal and degassing data, field observations, and seismic records, we interpret strong pre‐eruptive seismicity at Sabancaya as a consequence of magmatic intrusions destabilizing tectonic faults critically stressed by regionally high fluid pressures. High fluid pressure likely also promotes fault creep driven by static stress transfer from the inflation source. We speculate that combining high pore fluid pressures with sufficiently large, offset magmatic inflation can promote strong earthquakes during volcanic unrest.

Evaluation of Seismicity Triggering: Insights from the Coulomb Static Stress Changes after the 30 August 1968 Dasht-e-Bayaz Earthquake (Mw = 7.1), Eastern Iran

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.

Geomechanical model for a seismically active geothermal field: Insights from the Tinguiririca volcanic-hydrothermal system

In this work, we develop a multidisciplinary approach to investigate a geothermal system located at the volcanic arc of a subduction-related orogen and highlight the interplay between active tectonism, stress field and fluid migration. By using results of field investigations from the Tinguiririca geothermal field in the High Andes of Chile (35°S), empirical analysis, and numerical models of static stress variations, we proposed a geomechanical model for evaluating the distribution of hydrothermal manifestations in a seismically-active region. The present geomechanical model follows four major steps: (1) development of the 3D structural model of fault pattern; (2) estimation of the in-situ stress field; (3) calculation of the resolved-shear-to-normal-stress ratio (slip tendency) on each fault with varying geomechanical parameters (coefficient of friction, pore pressure and cohesion) as inputs; and (4) estimation of Coulomb static stress changes as a consequence of failure in a nearby fault. Through combination of all these analyses, we characterize in detail both the active deformation in the geothermal field and its relationship with hot fluid migration.

Small Earthquakes Matter in Injection‐Induced Seismicity

AbstractA simplistic triggering mechanism, pore pressure increase from injection, has been the focus of injection‐induced seismicity studies for decades. Research into other possible mechanisms, like poroelastic stress changes, is ongoing, but there has been relatively little focus on earthquake interaction. While studies have looked at how moderate‐magnitude events (M ≥ 3.0) may trigger larger magnitude‐induced seismicity, research into the cumulative effect of the hundreds to thousands of small‐magnitude (M ≤ 3.0) events is lacking. Here we use generic models to compare the possible stress changes from pore pressure increase and from earthquake interactions of small‐magnitude events. We find that the area of increased pore pressure is much larger than that of positive Coulomb static stress transfer; however, maximum Coulomb static stress change is larger than maximum pore pressure increase. We argue that, yes, small earthquakes do matter, and their interaction may be an important triggering mechanism to consider.

Evidence for Static and Dynamic Triggering of Seismicity Following the 24 August 2016, M W = 6.0, Amatrice (Central Italy) Earthquake

The recent 24 August 2016, MW = 6.0, Amatrice earthquake (central Italy) and the resulting aftershock sequence represents a test case to investigate the static and dynamic stress/strain contribution to the triggering of the sequence. Here we test the hypothesis that either static or dynamic stress contribution alone may not be sufficient to explain the aftershock distribution. We first inferred the horizontal source rupture directivity together with an estimate of the surface fault projection from the analysis of the peak-ground velocity. We computed the peak-dynamic strain field, modified by source directivity, using the peak-ground velocity as a strain proxy and the Coulomb static stress change. Finally, we compared the seismicity rate map (β-statistic) with each of the estimated fields to investigate correlations with the aftershock pattern. We found that the area of the highest values of the estimated peak-dynamic strain field better reproduces the observed asymmetry in the aftershock distribution. This suggests that, in addition to Coulomb static stress change, dynamic strain enhanced by source directivity, contributed to the triggering of the Amatrice earthquake aftershocks.

Investigating Triggering of the Aftershocks of the 2014 Napa Earthquake

Abstract The occurrence of the M w 6.0 South Napa California earthquake, on 24 August 2014 at 03:20 a.m. local time, triggered discussion in the seismological community about the level of damage associated with such a moderate‐magnitude event and about the geometry and orientation of the causative fault. In addition, coulomb static stress change mapping does not seem to be able to fully explain near‐source aftershock distribution. Here, we find clear evidence of a north‐northwest source directivity from the analysis of the spatial distribution of peak ground motion. The area of the highest values of the estimated peak dynamic strain field, computed accounting for fault extent and source directivity, agrees with the near‐source aftershock distribution. This might suggest that, in addition to coulomb static stress change, dynamic strain also contributed to the triggering of near‐source Napa earthquake aftershocks. The approach used here might be useful to identify areas likely prone to aftershock occurrence.

Structural context of the 2015 pair of Nepal earthquakes (Mw 7.8 and Mw 7.3): an analysis based on slip distribution, aftershock growth, and static stress changes

The Great Himalayan earthquakes are believed to originate on the Main Himalayan Thrust, and their ruptures lead to deformation along the Main Frontal Thrust (MFT). The rupture of the April 25, 2015 (Mw 7.8), earthquake was east-directed, with no part relayed to the MFT. The aftershock distribution, coseismic elevation change of similar to 1 m inferred from the InSAR image, and the spatial correspondence of the subtle surface deformations with PT2, a previously mapped out-of-sequence thrust, lead us to explore the role of structural heterogeneities in constraining the rupture progression. We used teleseismic moment inversion of P- and SH-waves, and Coulomb static stress changes to map the slip distribution, and growth of aftershock area, to understand their relation to the thrust systems. Most of the aftershocks were sourced outside the stress shadows (slip > 1.65 m) of the April 25 earthquake. The May 12 (Mw 7.3) earthquake that sourced on a contiguous patch coincides with regions of increased stress change and therefore is the first known post-instrumentation example of a late, distant, and large triggered aftershock associated with any large earthquake in the Nepal Himalaya. The present study relates the slip, aftershock productivity, and triggering of unbroken stress barriers, to potential out-of-sequence thrusts, and suggests the role of stress transfer in generating large/great earthquakes.

The Aftershock Sequence of the 2011 Mineral, Virginia, Earthquake: Temporal and Spatial Distribution, Focal Mechanisms, Regional Stress, and the Role of Coulomb Stress Transfer

Abstract The aftershocks of the 23 August 2011 M w 5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10–20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and ∼80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north‐northwest‐trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north‐northeast‐trending nodal planes. A concentration of aftershocks in the 4–6 km depth range near the mainshock are mostly of reverse type but display a 90° range of nodal‐plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depth‐dependent stress field prior to the occurrence of the mainshock. Analysis of earthquake occurrence using a new magnitude scale () indicates a Gutenberg–Richer law b ‐value of 0.864 and an Omori law p ‐value of 1.085, indicative of a typical aftershock sequence. Online Material: Catalogs of aftershock location, magnitude, and focal mechanisms.

Study on Aftershocks Triggering by Static Stress Changes of the Minxian-Zhangxian 6.6 Earthquake

On July 22, 2013, an Ms6.6 earthquake occurred in Minxian-Zhangxian, Gansu Province, China, which caused serious damages. Because of the abundance and clear relationship with the main shocks, aftershocks sequences are typical types of behavior and provide useful data sets. To better understand the aftershocks triggering by static stress changes of the main earthquake, based on Okada’s elastic half-space model, we used accept fault plane consistent with the source and accept fault plane as the optimal models to calculate the stress changes on aftershock focuses by the Minxian-Zhangxian 6.6 Earthquake respectively. The results show that the latter model is better than former, more than 90% of aftershocks located in NWW and SEE, the stress increased areas, which is consistent with strike of Lintan-Tanchang fault (LTF), in other words, the Coulomb static stress changes of the main shock can induce the aftershocks.

Renewal models and coseismic stress transfer in the Corinth Gulf, Greece, fault system

AbstractWe model interevent times and Coulomb static stress transfer on the rupture segments along the Corinth Gulf extension zone, a region with a wealth of observations on strong‐earthquake recurrence behavior. From the available information on past seismic activity, we have identified eight segments without significant overlapping that are aligned along the southern boundary of the Corinth rift. We aim to test if strong earthquakes on these segments are characterized by some kind of time‐predictable behavior, rather than by complete randomness. The rationale for time‐predictable behavior is based on the characteristic earthquake hypothesis, the necessary ingredients of which are a known faulting geometry and slip rate. The tectonic loading rate is characterized by slip of 6 mm/yr on the westernmost fault segment, diminishing to 4 mm/yr on the easternmost segment, based on the most reliable geodetic data. In this study, we employ statistical and physical modeling to account for stress transfer among these fault segments. The statistical modeling is based on the definition of a probability density distribution of the interevent times for each segment. Both the Brownian Passage‐Time (BPT) and Weibull distributions are tested. The time‐dependent hazard rate thus obtained is then modified by the inclusion of a permanent physical effect due to the Coulomb static stress change caused by failure of neighboring faults since the latest characteristic earthquake on the fault of interest. The validity of the renewal model is assessed retrospectively, using the data of the last 300 years, by comparison with a plain time‐independent Poisson model, by means of statistical tools including the Relative Operating Characteristic diagram, the R‐score, the probability gain and the log‐likelihood ratio. We treat the uncertainties in the parameters of each examined fault source, such as linear dimensions, depth of the fault center, focal mechanism, recurrence time, coseismic slip, and aperiodicity of the statistical distribution, by a Monte Carlo technique. The Monte Carlo samples for all these parameters are drawn from a uniform distribution within their uncertainty limits. We find that the BPT and the Weibull renewal models yield comparable results, and both of them perform significantly better than the Poisson hypothesis. No clear performance enhancement is achieved by the introduction of the Coulomb static stress change into the renewal model.

Comparison of characteristic and Gutenberg-Richter models for time-dependent M ≥ 7.9 earthquake probability in the Nankai-Tokai subduction zone, Japan

SUMMARY Earthquake forecasts are usually underinformed, and can be plagued by uncertainty in terms of the most appropriate model, and parameter values used in that model. In this paper, we explore the application of two different models to the same seismogenic area. The first is a renewal model based on the characteristic earthquake hypothesis that uses historical/palaeoseismic recurrence times, and fixed rupture geometries. The hazard rate is modified by the Coulomb static stress change caused by nearby earthquakes that occurred since the latest characteristic earthquake. The second model is a very simple earthquake simulator based on plate-motion, or fault-slip rates and adoption of a Gutenberg–Richter magnitude–frequency distribution. This information is commonly available even if historical and palaeoseismic recurrence data are lacking. The intention is to develop and assess a simulator that has a very limited parameter set that could be used to calculate earthquake rates in settings that are not as rich with observations of large-earthquake recurrence behaviour as the Nankai trough. We find that the use of convergence rate as a primary constraint allows the simulator to replicate much of the spatial distribution of observed segmented rupture rates along the Nankai, Tonankai and Tokai subduction zones. Although we note rate differences between the two forecast methods in the Tokai zone, we also see enough similarities between simulations and observations to suggest that very simple earthquake rupture simulations based on empirical data and fundamental earthquake laws could be useful forecast tools in information-poor settings.

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.

Seismicity Rate Changes along the Central California Coast due to Stress Changes from the 2003 M 6.5 San Simeon and 2004 M 6.0 Parkfield Earthquakes

Abstract We investigated the relationship between seismicity rate changes and modeled Coulomb static stress changes from the 2003 M 6.5 San Simeon and the 2004 M 6.0 Parkfield earthquakes in central California. Coulomb stress modeling indicates that the San Simeon mainshock loaded parts of the Rinconada, Hosgri, and San Andreas strike-slip faults, along with the reverse faults of the southern Los Osos domain. All of these loaded faults, except for the San Andreas, experienced a seismicity rate increase at the time of the San Simeon mainshock. The Parkfield earthquake occurred 9 months later on the loaded portion of the San Andreas fault. The Parkfield earthquake unloaded the Hosgri fault and the reverse faults of the southern Los Osos domain, which both experienced seismicity rate decreases at the time of the Parkfield event, although the decreases may be related to the decay of San Simeon-triggered seismicity. Coulomb stress unloading from the Parkfield earthquake appears to have altered the aftershock decay rate of the southern cluster of San Simeon aftershocks, which is deficient compared to the expected number of aftershocks from the Omori decay parameters based on the pre-Parkfield aftershocks. Dynamic stress changes cannot explain the deficiency of aftershocks, providing evidence that static stress changes affect earthquake occurrence. However, a burst of seismicity following the Parkfield earthquake at Ragged Point, where the static stress was decreased, provides evidence for dynamic stress triggering. It therefore appears that both Coulomb static stress changes and dynamic stress changes affect the seismicity rate.

Seismic triggering in a stable continental area: The Lugo 1995–1997 seismic sequences (NW Spain)

The northwestern area of the Iberian Peninsula traditionally is considered a seismically stable area. However, since 1979 this region has experienced a significant increase in seismic activity. In 1995 and 1997, two earthquakes, with magnitude M w 4.2 and 5.1, respectively, caused significant damage in the region (intensity EMS: V). In this work, we analysed the focal mechanisms that had the most coherence with the local structures and tectonics. Modelling of the coseismic Coulomb static stress changes generated by the 1995 sequence was performed in order to find a possible genetic relationship with the 1997 sequence. Models using oblique (reverse strike-slip) movement on a fault with the same geometry and orientation as the Baralla fault, mapped at the surface, yield the best fit with the spatial distribution of the 1997 sequence. The existence of causal relationships between two close seismic sequences is important for the improvement of future seismic hazard assessments methodologies.

Induced deformation during tunnel excavation: Evidence from geodetic monitoring

During tunnel excavation, convergence, i.e., a tendency for reduction of the section area of the void formed, is observed due to the stress redistribution around the excavation free surface. This deformation accumulates with a gradually decreasing rate until an equilibrium level is obtained, its amplitude depends on the rock mass conditions, the support system and the excavation procedure, as well as on the state of stress, and is usually recognized as a two dimensional effect; i.e., a deformation confined to a plane normal to the tunnel axis, not affecting neighboring sections. In the last years, electronic theodolites, a new generation of geodetic instruments which can describe in detail the 3-D changes in the shape of tunnels during their excavation stage relative to fixed reference frames, became available, making possible the understanding of the detailed pattern of such changes; especially the effect of rejuvenation of deformation in previously stabilized sections while tunnel excavation is progressing. Analysis of geodetic monitoring records of certain recently constructed tunnels indicates that, occasionally, large deformation from specific ‘source’ sections is transferred along the tunnel axis to “host”, neighboring sections, causing additional time-delayed deformation of previously stabilized sections and necking of the tunnel. This “induced” deformation is not due to effects such as nearby excavations, changes in the hydrological conditions, etc., and to tertiary creep; its distribution along the tunnel axis seems to depend on the potential of “host” sections to accommodate additional stresses from neighboring deformation “source” sections, and it may lead to a progressive, domino-type failure. The overall process is reminiscent of the triggering of new earthquakes by Coulomb static stress changes caused by a certain earthquake, on the condition that the stress level in the epicentral areas of the new events was close to the rupture limit.