Coseismic Stress Changes Research Articles

TheMw6.5 offshore Northern California earthquake of 10 January 2010: Ordinary stress drop on a high-strength fault

The 10 January 2010 Mw 6.5 earthquake offshore Northern California is one of the first intraplate earthquakes in oceanic lithosphere to be well captured by a GPS network. It presents an opportunity to evaluate rupture mechanics on a high-strength fault. Static inversion of the coseismic displacements shows that the slip peaks at the same depth as the expected strength envelope, where the differential stresses can be as high as 600 MPa. Laboratory experiments on peridotite predict dramatic dynamic weakening at these conditions. The observed ordinary stress drop, 2–20 MPa, may indicate that the lithosphere is much weaker than strength envelope predicts or that the failure mechanisms seen in the laboratory are not occurring during the rupture. The GPS observations show very little postseismic signal indicating that if a shear zone exists beneath the coseismic rupture, it operates at significantly greater stress levels than the coseismic stress change.

Slow Slip Acceleration beneath Andaman Islands Triggered by the 11 April 2012 Indian Ocean Earthquakes

The M-w 8.6 and 8.2 strike-slip earthquakes that struck the northeast Indian Ocean on 11 April 2012 resulted in coseismic deformation both at near and distant sites. The slip distribution, deduced using seismic-wave analysis for the orthogonal faults that ruptured during these earthquakes, is sufficient to predict the coseismic displacements at the Global Positioning System (GPS) sites, such as NTUS, PALK, and CUSV, but fall short at four continuous sites in the Andaman Islands region. Slip modeling, for times prior to the events, suggests that the lower portion of the thrust fault beneath the Andaman Islands has been slipping at least at the rate of 40 cm/yr, in response to the 2004 Sumatra-Andaman coseismic stress change. Modeling of GPS displacements suggests that the en echelon and orthogonal fault ruptures of the 2012 intraplate oceanic earthquakes could have possibly accelerated the ongoing slow slip, along the lower portion of the thrust fault beneath the islands with a month-long slip of 4-10 cm. The misfit to the coseismic GPS displacements along the Andaman Islands could be improved with a better source model, assuming that no local process contributed to this anomaly.

The Effect on the Nucleation and Failure of MS7.0 Lushan Earthquake Induced by the MS8.0 Wenchuan Earthquake

AbstractBased on calculations of both the post‐seismic Coulomb failure stress change and coseismic dynamic Coulomb failure stress change caused by the 2008 Wenchuan Ms8.0 earthquake, we simulate the their effects on the time of the nucleation and failure of the 2013 Lushan earthquake fault. Our calculation uses the rate‐ and state‐dependent friction law and viscoelastic relaxation in the middle‐lower crust and upper mantle. Furthermore, the triggering relationship between the Wenchuan and Lushan earthquakes is studied. The results show that the peak value of dynamic stress change on the Lushan earthquake fault induced by Wenchuan event is 0.127 MPa. With the 5‐year accumulation of the viscoelastic relaxation in the middle‐lower crust and upper mantle, the post‐seismic stress change on the Lushan earthquake fault is 0.025 MPa. The results indicate that the process of the nucleation and failure of the Lushan earthquake fault is affected by the post‐seismic Coulomb failure stress change, dynamic stress change and static stress change induced by the Wenchuan earthquake, and the effect of dynamic stress change is more obvious than others. Finally, we investigate the field of stress changes in the study area after the Lushan earthquake.

Probing the lithospheric rheology across the eastern margin of the Tibetan Plateau

The fundamental geological structure, geodynamics, and rheology of the Tibetan Plateau have been debated for decades. Two end-member models have been proposed: (1) the deformation of Tibet is broadly distributed and associated with ductile flow in the mantle and middle or lower crust, (2) the Tibetan Plateau formed during interactions between rigid lithospheric blocks with localization of deformation along major faults. The nature and distribution of continental deformation are governed by the varying rheology of rocks and faults in the lithosphere. Insights into lithospheric rheology can be gained from observations of postseismic deformation, which represents the response of the Earth's interior to coseismic stress changes. Here we use up to 2 years of InSAR and GPS measurements to investigate postseismic displacements following the 2008 Mw 7.9 Wenchuan earthquake in eastern Tibet and probe the differences in rheological properties across the edge of the Plateau. We find that near-field displacements can be explained by shallow afterslip on the Beichuan Fault, which is anti-correlated with the coseismic slip distribution. Far-field displacements cannot be explained by a homogeneous rheology, but instead require a viscoelastic lower crust (from 45 to 60 km depth) beneath Tibet with an initial effective viscosity of 4.4×1017 Pas and a long-term viscosity of 1018 Pas, whereas the Sichuan Basin block has a high-viscosity upper mantle (>1020 Pa s) underlying an elastic 35-km-thick crust. The inferred strong contrast in lithospheric rheologies between the Tibetan Plateau and the Sichuan Basin is consistent with models of ductile lower crustal flow that predict maximum topographic gradients across the Plateau margins where viscosity differences are greatest.

Postseismic deformation following theMw7.2, 23 October 2011 Van earthquake (Turkey): Evidence for aseismic fault reactivation

Geodetic measurements following the 23 October 2011, Mw = 7.2 Van (eastern Turkey) earthquake reveal that a fault splay on the footwall block of the coseismic thrust fault was reactivated and slipped aseismically for more than 1.5 years following the earthquake. Although long-lasting aseismic slip on coseismic ruptures has been documented following many large earthquakes, long-lasting, triggered slip on neighboring faults that did not rupture during the earthquake has not been reported previously. Elastic dislocation and Coulomb stress modeling indicate that the postseismic deformation can be adequately explained by shallow slip on both the coseismic and splay fault and is likely driven mostly by coseismic stress changes. Thus, the slip deficit on the shallow section of the coseismic fault indicated by interferometric synthetic aperture radar-based models has been partially filled by aseismic slip, suggesting a lower likelihood for a large earthquake on the shallow section of the Van fault than suggested by previous studies.

Quasi-Static Strain and Stress Fields due to a Moment Tensor in Elastic–Viscoelastic Layered Half-Space

We derived explicit expressions in the time domain for 3-D quasi-static strain and stress fields, due to a point moment tensor source in an elastic surface layer overlying viscoelastic half-space under gravity. The expressions of strain in the elastic surface layer were directly obtained from the expressions of displacement in our previous paper. The conversion of strain into stress is easy, because the stress–strain relation of elastic material is linear. In the viscoelastic substratum, the expressions of strain were obtained by applying the correspondence principle of linear viscoelasticity to the associated elastic solution. The strain–stress conversion is not straightforward, as the stress–strain relation of viscoelastic material is usually given in a differential form. To convert strain into stress, we used an integral form of the stress–strain relation instead of the usual differential form. The expressions give the responses of elastic half-space at \( t = 0 \), and the responses of an elastic plate floating on non-viscous liquid at \( t = \infty \). The moment tensor is rationally decomposed into the three independent force systems, corresponding to isotropic expansion, shear faulting and crack opening, and so the expressions include the strain and stress fields for these force systems as special cases. As the first numerical example, we computed the temporal changes in strain and stress fields after the sudden opening of an infinitely long vertical crack cutting the elastic surface layer. Here, we observe that the stress changes caused by the sudden crack opening gradually decay with time and vanish at \( t = \infty \) everywhere. After the completion of stress relaxation, a characteristic pattern of shear strain remains in the viscoelastic substratum. Since the strain and stress fields at \( t = \infty \) can be read as the strain- and stress-rate fields caused by steady crack opening, respectively, this numerical example demonstrates the realization of a steady stress state supported by steady viscous flow in the asthenosphere, associated with steady seafloor spreading at mid-ocean ridges. For the second numerical example, we computed the temporal changes in strain and stress fields after the 2011 Tohoku-oki mega-thrust earthquake, which occurred at the North American-Pacific plate interface. In this numerical example, the stress changes caused by coseismic fault slip vanish at \( t = \infty \) in the viscoelastic substratum, but remain in the elastic surface layer. The coseismic stress changes (and also strain changes) in the elastic surface layer diffuse away from the source region with time, due to gradual stress relaxation in the viscoelastic substratum.

Short- and long-term earthquake triggering along the strike-slip Kunlun fault, China: Insights gained from the Ms 8.1 Kunlun earthquake and other modern large earthquakes

Abstract Following the 2001 Ms8.1 Kunlun earthquake, earthquake records of more than 10 years, in addition to more than one century's records of large earthquakes, provide us with a chance to examine short-term (days to a few years) and long-term (years to decades) seismic triggering following a magnitude ~ 8 continental earthquake along a very long strike-slip fault, the Kunlun fault system, located in northern Tibet, China. Based on the calculations of coseismic Coulomb stress changes (ΔCFS) from the larger earthquake and post-seismic stress changes due to viscoelastic stress relaxation in the lower crust and upper mantle, we examined the temporal evolution of seismic triggering. The ETAS (epidemic type aftershocks sequence) model shows that the seismic rate in the aftershock area over ~ 10 years was higher than the background seismicity before the mainshock. Moreover, we discuss long-term (years to decades) triggering and the evolution of stress changes for the sequence of five large earthquakes of M ≥ 7.0 that ruptured the Kunlun fault system since 1937. All subsequent events of M ≥ 7.0 occurred in the regions of positive accumulated ΔCFS. These results show that short-term (up to 200 days in our case) triggering along the strike-slip Kunlun fault is governed by coseismic stress changes, while long-term triggering is somewhat due to post-seismic Coulomb stress changes resulting from viscoelastic relaxation.

Inversion for absolute deviatoric crustal stress using focal mechanisms and coseismic stress changes: The 2011 M9 Tohoku‐oki, Japan, earthquake

AbstractThe absolute magnitude of stress in the crust and the shear strength of faults are poorly known, yet fundamental quantities, in lithospheric dynamics. While stress magnitude cannot be measured directly, deviatoric stress state can be inferred indirectly from focal mechanism solutions collected before and after an earthquake. We extend a standard stress inversion for normalized stresses to invert for the 3‐D spatial distribution of absolute deviatoric stress and variation of fault strength with depth using focal mechanism solutions and coseismic stress changes produced by large earthquakes. We apply the method to the 2011 M9 Tohoku‐oki, Japan, earthquake. The northern Japan fore‐arc crust between 5 and 15 km depths appears to be weak with fault strength of 40–90 MPa, consistent with a coefficient of friction of 0.2–0.5. The M9 Tohoku‐oki coseismic stress change was large enough, relative to the ambient stress, to rotate the principal stress directions typically ~20° in the upper 20 km of the crust. The data from Japan require a heterogeneous ambient deviatoric stress field with short‐wavelength (~20–50 km) fluctuations in principal stress orientations.

Analysis of the Wenchuan Ms8.0 Earthquake's Co‐seismic Stress and Displacement Change by Using the Finite Element Method

Abstract:The variation of in situ stress before and after earthquakes is an issue studied by geologists. In this paper, on the basis of the fault slip dislocation model of Wenchuan Ms8.0 earthquake, the changes of co‐seismic displacement and the distribution functions of stress tensor around the Longmen Shan fault zone are calculated. The results show that the co‐seismic maximum surface displacement is 4.9 m in the horizontal direction and 6.5 m in the vertical direction, which is almost consistent with the on‐site survey and GPS observations. The co‐seismic maximum horizontal stress in the hanging wall and footwall decreased sharply as the distance from the Longmen Shan fault zone increased. However, the vertical stress and minimum horizontal stress increased in the footwall and in some areas of the hanging wall. The study of the co‐seismic displacement and stress was mainly focused on the long and narrow region along the Longmen Shan fault zone, which coincides with the distribution of the earthquake aftershocks. Therefore, the co‐seismic stress only affects the aftershocks, and does not affect distant faults and seismic activities. The results are almost consistent with in situ stress measurements at the two sites before and after Wenchuan Ms8.0 earthquake. Along the fault plane, the co‐seismic shear stress in the dip direction is larger than that in the strike direction, which indicates that the faulting mechanism of the Longmen Shan fault zone is a dominant thrust with minor strike‐slipping. The results can be used as a reference value for future studies of earthquake mechanisms.

Coulomb static stress changes before and after the 23 October 2011 Van, eastern Turkey, earthquake (<i>M</i><sub>W</sub>= 7.1): implications for the earthquake hazard mitigation

Abstract. Coulomb stress changes before and after the 23 October 2011 Van, eastern Turkey, earthquake have been analysed using available data related to the background and the aftershock seismicity and the source faults. The coseismic stress changes of the background seismicity had slightly promoted stress over the rupture plane of the 2011 Van earthquake, while it yielded a stress shadow over the Gürpı nar Fault which has been argued to have produced the 7 April 1646 Van earthquake. The stress shadow over the Gürp\\i nar fault has become more pronounced following the occurrence of the 2011 Van earthquake, meaning that the repetition of the 1646 Van earthquake has been further suppressed. Spatial distribution and source mechanisms of the 2011 Van earthquake's aftershocks have been utilised to define four clusters with regard to their relative location to the mainshock rupture. In addition, the aftershock sequence covers a much broader area toward the northeast. Correlations between the observed spatial patterns of the aftershocks and the coseismic Coulomb stress changes caused by the mainshock are determined by calculating the stress changes over both optimally oriented and specified fault planes. It is shown here that there is an apparent correlation between the mainshock stress changes and the observed spatial pattern of the aftershock occurrence, demonstrating the usefulness of the stress maps in constraining the likely locations of the upcoming aftershocks and mitigating earthquake hazard.

Potential of future seismogenesis in Hebei Province (NE China) due to stress interactions between strong earthquakes

Northeast China, a densely populated area, is affected by intense seismic activity, which includes large events that caused extensive disaster and tremendous loss of life. For contributing to the continuous efforts for seismic hazard assessment, the earthquake potential from the active faults near the cities of Zhangjiakou and Langfang in Hebei Province is examined. We estimate the effect of the coseismic stress changes of strong (M⩾5.0) earthquakes on the major regional active faults, and mapped Coulomb stress change onto these target faults. More importantly our calculations reveal that positive stress changes caused by the largest events of the 1976 Tangshan sequence make the Xiadian and part of Daxing fault, thus considered the most likely sites of the next strong earthquake in the study area. The accumulated static stress changes that reached a value of up to 0.4bar onto these faults, were subsequently incorporated in earthquake probability estimates for the next 30years.

Spatially variable fault friction derived from dynamic modeling of aseismic afterslip due to the 2004 Parkfield earthquake

AbstractWe investigate fault friction from dynamic modeling of fault slip prior to and following the Mw 6.0 earthquake which ruptured the Parkfield segment of the San Andreas Fault in 2004. The dynamic modeling assumes a purely rate‐strengthening friction law, with a logarithmic dependency on sliding rate: . The initial state of stress is explicitly taken into account, and afterslip is triggered by the stress change induced by the earthquake source model given a priori. We consider different initial stress states and two coseismic models, and invert for the other model parameters using a nonlinear inversion scheme. The model parameters include the reference friction μ*, the friction rate dependency characterized by the quantity a‐b, assumed to be either uniform or depth dependent. The model parameters are determined from fitting the transient postseismic geodetic signal measured at continuous GPS stations. Our study provides a view of frictional properties at the kilometers scale over the 0–15 km depth illuminated by the coseismic stress change induced by the Parkfield earthquake. The reference friction is estimated to be between 0.1 and 0.5. With independent a priori constraints on the amplitude of differential stress, the range of possible values narrows down to 0.1–0.17. The friction rate coefficient a‐b is estimated to be ∼ 10− 3 − 10− 2 with a hint that it increases upward from about 1–3 × 10–3 at 3–7 km depth to about 4–7 × 10–3 at 0–1 km depth. It is remarkable that our results are consistent with frictional properties measured on rock samples recovered from the fault zone thanks to the San Andreas Fault Observatory at Depth experiment.

Stress changes on major faults caused by 2013 Lushan earthquake and its relationship with 2008 Wenchuan earthquake

On April 20, 2013, an M s7.0 earthquake occurred in Ya’an-Lushan region, Sichuan Province, China, killing and injuring more than one thousand people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settlement as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co- and post-seismic stress changes caused by the Wenchuan and Lushan earthquakes to discuss the relationship between M w7.9 Wenchuan earthquake and M s7.0 Lushan earthquake, the influences on the distribution of aftershock caused by the Lushan earthquake, and the stress changes on major faults in this region. It is shown that the Coulomb failure stress increment on the hypocenter of Lushan earthquake caused by the Wenchuan earthquake is about 0.0037–0.0113 MPa. And the possible maximum value (0.0113 MPa) is larger than the threshold of stress triggering. Therefore, the occurrence of Lushan earthquake is probably effectively promoted by the Wenchuan earthquake. The aftershock distribution is well explained by the co-seismic stress changes of Lushan earthquake. By the two ends of the rupture of Lushan earthquake with increased Coulomb failure stress, a lack of aftershock recordings indicates the high seismic hazard. The stress accumulation and corresponding seismic hazard on the Kangding-Dafu segment of the Xinshuihe fault, the Beichuan-Yingxiu fault, the Pengxian-Guanxian fault, and the Ya’an fault are further increased by the Lushan earthquake and post-seismic process of Wenchuan earthquake.

Simulations of tremor‐related creep reveal a weak crustal root of the San Andreas Fault

AbstractDeep aseismic roots of faults play a critical role in transferring tectonic loads to shallower, brittle crustal faults that rupture in large earthquakes. Yet, until the recent discovery of deep tremor and creep, direct inference of the physical properties of lower‐crustal fault roots has remained elusive. Observations of tremor near Parkfield, CA provide the first evidence for present‐day localized slip on the deep extension of the San Andreas Fault and triggered transient creep events. We develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near Parkfield is consistent with triggered fault creep governed by laboratory‐derived friction laws between depths of 20–35 km on the fault. Simulated creep and observed tremor northwest of Parkfield nearly ceased for 20–30 days in response to small coseismic stress changes of order 104 Pa from the 2003 M6.5 San Simeon Earthquake. Simulated afterslip and observed tremor following the 2004 M6.0 Parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust. These creep responses require very low effective normal stress of ~1 MPa on the deep San Andreas Fault and near‐neutral‐stability frictional properties expected for gabbroic lower‐crustal rock.

1954年山丹7 1/4 级地震的孕震构造和发震机制探讨

Located in the front margin of the northeastern Tibetan Plateau, the Longshoushan fault zone is one of the boundary faults between the Hexi corridor and the Alxa block. Although the resolution of observation is limited, the 1954 M7(1/4) earthquake on the Longshoushan fault zone is a rare big event in this zone with modern seismological observation and documentation. During this earthquake, one continuous dextral-normal rupture occurred with a length of only 7 km along a transfer fault connecting two secondary faults of the Longshoushan fault zone. However, the dextral-normal sense of this surface rupture is contrary to the tectonic kinematics in this area. Based on recent field investigation and mapping, two phenomena were discovered. Firstly, except the one appeared on the transform fault, no surface rupture zone is found on the branches of the Longshoushn fault zone. Secondly, the main damage caused by earthquake is distributed on the region between the northern and the southern branches of the Longshoushan fault zone. These two findings indicate that the 1954 earthquake was caused by the main faults rather than the transfer fault of the Longshoushan fault zone, and the co-seismic surface rupture on the transfer fault is only a form of stress release at local portion. According to calculation of static co-seismic Coulomb stress change, the surface rupture could be triggered by an earthquake on the southern branch of the Longshoushan fault zone. Furthermore, in consideration of the possibility of dynamic triggering along the northern branch of the Longshoushan fault zone, the surface rupture should be the co-seismic response to the earthquake occurred on the main branches of the Longshoushan fault zone. Relocation of the small earthquakes also shows that a narrow inverted north-extending triangle about 10 km wide was formed between the northern and the southern faults of the Longshoushan fault zone.

Postseismic deformation following the 1999 Chi‐Chi earthquake, Taiwan: Implication for lower‐crust rheology

On 1999 September 21, the Mw 7.6 Chi‐Chi earthquake ruptured a segment of the Chelungpu Fault, a frontal thrust fault of the Western Foothills of Taiwan. The stress perturbation induced by the rupture triggered a transient deformation across the island, which was well recorded by a wide network of continuously operating GPS stations. The analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements, with relaxation times varying by a factor of more than ten, and large cumulative displacements at great distances, in particular along the Longitudinal Valley in eastern Taiwan, where relaxation times are also longer. We show that while afterslip is the dominant relaxation process in the epicentral area, viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale. We model the spatiotemporal behavior of the transient deformation as the result of afterslip on the décollement that extends downdip of the Chelungpu thrust, and viscoelastic flow in the lower crust and in the mid‐crust below the Central Range. We construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled. The model is compatible with the shorter relaxation times observed in the near field, which are due to continued fault slip, and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the Central Range. Our preferred model shows a viscosity of 0.5–1 × 1019Pa s at lower‐crustal depths and 5 × 1017Pa s in the mid‐crust below the Central Range, between 10 and 30 km depth. The low‐viscosity zone at mid‐crustal depth below the Central Range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures, estimated to be between 300°C and 600°C from the modeling of thermo‐chronology and surface heat flow data.

Challenging the rate‐state asperity model: Afterslip following the 2011 M9 Tohoku‐oki, Japan, earthquake

Prior to the 2011 M9 Tohoku‐oki earthquake, subduction at the Japan Trench was characterized by M7‐8 earthquakes, sometimes rupturing the same source regions (seismic asperities), followed by extensive afterslip detected by GPS measurements. A physically‐based model consisting of velocity‐weakening asperities surrounded by aseismic creep on velocity‐strengthening regions (the ‘rate‐state asperity model’) became the prevailing conceptual model for earthquakes in this region. Theory and numerical simulation indicates that velocity‐weakening areas do not exhibit sustained afterslip, while velocity‐strengthening regions do not accumulate stress interseismically. Here we demonstrate that the rate‐state asperity model is contradicted by models of postseismic deformation following the Tohoku‐oki earthquake: afterslip in the first eight months either occurred on historical seismic asperities or stress accumulated in regions surrounding the asperities. Unsmoothed inversions of cumulative 8‐month postseismic GPS displacements that restrict afterslip to areas outside of historical ruptures cannot fit the data without afterslip exceeding the slip that fully relaxes the coseismic stress change. In contrast, similarly constrained inversions allowing slip within historical ruptures can satisfactorily fit the postseismic displacements. These results require a modification of the rate‐state asperity model and raise new questions about physical processes and properties of the subduction interface.

The role of triggering by static stress transfer during geothermal reservoir stimulation

During creation of an Enhanced Geothermal System, massive fluid injections are conducted to induce fracture shear which generates reservoir permeability. In this study we analyze coseismic static stress transfer caused by induced seismic events during a stimulation at the European research project at Soultz‐sous‐Forêts (Alsace, France). For this purpose we developed an efficient method to calculate coseismic static stress changes from an elliptical slip distribution on a circular fracture using superposition of rectangular sources, which enables us to apply an analytical solution for fast computation. This method is applied on a data set of 715 focal mechanisms derived from seismic recordings of the stimulation of the well GPK2 to calculate temporal evolution of static stress transfer. We find that the resulting structure of coseismic stress changes can be divided into three parts: a quiet zone where no spreading of seismicity occurs, an active zone within the created reservoir with ongoing fracturing and a process zone where the growth of the reservoir occurs. Static stress changes in the active zone are of the order of 1 MPa, both positive and negative, but may exceed this value considerably on a local scale. Analysis of stress changes from a cluster of events that occurred after shut‐in lets us conclude that triggering by coseismic static stress changes is possible for some events. Our analysis shows that triggering by static stress transfer plays a minor role for injection induced seismicity in a volumetric reservoir, whereas it can be quite effective for rupture propagation along single large fault zones.

Response of seismic activity in mines to the stress changes due to mining induced strong seismic events

The influence of the static stress transfer due to two stronger seismic events from one mine section on subsequent seismicity linked to the same section is studied. The first strong tremor was provoked by destressing blasting. The static stress transfer caused by this tremor had apparently a triggering effect for the occurrence of the second strong event. This second event occurred in less than three weeks after the first one and on the area of simultaneous enhancement of both shear and normal components of the Coulomb stress change, caused by the first event. It appears also that in the next few months the cumulative stress change due to both the events influenced significantly the location and rate of smaller subsequent events. Interestingly, contrary to the results of previous analyses, the negative Coulomb stress changes turn out to have a major role in this respect. The coseismic stress change impact even seems to prevail over the influence of time-varying stress alteration caused by advancement of the mining front.

Andaman Postseismic Deformation Observations: Still Slipping after All These Years?

More than six years after the great ( M w 9.2) Sumatra–Andaman earthquake, postevent processes responsible for relaxation of the coseismic stress change remain controversial. Modeling of Andaman Islands Global Positioning System (GPS) displacements indicated early near‐field motions were dominated by slip down‐dip of the rupture, but various researchers ascribe elements of relaxation to dominantly poroelastic, dominantly viscoelastic, and dominantly fault slip processes, depending primarily on their measurement sampling and modeling tools used. After subtracting a pre‐2004 interseismic velocity, significant transient motion during the 2008.5–2010.5 epoch confirms that postseismic relaxation processes continue in Andaman. Modeling three‐component velocities as viscoelastic flow yields a weighted root‐mean‐square (wrms) misfit that always exceeds the wrms of the measured signal (26.3 mm/yr). The best‐fitting models are those that yield negligible deformation, indicating the model parameters have no real physical meaning. GPS velocities are well fit (wrms 4.0 mm/yr) by combining a viscoelastic flow model that best fits the horizontal velocities with ∼50 cm/yr thrust slip down‐dip of the coseismic rupture. Both deep slip and flow respond to stress changes, and each can significantly change stress in the realm of the other; it therefore is reasonable to expect that both transient deep slip and viscoelastic flow will influence surface deformation long after a great earthquake.