Coseismic Stress Changes Research Articles

Abruzzo, Italy, Earthquakes of April 2009: Heterogeneous Fault-Slip Models and Stress Transfer from Accurate Inversion of ENVISAT-InSAR Data

Abstract The seismic sequence that occurred in the Abruzzo Apennines near L’Aquila (Italy) in April 2009 caused extensive damage and a large number of casualties (more than 300). The earthquake struck an area in the Italian Apennines chain where several faults, belonging to adjacent seismotectonic domains, create a complex tectonic regime resulting from the interaction among regional stress buildup, local stress changes caused by individual earthquakes, and viscous-elastic stress relaxation. Understanding such complex interaction in the Apennines can lead to a large step forward in the seismic risk mitigation in Italy. The Abruzzo earthquake has been very well recorded by interferometric synthetic aperture radar (InSAR) data, much better than the first Italian earthquake ever recorded by satellites, namely, the 1997 Umbria–Marche earthquake. ENVISAT (ENVIronmental SATellite) data for the Abruzzo earthquake are, in fact, very clear and allow an accurate reconstruction of the faulting mechanism. We present here an accurate inversion of vertical deformation data obtained by ENVISAT images, aimed to give a detailed reconstruction of the fault geometry and slip distribution. The resulting fault models are then used to compute, by a suitable theoretical model based on the elastic dislocation theory, the stress changes induced on the neighboring faults. The correlation of the subsequent mainshocks and aftershocks of the Abruzzo sequence with the volumes undergoing increasing Coulomb stress clearly evidence the triggering effect of coseismic stress changes on further seismicity. Moreover, this analysis put in evidence which seismotectonic domains have been more heavily charged by stress released by the Abruzzo mainshocks. The most important faults significantly charged by the Abruzzo sequence belong to the Sulmona and Avezzano tectonic domains. Taking into account the average regional stress buildup in the area, the positive Coulomb stress changes caused by this earthquake can be seen as anticipating the next earthquakes in the neighboring domains of 15–20 yr.

The co-seismic Coulomb stress change and expected seismicity rate caused by 14 April 2010 Ms = 7.1 Yushu, China, earthquake

A moderate and widely felt earthquake of Ms7.1 (Mw6.9) occurred on April 14, 2010 in Yushu district of Qinghai province (China) which destroyed a large number of buildings and killed more than two thousand people. The distribution of the aftershock sequence is sparse, but with relatively large magnitudes (Ms6.3 and Ms5.7 aftershocks were recorded), strong aftershocks are still highly potential to occur in the near future. Therefore, it is critical to delineate areas where potential aftershocks could occur. Based on static stress triggering theory, we calculated the coseismic stress changes induced by the mainshock in Yushu County and its surrounding regions by adopting elastic dislocation theory and a multilayered crustal model. According to the rate- and state-variable friction law, we calculated the expected seismicity rate and probability of occurrences of M ≥ 5.0 earthquake in the next ten years. It is observed that the Coulomb stress changes increase apparently on the segment of the Wudaoliang-Qumalai Fault, the Batang-Luoxu and Dangjiang-Longbao segments of the Ganzi-Yushu Fault and the whole Wulanwula Lake-Yushu Fault. Based on the theory of Coulomb stress triggering, the probability of earthquake occurrence on these faults would increase in the near future. Considering the historical seismicity, the strain accumulation and the Coulomb stress changes, the probability of M ≥ 5 earthquake and M ≥ 6 earthquake occurrence in the entire study area in the next 10 years are 79–87% and 26.7–33.6%, respectively. Actually, a moderate earthquake of Ms5.3 occurred to the northwest of the Nangqian County on June 26, 2011.

Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003Mw= 6.9 Zemmouri, Algeria, earthquake

[1] The essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated, first through idealized examples and then by study of thrust faulting in Algeria. We calculated coseismic stress changes caused by the 2003 Mw = 6.9 Zemmouri earthquake, finding that a large majority of the Zemmouri afterslip sites were brought several bars closer to Coulomb failure by the coseismic stresses, while the majority of aftershock nodal planes were brought closer to failure by an average of ∼2 bars. Further, we calculated that the shallow portions of the adjacent Thenia tear fault, which sustained ∼0.25 m slip, were brought >2 bars closer to failure. We calculated that the Coulomb stress increased by 1.5 bars on the deeper portions of the adjacent Boumerdes thrust, which lies just 10–20 km from the city of Algiers; both the Boumerdes and Thenia faults were illuminated by aftershocks. Over the next 6 years, the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity. The stress also increased by 0.4 bar on the east Sahel thrust fault west of the Zemmouri rupture. Algiers suffered large damaging earthquakes in A.D. 1365 and 1716 and is today home to 3 million people. If these shocks occurred on the east Sahel fault and if it has a ∼2 mm/yr tectonic loading rate, then enough loading has accumulated to produce a Mw = 6.6–6.9 shock today. Thus, these potentially lethal faults need better understanding of their slip rate and earthquake history.

Contemporary deformation and stressing rates in Southern Alaska

SUMMARY The subduction of the Pacific Plate beneath North America induces broad scale stressing of the Alaskan crust that has led to the development of the highest mountains in North America, the highest slip rates along some of the longest strike-slip faults on Earth, and widespread seismicity that includes the 1964 M9.2 Alaska earthquake, the second largest ever recorded. These features are a consequence of deformation associated with three primary processes, interseismic loading due to relative plate motions, large earthquakes and postseismic processes. How these mechanisms contribute to the evolution of stress in the Alaskan crust is not well understood. Here we use observed contemporary surface velocities to constrain 2-D and 3-D viscoelastic numerical models of relative Pacific/North American plate motions, coseismic slip associated with the 1964 (M9.2) megathrust event and strike-slip earthquakes on the transform boundary in 1949 (M8.1), 1958 (M7.8) and 1972 (M7.6) (the four largest events prior to the 2002 M7.9 Denali quake), viscoelastic relaxation following these events, and afterslip, to gain insight into how these processes are shaping Alaska today. Results suggest that interseismic deformation and on-going post-seismic deformation following the 1964 earthquake both contribute significantly to the GPS measured contemporary velocity field. Viscoelastic relaxation associated with a mantle with a viscosity of � 10 19 Pa s is required to explain southerly directed velocities that are observed in the Cook Inlet region to well north of the Denali fault. Results also suggest that subduction of the Pacific Plate leads to a broad zone of deformation with high stressing rates concentrated in a band that lies several hundred kilometres from the plate boundary, coeval with the inboard location of the maximum locking depth of the megathrust. Interseismic deformation and stressing rates remain high further inland across the Yakutat microplate, where flat subduction extends the width of the locked plate interface. Calculations show that post-seismic relaxation following the large strike-slip events serves to reload these rupture surfaces while relieving stress on the eastern Denali Fault. Post-seismic relaxation following the 1964 earthquake combined with coseismic stress changes, promoted the triggering of the 2002 Denali quake. Calculations also suggest that over the past 50 years high stress has accumulated on part of the thrust interface to the west and east of the 1964 rupture surface and along the Queen-Charlotte Fault to the south of the 1949 rupture surface.

Temporal and spatial variation of stress field in Taiwan from 1991 to 2007: Insights from comprehensive first motion focal mechanism catalog

The total amount of 4,761 focal mechanisms was determined based on P-wave first motion polarities from 1991 to 2007 in Taiwan region. This dataset offers us a good opportunity to examine temporal and spatial variability of the stress field. We find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the Chi-Chi earthquake. The orientation of the maximum horizontal compressive stress axes (S H) shows a general agreement with the direction of plate motion between a depth range of 0–30 km. The 20° anticlockwise rotation of S H from the Longitudinal Valley (LV) to western Taiwan is probably caused by the left-lateral motion on the Longitudinal Valley Fault (LVF) that has consumed part of the oblique motion of plate convergence. On the other hand, part of the oblique convergence is transferred into the Central Range and the Hsuehshan Range judging from counterclockwise rotation of S H from east to west and strike-slip faulting in the Hsuehshan Range. Most events with a depth greater than 30 km occur offshore eastern Taiwan and the azimuth of S H is close to E–W directed, different from NW–SE directed at shallow depths. This may infer the existence of the transition of lithosphere rheology in offshore eastern Taiwan. The trends of S H in the depth of 0–10 km are strongly affected by the coseismic stress change of the Chi-Chi earthquake. In the northern half of the Chi-Chi rupture area, the trends of S H rotate 30° clockwise and the stress ratio increased by a factor of six after the mainshock. The orientations of S H still differ by 30° in 2007 comparing to that in the period before the Chi-Chi earthquake. The variation of S H trend is more diverse in the southern half of the rupture area, showing 20° counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation. The trend of S H returns to the pre-seismic direction of 110° in 2001. These notable changes of S H before and after the Chi-Chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop. We also recognize a significant 10° counterclockwise rotation of S H in the entire Chi-Chi rupture area between 1991 and 1999 before the earthquake took place. However, to the south of the Chi-Chi rupture, the trends of S H remain little changed before and after the Chi-Chi earthquake.

A unified continuum representation of post-seismic relaxation mechanisms: semi-analytic models of afterslip, poroelastic rebound and viscoelastic flow

We present a unified continuum mechanics representation of the mechanisms believed to be commonly involved in post-seismic transients such as viscoelasticity, fault creep and poroelasticity. The time-dependent relaxation that follows an earthquake, or any other static stress perturbation, is considered in a framework of a generalized viscoelastoplastic rheology whereby some inelastic strain relaxes a physical quantity in the material. The relaxed quantity is the deviatoric stress in case of viscoelastic relaxation, the shear stress in case of creep on a fault plane and the trace of the stress tensor in case of poroelastic rebound. In this framework, the instantaneous velocity field satisfies the linear inhomogeneous Navier's equation with sources parametrized as equivalent body forces and surface tractions. We evaluate the velocity field using the Fourier-domain Green's function for an elastic half-space with surface buoyancy boundary condition. The accuracy of the proposed method is demonstrated by comparisons with finite-element simulations of viscoelastic relaxation following strike-slip and dip-slip ruptures for linear and power-law rheologies. We also present comparisons with analytic solutions for afterslip driven by coseismic stress changes. Finally, we demonstrate that the proposed method can be used to model time-dependent poroelastic rebound by adopting a viscoelastic rheology with bulk viscosity and work hardening. The proposed method allows one to model post-seismic transients that involve multiple mechanisms (afterslip, poroelastic rebound, ductile flow) with an account for the effects of gravity, non-linear rheologies and arbitrary spatial variations in inelastic properties of rocks (e.g. the effective viscosity, rate-and-state frictional parameters and poroelastic properties).

Triggering Effect of M 4-5 Earthquakes on the Earthquake Cycle of Repeating Events at Parkfield, California

Stress perturbations influence earthquake recurrence and are of funda- mental importance to understanding the earthquake cycle and determining earthquake hazard. The large population of repeating earthquakes on the San Andreas fault at Parkfield, California, provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes. Using 187 M 0:4 to ∼1:7 repeating earthquake sequences from the High Resolution Seismic Network cat- alog, we find that the time to recurrence of repeating events subsequent to nearby M 4-5 earthquakes is shortened, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of ∼5 km, corresponding to static coseismic stress changes of > 0:6 26:6 kPa, and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s. Online Material: Figures of event chronologies of RES, estimates of dt, and coseismic static stress changes.

Stress evolution and fault interactions before and after the 2008 Great Wenchuan earthquake

The 12 May 2008 Wenchuan earthquake (Mw 7.9) ruptured ∼ 300 km of the Longmen Shan fault, claiming ∼ 90,000 lives and devastating many cities in the Sichuan province, China. The coseismic stress changes due to the Wenchuan earthquake have been studied in kinematic models using the inferred coseismic fault slips, but the cause of the fault slips, the impact of other large earthquakes, and the mechanical interactions between the faults in eastern Tibet are uncertain. Here we explore these issues using a three-dimensional viscoelastoplastic dynamic model that calculates the regional stresses from tectonic and topographic loading, and simulates earthquakes and their stress perturbations. Our calculated coseismic changes of the Coulomb stress associated with the Great Wenchuan earthquake are similar to those in previous models. However, we show that the cumulative Coulomb stress changes, hence the implied earthquake risks, are significantly different when previous large earthquakes in the region are included in the model. Particularly, we show that in spite of stress increase from the Wenchuan earthquake, the southeastern segments of the Xianshuihe fault stay in a stress shadow because of the stress release by six M ≥ 6.9 events in this part of the Xianshuihe fault since 1893. We also found that interseismic locking on the Xianshuihe fault can increase the loading rate on the Longmen Shan fault by up to ∼ 50 Pa/year.

A subsurface stress analysis and its possible relation with seismicity near the Itoiz Reservoir, Navarra, Northern Spain

We study the effects of surface water loads and moderate-magnitude earthquakes on the subsurface state of stress and its relation with seismicity, near the newly constructed Itoiz reservoir in the western Pyrenees, northern Spain. To do this, we initially compute the evolution of the stress changes in the subsoil due to the water load distribution during the impounding of the reservoir, and relate it with the main seismicity occurred after the beginning of impoundment. After this, we compute the coseismic and postseismic stress changes produced by the main events of the seismic series and study its influence on the triggering of the aftershocks. Stress changes due to the water loads are computed by means of the Boussinesq solution for a homogeneous elastic half-space whereas the coseismic stress changes due to earthquakes are obtained using the equations for an internal elastic dislocation in a half space. The effects over the possible seismogenic faults are in both cases, accounted by means of the Coulomb Failure Stress criterion (ΔCFS). We considered different threshold values and apparent friction coefficients of ΔCFS for the effective static triggering. Then, we calculate the pore pressure changes produced by the time evolution of the surface water loads, as also the pore pressure changes generated by the elastic dislocations due to the largest earthquake of the series (mainshock). The fault parameters for the computation of the ΔCFS of the perturbing seismic events were selected from published focal solutions. To define the target fault parameters we took into account the focal solutions of the earthquakes and aftershocks themselves, the parameters of mapped faults based on the tectonic setting and those of optimally oriented faults. Due to the absence of reliable estimations for the slip distributions of the main earthquakes, we assumed for stress computations a semi-elliptical distribution of slips over the fault. Results show the existence of a mainly positive stress influence of the surface water loads on the main earthquake of the series, in two different senses: the elastic Coulomb failure stress and the change in pore pressure. However, due to different uncertainties on the possible values of the apparent coefficient of friction ( µ') for the zone, there is not a clear indication of an effective earthquake triggering of the mainshock due to the elastic stress changes by the surface water loads. On the other hand, a more clear effect is found on the possible triggering of aftershocks due to the largest earthquakes of the series. It is also found a positive pore pressure influence of the two largest events on most of the aftershocks.

Postseismic deformation due to the Mw 6.0 2004 Parkfield earthquake: Stress‐driven creep on a fault with spatially variable rate‐and‐state friction parameters

We investigate the coseismic and postseismic deformation due to the Mw 6.0 2004 Parkfield, California, earthquake. We produce coseismic and postseismic slip models by inverting data from an array of 14 continuous GPS stations from the SCIGN network. Kinematic inversions of postseismic GPS data over a time period of 3 years show that afterslip occurred in areas of low seismicity and low coseismic slip, predominantly at a depth of ∼5 km. Inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane. The kinetics of afterslip is consistent with a velocity‐strengthening friction generalized to include the case of infinitesimal velocities. We performed simulations of stress‐driven creep using a numerical model that evaluates the time‐dependent deformation due to coseismic stress changes in a viscoelastoplastic half‐space. Starting with a coseismic slip distribution, we compute the time‐dependent evolution of afterslip on a fault plane and the associated displacements at the GPS stations. Data are best explained by a rate‐strengthening model with frictional parameter (a − b) = 7 × 10−3, at a high end of values observed in laboratory experiments. We also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks. The rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip, suggesting that postearthquake seismicity is governed by loading from the nearby aseismic creep. The San Andreas fault around Parkfield is deduced to have large along‐strike variations in rate‐and‐state frictional properties. Velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity‐weakening patches after the 2004 rupture.

Stress changes on major faults caused by M w7.9 Wenchuan earthquake, May 12, 2008

On May 12, 2008, a magnitude 7.9 earthquake ruptured the Longmenshan fault system in Sichuan Province, China, collapsing buildings and killing tens of thousands people. As predicted, aftershocks may last for at least one year, and moreover, large aftershocks are likely to occur. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settling people as to avoid future disasters. It is demonstrated that the redistribution of stress induced by an earthquake should trigger successive seismic activity. Based on static stress triggering theory, we calculated the coseismic stress changes on major faults induced by the Wenchuan earthquake, with elastic dislocation theory and the multilayered crustal model. We also discuss the stress distribution and its significance for future seismic activity under the impact of the Wenchuan earthquake. It is shown that coulomb failure stress (CFS) increases obviously on the Daofu-Kangding segment of the Xianshuihe Fault, the Maqu and Nanping segment of the Eastern Kunlun Fault, the Qingchuan Fault, southern segment of the Minjiang Fault, Pengxian-Guanxian Fault, Jiangyou-Guangyuan Fault, and Jiangyou-Guanxian Fault. The increased stress raises the probability of earthquake occurrence on these faults. Since these areas are highly populated, earthquake monitoring and early disaster alarm system are needed. CFS increases with a magnitude of 0.03–0.06 MPa on the Qingchuan Fault, which is close to the northern end of the rapture of Wenchuan earthquake. The occurrence of some strong aftershocks, including three events with magnitude higher than 5.0, indicates that the seismic activities have been triggered by the main shock. Aftershocks seem to migrate northwards. Since the CFS change on the Lueyang-Mianxian Fault located on the NEE of the Qingchuan Fault is rather small (±0.01 MPa), the migration of aftershocks might be terminated in the area near Hanzhong City. The CFS change on the western Qinling Fault is around 10 Pa, and the impact of static triggering can be neglected. The increment of CFS on the Pengxian-Guanxian Fault and Beichuan-Yingxiu Fault southwest to the main rupture is 0.005–0.015 MPa, which would facilitate earthquake triggering in these areas. Very few aftershocks in these areas indicate that the accumulated stress has not been released sufficiently. High seismic risk is predicated in these areas due to co-seismic CFS loading. The Wenchuan earthquake released the accumulated CFS on the Fubianhe Fault, the Huya Fault, the Ha’nan-Qingshanwan Fault, and the Diebu-Bailongjiang Fault. The decrement of CFS changes on the Longquanshan Fault east to Chengdu City is about 0.002 MPa. The seismic activity will be depressed by decrement of CFS on these faults.

Can compliant fault zones be used to measure absolute stresses in the upper crust?

Geodetic and seismic observations reveal long‐lived zones with reduced elastic moduli along active crustal faults. These fault zones localize strain from nearby earthquakes, consistent with the response of a compliant, elastic layer. Fault zone trapped wave studies documented a small reduction in P and S wave velocities along the Johnson Valley Fault caused by the 1999 Hector Mine earthquake. This reduction presumably perturbed a permanent compliant structure associated with the fault. The inferred changes in the fault zone compliance may produce a measurable deformation in response to background (tectonic) stresses. This deformation should have the same sense as the background stress, rather than the coseismic stress change. Here we investigate how the observed deformation of compliant zones in the Mojave Desert can be used to constrain the fault zone structure and stresses in the upper crust. We find that gravitational contraction of the coseismically softened zones should cause centimeters of coseismic subsidence of both the compliant zones and the surrounding region, unless the compliant fault zones are shallow and narrow, or essentially incompressible. We prefer the latter interpretation because profiles of line of sight displacements across compliant zones cannot be fit by a narrow, shallow compliant zone. Strain of the Camp Rock and Pinto Mountain fault zones during the Hector Mine and Landers earthquakes suggests that background deviatoric stresses are broadly consistent with Mohr‐Coulomb theory in the Mojave upper crust (with μ ≥ 0.7). Large uncertainties in Mojave compliant zone properties and geometry preclude more precise estimates of crustal stresses in this region. With improved imaging of the geometry and elastic properties of compliant zones, and with precise measurements of their strain in response to future earthquakes, the modeling approach we describe here may eventually provide robust estimates of absolute crustal stress.

Coupled afterslip and viscoelastic flow following the 2002 Denali Fault, Alaska earthquake

Models of postseismic deformation following the 2002 M 7.9 Denali Fault, Alaska earthquake provide insight into the rheologic structure of the Alaskan lithosphere and the physical processes activated following a large earthquake. We model coseismic GPS displacements and 4 yr of postseismic GPS position time-series with a coupled model of afterslip on the fault in the lithosphere and distributed viscous flow in the asthenosphere. Afterslip is assumed to be governed by a simplified version of a laboratory-derived rate-strengthening friction law that is characterized with a single parameter, σ(a − b), where σ is the effective normal stress on the fault and a − b is a dimensionless empirical parameter. Afterslip is driven by coseismic stress changes on the fault generated by the main shock. The lithosphere is modelled as an elastic plate overlying a linear, Maxwell, viscoelastic asthenosphere. We devise a scheme to simultaneously estimate the distributions of coseismic slip and afterslip, friction parameters, lithosphere thickness and asthenosphere viscosity. The postseismic GPS time-series are best reproduced with a 45–85 km thick elastic lithosphere overlying an asthenosphere of viscosity 0.6–1.5 × 1019 Pa s. The 45–85 km elastic lithosphere thickness is greater than or equal to the average crustal thickness in the region suggesting that distributed postseismic flow occurs largely within the mantle while postseismic deformation in the crust is confined largely to slip on a discrete fault penetrating the entire crust and perhaps into the uppermost mantle. For typical laboratory values of a − b of order 10−2 at temperatures corresponding to the inferred depth of afterslip, the estimated effective normal stress on the fault is ∼50 MPa, which is about an order of magnitude lower than effective normal stresses at hydrostatic pore pressure. Previous studies showed that models with linear Newtonian rheology cannot reproduce the observed GPS time-series but that models incorporating non-linear or biviscous flow of the mantle do fit the data. We show that a model with afterslip governed by a non-linear fault zone rheology coupled to Newtonian mantle flow is sufficient to reproduce the GPS time-series.

Modeling seismicity rate changes during the 1997 Umbria‐Marche sequence (central Italy) through a rate‐ and state‐dependent model

We model the spatial and temporal pattern of seismicity during a sequence of moderate‐magnitude normal faulting earthquakes, which struck in 1997 the Umbria‐Marche sector of Northern Apennines (Italy), by applying the Dieterich (1994) rate‐ and state‐dependent constitutive approach. The goal is to investigate the rate of earthquake production caused by repeated coseismic stress changes computed through a 3‐D elastic dislocation model in a homogeneous half‐space. The reference seismicity rate is assumed time independent, and it is estimated by smoothing the seismicity that occurred in the previous decade without declustering. We propose an analytical relation for deriving the stressing rate directly from the reference seismicity rate. This allows us to perform a tuning of the constitutive parameter Aσ (where A accounts for the direct effect of friction in the rate‐ and state‐dependent model and σ is the effective normal stress) into the Dieterich model through a maximum likelihood method, which yields for this seismic sequence a best fitting value equal to 0.04 MPa. Our computations show that, although seven out of eight main shocks are located in areas of increased rate of earthquake production, numerous aftershocks are located in seismicity shadows. Our simulations point out that the adopted value of Aσ strongly affects the pattern of both seismicity shadow and areas of enhanced rate of earthquake production. We conclude that solely accounting for static stress changes caused by the main shocks of this seismic sequence is not sufficient to forecast the complex spatial and temporal evolution of seismicity.

Postseismic Relaxation Along the San Andreas Fault at Parkfield from Continuous Seismological Observations

Seismic velocity changes and nonvolcanic tremor activity in the Parkfield area in California reveal that large earthquakes induce long-term perturbations of crustal properties in the San Andreas fault zone. The 2003 San Simeon and 2004 Parkfield earthquakes both reduced seismic velocities that were measured from correlations of the ambient seismic noise and induced an increased nonvolcanic tremor activity along the San Andreas fault. After the Parkfield earthquake, velocity reduction and nonvolcanic tremor activity remained elevated for more than 3 years and decayed over time, similarly to afterslip derived from GPS (Global Positioning System) measurements. These observations suggest that the seismic velocity changes are related to co-seismic damage in the shallow layers and to deep co-seismic stress change and postseismic stress relaxation within the San Andreas fault zone.

Coulomb stress evolution in Northeastern Caribbean over the past 250 years due to coseismic, postseismic and interseismic deformation

SUMMARY The Northeastern Caribbean region accommodates ∼20 mm yr −1 of oblique convergence between the North American and Caribbean plates, which is distributed between the subduction interface and major strike-slip faults within the overriding plate. As a result, this heavily populated region has experienced eleven large (M ≥ 7.0) earthquakes over the past 250 yr. In an effort to improve our understanding of the location and timing of these earthquakes, with an eye to understand where current seismic hazards may be greatest, we calculate the evolution of Coulomb stress on the major faults since 1751 due to coseismic, postseismic, and interseismic deformation. Our results quantify how earthquakes serve to relieve stress accumulated due to interseismic loading and how fault systems communicate with each other, serving both to advance or retard subsequent events. We find that the observed progressive westwards propagation of earthquakes on the Septentrional and Enriquillo strike-slip faults and along the megathrust was encouraged by coseismic stress changes associated with prior earthquakes. For the strike-slip faults, the loading of adjacent segments was further amplified by postseismic relaxation of a viscoelastic mantle in the decades following each event. Furthermore, earthquakes on the Septentrional fault relieve a small level of Coulomb stress on the parallel Enriquillo fault to the south (and vice versa), perhaps explaining anticorrelated timing of events on these respective fault systems. The greatest net build-up of Coulomb stress changes over the past 250 yr occurs along the central and eastern segment of the Septentrional and the Bowin strike-slip faults (65 ◦ –71 ◦ W), as no recent earthquake has relieved stress in these regions. For oblique thrust faults, net stress build-up over the past 250 yr is the largest on the North American/Caribbean megathrust west of 70.5 ◦ W. High Coulomb stress has also developed east of 65.5 ◦ W, where no historic events have been inferred to have relieved stress, though uncertainties in fault slip rates from our block model associated with a lack of GPS observations in this region may have led to an over-estimation of stress changes.

Stress changes from the 2008 Wenchuan earthquake and increased hazard in the Sichuan basin

On 12 May 2008, the devastating magnitude 7.9 (Wenchuan) earthquake struck the eastern edge of the Tibetan plateau, collapsing buildings and killing thousands in major cities aligned along the western Sichuan basin in China. After such a large-magnitude earthquake, rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. The mainshock of the 12 May earthquake ruptured with as much as 9 m of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike-slip and thrust motion that characterizes the region. The Sichuan basin and surroundings are also crossed by other active strike-slip and thrust faults. Here we present calculations of the coseismic stress changes that resulted from the 12 May event using models of those faults, and show that many indicate significant stress increases. Rapid mapping of such stress changes can help to locate fault sections with relatively higher odds of producing large aftershocks.

Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site

Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies.

Stress relations in three recent seismic series in the Murcia region, southeastern Spain

In this work we study the relation between the seismicity belonging to three recent seismic series in the Murcia region, southeastern Spain, with the coseismic stress changes due to their largest shocks. The first series occurred in 1999 near the town of Mula, with a mainshock magnitude of Mw = 5.1; the second series occurred in 2002, in the region of Bullas 35 km SW from the 1999 series, with events of magnitudes Mw ≤ 5.0. The last one occurred three years later (2005), 5 km SE from the 2002 series with magnitudes of events Mw ≤ 4.8. For the analysis, we relocated the seismicity of each series, next we computed the coseismic stress change produced by its main events and a foreshock and finally tested the possible stress relations inside and among the series based on the Coulomb Failure Stress changes. Our results show that the Mula 1999 main earthquake, associated in different publications to the Crevillente fault, was positively influenced for triggering by the foreshock. We also find that the 1999 series aftershocks were jointly influenced for triggering by both the foreshock and the mainshock events. From a comparative analysis of P wave arrivals, the series seems to be associated to different neighbouring seismogenic zones belonging to the Crevillente system. For the Bullas 2002 and the Bullas–La Paca series, we found that it is unlikely that any of them have been triggered by the previous Mula 1999 event, but it appears that the main event in 2002 had an important influence on the triggering of its aftershock activity and the promotion of the mainshock and aftershock seismicity in the zone of the 2005 series. Summing the effects of the last two main events, it may have been produced different zones of high stress increase on the aftershock area of the 2002 and 2005 series. Finally, the seismicity of the two latest series could be associated to different neighbouring seismogenic zones.

Modelling instantaneous dynamic triggering in a 3-D fault system: application to the 2000 June South Iceland seismic sequence

SUMMARY We present a model of seismogenesis on an extended 3-D fault, subject to the external perturba- tions of coseismic stress changes due to an earthquake occurring on another fault (the causative fault). As an application, we consider the spatio-temporal stress redistribution produced on the Hvalhnukur fault by the M S 6.6 2000 June 17 mainshock in the South Iceland Seismic Zone (SISZ). The latter is located nearly 64 km from the causative fault and failed 26 s after the main shock with an estimated magnitude M w = 5.25 ± 0.25, providing an example of instantaneous dynamic triggering. The stress perturbations are computed by means of a discrete wavenumber and reflectivity code. The response of the perturbed fault is then analysed solving the truly 3-D, fully dynamic (or spontaneous) problem accounting for crustal stratification. In a previous study, the response of the Hvalhnukur fault was analysed by using a spring-slider fault model (SS fault model), comparing the estimated perturbed failure time with the observed origin time. In addition to the perturbed failure time, this model can provide numerical estimates of many other dynamic features of the triggered event, which can be compared with available observations—the rupture history of the whole fault plane, its final extent and the seismic mo- ment of the induced event. We show the key differences existing between a mass-spring model and this extended fault model; in particular, we show the essential role of the load exerted by the neighbouring slipping points of the fault. By considering both rate- and state-dependent laws and non-linear slip-dependent law, we show how the dynamics of the 26 s fault strongly depend on the assumed constitutive law and initial stress conditions. In the case of rate- and state-dependent friction laws, assuming an initial effective normal stress distribution that is suitable for the SISZ and consistent with previously stated conditions of instantaneous dynamic triggering of the Hvalhnukur fault, we obtain results in general agreement with observations.