Coulomb Failure Stress Change Research Articles

Evidence of a Large Triggered Event in the Nepal Himalaya Following the Gorkha Earthquake: Implications Toward Enhanced Seismic Hazard

A DC (double couple) constrained multiple point-source moment-tensor inversion is performed on the band-passed (0.008–0.10 Hz) displacement data of the 25 April (Mw 7.8) 2015 Nepal mainshock, from 17 broadband stations in India. Our results reveal that the 25 April event (strike = 324°, dip = 14°, rake = 88°) ruptured the north-dipping main Himalayan thrust (MHT) at 16 km depth. We modeled the Coulomb failure stress changes (ΔCFS) produced by the slip on the fault plane of the 25 April Nepal mainshock. A strong correlation with occurrences of aftershocks and regions of increased positive ΔCFS is obtained below the aftershock zone of the 2015 Nepal mainshock. We notice that predicted ΔCFS at 16 km depth show a positive Coulomb stress of 0.06 MPa at the location of the 12 May 2015 event. These small modeled stress changes can lead to trigger events if the crust is already near to failure, but these small stresses can also advance the occurrence of future earthquakes. The main finding of our ΔCFS modeling implies that the 25 April event increased the Coulomb stress changes by 0.06 MPa at 16 km depth below the site of the 12 May event, and thus, this event can be termed as triggered. We propose that the seismic hazard in the Himalaya is not only caused by the mainshock slip on the MHT; rather, the occurrence of large triggered event on the MHT can also enhance our understanding of the seismic hazard in the Nepal Himalaya.

Transient poroelastic stress coupling between the 2015 M7.8 Gorkha, Nepal earthquake and its M7.3 aftershock

The large M7.3 aftershock occurred 17 days after the 2015 M7.8 Gorkha earthquake. We investigate if this sequence is mechanically favored by the mainshock via time-dependent fluid migration and pore pressure recovery. This study uses finite element models of fully-coupled poroelastic coseismic and postseismic behavior to simulate the evolving stress and pore-pressure fields. Using simulations of a reasonable permeability, the hypocenter was destabilized by an additional 0.15 MPa of Coulomb failure stress change (∆CFS) and 0.17 MPa of pore pressure (∆p), the latter of which induced lateral and upward diffusive fluid flow (up to 2.76 mm/day) in the aftershock region. The M7.3 location is predicted next to a local maximum of ∆p and a zone of positive ∆CFS northeast of Kathmandu. About 60% of the aftershocks occurred within zones having either ∆p > 0 or ∆CFS > 0. Particularly in the eastern flank of the epicentral area, ~83% of the aftershocks experienced postseismic fluid pressurization and ~88% of them broke out with positive pore pressure, which are discernibly more than those with positive ∆CFS (71%). The transient scalar field of fluid pressurization provides a good proxy to predict aftershock-prone areas in space and time, because it does not require extraction of an assumed vector field from transient stress tensor fields as is the case for ∆CFS calculations. A bulk permeability of 8.32 × 10−18 m2 is resolved to match the transient response and the timing of the M7.3 rupture which occurred at the peak of the ∆CFS time-series. This estimate is consistent with the existing power-law permeability-versus-depth models, suggesting an intermediately-fractured upper crust coherent with the local geology of the central Himalayas. The contribution of poroelastic triggering is verified against different poroelastic moduli and surface flow-pressure boundaries, suggesting that a poroelastic component is essential to account for the time interval separating the mainshock and the M7.3 aftershock.

Preliminary analysis on the source properties and seismogenic structure of the 2017 Ms7.0 Jiuzhaigou earthquake

At GMT time 13:19, August 8, 2017, an M s7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5–20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3–13 km, with a moment magnitude ( M w) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°–89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake.

What Is Better Than Coulomb Failure Stress? A Ranking of Scalar Static Stress Triggering Mechanisms from 105 Mainshock‐Aftershock Pairs

AbstractAftershocks may be triggered by the stresses generated by preceding mainshocks. The temporal frequency and maximum size of aftershocks are well described by the empirical Omori and Bath laws, but spatial patterns are more difficult to forecast. Coulomb failure stress is perhaps the most common criterion invoked to explain spatial distributions of aftershocks. Here we consider the spatial relationship between patterns of aftershocks and a comprehensive list of 38 static elastic scalar metrics of stress (including stress tensor invariants, maximum shear stress, and Coulomb failure stress) from 213 coseismic slip distributions worldwide. The rates of true‐positive and false‐positive classification of regions with and without aftershocks are assessed with receiver operating characteristic analysis. We infer that the stress metrics that are most consistent with observed aftershock locations are maximum shear stress and the magnitude of the second and third invariants of the stress tensor. These metrics are significantly better than random assignment at a significance level of 0.005 in over 80% of the slip distributions. In contrast, the widely used Coulomb failure stress criterion is distinguishable from random assignment in only 51–64% of the slip distributions. These results suggest that a number of alternative scalar metrics are better predictors of aftershock locations than classic Coulomb failure stress change.

Coseismic Coulomb failure stress changes caused by the 2017 M7.0 Jiuzhaigou earthquake, and its relationship with the 2008 Wenchuan earthquake

On August 8, 2017, a M 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China, resulting in significant casualties and property damage. Therefore, it is critical to identify the areas of potential aftershocks before reconstruction and re-settling people to avoid future disasters. Based on the elastic dislocation theory and a multi-layered lithospheric model, we calculate the Coulomb failure stress changes caused by the Wenchuan and Jiuzhaigou earthquakes, discuss the relationship between the M w7.9 Wenchuan and M 7.0 Jiuzhaigou earthquakes, and analyze the influence of the aftershock distribution and stress changes on the major faults in this region caused by the Jiuzhaigou earthquake. The co- and post-seismic stress changes caused by the Wenchuan earthquake significantly increased the stress accumulation at the hypocenter of the Jiuzhaigou earthquake. Therefore, the occurrence of the Jiuzhaigou earthquake was probably stimulated by the Wenchuan earthquake. The aftershock distribution is well explained by the co-seismic stress changes of the Jiuzhaigou earthquake. The stress accumulation and corresponding seismic hazard on the Maqu-Heye segment of the East Kunlun fault and the northern extremity of the Huya fault has been further increased by the Jiuzhaigou earthquake.

3-D dynamic rupture simulations of the 2016 Kumamoto, Japan, earthquake

Using 3-D dynamic rupture simulations, we investigated the 2016 Mw7.1 Kumamoto, Japan, earthquake to elucidate why and how the rupture of the main shock propagated successfully, assuming a complicated fault geometry estimated on the basis of the distributions of the aftershocks. The Mw7.1 main shock occurred along the Futagawa and Hinagu faults. Within 28 h before the main shock, three M6-class foreshocks occurred. Their hypocenters were located along the Hinagu and Futagawa faults, and their focal mechanisms were similar to that of the main shock. Therefore, an extensive stress shadow should have been generated on the fault plane of the main shock. First, we estimated the geometry of the fault planes of the three foreshocks as well as that of the main shock based on the temporal evolution of the relocated aftershock hypocenters. We then evaluated the static stress changes on the main shock fault plane that were due to the occurrence of the three foreshocks, assuming elliptical cracks with constant stress drops on the estimated fault planes. The obtained static stress change distribution indicated that Coulomb failure stress change (ΔCFS) was positive just below the hypocenter of the main shock, while the ΔCFS in the shallow region above the hypocenter was negative. Therefore, these foreshocks could encourage the initiation of the main shock rupture and could hinder the propagation of the rupture toward the shallow region. Finally, we conducted 3-D dynamic rupture simulations of the main shock using the initial stress distribution, which was the sum of the static stress changes caused by these foreshocks and the regional stress field. Assuming a slip-weakening law with uniform friction parameters, we computed 3-D dynamic rupture by varying the friction parameters and the values of the principal stresses. We obtained feasible parameter ranges that could reproduce the characteristic features of the main shock rupture revealed by seismic waveform analyses. We also observed that the free surface encouraged the slip evolution of the main shock.

Postseismic deformation associated with the 2008 Mw 7.9 Wenchuan earthquake, China: Constraining fault geometry and investigating a detailed spatial distribution of afterslip

We reconstruct two types of fault models associated with the 2008 Mw 7.9 Wenchuan earthquake, one is a listric fault connecting a shallowing sub-horizontal detachment below ∼20km depth (fault model one, FM1) and the other is a group of more steeply dipping planes further extended to the Moho at ∼60km depth (fault model two, FM2). Through comparative analysis of the coseismic inversion results, we confirm that the coseismic models are insensitive to the above two type fault geometries. We therefore turn our attention to the postseismic deformation obtained from GPS observations, which can not only impose effective constraints on the fault geometry but also, more importantly, provide valuable insights into the postseismic afterslip. Consequently, FM1 performs outstandingly in the near-, mid-, and far-field, whether considering the viscoelastic influence or not. FM2 performs more poorly, especially in the data-model consistency in the near field, which mainly results from the trade-off of the sharp contrast of the postseismic deformation on both sides of the Longmen Shan fault zone. Accordingly, we propose a listric fault connecting a shallowing sub-horizontal detachment as the optimal fault geometry for the Wenchuan earthquake. Based on the inferred optimal fault geometry, we analyse two characterized postseismic deformation phenomena that differ from the coseismic patterns: (1) the postseismic opposite deformation between the Beichuan fault (BCF) and Pengguan fault (PGF) and (2) the slightly left-lateral strike-slip motions in the southwestern Longmen Shan range. The former is attributed to the local left-lateral strike-slip and normal dip-slip components on the shallow BCF. The latter places constraints on the afterslip on the southwestern BCF and reproduces three afterslip concentration areas with slightly left-lateral strike-slip motions. The decreased Coulomb Failure Stress (CFS) change ∼0.322 KPa, derived from the afterslip with viscoelastic influence removed at the hypocentre of the Lushan earthquake, indicates that the postseismic left-lateral strike-slip and normal dip-slip motions may have a mitigative effect on the fault loading in the southwestern Longmen Shan range. Nevertheless, it is much smaller than the total increased CFS changes (∼8.368 KPa) derived from the coseismic and viscoelastic deformations.

CALCULATION OF THE CO‐SEISMIC DEFORMATION AND STRESS CHANGES OF THE KAIKOURA Mw7.8 EARTHQUAKE, NOV 13, 2016

AbstractOn 13 November, 2016, an Mw7.8 earthquake occurred in Kaikoura, northern South Island, New Zealand. The Mw 7.8 earthquake caused strong surface deformation, massive landslides and tsunami. Based on the fault slip model released by United States Geological Survey, the co‐seismic deformation and stress changes by Kaikoura Mw7.8 earthquake were computed with the global heterogeneous ellipsoid Earth model and high precision topography. The preliminary results show that the hanging wall of the Mw7.8 earthquake uplifted to the northeast, while the footwall subducting southwest. The co‐seismic deformation caused by this earthquake is up to several centimeters from Kaikoura to Campbell and the capital city of Wellington. The maximum co‐seismic horizontal displacement is about 1.2 m whereas the vertical is about 1.1 m. Although the accumulated strain along seismogenic faults had been released by this earthquake, the compressional force at the both ends of the seismogenic faults was increased due to co‐seismic stress changes and the maximum value of the co‐seismic stress changes reaches the order of MPa. The Coulomb failure stress changes are also up to MPa that are concentrated in the vicinity of epicenter. At the same time, the risk for NE‐SW dextral slip faults increases due to the co‐seismic shear stress. We also calculated the Coulomb Failure Stress changes in the North Island and South Island with local fault systems respectively. Our results show that Coulomb failure stress changes are positive in both regions, which means subsequent earthquakes are possible.

Slow slip events in the early part of the earthquake cycle

AbstractIn February 2014 a Mw = 7.0 slow slip event (SSE) took place beneath the Nicoya Peninsula, Costa Rica. This event occurred 17 months after the 5 September 2012, Mw = 7.6, earthquake and along the same subduction zone segment, during a period when significant postseismic deformation was ongoing. A second SSE occurred in the middle of 2015, 21 months after the 2014 SSE and 38 months after the earthquake. The recurrence interval for Nicoya SSEs was unchanged by the earthquake. However, the spatial distribution of slip for the 2014 event differed significantly from previous events, having only deep (~40 km) slip, compared to previous events, which had both deep and shallow slip. The 2015 SSE marked a return to the combination of deep plus shallow slip of preearthquake SSEs. However, slip magnitude in 2015 was nearly twice as large (Mw = 7.2) as preearthquake SSEs. We employ Coulomb Failure Stress change modeling in order to explain these changes. Stress changes associated with the earthquake and afterslip were highest near the shallow portion of the megathrust, where preearthquake SSEs had significant slip. Lower stress change occurred on the deeper parts of the plate interface, perhaps explaining why the deep (~40 km) region for SSEs remained unchanged. The large amount of shallow slip in the 2015 SSE may reflect lack of shallow slip in the prior SSE. These observations highlight the variability of aseismic strain release rates throughout the earthquake cycle.

Integrated geophysical characteristics of the 2015 Illapel, Chile, earthquake

AbstractOn 16 September 2015, a Mw 8.3 earthquake ruptured the subduction zone offshore of Illapel, Chile, generating an aftershock sequence with 14 Mw 6.0–7.0 events. A double source W phase moment tensor inversion consists of a Mw 7.2 subevent and the main Mw 8.2 phase. We determine two slip models for the mainshock, one using teleseismic broadband waveforms and the other using static GPS and InSAR surface displacements, which indicate high slip north of the epicenter and west‐northwest of the epicenter near the oceanic trench. These models and slip distributions published in other studies suggest spatial slip uncertainties of ~25 km and have peak slip values that vary by a factor of 2. We relocate aftershock hypocenters using a Bayesian multiple‐event relocation algorithm, revealing a cluster of aftershocks under the Chilean coast associated with deep (20–45 km depth) mainshock slip. Less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region. Most aftershocks are thrust‐faulting events, except for normal‐faulting events near the trench. Coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip; other aftershocks are more clearly associated with loading from the mainshock. These observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths (associated with many aftershocks) and with homogeneous slip (and few aftershocks) up to the trench. This event likely triggered seismicity separate from the main slip region, including along‐strike events on the megathrust and intraplate extensional events.

INFLUENCE OF STRESS TRANSFER IN PROBABILITY ESTIMATES OF M≥6.5 EARTHQUAKES IN GREECE AND SURROUNDING AREAS

The coseismic stress changes due to strong earthquakes (M>6.5) that occurred in Greece and its adjacent areas since the beginning of the 20th century (instrumental era) are calculated and the future seismic hazard expressed in terms of the probabilities of occurrence of M≥6.5 events is assessed. Calculations of the change in Coulomb failure stress reveal that 61 out of 67 ruptures were brought closer to failure by the preceding shocks. A new insight on the evaluation of future seismic hazards, in the study area, is given by translating the calculated coseismic stress changes into earthquake probability. We incorporated the effect of stress change into the time–depended probability estimates using an earthquake nucleation constitutive relation, which includes both permanent and transient effects of stress changes. Taking into account the current stress changes onto each major fault, the probability calculations were performed and given for the whole study area during the next 30 years

Strong Aftershock Study Based on Coulomb Stress Triggering—A Case Study on the 2016 Ecuador Mw 7.8 Earthquake

The 2016 Ecuador M 7.8 earthquake ruptured the subduction zone boundary between the Nazca plate and the South America plate. This M 7.8 earthquake may have promoted failure in the surrounding crust, where six M ≥ 6 aftershocks occurred following this mainshock. These crustal ruptures were triggered by the high coulomb stress changes produced by the M 7.8 mainshock. Here, we investigate whether the six M ≥ 6 aftershocks are consistent with the positive coulomb stress region due to the mainshock. To explore the correlation between the mainshock and the aftershocks, we adopt a recently published high-quality finite fault model and focal mechanisms to study the coulomb stress triggers during the M 7.8 earthquake sequence. We compute the coulomb failure stress changes (ΔCFS) on both of the focal mechanism nodal planes. We compare the ΔCFS imparted by the M 7.8 mainshock on the subsequent aftershocks with the epicenter location of each aftershock. In addition, the shear stress, normal stress, and coulomb stress changes in the focal sources of each aftershock are also computed. Coulomb stress changes in the focal source for the six M ≥ 6 aftershocks are in the range of −2.17–7.564 bar. Only one computational result for the M 6.9 aftershock is negative; other results are positive. We found that the vast majority of the six M ≥ 6 aftershocks occurred in positive coulomb stress areas triggered by the M 7.8 mainshock. Our results suggest that the coulomb stress changes contributed to the development of the Ecuador M 7.8 earthquake sequence.

An investigation into the remote triggering of the Oita earthquake by the 2016 Mw 7.0 Kumamoto earthquake using full wavefield simulation

High-amplitude seismic waves from the Mw 7.0 Kumamoto earthquake of April 16, 2016, triggered another large earthquake 80 km to the NE roughly 30 s later. The source was located at shallow depths beneath the Yufuin geothermal field, Oita Prefecture, Japan, and the event magnitude was approximately 5.9. To date, this is one of the clearest known examples of a remotely triggered large earthquake. The triggered Oita event was followed by significant seismicity, which was distinct from the aftershocks of the Kumamoto earthquake. The Coulomb failure stress change around the hypocenter, calculated for the passing waves of the Kumamoto earthquake by full wavefield simulation, was about 0.7 MPa when the Oita earthquake was triggered, with the static stress change being an order of magnitude smaller. The dynamic stress changes likely played an important role in triggering. A return to low seismicity levels 1 month after the triggered earthquake may have important implications for seismic hazard due to dynamic triggering.

Fault dip angle determination with the jℛi criterion and coulomb stress changes associated with the 2015 Mw 7.9 Gorkha Nepal earthquake revealed by InSAR and GPS data

Minimizing data misfit has been widely used in geodetic determination of fault dip angle, however, it ignores the contribution from data noise. Here, we use the jℛi criterion, which takes into account both data misfit and the contribution from data noise, to determine dip angle. Synthetic tests show dip angle estimates with the jℛi criterion are more accurate and robust than those with data misfit minimization. We applied this jℛi criterion to the determination of the dip angle of the 2015 Mw 7.9 Gorkha Nepal earthquake using Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data. The results show that the event ruptured to the north of Kathmandu with a maximum slip value of 5.8m and a dip angle of 9.5°. We also calculated the coulomb failure stress changes resolved onto the receiver faults that are generally consistent with the fault geometry of this event, suggesting the possible triggering relation to aftershocks with low dip angles and a zone of seismic potential located to the west of the rupture zone.

Coulomb stress evolution in a diffuse plate boundary: 1400 years of earthquakes in eastern California and western Nevada, USA

Diffuse plate boundaries are characterized by deformation distributed over a wide area in a complex network of active faults and by relatively low strain rates. These characteristics make it difficult to understand the spatial and temporal distribution of seismicity. The area east of the Sierra Nevada, between longitudes 121 degrees W and 116 degrees W, is part of a diffuse plate boundary. At least 17 major surface-rupturing earthquakes have happened here in the last 1400 years. Our purpose is to determine whether these events influence each other or whether they are randomly distributed in time and space. We model the evolution of coseismic and postseismic Coulomb failure stress changes (Delta CFS) produced by these earthquakes, and we also model interseismic stresses on the entire fault network. Our results show that 80% of the earthquake ruptures are located in areas of combined coseismic and postseismic Delta CFS >= 0.2 bar. This relationship is robust, as shown by the control tests that we carried out using random earthquake sequences. We also show that the Fish Lake Valley, Pyramid Lake, and Honey Lake faults have accumulated 45, 37, and 27 bars, respectively, of total Delta CFS (i.e., coseismic + postseismic + interseismic) in the last 1400 years. Such values are comparable to the average stress drop in a major earthquake, and these three faults may be therefore close to failure.

Geodetically constrained models of viscoelastic stress transfer and earthquake triggering along the North Anatolian fault

AbstractOver the past 80 years, 8 MW > 6.7 strike‐slip earthquakes west of 40° longitude have ruptured the North Anatolian fault (NAF) from east to west. The series began with the 1939 Erzincan earthquake in eastern Turkey, and the most recent 1999 MW = 7.4 Izmit earthquake extended the pattern of ruptures into the Sea of Marmara in western Turkey. The mean time between seismic events in this westward progression is 8.5 ± 11 years (67% confidence interval), much greater than the timescale of seismic wave propagation (seconds to minutes). The delayed triggering of these earthquakes may be explained by the propagation of earthquake‐generated diffusive viscoelastic fronts within the upper mantle that slowly increase the Coulomb failure stress change ( CFS) at adjacent hypocenters. Here we develop three‐dimensional stress transfer models with an elastic upper crust coupled to a viscoelastic Burgers rheology mantle. Both the Maxwell (ηM = 4 × 1018−1 × 1019 Pa s) and Kelvin (ηK = 1 × 1018−1 × 1019 Pa s) viscosities are constrained by studies of geodetic observations before and after the 1999 Izmit earthquake. We combine this geodetically constrained rheological model with the observed sequence of large earthquakes since 1939 to calculate the time evolution of CFS changes along the North Anatolian fault due to viscoelastic stress transfer. Apparent threshold values of mean CFS at which the earthquakes in the eight decade sequence occur are between ∼0.02 to ∼3.15 MPa and may exceed the magnitude of static CFS values by as much as 177%. By 2023, we infer that the mean time‐dependent stress change along the northern NAF strand in the Marmara Sea near Istanbul, which may have previously ruptured in 1766, may reach the mean apparent time‐dependent stress thresholds of the previous NAF earthquakes.

Influence of the impoundment of the Three Gorges Reservoir on the micro-seismicity and the 2013 M5.1 Badong earthquake (Yangtze, China)

On December 16, 2013, right after the Three Gorges Reservoir (TGR) reached its highest annual water level, a powerful M5.1 earthquake occurred in Badong County, China’s Hubei Province. The epicenter is 5.5km away from the upstream boundary and 100km from the dam. Was this earthquake triggered by the impoundment of the TGR, and what are its subsequences? To answer these questions, we constructed a coupled three-dimensional poroelastic finite element model to examine the ground surface deformation, the Coulomb failure stress change (ΔCFS) due to the variation of elastic stress and pore pressure, and the elastic strain energy potential accumulation in the TGR region upon the occurrence of this event. Our calculated maximum surface deformation values beneath the TGR compare well with GPS observations, which validates our numerical model. At the hypocenter of the earthquake, ΔCFS is around 8.0∼11.0kPa, revealing that it may be eventually triggered by the impoundment. We also discovered that the total elastic strain energy potential accumulation due to the impounded water load is around 1.7×1012J, merely equivalent to 0.01% of the total energy released by this event, indicating that this earthquake is predominately controlled by the typical regional tectonic settings as well as the weak fault zones, and the reservoir impoundment might only facilitate its procedure or occurrence. Furthermore, the stress level in this region remains high after this earthquake and the subsequent reservoir-triggered micro-seismicity or even bigger event are highly possible.

The implications of regional microseismic activities: A lesson from 2008 Ms. 8.0 Wenchuan earthquake

The Ms. 8.0 Wenchuan earthquake has greatly perturbed the crustal stress in Longmenshan fault zone (LFZ) and its neighboring regions, and hence significantly changed the regional seismicity pattern. Convectional coulomb stress change model based on half-space dislocation source of layered elastic or viscoelastic crust can only provide brief reference to estimate the after-shock sequences roughly. In this study, we apply the Z-test statistical method to examine the earthquake catalogue five years before and after this event to detect the changes of regional seismicity intensity and their statistical significance in LFZ and its neighboring regions. We analyze changes of the Z values with respect to time and seismic magnitude in each region. Our results reveal that, after the Wenchuan earthquake, seismicity intensity has significantly increased in Eastern Kunlun fault, southern Longmenshan fault, Longquanshan fault and Huayingshan fault in Sichuan basin, but has significantly decreased in northern and central Longmenshan fault, vicinity of the Songpan-Minjiang fault, and Fubianhe fault in Bayankala block, respectively. Seismicity rate change in northern and central Anninghe-Zemuhe fault zone is not correlated with the Wenchuan earthquake. These results remain within 99% confidence level of the Z-test statistical method. We compare our statistical analysis results with calculations of coseismic Coulomb failure stress change due to the Wenchuan earthquake from previous studies. In most of the regions and fault zones, the sign of the seismicity rate change agrees with the sign of the Coulomb stress change.

Foreshock triggering of the 1 April 2014 Mw 8.2 Iquique, Chile, earthquake

Abstract On April 1st, 2014, a Mw 8.2 (U.S. Geological Survey moment magnitude) earthquake occurred in the subduction zone offshore northern Chile. In the two weeks leading up to the earthquake, a sequence of foreshocks, starting with a Mw 6.7 earthquake on March 16th and including three more Mw 6.0+ events, occurred predominantly south of the April 1st mainshock epicenter and up-dip of the area of significant slip during the mainshock. Using earthquake locations and source parameters derived in a previous study ( Hayes et al., 2014 ) and a Coulomb failure stress change analysis of these events, we assess in detail the hypothesis that the earthquakes occurred as a cascading sequence, each event successively triggering the next, ultimately triggering the rupture of the mainshock. Following the initial Mw 6.7 event, each of the three largest foreshocks (Mw 6.4, 6.2 and 6.3), as well as the hypocenter of the mainshock, occurred in a region of positive Coulomb stress change produced by the preceding events, indicating these events were brought closer to failure by the prior seismicity. In addition, we reexamine the possibility that aseismic slip occurred and what role it may have played in loading the plate boundary. Using horizontal GPS displacements from along the northern Chile coast prior to the mainshock, we find that the foreshock seismicity alone likely does not account for the observed signals. We perform a grid search for the location and magnitude of an aseismic slip patch that can account for the difference between observed signals and foreshock-related displacement, and find that a slow slip region with slip corresponding to a Mw ∼ 6.8 earthquake located coincident with or up-dip of the foreshock seismicity can best explain this discrepancy. Additionally, such a slow slip region positively loads the mainshock hypocentral area, enhancing the positive loading produced by the foreshock seismicity.

Graben formation and dike arrest during the 2009 Harrat Lunayyir dike intrusion in Saudi Arabia: Insights from InSAR, stress calculations and analog experiments

AbstractDetailed spatial and temporal accounts of propagating dikes from crustal deformation data, including their interplay with faulting, are rare, leaving many questions about how this interplay affects graben formation and the arrest of dikes unanswered. Here we use interferometric synthetic aperture radar (InSAR) observations, stress calculations, and analog experiments to investigate the interaction between an intruding dike and normal faulting during the 2009 Harrat Lunayyir dike intrusion in western Saudi Arabia. We generated five displacement maps from InSAR data to unravel the temporal evolution of deformation covering the majority of the intrusion. We find that the observed surface displacements can be modeled by a ~2 m thick dike with an upper tip ~2 km below the surface on 16 May 2009, 4 weeks after the onset of seismic unrest. In the following three days, the dike propagated to within ~1 km of the surface with graben‐bounding normal faulting dominating the near‐field deformation. The volume of the dike doubled between mid‐May and mid‐June. We carried out analog experiments that indicate that the wedge‐shaped graben grew outward with the faulting style changing progressively from normal faulting to oblique. Coulomb failure stress change calculations show that the intruding dike caused two zones of shallow horizontal tension on both sides of the dike, producing two zones of fissuring and normal faulting at the surface. In return, the faulting provoked compression around the upper tip of the dike, holding back its vertical propagation.