Static Coulomb Failure Stress Research Articles

The 2023 Mw 7.8 and 7.6 Earthquake Doublet in Southeast Türkiye: Coseismic and Early Postseismic Deformation, Faulting Model, and Potential Seismic Hazard

Abstract On 6 February 2023, Mw 7.8 and 7.6 earthquakes struck southeast Türkiye and northwest Syria. They are the largest earthquakes in Türkiye in over 80 yr, causing significant damage and fatalities. We used Advanced Land Observation Satellite-2 and Sentinel-1 Synthetic Aperture Radar images to obtain near-field coseismic displacements by Differential Interferometric Synthetic Aperture Radar (DInSAR) and pixel offset tracking (POT). Discontinuities of the surface deformation suggest that the Mw 7.8 event ruptured ∼320 km along the East Anatolian fault, and the Mw 7.6 event ruptured ∼150 km near Elbistan, southern Türkiye. We inverted these earthquakes' fault geometry and slip distribution based on Global Navigation Satellite Systems, DInSAR, and POT displacements. The estimated fault slip model shows that the first Mw 7.8 event ruptured a steeply southeast-dipping fault, and the seismogenic fault of the second Mw 7.6 event is north-dipping with complex geometry. The dip angle of subfaults of the Mw 7.6 earthquake decreases from east to west. Faults responsible for the two earthquakes are dominated by left-lateral strike-slip motion, with the maximum slip of ∼9.1 m. Early postseismic deformation within two months exhibits displacement discontinuities in the Amanos and Pazarcık segments and the Çardak fault, suggesting that the afterslip partially compensated the coseismic slip deficit at the shallow depths. Furthermore, static Coulomb failure stress changes induced by the two earthquakes indicate that the southwestern Pütürge segment of the East Anatolian fault has a high risk of future rupture.

Source mechanisms and triggering process for the April 12th and 13th, 2014 earthquake doublet in the Solomon Islands

Mw 7.6 and Mw 7.4 earthquakes occurred 16 h apart on April 12 and April 13, 2014, at depths of 15 km and 35 km, respectively, southwest of Makira Island (San Cristobal) in the Solomon Islands. We study the source mechanisms to investigate the interrelation between the two events, and to investigate why the Solomon Islands have a historically high rate of doublet earthquakes. Teleseismic P waveform data for both events were used to obtain the slip distributions for the two earthquakes by a finite fault slip inversion method. The Mw 7.6 event is an east–west left-lateral strike slip, where the maximum slip is observed 8–24 km east of the hypocenter. The Mw 7.4 earthquake is a thrust event with north and south-dipping nodal planes. The fault plane is not clear from the aftershock distribution, but the north-dipping plane considered to be the likely fault plane because of the simpler slip distribution compared to the south-dipping plane. The static Coulomb failure stress changes caused by the first earthquake were calculated in the region of the second earthquake. The results show that there was a + 48.59 kPa change at the hypocenter for the assumed north-dipping plane and + 18.24 kPa for the assumed south-dipping plane of the Mw 7.4 event. The temporal pattern of aftershocks shows a possible rate increase of stress prior to the occurrence of the Mw 7.4 events, which may also contribute to the triggering of the second event by static Coulomb Stress changes. We propose a model that supports the geological complexity of the region that may encourage such doublet events.Graphical

Source Model of the 2023 Turkey Earthquake Sequence Imaged by Sentinel-1 and GPS Measurements: Implications for Heterogeneous Fault Behavior along the East Anatolian Fault Zone

On 6 February 2023, a devastating doublet of earthquakes with magnitudes of Mw 7.8 and Mw 7.6 successively struck southeastern Turkey near the border of Syria. The earthquake sequence represents the strongest earthquakes in Turkey during the past 80 years and caused an extensive loss of life and property. In this study, we processed Sentinel-1 and GPS data to derive the complete surface displacement caused by the earthquake sequence. The surface displacements were adopted to invert for the fault geometry and coseismic slip distribution on the seismogenic faults of the earthquake sequence. The results indicate that the coseismic rupture of the Turkey earthquake sequence was dominated by left-lateral strike slips with a maximum slip of ~10 m on the East Anatolian Fault Zone (EAFZ) and the Sürgü fault (SF). Significant surface ruptures are recognized based on the geodetic inversion, which is consistent with the analysis of post-earthquake satellite images. The cumulative released moment of the two earthquakes reached 9.62 × 1020 Nm, which corresponds to an event of Mw 7.95. Additionally, the interseismic fault slip rates and locking depths along the central and western segments of the EAFZ were estimated using the high-resolution long-term velocity field. The results reveal significant lateral variations of fault slip rates and locking depths along the central and western segments of the EAFZ. Generally, the estimated fault locking zone showed good spatial consistency with the coseismic fault rupture of the Mw 7.8 shock on the EAFZ. The static coulomb failure stress (CFS) change due to the Mw 7.8 earthquakes suggests that the subsequent Mw 7.6 event was certainly promoted by the Mw 7.8 shock. The stress transfers from the fault EAFZ to the fault SF were realized by unclamping the interface of the fault SF, which significantly reduces the effective normal stress on the fault plane. Large CFS increases in the western Puturge segment of the EAFZ, which was not ruptured in the 2020 Mw 6.8 and the 2023 Mw 7.8 earthquakes, highlight the future earthquake risk in this fault segment.

Coseismic and postseismic deformation of the 2016 Mw 6.0 Petermann ranges earthquake from satellite radar observations

Continental earthquakes, especially shallow earthquakes, can cause disasters in populated areas. On 20 May 2016, a moderate magnitude earthquake (Mw 6.0) ruptured on Northern Territory of Australia. With the aim of seismic hazard assessment, coseismic and postsesmic deformation of the Petermann earthquake has been determined using ALOS-2 ascending and Sentinel-1A descending images. The coseismic deformation field suggests that the earthquake ruptured in the NNW direction with a length of about 20 km, a rake angle of 57.1° and a moment magnitude of Mw 6.06. The maximum slip of 1.31 m can be observed at the depth of 1.25 km. The small baseline subset (SBAS) InSAR was utilized to obtain the postseismic deformation from 69 Sentinel-1A images in a descending orbit. The spatiotemporal evolution of the postseismic deformation presents a gradually decaying trend, with a maximum displacement of 5.4 cm within 858 days after the mainshock. Our afterslip model suggests that the maximum slip of 0.28 m occurred at the depth of 0.25 km with a slight shift to the northwest compared to the coseismic slip. The static Coulomb failure stress (CFS) changes at various depths were calculated, indicating the consistency between high loaded stress zones and aftershock distribution.

Spatiotemporal Evolution of Pore Pressure Changes and Coulomb Failure Stress in a Poroelastic Medium for Different Faulting Regimes

AbstractSpatial distribution of aftershocks has been explained by the co‐seismic static Coulomb Failure Stress changes (ΔCFS) to some extent. In practice, the earth's crust is porous medium saturated with fluid filling pores, cavities, cracks, and faults. The pore fluid flow plays an important role in the evolution of the stress field in porous medium of the earth. The ΔCFS can be influenced by the co‐seismic pore pressure changes and the post‐seismic pore fluid flow. In order to understand such impacts, we built 3D fault‐slip finite element models to study the evolution of the pore pressure changes and the ΔCFS based on the fully coupled poroelastic theory. The strike‐slip, reverse, and normal faulting models were investigated separately. The patterns of co‐seismic ΔCFS were similar to the elastic stress changes on a homogeneous half‐space, but there were considerable differences in details especially in the near‐fault area when considering co‐seismic pore pressure change. Due to post‐seismic pore fluid flow, the near‐field stress shadow narrowed gradually in the strike‐slip model. In the reverse faulting model, both the near‐field stress shadow and enhanced areas expanded. In the normal faulting model, the stress shadow near the epicenter reduced. The pore pressure changes decayed sharply at the beginning and then gradually approached to zero because the system evolves to drained conditions. These results can help us to understand the effect of pore fluid flow on Coulomb Failure Stress evolution and might improve the understanding of earthquake triggering and aftershock forecasting.

Heterogeneous Interseismic Coupling Along the Xianshuihe‐Xiaojiang Fault System, Eastern Tibet

AbstractWe use Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data to quantify the interseismic coupling along the Xianshuihe‐Xiaojiang fault system (XXFS). Our results confirm 7–11 mm/a of left‐lateral strike‐slip motion along the XXFS. South of the Shimian County, high interseismic coupling is found down to 20 km‐depth along the Anninghe‐Zemuhe fault. A second 130 km‐long section, highly coupled down to 10–15 km‐depth, is identified along the northern Xiaojiang fault. North of the Kangding County, the Xianshuihe fault appears to be predominantly creeping at depth. Previously identified long‐lasting surface creep following the 1973 M 7.6 Luhuo earthquake has now ended, possibly marking the initiation of a new period of stress accumulation and progressive relocking of the fault. Farther south along the fault, we identify a ∼30 km‐long section (∼30.2°–30.4°N) creeping at ∼7 mm/yr. Furthermore, creep accelerated by ∼2 mm/yr during the 2008–2014 period, possibly as a result of the static Coulomb failure stress increment induced by the 2008 Mw 7.9 Wenchuan earthquake, that occurred ∼200 km northeast of it. The 2014 Mw 5.9 Kangding earthquake occurred along the creeping section at the end of the period of accelerated creep. Finally, taking advantage of the long historical records of past large earthquakes available since 1327, we quantitatively compare the moment budget along the XXFS. Results highlight that along the Anninghe, Zemuhe, and Xiaojiang faults, moment deficit for Mw > 7 earthquakes has now accumulated.

Delineation of the Causative Fault of Recent Earthquakes (April–May 2020) in Delhi from Seismological and Morphometric Analysis

Recently, amid the pandemic of COVID-2019, the north-east Delhi region experienced two small earthquakes in a short span of 1 month; the first occurred on 12th April 2020 (Mw 3.5) and the other on 10th May 2020 (Mw 3.4). These events were followed by 4 aftershocks of magnitude Mw ≤ 3.0. We carried out morphotectonic (high stream length-gradient index) and static Coulomb stress failure analyses to delineate the hidden causative fault(s) in the region. In the study, ASTER DEM data of 30 m resolution and Survey of India (SoI) toposheets on 1:50,000 scales were used for morphotectonic analysis. The analysis depicted a very high stream length-gradient (SL) and fall in elevation in the epicentral area, suggesting the area to be tectonically active with a NE-SW trending fault line. In addition, the nature of static Coulomb failure stress contours for both the main events, Mw 3.5 and Mw 3.4, suggests an NNE-SSW trending high Coulomb stress regime. Such a high coulomb stress regime is obvious at the location where a high SL index and fall in elevation were marked, which clearly indicates the presence of NNE-SSW trending a causative fault, named ‘Khanpur-Japti fault’.

Fault Instability and Its Relation to Static Coulomb Failure Stress Change in the 2016 Mw 6.5 Pidie Jaya Earthquake, Aceh, Indonesia

Herein, we applied the fault instability criterion and integrated it with the static Coulomb stress change (ΔCFS) to infer the mechanism of the 2016 Mw 6.5 Pidie Jaya earthquake and its aftershock distribution. Several possible causative faults have been proposed; however, the existence of a nearby occurrence, the 1967 mb 6.1 event, created obscurity. Hence, we applied the fault instability analysis to the Pidie Jaya earthquake 1) to corroborate the Pidie Jaya causative fault analysis and 2) to analyze the correlation between ΔCFS distribution imparted by the mainshock and the fault instability of the reactivated fault planes derived from the focal solution of the Pidie Jaya aftershocks. We performed the fault instability analysis for two possible source faults: the Samalanga-Sipopok Fault and the newly inferred Panteraja Fault. Although the maximum instability value of the Samalanga-Sipopok Fault is higher, the dip value of the Panteraja Fault coincides with its optimum instability. Therefore, we concluded that Panteraja was the causative fault plane. Furthermore, a link between the 1967 mb 6.1 event and the 2016 Mw 6.5 earthquake is discussed. To analyze the correlation between the fault instability and the ΔCFS, we resolved the ΔCFS of the Pidie Jaya mainshock on its aftershock planes and compared the ΔCFS results with the fault instability calculation on each aftershock plane. We discussed the possibility of conjugate failure as shown by the aftershock fault instability. Related to the ΔCFS and fault instability comparison, we found that not all the aftershocks have positive ΔCFSs, but their instability value is high. Thus, we suggest that the fault plane instability plays a role in events that do not occur in positive ΔCFS areas. Apart from these, we also showed that the off-Great Sumatran Fault (Panteraja and Samalanga-Sipopok Faults) are unstable in the Sumatra regional stress setting, thereby making it more susceptible to slip movement.

Stress drop, earthquake aftershocks and regional stress relation based on synthetic static Coulomb failure stress model

Coulomb failure criterion has been applied widely in the scope of earthquake science to explain earthquake interactions base on stress change, with the well-known method named Coulomb failure stress change ΔCFS). Preceding studies have showed: increase ΔCFS, depicted as positive stress lobes, has correlation with occurrence of following events. However in the calculation process, ratio between regional stress and earthquake stress drop would affect stress distribution. Based on preceding researches, earthquake stress drop with similar magnitude to regional stress, would give results positive stress lobes along and at the base of the fault. Those stress distribution, could explain events interaction and mechanism of earthquake. This work carries out synthetic modeling of static ΔCFS upon varying earthquake stress drop and regional stress using COULOMB3.3. In accord with preceding studies, the results show positive ΔCFS along the fault when stress drop is comparable to regional stress. And yet, positive ΔCFS would take place at the top and at the base of the fault, expanding to the center of the fault -where the hypocenter is assumed- as the stress drop reaching regional stress in magnitude. This could explain the separated clusters of aftershock in depth observed in some earthquakes.

More major earthquakes at the Nepal Himalaya? – Study on Coulomb stress perspective

On April 2015 a major earthquake of 7.9 Mw occurred in the Nepal Himalaya, followed by 553 earthquakes of local magnitude greater than 4.0 within the first 43 days including another major event of 7.3 Mw. We resolve the static coulomb failure stress (CFS) change onto the finite fault models of 7.9 Mw after Elliott et al. (2016) and Galezka et al. (2015) and its effect on associated receiver faults. Correlation of aftershocks with the enhanced CFS condition shows that the Elliott et al. (2016) model explains 60.4% and the Galezka et al. (2015) model explains about 47.7% of the aftershocks in high stress regions. Aftershocks were poorly spatially correlated with the enhanced CFS condition after the 7.9 Mw main shock and can be explained by correlation with release of seismic energy from the associated secondarily stressed prominent thrust planes and transverse faults. Stress resolved on the associated receiver faults show increased stress on both transverse and thrust fault systems with the potential of triggering significant aftershocks or subsequent main shocks.

Source mechanism of the 1998 Mw 7.4 intraplate strike-slip earthquake in the West Philippine Basin revealed by Coulomb stress changes

On April 11, 2012, Mw 8.6 and Mw 8.2 earthquakes occurred off the west coast of northern Sumatra as a result of strike-slip type events within the Indo-Australia plate. It is suggested that these two earthquakes were linked to the occurrence of the 2004 great Sumatra–Andaman earthquake. Over the last few decades, several strike-slip type earthquakes have been observed within the West Philippine Basin (WPB). However, little investigation has been conducted to understand these strike-slip earthquakes and the possible geohazard potential that they may bring. In our study, to determine the rupture plane for these strike-slip events, we calculated the static Coulomb Failure Stress changes (ΔCFS) induced by the 1998 Mw 7.4 strike-slip earthquake and compared them with the distribution of aftershocks. Our results show that when the fault plane is trending in the NW–SE direction, the aftershocks that occurred in the region show a good agreement with the maximal positive ΔCFS. Consequently, instead of sliding along the pre-existing oceanic fracture zones, the 1998 strike-slip event that occurred in the WPB should have ruptured in the NW–SE direction. Moreover, we also observed a correlation between the activities of the normal faulting type earthquakes along the trench and the positive ΔCFS induced by the 1998 Mw 7.4 event, which suggests that the occurrence of intraplate strike-slip events in oceanic plates might affect the distribution of earthquakes along the nearby subduction systems.

Co-seismic and post-seismic effects of the Tohoku-Oki M W 9.0 earthquake in North and Northeast China

Using the US Geological Survey (USGS) finite fault model, the co-seismic horizontal displacement and horizontal principal stress field in North and Northeast China generated by the Tohoku-Oki M W 9.0 earthquake of March 11th, 2011, were retrieved by linear elastic finite element numerical simulation. On this basis, the static Coulomb failure stress variations of the main active faults and tectonic zones in North and Northeast China were calculated, and the triggering effect of the earthquake on seismicity in North and Northeast China was discussed. Results were as follows: (1) the co-seismic horizontal displacement field caused by the great earthquake was eastward in North China; the maximum value, which occurred in the Shandong Peninsula and north Jiangsu Province, was about 12 mm; and the values gradually decreased from east to west. The co-seismic horizontal displacement field in Northeast China was SE–SEE and the maximum value, which occurred in northeast Jilin Province, was about 30 mm. The simulated results for the co-seismic horizontal displacement field were in good agreement with GPS observational results. (2) The near-surface co-seismic maximum horizontal principal stress (ΔσHmax) was tensile stress ranging from 0 to 2.0 KPa. From Northeast China to North China, the direction gradually altered from NW to NWW to nearly EW to NEE. The co-seismic minimum horizontal principal stress (ΔσHmax) was compressive as well as tensile stress, with a magnitude of 0–0.75 KPa, which is approximately equal to zero. From Northeast China to North China, the direction gradually changed to NNW from NE to NNE to nearly NS. Although the co-seismic stress effect generated by the great earthquake had some effect on the present tectonic stress field of North and Northeast China, its influence was too slight to change the present framework of the regional tectonic stress field. (3) Coulomb failure stress variations on active fault zones in North and Northeast China mainly increased; however, the amount of increase was less than 1 KPa. The increases were largest in the middle and northeastern segments of the Tancheng–Lushan fault zone and the Mishan–Dunhua fault zone. The Tohoku-Oki Mw9.0 earthquake did have some triggering effect on seismic activity in North and Northeast China, but the effects were limited.

Normal upper plate fault reactivation in northern Chile and the subduction earthquake cycle: From geological observations and static Coulomb Failure Stress (CFS) change

The northern Chile subduction zone constitutes a seismic gap where no great interplate earthquakes (Mw>8.8) have nucleated since 1877. Active normal faulting occurs conspicuously in the Coastal Forearc at the southern end of this gap, between 22.7°S and 24°S. There, upper crustal extension is present right above a suggested subduction segment boundary of the Nazca-South America interplate contact. In this work, by using Coulomb Failure Stress Change (CFS) models, we explore the effects of coseismic and interseismic stages of the subduction earthquake cycle on the reactivation of pre-existing upper plate normal faults in this area. We attempt to unravel the first-order interplate conditions required to explain this extensional pattern, putting specific focus on the influence of the megathrust segmentation defined by the Mejillones Peninsula. It is suggested that the restricted occurrence of active normal faulting in the study area is tightly related to the structure of the interplate seismogenic zone. Positive CFS values are induced in the study area by the coseismic and interseismic stages of the subduction earthquake cycle, in a region characterized by lower coupling beneath the Mejillones Peninsula. We propose that fault reactivation in this zone is particularly favored during the coseismic stage by earthquakes occurring offshore of the Mejillones Peninsula and surrounding areas. In the adjacent subduction segments, coseismic and interseismic CFS values tend to counterbalance each other, resulting in no tensional conditions capable to cause normal fault reactivation. We suggest that the coupling variation in the seismogenic zone is long term controlled by the geological structure of the upper plate.

Insights on a Key Parameter of Earthquake Forecasting, the Coefficient of Variation of the Recurrence Time, Using a Simple Earthquake Simulator

Online Material: Tables of static Coulomb failure stress variation and regression coefficients. Probabilistic fault‐based and time‐dependent seismic‐hazard studies are commonly used to forecast the time between consecutive earthquakes; however, the correct evaluation of key parameters is critical for obtaining accurate results. Probabilities of occurrence must be input to formulate seismic design maps and to develop national recommendations for building codes around the world (e.g., Field et al. , 2009; Stucchi et al. , 2011; Stirling et al. , 2012). The basic requirements for computing earthquake probabilities are the mean recurrence time and its variability. To understand the statistical behavior of earthquakes, data on long earthquake sequences is necessary. However, worldwide during the instrumental period (approximately the past century), no major active fault segment has entirely ruptured more than once. Additional information on the recurrence of such events can be found in historical documents and damaged archaeological structures. Surface ruptures can also be preserved in sedimentary deposits, leading to paleoseismological records. In many cases, the compilation of these data sources for a given fault segment may yield catalogs of large earthquakes that include 3–4 events, with exceptional examples reaching 15 events (e.g., the San Andreas fault system, Working Group on California Earthquake Probabilities [WGCEP], 2007; the Jordan valley fault, Ferry et al. , 2011). Therefore, considering that the period over which historical and palaeohistorical earthquake records are available is relatively short compared to the recurrence time of major earthquakes, synthetic catalogs generated from numerical simulation models are needed. Earthquake simulations model long earthquake histories using various approximations of what is known about the physics of stress transfer due to fault slip and the rheological proprieties of faults (Tullis, 2012a). Today, the scientific knowledge regarding earthquake dynamics is far from complete; moreover, a realistic simulation of a full earthquake cycle over sufficient …

2011年4月11日福島県浜通りの地震（<I>M</I><SUB>j</SUB>7.0）の震源過程

The rupture process of the April 11, 2011 Fukushima Hamadori earthquake (Mj 7.0), which is one of the induced earthquakes by the 2011 Tohoku earthquake (Mw 9.0), is investigated using near-field strong motion waveforms. After the Fukushima Hamadori earthquake, distinct normal fault type surface ruptures were recognized in the source area. Those surface ruptures line up along the previously-known fault traces, Idosawa fault and Yunotake fault, which exist about 10km apart each other in echelon. We first apply the iterative decomposition method to confirm that these faults acted at the almost same instant whether or not. Additionally, relocation of hypocenters of aftershocks using double-difference method is performed. These results show that the two normal faults slipped with the delay of about 10 s. Two fault segments are assumed according to those results and the multi time window inversion method is applied to determine detailed rupture process on the faults. As the result, total moment of 1.1×1019Nm (Mw 6.6) and the maximum slip of 2.5m on the Idosawa segment are recovered. The result shows that the Idosawa segment acted first and the Yunotake segment slipped after that. Finally, the static Coulomb failure stress change (ΔCFF) on the Yunotake segment gained by the slip on the Idosawa segment is calculated. The ΔCFF in the large slip area on the Yunotake segment is positive and it suggests that the stress change gained by the Idosawa segment induced the slip on the Yunotake segment.

The 2008 May 29 earthquake doublet in SW Iceland

SUMMARY On 2008 May 29 an earthquake doublet shook the southwestern part of Iceland. The first main shock originated beneath Mt Ingolfsfjall, located near the western margin of the South Iceland Seismic Zone (SISZ) approximately 40 km east of the capital Reykjavik. Immediate aftershock activity was recorded by the SIL seismic network, operated by the Icelandic Meteorological Office (IMO), with both N–S and E–W structures illuminated over a broad area. A continuous GPS (CGPS) network, also operated by the IMO, recorded coseismic offsets with up to 200 mm of horizontal motion at the closest stations. We estimate the coseismic surface deformation observed by campaign and continuous GPS and satellite radar data (InSAR). We invert the geodetic data to find the optimal geometry, location and slip on the main faults, accounting for variation in the elastic parameters of the crust with depth. Our models indicate that most of the slip occurred on two N–S structures spaced ∼5 km apart. From a joint inversion of GPS and InSAR data for variable slip models we find that most of the slip for the first (Ingolfsfjall) event was concentrated at 2–4 km depth with a maximum of 1.9 m, whereas the slip on the second (Kross) fault was located deeper, at 3–6 km depth with up to 1.4 m of motion. The models give similar geodetic moments for the two main events, equivalent to a moment magnitude of Mw5.8 and Mw5.9 for the first and second event, respectively. Our estimated composite moment therefore equals a Mw6.1 for the doublet, smaller than the Mw6.3 estimated from teleseismic data (e.g. NEIC and Harvard). The geodetic data support rupture on two main faults and analysis of high-rate (1 Hz) CGPS data suggests that slip on the second fault initiated within 3 s of the first main shock. Static Coulomb failure stress calculations indicate that the first event caused a stress increase in the area of the main asperity (i.e. at the location of the largest slip patch) on the second fault. However, we cannot rule out dynamic stress triggering due to the short time between the two main events. The 2008 May 29 earthquake doublet appears to be a continuation of the earthquake sequence that started in 2000 June, when two Mw6.5 events struck the eastern and central part of the South Iceland Seismic Zone, in the span of 81 hr. The 2000 June–2008 May sequence has released about half of the moment accumulated by plate motion since the previous earthquake sequence in 1896–1912. Therefore, continued earthquake activity with moderate size events rupturing N–S faults in the SISZ in the coming decades is likely.

Some Aspects of Static Stress Change Calculation—Case Study on the Wenchuan Earthquake

AbstractWhen a major earthquake occurs, how aftershocks distribute catch the attention of the public. Recently, the Coulomb failure stress is used frequently to study the earthquake triggering of main shock. However, there are some pendent problems in the calculation and application of Coulomb stress. In conventional calculation of Coulomb stress, the shear stress change is approximated to be the changed value along the slip on the rupture of main shock. In fact, the stress orientations may change after the event. It affects the shear stress on ruptures. We improved the formula of Coulomb stress and made comparison. We applied the improved method to discussion of the relation between the Wenchuan earthquake and aftershocks. According to the stress measurement in the Longmen Shan, we calculated static Coulomb failure stress of the Wenchuan earthquake. The results show big differences between conventional and improved calculation around the fault. Our investigation reminds us to concern the uncertainty of the rupture models when we use Coulomb stress to predict the aftershocks.

Features of seismicity in Xinjiang and its possible reason after the Yutian MS7.4 earthquake, 2008

The paper discusses quantitatively the influence of the Yutian MS7.4 earthquake of March 21, 2008 and Wuqia MS6.9 earthquake of October 5, 2008 on regional seismicity in Xinjiang, and explains primarily the possible reason of earthquake activity feature in Xinjiang after the Yutian MS7.4 earthquake by analyzing the static Coulomb failure stress change produced by the Yutian MS7.4 earthquake and Wuqia MS6.9 earthquake, and the seismicity feature of MS≥3 earthquakes in the positive Coulomb stress change region of Kashi-Wuqia joint region, the central segment of Tianshan Mountain and Kalpin block. The result shows that the Yutian MS7.4 earthquake of March 21, 2008, may encourage the Wuqia MS6.9 earthquake of October 5, 2008, and the Yutian MS7.4 earthquake and Wuqia MS6.9 earthquake may change the seismicity state in the central segment of Tianshan Mountain, Kalpin block and Kashi-Wuqia joint region, and encourage the subsequent MS≥3 earthquakes.

Static Coulomb failure function and aftershocks of 1995 Kobe earthquake: A statistical test

The hypothesis that aftershocks are triggered by a change of the static Coulomb Failure stress is statistically tested for 1995 Kobe (Hyogo‐ken nanbu) Earthquake. We focus on the focal mechanisms of the aftershocks for the first 7 months and investigate whether these mechanisms are consistent with the static CFF triggering hypothesis. Two slip distribution models for the mainshock are tested and both explain focal mechanisms of more than 3/4 of the aftershocks. From a bootstrap statistical test, however, we find that such a high consistency rate between the observation and CFF based prediction is not statistically significant. The static CFF triggering is thus not a statistically strong hypothesis for 1995 Kobe aftershocks.

Coseismic stress changes and crustal deformation on the Reykjanes Peninsula due to triggered earthquakes on 17 June 2000

A large (Mw = 6.5) earthquake struck the South Iceland Seismic Zone (SISZ) on 17 June 2000. The 17 June main shock triggered increased seismicity over a large area and significant slip on at least three distinct faults on the Reykjanes Peninsula, up to 87 km to the west of the event. A second large (Mw = 6.4) earthquake in the SISZ occurred on 21 June 2000, about 17 km west of the 17 June main shock. This event does not appear to have triggered as much activity on the Reykjanes Peninsula as the 17 June main shock, although the epicenter was closer. Crustal deformation signals due to the June 2000 earthquakes on the Reykjanes Peninsula were observed with campaign and continuous GPS and synthetic aperture radar interferometry, with the largest coseismic deformation signal near lake Kleifarvatn. We model the faults using three uniform slip rectangular dislocations in an elastic half‐space. Best fit uniform slip models consistent with seismic and geodetic data indicate that all three faults trend N‐S and the motion on them was primarily right‐lateral strike slip. Our study suggests that the event near Kleifarvatn had a significantly larger moment than seismic estimates, indicating a component of aseismic slip on the fault lasting no more than several hours. Static Coulomb failure stress change calculations indicate that the event at Kleifarvatn increased the Coulomb stress at the hypocenter of the Núpshlídarháls event by 0.1–0.2 MPa as well as loading the Hvalhnúkur fault.