Static Coulomb Stress Changes Research Articles

Cascade processes of induced and triggered earthquakes-Case study in the Weiyuan shale gas development area in Sichuan Basin, China

Identifying accurate seismogenic faults is critical for studying the mechanisms of induced earthquakes. On February 24th and 25th, 2019, three moderate earthquakes with magnitudes of MS 4.7, MS 4.3, and MS 4.9 occurred successively in the shale gas development area of Weiyuan, China. We utilized high-resolution three-dimensional (3D) seismic data to identify two pre-existing faults (F1 and F2) that were responsible for the three moderate earthquakes. InSAR data were used to validate the rationality of the two seismogenic faults. Furthermore, we analyzed the impact of fluid diffusion on fault F1 near the fracturing well and calculated the Coulomb failure stress (CFS) generated on fault F2 by the MS 4.7 and MS 4.3 earthquakes to analyze the interactions between these events. The results indicated that fluid diffusion caused by hydrofracturing induced the MS 4.3 and MS 4.7 earthquakes on F1. The static Coulomb stress changes from these two earthquakes subsequently triggered the larger MS 4.9 earthquake on F2. This study provides a case of a cascading process in which induced earthquake events triggered a more distant and higher-magnitude earthquake. This triggering scenario reminds us that earthquake-to-earthquake interactions may be more hazardous than a “typical” inducing mechanism and challenges current risk management practices.

Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Abstract The analysis of Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay subsequent earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip and transient postseismic processes such as poroelastic effects and viscoelastic relaxation. However, the combined influence of poroelastic effects and viscoelastic relaxation on co- and postseismic Coulomb stress changes has not been systematically studied so far. Here, we use three-dimensional finite-element models with arrays of normal and thrust faults to investigate how pore fluid pressure changes and viscoelastic relaxation overlap during the postseismic phase. In different experiments, we vary the permeability of the upper crust and the viscosity of the lower crust or lithospheric mantle while keeping the other parameters constant. In addition, we perform experiments in which we combine a high (low) permeability of the upper crust with a low (high) viscosity of the lower crust. Our results show that the coseismic (i.e., static) Coulomb stress changes are altered by the signal from poroelastic effects and viscoelastic relaxation during the first month after the earthquake. For sufficiently low viscosities, the Coulomb stress change patterns show a combined signal from poroelastic and viscoelastic effects already during the first postseismic year. For sufficiently low permeabilities, Coulomb stress changes induced by poroelastic effects overlap with the signals from viscoelastic relaxation and interseismic stress accumulation for decades. Our results imply that poroelastic and viscoelastic effects have a strong impact on postseismic Coulomb stress changes and should therefore be considered together when analyzing Coulomb stress transfer between faults.

Focal Mechanism and Regional Fault Activity Analysis of 2022 Luding Strong Earthquake Constraint by InSAR and Its Inversion

On 5 September 2022, an Ms6.8 magnitude earthquake occurred in Luding County, Sichuan Province, China. Based on Sentinel-1 SAR images, this paper uses the D-InSAR approach to obtain the displacement field of the earthquake, invert the coseismic sliding distribution, and then calculate the static coulomb stress changes of the coseismic deformation on the aftershock distribution and surrounding faults. Further, the seismic structure is analyzed and discussed. The InSAR coseismic deformation field demonstrates that the maximum LoS displacement of the surface deformation caused by the Luding earthquake is about 15 cm. The Luding Ms 6.8 earthquake is dominated by the Moxi fault, which is a left-lateral strike-slip fault that ruptures along the NNW-SSE trend at about 160.3°, and the dip is 81°. The fault depth is mainly 5~15 km, the maximum sliding amount is about 174.8 cm, and the corresponding depth is 8.5 km. The seismic moment tensor obtained by inversion is 1.06 × 1019 Nm, Mw = 6.65. The Coulomb stress generated by the Luding earthquake on the northern end of the Anninghe fault zone exceeded the trigger threshold. The risk of the Anninghe fault’s future earthquake was greater, and continuous monitoring and risk assessment were required.

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

The 2022 Delingha, China, Earthquake Sequence and Implication for Seismic Hazard near the Western End of the Qilian–Haiyuan Fault

Abstract Although the Qilian–Haiyuan fault is known to be responsible for major earthquakes up to M 8, the potential of damaging earthquakes near its western end is not well understood. Since January 2022, three moderate earthquakes (M 5.8, M 6.0, and M 5.4) occurred around the Halahu region of Delingha, China, near the western end of the Qilian–Haiyuan fault. These earthquakes are unusual M 5+ events in this low-seismicity region, and both the U.S. Geological Survey and Global Centroid Moment Tensor solutions suggest that the focal mechanisms of the three mainshocks are distinct from the activity characteristics of the nearby mapped faults. Thus, determining the precise source parameters and identifying the causative fault of this earthquake sequence are important to analyze its seismogenic settings and seismic hazard in this region. In this article, we determined the point-source parameters of the three moderate events via regional waveform modeling and found that these earthquakes are strike-slip events with the nodal planes striking nearly north–south and east–west directions. We then resolved its seismogenic faults by analyzing the aftershock distribution and the rupture directivity. The results show that both the M 5.8 and M 6.0 events ruptured along the north–south nodal plane and expanded toward the south for ∼4 km and ∼3 km, respectively, and the ruptured fault of the M 5.4 is difficult to distinguish. Furthermore, we constrained the relative location using the Interferometric Synthetic Aperture Radar observation, and the result is consistent with that obtained from seismic waveform data. We proposed that this earthquake sequence ruptured along an unmapped dextral fault, which forms a conjugate fault system with the sinistral strike-slip Qilian–Haiyuan fault. Its deformation mechanism may be controlled by long-term, protracted, nearly north–south-trending, right-lateral simple shear in the Qilian Shan fold-thrust belt. The accumulated static Coulomb stress changes resulted in the northwest of this ruptured fault being closer to failure in the future.

Cascading Hazards in a Migrating Forearc‐Arc System: Earthquake and Eruption Triggering in Nicaragua

AbstractStrain partitioning in oblique convergent margins results in margin‐parallel shear in the overriding plate. Margin‐parallel shear is often accommodated by margin‐parallel strike‐slip faults proximal to active volcanic arcs. Along the Nicaraguan segment of the Central American Forearc (CAFA) in the Cocos‐Caribbean plate convergent margin, there are no well‐expressed right‐lateral faults that accommodate CA‐CAFA relative motion. Instead, historical earthquakes and mapped fault orientations indicate that the ∼12 mm/yr of dextral motion is accommodated on arc‐normal, left‐lateral faults (i.e., bookshelf faults). We investigate three upper‐plate earthquakes; the 10 April 2014 (Mw 6.1), 15 September 2016 (Mw 5.7), and 28 September 2016 (Mw 5.5), using Global Position System co‐seismic displacements and relocated earthquake aftershocks. Our analyses of the three earthquakes indicate that the 10 April 2014 earthquake ruptured an unmapped margin‐parallel right‐lateral fault in Lago Xolotlán (Managua) and the September 2016 earthquakes ruptured arc‐normal, left‐lateral and oblique‐slip faults. These earthquakes represent a triggered sequence whereby the 10 April 2014 earthquake promoted failure of the faults that ruptured in September 2016 by imparting a static Coulomb stress change (ΔCFS) of 0.02–0.07 MPa. Likewise, the 15 September 2016, earthquake additionally promoted failure (ΔCFS of 0.08–0.1 MPa) on sub‐parallel faults that ruptured in two subsequent earthquakes. We also present an instance of magma‐tectonic interaction whereby the 10 April 2014 earthquake dilated (10s of μStrain) the shallow magmatic system of Momotombo Volcano, which led to magma injection, ascent, and eruption on 1 December 2015, after ∼100 years of quiescence.

The 2015–2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, fault interaction and fluid processes

SUMMARYA sequence of three strong (MW7.2, 6.4, 6.6) earthquakes struck the Pamir of Central Asia in 2015–2017. With a local seismic network, we recorded the succession of the foreshock, main shock and aftershock sequences at local distances with good azimuthal coverage. We located 11 784 seismic events and determined 33 earthquake moment tensors. The seismicity delineates the tectonic structures of the Pamir in unprecedented detail, that is the thrusts that absorb shortening along the Pamir’s thrust front, and the strike-slip and normal faults that dissect the Pamir Plateau into a westward extruding block and a northward advancing block. Ruptures on the kinematically dissimilar faults were activated subsequently from the initial MW 7.2 Sarez event at times and distances that follow a diffusion equation. All main shock areas but the initial one exhibited foreshock activity, which was not modulated by the occurrence of the earlier earthquakes. Modelling of the static Coulomb stress changes indicates that aftershock triggering occurred over distances of ≤90 km on favourably oriented faults. The third event in the sequence, the MW 6.6 Muji earthquake, ruptured despite its repeated stabilization through stress transfer in the order of –10 kPa. To explain the accumulation of MW > 6 earthquakes, we reason that the initial main shock may have increased nearby fault permeability, and facilitated fluid migration into the mature fault zones, eventually triggering the later large earthquakes.

Source Model for the 2022 Qinghai Menyuan Ms 6.9 Earthquake Based on D-InSAR

On 8 January 2022, an Ms 6.9 earthquake occurred in Menyuan County, Qinghai Province, China. This earthquake ruptured a patch of the Qilian-Haiyuan fault in the northeast margin of the Qinghai-Tibet Plateau. In order to understand the seismogenic structure of this earthquake and analyze the fault activity, we use differential synthetic aperture radar interferometry (D-InSAR) technology to obtain a complete co-seismic displacement field on the surface, and use pixel tracking algorithm to extract the trace of the ruptured fault. The slip distribution of the seismogenic fault was inverted using the steepest descent method, and the Coulomb stress change caused by the earthquake was also calculated. Surface deformation results show that the Menyuan earthquake produced obvious surface displacements in an area of 50 × 40 km2. The displacements are mainly distributed in the western segment of the Lenglongling fault and the eastern segment of the Tolaishan fault. The maximum displacements in the ascending and descending orbits in the LOS direction are 59.7 and 94.7 cm, respectively. The co-seismic slip results show that the strike, dip and average slip angles of the seismogenic fault are 108°, 79° dipping to SW, and 4°, respectively. On the whole, the fault is mainly of left-lateral, with a small amount of thrust component and only one co-seismic rupture center in our inversion result. The rupture center is located at a depth of ∼5 km below the surface, and the maximum slip is 3.1 m. The total seismic moment released by this earthquake is 1.28 × 1019 N·m, and the corresponding moment magnitude is 6.67. Finally, the static Coulomb stress change results show that parts of the Lenglongling fault, the Tolaishan fault, the Sunan-Qilian fault and the Minyue-Damaging fault are loaded, emphasizing the importance for earthquake risk assessment of these fault.

Earthquakes in the Shadows: Why Aftershocks Occur at Surprising Locations

Abstract For decades there has been a debate about the relative effects of dynamic versus static stress triggering of aftershocks. According to the static Coulomb stress change hypothesis, aftershocks should not occur in stress shadows—regions where static Coulomb stress has been reduced. We show that static stress shadows substantially influence aftershock occurrence following three M ≥ 7 California mainshocks. Within the modeled static Coulomb stress shadows, the aftershock rate is an order of magnitude lower than in the modeled increase regions. However, the earthquake rate in the stress shadows does not decrease below the background rate, as predicted by Coulomb stress change models. Aftershocks in the stress shadows exhibit different spatial–temporal characteristics from aftershocks in the stress increase regions. The aftershock rate in the stress shadows decays as a power law with distance from the mainshock, consistent with a simple model of dynamic stress triggering. These aftershocks begin with a burst of activity during the first few days after the mainshock, also consistent with dynamic stress triggering. Our interpretation is that aftershock sequences are the combined result of static and dynamic stress triggering, with an estimated ∼34% of aftershocks due to dynamic triggering and ∼66% due to static triggering.

Quantitative Evaluation of Coseismic Deformations Induced by Seismogenic Faulting in Mining Exploration Area During the 2018 Xingwen and 2019 Changning Earthquakes, Sichuan, China

In the period between December 2018 and July 2019, a series of earthquakes (EQs), including the 16 December 2018 Ms 5.7 Xingwen mainshock and the 17 June 2019 Ms 6.0 Changning mainshock, struck the Changning shale gas exploration field in the southern margin of the Sichuan Basin. The Xingwen and Changning EQs both occurred on concealed faults, which led to hundreds of casualties, and affected a total of over 160 thousand people in southern Sichuan. The aftershock sequences following the Xingwen and Changning EQs were clustered in the vicinity of the Jianwu syncline and Changning anticline, respectively, and occurred mostly at depths of 3–7 km. In this study, coseismic surface deformation measurements obtained through differential interferometric synthetic aperture radar (D-InSAR) data were used to identify the faulting geometries and distributions. The coseismic deformation maps have maximum line-of-sight (LOS) displacements of ∼4.53 cm on the northwest side of the Xingwen EQ source fault and ∼7.84 cm on the southwest side of the Changning EQ source fault. The calculated static Coulomb stress changes indicated that most aftershocks occurred in increasing stress zones following the mainshock ruptures. From the InSAR deformation field, a complicated concealed seismogenic doublet fault was inferred, which predominately exhibited left-lateral strike-slip motion during the Xingwen and Changning EQs. The footwall ramp of the basement fault reactivated first, and resulted in the Xingwen EQ and concentrating stresses beneath the Changning anticline, which induced the Changning EQ half a year later. Compared with previous studies, we proposed that the fault network was lubricated by water that was injected during shale gas exploration, facilitating the occurrence of the Xingwen and Changning EQs. Such work evaluated the coseismic deformations of the Xingwen and Changning EQs, and derived the regional faulting distribution from aftershock sequences. It could provide useful information for monitoring and analyzing seismic activity around the hinge zones of folds in mining exploration areas, which contributes to effective risk assessment of disasters associated with seismic geo-environments.

Lithospheric Deformation Due To the 2015 M7.2 Sarez (Pamir) Earthquake Constrained by 5 years of Space Geodetic Observations

AbstractThe 2015 M7.2 Sarez (Pamir) earthquake occurred at the north‐west margin of the Tibetan Plateau. We use Sentinel‐1 and ALOS‐2 Synthetic Aperture Radar and Global Navigation Satellite System data to investigate coseismic and postseismic deformation due to the Sarez earthquake. Kinematic inversions show that the earthquake ruptured a ∼80 km long, sub‐vertical fault producing the maximum surface offset of 3–4 m on the south‐west and central fault segments. In contrast, the largest postseismic displacements are observed at the north‐east end of the earthquake rupture, predominantly on the west (hanging wall) side of the fault with an average rate of 20–30 mm/yr in the satellite line of sight. We use the derived coseismic and postseismic slip models to investigate mechanisms of time‐dependent relaxation, stress transfer and possible triggering relationships between the Sarez earthquake and a sequence of strong M6+ events that occurred within ∼100 km of the 2015 earthquake. We find that the near‐field postseismic displacements are best explained by shallow afterslip driven by the coseismic stress changes. The data also allow some contribution from poroelastic rebound, but do not show a clear signature of viscoelastic relaxation in the lower crust and upper mantle during the observation period, suggesting a lower bound on the effective viscosity of ∼1019 Pa s. A pair of M6+ events that occurred within 100 km and several months of the 2015 mainshock have experienced near‐zero and in some cases negative static Coulomb stress changes, suggesting either delayed dynamic triggering, or no relation to the mainshock.

The 21 May 2021 Mw 6.1 Yangbi Earthquake—A Unilateral Rupture Event with Conjugately Distributed Aftershocks

Abstract An Mw 6.1 earthquake occurred in Yangbi, Yunnan province, China, at 13:48:34 (UTC time) on 21 May 2021. It exhibited a typical foreshock–mainshock–aftershock sequence in the northwestern Yunnan rifting area. We used the first P-arrival time from regional broadband seismic stations to check the location of the mainshock’s hypocenter. The updated location of the epicenter is 99.88° E, 25.66° N. The focal mechanisms of nine earthquakes, including the mainshock and the largest foreshock and aftershock, were investigated using the general cut-and-paste method based on regional seismic waveforms to set up the fault geometry and to calculate the static coulomb stress change. The rupture process of the mainshock was constrained by 33 unclipped regional broadband seismic records from 15 stations and nine static displacements from three Global Navigation Satellite System stations. The results showed that the Yangbi earthquake was a unilateral rupture event propagating toward the southeast along-strike direction. The moment was 1.81×1018 N·m, and the corresponding magnitude was Mw 6.1. The peak slip value, the weighted rupture velocity, and the duration time of the mainshock were 0.95 m, 2.2 km/s, and 9.1 s, respectively. At 10 km southeast of the mainshock’s epicenter, the conjugately distributed aftershocks were observed, and the moment density of the mainshock along the strike direction decreased sharply (78% of the total moment released in the first 10 km), suggesting that the sharply decelerating rupture could be the reason for the conjugately distributed aftershocks. The changes in static coulomb stress due to the largest foreshock and mainshock were calculated. The results showed positive Coulomb stress on the major slip zone in the mainshock fault plane triggered by the largest foreshock. Moreover, the 0.01 MPa contour due to the mainshock radiated out of the bend zone of the Weixi–Qiaohou–Weishan fault.

The 2009 Ms 6.0 earthquake in Yao'an, Yunnan Province: Source constraints using relocated aftershocks and InSAR

An Ms 6.0 earthquake occurred in Yao'an County, Yunnan Province, on 9 June 2009. According to the results of relocated aftershocks, we find that the distribution of aftershocks has differences in the strike direction and depth in local areas. Therefore, the fault is divided into two segments. Combined with the surface deformation information obtained from InSAR Data as the constraint condition of the slip model, we inverse the three-dimensional coseismic slip distribution of the seismogenic fault of the Yao'an earthquake and discussed the influence of the Yao'an earthquake on the surrounding faults and the rupture direction of the event. The main research results are as follows: 1) we quickly obtain reasonable fault parameters, model, and slip distributions through relocated aftershocks and InSAR data, and the results obtained after inversion are consistent with the results of the geological structure and scholars' research. At the same time, the abnormal deep slip is found, which proves that there is a complex structure underground in the Yao'an area. 2) The results of static Coulomb stress change show that the aftershocks are basically distributed in the area where the static Coulomb stress increases. Faults with a relatively significant increase of Coulomb stress include the northwest section of Honghai Fault, the middle of Chenghai Fault, the middle of Hongge Fault, the middle of Nanhua-Chuxiong Fault, and the north of Yuanmou Fault. The future seismic risk of these faults should be considered. 3) The main direction of the Yao'an earthquake rupture is controlled by regional stress, and the abnormal rupture direction is affected by the complex underground structure.

Pore Fluid Pressure Imaging of the Mt. Pollino Region (Southern Italy) From Earthquake Focal Mechanisms

AbstractFocal mechanisms of selected earthquakes, recorded in the Mount Pollino region (southern Italy) from 2010 through 2014, are used to infer the pore fluid pressure at hypocenter depths. The 3‐D excess pore pressure field provides evidence that the sequence occurs in a fluid‐filled volume with values reaching 35 MPa. The mechanisms underlying this swarm‐like sequence and the triggering of earthquakes are investigated by computing the cumulative static Coulomb stress change at hypocenter depths and analyzing the pore‐pressure diffusion mechanism. The results indicate that static Coulomb stress change was lower than 0.01 MPa, which is the value generally assumed as threshold for the triggering, and seismicity distribution was actually driven by pore‐pressure diffusion with relatively low diffusivity value. This latter mechanism could also explain the delayed triggering of the two larger eventsML4.3 andML5.0, respectively, that occurred about 150 days apart.

New insights on faulting and intrusion processes during the June 2007, East Rift Zone eruption of Kīlauea volcano, Hawai'i

The East Rift Zone (ERZ) of Kīlauea Volcano, Hawai'i, represents one of the most volcanically active regions in the world. The 2007 Father's Day (FD) dike intrusion, eruption, and accompanying slow-slip event (SSE) has been previously modeled using geodetic data to constrain the geometry of the intrusion and the timing and magnitude of the SSE. Here, we perform inversions of three interferometric synthetic aperture radar (InSAR) datasets and a new intensity offset tracking dataset to assess the effect of integrating intensity cross-correlation offsets into inversion problems and explore additional potential models for the intrusion geometry of the FD event based on this additional data. The overall lowest misfit single Okada model for all datasets opens 2.3 m, strikes 73 degrees while dipping sub-vertically at 83 degrees, and extends approximately 2.9 km to the ENE and 2.4 km downdip. The differences are minor between complex en-echelon distributed Okada and decollement model of (Montgomery-Brown et al., 2010) or 3D-MBEM breaching models including multiple surface breaches and free-slipping decollement movement. Finally, we examine the static Coulomb stress changes for the proposed decollement fault created by our preferred model and a representative model of deep rift opening and find that deep rift zones dilation, not shallow ERZ intrusions, are likely modulating slip on the decollement.

Decades of Ground Deformation in the Weihe Graben, Shaanxi Province, China, in Response to Various Land Processes, Observed by Radar Interferometry and Levelling

Ground deformation is usually used as direct evidence for early warning of geological hazards. The Weihe Graben, located in the southern margin of the Ordos Plateau, is surrounded by many active faults. Earthquakes (e.g., the 1556 Huaxian M 8 earthquake), mine accidents and ground fissures are the major hazards that pose great threats to this densely populated region. In order to characterise both tectonic and anthropogenic activities in the Weihe Graben, we use Envisat data from 2003 to 2010 and Sentinel-1 data from 2014 to 2021, combined with levelling data from 1970 to 2014, to investigate the long-term ground deformation. We generate four InSAR rate maps using the small-baseline subset (SBAS) algorithm. The uncertainties of the InSAR rates are 1–2 mm/year by calculating the differences between the InSAR and levelling measurements. From the deformation time series, we found that most of the faults surrounding the Weihe Graben move at a relatively slow rate (<3 mm/year). Elastic dislocation modelling based on the InSAR and levelling data yields a slip rate of 2.3 ± 0.3 mm/year for the Huashan Fault, the seismogenic fault for the 1556 Huaxian earthquake. Anthropogenic deformation is much stronger than the tectonic deformation. We identified localised subsidence of 12 mines with a deformation rate ranging from 5 to 17 mm/year. The cities of Xi’an and Xianyang also show evident subsidence, which is likely to be caused by groundwater extraction. Land subsidence in Xi’an has slowed down from an average rate of 10–20 mm/year between 2003 and 2010 to about 5–10 mm/year between 2017 and 2020, but in Xianyang, subsidence has increased dramatically in the past five years from 1 mm/year to 7 mm/year. This is because new industrial and urban development centres have gradually moved from Xi’an to Xianyang. We identified a region bounded by the Kouzhen-Guanshan and Fufeng-Liquan Faults with strong subsidence, as a result of excessive extraction of groundwater. To quantify the effects of crustal groundwater unloading on faults, we calculated the static Coulomb stress changes on the two faults and found that Coulomb stress changes are localised in the upper 5 km with a magnitude of 0.01–0.02 bar/year. The Coulomb stress changes might be large enough (0.1 bar) to affect local seismicity if such excessive extraction of groundwater continued for 10 years.

Recent Seismic Activity in the Bejaia–Babors Region (Northeastern Algeria): The Case of the 2012–2013 Bejaia Earthquake Sequences

From November 2012 to May 2013, the coastal region of Bejaia (northeastern Algeria), experienced three distinct seismic sequences in about 6 months. The first one was on 28 November 2012 started with a mainshock of magnitude Mw = 5.1, and the second one occurred on 22 February 2013, a few kilometers southeast from the first one and culminated with a magnitude Mw = 4.3 earthquake. The last sequence comprised two shocks of moderate magnitude: a Mw 5.2 event on 19 May 2013, followed by a Mw 5.0 event on 26 May. Earthquakes were relocated using the double-difference method with differential times derived from phase-picked data and waveform cross-correlation delays. We compiled a dataset of 252 accurately relocated events that are associated with focal mechanism solutions consistent with a near-vertical, right-lateral strike-slip fault striking NW–SE. The data analysis is coherent with a new fault extending over 35 km from the Gulf of Bejaia to the northeast of Kherrata. This newly discovered active fault, we name it the Babors Transverse Fault, appears to be segmented in four segments (S1–S4) evidenced by separate earthquake clusters. Static Coulomb stress changes and the spatiotemporal evolution of seismicity suggest static stress triggering, with the rupture first activated in the S1 segment in the northwest, migrating to S2, and terminating in S3 and S4 in the southeast. Stress tensor inversion from focal mechanisms implies that the dominant local stress field is a pure strike-slip regime, with N–S maximum horizontal compression in good agreement with the velocity field displaying reorientation compared to the oblique convergence of Africa–Eurasia. In this study, the newly identified Babors Transverse and Tizi N’Berber–Darguinah faults, in association with the NNW–SSE offshore active fault of the Greater Kabylian block (in the north) and the NNW–SSE right-lateral strike-slip Lâalam fault (in the south), form an en echelon fault system that acts as a transfer zone between offshore thrust faulting and the Ghardimaou–North Constantine strike-slip fault.

Preliminary Report on the 18 May 2020 Ms 5.0 Qiaojia Earthquake, Yunnan, China

AbstractThe 18 May 2020 Ms 5.0 Qiaojia earthquake occurred in Qiaojia County, Yunnan Province, ∼25 km away from the 3 August 2014 Ms 6.5 Ludian earthquake. This earthquake was well recorded by dense local seismic stations of the Qiaojia array constructed near the Xiaojiang fault zone. The focal mechanism of the mainshock exhibited strike-slip motion with a centroid depth of 8 km. We determined the seismogenic fault of the Qiaojia earthquake using aftershock relocation with local dense seismic arrays. The mainshock is located on a previously unmapped fault. Aftershocks clearly delineated east–west rupture plane, which was not revealed by the regional seismic network due to relatively sparse stations. The length and width of the aftershock zone are ∼5 km and 3 km, respectively. The focal mechanisms of 70 aftershocks with magnitudes ML≥1.0 showed similar focal mechanism with the mainshock. The stress field inverted from focal mechanisms of the aftershocks is consistent with the tectonic stress field. The coseismic and postseismic static coulomb stress changes show that the Ludian earthquake has a negative impact on the Qiaojia earthquake with a value of −0.01 MPa, implying that the Qiaojia earthquake was unlikely statically triggered by the Ludian earthquake. The Qiaojia earthquake sequence was characterized by low b-value and low-decay rate in the aftershock area, indicating high-seismic risk in this region. The dense seismic observation allows us to study the moderate earthquake in detail and provides us with valuable information of near-fault seismicity to analyze earthquake hazard and the potential of large earthquakes in the future.

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.

Coseismic deformation and multi-fault slip model of the 2019 Mindanao earthquake sequence derived from Sentinel-1 and ALOS-2 data

From October to December 2019, a sequence of four earthquakes of Mw > 6.0 occurred in southern Mindanao, the Philippines, and revealed complex tectonics in this region. We used Advanced Land Observing Satellite (ALOS)-2 and Sentinel-1A/B Synthetic Aperture Radar (SAR) data to map the coseismic deformation and to constrain both the fault geometry and kinematics of coseismic slip distributions during the four earthquakes. Our observed coseismic deformation field shows that these four earthquakes ruptured two intersected conjugate faults with NE and NW strikes. Based on ascending and descending Interferometric SAR (InSAR) data, we constructed elastic half-space models to search for the optimal fault dip angle and inverted the slip distributions of the four earthquakes. The results show that these four earthquakes were all dominated by left- and right-lateral strike-slip motion. The two conjugate faults clearly had significant differences in terms of slip behaviour and fault kinematics. The static Coulomb stress change indicates that earlier earthquakes had a significant triggering effect on subsequent earthquakes. The static Coulomb stress change induced by the four earthquakes suggests a significant stress loading effect on NW-trending faults. The results of this study show that the potential for future earthquakes on NW-trending faults cannot be ignored; however the magnitudes of future earthquakes should be limited by complex fault geometry and segmentation.