Coulomb Stress Changes Research Articles

Analysis of Coulomb Stress Changes in Aceh Earthquake on Sibayak Volcano

Based on geological conditions, Indonesia is located between the Indo-Australian plate, the Eurasian plate, and the Pacific plate, making Indonesia prone to tectonic earthquakes. However, since the big earthquake that occurred in Aceh on December 26, 2004, the geological conditions in Sumatra have undergone significant changes marked by stressful situations in the western to southern regions of Sumatra, especially on Sibayak Volcano. This study used data from BMKG and Global CMT, which included magnitude (Mw), depth, earthquake coordinates (longitude and latitude), type of earthquake, strike, dip, and rake. From the analysis using Coulomb 3.3, the highest coulomb stress value of Sibayak Volcano was obtained in 2015 with an average change in coulomb stress of 0.235 bar, shear 0.1909 bar, normal 0.1106 bar. However, the lowest coulomb stress value occurs in 2021 with a moderate shift in coulomb stress of 0.0593 bar, shear 0.0251, normal 0.0849 bar.

Slip Models of the 2016 and 2022 Menyuan, China, Earthquakes, Illustrating Regional Tectonic Structures

As one of the large-scale block-bounding faults in the northeastern Tibetan Plateau, the Qilian-Haiyuan fault system accommodates a large portion of north-eastward motion of the Tibetan Plateau. In 2016 and 2022, two strong earthquakes of Mw6.0 and Mw6.6 occurred in the Menyuan area near the Lenglongling fault (LLLF) at the western segment of the Qilian-Haiyuan fault. These two adjoining events, only 40 km apart, exhibited notable differences in focal mechanisms and rupture kinematics, indicating complex fault geometries and tectonic structures in the region, which are still poorly known. Here, we obtained an interseismic velocity map spanning 2014–2020 in the Menyuan region using Sentinel-1 InSAR data to probe strain accumulation across the LLLF. We obtained the coseismic deformation fields of the two Menyuan earthquakes using InSAR data and inverted out their slip distributions. We calculated the Coulomb stress changes to examine the interactions and triggering relationship between two ruptures and to access regional seismic potential. We found that the 2016 earthquake was a buried thrust event that occurred on the northern LLLF, whilst the 2022 earthquake was a left-lateral strike-slip event that occurred on the western end of the LLLF. We indicated there may be no direct triggering relationship between two spatiotemporally adjacent earthquakes. However, the 2022 earthquake caused a remarkable stress perturbation to the surrounding area. Particularly, a large area with notable stress increase stands out along the Tuolaishan fault and the LLLF, likely posing a high seismic hazard in the region.

Neotectonic faults in the Southern Chile intra-arc (38°S–40.5°S): Insights about their seismic potential and the link with the megathrust earthquake cycle

Megathrust earthquakes are a primary geological hazard in subduction zones. In oblique subduction margins, however, seismic threat must be also considered concerning crustal strike-slip faults driving the parallel-to-the-trench component of the convergent vector. These faults have proven their capacity to produce moderate-to-large shallow earthquakes, causing severe damage for surrounding areas. The Southern Chile Subduction Zone (SCSZ) is characterized by the oblique convergence between the Nazca and the South America plates. Between 37°S and 46°S, slip partitioning is significantly led by intra-arc faults like the Liquiñe-Ofqui Fault System (LOFS) and subsidiary NW to NE striking structures. Despite it has been demonstrated that the behavior of these faults is seismogenic, the inland evidence of Holocene deformation along them is scarce. This has concealed to accurately define individual active faults, discussing their seismic potential, and addressing their link with the megathrust earthquake cycle. In this contribution, we first present field evidence for Holocene deformation on intra-arc faults in the area. Based on this, we define 5 active faults and discuss their seismic capacity. To assess the link with the megathrust earthquake cycle, we calculate the Coulomb stress changes induced on these faults by the Mw 9.5 Valdivia Earthquake slip distribution and an interseismic period of 300 years. We conclude that 1) the identified neotectonic faults have the potential for producing moderate-to-large earthquakes (Mw 5.5 to 6.8) and 2) their occurrence would be enhanced by the coseismic and the interseismic stages of the subduction cycle. With this, we propose that the intra-arc faults addressed in this paper, and certainly others in the SCSZ, must be considered as significant sources of seismic hazard.

A previously unidentified fault revealed by the February 25, 2022 (Mw 6.1) Pasaman Earthquake, West Sumatra, Indonesia

A destructive earthquake (Mw 6.1) struck Pasaman, West Sumatra, Indonesia, on 25 February 2022, resulting in at least 18 deaths and damage to 1765 buildings. Our relocated foreshock, mainshock, and aftershocks and their source mechanisms reveal a previously unknown ∼20 km long segment of the Sumatran Fault as a result of dextral strike-slip motion (strike N132oE and dip 72oSW) along what we have called the Kajai Fault. The inverted rupture model indicates a single, compact asperity with an approximate depth range of 2–11 km. This asperity extends ∼14 km along strike, and ∼9 km in the down-dip direction. The Coulomb stress change of the mainshock shows that areas to the north and south experienced an increase in stress, which is consistent with the observed aftershock pattern. The nearby Great Sumatran Fault segments (Angkola and Sumpur) experienced a significant increase in stress without any accompanying aftershocks, which likely increases the risk of them rupturing in the future.

2019 Mw 6.0 Mesetas (Colombia) Earthquake Sequence: Insights From Integrating Seismic and Morphostructural Observations

AbstractThe boundary between the Northern Andean block and the South American plate is a first‐order tectonic structure in Colombia with historically M > 7 earthquakes. However, details about individual sections of the system remain unknown. We illuminate the seismotectonic of the Algeciras fault by investigating an earthquake sequence that started on 24 December 2019. Using seismic networks at region, we estimate foreshocks and aftershocks focal mechanisms, local stress field, kinematic slip models of the largest events, and Coulomb stress changes. We integrate seismological and morphostructural observations to characterize source properties and reinterpret local faults. Two mainshocks (doublet of Mw 6.0 and 5.8) occurred within 16 min, rupturing just a few kilometers from each other. Discrimination of causative faults among the centroid moment‐tensor nodal planes is difficult because the focal zone is a complex tectonic environment. The focal mechanisms and the local stress field obtained are consistent with a regional NE trending dextral transpressive shear. The relocated aftershocks show a cluster with an L‐shaped pattern concentrated in a ∼7 × 7 km area. Our model defines the Algeciras fault with two structural styles for its northern termination. The NW part is characterized by a duplex‐style of right‐lateral strike‐slip with inner secondary faults of the same sense of movement, and the SE zone by a domino‐style system with inner minor faults of sinistral kinematics. The earthquake doublet is in the zone characterized by the duplex style. In contrast, the southern part of the aftershocks is located in the zone characterized by the domino style.

Landslide Hazard Assessments of A Potential Earthquake-triggered in Central Taiwan Using Newmark's Model with the Stochastic Semi-Empirical Technique

This research aims to evaluate the probability distribution of landslides triggered by a potential earthquake. Based on the evaluations, successive disaster prevention and mitigation can be well-prepared in advance. Deriving the critical acceleration value depends on the geometry and strength of the material on the slope. Thus, the Newmark's displacement of slope can be determined using the critical acceleration and strong motion recording of an earthquake. A threshold value of Newmark's displacement is assumed as 5cm, which means a landslide may occur if Newmark's displacement is greater than it. To identify the feasibility, we synthesize two waveforms of the 1999 Chi-Chi earthquake (M_w 7.6) and one for the 2013 Nantou earthquake (M_w 6.2) to compute the Newmark's displacements at Jiou-Jiou Peaks and Shinyi town in Nantou County, Taiwan by using the stochastic semi-empirical technique. The Newmark's displacements are 9.75 averaged and 6cm respectively, which are coincident with the earthquake-triggered landslides occurring actually. By counting accumulations of Coulomb's stress changes of Nantou earthquakes in 2013 in central Taiwan, we predict the epicenter of a potential earthquake. The regression relation between critical acceleration with Arias intensity of the seismic waveform is employed to calculate Newmark's displacements in the Nontou County. Accompanied by Newmark's displacement, the empirical probabilities of earthquake-induced landslides are derived. Accordingly, there are four populated densely areas with higher Newmark's displacements and probabilities, indicating that high landslide risk is existing potentially in these areas and the corresponding disaster prevention management and mitigation must be carefully regulated.

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 and Triggered Aseismic Faulting of the 2021 Mw 7.3 Maduo Earthquake Revealed by the UAV-Lidar/Photogrammetry, InSAR, and Field Investigation

Multi-source datasets, including the UAV-Lidar/Photogrammetry, InSAR, and field investigation data, have been used for revealing the complex surface displacement pattern and focal mechanism of the 2021 Maduo Mw 7.3 earthquake. First, a co-seismic surface deformation field was extracted from the Synthetic Aperture Radar (SAR) images captured by ALOS-2 and Sentinel-1 satellites. Second, the SAR pixel offset tracking results were adopted to detect the initial location of the seismogenic fault. Then, the Lidar digital elevation model with high spatial resolution and field investigation were employed to refine and verify the location of the seismogenic fault. It was found that bifurcated strike-slip rupture should account for the 2021 Maduo earthquake. As indicated by the estimated faulting model based on the InSAR data, the maximal fault slip was ~6.2 m, occurring in the southeast of the main seismogenic fault, and five remarkable slip concentrations controlled the surface displacement of the 2021 Maduo earthquake. Furthermore, the co- and post-seismic InSAR deformation, dilatation, shear strain, Coulomb failure stress, and aftershock sequence suggest that the co-seismic rupture of the two main seismogenic faults have triggered the aseismic slip along the Changmahe fault. Lastly, according to the Coulomb failure stress change due to the historical earthquakes and the 2021 Maduo earthquake, the 1937 M 7.8 earthquake predominantly controlled the Coulomb failure stress change along the Kunlun fault, and the Xidatan-Alake Lake and Maqin segments had a higher risk of future earthquake than the other segments.

Raised potential earthquake and tsunami hazards at the North Sulawesi subduction zone after a flurry of major seismicity

The North Sulawesi subduction zone (NSSZ) was ruptured by a series of large earthquakes (Mw >7) since 1990s, generally triggered small to moderate tsunamis in the surrounding seas and ocean. But the 2018 Sulawesi Mw 7.5 earthquake ruptured the west subduction zone and induced a large tsunami that displaced hundreds of people. These large earthquakes, especially the deeper thrust events, generated stress loading to the shallow megathrust that could rupture to excite exceptional tsunami hazard as observed in Sumatra, Japan and the other subduction zones. Whether the stress loading from downdip events can trigger future failure of a shallow tsunami earthquake and its ensuring tsunami hazard impact remains elusive. Here we investigate the potential of earthquake and tsunami by analyzing the historical earthquake characteristics, calculating Coulomb stress changes and simulating hundreds of hypothetical earthquake ruptures to assess the plausible tsunami hazard in the NSSZ. Our results show that a series of Mw 7+ downdip megathrust earthquakes have loaded most of the megathrust, especially the shallow portion (<10 km), with increased stress >10 kPa, implicating a high potential of future large earthquakes and ensuring outsize tsunamis. Our modeled tsunami wave heights vary between >0 and 43 m along the North Sulawesi coastlines. One intriguing fact is that the end point of the tsunami wave energy dissipation path gains a comparable tsunami impact as the region in the rupture zone for all the magnitude considered, highlighting a dual-pattern threatened regions in the Celebes Sea. Importantly, the thrust and fold belt structure in the wide outer wedge of the accretionary prism and the strong seafloor bathymetry variation offshore Sulawesi Island could serve an efficient wave-amplification tool that need to be considered in future hazard assessment. Our findings alert that the earthquake and tsunami hazard potential are largely raised by these downdip major earthquakes.

Study of Coulomb Stress Change (CSC) Earthquake in the Segment Area of West Seram – Ambon Island

Long beans are short-lived plants that are widely cultivated in Indonesia. To stimulate the growth of long beans, it is necessary to fertilize with the appropriate dose for its growth. Research on the effect of vermicompost and NPK fertilizer has been carried out in Mertak Umbak Village, Central Lombok. The purpose of the research was to determine the effect of the application of vermicompost on the growth of long beans, the effect of the application of NPK fertilizer on the growth of long beans and the interaction effect of the application of vermicompost and NPK fertilizer on the growth of long beans. The research was carried out with a 2-factor design and 4 replications. The research data were analyzed by analysis of variance. In this study, the results showed that the treatment of vermicompost significantly increased the number of leaves and plant height, but could not increase the length and width of the long bean leaves, the application of NPK fertilizer could increase the leaf length, leaf width, number of leaves and stem length of long beans and the interaction of vermicompost and NPK fertilizer treatment did not significantly affect all measured long bean growth parameters

Stress evolution on major faults in Tien Shan and implications for seismic hazard

Tien Shan tectonic belt has experienced intense seismicity and a series of destructive strong earthquakes. However, earthquake triggering effects and faulting interactions in this area are poorly understood. A 3D finite element model of Tien Shan tectonic belt is constructed, to investigate stress evolutions on major faulting zones driven by interseismic tectonic loading and historical strong earthquakes with M≥ 6.0 since 1900. The numerical results show Tien Shan is dominated by nearly N-S compression, with higher tectonic loading rate in southwest Tien Shan. 1906 Manas M7.7 earthquake exerted pronounced Coulomb stress increase on its adjacent faulting zones, especially in the epicenter of 2016 Hutubi M6.0 earthquake. And three large earthquakes with M≥ 8.0, e.g., Chilik M8.3 earthquake in 1889, Kemin M8.0 earthquake in 1911 and Atushi M8.2 earthquake in 1902, increased the Coulomb stress by above 100 kPa in the epicenter of 1991 Keping M6.0 earthquake. While, stress perturbations by other strong earthquakes are limited, with slight Coulomb stress changes in the epicenters of their subsequent earthquakes. Overall, strong earthquakes with M> 7.0 in Tien Shan, induced substantial Coulomb stress changes on the adjacent faulting zones. Stress evolutions on major faults reveal higher stress accumulation in southwest Tien Shan, east KQX fault, west BoA fault, and HMT fault, indicating higher seismic risk.

The 2021 Mw6.7 Lake Hovsgol (Mongolia) Earthquake: Irregular Normal Faulting with Slip Partitioning Controlled by an Adjacent Strike-Slip Fault

In transtensional regions, structures striking obliquely to the extension direction generally exhibit oblique or partitioned slips. However, their on-fault partitioning patterns and controlling factors are less known, hindering our understanding of the evolution of the rifting process. Here, we study the slip distribution of the 2021 Mw6.7 Lake Hovsgol (Mongolia) earthquake occurred in a pull-apart basin using InSAR observations. Our preferred slip model shows a remarkable feature, with three zones exhibiting distinct slip directions at different depths. The Coulomb stress change analysis reveals that this pattern is likely controlled by the left-lateral motion of the Mondy Fault to the north, which also inhibits the growth of a boundary fault to the east of the lake, shaping the asymmetric graben structure in this region. Our results imply the important role of major strike-slip faults bounding the pull-apart basin in the formation and evolution of the oblique rift.

Comparative analysis on seismicity and stress triggering of strong earthquakes sequence in Central Tibet

Central Tibet constitutes part of the central part of the Qinghai–Tibetan Plateau, which is one of the highest seismically active areas in China. This paper discusses the regularity of seismic activity in this area. Based on a stratified viscoelastic earth model, we calculated the Coulomb stress changes imparted from four strong earthquakes (M ≥ 6.3) along the Bengco–southeastern piedmont of the Nyainqentanglha mountain fault zone in this region. The result shows that the study area may be entering a new active period from 2020. There was a trigger between the strong earthquakes (M ≥ 6.3) on the Bengco fault zone. The post-seismic viscous relaxation effect of a strong earthquake had a significant impact on subsequent earthquakes (M ≥ 6.3). In the next 100 years, the Coulomb stress loading is more than 1.0 MPa in the northwest section of the Bengco fault and the central part of Nimu segment of the southeastern piedmont of the Nyainqentanglha mountain fault. Thence, strong earthquakes may occur along these fault segments. The maximum magnitude of the earthquake could be M6.7 in the next 100 years.

Crustal response to inflation imaged by mapping of subsurface faults, slip directions, and stress changes during the 1993–1998 unrest in Hengill, SW-Iceland

Between 1993 and 1998, the Hengill volcanic region in southwest Iceland was subject to a significant volcano-tectonic event which caused a total of 8 cm uplift and induced >90 thousand earthquakes, most of magnitude smaller than 2 ML and a small intrusion has been proposed as its cause. In this paper, we study fault activation around the uplift center to analyze whether the Coulomb stress change induced by such an intrusion would cause the observed crustal response. The locations of a subset of 12,000 micro-earthquakes are significantly improved by a double-difference relocation algorithm and the use of cross-correlation of waveforms to refine phase arrival times. The vast majority of seismic activity is scattered, but twenty-five (25) rectangular prisms, consisting of 11 to 363 earthquakes and 200 to 3000 m long, were identified and are interpreted as faults. Most of these faults are sub-vertical (within 5° from vertical) and only two faults dip at angles of 62° and 65°, respectively. Focal mechanism analysis of individual events was applied to estimate slip direction on these faults. Simulation of the Coulomb stress changes reveals that the stress changes would have been able to induce the observed fault movement on 22 of 25 faults. Discrepancies on the remaining faults are likely caused by uncertainties in the rake directions. Our results emphasize that the seismicity caused by a volume change in the crust is controlled by the interplay between the local stress field and the induced stresses.

Applying unsupervised machine-learning algorithms and MUSIC back-projection to characterize 2018–2022 Hualien earthquake sequence

The city of Hualien in eastern Taiwan is located at the junction of the Luzon arc–Eurasian continent collision and the oblique subduction of the Philippine Sea plate along the Ryukyu Trench. This complex tectonic setting creates one of the most intense seismic hazards in the world. Since the mid-twentieth century, three earthquake sequences (1951, 1986–1992, and 2018–2022) have caused severe damages and revealed a diversity of focal mechanisms, demonstrating a complex seismogenic tectonic interactions of collisions and subductions. In this study, we used the two-dimensional back-projection method to examine rupture characteristics of three strong earthquakes during the latest sequence (2018–2022). We also studied the same earthquake sequence using unsupervised machine-learning via density-based spatial clustering of applications with noise (DBSCAN) and principal component analysis (PCA). Our results revealed two westward dipping seismogenic structures. The deeper one lies beneath the eastern flank of the Central Range and the shallower one rotates clockwise along the coastline from south to north. Overall, focal mechanisms within these two structures are consistent with planar geometries obtained using PCA. The deeper structure beneath the Central Range is likely an imbricated fault system. The coastline structure is characterized by a transition of strike and dip angles from south to north. We suggest that the subhorizontal nodal planes in earthquakes offshore Hualien are related to the shallow northward dipping patch in the northern part of the coastal structure. Our Coulomb stress change results revealed strong linkages between these two structures.

A Coulomb Stress Response Model for Time‐Dependent Earthquake Forecasts

AbstractSeismicity models are probabilistic forecasts of earthquake rates to support seismic hazard assessment. Physics‐based models allow extrapolating previously unsampled parameter ranges and enable conclusions on underlying tectonic or human‐induced processes. The Coulomb Failure (CF) and the rate‐and‐state (RS) models are two widely used physics‐based seismicity models both assuming pre‐existing populations of faults responding to Coulomb stress changes. The CF model depends on the absolute Coulomb stress and assumes instantaneous triggering if stress exceeds a threshold, while the RS model only depends on stress changes. Both models can predict background earthquake rates and time‐dependent stress effects, but the RS model with its three independent parameters can additionally explain delayed aftershock triggering. This study introduces a modified CF model where the instantaneous triggering is replaced by a mean time‐to‐failure depending on the absolute stress value. For the specific choice of an exponential dependence on stress and a stationary initial seismicity rate, we show that the model leads to identical results as the RS model and reproduces the Omori‐Utsu relation for aftershock decays as well stress‐shadowing effects. Thus, both CF and RS models can be seen as special cases of the new model. However, the new stress response model can also account for subcritical initial stress conditions and alternative functions of the mean time‐to‐failure depending on the problem and fracture mode.

A possible characteristic of foreshocks derived from the evaluation of loading/unloading induced by earth tides

The detection of the potential of a larger event immediately after a moderate earthquake is a quite difficult problem. In this work, we devised an approach to determine whether an earthquake is a foreshock to a larger mainshock by evaluating loading/unloading states. This is done by calculating the Coulomb failure stress (CFS) change induced by Earth tides along the tectonically preferred slip direction on the seismogenic fault surface of each of the candidates. The technique is based on the load/unload response ratio (LURR) method, but the determination of the sliding direction of CFS is different, which is derived from the moderate earthquake that has just occurred rather than the assumption from the regional stress setting. Using the approach, we tested since the year 2000 the Ms ≥ 4.0 foreshocks of the Ms ≥ 6.0 earthquakes on the Chinese mainland, and also the earthquake swarms where no subsequent larger mainshocks were expected in the previous 10 years. The former mostly occurred in the loading process, while the latter arose more frequently in a random process. The marked difference indicates that the occurrence of earthquakes during loading may imply the critical state of the seismogenic faults, and the derived stress is more likely to trigger a larger event than that of the earthquakes that occurred during unloading.

The role of fault interaction in earthquake migration in Central Sulawesi, Indonesia

Over the past 50 years, a sequence of destructive earthquakes, such as the 1968 (Mw 7.2), 1996 Tonimi (Mw 7.9), and 2018 Palu (Mw 7.5) earthquakes, occurred on the Palu-Koro strike-slip fault and the Minahassa Megathrust, which caused catastrophic disasters in Central Sulawesi, Indonesia. To explore the relationship between fault interaction and earthquake migration in this target area, we simulated the Coulomb stress change (ΔCFS) caused by 13 historical large earthquakes (M ≥ 6.0) since 1968. We show that the 1968 earthquake (Mw 7.2) on the Palu-Koro Fault promoted the 1996 earthquake sequence (Mw 7.9, Mw 6.6, and Mw 7.0) on the Minahassa Megathrust by transferring positive stress to its western edge with a maximum ΔCFS value of 124 kPa before 1996. Consequently, the 1996 earthquake (Mw 7.9) promoted six large earthquakes (M ≥ 6.0) including the 2018 Palu earthquake (Mw 7.5) on the Palu-Koro Fault between 1998 and 2020 via stress loading over the entire Palu-Koro Fault with a maximum ΔCFS value of 76 kPa. Via a positive feedback, these six earthquakes have increased stress at the western edge of the Minahassa Megathrust with a maximum ΔCFS value of 265 kPa up to 2020, resulting in increased seismic and consequent tsunami hazards in this region. Based on these risks, close attention should be paid to the future seismic hazards in Central Sulawesi. These findings suggest that the Palu-Koro Fault and the Minahassa Megathrust constitute a coupled fault system in which seismicity in either fault might bring the other closer to failure. Our results provide new insights into the causes of earthquake generation, migration, and hazard estimation in Central Sulawesi, and provide a basis for understanding stress transfer and seismic activity in strike-slip and thrust fault systems globally.

Bayesian Multi‐Model Estimation of Fault Slip Distribution for Slow Slip Events in Southwest Japan: Effects of Prior Constraints and Uncertain Underground Structure

AbstractWe consider a Bayesian multi‐model fault slip estimation (BMMFSE), which incorporates many candidates of the underground structure (Earth structure and plate boundary geometry) model characterized by a prior probability density function (PDF). The technique is used to study long‐term slow slip events (L‐SSEs) that occurred beneath the Bungo Channel, southwest Japan, in around 2010 and 2018. We here focus on the two advantages of BMMFSE: First, it allows for estimating slip distribution without introducing relatively strong prior information such as smoothing constraints, by combining a fully Bayesian inference and better consideration of model uncertainty to avoid overfitting. Second, the posterior PDF for the underground structure is also obtained during the fault slip estimation, which can be used as priors for the estimation of slip distribution for recurring events. The estimated slip distribution obtained using BMMFSE agreed better with the distribution of deep tectonic tremors at the down‐dip side of the main rupture area than those based on stronger prior constraints when the corresponding Coulomb failure stress changes are compared. This finding suggests a mechanical relationship between the L‐SSE and the synchronized tremors. The use of the posterior PDF of the underground structure estimated for the 2010 L‐SSE as prior PDF for the 2018 event resulted in more consistent estimation with the data, indicated by a smaller value of an information criterion.