Positive Coulomb Stress Changes Research Articles

Insights into Very Early Afterslip Associated with the 2021 M 8.2 Chignik, Alaska Earthquake Using Subdaily GNSS Solutions

Based on subdaily kinematic GNSS solutions, the fault slip properties during the very early postseismic phase after the 2021 M 8.2 Chignik earthquake are investigated in this paper. The very early postseismic deformations captured by near-field GNSS sites can be well depicted by the power model. The comparison of afterslip determined by daily and subdaily GNSS solutions suggests that neglecting very early afterslip can result in the underestimation of postseismic slip. Compared with coseismic slip, the cumulative afterslip of the first 24 h is mainly focused in the southeast of the hypocenter, and the shallow updip afterslip appears after this earthquake. The spatio-temporal evolution of the afterslip reveals that the patch of afterslip is immediately generated after the earthquake, and then the postseismic slip gradually grows along the afterslip patch. The magnitude of the afterslip patch varies remarkably within the 24 h following the earthquake, especially in the first several hours. Meanwhile, the spatio-temporal patterns of aftershocks and afterslip exhibit strong similarity during the first 24 h, suggesting that very early afterslip may be a possible driving factor of aftershocks. Moreover, most of the afterslip patches and aftershocks occurring immediately after this earthquake are situated in the area covered by positive Coulomb Stress Change (CSC), which implies that the immediate afterslip and aftershock activities can be influenced by the coseismic CSC. The following afterslip process further releases coseismic CSC and then influences the spatio-temporal variations of aftershock activities. Thus, the afterslip may be a possible triggering mechanism of very early aftershocks for this earthquake, alongside the effects of the CSC generated by coseismic rupture.

Revisiting the seismic hazards of faults surrounding the 2022 Ms6.8 Luding earthquake, Sichuan, China

The occurrence of the Ms6.8 Luding earthquake on 5 September 2022 filled the seismic gap in the southeastern segment of the Xianshuihe fault zone and transferred stress to the surrounding faults, which attracted much more attention to the seismic hazard. In this study, we calculate the coseismic Coulomb stress changes caused by the Ms6.8 Luding earthquake and analyze its relationship with the relocated aftershocks, and further explore the stress evolution before the Luding earthquake and seismic hazards on the surrounding faults after it. The results show significant coseismic Coulomb stress increases in the northern Anninghe fault zone and the coseismic unruptured section of the southeastern Xianshuihe fault zone. Aftershocks are highly correlated with the positive coseismic Coulomb stress change, wherein eight out of eleven aftershocks of M ≥ 3.0 occurred in areas with significant stress increases ranging from 0.03 MPa to 0.44 MPa, indicating the triggering effect. The Moxi fault experiences co- and post-seismic Coulomb stress changes due to historical M ≥ 7.0 strong earthquakes since 1515, which help understand the seismogenic mechanism of the Luding earthquake. Moreover, the Luding earthquake advanced the potential earthquakes in the northern section of the Anninghe fault zone by approximately 60 years, further highlighting the potential seismic hazards.

Properties of the Petrinja (Croatia) earthquake sequence of 2020–2021 – Results of seismological research for the first six months of activity

We present locations of almost 14,000 events from the first six months of the Petrinja (Croatia) earthquakes sequence (mainshock 29 December 2020, Mw 6.4). The catalogue is estimated complete for ML ≥ 1.20. Initially sparse local seismograph network was densified a week after the mainshock, which reduced uncertainties of focal locations by about 75%. Source-specific station corrections were used in an iterative location scheme. The hypocentres were located with 30 different sets of velocity models and program control parameters in order to gain insight into the epistemic uncertainty of each earthquake location. The bulk of epicentres are located close to, and to the SE of the causative, dextral strike-slip Petrinja fault, but considerable activity has been triggered on smaller faults, up to 25 km away. About a half of the 78 first-motion polarity focal mechanism solutions (FMS) from the full first year of activity were found to have mechanisms similar to the foreshock, the mainshock and the largest of aftershocks, while the rest indicated almost pure reverse faulting. Strike-slip mechanisms occurred more often close to the main fault. The P-axes of FMS for aftershocks were found to be on the average rotated by 16° clockwise with respect to the P-axis of the mainshock, regardless of the style of faulting. The spatial distribution of aftershocks coincides very well with the areas of positive Coulomb stress change caused by the mainshock rupture. The inferred cut-off seismogenic depth in the area (7–10 km), estimated on the basis of published values of the geothermal gradient and the assumption of predominantly granitic upper crust, is considerably shallower than the depth of the deepest reliably located aftershocks (16–18 km). This fact may be explained by considering ophiolite-dominated crust, and by increased strain rate during the sequence, which raises the ability of crustal rocks for brittle failure.

A shallow slow slip event in 2018 in the Semidi segment of the Alaska subduction zone detected by machine learning

Slow slip events (SSEs) have been discovered at shallow depth near the trench in some subduction zones and have been linked to the triggering of large earthquakes and the absence of tsunami. These shallow SSEs are invariably submarine, making it difficult to observe their temporal and spatial extent. Here, we report a shallow SSE in late 2018 near the west Semidi segment of the Alaska subduction zone, up-dip of and preceding the Mw 8.2 Chignik earthquake on July 29th, 2021. The SSE was detected in data from an offshore array of seafloor pressure gauges by a machine learning method. This detection is supported by the spatial pattern of simulated SSE deformation, the increased seismicity after the SSE in the positive Coulomb stress change area, the lack of shallow slip in Chignik earthquake, and the absence of a sizable tsunami following the Chignik earthquake. Our method has the potential to transform the way offshore SSEs are detected and to improve tsunami hazard assessment in subduction zones.

Impoundment‐Associated Hydro‐Mechanical Changes and Regional Seismicity Near the Xiluodu Reservoir, Southwestern China

AbstractFour large hydropower stations have recently been built downstream the Jinsha River in Southwestern China with a strong regional tectonic activity background. There is widely felt seismicity since the impoundment of the Xiluodu and Xiangjiaba reservoirs, increasing the public concern in this region. We begin with a criticality analysis of the faults near these reservoirs to quantify their susceptibility to triggered seismicity. Then we focus on the Xiluodu reservoir to investigate the correlation between the impoundment and seismicity nearby. We analyze the spatio‐temporal distribution of seismicity near the Xiluodu reservoir, and identify the plausible rapid and delayed seismic response due to the impoundment. According to the impoundment record, we explicitly model the hydro‐mechanical changes due to diffusion and reservoir water load, that is, pore pressure, elastic stress, and the resulting Coulomb stress. Our results show that the pore pressure changes can reach a level that may trigger fault reactivation and consequently, seismicity nearby. The water load can also induce the positive Coulomb stress changes on faults, depending on the fault orientation, which is especially important for understanding the earthquakes that occurred shortly after the impoundment and at more than 10 km distance from the reservoir. The combination of these two effects can induce positive Coulomb stress change over a larger area, which overlaps the majority of the events after the impoundment, including two M5+ events. While the causal relationship between the impoundment and seismicity warrants further analysis, we hope to inform the regional seismic impact of impoundment with this timely study.

AutoCoulomb: An Automated Configurable Program to Calculate Coulomb Stress Changes on Receiver Faults with Any Orientation and its Application to the 2020 Mw 7.8 Simeonof Island, Alaska, Earthquake

AbstractCoulomb stress change is the change in resultant force of shear stress and friction imposed on a receiver fault plane. The resulting stress change is often computed using the Coulomb 3.4 and the postseismic Green’s functions and postseismic components (PSGRN-PSCMP) programs. Notwithstanding both preferences, both have incomplete optimally oriented failure planes (OOPs) and are inconvenient to resolve Coulomb stress changes on various fault planes placed in varying depths. Here, we present an alternative program termed AutoCoulomb. It leverages the shell command-line tool to automatically batch-process Coulomb stress changes on all sorts of receiver fault planes. We first validate the program. We then apply it to the 2020 Mw 7.8 Simeonof Island, Alaska, earthquake, as a case study. Our results show that Coulomb stress changes resolved on fixed receiver faults, using the three programs, are in line with each other. So are those resolved on 3D OOPs using the PSGRN–PSCMP and the AutoCoulomb programs. Nevertheless, Coulomb stress changes on 2D OOPs, generated by the AutoCoulomb program, always outweigh those done by the Coulomb 3.4 program, indicating that 2D OOPs constrained by the latter are not the most optimal. Some nonoptimal 2D OOPs result in the reversal of the signs of Coulomb stress changes, posing a risk of misleading stress shadows with negative Coulomb stress changes. For the case study, the 28 July 2020 Mw 6.1 aftershock received a positive coseismic Coulomb stress change of ∼3.5 bars. In contrast, the compounded coseismic Coulomb stress changes at the hypocenters of the 1946 Mw 8.2, the 1948 Mw 7.2, and the 2020 Mw 7.8 earthquakes are within a range from −1.1 to 0.1 bar, suggesting that coseismic Coulomb stress changes promoted by preceding mainshocks alone are not responsible for these mainshocks. Other factors, such as postseismic viscoelastic relaxation, afterslip, and slow slip, may contribute to promoting their occurrence.

What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

ABSTRACTStrong mining-induced earthquakes are often followed by aftershocks, similar to natural earthquakes. Although the magnitudes of such in-mine aftershocks are not high, they may pose a threat to mining infrastructure, production, and primarily, people working underground. The existing post-earthquake mining procedures usually do not consider any aspects of the physics of the mainshock. This work aims to estimate the rate and distribution of aftershocks following mining-induced seismic events by applying the rate-and-state model of fault friction, which is commonly used in natural earthquake studies. It was found that both the pre-mainshock level of seismicity and the coseismic stress change following the mainshock rupture have strong effects on the aftershock sequence. For mining-induced seismicity, however, we need to additionally account for the constantly changing stress state caused by the ongoing exploitation. Here, we attempt to model the aftershock sequence, its rate, and distribution of two M≈2 events in iron ore Kiruna mine, Sweden. We could appropriately estimate the aftershock sequence for one of the events because both the modeled rate and distribution of aftershocks matched the observed activity; however, the model underestimated the rate of aftershocks for the other event. The results of modeling showed that aftershocks following mining events occur in the areas of pre-mainshock activity influenced by the positive coulomb stress changes, according to the model’s assumptions. However, we also noted that some additional process not incorporated in the rate-and-state model may influence the aftershock sequence. Nevertheless, this type of modeling is a good tool for evaluating the risk areas in mines following a strong seismic event.

Triggered crustal earthquake swarm across subduction segment boundary after the 2016 Pedernales, Ecuador megathrust earthquake

Megathrust ruptures and the ensuing postseismic deformation cause stress changes that may induce seismicity on upper plate crustal faults far from the coseismic rupture area. In this study, we analyze seismic swarms that occurred in the north Ecuador area of Esmeraldas, beginning two months after the 2016 Mw 7.8 Pedernales, Ecuador megathrust earthquake. The Esmeraldas region is 70 km from the Pedernales rupture area in a separate segment of the subduction zone. We characterize the Esmeraldas sequence, relocating the events using manual arrival time picks and a local a-priori 3D velocity model. The earthquake locations from the Esmeraldas sequence outline an upper plate fault or shear zone. The sequence contains one major swarm and several smaller swarms. Moment tensor solutions of several events include normal and strike-slip motion and non-double-couple components. During the main swarm, earthquake hypocenters increase in distance from the first event over time, at a rate of a few hundred meters per day, consistent with fluid diffusion. Events with similar waveforms occur within the sequence, and a transient is seen in time series of nearby GPS stations concurrent with the seismicity. The events with similar waveforms and the transient in GPS time series suggest that slow aseismic slip took place along a crustal normal fault during the sequence. Coulomb stress calculations show a positive Coulomb stress change in the Esmeraldas region, consistent with seismicity being triggered by the Pedernales mainshock and large aftershocks. The characteristics of the seismicity indicate that postseismic deformation involving fluid flow and slow slip activated upper plate faults in the Esmeraldas area. These findings suggest the need for further investigation into the seismic hazard potential of shallow upper plate faults and the potential for megathrust earthquakes to trigger slow-slip and shallow seismicity across separate segments of subduction zones.

Source Parameters of Three Moderate Size Earthquakes in Weiyuan, China, and Their Relations to Shale Gas Hydraulic Fracturing

AbstractOn 7 September 2019, an ML5.4 earthquake struck the Weiyuan area in southwestern China. This is the largest and most damaging event in the Weiyuan shale gas field. In the next 4 months, two earthquakes with ML4.3 and ML5.2 rattled the same area. In this study, we obtain source parameters of these earthquakes (e.g., centroid location, focal mechanism, and rupture directivity) to investigate whether they are induced by hydraulic fracturing of shale gas production. We first invert for high‐resolution centroid locations and focal mechanisms of two M ~ 3 earthquakes recorded by a temporary dense seismic array. We then adopt the M ~ 3 events as references to obtain reliable centroid locations and rupture directivity of those moderate‐size earthquakes. The results show that all earthquakes have centroid depths of no more than 3 km and are within 2 km to injection wells of shale gas production, which suggest that these earthquakes may be induced by local hydraulic fracturing. The ML5.2 earthquake has the same rupture directivity to NE as the ML5.4 event and is located in the zone of positive Coulomb stress change caused by the latter. However, the long time lapse of the ML5.2 earthquake and the existence of an injection well within 2 km suggest that it is more likely induced by hydraulic fracturing than triggered statically by the ML5.4 earthquake.

Is the Aftershock Zone Area a Good Proxy for the Mainshock Rupture Area?

ABSTRACTThe locations of aftershocks are often observed to be on the same fault plane as the mainshock and used as proxies for its rupture area. Recent developments in earthquake relocation techniques have led to great improvements in the accuracy of earthquake locations, offering an unprecedented opportunity to quantify both the aftershock distribution and the mainshock rupture area. In this study, we design a consistent approach to calculate the area enclosed by aftershocks of 12 Mw≥5.4 mainshocks in California, normalized by the mainshock rupture area derived from slip contours. We also investigate the Coulomb stress change from mainshock slip and compare it with the aftershock zone. We find that overall, the ratios of aftershock zone area to mainshock rupture area, hereinafter referred to as “aftershock ratio”, lie within a range of 0.5–5.4, with most values being larger than 1. Using different slip-inversion models for the same mainshock can have a large impact on the results, but the ratios estimated from both the relocated catalogs and Advanced National Seismic System catalog have similar patterns. The aftershock ratios based on relocated catalogs of southern California fall between 0.5 and 4.3, whereas they exhibit a wider range from 1 to 5.4 for northern California. Aftershock ratios for the early aftershock window (within one-day) show a similar range but of smaller values than using the entire aftershock duration, and we propose that continuing afterslip could contribute to the expanding aftershock zone area following several mainshocks. Our results show that areas with positive Coulomb stress change scale with aftershock zone areas, and spatial distribution of aftershocks represents stress release from mainshock rupture and continuing postseismic slip.

Effects of imparted Coulomb stress changes in the seismicity and cluster of the December 2017 Hojedk (SE Iran) triplet

The Hojedk area in the southeastern Iran experienced three earthquakes during 12 days in December 2017. These events occurred in proximity to a region that has experienced several moderate and large events in the past. In this paper, the role of Coulomb stress changes in occurrence of the Hojedk cluster was investigated, and also the produced stress changes due to these events on the surrounding and optimally oriented faults was calculated. Previous earthquakes (events in the Golbaf–Sirch region, the Shahdad slip, and the Dahuiyeh–Zarand earthquake) imparted positive stress changes on the fault planes of the Hojedk earthquakes, especially on the ruptured plane of the first main shock. The Hojedk first main shock triggered the second one by imparting maximum stress changes of about 11.7 MPa on the fault plane of this event, and these two events brought the third main shock to failure by producing more than 6.0 MPa stress changes on its ruptured plane. Among the major active faults in the surrounding area, the middle part of the Nayband fault, northern part of the Golbaf–Sirch fault, and southern part of the Kuhbanan fault received positive stress changes and southen parts of the Ravar and all parts of the Chatrud faults received negative stress changes due to the Hojedk earthquakes. Furthermore, our results for correlation between Coulomb stress changes and seismicity distribution showed that following the Dahuiyeh–Zarand earthquake, the majority of the seismicity located on the positive stress area and Coulomb stress changes have a controlling role on the spatial distribution of seismicity even after a decade. The occurrence of the Hojedk first main shock affects the spatial distribution of aftershocks until the occurrence of the next main hocks. By triggering the next one, the stress patterns change in the area, and seismicity follows from the stress patterns of the last event.

Finite Slip Models of the 2019 Ridgecrest Earthquake Sequence Constrained by Space Geodetic Data and Aftershock Locations

ABSTRACTThe July 2019 Ridgecrest, California, earthquake sequence involved two large events—the M 6.4 foreshock and the M 7.1 mainshock that ruptured a system of intersecting strike-slip faults. We present analysis of space geodetic observations including Synthetic Aperture Radar and Global Navigation Satellite System data, geological field mapping, and seismicity to constrain the subsurface rupture geometry and slip distribution. The data render a complex pattern of faulting with a number of subparallel as well as cross-cutting fault strands that exhibit variations in both strike and dip angles, including a “flower structure” formed by shallow splay faults. Slip inversions are performed using both homogeneous and layered elastic half-space models informed by the local seismic tomography data. The inferred slip distribution suggests a moderate amount of the shallow coseismic slip deficit. The peak moment release occurred in the depth interval of 3–4 km, consistent with results from previous studies of major strike-slip earthquakes, and the depth distribution of seismicity in California. We use the derived slip models to investigate stress transfer and possible triggering relationships between the M 7.1 mainshock and the M 6.4 foreshock, as well as other moderate events that occurred in the vicinity of the M 7.1 hypocenter. Triggering is discouraged for the average strike of the M 7.1 rupture (320°) but encouraged for the initial orientation of the mainshock rupture suggested by the first-motion data (340°). This lends support to a scenario according to which the earthquake rupture nucleated on a small fault that was more optimally oriented with respect to the regional stress and subsequently propagated along the less-favorably oriented pre-existing faults, possibly facilitated by dynamic weakening. The nucleation site of the mainshock experienced positive dynamic Coulomb stress changes that are much larger than the static stress changes, yet the former failed to initiate rupture.

A reappraisal of the 2005 Kashmir (Mw 7.6) earthquake and its aftershocks: Seismotectonics of NW Himalaya

We study the source properties of the 2005 Kashmir earthquake and its aftershocks to unravel the seismotectonics of the NW Himalayan syntaxis. The mainshock and larger aftershocks have been simultaneously relocated using phase data. We use back-projection of high-frequency energy from multiple teleseismic arrays to model the spatio-temporal evolution of the mainshock rupture. Our analysis reveal a bilateral rupture, which initially propagated SE and then NW of the epicenter, with an average rupture velocity of ~2 kms−1. The area of maximum energy release is parallel to and bound by the surface rupture. Incorporating rupture propagation and velocity, we model the mainshock as a line source using P- and SH-waveform inversion. Our result confirms that the mainshock occurred on a NE dipping (~35°) fault plane, with centroid depth of ~10 km. Integrated source time function show that majority of the energy was released in the first ~20 s, and was confined above the hypocenter. From waveform inverted fault dimension and seismic moment, we argue that the mainshock had an additional ~25 km blind rupture beyond the NW Himalayan syntaxis. Combining this with findings from previous studies, we conjecture that the blind rupture propagated NW of the syntaxis underneath a weak detachment, overlain by infra-Cambrian salt layer, and terminated in a wedge thrust. All moderate-to-large aftershocks, NW of the mainshock rupture, are concentrated at the edge of the blind rupture termination. Source modeling of these aftershocks reveal thrust mechanism with centroid depths of 2–10 km, and fault planes oriented sub-parallel to the mainshock rupture. To study the influence of mainshock rupture on aftershock occurrence, we compute Coulomb failure stress on aftershock faults. All these aftershocks lie in the positive Coulomb stress change region. This suggest that the aftershocks have been triggered by either co-seismic or post-seismic slip on the mainshock fault.

Coulomb Stress Analysis of Halmahera Earthquake on June 7th 2016, Mw=6.3 and Its Correlation with Aftershocks

Main earthquake with magnitude Mw 6.3, has been occurred in the West of Halmahera Island, Northern Molucca on June 7th, 2016. The activity of the Pacific plate, Eurasia plate and Indo-Australia plates trigger some earthquake around Halmahera. The purpose of the current study was to determine the Coulomb stress change of the main earthquake and predict the aftershocks’ location around it. For understanding an interaction of the faults, this study used Coulomb stress change model. Three component seismic waveform data recorded by TNTI station within 132 km, SANI station within 381.2 km, LUWI station within 478.3 km, and TOLI2 station within 617.6 km of the epicenter. This study used software ISOLA-GUI to obtain the parameters of the earthquake source. The parameters of the earthquake source were then used to determine fracture orientation, length, width and slip displacement. It was also used to calculate the Coulomb stress changes around the main shock, using software Coulomb 33. The result shows that the orientation of the earthquake fault has a dip angle of 48° to the horizontal plane and has a strike of 210° against the North. The fault length is 19.49 km, the fault width is 11.59 km and slip displacement is 49.43 cm. Based on plotting result, the main earthquake in Northern Molucca on June 7th, 2016 has positive Coulomb stress change that spread across the Northeast to the Southwest. This change is likely triggered three aftershocks with a range of Mw 4.7 to 6.2.

Cephalonia-lefkas Transform Fault Zone (CLTFZ) Complexity: Insights from 2015 Lefkas Earthquake Sequence

In order to define a better model for the Cephalonia-Lefkas Transform Fault Zone the sequence of 2015 Lefkas earthquake was examined. On 17 November 2015 (07:10 GMT) a major earthquake (Mw=6.4) occurred on the central-western part of Lefkas island. Several destructive events were located in the past in this fault zone, so an extensive seismotectonic study is feasible for that area. Manual analysis was performed using a custom velocity model that was determined for that purpose, applying the average travel-time residuals and location uncertainties errors minimization method. Several clusters belonging to the aftershock sequence were identified, whereas three are directly related to the causative fault, covering an area of about 25 km. The central one, which includes the mainshock, comprises of only a few aftershocks. The northern, within which the majority of aftershocks are located, lies in the central part of Lefkas island and the southern occurred close to the SW edge of the island. In addition, offshore clusters with distinct characteristics have been identified to the south, between Lefkas and Cephalonia islands. The temporal evolution of the aftershock sequence indicates that no migration was observed, given that after the occurrence of the mainshock the entire epicentral area was activated. Focal mechanisms of the Seismological Laboratory of the University of Athens showed dextral strike-slip faulting for both mainshock and major aftershocks of the sequence. Taking into account the spatial distribution of the aftershocks, supported by the tectonic and geomorphological settings of the region, a deformation pattern, consisting of the Cephalonia-Lefkas and Ithaca-Lefkas major fault zones which converge in the area of Vassiliki bay is proposed. The appearance of the southernmost clusters was interpreted by the positive Coulomb stress changes transfer due to major earthquake Mw=6.4.

Evidence for Slip on a Border Fault Triggered by Magmatic Processes in an Immature Continental Rift

AbstractContinental rifting evolves through repeated tectonic and magmatic processes. Here we investigate the 2007–2008 northern Tanzania rifting episode to understand the interactions between magmatism and border fault slip during immature continental rifting. We compare modeled stress changes with geodetic observations from the western adjacent border fault of the Natron basin. We model six distinct phases of the rifting episode, including the eruptive activity of Ol Doinyo Lengai. The southern portion of the border fault experienced a positive cumulative Coulomb stress change, consistent with newly processed Global Positioning System data showing a distinct instance of slip during this time period. Our work suggests active volcanism and faulting are correlated in time, indicating magmatism associated with volcanic activity influences slip on border faults in immature continental rifts.

Time-dependent stress increase along the major faults in eastern Turkey

The co-seismic, and post-seismic effects of 18 earthquakes that occurred since 1822 were modelled in conjunction with a regionally variable secular inter-seismic effect in order to investigate how the Coulomb stress field in the eastern part of the North Anatolian and the East Anatolian Fault Zones, have evolved over the past two centuries. Results show that viscoelastic post-seismic relaxations caused by the 1939 Ms 7.9 Erzincan earthquake is still strong and influencing the seismic activity in the region. Post-seismic effects have increased the stress on the Yedisu segment as much as 5.3 bars, whereas supressed stress on the Kahramanmaras-Malatya segment. In general, increases in stress due to viscous-elastic effects are comparable, or in certain cases even larger than the increases in stress caused by co-seismic deformations. This indicates that the additional stress changes derived from viscoelastic deformations could be significant and should be taken into consideration when assessing seismic hazard along the active seismic zones. Two regions are identified as having high positive Coulomb stress changes therefore high seismic hazard; a segment close to Bingol Province in the East Anatolian Fault Zone and the Yedisu segment of the North Anatolian Fault Zone.

Kinematics of the 2012 Ahar–Varzaghan complex earthquake doublet (Mw6.5 and Mw6.3)

On 2012 August 11, an earthquake doublet (Mw6.5 and Mw6.3), separated in time by 11 min, occur in the northwest of Iran. The hypocentres of these earthquakes are close (∼6 km) and located near the cities of Ahar and Varzaghan. The rupture process of both main shocks is retrieved by inverting the near-field strong motions data and using the elliptical subfault approximation method. Our calculations show that the two earthquakes are occurring on two distinct fault planes: the first main shock (M1) has nucleated at a depth of ∼8.5 km, and is located ∼4 km east of the eastern termination of the E-W trending surface rupture. The slip reaches the ground surface west of the hypocentre on an E-W striking fault (N88°E) that dips almost vertically (80°S). This earthquake exhibits a right-lateral strike-slip mechanism. The entire slip is imaged on a single patch that ruptures with an average speed of 2.4 km s−1. The rupture duration is ∼5.6 s and the earthquake releases a seismic moment of ∼8.41E + 18 N·m. The slip reaches the surface with a right-lateral dislocation value of ∼1 m, which is consistent with the observed surface rupture. About 11 min later, the second main shock (M2) nucleates ∼5 km to the west and 4 km to the north with respect to the hypocentre of the M1, and at a depth of ∼16.5 km. The M2 rupture evolves toward shallower depths and to the west on an ENE-WSW oriented fault plane (strike ∼256°) with a dip of ∼60° northward. The slip is essentially distributed on two distinct patches with strike-slip and reverse mechanisms, respectively. The first patch has a pure right-lateral strike-slip mechanism, and ruptures at a relatively fast speed of over 2.8 km s−1, and last for about 2.6 s until it reaches the second patch. The latter has a reverse mechanism (rake∼112°) and extends the rupture toward shallow depths, and to the west at a speed of ∼2.5 km s−1, and its rupture lasts for ∼2.5 s. The top of the slip distribution of M2 stops at a depth of ∼8 km. We observe that aftershocks surround the M1 and most of the M2 slip models. They are not distributed in the region of high slip (∼3.1 m) of M1. We show that the rupture of M2 is controlled by the static Coulomb stress changes caused by M1, with the maximum slip of M2 located in the positive Coulomb stress caused by M1. The M2 rupture stops where it reaches the area of high negative Coulomb stress change (over −10 bars). The cumulative Coulomb stress fields of both main shocks show a transfer of positive static Coulomb stress change of >0.1 bars on the eastern segment of the North Tabriz Fault. This segment did not rupture since the 1721 M∼7.6–7.7 event that has destroyed the city of Tabriz, and that currently hosts 2 million people. The occurrence of this earthquake doublet with different mechanisms reveals the slip partitioning of the oblique convergence regime of NW Iran on the Ahar–Varzaghan complex fault system.

Stress triggering and aftershock properties induced by the Dalparri blind thrust fault at NW Zagros: The 18 August 2014 Mormori earthquake

Abstract The Coulomb stress changes induced by the Mormori earthquake and their influences on the neighboring regions have been investigated. The central section of the Siah Kuh anticline and the southern section of the Kaseh Mast anticline have been encouraged to failure while, the Kabir Kuh anticline, the eastern and western sections of the Siah Kuh anticline and the northern section of the Kaseh Mast anticline have been inhibited from failure. A limited area of the positive Coulomb stress change zone on the top right of the Dalparri blind thrust has been detected in the cross-sectional view of the Coulomb stress changes that can be considered as the location of a back thrust. The Mormori aftershocks have been plotted on the stress change map and it is found that a little less than half of the events occurred in the increased Coulomb stress zones along the strike of the Dalparri anticline axis. Six out of nine large aftershocks with M ≥ 5 occurred in the positive Coulomb stress lobe. The estimated b-value map of the Mormori region has been compared with the Coulomb stress change zones and it is observed that the low and high b-value regions have some consistency with the positive and negative Coulomb stress zones near the north and northwest of the main shock, west of the Kaseh Mast anticline and the south and southwest of the western end of the Dalparri Fault, respectively. In contrast, the central and northern negative Coulomb stress change lobes have not consistency with the low b-values in these regions. By examining the modified Omori relation of the aftershocks, it is inferred that the moderate-fast decay rate in the Mormori region may be related to the local stress reduction due to the occurrence of several large aftershocks.

Postseismic Coulomb stress changes on intra-continental dip-slip faults due to viscoelastic relaxation in the lower crust and lithospheric mantle: insights from 3D finite-element modelling

Earthquakes in the brittle upper crust induce viscoelastic flow in the lower crust and lithospheric mantle, which can persist for decades and lead to significant Coulomb stress changes on receiver faults located in the surrounding of the source fault. As most previous studies calculated the Coulomb stress changes for a specific earthquake in nature, a general investigation of postseismic Coulomb stress changes independent of local geological conditions is still lacking for intra-continental dip-slip faults. Here we use finite-element models with normal and thrust fault arrays, respectively, to show that postseismic viscoelastic flow considerably modifies the original coseismic Coulomb stress patterns through space and time. Depending on the position of the receiver fault relative to the source fault, areas with negative coseismic stress changes may exhibit positive postseismic stress changes and vice versa. The lower the viscosity of the lower crust or lithospheric mantle, the more pronounced are the transient stress changes in the 1st years, with the lowest viscosity having the largest effect on the stress changes. The evolution of postseismic Coulomb stress changes is further controlled by the superposition of transient stress changes caused by viscoelastic relaxation (leading to stress increase or decrease) and the interseismic strain accumulation (leading to a stress increase). Stress changes induced by viscoelastic relaxation can outweigh the interseismic stress increase such that negative Coulomb stress changes can persist for decades. On some faults, postseismic relaxation and interseismic strain accumulation can act in concert to enhance already positive Coulomb stress changes.