Strike-slip Events Research Articles

Kinematics of the 2023 Kahramanmaraş Earthquake Doublet: Biased Near-Fault Data and Shallow Slip Deficit

Abstract Accurate estimate of the shallow slip deficit (SSD) for large strike-slip events is highly dependent on near-fault data. Previous studies have estimated the SSD of the 2023 Kahramanmaraş earthquake doublet, which may vary from a few percent to about 50%. Whether this reduced shallow slip is real or artificial is crucial for understanding the seismic hazards during and following the earthquake doublet. In this study, we inverted for the kinematic slip of this earthquake doublet with refined near-fault Interferometric Synthetic Aperture Radar observations and compared the results with the source model without further data processing. The model that excludes nonphysical data produced only ∼6% and ∼22% SSD for the M7.8 and 7.6 events, respectively, compared to ∼44% and ∼53% SSD of the model using original data. The increased data gap generally leads to overestimated SSD, but when the data coverage is almost complete, our result shows that the SSD is very sensitive to biased near-fault data which may induce significant artificial SSD. Our results suggest that overestimated SSD may be accompanied by slip migration from shallow toward deeper depths, to meet the total moment or energy release constrained by surface displacement data. The M7.8 event increased static Coulomb stress at where the second M7.6 event nucleated and thus may have triggered the following event.

Riedel shear structures reactivation may induce earthquakes through long-term steam injection: A case study of a heavy oil production field in northwestern China

Conventional heavy oil exploitation methods involve steam stimulation and flooding. An oil field in northwestern China has been producing heavy oil via steam injection for several decades. The production area has been seismically quiet until an increase in seismicity occurred several years ago. An array of 40 seismographs has been deployed between July and October 2021 to monitor seismicity and resolve the possible causes of the recent seismicity. Using an end-to-end machine-learning-based high-precision earthquake location workflow, we analyze a microseismic sequence comprised of 178 events that occurred in the study area. Numerical simulations incorporating Coulomb failure stress suggest that prolonged steam injection can reactivate faults and induce seismic events. Similarly, fluid diffusion through conduits may achieve the same effect. Analysis of the focal mechanism solutions of 21 strike-slip and thrust events with [Formula: see text] in conjunction with the background stress regime ([Formula: see text] azimuth = N15°W) reveal that the stress distribution is compatible with a left-lateral Riedel shear structure (RSS) model. Therefore, we can speculate that steam injection may induce earthquakes by reactivating preexisting RSS fault structures. To conclude, the recent seismic events could have been induced by two possible mechanisms: (1) long-term steam injection may cause the static stress level on the faults beneath the reservoir to build up to critical levels, following which a slight stress disturbance can trigger an earthquake and (2) fluid conduits may transport condensed water to basement faults, weakening the faults through fluid diffusion.

Paleostress constraints on the tectonic evolution of the Central High Atlas orogen

The Central High Atlas is distinguished by map-scale, sigmoid, and narrow magmatic-cored ridges, which separate wide and open synclines. The origin of these structures has been debated for years. This study addresses this issue from a paleostress perspective, using mesostructural analysis in the Imilchil region and incorporating insights from previous research.Our analysis reveals that the Central High Atlas ridges developed through two main structural stages from the Jurassic to the Cenozoic. The first stage involved an extensional event characterized by NW-SE-oriented σ3, coeval with the Early Jurassic Tethyan extension. This was followed by wrench tectonics driven by oblique left-lateral motion of Africa with respect to Europe. This events is marked within the Central High Atlas by ENE-WSW-oriented σ1 and NNW-SSE-oriented σ3. The basin-scale left-lateral motion likely drove the formation of sigmoid stepovers, facilitating the emplacement of magmatic bodies from the Middle Jurassic to the Early Cretaceous. The second deformation stage is likely associated with the convergence between Africa and Europe and the consecutive Alpine orogeny. In the study area, this stage consists of three events: a pre-folding strike-slip event with NNE-SSW to N–S-oriented σ1, and a pre-folding strike-slip event with NW-SE-oriented σ1. Near the diapirs, the analyzed mesostructures display syn-to post-folding patterns in some locations, indicating pre-Alpine layer tilting driven by salt tectonics. The final event is marked by significant post-folding NW-SE compression, associated with the Central High Atlas basin inversion and the compaction of pre-existing magmatic-cored salt diapirs.This study highlights the complex deformation history that has influenced the evolution of the Central High Atlas ridges and provides evidence of the significant role of strike-slip tectonics during the post-rift period and magmatic emplacement, as well as in the early phases of basin inversion.

Understanding of landslides induced by 2022 Luding earthquake, China

On September 5, 2022, at least 10,855 landslides had been triggered by a magnitude Mw 6.7 (Ms 6.8) earthquake on the eastern margin of the Tibetan Plateau. Unfortunately, a detailed analysis of the spatial patterns of landslides in the eastern margin of the Baryan Har block is lacking. The observations show that the highest landslide concentrations are distributed along the seismogenic fault (Moxi fault) and Dadu River valley, coinciding with the effects of the hanging wall and microepicenter. Seismogenic tectonics controlled the regional distribution of new landslides, and the local topography influenced the detailed positions on the slopes. The total landslide mass wasting volume was 223.1 × 106 m3, and the maximum occurred in the Wandong Basin (value of 74 × 106 m3). Thirty landslide dams were temporarily existing. Although some local collapses occurred at the toe of the Hailuogou glacier, seismic shaking had no obvious influence on the overall stability of the glacier. A post debris flow assessment indicates that some large basins contained much loose material and that some steep small basins had high debris flow susceptibility. On the eastern margin of the Bayan Har block, the landslide-triggering thrust and strike-slip events both follow the distributions of the hanging wall.

Immature characteristics of the East Anatolian Fault Zone from SAR, GNSS and strong motion data of the 2023 Türkiye–Syria earthquake doublet

On February 6, 2023, an Mw 7.9 earthquake occurred in the western section of the East Anatolia Fault Zone (EAFZ). It was subsequently followed by an Mw 7.7 earthquake on the northern branch of the EAFZ, known as the Sürgü Fault Zone. Coseismic deformation fields were derived for these earthquakes using joint evaluation of near-field strong motion data, Global Navigation Satellite System data, and Synthetic Aperture Radar datasets. The coseismic slip distribution model was determined through the joint kinematic finite fault inversion. The Mw 7.9 earthquake was a left-lateral strike-slip event, predominantly occurring at depths up to 20 km. The earthquake displayed three distinct asperities that correlate well with bends and stepovers along the EAFZ. The Mw 7.7 earthquake also exhibited left-lateral strike-slip characteristics, with a major asperity along the Çardak Fault featuring a maximum slip of approximately 9.5 m at depths between 0 and 24 km. The occurrence of this unanticipated large Mw 7.9 catastrophic seismic event on a fault with low-intermediate structural maturity is noteworthy. In the vicinity of immature faults with multiple jogs, stress tends to accumulate at barrier locations. When the accumulated stress near several adjacent barriers reaches a certain threshold, it may result in the transformation of multiple barriers into asperities, triggering cascading ruptures.

The 2022 Mw 6.6 Menyuan earthquake: An early-terminated runaway rupture by the complex fault geometry

The 2022 Mw 6.6 Menyuan earthquake, which caused unexpectedly large surface offsets (about 2.6–3.5 m) relative to the moment magnitude of this event, is one of the best-recorded strike-slip events in the northeastern Tibetan plateau. Although previous studies have revealed many kinematic features of this event, the mechanism and rupture dynamic characteristics underlying unexpectedly large surface dislocations need to be clarified. In this study, based on the grid search method, we conducted 61 dynamic rupture simulations considering terrain relief to screen out a preferred dynamic model that fits the geodetic, seismic, and geological observations well. The results show that the average stress drop of the nucleation part is relatively high (11 MPa), which could provide high initial rupture energy and give this event a chance to develop into a large earthquake. However, the rupture propagating eastward is slowed and terminated by two consecutive fault bends of about 10° each, and the westward propagating one might concentrate at a shallow depth (< 3 km) on the south secondary fault after it jumps over the stepover. These results imply that the complex fault geometry (the fault bend and stepover) might terminate the rupture early so that the observed magnitude of this event (Mw 6.6) is smaller than the estimated magnitude (Mw 7.1–7.2) based on the surface offsets. Our study again shows that, due to advances in computational power and observations (coverage and quality), physics-based dynamic rupture simulations have entered an era where it is possible to fit the observed data well, which is vital for further understanding the mechanisms of earthquakes.

Simultaneous magnitude and slip distribution characterization from high-rate GNSS using deep learning: case studies of the 2021 Mw 7.4 Maduo and 2023 Turkey doublet events

SUMMARY Rapid and accurate characterization of earthquake sources is crucial for mitigating seismic hazards. In this study, based on 18 000 scenario ruptures ranging from Mw 6.4 to Mw 8.3 and corresponding synthetic high-rate Global Navigation Satellite System (HR-GNSS) waveforms, we developed a multibranch neural network framework, the continental large earthquake agile response (CLEAR), to simultaneously determine the magnitude and slip distributions. We apply CLEAR to recent large strike-slip events, including the 2021 Mw 7.4 Maduo earthquake and the 2023 Mw 7.8 and Mw 7.6 Turkey doublet. The model generally estimates the magnitudes successfully at 32 s with errors of less than 0.15, and predicts the slip distributions acceptably at 64 s, requiring only approximately 30 ms on a single CPU (Central Processing Unit). With optimal azimuthal coverage of stations, the system is relatively robust to the number of stations and the time length of the received data.

Distribution pattern, geometric characteristics and tectonic significance of landslides triggered by the strike-slip faulting 2022 Ms 6.8 Luding earthquake

On September 5th, 2022, an Ms 6.8 earthquake struck the Luding area of Gaze Tibetan Autonomous Prefecture, Sichuan Province, China. This earthquake with a focal depth of 16 km at 29.49°N, 102.22°E (hereinafter called the Luding earthquake) was generated by a strike-slip faulting and triggered at least 5000 landslides over an area of 5000 km2. Over 80 % of landslides are distributed in areas with a seismic intensity of IX, which mainly consist of massive shallow landslides with a few topples, rockfalls, and debris flows. Statistically, coseismic landslides are prone to occur in areas with low elevations, steep hillslope gradient, and high ground motion, and most of them are distributed in four landslide abundance areas, including the Moxi area (Sub-1), the Wandong area (Sub-2), the Wanggangping area (Sub-3) and Xingfu area (Sub-4). Among them, the landslide scale and abundance index peaks in the Sub-2 and Sub-4 areas featured by the low-elevation valley landform with large relief and steep slopes. The geometric parameters of landslides show that the majority of landslides have the height difference between 15 and 150 m and a movement distance between 50 and 250 m. As the landslide area increases, the length and height of the landslide follow a growth trend which can be described by the logarithmic relationship. Compared to other strike-slip seismic events, the Luding event has the more obvious ability to trigger coseismic landslides. The spatial distribution of coseismic landslides is controlled by the seismogenic fault, and over 85 % of landslides are roughly distributed within a range of 5 km on both sides of the seismogenic fault. Overall, the west side of the seismogenic fault has higher landslide abundance and pronounced near-surface rupture, and it thus can be inferred that the seismogenic fault dips slightly westward based on the source mechanism solution and actual landslide distribution.

Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit, North China: Evidence from geochronology and noble gas isotopes

The Haoyaoerhudong gold deposit, located in the northwestern part of the North China craton (NCC), has produced over 120 metric tonnes (t) of gold since 2007. It has a total reserve of > 240 t at average gold grade of 0.62 g/t, making it one of the largest open pit gold mines in China. The steeply dipping, large-tonnage, low-grade, vein- or veinlet-type gold orebodies are hosted in strongly-deformed Mesoproterozoic carbonaceous schist of the Bayan Obo Group. The laminated/boudinaged veins/veinlets in the sinistral ductile–brittle shear zones are dominated by quartz, biotite, gold-bearing löllingite, pyrrhotite, (arseno)pyrite, with minor native gold, titanite and xenotime. In this paper, we present new in situ U–Pb geochronological data on magmatic zircon from the pre-ore dikes, on metamorphic and hydrothermal xenotime, and on hydrothermal titanite from the hydrothermally altered carbonaceous schist and auriferous quartz–sulfides veins/veinlets, as well as He-Ar isotopic analysis on gold-bearing (arseno)pyrite in the syn-ore stage. The metamorphic xenotime U–Pb age of 426 ± 6.0 Ma (2σ) records a regional metamorphic event, possibly related to the accretion of the Bainaimiao arc onto the NCC. Two pre-ore andesitic dikes yielded similar emplacement ages at ∼ 278 Ma constrained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon data. Hydrothermal xenotime grains from the altered carbonaceous schist and auriferous quartz–sulfides veins yielded U–Pb ages of 256.0 ± 4.1 Ma (2σ) and 254.4 ± 2.1 Ma (2σ), respectively, overlapping with that of the hydrothermal titanite at 255.4 ± 0.8 Ma (2σ) from the laminated quartz–sulfides veinlets. This indicates that the gold mineralization occurred at ca. 255 Ma. The ∼ 255 Ma gold mineralization age is much younger than the previously reported Early–Middle Permian regional magmatic activity (ca. 291 Ma to 268 Ma), and may be associated with the regional sinistral strike-slip event in the late orogenic cycle related to the collision between the Siberian craton and the NCC. The 3He/4He (R/Ra) and 40Ar/36Ar values of the gold-bearing (arseno)pyrite are 0.04 to 0.09 (average = 0.07) and 375.8 to 2023 (average = 1045), which reveal the ore-forming fluids dominantly originated from the crustal rocks, with limited involvement from the mantle. Collectively, our new geochronological data, noble gas isotopic analyses, and geological evidence support a typical orogenic gold deposit model at Haoyaoerhudong.

Regional tectonic dynamics in Central Iran: Unveiling the interplay of fault systems through morphotectonic and seismological analyses in the Shotori mountains

Iran, located at the convergence of the Eurasian and Arabian tectonic plates, is a region of significant geologic and tectonic interest. This setting gives rise to complex tectonic dynamics that are crucial for understanding the seismicity and geological evolution of the area. This study provides novel insights into the tectonic dynamics of Central Iran, with a particular focus on the Shotori Mountains. Employing a combination of satellite imagery, earthquake catalog analysis, and field observations, we have studied the complex interactions among active fault systems in this part of Iran. Our investigation highlights the significant interactivity of these faults, especially at the convergence point of the Nayband strike-slip fault with the Esfandiar and Shotori thrust faults. This area is characterized by a high frequency of reverse mechanism earthquakes, primarily occurring in shallower crustal layers. A notable aspect of our findings is the identification of a distinct pattern of seismic activity linked to the interaction between different fault types. The Nayband shear zone, for instance, is critically responsible for right-lateral, oblique reverse faulting, and strike-slip events. This study also reveals a novel aspect of tectonic behavior in Central Iran: the correlation between strike-slip fault systems and their thrust terminations, influenced by the rotational dynamics of these faults. Our analysis suggests that the activation of strike-slip faults can precipitate the activation of their thrust terminations, a phenomenon particularly evident at their intersection points. This insight is crucial for understanding regional tectonic processes and could significantly impact seismic hazard assessments in Central Iran. Furthermore, the interaction between the Nayband strike-slip fault and its thrust terminations, such as the Shotori and Esfandiar faults, highlights areas of heightened tectonic activity. This study not only advances our understanding of the seismic and tectonic landscape of Central Iran but also has broader implications for regional tectonic studies and seismic risk management.

‘Double Puzzle’ at the Shumagin seismic gap, Alaska Peninsula: intraslab strike-slip faulting loaded by lateral variations in megathrust fault friction

SUMMARY Along the Aleutian megathrust, the Shumagin seismic gap (162° W–158.5° W) had not hosted a large megathrust earthquake during the observational period 1946–2020. Geodetic evidence suggests a prominent trench-parallel transition from strong to weak kinematic coupling in this segment, indicating varying frictional properties of the megathrust. In 2020 July and October, the occurrence of two large, but dissimilar earthquakes in this seismic gap (the Mw 7.8 Simeonof Island thrust event on 2020 July 22 followed by the Mw 7.6 Sand Point intraslab strike-slip event on 2020 October 19) presented a unique opportunity to examine the interaction between stresses on the megathrust and within the downgoing slab. We use geodetic and geophysical evidence to derive a more accurate kinematic coupling model of the megathrust in this area and show that the Mw 7.6 Sand Point earthquake within the downgoing Pacific slab, likely occurred as a result of both trench-perpendicular interseismic shear stress caused by variable kinematic coupling of the megathrust and coseismic stress changes resulting from the Simeonof Island event. Furthermore, we show that the location of the strike-slip event coincides with along-strike change in the megathrust gravitational anomaly and flexural bending of the downgoing plate, suggesting a long-term interaction between megathrust frictional properties and the structure of the plate interface and downgoing slab.

Coseismic Deformation of the 2023 Türkiye Earthquake Doublet from Sentinel-1 InSAR and Implications for Earthquake Hazard

Abstract The 6 February 2023 Türkiye earthquake doublet occurred on the east Anatolian fault system, which marks the tectonic boundary between the Arabia plate and the Anatolian microplate. This earthquake doublet consists of the Mw 7.8 Pazarcik earthquake along the east Anatolian fault and the Mw 7.6 Çardak earthquake along the Savrun–Çardak fault. Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) satellite successfully imaged the surface deformation caused by this earthquake doublet. The pixel offset from cross correlation of two Synthetic Aperture Radar images complements the interferograms in mapping the surface ruptures and the near-field deformation. We inverted for a coseismic slip model in elastic half-space using the InSAR phase and the range offset data. The variance reduction of the inversion reaches ∼90%. The coseismic slip model shows that the 2023 Türkiye earthquake doublet are left-lateral strike-slip events. The peak slip is located near Nurhak in southern Türkiye along the Savrun–Çardak fault. From measuring discontinuities in the pixel offset images we found that the surface rupture length of the Pazarcik earthquake is ∼300 km and the surface rupture length of the Çardak earthquake is ∼100 km. To first order, the faults are dipping vertically. “Slip gaps” are identified by our modeling, and they might be the source regions of future large earthquakes.

Multiple segmentation and seismogenic evolution of the 6th February 2023 (Mw 7.8 and 7.7) consecutive earthquake ruptures and aftershock deformation in the Maras triple junction region of SE-Anatolia, Turkey

On 6th February 2023 (UTC), two consecutive and catastrophic earthquakes with moment magnitudes (Mw) 7.8 and 7.7 struck the Maras Triple Junction (MTJ) region in SE Anatolia along with dozens of aftershocks, causing numerous casualties and significant building damage, and generating the most complex and longest surface ruptures ever observed in Turkey. The main driving mechanisms of this complex double event are still unresolved and remain controversial, even though they are likely linked with conventional fault activations, recurrence intervals and seismic gaps. Here, the aim was to gain insight into the source regimes and rupture processes of both events and their relationship with resolved fault focal solutions for the observed aftershocks, and to present an interpretation that accounts for the most puzzling aspects of the fault rupture models. In line with this, the co-seismic slip distributions of these two events were examined by joint analyses of centroid moment tensor (CMT) and finite-fault source inversions using regional and teleseismic broadband observations. Inversion results indicate that both earthquakes were left-lateral strike-slip events, and the main ruptures extended mainly from close to NNE to SSW and E to W, with maximum slips of ∼6.5–10 m, mostly confined to a shallow depth range of ≤ ∼10–15 km and extending to the surface, indicating bilateral source processes with an average rupture velocity of ∼3.5–5.5 km/s. The estimated total seismic moment range was 4.94–8.22 × 1020 N m, associated with ∼352–152 km long (along strike) and ∼ 25 km wide (along dip) fault planes at focal depth of ∼10 km. Regional CMT results indicate nearly pure normal-slip and left-lateral normal oblique-slip focal mechanisms and shallow centroid depths (≤ ∼15 km) for the early aftershock distribution that are obviously complementary with the co-seismic bilateral rupture propagations. This result highlights that double pull-apart branching of focal mechanisms for aftershock occurrence implies interacting fault ruptures embedded in the MTJ area, where two sub−/supershear-rupturing faults meet, thus explaining multiple segmentation and seismogenic evolutions of two interrelated mainshocks, i.e. “triple junction earthquakes”. The results reveal that the MTJ tends to migrate to the SSW and likely drives the SSW-stepping of the left-lateral strike-slip shear (∼136 km). This accounts for the peak slips, long co-seismic fault ruptures and the associated faulting styles. Hence, the co-seismic faulting apparently distributed across the MTJ may reflect triple junction migration, and thus large extension at the core of the Anatolian-Arabian plates, leading to very high seismic hazard in similar junction regions of the country.

Rupture process of the January 8, 2022, Menyuan M 6.9 earthquake

After the occurrence of destructively strong earthquakes, rapid acquisition of the source rupture process can provide important reference information for post-earthquake disaster relief and aftershock trend determination. An M 6.9 earthquake occurred in Menyuan County, Qinghai Province on January 8, 2022. The epicenter is located in the seismic gap in the middle section of the Haiyuan fault belt. Such a typical strong earthquake was taken as an example to investigate the rupture process of strong earthquakes. Three days after the earthquake, the InSAR (Interferometric Synthetic Aperture Radar) coseismic deformation field was obtained by Sentinel radar, indicating that the surface ruptured obviously. The southern block of the earthquake faces towards the satellite about 95 ​cm along the LOS (line of sight) direction, and the northern block is away from the satellite by ​∼ ​74 ​cm, consistent with the characteristic of left-lateral strike-slip motion. In this study, InSAR coseismic deformation data and far-field waveform data were used to jointly invert the earthquake rupture process, and a four-segment finite fault model was constructed by referring to the surface deformation. The inversion results show that the focal depth of the Menyuan earthquake is about 7 ​km, and the strike of the seismogenic fault is 89.0°, 104.0°, 119.0° and 131.0° from west to east, respectively. It is a high-dip left-lateral strike-slip earthquake event lasting about 14 ​s. The rupture propagation mode is a bilateral extension. The maximum slip along the fault is about 380 ​cm, and the seismic moment magnitude is 6.7. The surface rupture length is about 24 ​km, which is consistent with that measured in the field survey. The detailed seismic source model can provide basic data for the aftershock trend determination and seismic risk analysis of the adjacent active faults.

Predictive Displacement Models Considering the Probability of Pulse-Like Ground Motions for Earthquake-Induced Landslides Hazard Assessment

ABSTRACT Predictive displacement models (PDMs) based on the Newmark sliding block method have been widely applied in earthquake-induced landslide hazard assessment (ELHA). Pulse-like ground motions (PLGMs) can cause significant structural damage. In this study, four PDMs that consider the probability of occurrence of PLGMs (PP) were created for strike-slip and non-strike-slip events. The results demonstrate that the model with PP has greater accuracy in non-strike-slip events than the model without PP, however, the effect of PP may be disregarded in strike-slip events. This result is applicable to the ELHA and can provide a basis for post-earthquake emergency and geological disaster mitigation.

The 2022 Ms 5.8 and 6.0 Maerkang Earthquakes: Two Strike-Slip Events Occurred on V-Shaped Faults

ABSTRACT Within four hours on 10 June 2022, three consecutive earthquakes of Ms≥5.0 with a maximum magnitude up to Ms 6.0 struck Maerkang, Sichuan, where is supposedly less prone to earthquakes. This article uses seismic observations recorded by the Sichuan Seismic Network to relocate the earthquake sequences and refine velocity models in Maerkang using a double-difference seismic tomography method. The results show that the aftershocks align along northwest and north-northwest directions with a V-shaped pattern, and the Ms 5.8 and 6.0 earthquakes are located in the center of the sequences. The seismogenic faults of the Maerkang earthquake are the two intersecting secondary faults on the east side of the Songgang fault that dip to the northeast and southwest, respectively, with dip angles of about 80°. Analysis of the seismicity parameters and Coulomb stress changes revealed that the Ms 5.8 and 6.0 earthquakes occurred on two separate faults, and the Ms 5.8 earthquake triggered the Ms 6.0 earthquake, while the Ms 6.0 earthquake prevented the rupture of the Ms 5.8 earthquake. The apparent high-velocity anomaly in the upper crust and the low-velocity and high-conductivity layer in the middle-lower crust of the source region suggest that drag forces exerted by the ductile middle-lower crust may have caused the brittle upper crust to move under the eastward extrusion of the Tibetan plateau, leading to the stresses accumulate and release on the causative faults. The Maerkang earthquake highlighted the lateral movement of deep materials and energy redistribution in the intrablock deformation of the Bayan Har block.

Variation of b-Value of Normal, Reverse and Strike-Slip Faulting Earthquakes with Focal Depth

A crucial factor in evaluating seismic activity, seismotectonics, and seismic hazard assessment is the distribution of earthquake sizes (b-value). The goal of this research is to explore the relation between depth and the b-constant values of normal, inverse, and strike-slip faulting earthquakes. The earthquake catalog used in this work was extracted from the Global Centroid Moment Tensor Catalog (GCMTC) and covers the period from 2008 to 2017. The focal depth of the selected tremors ranges from zero to 700 km. The magnitude of completeness for normal and reverse faulting events is 5.2 and 5.1 for strike-slip faulting events. The value of the b- constant was estimated using the maximum likelihood technique. The findings of regression analysis showed that there is an insignificant negative poor correlation relation between the b- value of the normal and strike-slip faulting events and depth, while that relation is an insignificant positive poor correlation for reverse faulting. The depth of the turning point for the b-constant value of normal and reverse faulting earthquakes is near the mantle seismic discontinuities 410 and 520 km. The depth of inflection points of b-constant value strike-slip faulting earthquakes is near to lithosphere-asthenosphere boundary (LAB) at depth of 250km and the mantle seismic discontinuity of 520km.

Present-Day Tectonic Stress Evolution in Southern Yunnan Based on Focal Mechanisms.

Tectonic extrusion bypassing the eastern Himalayan syntaxis results in a significant increase in regional stress instability and the associated frequent occurrence of earthquakes in southern Yunnan, China. However, the stress field, and the relationship between the focal mechanism of earthquakes and stress evolution in southern Yunnan, remain enigmatic. In this paper, using a modified grid point test method, we calculated the focal mechanism of ML ≥ 2.5 earthquakes in southern Yunnan (22-25° N, 100-104° E) from January 2009 to June 2023. Utilizing the solutions of historical earthquake focal mechanisms, we obtained the present-day regional tectonic stress field in southern Yunnan via inversion. The results indicate complex and diverse seismic focal mechanisms, and the main types of earthquakes are strike-slip events, followed by normal fault and reverse fault events. The orientations of the maximum and minimum principal stress axes rotate in a clockwise direction from northeast to southwest. The internal stress orientation distribution of the rhombic Sichuan-Yunnan block in the study area is consistent, and the block boundary zone is the site where stress deflection occurs, and the regional tectonic stress field is influenced by the interaction among different blocks. The distribution of R-value in the Lamping-Simao block gradually increases from north to south, indicating that the compressive stress required for material transport becomes relatively small. Combined with the geological and tectonic background of the study area, our results suggest that the speed of block movement gradually decreases from north to south; the distribution of R-value in the South China block is significantly smaller than that of the interior of the Sichuan-Yunnan rhombus, and the proportion of compressive stresses is larger, indicating a stronger extrusion in this region, which may be related to the fact that the Sichuan-Yunnan rhombus is strongly resisted by the South China block in the east.

A top-down control on upper crustal inheritance on the south-eastern Tibetan Plateau

Mantle tectonics, such as asthenospheric upwelling and lithospheric decoupling, usually controls subsequent fluid formation and migration from deep to shallow levels in the lower lithosphere and promotes rock failure and deformation. Based on the concept whereby the fluid migration location defines the structural inheritance, we aim to relate crustal processes to mantle tectonics using fluids revealed by a new magnetotelluric (MT) array across the Ailao Shan-Red River belt. As imaged via a 3D resistivity model, two lower-lithospheric conductive anomalies are determined as consistent with the Dian-Qiong (DQ) suture and Song Da (SD) belt and interpreted to contain interconnected melt. Because potassic magmatism was enabled by partial melting in the lower lithosphere, we could infer that the DQ and SD have reworked as the major fluid migration channels. Fluid migration is considered to drive lithospheric decoupling at a low-viscosity conductive layer, which is inferred feeding by aqueous and melt fluids originating from the two channels and diffusing at depths from 15 to 20 km. Constrained by the geochronology results, this weak layer could have provided convenience to induce the entirely upper crustal translation-rotation within the time interval between potassic magmatism and strike-slip of the Red River fault. This translation-rotation process is inherited from the underlying mantle processes and may further be remotely affected by the upper crustal movement of the Tibetan Plateau, conforming with the crustal rotation observed to the north of our study region. This work provides a compelling example of the tectonic control of the mantle on inherited responses in the crust.Plain Language Summary Crustal deformation is usually controlled by mantle tectonic processes, especially the southeastern Tibetan Plateau. Some models have proposed that lithospheric shearing and lower-crustal flow have controlled the crustal deformation in the Cenozoic. However, crustal rotation models show less possibility of a channel flow and relate the crustal deformation to the remote effect of India-Asia collision. Our study investigates above hypotheses by imaging the electrical resistivity of rocks beneath the major Red River Fault area in southeastern Tibet, using magnetotelluric data with high quality in a dense array. The lithosphere of the study area is interpreted to have been divided into two horizontal systems by a fluid diffusion layer at the bottom of upper crust. Resistivity low in the lower lithosphere is inferred due to partial melting of deep lithosphere and reworking of the paleo sutures in the Late Cenozoic, which have spread at the bottom of upper crust. Rather than the channel flow, this fluid migration process sensitively reflected in our model relates the mantle tectonics to crustal rotation by providing rheological conditions. We, hence, propose an inherited structure model featured by lithospheric decoupling and upper-crustal translation-rotation that may occur before the major strike-slip event.

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.