Earthquake Focal Mechanism Solutions Research Articles

Can the regional 3D stress field according to the Wallace–Bott hypothesis predict fault slip directions of future large earthquakes?

When evaluating strong ground motions and tsunamis from specified source faults, it is required that the input parameters, such as fault geometry, rake angle, and slip amount, do not deviate from those of a real earthquake. Recently, a regional three-dimensional (3D) tectonic stress field was used to estimate rake angles for mapped submarine faults with the Wallace–Bott hypothesis (WBH), the direction of fault slip was parallel to the resolved stress vector on a preexisting fault, and strong ground motions and tsunamis were simulated. However, this modeling technique has not been adequately validated. Additionally, it is necessary to examine how the stress field estimated from seismological data for a limited period (~ 10 years) can be used as a proxy for the long-term tectonic stress field. In this study, to provide such validation, we utilized two catalogs of focal mechanism solutions for earthquakes and compared the observed rake angles with those calculated from the regional 3D tectonic stress field with the WBH by fixing the fault strike and dip angles according to those from the focal mechanism data. The resulting misfit angles between the observed and calculated rake angles are generally small (ranging between − 30° and 30°), excluding several regions (e.g., the source and surrounding regions of the 2011 off the Pacific coast of Tohoku earthquake and swarm-like activities activated after the 2011 quake). We also confirmed that the calculated rake angles and classified fault types are consistent with geomorphologically and geologically evaluated types of faulting for major Quaternary active faults in the Kyushu district of southwest Japan. These results support the validity and effectiveness of estimating rake angles for a specific fault with known geometry from the above method and data, while also showing that close attention is needed to apply this method to, for example, seismically inactive regions where the inverted stress field includes significant uncertainties and/or near sites of recent and large earthquakes where the stress field has been perturbed.Graphical

Stress field and tectonic regime of the Eastern Mediterranean from inversion of earthquake focal mechanisms

The formal stress inversions of 1258 earthquake focal mechanism solutions reveal the presence of six distinct stress regimes. However, the predominant tectonic regimes comprise normal faulting (39%), strike-slip faulting (29%), thrust faulting (25%), and unspecified oblique faulting (7%). The division of the study area into zones based on structural and deformation patterns has provided valuable insights into the variations in the stress field and tectonic processes within this seismically active region. For each designated zone, reduced stress tensors were derived using the improved right dihedron method and the rotation optimization technique implemented by the Win-Tensor software.The stress field derived from the entire dataset is characterized by R′ = 1.0 and an NW-SE SHmax orientation, strongly influenced by the tectonic stress regime of the Hellenic Arc. In specific regions such as CYS, CEM, and WEM, the prevailing tectonic regimes is characterized by a predominant compressive setting with a stress index (R' = 2.0 to 2.5). This pattern aligns with neotectonic deformation associated with the subduction of the African Plate beneath the Eurasian Plate. The EMZ Zone exhibits a strike-slip tectonic regime with an R' of 1.50, primarily influenced by the Dead Sea Transform Fault. This fault accommodates differential motion between the African and Arabian plates in the NNE direction, resulting in left-lateral strike-slip motion along the plate boundary. The mean slip tendency values, varying between 0.62 and 0.66, suggests that the faults in the study area are appropriately oriented for slip or shear displacement, indicating an elevated probability of reactivation.

Contribution of mine borehole data toward high-resolution stress mapping: An example from northern Bowen Basin, Australia

Most of in-situ stress data in the Australian continent comes from wellbore stress analysis in deep hydrocarbon reservoirs, and earthquake focal mechanism solutions near the Australian plate boundaries, where geophysical tools facilitate understanding of the present-day stress patterns. This resulted in a paucity of stress information in many other regions such as the northern Bowen Basin, which is an active mining province, but with low seismicity rates and limited deep petroleum exploration. The mining industry runs several hundred kilometres of image logs annually to characterise geotechnical attributes. These logs provide an image from the borehole wall, which facilitates analysis of stress-related borehole deformations for in-situ stress characterisation. This paper examines the orientation of horizontal in-situ stress using different types of image logs in mine boreholes across the northern Bowen Basin. Analyses of 128 km of image logs in 680 vertical boreholes resulted in the interpretation of 9046 pairs of stress-related indicators including 735 drilling induced fractures and 8311 borehole breakouts. Our comprehensive database comprises 890 quality-ranked data records for the orientation of maximum horizontal stress (SHmax), which makes the Bowen Basin as a basin with the highest data density in the world in terms of quality-ranked stress information according to the World Stress Map. Statistical analysis of SHmax orientation reveals that the mean SHmax orientation in northern Bowen Basin is N018° ± 16°. The results show that this orientation is consistent over long distances, which is in contrast with several eastern Australian basins. This uniform stress pattern agrees well with plate-scale geomechanical model predictions, which further highlights the impact of plate boundary forces in the contemporary stress pattern of this region. Detailed image log investigation did not show any systematic rotation of stress; however, some small-scale stress perturbations were observed in the vicinity of sharp stiffness contrasts and geological structures.

Variations and causes of in-situ stress orientations in the Dibei-Tuziluoke Gas Field in the Kuqa Foreland Basin, western China

The Kuqa Foreland Basin, adjacent to the southern Tianshan Mountains, is an important gas-producing basin in northwestern China. The development of the basin is dominated by a regional stress field with orientations of the maximum horizontal stress (SHmax) in the north-south direction, as a result of the collision between the Indian and Eurasian Plates. Previous studies have noted variations in the stress state in the Kuqa Foreland Basin; however, little attention has been paid to understanding the mechanisms causing such variations. In this study, a systematic analysis of in-situ SHmax orientations and fractures has been carried out using image logs from eight wells and XMAC/DSI logs from five wells. These wells are located in an East-West (E-W) trending fold-thrust in the foreland basin. The SHmax orientations (having quality ranked between A and D) obtained from borehole breakouts and drilling-induced fractures are highly variable and occur as two evidently different stress patterns on a scale of less than 10 km. The first stress pattern with SHmax orientations between NNW and NNE (165.08 and 188.59 °N) is broadly comparable to the regional stress field inferred from earthquake focal mechanism solutions in northwestern China from the World Stress Map (WSM). The stress pattern is interpreted to be controlled by plate boundary forces related to the collision between the Indian and Eurasian Plates. The SHmax orientations for the second stress pattern, however, ranging from NNE to ENE (26.65–57.77 °N), are clearly inconsistent with the regional stress filed and have been influenced by local geological factors. Faults and hinge-parallel fractures (trending in an approximately W-E direction) formed during the syn-folding stage probably resulted in heterogeneity of elastic properties and thus deflected SHmax orientations, both laterally and vertically, to generate the second stress pattern. The NE-striking hinge-oblique fractures are interpreted to have formed in response to the stress reorientation. Observations in this study suggest that there may exist a close relationship between fracturing (faulting) and stress variations, and pre-existing faults and fractures can influence subsequent fracturing by regulating local stress fields on a scale of several kilometers. The coupling between fracturing or faulting and stress produced fracture swarms, which can significantly enhance fluid flow and hence petroleum production.

Moment tensor inversion implementation in determining focal mechanism solution of Palu-Koro and Matano fault events: processing strategy

Earthquake focal mechanisms are helpful in analyzing seismotectonic features in a specific area, as they can depict subsurface structures. Earthquake focal mechanism solution can result from the first polarity or moment tensor inversion method. The moment tensor inversion can produce complete information of focal mechanism solution than the first polarity method. However, the procedure is slightly more complicated, for it is based on waveform inversion. Several problems related to signal processing might arise. Strategies should be applied to overcome the difficulties and obtain reliable focal mechanism solutions. In this study, we used moment tensor inversion to produce focal mechanisms data of Palu-Koro and Matano Fault events. We processed the focal mechanism of events with a depth of less than 60 km and with a minimum magnitude of Mw 4.7. The moment tensor inversion is conducted using Isolated Asperities (ISOLA) software. Full waveform inversion of stations located near Palu-Koro and Matano Fault is applied. The cause of waveform fitting difficulties using the data in this study area and the solution are discussed. For instance, the existence of unseen signal noise and clear-yet-disturbing signals are observed. Improved solutions can be achieved by station reselection and bandpass frequency adjustment.

The Origin of Seismic and Tectonic Activity Underlying the Sichuan Basin, Central China

AbstractThe Sichuan basin, located adjacent to the eastern margin of the Tibetan Plateau, serves as an ideal marker for testing the extrusion process of the plateau. The basin is seismically active, with the strongest earthquake, the 2008 Mw 7.9 Wenchuan earthquake, occurring in the Longmen Shan range along its northwestern edge. A new regional compilation of focal mechanism solutions of earthquakes in and surrounding the basin reveals that a large fraction of the events have focal depths ranging between 8 and 25 km, corresponding to the crystalline basement of the basin. Seismic deformation involves right‐lateral oblique reverse faults, mostly trending northeast–southwest, similar to the kinematics of the mainshock of the Wenchuan earthquake. Shallow earthquakes (3–8 km) suggest that some of the seismic faults rupturing the crystalline basement are growing toward the surface. To the southwest, the seismicity transitions to activity along the left‐lateral Xianshuihe–Xiaojiang fault zone. The spatial relationship between these two sets of fault zones is consistent with a model in which the Sichuan basin responds to the southeast extrusion of the Chuan‐Dian block at the southeast margin of the plateau by a counterclockwise bookshelf rotation of the crystalline basement. This deformation pattern initiated ∼4–2 Ma, as shown by the age of the Xiaojiang fault segment. The history of left‐lateral movement along the Xianshuihe‐Xiaojiang fault can be traced back to ∼12 Ma when the eastward growth of the Tibetan Plateau resulted in the shortening of both the Longmen Shan thrust belt and the sedimentary over the Sichuan basin.

Updated stress dataset of the Circum-Pannonian region: Implications for regional tectonics and geo-energy applications

In this paper we present the updated stress database of the Circum-Pannonian region in Central-Eastern Europe, containing stress indicators from earthquake focal mechanism solutions, borehole breakout analysis, overcoring measurements and geologic fault-slip data. Recent improvements in seismic network density in the framework of the AlpArray project, as well as methodological advancements, facilitated the calculation of focal mechanism solutions of small magnitude events, contributing to the more detailed reconstruction of the stress field in the area. The new database consists of 368 A – C quality stress indicators that we interpolate to a regular grid to produce stress orientation and relative stress magnitude maps. Interpolated maximum horizontal stress (SH) orientations are in general marked by low uncertainties (mostly between 10° – 30°), indicating that the interpolation results can be considered reliable for the majority of the study area. Inferred SH directions show a good fit with previous studies, confirming earlier findings that the Pannonian basin is under compression and transpression, controlled by the combination of its landlocked position with the N-NE directed motion of the Adriatic microplate. The new tectonic regime map highlights several new features, such as the complete absence of extension in the entire Pannonian basin. The determined stress regimes and trajectories calculated from interpolated SH directions generally agree well with the orientation and kinematics of neotectonic structures in the Pannonian region. Additionally, measured and interpolated SH directions and faulting styles can provide boundary conditions for geomechanical studies, serving as valuable input for geo-energy applications.

FocMech-Flow: Automatic Determination of P-Wave First-Motion Polarity and Focal Mechanism Inversion and Application to the 2021 Yangbi Earthquake Sequence

P-wave first-motion polarity is important for the inversion of earthquake focal mechanism solutions. The focal mechanism solution can further contribute to our understanding of the source rupture process, the fault structure, and the regional stress field characteristics. By using the abundant focal mechanism solutions of small and moderate earthquakes, we can deepen our understanding of fault geometry and the seismogenic environment. In this paper, we propose an automatic workflow, FocMech-Flow (Focal Mechanism-Flow), for identifying P-wave first-motion polarity and focal mechanism inversion with deep learning and applied it to the 2021 Yangbi earthquake sequence. We use a deep learning model named DiTingMotion to detect the P-wave first-motion polarity of 2389 waveforms, resulting in 98.49% accuracy of polarity discrimination compared with human experts. The focal mechanisms of 112 earthquakes are obtained by using the CHNYTX program, which is 3.7 times more than that of the waveform inversion method, and the results are highly consistent. The analysis shows that the focal mechanisms of the foreshock sequence of the Yangbi earthquake are highly consistent and are all of the strike-slip type; the focal mechanisms of the aftershock sequence are complex, mainly the strike-slip type, but there are also reverse and normal fault types. This study shows that the deep learning method has high reliability in determining the P-wave first-motion polarity, and FocMech-Flow can obtain a large number of focal mechanism solutions from small and moderate earthquakes, having promising application in fine-scale stress inversion.

Depth‐Dependent Crustal Stress Rotation and Strength Variation in the Charlevoix Seismic Zone (CSZ), Québec, Canada

AbstractIntraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new data set of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River‐parallel (∼54°) near the surface to river‐perpendicular (∼120°) in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ∼13 and ∼26 km depth, where glacial rebound induced stress perturbation is further amplified by a “weaker” middle crust. Finally, depth‐dependent b‐values confirm the rheological difference between upper and middle crust in the CSZ.

The 2022 Twin Earthquakes and Tectonic Environment in Kyaing Tong Basin, Eastern Myanmar

The first earthquake of magnitude 4.8 struck at 57 km south of Kyaing Tong, latitude 21.1141°N longitude 99.850°E, on 21st July 2022, at 16:40:08 UTC ( 11:07 a.m. local time ) at the depth of 10 km. Immediately after 30 minutes on the same day of 21st July 2022, the second earthquake with magnitude 5.9 occurred, 53 km south of Kyaing Tong at latitude 21.150°N 99.863°E, 17:07:26 UTC (11:37 a.m. local time) at 10 km depth. Focal mechanism solution for the first event was left-lateral strike-slip faulting and the second by normal faulting with a component of strike-slip component (United States Geological Survey). Many lesser aftershocks followed the main shocks and continued since July and still ongoing up to date in 2022. Twin earthquakes ruptured along the Wan Ha fault which bounds the southeast side of the Kyaing Tong basin, eastern Myanmar. The rupture propagated southward. The Kyaing Tong basin is one of the tectonic basins in the Paleo-Tethys suture zone in eastern Myanmar. These earthquakes are due to the movement of Wan Ha fault, affected the town Kyaing Tong where it caused some houses and schools collapsed. No casualty was reported. According to data from USGS, and other earthquake agencies, there is a clear increase in detected earthquakes, particularly smaller quakes of magnitude around ≥ 1.0 and ≥ 2.0 at shallower depth of 1000 meter. From the studies of satellite images, aerial photographs, shaded relief map and Digital Elevation Model map, several lineaments are visible and detailed mapping of these lineaments show the strike-slip faults in ENE-WSW direction and normal faults in NE-to NNE-striking direction. NW-SE extension is evident by the occurrence of brittle normal faults and NE-to ENE-oriented sinistral strike-slip fault. Within this zone is the trace of regional compression in a NE-SW direction. This region is presently undergoing active tectonic deformation, as indicated by current seismicity and earthquake focal mechanism solution.

Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults

The Xiluodu (XLD) reservoir is the second largest reservoir in China and the largest in the Jinsha River basin. The occurrence of two M > 5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators. We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment. Following impoundment, earthquake activity increased significantly. The occurrence of M ≥ 3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level, though this correlation weakened after five years because earthquake activity was far from the reservoir area. Earthquakes in the XLD reservoir area are clustered; near the dam (Area A), small faults are intermittently distributed along the river, while Area B is composed of multiple NW-trending left-lateral strike-slip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EW-trending right-lateral strike-slip minor fault. The geometries of the deep and the shallow parts of the NW-trending fault differ. Under the action of the NW-trending background stress field, a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment. The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment.

Fault Orientation and Relocated Seismicity Associated with the 12 December 2018 Mw 4.4 Decatur, Tennessee, Earthquake Sequence

AbstractSmall-to-moderate size earthquakes have occurred along the Eastern Tennessee Seismic Zone (ETSZ) for decades, resulting in the second highest seismicity rate in Central and Eastern United States following the New Madrid Seismic Zone. However, the underlying tectonic setting of seismicity in the ETSZ remains unclear. The recent December 2018 Mw 4.4 earthquake near Decatur, Tennessee, provides an opportunity to study one of the largest recent events in this region, after the 2003 Mw 4.6 Fort Payne, Alabama, earthquake. Here, we use a matched filter technique to detect additional microearthquakes around the Mw 4.4 mainshock. We create templates from 967 cataloged earthquakes spanning over 15 yr (January 2005–July 2020) in the ETSZ. These templates are used to detect missing events four weeks before to four weeks after the mainshock. We calculate the magnitudes of new events using principle component fitting between templates and newly detected events. Two relocation algorithms, XCORLOC and hypoDD, are used comparatively to relocate detected events. We construct focal mechanism solutions for earthquakes around the mainshock using HASHpy, and refine the focal mechanism and depth solution of the mainshock using the cut-and-paste method. The mainshock appears to have occurred at a depth of approximately 5–6 km, which is deeper than the decollement (4–5 km) that separates the Grenville-age basement rock and the Paleozoic sedimentary rock. In addition, the mainshock appears to rupture bilaterally along an east–northeast-trending strike-slip fault, which is consistent with the relocated aftershock locations and one of the nodal planes of the mainshock. The occurrence of the 2018 Mw 4.4 mainshock at a relatively shallow depth and its close proximity to the Watts Bar Nuclear Power Plant highlight the need to reevaluate seismic hazard associated with moderate-size earthquakes along the ETSZ.

Seasonal terrestrial water load modulation of seismicity at the southeastern margin of the Tibetan Plateau constrained by GNSS and GRACE data

SUMMARY The seasonal terrestrial water load modulation of seismicity is an important phenomenon to understand the mechanism behind earthquake triggering and nucleation. The presence of high-level seismicity and large seasonal water load amplitudes at the southeastern margin of the Tibetan Plateau provides a natural experimental environment for studying the modulation mechanism. The spatiotemporal distribution of the water load was inverted using Global Navigation Satellite System (GNSS) and Gravity Recovery and Climate Experiment (GRACE) data, and an earthquake catalogue (M ≥ 2.5) was declustered to obtain the background seismicity using the Epidemic Type Aftershock Sequences (ETAS) model. The Multichannel Singular-Spectrum Analysis (M-SSA) is adopted to the time-series of monthly averaged terrestrial water load and background seismicity rates, and the results show 1- and 2-yr periodicities in the seismicity rates and water load. The 1-yr periodicity in the seismicity rate is correlated with the rate of change of the monthly averaged water load. To evaluate the seasonal principal stress perturbations on the tectonic background stress orientations, the stress changes caused by the seasonal water load changes are projected onto the tectonic background stress field orientations constraining by 8 yr of earthquake focal mechanism solutions. The results show that the largest change of the seasonal principal stress perturbations is about 16 kPa. The number of excess earthquakes is evaluated with the background seismicity rates for discrete stress intervals. The results indicate a ∼10 per cent increase in the seismicity rates that correlate with the rates of the minimum and mean principal stress perturbations. The results above can be explained by the model of harmonic stress perturbations on rate-and-state fault. Based on this model, the nucleation period of the seasonal seismicity should be less than 1 yr at the southeastern margin of the Tibetan Plateau.

Late Cenozoic Stress State in Gulf of Güllük and Surroundings, SW Anatolia

This study was performed with the aim of revealing the Late Cenozoic stress state of the Gulf of Güllük and close surroundings. In this study, the tectonic regime and stress states from the Pliocene to the present day were determined. Mesoscopic fault plane data were collected from outcrops at 19 different stations in the region between the Gulf of Güllük and Milas. Additionally, focal mechanism inverse solutions were calculated for 12 earthquakes larger than M:3.0 occurring from 2004 to 2015 to reveal the current tectonic regime. According to kinematic analysis results from fault assemblages and focal mechanism solutions for earthquakes, two tectonic regimes affected the region before the Pliocene and at present. In the first tectonic regime, strike-slip faulting developed under a NW-SE oriented compressional regime. There was an R ratio of 0.426 between the principal stress axes and this shows faulting had transtensional character. The region converted to a NE-SW oriented extensional regime dominated by normal faulting in the Quaternary. This currently effective extensional regime was understood from focal mechanism solutions of earthquakes developing in the region. The reason for this regime being effective is the rapid pull by the African plate on the Anatolian plate, which rides above the African plate, and horizontal extension in the Anatolian plate.

FOCAL MECHANISMS OF EARTHQUAKES OF SOUTHERN BAIKAL REGION AND NORTHERN MONGOLIA

The paper presents previously unpublished earthquake focal mechanism solutions for earthquakes which occurred in South Pribaikalye, Transbaikalia, Tuva and Northern Mongolia derived from the first-motion polarities of P-waves recorded by the Baikal, Buryat, Altai-Sayan networks of the Geophysical Survey RAS and Mongolian national network.

Late Quaternary subsidence records from the Datça graben and Cnidus ancient city (SW Turkey): sea-level changes versus tectonics

The Datça graben in southwestern Anatolia is a WNW-trending seismically active depression, with tectonic activity since Pliocene time. This tectonic activity is controlled by normal faults, which have effected ancient settlements. The Cnidus city (old and modern) –an ancient mercantile centre during the Hellenistic, Roman and Byzantine periods– is one of the places that has recorded this activity. The ancient harbour walls of Cnidus, lying 2.2-4.0m below sea level, contain important traces about sea-level changes and tectonics over the past 2.6kyr. Palaeostress analysis along boundary faults in the Datça graben yields an almost N–S oriented pure tensional regime, compatible with earthquake focal mechanism solutions located around the Datça Peninsula. Additionally, an almost E−W trending surface rupture related to a historical earthquake in modern Cnidus, which shows normal fault characteristics, gives further support to the ongoing extension along the Kızlan, Karaköy and Cnidus fault zones. Previous studies on late Quaternary sea-level changes around the Datça Peninsula suggest that 2.6kyr ago sea level was 1.0-1.25m lower than today. From the present-day depth of the Old Cnidus harbour remains and regional sea-level records, it can be inferred that tectonics has played a significant role. Our calculations show that the Datça graben is subsiding at rates of 0.36-0.46mm/yr in the central part and 1.05-1.15mm/yr in the southern part. These values match those found in other areas around the Datça Peninsula.

Recent Tectonic Stress Field in Northwest Yunnan Province

Based on the collected focal mechanism solutions of M≥3.0 earthquakes in Northwest Yunnan from 2000 to 2018, we used Michael’s linear inversion method to perform stress tensor inversion, and attempted to analyze the temporal and spatial evolution characteristics of the tectonic stress field in Northwest Yunnan. The results show that the types of focal mechanism solutions of earthquakes in Northwest Yunnan are complex, mainly strike-slip faulting earthquakes (46%), followed by normal faulting earthquakes (27%). The direction of maximum principal stress in the study area is NNW near NS direction, and the direction of minimum principal stress is ENE direction. It shows that the study area is subjected to horizontal compression from NNW near NS direction. According to the variation of the stress tensor variance over time, M≥5.0 earthquakes in the study area mainly occurred when the stress tensor variance was lower than 0.2. Most earthquakes occur when the variance of the stress tensor decreases, that is, when the focal mechanism solutions tend to be consistent. And these earthquakes mostly occur in the low value area of the variance and its edge.

In situ stress measurement and analysis of the stress accumulation levels in coal mines in the northern Ordos Basin, China

For non-directional drilling cores, selection of samples and the test methods for in situ stress measurements to evaluate the Kaiser effect (KE) were proposed, and the magnitude and direction of the principal stresses were derived from first principles. Based on this approach, the KE for 423 samples in the Burtai and Baode coal mines in the northern Ordos Basin (NOB), China, have been investigated. The results show that the maximum horizontal principal stress (σH), the minimum horizontal principal stress (σh) and the vertical stress (σv) varied with depth and location, and the values increase with increasing depth. Generally, the horizontal stresses play a leading role. For the main stress regimes in the NOB, σH > σh > σv (Burtai Mine, < 172 m; Baode Mine, < 170 m) and σH > σv > σh (Burtai Mine, 170–800 m; Baode Mine, 170–400 m), and the σv > σH > σh stress regimes are mainly distributed in moderately deep to deep coal mines. For rock masses with a depth of 350 m, k ((σH + σh)/2σv) tends to 1, indicating that a deep critical state will gradually emerge. The test results are compared with those for the overcoring (OC) method, the anelastic strain recovery (ASR) method and micro-hydraulic fracturing (HF). The relative errors for σH, σh and σv were 14.90%, 19.67%, 15.47% (Burtai Mine) and 10.74%, 22.76%, 19.97% (Baode Mine), respectively, and the errors are all within an acceptable range, thus verifying the reliability of the KE method. The dominant orientation for the σH (Burtai mine, NE-NNE; Baode Mine, NEE) is obtained via paleomagnetic technology, and the data are consistent with those (NE-NEE) of the earthquake focal mechanism solutions for the area. Based on the Byerlee–Anderson theory, a discussion is given on the levels of stress accumulation in the rock mass of the mines. For dry rocks or hydrostatic pressure rocks, the friction coefficients of the faults are low for both locations, and the values are less than the lower limit (0.6) of the strike-slip faults slip, indicating that the stress fractures at a low level around the study areas are lower than the friction limit stress. The stress accumulation levels in the Baode Mine are slightly larger than those in the Burtai Mine.

Seismotectonics of Vlora-Elbasani-Dibra Transversal Fault Zone (Albania): A Review

The Vlora-Elbasani-Dibra transversal fault zone crosses from south-west to northeast the Albanides collision zone starting from Vlora area to the Dumre diapir and continuing from Elbasani Quaternary graben to Dibra area, within the internal Albanides extensional zone. The Vlora-Elbasani-Dibra seismogenic fault zone is expressed by an alignment of important historical and instrumental seismicity with 15 Ms > 6 earthquakes. The Vlora-Berati-Elbasani segment of this fault zone during the year 1851 has been activated along all is length. The geological and neotectonic investigations and the focal mechanism solutions of earthquakes that occurred along the Vlora-Elbasani-Dibra fault zone prove its division into two distinct fault zones: a) the Mali i Kanalit Block - Dumre diapir strike-slip fault zone NNE trending in transpression, and b) the Elbasan-Dibra normal fault zone NE trending in transtension. The Mali i Kanalit Block-Dumre diapir strike-slip zone NNE trending in transpression offsets dextrally and to SW the folds and thrusts of the Çika, Kurveleshi and Berati anticlinal belts of Ionian zone. The Neogene (mainly since Middle Miocene) offset of the Çika, Kurveleshi and Berati anticlinal belts from generally NW-SE trending to near N-S one has led to thrust’s steepening and turning into high-angle strike-slip faults due to NE-SW oblique compression from the Albanides collision zone. The Othoni Island-Dhermi strike-slip fault offsets the dextrally dragged Devolli (to the east of Dumre diapir), Osumi (near to Berati town), and Vjosa (west of Shkoza village) rivers’ valleys formed from the Middle Pleistocene (180,000 years) to historic times. The Othoni Island-Dhermi dextral strike-slip fault, that reaches the maximum velocity of greater than 5 mm/year to the south of it, is the only leading fault that shaped the Mali i Kanalit Block-Dumre diapir dextral strike-slip fault zone. Focal mechanism solutions of earthquakes that occurred on the Mali i Kanalit Block - Dumre diapir dextral strike-slip zone show their generation from strike-slip faults in transpressional motions. Focal mechanism solutions of earthquakes that occurred on the Elbasan-Dibra were generated from normal faults with dextral strike-slip movements.

Driving stress and seismotectonic implications of the 2013 Mw5.8 Awaji Island earthquake, southwestern Japan, based on earthquake focal mechanisms before and after the mainshock

A driving stress of the Mw5.8 reverse-faulting Awaji Island earthquake (2013), southwest Japan, was investigated using focal mechanism solutions of earthquakes before and after the mainshock. The seismic records from regional high-sensitivity seismic stations were used. Further, the stress tensor inversion method was applied to infer the stress fields in the source region. The results of the stress tensor inversion and the slip tendency analysis revealed that the stress field within the source region deviates from the surrounding area, in which the stress field locally contains a reverse-faulting component with ENE–WSW compression. This local fluctuation in the stress field is key to producing reverse-faulting earthquakes. The existing knowledge on regional-scale stress (tens to hundreds of km) cannot predict the occurrence of the Awaji Island earthquake, emphasizing the importance of estimating local-scale (< tens of km) stress information. It is possible that the local-scale stress heterogeneity has been formed by local tectonic movement, i.e., the formation of flexures in combination with recurring deep aseismic slips. The coseismic Coulomb stress change, induced by the disastrous 1995 Mw6.9 Kobe earthquake, increased along the fault plane of the Awaji Island earthquake; however, the postseismic stress change was negative. We concluded that the gradual stress build-up, due to the interseismic plate locking along the Nankai trough, overcame the postseismic stress reduction in a few years, pushing the Awaji Island earthquake fault over its failure threshold in 2013. The observation that the earthquake occurred in response to the interseismic plate locking has an important implication in terms of seismotectonics in southwest Japan, facilitating further research on the causal relationship between the inland earthquake activity and the Nankai trough earthquake. Furthermore, this study highlighted that the dataset before the mainshock may not have sufficient information to reflect the stress field in the source region due to the lack of earthquakes in that region. This is because the earthquake fault is generally locked prior to the mainshock. Further research is needed for estimating the stress field in the vicinity of an earthquake fault via seismicity before the mainshock alone.