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This study explores the feasibility of using fluctuations in the recurrence magnitude dispersion factor (b-value) as a seismic precursor for the Mizoram earthquake that occurred on November 26, 2021, in the Indo-Burma region of northeast India. Employing a comprehensive and homogeneous earthquake catalog spanning from 1900 to 2020, the seismic analysis involved delustering and completeness testing. The research implements a sub-sectional b-value calculation method, dividing the study area into uniformly sized grid cells (2° × 2°) and performing temporal b-value mapping for each grid. The epicenter of the Mizoram earthquake was located within a grid cell characterized by an intermediate b-value. Time-series analysis of the b-value indicated a notable decline preceding the main event, suggesting its potential as a seismic precursor. The study also examines depth-dependent variations in the b-value, revealing an inverse relationship between the b-value and crustal stress. To evaluate the significance of b-value anomalies, the Kolmogorov–Smirnov (K-S) statistic was employed instead of visual inspection. Additionally, the research provides probabilistic estimates of seismic hazard parameters, including the most probable maximum yearly earthquake, mean return period, and probabilities of earthquakes of varying magnitudes. These findings contribute to a deeper understanding of the complex seismotectonic framework and high lithospheric variability in the investigated region. [ABSTRACT FROM AUTHOR]

There are two seismic gaps (Dayi seismic gap and Tianquan-Kangding seismic gap) on the Longmen Shan fault (LMSF), despite the successive occurrence of the 2008 Mw7.9 Wenchuan and 2013 Mw6.6 Lushan earthquakes. To analyze the effects of the Wenchuan and Lushan earthquakes on the LMSF (especially on seismic gaps along the LMSF) and regional seismic hazards, we calculate Coulomb stress changes caused by the Wenchuan and Lushan earthquakes based on a three-dimensional viscoelastic finite element model. Additionally, we calculate the spatial distribution of regional b-values based on the instrumental seismic catalog before the Wenchuan earthquake. By utilizing the inverse correlation between b-value and stress level, we infer the regional background stress level. The results show that regional earthquakes (including the 2008 Mw7.9 Wenchuan earthquake, 2013 Mw6.6 Lushan earthquake, 2014 Mw6.1 Kangding earthquake, 2017 Mw6.5 Jiuzhaigou earthquake, and 2022 Mw6.6 Luding earthquake) occurred in regions characterized by low b-values. Meanwhile, subsequent earthquakes occurred in regions where Coulomb stress changes caused by the Wenchuan and Lushan earthquakes were positive. This suggests that regions with both low b-values and positive Coulomb stress changes may pose higher seismic hazards. We found that there are four regions (southern Xianshuihe fault, Dongkunlun fault, northern Xiaojinhe fault, and Hanan-Qingshanwan fault) with both positive Coulomb stress changes caused by the Wenchuan and Lushan earthquakes and low b-values, which may indicate high stress accumulation and high seismic hazard in the future. The results also show that Coulomb stress changes caused by the Wenchuan and Lushan earthquakes increased significantly in the Dayi seismic gap (+ 0.216 ~ + 2.607 MPa) and Tianquan-Kangding seismic gap (+ 0.021 ~ + 0.211 MPa), while the result of the high b-values for the Dayi and Tianquan-Kangding seismic gaps indicate less background stress accumulation. However, with continued tectonic loading, seismic hazards on both seismic gaps should attract our attention. [ABSTRACT FROM AUTHOR]

Moment tensor inversion was performed for earthquake of moderate magnitude occurring in May 2022 near Khuzdar, Pakistan. According to local news agencies, the event caused damages to engineered structures and collapse of several mud houses. Faulting style depicted from inversion implies that the event may be associated with Ornach-Nal strike-slip fault which is part of western boundary of Indian plate. To understand the tectonic setting of the area, the present study was augmented by inclusion of results from similar studies. Principal stress axis (SHmax) was determined by performing stress inversion. Based on 38 events focal mechanism solutions from the global Harvard Centroid Moment Tensor (CMT) of the Khuzdar region depict NW–SE orientation of SHmax direction. On the basis of their stress homogeneity, reduced stress tensors obtained from formal stress inversion have been divided into two subsets resulting in thrust and strike-slip faulting. The present-day stress state conforms to the oblique convergence of Indian and Arabian plates beneath Eurasian. Shear strain produced by strike-slip movement of plate boundary (Chaman fault system) is being accommodated in Kirthar range within the Indian plate. Before this event, a lower b-value (0.7) and accelerated earthquake sequence were observed in the Khuzdar region, which is (in our view evidence of presence of stress loaded asperities along this fault system) representation of stress loaded asperities exist. The spatial distribution of b-value depicts the lowest value in this region before the occurrence of Awaran earthquake of 2013 that took place about 70 km west of the recent event. [ABSTRACT FROM AUTHOR]

The Yedisu Seismic Gap is one of the most important seismic gaps throughout the North Anatolian Fault Zone since it has not produced destructive earthquakes for a long time. To analyze the characteristics of future seismic hazards, the interrelationships between seismotectonic b-values, Coulomb stress changes, and S-wave velocity models of crust are presented in and around the Yedisu Seismic Gap located northwest of the Karlıova Triple Junction. For this purpose, the most up-to-date earthquake catalog and the focal mechanism solutions of recent earthquakes are used to image the different depth intervals. Results show that the relatively positive stresses are accumulating along the Varto Fault Zone and Kargapazarı and Yedisu Segments between 5 and 15 km depth intervals. At the same time, the lower b-values between 0.6 and 1.0 are found in the same segments. However, in the volcanic regions around the Karlıova Triple Junction, the low S-wave velocity zones may be related to high b-values, negative stress changes, and volcanic structures. The region between the Turnadağ volcano and the Varto caldera shows scattered stress and b-value changes in the upper crust. Moreover, the probability of earthquakes for Mw = 6.0, 7.0, and 7.7 in the intermediate term (10 years) is estimated as ~ 65%, ~ 17%, and ~ 5%, respectively. Recurrence of earthquakes with Mw = 6.0, 7.0, and 7.7 are calculated as ~ 10, ~ 55, and ~ 187 years, respectively. Consequently, the regions characterized by low strong b-values and positive stress loading reveal high earthquake hazard potential on the whole in the next decade. [ABSTRACT FROM AUTHOR]

The Gulf of Aqaba is considered one of the most seismotectonically active regions in the Middle East. This study analyzes the seismic activity from 1983 to 2018 to investigate the spatio-temporal distribution of the events in the Gulf's three basins. The seismic activity is characterized by conventional mainshock–aftershock sequences and swarm activity. Most activity is located in the Aragonese basin, while the Dakar basin shows less seismic activity. The b-value was calculated for the three basins from the declustered catalogue, representing the fingerprint of the transform fault. Within the Aragonese, three significant earthquakes occurred in 1993, 1995, and 2015 with moment magnitude (Mw) of 5.8, 7.2, and 5.2, respectively. An intermediate seismic quiescence preceded the three earthquakes. The quiescence time for the 1995 earthquake began at the tail of the aftershocks of the 1993 mainshock, suggesting that the two earthquakes were not entirely independent. At the tail of the 1995 earthquake aftershocks, the seismic activity spread across the Gulf, increasing in frequency and decreasing in magnitude. A similar pattern was observed for the seismic quiescence preceding the 2015 mainshock. The occurrence of the three significant and proximal earthquakes in the Aragonese region over a 22-year interval, with the described similarities, suggests a link between these earthquakes. Finally, the Coulomb stress transfer shows that the 1993 source fault seems to increase the stress loading on the 1995 left-lateral strike-slip fault, and the latter would also increase the stress transfer on the 2015 left-lateral strike-slip fault. [ABSTRACT FROM AUTHOR]

The magnitude frequency distribution of earthquakes is generally observed to depend on regional differential stress. The typical signature is a decrease of the b-value with increasing differential stress, corresponding to an increased probability of larger events. At the scale of a seismogenic fault, regional stress changes translate into shear and normal stress variations that both influence the nucleation process and the maximum extent of earthquake ruptures. Recent models have shown that normal stress changes cannot explain the observed b-value dependency, suggesting that b-value is primarily controlled by shear stress conditions on faults resulting from regional loading. The relationship between shear stress and b-value is however still not clear. Here, I investigate numerically the shear stress contribution on magnitude statistics by analyzing the b-value evolution with shear stress changes along a planar rate-and-state interface containing hierechical asperities and forced by slow tectonic loading under constant normal stress. The b-value is shown to decrease from 1.2 to 0.7 when shear stress increases by 1 MPa which is in agreement with fracture mechanics predictions. An approximate expression relating shear stress and b value is proposed and validated by the simulations, where the rate of b-value decrease with shear stress is provided by the inverse stress drop of typical earthquakes. The effect of shear stress along a planar hierarchical interface is therefore too strong to explain current observations. [ABSTRACT FROM AUTHOR]

Spatiotemporal b-value maps are presented for an extensive region from the northern Okinawa trough to off southern Kyushu Island, Japan. This region shows high seismic activity associated with a tectonic setting characterized by seafloor spreading caused by the opening of a back-arc basin in the Ryukyu arc–trench system and the presence of a left-lateral shear zone on southern Kyushu Island. The obtained spatial and temporal distributions of b-value for the analyzed period are highly heterogeneous, reflecting the influence of the tectonic features and processes of the region. A comparison of the results with other geophysical observations suggests deep fluids associated with mantle upwelling below the seismogenic layer are the dominant control on the observed b-value variations. In addition, the hypocentral area of the largest earthquake (M7.1), which occurred in 2015, corresponds approximately to a region with low b-values (b = 0.5–0.7). Another region with low b-values (b = 0.5–0.7) occurs in the west of the analyzed region off southern Kyushu Island, where the occurrence of large-magnitude events is not clearly recognized. During the analysis period, there were few moderate to large earthquakes in the analyzed region. Furthermore, some active faults that have the potential to generate large earthquakes of ~ M7 are located in those parts of the analyzed region with low b-values. These findings suggest a high likelihood of nucleation of large earthquakes in the focal region. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between the b value of the Gutenberg–Richter law and the so-called k–M slope, a topological parameters, derived from the visibility graph analysis, that recently has been gaining great attention in describing the time dynamics of seismicity. The relationship has been already found to be nearly linear for several seismic areas. In this paper, we investigate such relationship in two of the most seismically active areas of the world: Italy and Taiwan. Our findings indicate that the relationship between the Gutenberg–Richter b value and k–M slope is nearly linear with a slope of regression line of about 0.07. Furthermore, analysing the seismicity by using the directed visibility graph, the role of the aftershocks is evidenced by the value of two new parameters, defined as the difference and the ratio between the forward directed and the backward directed k–M slopes, which are higher for the whole catalogues than for the aftershock-depleted ones. [ABSTRACT FROM AUTHOR]

The b-value is closely related to the crustal stress. By studying the b-value trends of regions before large earthquakes, we can obtain the evolution of crustal stress, which is generally considered useful earthquake precursor information. A quake is closely related to fault tectonic movement. In the past, the traditional b-value calculation method did not consider the spatial distribution of the fault. Here, we propose a new algorithm based on the fault's buffer area, combining the spatial distribution and trend of the fault. This algorithm can accurately calculate the b-value changes for a specific fault using the fault as the basic independent unit. This method provides a useful reference for earthquake hazard judgment. Based on this algorithm, we use a total of 428,963 earthquakes from 1980 to 2013 after removal of the aftershock sequences and combine these quakes with the active fault distribution data to study the b-value changes before major earthquakes in western Sichuan. The results show that the b-values of the Fubianhe fault, Longmenshan main fault, and Shuangshi–Pengguan fault in western Sichuan decreased for at least 2 years before the earthquakes. The Longmenshan main fault area began to decrease slowly 5 years before the Wenchuan Ms8.0 earthquake and sharply decreased in the year before the quake, which reflects a significant change in crustal stress during this time. This conclusion and the method are available for long-term earthquake hazard assessment as a reference. [ABSTRACT FROM AUTHOR]

The main topic of this study is to characterize the seismic activity in the studied region by performing qualitative and quantitative analyses of several selected seismic sequences (seismic swarms and aftershock sequences). Clustering analysis of the selected seismic sequences, located in recent years in the Ibero-Maghrebian region, has been performed using the concept of fractal dimension. Analyses of the temporal variation of the fractal dimensions D - 2 (T) , D 0 (T) and D 2 (T) , as well as the spectral slope, have been also performed in order to characterize each analyzed sequence. The variation of the b-value parameter of the Gutenberg-Richter relationship with time has been derived using the windowing method, and a comparative analysis of the variation in time and space of the D 2 -value is assessed. Taking into account that b-value variations are often used in seismic hazard and earthquake forecasting studies, this type of work highlights the importance of fractal dimension research in seismic sequences. From our dataset, a positive correlation between b-value and the fractal dimension D 2 has been obtained, as shown in the following relations D 2 = 1.36 ± 0.27 b + 0.59 (± 0.26) and b = 0.74 ± 0.13 D 2 - 0.44 (± 0.24). [ABSTRACT FROM AUTHOR]

In our study, we used an improved Bayesian method based on a basis spline (B-spline) function to estimate the spatial and temporal variations of b-values in Southwest China. We propose that b-values combined with seismicity and tectonic background can provide a fairly clear stress condition and can be used in earthquake hazard analysis. Very low b-values in Southwest China appear to be related to the 2008 $$M_s\,8.0$$ Wenchuan earthquake. We also suggest that the decreasing b-value trend in the Longmen Shan area from 2000 to 2008 may be an indicator of the 2008 $$M_s\,8.0$$ Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

Fault zones contain structural complexity on all scales. This complexity influences fault mechanics including the dynamics of large earthquakes as well as the spatial and temporal distribution of small seismic events. Incomplete earthquake records, unknown stresses, and unresolved fault structures within the crust complicate a quantitative assessment of the parameters that control factors affecting seismicity. To better understand the relationship between fault structure and seismicity, we examined dynamic faulting under controlled conditions in the laboratory by creating saw-cut-guided natural fractures in cylindrical granite samples. The resulting rough surfaces were triaxially loaded to produce a sequence of stick-slip events. During these experiments, we monitored stress, strain, and seismic activity. After the experiments, fault structures were imaged in thin sections and using computer tomography. The laboratory fault zones showed many structural characteristics observed in upper crustal faults, including zones of localized slip embedded in a layer of fault gouge. Laboratory faults also exhibited a several millimeter wide damage zone with decreasing micro-crack density at larger distances from the fault axis. In addition to the structural similarities, we also observed many similarities between our observed distribution of acoustic emissions (AEs) and natural seismicity. The AEs followed the Gutenberg-Richter and Omori-Utsu relationships commonly used to describe natural seismicity. Moreover, we observed a connection between along-strike fault heterogeneity and variations of the Gutenberg-Richter b value. As suggested by natural seismicity studies, areas of low b value marked the nucleation points of large slip events and were located at large asperities within the fault zone that were revealed by post-experimental tomography scans. Our results emphasize the importance of stick-slip experiments for the study of fault mechanics. The direct correlation of acoustic activity with fault zone structure is a unique characteristic of our laboratory studies that has been impossible to observe in nature. [ABSTRACT FROM AUTHOR]

Over the last 25 years mining-induced seismicity in the Ruhr area has continuously been monitored by the Ruhr-University Bochum. About 1,000 seismic events with local magnitudes between 0.7 ≤ ML ≤ 3.3 are located every year. For example, 1,336 events were located in 2006. General characteristics of induced seismicity in the entire Ruhr area are spatial and temporal correlation with mining activity and a nearly constant energy release per unit time. This suggests that induced stresses are released rapidly by many small events. The magnitude–frequency distribution follows a Gutenberg–Richter relation which is a result from combining distributions of single longwalls that themselves show large variability. A high b-value of about 2 was found indicating a lack of large magnitude events. Local analyses of single longwalls indicate that various factors such as local geology and mine layout lead to significant differences in seismicity. Stress redistribution acts very locally since differences on a small scale of some hundreds of meters are observed. A regional relation between seismic moment M0 and local magnitude ML was derived. The magnitude–frequency distribution of a single longwall in Hamm was studied in detail and shows a maximum at ML = 1.4 corresponding to an estimated characteristic source area of about 2,200 m2. Sandstone layers in the hanging or foot wall of the active longwall might fail in these characteristic events. Source mechanisms can mostly be explained by shear failure of two different types above and below the longwall. Fault plane solutions of typical events are consistent with steeply dipping fracture planes parallel to the longwall face and nearly vertical dislocation in direction towards the goaf. We also derive an empirical relation for the decay of ground velocity with epicenter distance and compare maximum observed ground velocity to local magnitude. This is of considerable public interest because about 30 events larger than ML ≥ 1.2 are felt each month by people living in the mining regions. Our relations, for example, indicate that an event in Hamm with a peak ground velocity of 6 mm/s which corresponds to a local magnitude ML between 1.7 and 2.3 is likely to be felt within about 2.3 km radius from the event. [ABSTRACT FROM AUTHOR]

In June/July 2000, a hydraulic stimulation experiment took place at the geothermal EGS site of Soultz-sous-Forêts (Alsace, France) in order to enhance the permeability of the fractured granitic massif at 5 km depth. As it is well known that fluid injections tend to induce microseismic events, a downhole and a surface seismological network have been installed to monitor the seismic activity during the stimulation test. 23400 m3 of fluid have been injected in the rock volume through the open-hole section (4400 m–5000 m) of the well GPK2 at increasing rates of 30 l.s−1, 40 l.s−1 and then 50 l.s−1. More than 7200 microseismic events in the magnitude range –0.9 to 2.6 have been precisely located through a simultaneous inversion of the seismic velocity structure and location parameters. The analysis of the behavior of the seismicity relative to the hydraulic parameters gives important information about the geothermal reservoir. It appears that the evolution of the seismicity strongly depends on the variations of the injection rate: An increase or a decrease leads to changes of the velocity structure, the number and magnitude of microseismic events. This involves different hydro-mechanical processes between the fluid flow and the fracture planes, which will control the final shape of the microseismic cloud. Moreover, the study of the variations of the b-value with time suggests that the stimulation experiment produces a large proportion of small earthquakes, but records of events of magnitude higher than 2 indicate that fluid injection could reactivate structures whose dimensions allow the generation of such earthquakes. [ABSTRACT FROM AUTHOR]

We employed layered model joint hypocentral determination (JHD) with station corrections to improve location identification for the 26 January, 2001 Mw 7.7 Bhuj early and late aftershock sequence. We relocated 999 early aftershocks using the data from a close combined network (National Geophysical Research Institute, India and Center for Earthquake Research Institute, USA) of 8–18 digital seismographs during 12–28 February, 2001. Additionally, 350 late aftershocks were also relocated using the data from 4–10 digital seismographs/accelerographs during August 2002 to December 2004. These precisely relocated aftershocks (error in the epicentral location<30 meter, error in the focal depth estimation < 50 meter) delineate an east-west trending blind thrust (North Wagad Fault, NWF) dipping (~ 45°) southward, about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake. The aftershock zone is confined to a 60-km long and 40-km wide region lying between the KMF to the south and NWF to the north, extending from 2 to 45 km depth. Estimated focal depths suggest that the aftershock zone became deeper with the passage of time. The P- and S-wave station corrections determined from the JHD technique indicate that the larger values (both +ve and -ve) characterize the central aftershock zone, which is surrounded by the zones of smaller values. The station corrections vary from −0.9 to +1.1 sec for the P waves and from −0.7 to +1.4 sec for the S waves. The b-value and p-value of the whole aftershock (2001–2004) sequences of Mw ≥ 3 are estimated to be 0.77 ± 0.02 and 0.99 ± 0.02, respectively. The p-value indicates a smaller value than the global median of 1.1, suggesting a relatively slow decay of aftershocks, whereas, the relatively lower b-value (less than the average b-value of 1.0 for stable continental region earthquakes of India) suggests a relatively higher probability for larger earthquakes in Kachchh in comparison to other stable continental regions of the Indian Peninsula. Further, based on the b-value, mainshock magnitude and maximum aftershock magnitude, the Bhuj aftershock sequence is categorized as the Mogi's type II sequence, indicating the region to be of intermediate level of stresses and heterogeneous rocks. It is inferred that the decrease in p-value and increase in aftershock zone, both spatially as well as depth over the passage of time, suggests that the decay of aftershocks perhaps could be controlled by visco-elastic creep in the lower crust. [ABSTRACT FROM AUTHOR]

The correlation between theb-values of acoustic emissions (AEs) and the phase of the moon was investigated at the Underground Research Laboratory (URL) in Canada. The same data as those used in Iwata (2002) were examined, which showed that the occurrence of AEs is correlated with the phase of the moon. It was expected, therefore, that theb-value of the AEs would also be sensitive to tidal stress/strain fluctuations. We investigated the variation of theb-values as a function of the phase of the moon. Results show thatb-values immediately following the times of full/new moon are higher than those at other times. Using AIC (Akaike Information Criterion) and random (Monte Carlo) simulations, it was confirmed that this feature is statistically significant. We also investigated whether or not there was a change in theb-values immediately before the times of full/new moon, but no statistically significant change was observed. The results suggest that the effect of stress/strain fluctuations on AE occurrences at the URL is asymmetric to the times of full/new moon. [ABSTRACT FROM AUTHOR]