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To study the morphological characteristics of the largest canyons in the Southern and Central Basins of Lake Baikal, we analysed a digital relief model based on high-resolution multibeam bathymetric surveying. We evaluate the role of tectonic, lithological and climatic controlling factors in the formation and evolution of canyons. Tectonic activity and seismicity act as a catalyst for canyon formation and for the development of sublacustrine gravity flows and turbidity currents, and alter the morphological appearance of already existing canyons. The influence of tectonics on the formation of the Baikal canyons is particularly noticeable on the western slope of Lake Baikal, where canyons are mainly located along faults and follow their directions. Gravity flows and turbidity currents become more important during earthquakes and periods of lower lake level. When canyon heads are located close to river mouths, riverine processes have a significant impact on the evolution of the canyon. The topography of the original surface and the lithology of the substrate also play a significant role in the evolution of canyons and in shaping the canyon morphology. During periods of lower lake level (during Pleistocene glacial periods) riverine processes were responsible for initiating the formation of many canyons in Lake Baikal. The canyons developed under the complex influence of the factors considered. One or another factor could be dominant at different stages of their evolution, while the role of the other factors became secondary. These roles could interchange over time. The Baikal canyons show a great deal of resemblance to canyons in marine environments, in terms of morphological characteristics as well as of canyon-forming processes. We propose a morphogenetic classification of the underwater valleys and canyons of Lake Baikal.

The mechanical failure of fault plane during fluid injection can be conveniently approached by numerical methods, and the results can be applied in fault slip analysis to determine the corresponding magnitude of induced seismicity. During hydrofracturing, when faults are present and the fluid is injected into the fault, micro-seismic events are possible, although the magnitude is often somewhat larger than those associated with micro-seismic events produced from regular hydraulic fracturing because of the larger surface area available for fault rupture. This study considers the rate at which the changing elastic properties of materials influences the magnitude of seismic event during fault injection. The simulation is carried out under varying injection flow rates from 0.18 kg/s to 0.3 kg/s, and the thermo-hydro-mechanical (THM) model in FLAC3D is adopted. As the material elastic moduli increase significantly under isothermal injection, the resulted non-uniformity in the fault slip timing affects the magnitude of injection-induced seismicity. Rocks with lower moduli produced higher slip distance and seismicity during shear failure. However, in the coupled thermal case, the magnitudes of seismicity during injection are largely enhanced at lower elastic properties, which suggests that the energy of accumulated fluid pressure produces a larger rupture and longer slip displacement in cold injection than in the isothermal case. The resulting volumetric strain, both in the fault zone and in the matrix, is higher in lower moduli, meanwhile, it is much developed in non-isothermal injection as a result of the rock's response to the sum effect of thermal strain and the stress-induced strain.

Mantle plumes are one of the most influential first-order tectonic systems on Earth, playing a prominent role in controlling local magmatic activity and seismicity. Recent studies have shown that the Hainan mantle plume originated from the lower mantle and its volcanic activity is still intense during the Holocene. However, the effect of the Hainan plume on local seismicity is still unclear. In this study, we conducted the first wide-angle seismic survey in the source area of the 1969 earthquake doublet (M 5.1 and 5.2) offshore Hainan Island. A high-resolution 3-D P-wave velocity model of the crust is obtained using a large amount of Pg and PmP wave arrival times recorded by a portable array consisting of 17 ocean bottom seismographs (OBSs). Our results show that the 1969 earthquake doublet occurred in high-to-low velocity transitional zones which correspond to active faults. Prominent low-velocity anomalies are revealed in the crust of the study area, which may reflect magmatic intrusion channels. On the basis of multiple geophysical evidence including multi-channel seismic reflection profiles, geothermal data, and seismic tomography results, we infer that the low-velocity anomalies in the source area may reflect magmas and fluids from the Hainan plume, and high-angle normal faults in the crust serve as conduits for ascending of the magmas and fluids. The 1969 earthquake doublet offshore Hainan Island can be attributed to the magma-hydrothermal alteration processes in the fault zones, which led to significant reduction of the fault strength and friction coefficient, facilitating the shear failure propagation. Furthermore, the seismicity in and around Hainan Island exhibits distinct swarm-like behaviors, which is also evidence for the Hainan plume affecting the crustal seismicity.

All underwater drilling and blasting operations generate seismic waves. However, due to a lack of suitable vibration sensing instruments, most studies on the propagation of seismic waves have been limited to shorelines near construction areas or wharfs, whereas comparatively few studies have been conducted on the larger seafloor itself. To address this gap, a seafloor vibration sensor system was developed and applied in this study that consists of an autonomous acquisition storage terminal, software platform, and hole-plugging device that was designed to record the blasting vibration intensities received through submarine rocks at a given measurement point. Additionally, dimensional analyses were used to derive a predictive equation for the strength of blast vibrations that considered the influence of the water depth. By combining reliable vibration data obtained using the sensor system in submarine rock and the developed predictive equation, it was determined that the water depth was an important factor influencing the measured vibration strength. The results using the newly derived equation were compared to those determined using the Sadowski equation, which is commonly used on land, and it was found that predictions using the derived equation were closer to the experimental values with an average error of less than 10%, representing a significant improvement. Based on these results, the developed sensor system and preliminary theoretical basis was deemed suitable for studying the propagation behavior of submarine seismic waves generated by underwater drilling and blasting operations.

Fluid pressurization within the fault zone generates increasing pore pressure and stress change which is liable to create shear and/or brittle fractures within the reservoir volumes and subsequently generating earthquakes of varying magnitudes. Here, we explored time-dependent fault weakening processes in the fault zone which are dependent on several factors, including the rate of cold-water injection, modes of injection (hydromechanical (HM) and thermo-hydro-mechanical (THM) interactions), and changing fault spatial configurations using data from Niger Delta Basin. The variation in the stability of different fault models in withstanding stresses induced by HM and THM fluid interactions is evident. Fault permeability enhancement and the behaviour of slip event under isothermal and non-isothermal conditions revealed that stress and pore pressure perturbations have a first order control on the rate of fault dilation and compression. It is observed that the progressive cooling of the reservoir induced thermal stress which induced the timing of slip by unloading the fault to earlier seismic rupture in the non-isothermal case, and accelerates the magnitude of the fault reactivation and the accompanied induced seismicity. Owing to increased tendency of shear failure during injection, fracture opening through shear dilation is more enhanced in THM simulation as the fracture permeability is significantly higher than in HM. This effect becomes increasingly more dominant with intermediate fault angle and joint orientation. Certain fault/joint configurations which were resistant to shear failure under isothermal injection had their frictional resistance broken by thermal stress. The results also indicate that there is higher pore pressure build-up in THM than in HM as the injection rate increases and reservoir temperature drops during cold injections.. This study has demonstrated the importance of fully characterizing the fracture geometries and configurations of normal faulting regime in addition to fluid injection conditions when developing fractured reservoirs to mitigate seismic risks and hazards that could result from early fault reactivation.

Volcano seismicity is an important tool in remotely monitoring and forecasting activity at volcanoes around the world. Volcanic earthquakes show diverse spectral characteristics, with shallow Long Period (Low Frequency) seismicity and long duration tremor generally interpreted as indicators of fluid migration, and as potential precursors to eruption. Here we show that a common low-cohesion volcanic sediment from Campi Flegrei caldera (Italy) produces Low Frequency and long duration seismicity whilst undergoing deformation in dry conditions. We employ acoustic-emission rock deformation experiments at a range of strain rates to produce events which are spectrally indistinguishable when normalised for scale from Long Period and tremor seismicity observed in natural volcanic settings. Generation of these signals is enhanced at lower strain rates. Correlated X-Ray tomography of samples before and after deformation constrain the source as sub-grain scale diffuse damage. Given the ubiquitous nature of slow edifice deformation, and the frequent occurrence of such low cohesion materials in the upper edifice of volcanoes, we suggest low frequency seismicity and tremor in volcanic settings do not require fluid movement. Instead, these characteristic signals can be an indicator that deformation within the edifice is being accommodated by weak volcaniclastic materials.

Eastern Himalayan Syntaxis (EHS) is a tectonically active region that undergoes continuous geomorphic changes. Large landslides are predominant in this region. A giant landslide called Jiaobunong landslide on the northwestern flank of the EHS were studied and simulated to investigate the formation mechanism, evolutionary process, and failure mechanism of the landside, so that we could better understand the large complex ancient landslides in this region. Field investigation, geological background analyses, and numerical modeling were conducted to reveal the natural and seismic characteristics, as well as dynamic process of the landslide. The results show that the Jiaobunong landslide was the result of long-term geological and geomorphic evolution. Uplift, river incision, weathering, fault creep, glaciation, and earthquakes play key roles in the formation of landslides. Given the huge landslide volume, strong seismicity of the study area, proximity to an active fault, and the need for extra forces to induce landsliding, the Jiaobunong landslide was triggered by a paleo-earthquake. Using numerical simulation based on the discrete element method, the slope dynamic response of the earthquake as well as the mass movement and accumulation process was reproduced. Simulation results showed that the landslide movement experienced four stages: initiation phase (0–5 s), acceleration phase (5–35 s), deceleration phase (35–95 s), and the compaction and self-stabilization stage (after 95 s). The rock mass was disintegrated and experienced strong collisions during the movement. The dammed lake gradually disappeared because of long-term river incision by the overtopping river water. These processes play a vital role in the evolution of landforms in the region of EHS.

Frequency-magnitude distribution was investigated in Elazig city and the surrounding area (coordinates ranging from 38.1° N to 39° N for latitudes and 39° E to 40° E for longitude) to reveal the present-day crustal stress within the region. For this study, a complete set of 5603 earthquakes of Md ≥ 1 from 1 January 2000 to 31 August 2020 were localized from the Bogazici University, Kandilli Observatory and Earthquake Research Institute, Regional Earthquake-Tsunami Monitoring Center (KOERI) catalogue. For the whole study region, spatial mapping of the frequency-magnitude distribution was thus produced. The frequency-magnitude distribution (b-value) for seismicity within the study area was not homogeneous. Outstanding variations in the b-value were detected with b ranging from b ≈ slightly more than 0.3–2. In this region, low b-values are mostly dominant in the region except for a few local areas. This study also shows that accumulated stresses are high particularly at the Puturge segment in contrast to the fault drawn in the south, which represents the main thrust belt that runs through Hani-Lice and stretches to Cungus. As expected, stress accumulation in reverse faults is fairly higher. However, the sharing of the tension formed in the compression zone of the Arabian Plate and the Anatolian plate (i.e. stress partitioning in a sense), results in a low share in the reverse fault belt.

Research subject and methods. The seismicity of the Krilyon Peninsula (Sakhalin Island) after the earthquake on April 23, 2017 (M = 5.0) during the 2018–2021 period was investigated by the method of self-developing processes (SDP).Results. The seismic activity showed a damping trend after the earthquake (and several following aftershocks), with the activation of SDP seismicity observed in two cases. In the study area (40 km within the radius from the epicenter of the Krillon earthquake) and its immediate vicinity (up to 80 km), earthquakes with M = 3.9 and M = 4.3 were observed 74 and 26 days after seismic activations. These earthquakes are considered to be the strongest events occurred in the study area in the period under consideration. The results of the study were compared with the monitoring data of the Kamchatka branch of the Geophysical Survey Russian Academy of Sciences (KB GS RAS) on the volumetric activity of subsoil radon (OA Rn). It was revealed that the OA Rn anomalies detected in November 2018 and January 2020 had appeared several weeks after the identifed SDP. The anomalies were registered at observation points remote from the study area (as well as from the above-mentioned earthquakes) by no more than 50 km. Earthquakes in 2018 (M = 3.9) and 2020 (M = 4.3) occurred 19 and 32 days after the appearance of OA Rn anomalies. In general, the current situation can be assessed as calm, since, according to the analysis data, the seismic activity has no prerequisites for the transition from the decaying stage to the stage of stabilization. As shown in the work, this is a sign for preparing an earthquake with a magnitude higher than 5. Nevertheless, activation and (as a result) earthquakes of moderate strength (M < 5) are possible on a damping trend. Conclusion. It is proposed to take into account the self-developing processes of activation as a primary sign for a change in the geodynamic state of the environment, under which the appearance of OA Rn anomalies is possible. These anomalies are associated with the forecast of moderate seismicity during foreshock or aftershock activations.

The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotectonics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tectonic transient that most affected the whole Puerto Rico Island.

Solid earth : SE 12(10), 2211-2234 (2021). doi:10.5194/se-12-2211-2021, Published by Copernicus Publ., Göttingen

To study the seismicity of the Red River fault, observational GPS data from the Crustal Motion Observation Network in China from 1999 to 2018 were selected to calculate the characteristics of strain accumulation based on interpolation. The block-dislocation model was used to study the slip rate, locking degree, and slip deficit of the Red River fault. We used the negative value of the product of the obtained fault slip deficits and time to calculate the fault’s interseismic Coulomb stress accumulation rate. The results revealed that the fault’s dextral strike-slip rate of its northern section is 4.7 mm/year, which is higher than that of its middle and southern sections. The surface expansion rate is mainly manifested by alternating high values of surface expansion and low values of surface extrusion. The maximum shear strain rate, locking degree, slip deficit, and loading rate of interseismic Coulomb stress are all measurably higher in the northern section. In the middle section, the dextral strike-slip rate, surface expansion rate, maximum shear strain rate, and slip deficit are all lower. In the southern section, the slip deficits are large but the locking degree is lower, the maximum shear strain rate is low but high in the surrounding area, and the interseismic Coulomb stress in most parts is positive. The moderate or strong earthquakes probably occur in the northern part of the Red River fault and rarely occur in the middle section, but there is risk of moderate or strong earthquakes in the southern section.

The Zagros suture zone is seismically active region in Iranian plateau. This region is of high importance in terms of seismicity, since it is a vast and populated region and in recent years the earthquakes with high intensities have frequently occurred and have caused extensive destruction and heavy human loss. The study of the focal mechanism is very important in understanding the seismotectonic characteristics. Focal mechanisms of Zagros were collected over a period of 20 years and they were classified by FMC software. Seven groups were considered for the type of faulting and Zagros was divided into three zones. For each zone, the frequency percentage of each group of faults was determined. The most of faulting are of the reverse and compression type with the strike-slip component. Finally, the role of nodal plane selection in determining the type of faulting was discussed and it was found that the selection of each nodal plane in determining the type of faulting has the same result.

SUMMARYThe brutal onset of seismicity offshore Mayotte island North of the Mozambique Channel, Indian Ocean, that occurred in May 2018 caught the population, authorities and scientific community off guard. Around 20 potentially felt earthquakes were recorded in the first 5 d, up to magnitude Mw 5.9. The scientific community had little pre-existing knowledge of the seismic activity in the region due to poor seismic network coverage. During 2018 and 2019, the MAYOBS/REVOSIMA seismology group was progressively built between four French research institutions to improve instrumentation and data sets to monitor what we know now as an on-going exceptional submarine basaltic eruption. After the addition of 3 medium-band stations on Mayotte island and 1 on Grande Glorieuse island in early 2019, the data recovered from the Ocean Bottom Seismometers were regularly processed by the group to improve the location of the earthquakes detected daily by the land network. We first built a new local 1-D velocity model and established specific data processing procedures. The local 1.66 low VP/VS ratio we estimated is compatible with a volcanic island context. We manually picked about 125 000 P and S phases on land and sea bottom stations to locate more than 5000 events between February 2019 and May 2020. The earthquakes outline two separate seismic clusters offshore that we named Proximal and Distal. The Proximal cluster, located 10 km offshore Mayotte eastern coastlines, is 20–50 km deep and has a cylindrical shape. The Distal cluster start 5 km to the east of the Proximal cluster and extends below Mayotte's new volcanic edifice, from 50 to 25 km depth. The two clusters appear seismically separated, however our data set is insufficient to firmly demonstrate this.

This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relationship between the distribution of the velocity and geoelectric heterogeneities in the crust and upper mantle, seismicity and the stress-strain state of the crust.

The results of seismic hazard zoning of the submerged crossing of the pipeline in the Nevelskoy Strait, between the village of Lazarev of the Nikolaevsky district, Khabarovsky krai, and Cape Pogibi near the village of Pogibi, Sakhalin oblast are presented. Along the water area, this section of the crossing is limited by a zone that is 500 m wide and 11 km long. It follows from the stock of geological documents of the Geophysical Service of the Russian Academy of Sciences and the general seismic zoning map (GSZ-2015) that the highest seismic activity in the Okhotsk region is observed in the area of the Kuril Islands, while the closest activity to the construction site manifests itself in the central zone of northern Sakhalin. Large earthquakes are recorded by a number of seismological stations in the immediate vicinity of the construction site. The foci of nearby earthquakes are displaced eastward of the construction site and are mostly confined to the systems of deep faults that result from the juncture between the West Sakhalin synclinorium and the Sikhote Alin volcanic belt. The main purpose of this research is related to the development of a procedure for forecasting seismic impacts for the areas that are prone to predicted strong earthquakes. In this context, an approach is proposed for the specific structure, which is based on the construction of a set of the most probable seismic models to reach the reference bedrock, setting of the pickup signal corresponding to initial seismicity of the area, calculating the main parameters of seismic impacts for them, and then, in the first approximation, the implementation of the technology for constructing an engineering–seismological cross-section along the pipeline construction site. The proposed approaches to forecasting seismic events for the section of the pipeline crossing in the Nevelskoy Strait are based on certain restrictions that made it possible to obtain a set of parameters of seismic impacts for predicted strong earthquakes that are necessary for design and construction of an earthquake-resistant linear structure. In improving the calculation methods and monitoring critically important objects, the advantage of this approach is the ability to recalculate seismic impacts on a structure using the constructed seismic ground-motion models.

We show that long-recognized seismicity in the central Virginia seismic zone of the eastern North American intraplate setting arises primarily from tectonic processes predicted by new, full...

—Numerical modeling of induced seismicity resulting from fluid injection into the subsurface is considered. One of important factors determining the slip dynamics of a tectonic fault during fluid injection loading is the form of the law of friction acting on the fault sides. The numerical analysis in the context of a slider model compared to the results of laboratory experiments has shown that a two-parameter rate-and-state friction law describes the widest spectrum of the observed sliding modes. A double porosity model and a slider model for fracture are used to simulate the induced seismicity in ​​the region of Basel, Switzerland, caused by geothermal stimulation. A physically complete model of embedded fractures which allows for calculating fluid flow in a rock with fractures or faults taking into account changes in the hydraulic properties of the latter is presented. Fault deformation process is described based on the displacement discontinuity method. Based on the model, the results of the field experiment on water injection into a fault in the southern France are analyzed. The development of fault slips is studied as a function of a number of parameters characterizing a fault, filtration properties of a rock, and injection pattern. The conditions for the emergence of seismic slip in the context of the proposed model are established.

The NW Iberian Peninsula is an intraplate region with significant seismicity, but it has rarely been studied. Despite the increase in seismicity during recent years, which has even forced changes to the building codes, a comprehensive statistical analysis of this seismicity has not been carried out. Thus, this study aims to characterize seismicity clusters on an intraplate region, based on comprehensive qualitative and quantitative analysis. The b value of the Gutenberg–Richter law is estimated and the Epidemic Type Aftershock Sequence model is employed. Additionally, qualitative analysis is carried out using the Mogi classification, and the results of this work are compared with those of other statistical studies in the literature. We identify clustered seismic activity in the NW Iberian Peninsula, which is classified into 18 swarms and eight sequences. A high b value and low efficiency of aftershock generation are characteristic of the swarms. These parameters, along with the similarities to other regions, point to fluid migration as the swarm seismic source mechanism. Earthquakes of higher magnitude than swarms and a b value close to 1 define the sequences of this region. The statistical results in sequences (high k, c, and p values) and the similarity to other regions would indicate a mix of tectonic origin and aseismic processes (such as fluid migration or slow slip events) as the driving force of seismic sequences. The presence of crustal fluids associated with fault activity in the NW Iberian Peninsula also supports the hypothesis of fluid migration as the source of seismic events.

The 1819 Allah-Bund earthquake (ABE) that occurred in the north-western part of the Kachchh rift, India, was a deadliest intraplate earthquake of the nineteenth century. In this work, we have used WGM 2012 global gravity model data to understand the tectonic framework in the epicentral zone of the 1819 ABE and surrounding regions. Total horizontal gradient analysis of the Bouguer anomaly (BA) is carried out to map major lineaments in the study area. The logarithmic amplitude spectra of the BA provide the average depth of 13 km between the interface of the upper crust and the basement while basement-sediment interface is found at 1.6 km. The low-pass filter anomaly of cut-off wavelength of 100 km is considered as the regional anomaly. The residual anomaly shows high value with a NW–SE trend which is similar to the trace of Allah-Bund (AB) on the surface. The upper crustal geometry of the region is provided for the first time by performing 2.5D forward gravity modelling along two profiles. These are constrained by seismological and well data. Our present model reveals that the upper crustal thickness increases towards the AB. Inversion of the BA reveals the NW dipping Moho in the study area. The basement is undulating, which is uplifted by 1–2 km over the AB. We also tried to explain the seismicity in the study area. The present study provided the first geophysical evidence of the ABF, and it is inferred that the fault is shallow in nature, dipping in the SW direction.

A permanent seismic station was for the first time installed on Severnaya Zemlya in the Russian Arctic. This made it possible to record low-magnitude earthquakes occurring within the Severnaya Zemlya archipelago and in the Taimyr Peninsula, which is the northernmost landmass part of the Eurasian continent. The present paper summarizes information on the seismicity in the area of study for the entire instrumental period of observation. The earthquakes that occurred during the earlier half of the twentieth century and for some earthquakes of the twenty-first century were relocated using all raw data and bulletins of seismic stations available today. Most earthquakes occurred in the mouth part of the Khatanga Bay with the Khatanga graben where the Khatanga-Lomonosov marginal continental fault zone originates and in the eastern Taimyr Peninsula. The resulting catalog with relocated earthquakes can form a basis for subsequent studies involving geodynamic models, the crustal state of stress and strain, and earthquake hazard assessment.

Earthquake precursor and earthquake monitoring are always important in the earthquake research field, even if there is still debatable about the existence of earthquake precursor. Laboratory rupture experiment is a useful technique to simulate and make an insight into the complex mechanisms of earthquakes. Five marble samples with prefabricated cracks are used for uniaxial loading experiments to investigate whether there is a precursory signal before rock fracture, and to simulate the rupture process of strike-slip fault and its surrounding area. The existence of precursory signal is confirmed by the coefficient of variation (CV) results, from which we can see two patterns which are known as seismicity acceleration and quiescence before an earthquake. Moreover, these CV findings are applied to determine the locations of large deformation sampling points on the rock surface at different loading stages, which indicates that such kinds of points migrate towards the prefabricated cracks as the rock approaches rupture. Similar results are obtained when we consider actual seismicity at the northern end of the San Andreas Fault in California. It provides a crucial evidence to prove the existence and role of precursor characteristics. In this case, three kinds of seismic monitoring models are designed to find out how to arrange the locations of seismic stations in the case of limited number of stations, so as to monitor these precursor characteristics more effectively.

Oceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep. The mode of slip varies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures

This paper makes an attempt to review historical and recent seismicity in Delhi and presents updated seismic hazard map of Delhi integrated with ward-wise classified population data. Earthquakes are not destructive unless other factors that enhance the damage prevail. Population density and types or quality of buildings in an area are two such important factors that affect the risk during any hazards, particularly an earthquake. This paper presents the details of the seismic hazard analysis for Delhi with the latest available information on earthquakes and seismic sources within 300 km radius around the city, using state-of-the-art deterministic approach. Ten different attenuation relationships developed for the active tectonic region were considered in the present study. They were ranked using log-likelihood method. Ward-wise population maps along with classified population maps were developed based on the 2011 census data. Hazard contour map and population maps are presented in this paper to revisit the seismic risk prevailing in the region. An attempt was made to overlay the hazard map with population maps showing the distribution of various classes of the population that reflects the seismic risk exposure to different wards of Delhi.

Between 2006 and 2020, earthquakes and other geohazards on volcano-dotted Java Island have caused about 7000 deaths, and another 1.8 million people were injured, displaced, or left homeless. In this study, we quantify the current state of earthquake hazard for 29 cities of Java, using seismicity statistics of a cumulative number of small events (natural times) between pairs of large earthquakes. This approach, known as earthquake nowcasting (Rundle et al., 2016), rests on the key concepts of elastic rebound and ergodic dynamics in earthquake fault networks. Our analysis of statistical inference shows that the estimated earthquake potential score (EPS) as on February 18, 2021 corresponding to M ≥ 6.5 events in a 300 km circular area ranges from 43 to 94%, with the scores of Jakarta (43), Surabaya (89), Bandung (69), Semarang (48), Serang (47), and Yogyakarta (59). This means, for example, that Surabaya has progressed significantly in the regional cycle of large earthquakes, whereas Yogyakarta is about midway in its seismic cycle. We observe that a change in magnitude threshold or geographic area has a consistent impact on the nowcast scores. These findings not only enable a rapid yet meaningful way to rank several cities based on their current exposure to earthquake hazards, but also empower earthquake scientists and policymakers towards better policymaking, land-use planning, earthquake insurance, disaster risk mitigation, and social awareness with respect to the seismically active island of Java.

Philippine archipelago (PA) has strong background seismicity, but there is no systematic study of earthquake triggering in this region. There are six earthquakes (Mw > 6) occurred between 2018/12/29 and 2019/09/29 in PA, which provides an excellent opportunity to investigate the triggering relationship among these events. We calculate the static Coulomb stress changes of the first five events, and find that the local seismicity after the 2018/12/29 Mw 7.0 earthquake is mostly associated with positive Coulomb stress changes, including the 2019/05/31 Mw 6.1 event, suggesting a possible triggering relationship. However, we cannot rule out the dynamic triggering mechanism, due to increased microseismicity in both positive and negative stress change regions, and an incomplete local catalog, especially right after the first Mw 7.0 mainshock. The dynamic stresses from these Mw > 6 events are large enough (from 5 kPa to 3 532 kPa) to trigger subsequent events, but a lack of seismicity and waveform evidence does not support delayed dynamic triggering among these events, even the shortest time interval is less than 24 hours. In the past 45 years, the released seismic energy shows certain peaks every 5–10 years. However, earthquakes with Mw > 6.0 were relatively infrequent between 2004 and 2018 at PA. Hence, it is possible that several regions are relatively late in their earthquake cycles, which would enhance their susceptibility of being triggered by earthquakes at nearby and regional distances.

The proposed site of the Diamer Bhasha Dam in northern Pakistan is situated in an active tectonic zone with intensive seismicity, which makes it necessary for seismic hazard analysis (SHA). Deterministic and probabilistic approaches have been used for SHA of the dam site. The Main Mantle Thrust (MMT), Main Karakaram Thrust (MKT), Raikot-Sassi Fault (RKSF) and Kohistan Fault (KF) have been considered as major seismic sources, all of which can create maximum ground shaking with maximum potential earthquake (MPE). Deterministically estimated MPE for magnitudes of 7.8, 7.7, 7.6, and 7.1 can be produced from MMT, MKT, RKSF and KF, respectively. The corresponding peak ground accelerations (PGA) of 0.07, 0.11, 0.13 and 0.05 g can also be generated from these earthquakes, respectively. The deterministic analysis predicts a so-called floating earthquake as a MPE of magnitude = 7.1 as close as 10 km away from the site. The corresponding PGA was computed as 0.38 g for a maximum design earthquake at the project site. However, the probabilistic analysis revealed that the PGA with 50% probability of exceedance in 100 years is 0.18 g. Thus, this PGA value related to the operational basis earthquake (OBE) is suggested for the design of this project with shear wave velocity (Vs30) equal to 760 m/s under dense soil and soft rock conditions.

Geothermal and volcanic areas are prone to earthquake triggering1,2. The Coso geothermal field in California lies just north of the surface ruptures driven by the 2019 Ridgecrest earthquake (moment magnitude Mw = 7.1), in an area where changes in coseismic stress should have triggered aftershocks3,4. However, no aftershocks were observed there4. Here we show that 30 years of geothermal heat production at Coso depleted shear stresses within the geothermal reservoir. Thermal contraction of the reservoir initially induced substantial seismicity, as observed in the Coso geothermal reservoir, but subsequently depleted the stress available to drive the aftershocks during the Ridgecrest sequence. This destressing changed the faulting style of the reservoir and impeded aftershock triggering. Although unlikely to have been the case for the Ridgecrest earthquake, such a destressed zone could, in principle, impede the propagation of a large earthquake. Thirty years of geothermal heat production at Coso in California depleted shear stresses within the geothermal reservoir, which changed its faulting style and inhibited aftershocks from the 2019 Ridgecrest earthquake.

In this article, we investigate the influence of the Arctic mid-ocean ridge system (AMORS), including the Gakkel and Mohns ridges, and the Knipovich ridge–Lena trough (KL) segment, on seismicity of the Novaya Zemlya archipelago area (NZ) and the northernmost margin of the East-European Platform (EEP) for 1980–2019. For each individual area, the annual seismic energy was obtained by adding the energies of all earthquakes. To do this, we have converted various types of magnitude by different seismic networks into moment magnitude Mw. We compiled the updated catalog for the NZ, the northern EEP, and the northern part of the Ural fold belt (UFB). As a result, we constructed time distributions of annual seismic energy releases for each composing ridges of AMORS, NZ, and EEP combined with UFB. A model based on the Elsasser’s one describing the transfer of lithospheric stress disturbances in the horizontal direction was built, and quantitative calculations of the disturbance propagations from AMORS were performed. Results are in good agreement with the annual seismic energy time lags between rifts and NZ and EEP together with the UFB. We calculated correlation coefficients between the seismic energy releases over the time for the structures, enabling identification of the characteristic excitation cycles and estimation of the interval of disturbance transfer from AMORS. As a result, disturbances from the Gakkel ridge appear 3 yr later in NZ, from the KL segment in 4 yr, and from the Mona ridge in 8 yr. For the EEP + UBF combined area, we found the following disturbances spreading lags as 7 yr for the Mona ridge, 4 yr for the KL segment, and 5 yr for the Gakkel ridge. The obtained damping amplitudes of the disturbance spreading from the arctic ridges are sufficient to affect the intraplate seismic activity.

Eastern North America (ENA) is part of a region with low-to-moderate seismicity; nonetheless, some significant seismic events have occurred in the last few decades. Recent events have reemphasized the need to review ENA seismicity and ground motion models, along with continually reevaluating and updating procedures related to the seismic safety assessment of hydroelectric infrastructures, particularly large dams in Québec. Furthermore, recent researchers have shown that site-specific characteristics, topography, and valley shapes may significantly aggravate the severity of ground motions. To the best of our knowledge, very few instrumental data from actual earthquakes have been published for examining the site effects of hydroelectric dam structures located in eastern Canada. This article presents an analysis of three small earthquakes that occurred in 1999 and 2002 at the Denis-Perron (SM-3) dam. This dam, the highest in Québec, is a rockfill embankment structure with a height of 171 m and a length of 378 m; it is located in a narrow valley. The ground motion datasets of these earthquakes include the bedrock and dam crest three-component accelerometer recordings. Ground motions are analyzed both in the time and frequency domains. The spectral ratios and transfer functions obtained from these small earthquakes provide new insights into the directionality of resonant frequencies, vibration modes, and site effects for the Denis-Perron dam. The crest amplifications observed for this dam are also compared with previously published data for large dams. New statistical relationships are proposed to establish dam crest amplification on the basis of the peak ground acceleration (PGA) at the foundation.

Peninsular Malaysia is tectonically situated on a stable craton (intraplate) and so far experiences relatively little earthquake activities, thus considered as a region with low seismicity. This study uses earthquake data from 59 events obtained from various sources in the period 1922 to 2020. The overall seismicity in the study area is low as expected due to the general intraplate setting. Earthquakes occurred onshore and offshore of Peninsular Malaysia between latitudes 1° and 7° N and longitudes 99° and 105° E. The seismicity pattern shows that the epicenters are distributed spatially in some parts of the peninsula and in the Malacca Strait with several epicenter zones. Most of earthquakes are associated with several preexisting faults and fault zones indicating that they are the major contributor to the local seismicity. Meanwhile, some further earthquakes were caused by activities related to reservoirs. Magnitudes are ranging from Mw 0.7 to 5.4 with the majority is Mw 1.0 + and 2.0 +. Hypocenters are located in between 1 and 167 km deep (shallow to intermediate earthquakes) with the majority being shallow earthquakes (1–70 km). The deepest earthquake located in the Straits of Malacca can be associated with a slab detachment broken off from the Sumatran Subduction Zone. Finally, this study contributes to the understanding of the intraplate seismicity of Peninsular Malaysia as a basis for seismic hazard and risk assessment.Article HighlightsEarthquake assessment over the last 100 year reveals low but clear seismicity with an associated seismic hazard and risk for certain areas.Shallow, low-magnitude earthquakes associated with reservoir activities and preexisting faults reactivated by the nearby subduction zone.A deeper, low-magnitude earthquake can be related to slab detachment from the Sumatran subduction zone toward the east.

This work presents the evaluation of earthquake resistance of the Arusha International ConferenceCentre (AICC) complex, in Tanzania. The evaluation included probabilistic seismic hazardanalysis (PSHA) and site response analysis. Seismic sources considered to constitute a seismichazard in this study were randomly occurring seismicity located within five tectonic provincesaround the site. For each province the seismic hazard is based on a cursory analysis of earthquakedata from compiled ESARSWG bulletins and temporary deployed networks within the NorthTanzania Divergence (NTD). Bedrock response signal together with the information of materialcharacteristics from boreholes around the AICC site were used in analysis of site response. PSHAresults indicated uniform hazard spectra values of 0.15, 0.2 and 0.27 g for return periods of 475,975 and 2475 years, respectively. The surface ground response results indicated a maximumamplification factor of 3.7 and a spectral response of 4.5 g for a wave period of 0.6 sec thatmatches the natural frequency of the 6-7 storey buildings of the AICC complex. It is thisresonance effect on the buildings that is assumed to have caused intense shaking in the earthquakeof December 5th 2005 from Lake Tanganyika. Keywords: Probabilistic seismic hazard analysis; Arusha International Conference Centre; EastAfrican Rift System; Uniform hazard spectra; Site effect.

Dinar Fault is an active breakaway fault that plays an essential role in the southwestern Anatolia extensional tectonic regime's neotectonic development. The Dinar Fault's seismicity was proven by October 1, 1995, Dinar Earthquake (Mw 6.2). It consists of three geometric segments called Gumussu, Kizilli and Dikici, respectively, from northwest to southeast. The October 1, 1995 Dinar Earthquake occurred on the Kizilli Segment, approximately 10 km long, located in the middle of the Dinar Fault. In this study, active tectonic and kinematic characteristics and paleoseismic history of the Dinar Fault were investigated. Based on the fault plane slip data obtained from the Dinar Fault, it is a normal fault with a minor right-lateral strike-slip component. A total of four trenches were excavated across the 1995 surface rupture. According to paleoseismological trench studies, the Dinar Fault's slip rate was estimated as 0.37–0.56 mm/year and the average earthquake recurrence interval was 523 years in the last 3660 years. A number of these earthquakes destroyed the ancient city of Apameia Kibotos (ancient Dinar). The last earthquake that occurred on the Dinar Fault is the October 1, 1995 Dinar Earthquake. The date of the penultimate event occurring on the Dinar Fault was determined as AD 1318–1486, while the antepenultimate event was determined as AD 916–1155. The oldest event occurring on the Dinar Fault was determined as AD 916–111 BC. Structural and paleoseismological data indicate that the other sub-segments of the Dinar Fault, which did not rupture in the 1995 earthquake, have the potential to produce earthquakes with magnitudes varying between Mw: 6.2 to Mw: 6.6.

SUMMARY The Kachchh Rift Basin of western India that hosted the devastating 2001 MW 7.7 Bhuj earthquake has been witnessing minor to moderate seismicity since then. The genesis of these intraplate earthquakes and geometry of the causative faults is still elusive. In this study, we relocated all the earthquakes recorded between 2006 and 2018 and utilized a set of 4285 best located earthquakes to provide rare insights into the orientation and depth distribution of the currently active faults. We also analysed previously computed source mechanisms, in perspective of our location results. It is revealed that the present seismic activity is primarily aligned along two planes—one steeply dipping (∼60°) to the northeast with a NW–SE strike and the other gently (∼34°) dipping to the SSW, striking WNW–ESE. The traces of these fault surfaces coincide with the Kachchh Mainland fault (KMF) and the North Wagad Fault (NWF) when extrapolated to the surface, respectively. Activity along the NWF has been shown in earlier studies, however, clear evidence of activity along the steep north-dipping KMF is presented for the very first time. Thrust earthquakes dominate the NWF while strike-slip earthquakes are seen across the KMF. Our results show that the two fault surfaces converge between 70.30°E and 70.43°E longitudes in the depth range 22–32 km, however, there is large E–W offset between the northern and southern extremities of the fault-system. This convergence zone hosted the largest earthquakes (ML 4.7–5.1) between 2006 and 2018 and the hypocentre of 2001 main shock also coincide with it. The earthquakes occurring in the interfault region show major strike-slip motion and are probably influenced by the relative motion between the NWF and KMF. Consistent seismicity, assisted probably by high conducting material, is seen across the NWF while intermittent seismicity is revealed along the KMF. Scrutiny of earlier studies revealed that the 2001 main shock and first few weeks of aftershocks were hosted on another south-dipping fault, the South Wagad Fault (SWF), however, activity along the SWF is found to be meagre during 2006–2018. Instead, seismicity has currently migrated to faults north and south of the SWF. These currently active faults most likely were reactivated during/after the 2001 main shock.

The results of surface morphology and InSAR analyses carried out along a NNE-SSW oriented (~7 km-long) transfer fault are presented. This structure actively accommodates differential motion across two blocks in the western Sub-Himalayan belt of India. Segmentation is most pronounced in this sector of the accretionary wedge, where minimal climatic variations and a three-fold sub-division (Kangra Renetrant-Nahan Salient-Dehradun Reentrant) are well recognized. Segmentation is here clearly elucidated by variation in surface topography, geologic structure and recorded seismicity over the last few decades. However, the transition from one segment to another is poorly documented and understood. This article primarily documents surface topography and drainage anomalies within the transition zone between the Kangra Reentrant and the Nahan Salient through the study of space imageries spanning nearly six-decades (i.e. early 1960's to present). Geomorphic observations and InSAR analyses demonstrate that the segmentation is largely governed by lateral variation in geological structure, deformation patterns and mechanical stratigraphy and accommodated by transverse movement along the transfer fault. It should be noted that transfer faults commonly represent a privileged pathway for the major drainage and therefore surface manifestations are rarely preserved, e.g. Yamuna Tear, Ganga Tear etc. A rare instance of preserved surface topography associated with the recent activity along the Sabilpur transfer fault at the transition between the Kangra Renetrant and the Nahan Salient is documented here. Our investigations demonstrate that the Sabilpur Active Fault (SAF) accommodates differential tectonic convergence between the two blocks predominantly by left-lateral strike-slip kinematics with a component of uplift related to thrusting at HFT.

Shillong plateau in the North East India resembles a kind of inselberg, confined in between the great Hilamalayan mountain belts in the north and the Arakan-Yoma towards east. The study area is governed by a system of complex seismotectonic features responsible for the intense seismicity in the region, significantly the devastation aftermath the 1897 great Assam Earthquake (Ms~8.7). Spatio-temporal distribution of seismicity triggering inside the plateau from 1966 to 2013 suggests distinctly variable depth-ranges of seismicity zones within the crust. The depth section counterparts of the seismicity monitoring within Shillong plateau and its periphery infers the bottom of the seismogenic zone to lie at ~40km depth with an average depth of earthquake occurrence at 22.31km; besides, the study also dictates the dominant earthquake magnitude as ≤4.99Mw, wherein most of the earthquakes are confined within magnitude 3-5 Mw with an average magnitude 3.42 MW. The study reveals intense seismic activity in the central and western part of Shillong plateau owing to the conjecture of a number of active faults. The present study incorporates both qualitative as well as quantitative approach while understanding the types of recent seismicity pattern in the Shillong massif.

Ocean swell interacting with Antarctic ice shelves produces sustained (approximately, 2×106 cycles per year) gravity-elastic perturbations with deformation amplitudes near the ice front as large as tens to hundreds of nanostrain. This process is the most energetically excited during the austral summer, when sea ice-induced swell attenuation is at a minimum. A 2014–2017 deployment of broadband seismographs on the Ross Ice shelf, which included three stations sited, approximately, 2 km from the ice front, reveals prolific swell-associated triggering of discrete near-ice-front (magnitude≲0) seismic subevents, for which we identify three generic types. During some strong swell episodes, subevent timing becomes sufficiently phase-locked with swell excitation, to create prominent harmonic features in spectra calculated across sufficiently lengthy time windows via a Dirac comb effect, for which we articulate a theoretical development for randomized interevent times. These events are observable at near-front stations, have dominant frequency content between 0.5 and 20 Hz, and, in many cases, show highly repetitive waveforms. Matched filtering detection and analysis shows that events occur at a low-background rate during all swell states, but become particularly strongly excited during large amplitude swell at rates of up to many thousands per day. The superimposed elastic energy from swell-triggered sources illuminates the shelf interior as extensional (elastic plate) Lamb waves that are observable more than 100 km from the ice edge. Seismic swarms show threshold excitation and hysteresis with respect to rising and falling swell excitation. This behavior is consistent with repeated seismogenic fracture excitation and growth within a near-ice-front damage zone, encompassing fracture features seen in satellite imagery. A much smaller population of distinctly larger near-front seismic events, previously noted to be weakly associated with extended periods of swell perturbation, likely indicate calving or other larger-scale ice failures near the shelf front.

Observations of glacier slip over till beds, across a range of spatial and temporal scales, show abundant seismicity ranging from Mw∼−2 microearthquakes and tremor (submeter asperities and millisecond duration) to Mw∼7 slow-slip events (∼50 km rupture lengths and ∼30 min durations). A complete understanding of the mechanisms capable of producing seismic signals in these environments represents a strong constraint on bed conditions. In particular, there is a lack of experimental confirmation of velocity-weakening behavior of ice slipping on till, where friction decreases with increasing velocity—a necessity for nucleating seismic slip. To measure the frictional strength and stability of ice sliding against till, we performed a series of double-direct-shear experiments at controlled temperatures slightly above and below the ice melting point. Our results confirm velocity-strengthening ice–till slip at melting temperatures, as has been found in the few previous studies. We provide best-fit rate-and-state friction parameters and their standard deviations from averaging 13 experiments at equivalent conditions. We find evidence of similar velocity-strengthening behavior with 50% by volume debris-laden ice slid against till under the same conditions. In contrast, velocity-weakening and linear time-dependent healing of ice–till slip is present at temperatures slightly below the melting point, providing an experimentally supported mechanism for subglacial seismicity on soft-beds. The stability parameter (a−b) decreases with slip velocity, and evolution occurs over large (mm scale) displacements, suggesting that shear heating and melt buildup is responsible for the weakening. These measurements provide insight into subglacial stiffness in which seismicity of this type might be expected. We discuss glaciological circumstances pointing to potential field targets in which to test this frozen seismic asperity hypothesis.

Magma viscosity strongly controls the style (for example, explosive versus effusive) of a volcanic eruption and thus its hazard potential, but can only be measured during or after an eruption. The identification of precursors indicative of magma viscosity would enable forecasting of the eruption style and the scale of associated hazards1. The unanticipated May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai'i2 displayed exceptional chemical and thermal variability in erupted lavas, leading to unpredictable effusion rates and explosivity. Here, using an integrated analysis of seismicity and magma rheology, we show that the orientation of fault-plane solutions (which indicate a fault's orientation and sense of movement) for earthquakes preceding and accompanying the 2018 eruption indicate a 90-degree local stress-field rotation from background, a phenomenon previously observed only at high-viscosity eruptions3, and never before at Kīlauea4-8. Experimentally obtained viscosities for 2018 products and earlier lavas from the Pu'u 'Ō'ō vents tightly constrain the viscosity threshold required for local stress-field reorientation. We argue that rotated fault-plane solutions in earthquake swarms at Kīlauea and other volcanoes worldwide provide an early indication that unrest involves magma of heightened viscosity, and thus real-time monitoring of the orientations of fault-plane solutions could provide critical information about the style of an impending eruption. Furthermore, our results provide insight into the fundamental nature of coupled failure and flow in complex multiphase systems.

This study presents a procedure for compilation of earthquake catalog for southern Pakistan (spreading over Sindh and Balochistan provinces) and frontal offshore area of Arabian Sea. The active tectonic plate margins of Indian, Eurasian and Arabian Plates and their associated fault lineaments are dominant seismic sources in study area. These seismotectonic sources are extending beyond the terrestrial boundaries of southern Pakistan and diversified in their seismic potential, accumulation of stresses and earthquake generating mechanism. We have compiled earthquake datasets for pre-instrumental period (325 B.C. to 1900) and instrumental observations during January 1901 to August 2020. The seismological datasets are gathered from open source agencies including ISC, GCMT, USGS, PMD and published catalogs. Since earthquake magnitude has a crucial role in seismological studies, the catalog has been thoroughly processed for magnitude homogenization to Mw scale. We have developed magnitude conversion correlations for Mb and Ms to Mw by orthogonal and least square regression analyses. The comparison of these correlations with previously developed regression equations by other authors in similar environments helped us to determine the validity of derived correlations for the study area. The best correlated relations are used for converging multimagnitude types in compiled catalog to Mw scale. Further the catalog is processed for removal of foreshocks- and aftershocks by Reasenberg declustering algorithm. The declustered catalog is used for integrated analysis of various parameters such as completeness analysis in time and magnitude, histograms of depth and magnitude, estimation of GR parameters. It is observed that declustered catalog has completeness period (1960–2020) which is significant for appropriate seismicity analysis.

Hot dry rock (HDR) geothermal energy has become promising resources for relieving the energy crisis and global warming. The exploitation of HDR geothermal energy usually needs an enhanced geothermal system (EGS) with artificial fracture networks by hydraulic fracturing. Fault reactivation and seismicity induced by hydraulic fracturing raise a great challenge. In this paper, we investigated the characteristics of fault slip and seismicity by numerical simulation. The study was based on a hydraulic fracturing project in the geothermal field of Yishu fault zone in China. It revealed that fluid injection during hydraulic fracturing can cause the faults that exist beyond the fluid-pressurized region to slip and can even induce large seismic event. It was easier to cause felt earthquakes when hydraulic fracturing was carried out in different layers simultaneously. We also examined the effects of the location, permeability, and area of the fracturing region on fault slip and magnitude of the resulting events. The results of the study can provide some useful references for establishing HDR EGS in Yishu fault zone.

The Great Sumatran Fault (GSF) activity is a severe threat to Banda Aceh development as the capital city of Aceh Province, Indonesia. The earthquakes originating along this fault trace, despite generating low strength, considerably threaten infrastructure and human lives. Therefore, a detailed study of the GSF activity and presence becomes critical. In this paper, we applied the Global Gravity Model plus (GGMPlus) to map the subsurface structure and modeling of two GSF segments with a resolution of 200 m/px, namely the Aceh and Seulimeum segments toward the north of the Sumatran Island. The Bouguer anomaly data are inconsistent with the geology of the study areas, dominated by igneous rocks on the Aceh segment and volcanic rocks on the Seulimeum segment. Further, the contrast between the Seulimeum segment in the northeast and the Aceh segment in the southwest can be demonstrated by high-pass filtering. The GGMPlus data validation results with field measurements using the Scintrex CG-5 Autograv, the root mean square error obtained via data comparison are 12.32% in the Krueng Raya fault zone, and 26.1% at the Seulawah Agam Volcano area, respectively. We also performed 2D gravity data modeling along with the Aceh and Seulimeum segments in the NW–SE direction. This model was then compared with the geological cross section, seismicity, and magnetotelluric data. The results of Singular Value Decomposition and Occam inversion show three vertical blocks of high densities with an interspersion of lower densities, which can be confirmed as the Aceh and Seulimeum segments. Based on data processing, it can be concluded that the GGMPlus satellite can improve the maps and images of the northernmost GSF structure.

From November 2012 to May 2013, the coastal region of Bejaia (northeastern Algeria), experienced three distinct seismic sequences in about 6 months. The first one was on 28 November 2012 started with a mainshock of magnitude Mw = 5.1, and the second one occurred on 22 February 2013, a few kilometers southeast from the first one and culminated with a magnitude Mw = 4.3 earthquake. The last sequence comprised two shocks of moderate magnitude: a Mw 5.2 event on 19 May 2013, followed by a Mw 5.0 event on 26 May. Earthquakes were relocated using the double-difference method with differential times derived from phase-picked data and waveform cross-correlation delays. We compiled a dataset of 252 accurately relocated events that are associated with focal mechanism solutions consistent with a near-vertical, right-lateral strike-slip fault striking NW–SE. The data analysis is coherent with a new fault extending over 35 km from the Gulf of Bejaia to the northeast of Kherrata. This newly discovered active fault, we name it the Babors Transverse Fault, appears to be segmented in four segments (S1–S4) evidenced by separate earthquake clusters. Static Coulomb stress changes and the spatiotemporal evolution of seismicity suggest static stress triggering, with the rupture first activated in the S1 segment in the northwest, migrating to S2, and terminating in S3 and S4 in the southeast. Stress tensor inversion from focal mechanisms implies that the dominant local stress field is a pure strike-slip regime, with N–S maximum horizontal compression in good agreement with the velocity field displaying reorientation compared to the oblique convergence of Africa–Eurasia. In this study, the newly identified Babors Transverse and Tizi N’Berber–Darguinah faults, in association with the NNW–SSE offshore active fault of the Greater Kabylian block (in the north) and the NNW–SSE right-lateral strike-slip Lâalam fault (in the south), form an en echelon fault system that acts as a transfer zone between offshore thrust faulting and the Ghardimaou–North Constantine strike-slip fault.

Palaeoseismology studies the footprints of ancient earthquakes to improve the knowledge about the modern seismicity of the territory. A ground‐penetrating radar (GPR), among other geophysical methods, is used for quick determination of shallow stratigraphy – displaced, oblique layers within the fault zone. GPR data interpretation from diverse and complex reflection patterns of the fault zone heavily depends on the interpreter's experience. The range of different fault zone parameters in which this method can be successfully applied has not yet been investigated. We used a numerical simulation of GPR data to determine how GPR images the elements of faults (fault plane, hanging wall, footwall) in comparison with other reflections. Furthermore, we studied which parameters have the most significant impact on GPR wave patterns. We performed a series of numerical models of a fault, changing its geometry with increasing complexity from elementary models to realistic ones. The resulting synthetic profiles allowed finding specific GPR signatures from the fault plane, the hanging wall and the footwall. We collected field GPR data from two different fault zones and examined them for verification.

We propose a new algorithm, focusing on spatial amplitude patterns, to automatically detect volcano seismic events from continuous waveforms. Candidate seismic events are detected based on signal-to-noise ratios. The algorithm then utilizes supervised machine learning to classify the existing candidate events into true and false categories. The input learning data are the ratios of the number of time samples with amplitudes greater than the background noise level at 1 s intervals (large amplitude ratios) given at every station site, and a manual classification table in which ‘true’ or ‘false’ flags are assigned to candidate events. A two-step approach is implemented in our procedure. First, using the large amplitude ratios at all stations, a neural network model representing a continuous spatial distribution of large amplitude probabilities is investigated at 1 s intervals. Second, several features are extracted from these spatial distributions, and a relation between the features and classification to true and false events is learned by a support vector machine. This two-step approach is essential to account for temporal loss of data, or station installation, movement, or removal. We evaluated the algorithm using data from Mt. Ontake, Japan, during the first ten days of a dense observation trial in the summit region (2017 November 1–10). Results showed a classification accuracy of more than 97 per cent., Online Published: 22 December 2020

Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower crust. The mainshock triggered a significant increase in the number of small-magnitude events spread over an unusually large area in the ECAB. The paper’s goal is to compute the source parameters of the earthquakes that occurred during the aforementioned sequence, by empirical application of Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple methods and seismicity evolution at regional scale is investigated. Our results highlight a still active deformation regime at the edge of the EE Craton, while the source parameters reveal a complex fracture of the mainshock and a very high-stress drop.

Recent efforts to characterize small (Mw

The Biga Peninsula, which has the most complex tectonic segments of the northwestern Aegean region, shows a remarkable dense seismicity. The earthquake hazard analysis of medium and large earthquakes occurred at differ-ent periods for the peninsula and its vicinity is the basis of this study. The basic earthquake parameters those are generally known as a and b-values, were determined, and evaluated for the earthquakes M> 2 during the instrumental period earthquake catalog, which is the database of this study. A homogenous earthquake catalog combined of two institutions COMU and KOERI data, which are quite dense and have high solution quality, play a remarkable role in this study. It was observed that the dense seismic activity after the earthquake that initiated in the event of Mw = 5.4 on February 6, 2017, especially in Ayvacik and its close vicinity, and which has been continuing, was clustered around Tuzla deformation zone in an area of approximately 25x10 km. Probabilistic parameters including b-value which can be related with local tectonic structures and stress states before and after this earthquake were also evaluated. In the short and long term, the decreasing and increasing tendencies of these parameters before and after the main shock were examined spatially and temporally in addition to local and regional dominant stress-es, heterogeneity in the upper crust and the contribution of the geothermal resources to the seismicity and seism tectonic structure. Considering the spatial density of earthquakes towards to Saroz-Ayvacik-Edremit, the pre-earthquake b-values increased inversely with the stresses, and then this decline in behavior was distinguished in the 2014 and 2017 earthquake sequences