Hypocenter Research Articles

An Iterative Linear Method with Variable Shear Stress Magnitudes for Estimating the Stress Tensor from Earthquake Focal Mechanism Data: Method and Examples

ABSTRACT Earthquake focal mechanism data provide information about the stress state at the origin of earthquakes. The inversion methods that are commonly used to infer the stress tensor from focal mechanisms have varying complexity but always rely on a number of assumptions. We present an iterative method built upon a classic linear stress tensor inversion that allows for relaxing the assumption on shear stress magnitudes while preserving the computational simplicity of the linear problem. Every iteration of our method computes the least-squares solution of the problem, which makes the method fast enough to estimate the inverted parameter errors with nonparametric resampling methods such as bootstrapping. Following previous studies, this method removes the fault plane ambiguity in focal mechanism data by selecting the nodal plane that best satisfies the Mohr–Coulomb failure criterion. We first test the performance and robustness to noise of the proposed method on synthetic data sets and then apply it to data from the Southern California and Geysers geothermal field data sets. We focus the study on investigating the consequences of relaxing the assumption of constant shear stress magnitudes. Our variable shear method successfully generalizes its constant shear counterpart: it is able to perform similarly when the constant shear assumption is a good approximation and provides more accurate results when it is not. We provide the Python package iterative linear stress inversion to implement the proposed method.

Hindu Kush intermediate-depth seismicity: A possible explanation for the preferential asymmetry displayed by river catchments within the corresponding epicentral area

Some conceptual models proposed for explaining the Hindu Kush intermediate-depth seismicity assumed that a subhorizontal fracture was migrating laterally, from east toward west, along the strike of a subducted tectonic plate. The present study has identified a distinct cluster of earthquake hypocenters, likely corresponding to the advancing tip of the subhorizontal fracture. Those hypocenters belonged to a set of fairly moderate earthquakes (5.6 ≤ M ≤ 6.5), which had not been, so far, explicitly addressed in the Hindu Kush seismo-tectonic interpretations, and which were proven to be virtually the only events displaying a failure regime characteristic to a laterally propagating breakoff – namely, reverse-fault focal mechanisms with subhorizontal compression axes parallel to the slab strike. A separate, upper-positioned hypocenter cluster was noticed to include all the Mw ≥ 7 intermediate-depth earthquakes: it was conjectured, relying on a previously published numerical model, that those strong events could be triggered by upward transfer of the viscoelastic stress induced by the slab pull increase caused by an episode of lateral breakoff advance. At the same time, as the inferred breakoff fracture enables the deeper segment of the plate to detach from the shallower segment, the plate region situated above the severed region is relieved of the gravitational pull that was imposed by the deeper, and currently detached, plate segment. In response to the resulting crustal rebound, the surface topography is expected to exhibit down-tilting to the west. Such an anticipation seems to be confirmed by the catchments of several streams which flow roughly perpendicular to the intermediate-depth earthquakes epicentral domain: relevant geomorphic indices (the swath profile average and maximum elevation curves; the transverse hypsometric integral – THi; the transverse topographic symmetry factor – T-vector) computed for the concerned valleys consistently suggested larger uplift rates in the east, as compared to those manifest further to the west.

Earthquake Focal Mechanisms and Geodynamic Modeling of the North and Central Caucasus

Earthquake Focal Mechanisms and Geodynamic Modeling of the North and Central Caucasus

Geophysical monitoring of the geodynamic regime of Central Armenia

Geophysical monitoring of the geodynamic regime of the territory of Armenia is carried out including observations of the chemical composition of groundwater, changes in the stress-strained state of the earth’s crust and current seismicity. Hydrogeochemical effects of changing the stress-strained state of the medium as indicators of modern geodynamic processes have been revealed. Based on published data, the authors suggest that the nature of the manifestation of geochemical anomalies depends on the parameters of the forthcoming seismic event – magnitude (M) and epicentral distance (Δ). A map of the spatial-temporal distribution of deformations of the earth’s crust because of earthquakes over the past three years in the region has been compiled. The main components of ionic (Cl–, HCO3, SO4, Mg2 +, total mineralization) and gas (CO2) composition of mineral waters in comparison with seismicity in the earth’s crust are studied. A map of the distribution of earthquake epicenters in space and in time has been compiled. The obtained graph shows the hypocenters dependence of magnitude of earthquakes that occurred during the investigated period on the depth of hypocenters. Earthquake catalogue is using to plot earthquake recurrence. Statistical analysis of observational data showed that the most informative predictive characteristic is the dispersion of carbon dioxide content in water. According to the time series of concentrations of chemical indicators, the effects associated with the geodynamic regime of the earth’s crust of Central Armenia were determined. A map of the distribution of deformations in the earth’s crust of central Armenia was compiling taking into account the depths of the hypocenters of earthquakes.

Geologic map of Slate Range Crossing area, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex Neogene–Quaternary deformation in the Slate Range Crossing area (California, USA) of the active dextral transtension of the Death Valley region and Walker Lane belt. This map integrates the late Cenozoic structures and geologic units with the Mesozoic geologic units and deformation as a data set to examine the controls on reactivation of older structures. These geologic data were collected to study pre-, syn-, and post-kinematic rocks to examine the deformation history of the area and to find palinspastic markers to examine the late Cenozoic fault displacement and displacement history across Panamint Valley to the east, as reported in Andrew and Walker (2009). The study focused on defining the Miocene and Pliocene rocks and deposits and examining lateral changes and depositional sources of clasts. There are two different volcanic-sedimentary sequences in this area. A Miocene section contains mafic to felsic volcanic units, numerous debris-flow to laharic deposits, and several associated conglomerates and breccias containing exotic clasts. The exotic clasts are matched to rocks in the Panamint Range on the east side of Panamint Valley as reported in Andrew and Walker (2009) as displacement vectors for palinspastic reconstructions. These Miocene strata ubiquitously dip eastward 20–40º. A younger volcanic-sedimentary sequence contains relatively thin mafic lava flows and associated locally derived, coarse-grained mass wasting deposits. These younger basaltic lavas generally have gentle dipping lava flow features and foliation. Numerous faults cut the different age deposits allowing a chronology of Neogene to Quaternary faulting; additionally, there are numerous fabrics associated with Jurassic contraction and Cretaceous(?) dextral shear. The area near Slate Range Crossing has a conspicuous zone of earthquake foci; this study found that some of this seismic activity coincides with a zone of southwest-striking, moderately dipping to the north, sinistral-oblique normal faults, which cut across the northernmost Slate Range. These faults form a structural boundary between the Argus and Slate Ranges and link the fault networks in Panamint Valley with those in Searles Valley. This mapping and structural data demonstrate the two-stage Neogene fault history of the Walker Lane belt deformation in this area and show that regional tilting of rocks occurred after ca. 13 Ma and before ca. 4 Ma; this eastward down-tilting appears to be a discrete event and may mark the change from extension to transtension. This detailed geologic mapping and collection of structural data for the rocks in the eastern Argus and northern Slate Ranges and Panamint Valley were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer. This map is a simplification of detailed geologic mapping data collected at 1:6000 scale and reduced to 1:20000 scale. Structural data includes kinematic and relative timing of deformation information.

Bayesian-based joint determination of earthquake hypocentres and 1-D velocity structures divided by a structural boundary

SUMMARY We propose a Bayesian approach for jointly determining the locations of earthquake hypocentres and velocity structures in a region divided by a structural boundary, such as a fault zone. To accommodate the Markov chain Monte Carlo (MCMC) method in a region containing lateral velocity contrast, we incorporated a data-driven station clustering scheme when estimating earthquake hypocentre locations, a 1-D velocity model assigned to each cluster and station corrections for traveltimes. The proposed method realizes the clustering of stations reflecting velocity structures beneath the station network in the framework of the MCMC method. We verified the performances of the proposed method using numerical experiments with different distributions of earthquake hypocentres relative to the structural boundary. The results demonstrated that the proposed method reconstructed earthquake hypocentre locations and velocity structures involving lateral contrast robustly to earthquake distributions. Additionally, the number of 1-D velocity models (the number of station clusters) was optimized by comparing resultant posterior probability proportional to distances between the observed and theoretical traveltimes. We also applied the proposed method to estimate the earthquake hypocentre locations and velocity structures around the source area of the 2004 Mid-Niigata prefecture earthquake, Japan, to examine the applicability of the proposed method to real observation data. The stations were divided into east and west clusters, coinciding with the lateral velocity variation related to the geological setting. Therefore, the proposed method accurately determines earthquake hypocentres in a region divided by a structural boundary without any knowledge regarding the geological structure.

Determination of coordinates of the earthquake hypocenter with a known structure of the earth's crust

Objective. The aim of the study is to develop methods for determining the coordinates of the hypocenter of an earthquake with a known structure of the earth's crust. Method. To solve the problem, the brute force method, the confidence interval, the comparative method and the enumeration of possible options on the grid are used. Result. A method has been developed for determining the coordinates of an earthquake source with a known structure of the earth's crust using points of various figures of the n-th order as geometric locations in calculations. Conclusion. The proposed method, with the known structure of the earth's crust, makes it possible to use all known methods that require information on the travel time differences of seismic waves to spaced seismic sensors and the difference in travel times of longitudinal and transverse seismic waves to the seismic sensor for calculating the coordinates of the earthquake hypocenter. The accuracy of determining the coordinates of the earthquake hypocenter depends on the quality of determining the structure of the earth's crust on which the seismic event was observed.

A study of the properties of the tectonic structure of the Kryvyi Rih city based on statistical analysis of seismicity

The article concerns results of studying two significant seismic events — an explosion (23.09.2020 09:00) and an earthquake (30.09.2020 20:00), which occurred in the area of the city of Kryvyi Rih. There were developed algorithms to process records by the Kryvyi Rih seismic station. There was done a comprehensive interpretation of the seismologic and geological-geophysical data. The seismic events’ coordinates were determined, the quality of seismological data processing was evaluated based on the expert method, the level of trustworthiness of the resulting parameters was estimated. The nature of the eathquake’s origin was analyzed based on the shape and the records’. The structure of the mechanism of the earthquake center shows that this was a natural seismic event of the induced type. The article also provides results of geotectonics study of the region of its origin and shows the connection of the geological structures with the epicenter’s parameters. The parameters of the stress-strain state of the crust in the Kryvyi Rih iron-ore basin are calculated, and the parameters of the epicenter of the induced earthquake (2020.09.30 20:00:40.836, UTC) are established. The experimentally calculated depth of the epicenter of local earthquake coincides with the hypothetical depth of 15 km at the lower edge of faults according to the model of the distribution of the depths of the earthquake hypocenters on the Eastern European platform. The proposed methods and algorithms open new opportunities for modern geophysical research in the long-term seismic hazard assessment of Kryvyi Rih. Meanwhile, for a more precise determination of the place, time and nature of local seismic events in the area it is necessary to develop, here and on adjacent territories, a network of seismic stations capable of recording events of comparable and lesser magnitude.

Reconstruction of seismotectonic deformations of Asia in different layers of the Earth’s crust

The calculation of seismotectonic deformations for different depth levels 1–15, 16–35, 36–70 km was performed according to the data of 1819 mechanisms of earthquake foci that occurred in Central Asia (φ = 25° – 60° N, λ=60° – 115° E) for the period from 1976 to the end of July 2020 with M>4.7. The orientation of the main axes of the strain tensor reconstructed from the mechanisms of earthquake foci with M>4.7 coincide at different depth levels with mainly submeridional and north-eastern shortening and varying elongation from sublatitude to north-western and near-vertical. The consistency of the orientation of the main axes of shortening and elongation reconstructed from seismological materials and from the published results of calculating GPS observations, is traced.

Self-Excitation of the Earthquakes

Numerous correlations between magnetic and seismic events unambiguously indicate that the magnetic control of the earthquakes is a fundamental phenomenon. It proceeds from the remarkable physics of magneto-plasticity of solids, which implies acceleration of dislocations by microwaves. The motion of dislocations provides release of dangerous elastic energy of the earthquake focus and transforms elastic energy into the safe energy of plastic deformation. Magneto-plasticity seems to be the most important mechanism of the magnetic control because the piezoelectric effect as a suggested mechanism of magnetic control should be excluded (Chelidze et al.). Magnetic control certifies earthquake focus as a receiver of microwaves; on the other side, numerous observations exhibit emission of microwaves generated by earthquake focus, so that it can be considered as a permanent generator of microwaves. The idea of this paper is to offer a mechanism of self-excitation of the focus: self-triggering is suggested to be induced by microwaves generated by earthquake focus itself. The more intensive is the crack formation, the higher is the density of microwaves, which accelerate dislocations and intensify crack formation: it is a feedback breeding, avalanche-like process. Both functions of earthquake focus, to be simultaneously generator and receiver of microwaves, are integrated into the same space and time. It excludes such limitation of the magnetic control as the penetrability of the rocks for microwaves.

Grid-characteristic method on Chimera meshes using a single background grid with varying elastic properties

Nowadays, traditional hydrocarbon reserves are depleted. The search for and production of oil and gas in hard-to-reach fields require the development of appropriate computing technologies. Therefore, there is a need to develop novel high-precision computational methods that make it possible to more accurately take into account the features of the geological model, and at the same time save computational resources. In this paper, we have proposed a novel modification of the grid-characteristic method on Chimera grids to take into account the complex geometry of the interfaces of geological rocks. This modification differs from those proposed earlier by using a single background grid with varying elastic parameters of the medium and imposes fewer restrictions on the geological model than the previously proposed options. The computational method proposed in the paper is applicable not only for modeling seismic waves in geological media, but also for modeling ultrasonic and sound study, non-destructive testing of various objects, and in medicine. Also, the proposed numerical method can be applied for a more accurate calculation of seismic waves from earthquake hypocenters and man-made sources.

Fluid Saturation in the Lithosphere of the Altai-Sayan Folded Region according to Magnetotelluric Data

Abstract —A model of deep electrical conductivity of the Altai–Sayan folded region is presented, which was compiled from the results of three-dimensional inversion of the invariant values of the impedance matrix and interactive selection of three-dimensional model curves of magnetotelluric sounding to experimental ones. The obtained electrical resistivity values were used to estimate the fluid saturation of the Earth’s crust by applying the Shankland–Waff equation. The NaCl salinity of the aqueous solution is taken equal to 170 g/L, for which the fluid saturation is most consistent with the seismic data. The electrical conductivity and fluid saturation of focal zones of earthquakes, activated blocks of the region and deep faults were studied. Most of the earthquake hypocenters are located above the top of the conductors and near deep low-resistivity faults. The position of low-resistivity anomalies correlates with the position of domains with the enhanced attenuation of converted earthquake waves and reduced velocities of compressional waves. The Teelin, Samagaltai, and Kaa-Khem earthquake sources are characterized by the maximum fluid content (0.5–0.9%), and the Altai, Shapshal, Shagonar and Bolsheporoshskii sources, by minimum ones (0.1–0.2%). The fluid saturation of deep faults reaches 1.2%. The faults characterized by tensile stresses oriented orthogonally to their strike, show the highest fluid content. A similar pattern is observed for the crustal blocks located beneath depressions. The high fluid content beneath the Kyzyl Basin and its surroundings correlate with the major compressional-wave velocity deficit beneath it.

A Detailed Earthquake Catalog for Banda Arc–Australian Plate Collision Zone Using Machine-Learning Phase Picker and an Automated Workflow

Abstract The tectonic setting of Timor–Leste and Eastern Indonesia comprises of a complex transition from oceanic lithosphere subduction to arc-continental collision. To better understand the deformation and convergent-zone structure of the region, we derive a new catalog of earthquake hypocenters and magnitudes from a temporary deployment of five years of continuous seismic data using an automated processing procedure. This includes a machine-learning phase picker, EQTransformer, and a sequential earthquake association and location workflow. We detect and locate ∼19,000 events during 2014–2018, which demonstrates that it is possible to characterize earthquake sequences from raw seismic data using a well-trained machine-learning picker for a complex convergent plate setting. This study provides the most complete catalog available for the region for the duration of the temporary deployment, which includes a complex pattern of crustal events across the collision zone and into the back-arc, as well as abundant deep slab seismicity.

ГЕОДИНАМІКА

The purpose of the work is to study the features of seismotectonics of the junction zone of the Oash and Transcarpathian faults in the Ukrainian Transcarpathians. The research methodology combines a complex analysis of geological-tectonic, seismological, geomorphological and geodetic data on the studied area. For the clarification of coordinates and depths of local earthquake foci we applied methods of their hypocenters specification, using a calculated seismological hodograph and kinematic corrections. Geological and geophysical data, in particular on regional profiles, were used to link seismic events to specific geological structures. It was established that a number of seismic events were recorded at depths of 40-52 km in the studied zone, in its northeastern part. They occurred simultaneously with traditional deep localization of seismic sources in the crust of the Transcarpathian depression (0-27 km deep), including at the depths of the sole of thrust sedimentary strata / roof of the pre-Neogene basement of the Transcarpathian depression (2-3 km deep) in the southwestern part of the studied area. Since these earthquakes are spatially localized to the northeast of the zone where the Transcarpathian deep fault is traced by geological and geomorphological data on the surface, they indicate the subduction of the crustal structures of the Transcarpathian trough in this direction under the thrusts of the Folded Carpathians. This is also evidenced by the corresponding slope of the seismofocal zone at the intersection of the Transcarpathian Deep Fault at greater (15-30 km) depths. These and other features of local seismotectonics reflect the so-called "crocodile" tectonics in the Ukrainian Carpathians. The features encompass the reverse, south-west slope of the seismofocal zone at shallower (0-12 km) depths, as well as the features of the relief of the Carpathians in the studied seismogenic zone. They fully correspond to the characteristics of the "alpine" and "terrain" geodynamics of the region, i.e. compression and displacement in the northeastern and eastern directions, respectively. For the first time, the research established the presence of relatively deeper (30-55 km) seismic activity in the earth's crust of the Ukrainian Transcarpathians – in the zone of subduction of the Moho border under the Carpathians in the area of junction of the Transcarpathian and Oash deep faults. Additionally, the geodynamic and tectonophysical justification for its presence was given in terms of combination of "alpine", terrane and asthenolitic geodynamics of the region. Taking into account the features of seismotectonics of the Oash and Transcarpathian faults junction zone will contribute to clarifying the assessment of the characteristics and peculiarities of the spatial distribution of natural geoecological, in particular, seismotectonic risks and hazards in the central part of the Ukrainian Transcarpathians.

The interrelationship between electrical resistivity and VP/VS ratio: A novel approach to constrain the subsurface resistivity structure in data gap areas in a seismogenic zone

Large man-made water reservoirs promote fluid diffusion and cause critically stressed fault zones underneath to trigger earthquakes. Electrical resistivity is a crucial property to investigate such fluid-filled fault zones. Therefore, we carry out magnetotelluric (MT) investigation to explore an intraplate earthquake zone, which is related to artificial reservoir-triggered seismicity. However, due to surface access restrictions, our data set has a gap in coverage in the middle part of the study area. This data gap region coincides with the earthquake hypocenter distribution in that intraplate earthquake zone. Therefore, it is vital to fill the data gap to obtain the electrical signature of the active seismic zone. To compensate for the data gap, we have developed a relation that connects resistivity with the ratio of seismic P- to S-wave velocity ([Formula: see text]). Using this relation, we estimate a priori resistivity distribution in the data gap region from known [Formula: see text] values during the inversion to compensate for the data gap. A comparison study of the root-mean-square misfits of inversion outputs (with and without data gap filled) proves the effectiveness of the established relation. The inversion output obtained using the established relation brings out fault signatures in the data gap region. To examine the reliability and accuracy of these fault signatures, we occupy a portion of the data gap with new MT sites. We compare the inversion output from this new setup with the inversion output obtained from the established relation and observe that the electrical signatures in both outputs are spatially correlated. Furthermore, a synthetic test on a similar earth model establishes the credibility and robustness of the derived relation.

SEISMICITY of the TERRITORY of BELARUS in 2015

An annual review of the seismicity in the territory of Belarus based on the data of two analog (operated in the first half-year) and seventeen digital stations is presented. A total of 80 events with Кd=4.6–8.4 were recorded all of them being confined to the southern part of the territory, the Soligorsk mining area included. The map of all the event epicenters for 2015 is given. The table of the distribution of the seismic events by their energy classes and seismic energy in months is presented. The maximum values of the seismic energy release fell in August, and the maximum number of the events was observed in July. The level of the seismic energy released in 2015 is the same as in 2014 but 2.18 times lower than its long-time average value for 1983–2014. The number of the events in 2015 is 1.4 times more than their number in 2014 and 1.86 times more than the Nср value for the previous 32 years. The distribution of the earthquakes in the depth intervals layers showed that the earthquake foci are mostly located in the upper 20 km part of the Earth’s crust. However, the foci of 47 earthquakes are located at depths above 10 km. The distribution of all the events in 2015 is represented in real-time. The quiet seismic periods and seismic activation periods were determined. The distribution of the seismic events in the hourly intervals showed the periods of the increase of the seismic events number. The maximum and minimum values N in the seismic event distribution by the days of the week were determined.

MUYAKAN EARTHQUAKE SEQUENCE in 2015 (Northern Baikal Region)

We present the results of studies of the seismic regime, focal mechanisms, and macroseismic data in the area of the largest Muyakan activation in 2015 (northern Baikal region). Due to the deployment of a network of temporary seismic stations, the number of registered earthquakes (KR≥3) increased significantly in 2015 and reached  30 thousand. Spatio-temporal development of the considered activation is characterized by stable low values of earthquake hypocenters and dividing the epicentral field into two clusters – eastern and north-western ones. Both clusters are connected with local stress-strain field (rift type and strike-slip, respectively), while the general regime of seismotectonic deformations of the crust in the activation area, calculated from the statistical analysis of 77 focal mechanisms of Muyakan earthquakes (KR≥9.2), demonstrates the predominance of submeri-dional horizontal extension. Macroseismic effects from the largest earthquakes of the Muyakan sequence were felt, predominantly, in Severomuysk settlement (=10–15 km, I=5). New data on strong motions, obtained from the records of the seismic station with the same name, have significantly complemented the database for the territory of the north-eastern flank of the Baikal rift zone. In general, the obtained results could be used to clarify the seismic hazard of the considered area and to improve the instrumental part of seismic scales.

A Far-Field Ground-Motion Model for the North Australian Craton from Plate-Margin Earthquakes

ABSTRACT The Australian territory is just over 400 km from an active convergent plate margin with the collision of the Sunda–Banda Arc with the Precambrian and Palaeozoic Australian continental crust. Seismic energy from earthquakes in the northern Australian plate-margin region are channeled efficiently through the low-attenuation North Australian craton (NAC), with moderate-sized (Mw≥5.0) earthquakes in the Banda Sea commonly felt in northern Australia. A far-field ground-motion model (GMM) has been developed for use in seismic hazard studies for sites located within the NAC. The model is applicable for hypocentral distances of approximately 500–1500 km and magnitudes up to Mw 8.0. The GMM provides coefficients for peak ground acceleration, peak ground velocity, and 5%-damped pseudospectral acceleration at 20 oscillator periods from 0.1 to 10 s. A strong hypocentral depth dependence is observed in empirical data, with earthquakes occurring at depths of 100–200 km demonstrating larger amplitudes for short-period ground motions than events with shallower hypocenters. The depth dependence of ground motion diminishes with longer spectral periods, suggesting that the relatively larger ground motions for deeper earthquake hypocenters may be due to more compact ruptures producing higher stress drops at depth. Compared with the mean Next Generation Attenuation-East GMM developed for the central and eastern United States (which is applicable for a similar distance range), the NAC GMM demonstrates significantly higher short-period ground motion for Banda Sea events, transitioning to lower relative accelerations for longer period ground motions.

Mantle earthquakes in the Crimea-Black Sea-Caucasus regions

The question of the existence of foci of deep earthquakes in the region of the Crimea-Black Sea-Caucasus is extremely important from the point of view of the geodynamics of the region. Previously it was thought that only crustal earthquakes could occur in this region. Recently, results have been obtained that show that earthquakes with depths of at least 300 km occur in this region. The article discusses the question of how plausible these results are and why they were not obtained earlier. Seven specific examples of the ambiguous determination of the depth of earthquake hypocenters in the Crimea-Black Sea-Caucasus region are considered. These examples clearly show that determining the coordinates of earthquake hypocenters using algorithms based on the Geiger method does not allow one to uniquely determine the depth of the hypocenters. The article gives an idea of the authors about the origin of mantle earthquakes in the Caucasian and Crimean-Black Sea regions. For the Caucasus region, mantle earthquakes are associated with two reasons: submersion of the lithospheric layer; in the asthenospheric layer, represented in the seismotomographic sections by a low-velocity anomaly, the nature of earthquake foci is associated with fluids formed during phase transition reactions. In the Crimean-Black Sea region, earthquake foci are located in the lithosphere layer, and the sliding of the lithosphere along the less viscous underlying layer of the upper mantle causes tectonic movements in the lithosphere accompanied by earthquakes. In addition, to determine the coordinates of the hypocenters of the Crimean and Caucasian earthquakes during routine processing, hodographs were used for depths not exceeding 35 km for the Crimea and 50 km for the Caucasus and 150 for the North Caucasus. This circumstance is the main reason why deep earthquakes could not be detected.

Pore Fluid Pressure Imaging of the Mt. Pollino Region (Southern Italy) From Earthquake Focal Mechanisms

AbstractFocal mechanisms of selected earthquakes, recorded in the Mount Pollino region (southern Italy) from 2010 through 2014, are used to infer the pore fluid pressure at hypocenter depths. The 3‐D excess pore pressure field provides evidence that the sequence occurs in a fluid‐filled volume with values reaching 35 MPa. The mechanisms underlying this swarm‐like sequence and the triggering of earthquakes are investigated by computing the cumulative static Coulomb stress change at hypocenter depths and analyzing the pore‐pressure diffusion mechanism. The results indicate that static Coulomb stress change was lower than 0.01 MPa, which is the value generally assumed as threshold for the triggering, and seismicity distribution was actually driven by pore‐pressure diffusion with relatively low diffusivity value. This latter mechanism could also explain the delayed triggering of the two larger eventsML4.3 andML5.0, respectively, that occurred about 150 days apart.