Earthquake Source Mechanisms Research Articles

Сейсмотектонические деформации Алтае-Саянской горной области. Часть 1

Seismotectonic deformations (STD) of the Earth’s crust in the Altai-Sayan mountain region were studied. The STD calculation was performed on the basis of the approaches proposed in the works of Yu.V. Riznichenko and S.L. Yunga. Estimation of seismicity distribution and calculation of the average annual STD velocity (STD intensity) IƩ were made on the basis of the catalog of earthquakes that occurred in 1997–2020 (15 669 seismic events). Areas of manifestation of intense seismotectonic deformations and seismic activity are identified. A high level of seismicity and the average annual STD velocity (10–7 year–1) is noted in the areas where strong seismic events occurred (Chuya earthquake on September 27, 2003, Tuva earthquakes on December 27, 2011 and February 26, 2012). The study of STD directionality is based on data on the focal mechanisms of earthquake sources (591 events) that occurred from 1963 to 2021. The classification of STD modes was used to construct the STD map. According to the STD maps, the direction of the shortening axes was determined, which changes from west to east from northwest to northeast. The study area is characterized by a variety of deformation conditions: compression, transpression, strike-slip, transtension, tension, etc. Based on the averaged strain tensors, the distributions of the Lode–Nadai coefficient, angle of generalized plane strain, and vertical component are calculated and plotted. The zones where various modes of deformation, such as simple compression, the predominance of simple compression, shear, the predominance of simple tension and simple tension are manifested, are distinguished in the study area. Both uplift and subsidence of the Earth’s crust are noted in the study area depending on the deformation mode.

Modern geodynamics and seismicity of the Azerbaijani sector of the Caspian Sea

The region’s oil fields are located within the South Caspian oil and gas basin, on the territory of the Absheron Peninsula and the adjacent waters of the Caspian Sea. Here are more than 80 oil and gas fields. To this end, this article presents the geological structure of the South Caspian Depression, as well as the Absheron-Balkhan zone. The analysis of oil and gas content and modern seismicity of the Caspian Sea is carried out. Increasing recoverable oil reserves, reducing water cut, increasing or even stabilizing production at this stage is the number one task for the oil industry. It has been established that in recent years, the level of seismic activity of individual sections of the Caspian water area has increased, and the amount of seismic energy released in the Central Caspian has increased by several dozen times, it can be assumed that the change in oil production in many offshore fields in the northern part of the Absheron-Balkhan fold system is associated with strong (ml>3.0) earthquakes characterized by fault-shear type of movements. Keywords: Caspian sea; oil and gas deposits; earthquake source mechanisms.

Results of local seismological monitoring in the Lena delta region

Refinement of the geodynamic situation in the area of the Lena River delta is relevant for understanding local geological processes. Previously, source seismology for this area was provided only by remote regional seismic stations. This paper shows the results of processing data from detailed seismological observations for 2018–2021. The distribution of earthquake hypocenters and their focal mechanisms are presented. The results of processing largely confirm the a priori geological data and do not contradict the results of regional observations. The accumulation of earthquakes in the area of the Kharaulakh Range most likely refers to the Primorsky Fault, which extends along the coast of the Bykovskaya channel. According to geological studies, the Primorsky Fault is a fault, which is confirmed by the focal mechanisms of earthquake sources in this area.

Stress State of the Earth’s Crust, Seismicity, and Prospects for Long-Term Forecast of Strong Earthquakes in Uzbekistan

Abstract —The current stress state of the Earth’s crust on the territory of Uzbekistan is studied using algorithms from the cataclastic analysis of displacements (CAD) with the application of a combined catalog of earthquake source mechanisms. The axis of maximum compression is nearly horizontal and orthogonal to the strike of tectonic structures for the larger part of the area under study, and the angle of penetration of the axis of minimum compression varies greatly for different parts of the territory. The areal distribution of the Lode–Nadai coefficient indicates the predominance of a stress state close to pure shift for the area under study. Without differentiation of the seismically active bed by depth, almost the entire territory of Uzbekistan is characterized by a geodynamic stress state corresponding to horizontal compression. There are significant differences in the stress state parameters for different deep beds of the Earth’s crust. With account for the representative recording periods of seismic events of different energy levels, the earthquake recurrence parameters for the entire territory as a whole and large seismically active zones are determined. It has been established that the epicenters of strong earthquakes recorded since the historical time are grouped in compact areas with linear dimensions of 50–80 km. The tectonophysical interpretation of the selected areas is given within the framework of the CAD, in which it is noted that strong earthquakes occur mainly in areas of lower effective confining pressure and maximum tangent stresses. This is due to the fact that the values of friction forces at faults in such areas are low, which creates the most favorable conditions for large-scale destruction. According to the results of the reconstruction of the natural stresses of the CAD, such areas on the territory of Uzbekistan are identified within faults and flexural-fault zones in South Fergana and North Fergana, as well as in the Gazli region. The current seismological situation in the selected areas of long-term forecast is estimated using a set of prognostic parameters of the seismic regime, and a map of the areas of expected seismic activity is compiled. The analysis of previous forecast maps developed within the framework of the presented approach shows high value in their information.

Possible Seismic Source Mechanism of the Catastrophic Tsunamigenic Earthquake on May 9, 1877 in Northwestern Chile

Possible Seismic Source Mechanism of the Catastrophic Tsunamigenic Earthquake on May 9, 1877 in Northwestern Chile

Sub‐Decadal Volcanic Tsunamis Due To Submarine Trapdoor Faulting at Sumisu Caldera in the Izu–Bonin Arc

AbstractThe main cause of tsunamis is large subduction zone earthquakes with seismic magnitudes Mw > 7, but submarine volcanic processes can also generate tsunamis. At the submarine Sumisu caldera in the Izu–Bonin arc, moderate‐sized earthquakes with Mw < 6 occur almost once a decade and cause meter‐scale tsunamis. The source mechanism of the volcanic earthquakes is poorly understood. Here we use tsunami and seismic data from the recent 2015 event to show that abrupt uplift of the submarine caldera, with a large brittle rupture of the ring fault system due to overpressure in its magma reservoir, caused the earthquake and tsunami. This submarine trapdoor faulting mechanism can efficiently generate tsunamis due to large vertical seafloor displacements, but it inefficiently radiates long‐period seismic waves. Similar seismic radiation patterns and tsunami waveforms due to repeated earthquakes indicate that continuous magma supply into the caldera induces quasi‐regular trapdoor faulting. This mechanism of tsunami generation by submarine trapdoor faulting underscores the need to monitor submarine calderas for robust assessment of tsunami hazards.

Characteristics of Seismogenic Structures and 3D Stress State of the Delaware Basin of West Texas as Constrained by Earthquake Source Mechanisms

Abstract Seismicity in the Delaware basin is suggested to be associated with oilfield operations. To better characterize the seismogenic structures revealed by the induced seismicity, in this study we determined source mechanisms for the Delaware basin of Texas and leveraged the obtained source mechanisms to perform stress inversion for evaluating the region’s stress state. Based on the seismogenic patterns and seismicity distribution, we identified seven distinctive seismogenic zones. Within each zone, earthquakes form several parallel-trending linear clusters. Most notably, there is an observable change in the seismicity trend on either side of the basin-bisecting Grisham fault zone. In addition, the extent of hypocentral depths vary drastically across the fault zone, with events located below the basin–basement interface north of the fault zone and more shallowly to the south. We also see spatial variations of source mechanism patterns and the direction of the maximum horizontal stress across the Delaware basin. A vast majority of seismic moment release can be attributed to the basement-rooted tectonic faults in the Culberson–Mentone seismogenic zone. A statistic deficit present in the magnitude–frequency distribution suggests that earthquakes with Mw 3.8–4.7 are needed to fulfill this scaling law. It indicates a strong possibility for future occurrence of earthquakes within this magnitude range.

An embedded ultrasonic sensor for monitoring acoustic emissions in laboratory earthquake experiments

The monitoring of acoustic emissions (AEs) is one of the most effective means to study the source mechanisms of laboratory earthquakes. However, conventional AE monitoring methods are usually suffered from aperture effect and weak coupling problems. Hence in this study, we propose a new method to monitor AEs using a kind ofultrasonic sensors made of polyvinylidene fluoridepiezoelectric film. These ultrasonic sensors are embedded in the sample. Elaborately designed calibration experiments are performed to obtain the instrument response. The calibration results show that the sensor has a broad frequency band (1.2–400 kHz), and there is a nearly flat response spectrum with respect to velocity. This means that the ultrasonic sensor can be regarded as a velocity sensor. In addition, our experimentsalso verify that the ultrasonic sensors can capture the source characteristics of the laboratory earthquakes. This method may facilitate the quantitative analysis of the source mechanism of AEs in laboratory investigations of earthquakes.

Persistent shallow micro-seismicity at Llaima volcano, Chile, with implications for long-term monitoring

Identifying the source mechanisms of low-frequency earthquakes at ice-covered volcanoes can be challenging due to overlapping characteristics of glacially and magmatically derived seismicity. Here we present an analysis of two months of seismic data from Llaima volcano, Chile, recorded by the permanent monitoring network in 2019. We find over 2000 repeating low-frequency events split across 82 families, the largest of which contains over 200 events. Estimated locations for the largest families indicate shallow sources directly beneath or near the edge of glaciers around the summit vent. These low-frequency earthquakes are part of an annual cycle in activity at the volcano that is strongly correlated with variations in atmospheric temperature, leading us to conclude that meltwater from ice and snow strongly affects the seismic source mechanisms related to glacier dynamics and shallow volcanic processes. The results presented here should inform future assessments of eruptive potential at Llaima volcano, as well as other ice-covered volcanoes in Chile and worldwide.

Bayesian slip inversion with automatic differentiation variational inference

SUMMARYThe Bayesian slip inversion offers a powerful tool for modelling the earthquake source mechanism. It can provide a fully probabilistic result and thus permits us to quantitatively assess the inversion uncertainty. The Bayesian problem is usually solved with Monte Carlo methods, but they are computationally expensive and are inapplicable for high-dimensional and large-scale problems. Variational inference is an alternative solver to the Bayesian problem. It turns Bayesian inference into an optimization task and thus enjoys better computational performances. In this study, we introduce a general variational inference algorithm, automatic differentiation variational inference (ADVI), to the Bayesian slip inversion and compare it with the classic Metropolis–Hastings (MH) sampling method. The synthetic test shows that the two methods generate nearly identical mean slip distributions and standard deviation maps. In the real case study, the two methods produce highly consistent mean slip distributions, but the ADVI-derived standard deviation map differs from that produced by the MH method, possibly because of the limitation of the Gaussian approximation in the ADVI method. In both cases, ADVI can give comparable results to the MH method but with a significantly lower computational cost. Our results show that ADVI is a promising and competitive method for the Bayesian slip inversion.

Estimation of the orientation of stress in the Earth\u2019s crust without earthquake or borehole data

Mechanical stress acting in the Earth’s crust is a fundamental property that is important for a wide range of scientific and engineering applications. The orientation of maximum horizontal compressive stress can be estimated by inverting earthquake source mechanisms and measured directly from borehole-based measurements, but large regions of the continents have few or no observations. Here we present an approach to determine the orientation of maximum horizontal compressive stress by measuring stress-induced anisotropy of nonlinear susceptibility, which is the derivative of elastic modulus with respect to strain. Laboratory and Earth experiments show that nonlinear susceptibility is azimuthally dependent in an anisotropic stress field and is maximum in the orientation of maximum horizontal compressive stress. We observe this behavior in the Earth—in Oklahoma and New Mexico, U.S.A, where maximum nonlinear susceptibility coincides with the orientation of maximum horizontal compressive stress measured using traditional methods. Our measurements use empirical Green’s functions and solid-earth tides and can be applied at different temporal and spatial scales.

Earthquake Source Mechanisms and Stress Field Variations Associated With Wastewater‐Induced Seismicity in Southern Kansas, USA

AbstractThe strong increase of seismicity rates in the contiguous USA over the last 10 years is linked to the injection of huge amounts of wastewater from oil and gas production in unconventional hydrocarbon reservoirs. We calculated 549 moment tensors of induced earthquakes (MW ≤ 4.9) in southern Kansas to study their source mechanisms and their relation to injection activity. Seventeen percent of the events analyzed contained significant volumetric (ISO%) components, and these events mostly occurred near the two largest local earthquakes during the 4 months of largest active wastewater disposal. Mapping the local stress field, we determined that most of the region lies within a transtensional stress regime, with a maximum horizontal stress σHmax trending N75°E. In the epicentral area of the MW 4.9 Milan earthquake, the σHmax trend is rotated to about S80°E. Locally, two areas display a change in the stress field orientation with depth, from transtensional above 5.5 km depth to strike slip deeper in the basement. Relating the resolved fault geometries to the obtained local stress field orientation, we find that most of the activated fault planes were optimally oriented to the current stress field and thus small stress perturbations caused by the water injection could lead to failure.

Earth\u2019s free oscillations excited by the 2011 Tohoku earthquake recorded in multiple GPS networks

We search in the continuous GPS 3-D displacement data for the signals of the normal modes of Earth’s free oscillation that were excited by the 2011 Mw 9.0 Tohoku earthquake. A previous study has reported such a detection; we here conduct a more comprehensive and detailed study. We use GPS data from three separate networks: (i) about 1000 stations from the Japan GEONET; (ii) about 600 stations from the western USA PBO; and (iii) about 140 stations of the global IGS, and solve and form records of 21 h length at 30-s sampling rate. We conduct various multiple-record stacking methods: the frequency-domain power spectrum stacking that reduces the variance of the noises, and the time-domain stackings that boost the SNR of target modes while suppressing the non-target modes. We find the time-domain stacking method of optimal sequence estimation (OSE) to be the most effective, which show clearly high sensitivity and detectability of the modes in the spectrum. For the near-field GEONET where all excited modes have anti-nodes, all the spheroidal fundamental modes 0S9–0S43 below 5 MHz and some of the lower-degree overtones as well as most of the low-degree toroidal fundamental modes show up as prominent spectral peaks against the PREM model eigenfrequencies. The PBO sees less strong (being far-field and generally off-antinodes), but still clearly identifiable spectral peaks of the fundamental modes. The global IGS network data detect barely a handful of these modes because of its sparsity and small numbers of stations. We thus demonstrate that GPS does actually record the tiny seismic signals that can be revealed by means of multiple-record stacking methods, potentially useful for studying earthquake source mechanisms exciting the normal modes.

Dynamics of SV-wave, SH-wave, and P-wave in a cross-anisotropic medium exhibiting exponential heterogeneity

This paper investigates the dynamics of SV -waves, SH-waves, and P-waves in a continuously heterogeneous cross-anisotropic material. Two case studies have been performed, each associated with the heterogeneity involved in Young’s moduli, shear modulus, and density of the medium by means of a function of depth. With suitable analytical treatment, the equation of motion reduces to an eigenvalue problem that further helps to achieve the closed-form expression for the phase velocities in the considered framework. Based on the numerical calculations and simulations, the propagation behavior of wave along with the above-stated variation has been scrutinized and discussed. This study unfolds the remarkable influence of inhomogeneity parameters on quasi-wave velocities. The prominent effect of the dependence of wave velocity on the degree of material anisotropy has been shown using 2-dimensional and 3-dimensional graphical presentations for two alternative cases. The impact of phase angle and depth parameter has also been remarkably established. This study may help geophysicists and civil engineers in the field of theoretical foundations and practical applications related to earthquake source mechanisms.

Seismic source characterization of the Arabian Peninsula and Zagros Mountains from regional moment tensor and coda envelopes

Reliable estimates of moment magnitude and source mechanism for seismic events in the Middle East can be challenging due to a small number of openly available stations, the complex tectonic setting, and regions of high attenuation. Access to high-quality waveform data from well-calibrated regional seismic stations is fundamental in producing robust and stable estimates of earthquake source parameters, particularly when measurements of absolute ground motion amplitudes are required. Earthquake source mechanism and moment magnitude are invaluable information in the assessment of seismic hazard, plate motions, and the characterization of faults and regional stress field. The expansion of the Kingdom of Saudi Arabia’s national seismic network in the past several decades provides an opportunity to develop the capabilities of routine focal mechanism and moment magnitude estimations. In this study, we use time-domain full waveform moment tensor inversion and coda envelope–derived amplitude measurements to solve for earthquake source mechanism, moment magnitudes, and their source type. We compared the moment magnitudes calculated from the two methods and publicly available earthquake catalogs and discuss the implications of the obtained source parameters.

Analysis of Tsunami Inundation due in Pangandaran Tsunami Earthquake in South Java Area Based on Finite Faults Solutions Model

On July 17, 2006 an earthquake with a magnitude of 7.7 triggered a tsunami that struck 500 km of the coast in the south of the island of Java. The tsunami generated is classified as an earthquake tsunami because the waves generated were quite large compared to the strength of the earthquake. The difference in the strength of the earthquake and the resulting tsunami requires a tsunami modeling study with an estimated fault area in addition to using aftershock and scaling law. The purpose of this study is to validate tsunamis that occur based on the estimation of the source mechanism and the area of earthquake faults. Determination of earthquake source mechanism parameters using the Teleseismic Body-Wave Inversion method that uses teleseismic waveforms with the distance recorded waveform from the source between Whereas, tsunami modeling is carried out using the Community Model Interface for Tsunami (commit) method. Fault plane parameters that obtained were strike , dip , and rake with dominant slip pointing up to north-north-west with a maximum value of 1.7 m. The fault plane is estimated to have a length of 280 km in the strike direction and a width of 102 km in the dip direction. From the results of the tsunami modeling, the maximum inundation area is 0.32 km2 in residential areas flanked by Pangandaran bays and the maximum run-up of 380.96 cm in Pasir Putih beach area. The tsunami modeling results in much smaller inundation and run-up from field observations, it was assumed that the fault plane segmentation had occurred due to the greater energy released than the one from the fault area, causing waves much larger than the modeling results.

Spatiotemporal Seismicity Evolution and Seismic Hazard Potentials in the Western East Sea (Sea of Japan)

The earthquakes in the western East Sea (Sea of Japan) mostly occur in the continental margin off the east coast of the Korean Peninsula. The seismic hazard potentials in and around the western Ease Sea are studied based on analyses of tectonic structures, seismicity features, earthquake source properties, Coulomb stress changes, and strong ground motions. The earthquake source mechanisms suggest that paleo-rifting structures in the western East Sea were activated by the current stress field. A low stress cumulation rate results in the occurrence of earthquakes with long recurrence intervals. The background seismicity suggests that earthquakes with magnitudes $$M_w$$ 5.0, 6.0, and 7.0 may occur within every $$\sim $$ 44, $$\sim $$ 336, and $$\sim $$ 2550 years at 95 % confidence level. The spatial distribution of earthquakes changes with time. Most earthquakes are clustered within $$\sim $$ 60 km from the coast. The seismicity analysis indicates an apparent increase of moderate-size ( $$M_w$$ 3–5) earthquakes since the 2011 $$M_w$$ 9.0 Tohoku-Oki megathrust earthquake. Static stress changes by moderate-size inland earthquakes induce offshore events. The seismicity and Coulomb stress changes suggest high seismic potentials around the western margin of the Ulleung basin. Earthquakes with magnitudes $$M_w$$ 6.0–7.0 in the western East Sea may produce peak ground accelerations of 0.2 g within the distance of $$\sim $$ 40–80 km, which includes the coastal regions.

Dynamics of Flexoelectric Materials: Subsonic, Intersonic, and Supersonic Ruptures and Mach Cone Formation

Abstract Motivated by recent, unexpected, experimental observations of “intersonic” rupture growth in which both shear and dilatational Mach fronts were observed at the tips of dynamic frictional ruptures propagating at rupture speeds below the dilatational wave speed of the surrounding solid, and we formulate the general dynamic flexoelectric problem and we investigate its plane strain/plane polarization specialization. The coupling of the mechanical problem is analogous to a problem of Toupin–Mindlin gradient elasticity, where two micromechanical characteristic lengths and two microinertial lengths emerge as a combination of the mechanical, dielectric, and flexoelectric constants. The solution of the rupture growth problem allows us to provide an explanation of the experimental results. This becomes possible since flexoelectricity predicts a new aspect that was not observed in the classical analysis: subsonic super shear and supersonic crack tip (or rupture) motions are not related exclusively with the problem being elliptic or hyperbolic, respectively. This is due to the influence of the microinertial lengths, which, in addition to the ratios of the rupture to the wave speeds, also affect the slopes of the Mach cones. Moreover, we are able to explain the experimental paradox of the observation of double Mach cone pairs at the tips of supershear, but subsonic, frictional, ruptures in poly-methyl-methacrtylate (PMMA) by demonstrating that both dilatational and shear Mach cones could appear in flexoelectric solids at rupture speeds below the material dilatation wave speed, something that is impossible from the classical elasticity analysis and is due to the dispersive nature of the present problem. Our analysis is of relevance to the dynamic deformation and fracture of both synthetic and naturally occurring flexoelectric materials and systems, with implications to both engineering and earthquake source mechanics.

Active Tectonics in Tuscany (Central Italy): Ten Years of Seismicity (2009-2019)

Strong earthquakes (moment magnitude MW ≥ 5.5) are uncommon in Tuscany and surroundings (central Italy). The last strong seismic event occurred a century ago (September 7, 1920 Garfagnana, MW = 6.53). The paucity of seismic instrumental recordings hinders the identification of the tectonic regime active in Tuscany. On the other hand, the geological and geomorphological pieces of evidence collected so far, concerning potential active and capable faults, are scarce, fragmentary and ambiguous. In this work I shed light on the active deformation of Tuscany by using two independent approaches: earthquake source mechanisms and GNSS (GPS) geodetic measurements. I have considered 41 small seismic events (MW ≤ 5.1) that occurred in the study area during the last decade. The related source mechanisms (retrieved by the Time Domain Moment Tensor method) define a relatively clear picture of the active deformation: extension along the northern Apennine watershed and strike-slip regime within inner Tuscany, up to the Tyrrhenian coast. This pattern broadly agrees with the horizontal strain field reconstructed by the geodetic velocity field. The latter has been constrained by a network of 840 GPS stations located in Italy and neighboring countries, operating in the last 20 years.

Ways to assess of seismicity of the territory of Syria

The article considers estimates of changes in the magnitudes of seismic events for two years (2005-2006) according to seismic catalogs. For this period, graphs of the law of repeatability of earthquakes were constructed and compared with each other. The analysis of the graphs of the frequency of occurrence of a number of zones built during seismic zoning was carried out, and the relationship between parameter A and the earthquake source mechanism was determined. Seismic zoning - the division of the territory into areas with varying degrees of intensity occurred and expected earthquakes. Seismic zoning data is used in the design and construction of earthquake-resistant structures and solving other practical problems in a seismically dangerous area. This is the applied aspect of seismic zoning. For the mapping of seismic zoning, historical data and instrumental observations of earthquakes, geological-tectonic and geophysical maps, as well as data on the movements of blocks of the earth’s crustare used. There are areas of possible occurrence of earthquake foci (OEF zones) with different depths. The greatest danger is represented by earthquakes with foci within the earth’s crust (at a depth of 3 to 30-50 km). The article provides a statistical analysis of the earthquakes of Syria for two years, taking into account historical earthquakes from the chronicles. The result of the work is the establishment of links between the identified earthquakes and the planetary events of the Earth, including solar activity. There is an increase in seismic activity in winter. Presented research should be used in forecasting future. It is worth noting that the basis for forecasting future natural events is the integration of analytical methods for analyzing data and theoretical ideas about the tectonics of the region in question.