Исмаиллинское землетрясение 5 февраля 2019

The Shamakhi-Ismailli seismogenic zone is known as the zone of the most powerful earthquakes in the Caucasus, which has been characterized by high seismic activity for centuries. Analysis of seismicity over the past 15 years has shown an increase in activity in this region. In October 2012, there was a devastating earthquake with a magnitude of 5.3. It is this earthquake that can be considered a trigger of activity in this region in subsequent years. In view of this, the task of studying seismicity, as well as the stress fields of the lithosphere of the region under study, seems to be especially urgent. The study of the seismicity of the Shamakhi-Ismailli zone provides additional information on the deep tectonic processes occurring in this region, which is important for seismic zoning. Aim. The article analyzes the seismic activity of the Shamakhi-Ismailli region, which began with an earthquake on February 5 at 19 h 19 min, with ml = 4.4, which occurred 11 minutes before the main shock with an intensity of 6 points, which occurred on February 5, 2019 at 19 h 31 m. Methods.The epicentral field was studied, as well as the distribution of foci in depth, solutions of the mechanisms of foci of the main shock and the most noticeable aftershock were constructed and analyzed. A diagram of the main elements of the rupture tectonics of the Shamakhi-Ismailli focal zone has been drawn, on which the mechanisms of the focal points of the lakes of the Ismailli field are plotted. Results. It has been established that the source area is located in the zone of intersection of the Vandam longitudinal fault with the West Caspian and transverse Akhsu strike-slip faults, which additionally characterizes the high seismic activity and deep penetration of the West Caspian right-sided orthogonal fault. Thus, it can be seen that, in terms of epicenters, they tend to the basement faults and the nodes of their intersection, i.e. The main shock that occurred on February 5, 2019, shows the agreement of the second nodal plane NP2 with the right-lateral Akhsu and West-Caspian transverse faults characterized by the type of displacement right-lateral strike-slip. An analysis of the orientation of the compression axes showed the NE-SW orientation, and the extension axes of the NW-SE orientation Шамахи-Исмаиллинская сейсмогенная зона известна как зона самых сильных землетрясений на Кавказе, которая на протяжении веков характеризовалась высокой сейсмической активностью. Анализ сейсмичности за последние 15 лет показал рост активности в этом регионе. В октябре 2012 года произошло разрушительное землетрясение магнитудой 5,3. Именно это землетрясение можно считать триггером активности в этом регионе в последующие годы. В связи с этим задача изучения сейсмичности, а также полей напряжений литосферы изучаемого региона представляется особенно актуальной. Изучение сейсмичности Шамахи-Исмаиллинской зоны дает дополнительную информацию о глубинных тектонических процессах, происходящих в этом регионе, что важно для сейсмического районирования. Цель работы.В статье проанализирована сейсмическая активность Шамахы-Исмаиллинского района, начавшаяся землетрясением 5 февраля в 19 ч 19 мин, с ml = 4,4, произошедшим за 11 минут до главного толчка с интенсивностью 6 баллов, произошедшего 5 февраля 2019 в 19 час 31 мин. Методы работы. Изучены эпицентральное поле, распределение очагов по глубине, построены и проанализированы решения механизмов очагов главного толчка и наиболее заметного афтершока. Составлена схема основных элементов разрывной тектоники Шамахы-Исмаиллинской очаговой зоны, на которой нанесены механизмы очагов озер Исмаиллинского месторождения. Результаты работы. Установлено, что очаговая область расположена в зоне пересечения Вандамского продольного разлома с Западно-Каспийским и поперечным Ахсуйским сдвигами, что дополнительно характеризует высокую сейсмическую активность и глубокое проникновение Западно-Каспийского правостороннего ортогонального разлома. Таким образом, видно, что в плане эпицентров они стремятся к разломам фундамента и узлам их пересечения, т.е. главный толчок, произошедший 5 февраля 2019 г., показывает совпадение второй узловой плоскости NP2 с правосторонним Ахсуйским и Западно-Каспийским поперечным разломом, характеризующимися правосторонним сдвиговым типом смещения. Анализ ориентации осей сжатия показал ориентацию СВ-ЮЗ, а оси растяжения – ориентацию СЗ-ЮВ.

Numerical Simulation of the Effects of Wedge Subduction on the Lithospheric Thermal Structure and the Seismogenic Zone South of Chile Triple Junction

In contrast to common subduction, the young and thin part of the Antarctic Plate subducts first to the south of the Chile Triple Junction (CTJ), followed by the old and thick part, corresponding to wedge subduction. A finite element model was used to simulate the wedge subduction of the Antarctic Plate and to compare it with the slab subduction of the Nazca Plate. The results show that the CTJ is not only a wedge subduction boundary but also an important factor controlling the lithospheric thermal structure of the overriding plate. The computed heat flow curves are consistent with the data observed near the trench of the two selected profiles. The different slab dips to the north and south of the CTJ are considered to be caused by wedge subduction. When the slabs are young and at the same age, the deep dip of the Antarctic slab is 22° smaller than the Nazca slab. Southward from the CTJ, the slab age of the wedge subduction increases, which leads to a larger slab dip, a colder slab, and a wider seismogenic zone. The effect of the slab age of wedge subduction on the focal depth is smaller than that of the convergence rate. A 4.8-cm/year difference in convergence rate of the wedge subduction results in an 11-km difference in the width of the seismogenic zone and a 10-km difference in the depth of the downdip limit. Among these controlling factors, the convergence rate plays a major role in the different focal depths south and north of the CTJ.

Present-day stress field pattern in the Vrancea seismic zone (Romania) deduced from earthquake focal mechanism inversion

Vrancea seismogenic zone in the South-Eastern Carpathians is characterized by localized intermediate-depth seismicity. Due to its complex geodynamics and large strain release, Vrancea represents a key element in the Carpatho-Pannonian system. Data from a recently compiled catalogue of fault plane solutions (REFMC) are inverted to evaluate stress regime in Vrancea on depth. A single predominant downdip extensive regime is obtained in all considered clusters, including the crustal layers located above the Vrancea slab. The prevalent stress regime confirms previous investigations and requires some mantle-crust coupling. The S3 principal stress is close to vertical, while S1 and S2 are horizontal, oriented perpendicularly and respectively tangentially to the Carpathians Arc bend. This configuration is present at any depth level. According to seismicity patterns, there are two main active segments in the Vrancea intermediate-depth domain, at 55 – 105 km and 105 – 180 km, both able to generate major events. The configuration of the tectonic stresses as resulted from inversion is similar in both segments. Also, high fault instability (I > 0.95) is characterizing the segments. The only notable difference is given by the friction and stress ratio parameters which drop down in the bottom segment from μ = 0.95 to μ = 0.55 and from R = 0.51 to R = 0.29. This variation is attributed to possible weakening processes activated below 100 km depth and can explain the intensification of seismicity production as earthquake rate and average energy release in the lower segment versus the upper segment.

Spatiotemporal Evolution of Earthquakes in Longmenshan Fault and Adjacent Area, before and after the 2008 Wenchuan Earthquake

Seismicity sequence following a main earthquake usually contains much meaningful information for unveiling the focal mechanism and predicting the reoccurrence interval of large earthquakes. The spatiotemporal evolution of earthquakes before and after the 2008 Wenchuan earthquake (Ms 8.0) is analysed comprehensively in this study. The frequency-magnitude relation of the 3493 earthquake events retrieved from the database of the International Seismological Centre indicates that the adopted catalogue is complete for magnitudes ≥Ms 3.4. The seismicity during the 10 years before the Wenchuan earthquake remained stable, including the magnitudes and focal depths. However, seismicity attenuated sharply in the year following the Wenchuan earthquake, and the magnitude of earthquakes before the Wenchuan earthquake decreased gradually. The area of the seismogenic zone of the 2008 Wenchuan earthquake was smaller than the earthquake stricken area. The earthquakes that occurred in the Longmenshan fault area and adjacent area in the study period were mainly shallow earthquakes. The focal depths of earthquakes in the study area became stable gradually after the Wenchuan earthquake, mainly within the range from 10 to 16 km. The earthquakes in the study area were mainly distributed with an along-dip distance of 0–20 km, and the seismicity was distributed uniformly along the fault strike.

The rise and demise of deep accretionary wedges: A long-term field and numerical modeling perspective

Several decades of field, geophysical, analogue, and numerical modeling investigations have enabled documentation of the wide range of tectonic transport processes in accretionary wedges, which constitute some of the most dynamic plate boundary environments on Earth. Active convergent margins can exhibit basal accretion (via underplating) leading to the formation of variably thick duplex structures or tectonic erosion, the latter known to lead to the consumption of the previously accreted material and eventually the forearc continental crust. We herein review natural examples of actively underplating systems (with a focus on circum-Pacific settings) as well as field examples highlighting internal wedge dynamics recorded by fossil accretionary systems. Duplex formation in deep paleo–accretionary systems is known to leave in the rock record (1) diagnostic macro- and microscopic deformation patterns as well as (2) large-scale geochronological characteristics such as the downstepping of deformation and metamorphic ages. Zircon detrital ages have also proved to be a powerful approach to deciphering tectonic transport in ancient active margins. Yet, fundamental questions remain in order to understand the interplay of forces at the origin of mass transfer and crustal recycling in deep accretionary systems. We address these questions by presenting a suite of two-dimensional thermo-mechanical experiments that enable unravelling the mass-flow pathways and the long-term distribution of stresses along and above the subduction interface as well as investigating the importance of parameters such as fluids and slab roughness. These results suggest the dynamical instability of fluid-bearing accretionary systems causes either an episodic or a periodic character of subduction erosion and accretion processes as well as their topographic expression. The instability can be partly deciphered through metamorphic and strain records, thus explaining the relative scarcity of paleo–accretionary systems worldwide despite the tremendous amounts of material buried by the subduction process over time scales of tens or hundreds of millions of years. We finally stress that the understanding of the physical processes at the origin of underplating processes as well as the forearc topographic response paves the way for refining our vision of long-term plate-interface coupling as well as the rheological behavior of the seismogenic zone in active subduction settings.

Seismicity recorded in hematite fault mirrors in the Rio Grande rift

Exhumed fault rocks provide a textural and chemical record of how fault zone composition and architecture control coseismic temperature rise and earthquake mechanics. We integrated field, microstructural, and hematite (U-Th)/He (He) thermochronometry analyses of exhumed minor (square-centimeter-scale surface area) hematite fault mirrors that crosscut the ca. 1400 Ma Sandia granite in two localities along the eastern flank of the central Rio Grande rift, New Mexico. We used these data to characterize fault slip textures; evaluate relationships among fault zone composition, thickness, and inferred magnitude of friction-generated heat; and document the timing of fault slip. Hematite fault mirrors are collocated with and crosscut specular hematite veins and hematite-cemented cataclasite. Observed fault mirror microstructures reflect fault reactivation and strain localization within the comparatively weaker hematite relative to the granite. The fault mirror volume of some slip surfaces exhibits polygonal, sintered hematite nanoparticles likely created during coseismic temperature rise. Individual fault mirror hematite He dates range from ca. 97 to 5 Ma, and ~80% of dates from fault mirror volume aliquots with high-temperature crystal morphologies are ca. 25–10 Ma. These aliquots have grain-size–dependent closure temperatures of ~75–108 °C. A new mean apatite He date of 13.6 ± 2.6 Ma from the Sandia granite is consistent with prior low-temperature thermochronometry data and reflects rapid, Miocene rift flank exhumation. Comparisons of thermal history models and hematite He data patterns, together with field and microstructural observations, indicate that seismicity along the fault mirrors at ~2–4 km depth was coeval with rift flank exhumation. The prevalence and distribution of high-temperature hematite grain morphologies on different slip surfaces correspond with thinner deforming zones and higher proportions of quartz and feldspar derived from the granite that impacted the bulk strength of the deforming zone. Thus, these exhumed fault mirrors illustrate how evolving fault material properties reflect but also govern coseismic temperature rise and associated dynamic weakening mechanisms on minor faults at the upper end of the seismogenic zone.

Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

Subduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution leads to coupling between fluid flow and slip behavior through decreases in porosity/permeability and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and the removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fluctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simulator with a fluid flow system depicts a subduction zone interface governed by feedbacks between fluid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fluid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fluid pressure that propagates trenchward.

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. We, therefore, carry out magnetotelluric (MT) investigation to explore an intra-plate earthquake zone, which is related to artificial reservoir triggered seismicity. However, due to surface access restrictions, our dataset 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 intra-plate earthquake zone. Therefore, it is vital to fill the data gap to get the electrical signature of the active seismic zone. To compensate for the data gap, we develop a relation that connects resistivity with the ratio of seismic P- to S-wave velocity ( VP/ VS). Utilizing this relation, we estimate a priori resistivity distribution in the data gap region from known vp/vs values during inversion to compensate for the data gap. A comparison study of the root mean square (RMS) misfits of inversion outputs (with and without data gap filled) proves the effectiveness of the established relation. The inversion outputs 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. Further, a synthetic test on a similar earth model establishes the credibility and robustness of the derived relation.

Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation

Although the study of spatiotemporal variation of a subsurface velocity structure is a challenging task, it can provide a description of the fault geometry as well as important information on the rheological changes caused by fault rupture. Our main objective is to investigate whether rheological changes of faults can be associated with the seismogenic process before a strong earthquake. For this purpose, a 3D tomographic technique is applied to obtain P- and S-wave velocity structures in central Taiwan using travel time data. The results show that temporal variations in the Vs structure in the source area demonstrate significant spatiotemporal variation before and after the Chi-Chi earthquake. We infer that, before the mainshock, Vs began to decrease (and Vp/Vs increased) at the hanging wall of the Chelungpu fault, which may be induced by the increasing density of microcracks and fluid. However, in the vicinity of the Chi-Chi earthquake’s source area, Vs increased (and Vp/Vs decreased), which may be attributed to the closing of cracks or migration of fluid. The different physical characteristics at the junctional zone may easily generate strong earthquakes. Therefore, seismic velocity changes are found to be associated with a subsurface evolution around the source area in Taiwan. Our findings suggest that monitoring the Vp and Vs (or Vp/Vs) structures in high seismic potential zones is an important ongoing task, which may minimize the damage caused by future large earthquakes.