Seismogenic Region Research Articles

CHERNOGORSKOE-I EARTHQUAKE on DECEMBER 12, 2020 with KR=12.9, Mw=4.8 (GCMT), I0=5–6 in the CHECHEN REPUBLIC

On December 12, 2020 at 21h29m GMT, a significant event with KR=12.9, Mw=4.8 occurred on the territory of the Urus-Martan district of the Chechen Republic. Its epicenter belongs to the area of junction of the southern side of the Terek-Caspian foredeep with the structures of the northern slope of the Greater Caucasus and is associated with the Chernogorskaya zone of Possible Earthquake Sources (Mmax=6.5) of the Terek-Sunzha seismogenic region. The earthquake of December 12, called Chernogorsk-I, was felt in 57 settlements with an intensity from 2 to 5–6 by MSK-64. Based on the results of macroseismic surveys, the maps of shaking intensity in populated areas and isoseisms of 5, 4 and 3 points were constructed. The calculated intensity at the epicenter coincides with the observed one in nearest settlements and amounts to I0=5–6 points. The source was dominated by compressive stresses oriented submeridionally. The type of movement in the source is a reverse fault along both planes of sublatitudinal strike. The closeness of orientation of the nodal planes to the Chernogorsky fault orientation allows us to draw a conclusion about movement along this fault at Chernogorskoe-I earthquake.

Possible Identification of Precursor ELF Signals on Recent EQs That Occurred Close to the Recording Station

The Lithospheric–Atmospheric–Ionospheric Coupling (LAIC) mechanism stands as the leading model for the prediction of seismic activities. It consists of a cascade of physical processes that are initiated days before a major earthquake. The onset is marked by the discharge of ionized gases, such as radon, through subterranean fissures that develop in the lead-up to the quake. This discharge augments the ionization at the lower atmospheric layers, instigating disturbances that extend from the Earth’s surface to the lower ionosphere. A critical component of the LAIC sequence involves the distinctive perturbations of Extremely Low Electromagnetic Frequencies (ELF) within the Schumann Resonances (SR) spectrum of 2 to 50 Hz, detectable days ahead of the seismic event. Our study examines 10 earthquakes that transpired over a span of 3.5 months—averaging nearly three quakes monthly—which concurrently generated 45 discernible potential precursor seismic signals. Notably, each earthquake originated in Southern Greece, within a radius of 30 to 250 km from the observatory on Mount Parnon. Our research seeks to resolve two important issues. The first concerns the association between specific ELF signals and individual earthquakes—a question of significant importance in seismogenic regions like Greece, where earthquakes occur frequently. The second inquiry concerns the parameters that determine the detectability of an earthquake by a given station, including the requisite proximity and magnitude. Initial findings suggest that SR signals can be reliably linked to a particular earthquake if the observatory is situated within the earthquake’s preparatory zone. Conversely, outside this zone, the correlation becomes indeterminate. Additionally, we observe a differentiation in SR signals based on whether the earthquake took place over land or offshore. The latter category exhibits unique signal behaviors, potentially attributable to the water layers above the epicenter acting as a barrier to the ascending gases, thereby affecting the atmospheric–ionospheric ionization process.

Automatic relocation of intermediate-depth earthquakes using adaptive teleseismic arrays

SUMMARY Intermediate-depth earthquakes, accommodating intraslab deformation, typically occur within subduction zone settings at depths between 60–300 km. These events are in a unique position to inform us about the geodynamics of the subducting slab, specifically the geometry of the slab and the stress state of the host material. Improvements in the density and quality of recorded seismic data enhance our ability to determine precise locations of intermediate-depth earthquakes, in order to establish connections between event nucleation and the tectonic setting. Depth phases (near-source surface reflections, e.g. pP and sP) are crucial for the accurate determination of earthquake source depth using global seismic data. However, they suffer from poor signal-to-noise ratios in the P wave coda. This reduces the ability to systematically measure differential traveltimes to the direct P arrival, particularly for the frequent lower magnitude seismicity which highlights considerable seismogenic regions of the subducted slabs. To address this limitation, we have developed an automated approach to group globally distributed stations at teleseismic distances into ad-hoc arrays with apertures of 2.5$^\circ$, before optimizing and applying phase-weighted beamforming techniques to each array. Resultant vespagrams allow automated picking algorithms to determine differential arrival times between the depth phases and their corresponding direct P arrival. Using these differential times we can then determine the depths of earthquakes, which in turn can be used to create a catalogue of relocated events. This will allow new comparisons and insights into the governing controls on the distribution of earthquakes in subducted slabs. We demonstrate this method by relocating intermediate-depth events associated with northern Chile and the Peruvian flat slab regions of the subducting Nazca plate. The relocated Chilean catalogue contains comparable event depths to an established catalogue, calculated using a semi-automated global methodology, which serves to validate our fully automatic methodology. The new Peruvian catalogue we generate indicates three broad zones of seismicity approximately between latitudes 1–7$^\circ$S, 7–13$^\circ$S and 13–19$^\circ$S. These align with flat to steep slab dip transitions and the previously identified Pucallpa Nest. We also find a regionally deeper slab top than indicated by recent slab models, with intraslab events concentrated at points where the slab bends, suggesting a link between slab flexure and intermediate-depth earthquake nucleation.

Characterization of seismicity and precursor monitoring in northern Baja California using RESNOM data

Northern Baja California, Mexico, is part of the boundary between the Pacific and North American plates, and the relative motion between these plates causes seismicity that has attained a recorded magnitude M7.3. This seismicity represents a hazard for towns and cities located in this area and, in order to be able to contribute to the hazard estimation, we propose to establish monitoring of possible precursors to major earthquakes.We will use data from the RESNOM seismological network, operated by CICESE, to monitor the area between longitudes −117.4° and −114.5°, and latitudes 30.5° and 33.2°.Since the study area presents a complex array of seismogenic regions, we define three regions for monthly precursor monitoring, based on long-term epicentral spatial distributions, together with geological and tectonic considerations. We propose monthly monitoring because it is a good tradeoff between useful temporal definition and having enough events for evaluation of statistical measures.We are monitoring monthly two types of measures, those that can be directly evaluated from data for each month (number of events/day, released seismic moment/day, and average magnitude and depth), and measures requiring more processing and larger amounts of data (direct clustering, clustering from apparent velocities, Poisson entropy, and fractal dimensions D0, D1, and D2).We estimate that coverage is adequate from 2005 to date (June 2023), and monitoring of data over this period suggests that, for two of the regions, it may be possible to identify precursory activity before earthquakes with magnitudes M≥7.0 and, possibly M≥6.0. The third region, however, needs improved network seismographic coverage for monitoring to be effective. There are not enough recorded strong and major earthquakes to be able to estimate reliability for precursory signals yet.Monitoring will continue indefinitely and may evolve to consider more or different measures and/or regions. We hope that this monitoring may help in identifying regions of interest for detailed observation and, eventually, to help in the estimation of seismic hazard in northern Baja California.

Earthquake forecasting model for Albania: the area source model and the smoothing model

Abstract. We proposed earthquake forecasting models for Albania, one of the most seismogenic regions in Europe, to give an overview of seismic activity by implementing area source and smoothing approaches. The earthquake catalogue was first declustered to remove foreshocks and aftershocks when they are within the derived distance windows and time windows of mainshocks. Considering catalogue completeness, the events with M≥4.1 during the period of 1960–2006 were implemented for the learning forecast model. The forecasting is implemented into an area source model that includes 20 sub-regions and a smoothing model with a cell size of 0.2∘ × 0.2∘ to forecast the seismicity in Albania. Both models show high seismic rates along the western coastline and in the southern part of the study area, consistent with previous studies that discussed seismicity in the area and currently active regions. To further validate the forecast performance of the two models, we introduced the Molchan diagram to quantify the correlation between models and observations. The Molchan diagram suggests that the models are significantly better than a random distribution, confirming their forecasting abilities. Our results provide crucial information for subsequent research on seismic activity, such as probabilistic seismic hazard assessment.

Study of the OLR Anomalies before the 2023 Turkey M7.8 Earthquake

Using the model of the additive tectonic stress from celestial tide-generating force, we studied the relationship between the seismogenic structure and celestial tide-generating stress in the M7.8 Turkey earthquake on 6 February 2023. We analyzed the daily continuous variation characteristics of OLR before and after the Turkey earthquake and discussed the correlation characteristics of tidal stress, OLR, and the earthquake. The results showed that the observed OLR anomaly according to the tidal stress variation cycle “C” (1–15 February) presented a phase change in time, which was synchronized with a continuous trough-to-peak change in the additional tectonic main pressure stress. The spatial distribution of OLR anomalies was mainly concentrated in the southwest section of the East Anatolian Fault Zone, which indicates that seismic tectonic movements were the main causes of OLR anomaly variation during this earthquake. An OLR anomaly change was related to this M7.8 “Swarm Type” of earthquake in Turkey. Impending earthquake OLR anomalies represent that the stress of the seismogenic structure in the seismogenic region has entered a critical state, which can provide stress monitoring and a seismogenic region indication for earthquakes induced by tidal force. The change cycle of the celestial tide-generating force provides a time indication for the identification of seismic thermal anomalies, and it indicates that the combination of the additional tectonic stress of the tidal force and the change of OLR anomaly has value for the research on the short-impending earthquake precursor.

Earthquake Forecasting Using Big Data and Artificial Intelligence: A 30-Week Real-Time Case Study in China

ABSTRACTEarthquake forecasting is one of the most challenging tasks in the field of seismology that aims to save human life and mitigate catastrophic damages. We have designed a real-time earthquake forecasting framework to forecast earthquakes and tested it in seismogenic regions in southwestern China. The input data are the features provided by the multicomponent seismic monitoring system acoustic electromagnetic to AI (AETA), in which the data are recorded using two types of sensors per station: electromagnetic (EM) and geo-acoustic (GA) sensors. The target is to forecast the location and magnitude of the earthquake that may occur next week, given the data of the current week. The proposed method is based on dimension reduction from massive EM and GA data using principal component analysis, which is followed by random-forest-based classification. The proposed algorithm is trained using the available data from 2016 to 2020 and evaluated using real-time data during 2021. As a result, the testing accuracy reaches 70%, whereas the precision, recall, and F1-score are 63.63%, 93.33%, and 75.66%, respectively. The mean absolute error of the distance and the predicted magnitude using the proposed method compared to the catalog solution are 381 km and 0.49, respectively.

The Analysis of Lithosphere–Atmosphere–Ionosphere Coupling Associated with the 2022 Luding Ms6.8 Earthquake

Taking the Luding Ms6.8 earthquake (EQ) on 5 September 2022 as a case study, we investigated the potential seismic anomalies of the ionosphere, infrared radiation, atmospheric electrostatic field (AEF), and hot spring ions in the seismogenic region. Firstly, we analyzed the multi-parameter anomalies in the ionosphere around the epicenter and found synchronous anomalous disturbances in the ground parameters, namely the global ionospheric map (GIM), GPS, TEC, and satellite parameters, such as the He+ and O+ densities on 26 August under relatively quiet solar–geomagnetic conditions (F10.7 < 120 SFU; Kp < 3; Dst > −30 nT; |AE| < 500 nT). Next, both the anomaly analysis of the infrared radiation and AEF, and the survey results of the Luding EQ scientific expedition on the hot spring ions showed pre-seismic anomalous variations at different time periods in the seismogenic region. The characteristics of Earth’s multi-sphere coupling anomalies in temporal evolution and spatial distribution were obvious, which validated the Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) mechanism. Finally, combining the analysis results and the LAIC mechanism, we suggested that the multi-sphere coupling anomalies were more likely associated with the Luding Ms6.8 EQ, and that the differential motion and the regional crustal stress accumulation between the Chuandian block and the Bayan Har block might have led to this EQ. Furthermore, remote sensing and ground-based monitoring technologies can play an important role in corroborating and compensating each other, while further study of the multi-sphere coupling mechanism will provide a clearer understanding of the seismogenic process for major EQs.

Fluid–rock interaction in the basement granitoids: A plausible answer to recurring seismicity

The Koyna–Warna Seismogenic Region in the western part of the Indian Subcontinent has been recognized as one of the most significant sites of Reservoir–Triggered–Seismicity (RTS) during the last five decades. The basement granitoids, overlain by the porous and vesicular Deccan Trap basalt, contain numerous interconnecting fractures which act as the ascending and descending pathways of fluid flow. As a result of this fluid flow along fractures, the host rock has been subjected to significant chemical alteration along with the subsequent formation of some new minerals at the expense of a few other pre–existing mineral phases. Mesoscopic observations followed by Optical microscopy in the core samples of the basement rocks upto 1.5 km depth retrieved from the borehole KBH1 near Rasati (about 4.7 km from the Koyna Dam) have revealed the presence of chlorite and the precipitation of calcite, whereas the bulk mineralogical XRD has reaffirmed the presence of chlorite, calcite along with illite at a certain depth. This entire secondary mineral assemblage resembles the propylitic kind of hydrothermal alteration at temperatures < 350°C under acid–to–neutral solution conditions and also indicates water channelization up to the deeper level in the basement granitoids (>1.5 km). In addition, the presence of the hydrophilic clay minerals along fault and fracture zones may be responsible for triggering the seismicity in the Koyna Seismogenic Region as their absorption of water reduces the shear strength of faults and their low frictional strength accelerates the fault weakening process causing the generation of slip surfaces. Thus, in addition to several seismotectonic features, fault geometry and existing stress pattern, the clay mineralisation along the pre–existing faults and fractures of the basement rocks may also be a factor behind the recurring seismicity in this region.

Understanding Anthropogenic Fault Rupture in the Eagle Ford Region, South-Central Texas

ABSTRACT There is a well-known occurrence of felt seismicity and smaller seismic events in areas where hydraulic fracturing (HF) operations occur. The Eagle Ford shale play of south-central Texas experienced an increase in the rate of felt seismicity from 2014 to 2019, temporally coincident with petroleum development in the region. By mid-2019, the rate of seismicity decreased alongside a reduction in the rate of well completions, thus, prompting this investigation of the relationship between HF operations and geologic conditions that contribute to induced earthquake hazards. The goals of this work included mapping and conducting a geomechanical characterization of faults that delineate seismogenic regions of the Eagle Ford to understand the conditions that lead to inducing fault rupture. An integrated regional dataset composed of published data, wells, earthquakes, and interpretations from operators provided input for a 3D structural framework. Earthquake relocation analyses helped constrain the distribution of earthquakes that correlate to interpreted faults and enable identification of those that have been seismogenic. In-situ stress state of faults was analyzed to determine fault sensitivity in-situ. A spatiotemporal analysis of HF operations and earthquakes further revealed induced-earthquake clusters that are linked to specific faults. We show how seismogenic and aseismogenic fault systems relate to earthquakes by determining which faults are more sensitive and which faults have been seismogenic. Faulting is dominated by northwest–southwest-striking normal faults with 21% having hosted induced earthquakes since 2017. Faults in the Eagle Ford region have a geologically quasi-stable in-situ stress state. Using a conservative scheme, we directly associate 45% of earthquake ruptures to HF to build our analysis dataset. Of those events, 70% are located within 1 km of a mapped fault. Stress conditions on seismogenic faults show a wide range of sensitivity to rupture. This suggests that all faults close to HF operations should be considered as candidates likely to rupture.

Earthquake Risk Probability Evaluation for Najin Lhasa in Southern Tibet

The probabilistic seismic hazard analysis (PSHA) method is effectively used in an earthquake risk probability evaluation in seismogenic regions with active faults. In this study, by focusing on the potential seismic source area in Najin Lhasa, southern Tibet, and by incorporating the PSHA method, we determined the seismic activity parameters and discussed the relationship of ground motion attenuation, the seismic hazard probability, and the horizontal bedrock ground motion acceleration peak value under different transcendence probabilities in this area. The calculation results show that the PSHA method divides the potential source area via specific tectonic scales and detailed tectonic markers, which reduces the scale of the potential source area and better reflects the uneven spatial distribution of seismic activity in the vicinity of Najin. The corrected attenuation relationship is also in line with the actual work requirements and is suitable for earthquake risk analysis. In addition, the major influences on the peak acceleration of ground motion in the study area are mainly in the potential source areas of Qushui (M7.5), Dangxiong (M8.5), and Kangma (M7.5). The peak horizontal ground motion acceleration (PGA) with a transcendence probability of 10% in 50 years is 185.9 cm/s2, and that with a transcendence probability of 2% in 50 years is 265.9 cm/s2.

Near-Field Seismic Motion: Waves, Deformations and Seismic Moment

The tensorial force acting in a localized seismic focus is introduced and the corresponding seismic waves are derived, as solutions of the elastic wave equation in a homogeneous and isotropic body. The deconvolution of the solution for a structured focal region is briefly discussed. The far-field waves are identified as P and S seismic waves. These are spherical-shell waves, with a scissor-like shape, and an amplitude decreasing with the inverse of the distance. The near-field seismic waves are spherical-shell waves, decreasing with the inverse of the squared distance. The amplitudes and the polarizations of the near-field seismic waves are given. The determination of the seismic-moment tensor and the earthquake parameters from measurements of the P and S seismic waves at Earth’s’ surface is briefly discussed. Similarly, the mainshock generated by secondary waves on Earth’s surface is reviewed. The near-field static deformations of a homogeneous and isotropic half-space are discussed and a method of determining the seismic-moment tensor from epicentral near-field (quasi-) static deformations in seismogenic regions is presented.

Lower crustal intraplate seismicity in Kachchh region (Gujarat, India) triggered by crustal magmatic infusion: Evidence from shear wave velocity contrast across the Moho

The genesis of intraplate seismicity remains a topic of debate among seismologists. Here, we address this issue by investigating shear wave velocity contrast across the Moho (δβM) in the Deccan volcanic-covered Kachchh seismogenic region, located in the Gujarat state of western India. This region, having complex geology, has witnessed several large damaging intraplate earthquakes since historical times. Most of the shallow focused earthquakes clustered in the eastern part of Kachchh often occurred due to the fluid pressure in the fractured rock matrix at the intersection of a fault network consisting of mainly three prominent faults, viz. Gedi, Kachchh Mainland, and North Wagad. However, the causes of lower crustal earthquakes are not well understood – whether it is due to the reactivation of faults or processes related to the fluid-filled fracked rock matrix of mafic/ultramafic material or magmatic infusion from the underlying mantle. Our results show that the δβM in the study area varies widely from ∼0.15 to 0.86 km/s. The lowest value of δβM (∼0.15 km/s) coincide with lower crustal seismicity. Further, the scaling relation between δβM and crustal thickness is negative, i.e., the thick crust has a low δβM, implying that the magmatic material has enveloped the lower crust, which ascended through the deep rifts during Deccan volcanism at ∼66 Ma. Higher values of δβM below a few stations have been ascribed to the pockets of trapped magmatic material or fluid below the lower crust. We suggest that the presence of mafic/ultramafic material in the crust-mantle transition zone, together with the fluid-filled fractured rock matrix in the intersecting fault geometry might accumulate enough strain to generate seismic activity in the lower crust. The presence of volatile influx or fluid pressure facilitates the triggering of deep-seated faults, and CO2-rich carbonatite-type magma percolated from the shallow asthenosphere.

Source mechanisms and rupture processes of the Jujuy seismic nest, Chile-Argentina border

The Altiplano-Puna plateau, in Central Andes, is the second-largest continental plateau on Earth, extending between 22° and 27°S at an average altitude of 4400 m. The Puna plateau has been formed in consequence of the subduction of the oceanic Nazca Plate beneath the continental South American plate, which has an average crustal thickness of 50 km at this location. A large seismicity cluster, the Jujuy cluster, is observed at depth of 150–250 km beneath the central region of the Puna plateau. The cluster is seismically very active, with hundreds of earthquakes reported and a peak magnitude MW 6.6 on 25th August 2006. The cluster is situated in one of three band of intermediate-depth focus seismicity, which extend parallel to the trench roughly North to South. It has been hypothesized that the Jujuy cluster could be a seismic nest, a compact seismogenic region characterized by a high stationary activity relative to its surroundings. In this study, we collected more than 40 years of data from different catalogs and proof that the cluster meets the three conditions of a seismic nest. Compared to other known intermediate depth nests at Hindu Kush (Afganisthan) or Bucaramanga (Colombia), the Jujuy nest presents an outstanding seismicity rate, with more than 100 M4+ earthquakes per year. We additionally performed a detailed analysis of the rupture process of some of the largest earthquakes in the nest, by means of moment tensor inversion and directivity analysis. We focused on the time period 2017–2018, where the seismic monitoring was the most extended. Our results show that earthquakes in the nest take place within the eastward subducting oceanic plate, but rupture along sub-horizontal planes dipping westward. We suggest that seismicity at Jujuy nest is controlled by dehydration processes, which are also responsible for the generation of fluids ascending to the crust beneath the Puna volcanic region. We use the rupture plane and nest geometry to provide a constraint to maximal expected magnitude, which we estimate as MW ∼6.7.

Fluid behaviors from the seismogenic region of Longmenshan fault zone, eastern Tibetan Plateau

Fluid behaviors from the seismogenic region of Longmenshan fault zone, eastern Tibetan Plateau

Landslide Susceptibility Mapping in the Vrancea-Buzău Seismic Region, Southeast Romania

This study presents the results of a landslide susceptibility analysis applied to the Vrancea-Buzău seismogenic region in the Carpathian Mountains, Romania. The target area is affected by a large diversity of landslide processes. Slopes are made-up of various types of rocks, climatic conditions can be classified as wet, and the area is a seismically active one. All this contributes to the observed high landslide hazard. The paper analyses the spatial component of the landslide hazard affecting the target area, the regional landslide susceptibility. First, an existing landslide inventory was completed to cover a wider area for the landslide susceptibility analysis. Second, two types of methods are applied, a purely statistical technique, based on correlations between landslide occurrence and local conditions, as well as the simplified spatial process-based Newmark Displacement analysis. Landslide susceptibility maps have been produced by applying both methods, the second one also allowing us to simulate different scenarios, based on various soil saturation rates and seismic inputs. Furthermore, landslide susceptibility was computed both for the landslide source and runout zones—the first providing information about areas where landslides are preferentially triggered and the second indicating where landslides preferentially move along the slope and accumulate. The analysis showed that any of the different methods applied produces reliable maps of landslide susceptibility. However, uncertainties were also outlined as validation is insufficient, especially in the northern area, where only a few landslides could be mapped due to the intense vegetation cover.

Relationships between macroseismic intensity and peak ground acceleration and velocity for the Vrancea (Romania) subcrustal earthquakes

The goal of this paper is to develop a new empirical relationship between observed macroseismic intensity and strong ground motion parameters such as peak ground acceleration (PGA) and velocity (PGV) for the Vrancea subcrustal earthquakes. The recent subcrustal earthquakes provide valuable data to examine these relationships for Vrancea seismogenic region. This region is one of the most active seismic zones in Europe and it is well-known for the strong subcrustal earthquakes. We examine the correlation between the strong ground-motion records and the observed intensities for major and moderate earthquakes with Mw ≥ 5.4 and epicentral intensity in the range VI to IX MSK degrees that occurred in Vrancea zone in the period 1977-2009. The empirical relationships between maximum intensity and ground parameters obtained and published by various authors have shown that these parameters do not always show a one-to-one correspondence, and the errors associated with the intensity estimation from PGA/PGV are sometimes +/-2 MSK degree. In the present study, the relation between macroseismic intensity and PGA/PGV will be given both as a mathematical equation, but also as corresponding ground motion intervals. Because of the intensity data spreading and errors related to mathematical approximations, it is necessary to systematically monitor not only the acceleration and velocity but also all the other ground motion parameters. The mathematical relation between these parameters might be used for the rapid assessment of ground shaking severity and potential damages in the areas affected by the Vrancea earthquakes.

Mineralogical and Geochemical Evidence of Fluid‐rock Interaction at the Shallow Crustal Level in Koyna Seismogenic Region, Maharashtra, India: Impact and Implications

Mineralogical and Geochemical Evidence of Fluid‐rock Interaction at the Shallow Crustal Level in Koyna Seismogenic Region, Maharashtra, India: Impact and Implications

Hydraulically conductive fault zone responsible for monsoon triggered earthquakes in Talala, India

The Deccan volcanic province (DVP) in the northwestern part of the Indian subcontinent is the largest continental volcanic province in the world. Talala, located in the middle of the DVP, experience seismicity with swarm-like behavior towards the end of, or shortly after, the annual monsoon period. Between the years 2007 and 2012, seismic activity was recorded with moment magnitudes up to Mw 5.1. Although there is a statistical correlation between seismic activity and monsoon rainfall, there is little information about the geometry or attributes of the potentially causative fault system to date. We conducted geophysical surveys in the Talala seismogenic region to investigate the structure of the causative fault, with two Time-Domain Electromagnetic (TDEM) profile measurements, and Magnetotelluric (MT) measurements across the fracture zone to characterize deeper structures. TDEM measurements in the E-W direction revealed an electrically conductive zone within highly resistive surroundings, and the MT measurements revealed high electrical resistance along a curved structure that correlates with the seismogenic area and is likely associated with fractured crystalline rock. These fracture systems or networks can act as conduits for channeling and focusing intense rainfall events and develop significant fluid over-pressures to drive the swarm sequences. Numerical modelling results of fluid pressure evolution indicate increased groundwater and reservoir levels are likely responsible for earthquake generation.

Thermal and Physical Properties of Deccan Basalt and Neoarchean Basement Cores From a Deep Scientific Borehole in the Koyna−Warna Seismogenic Region, Deccan Volcanic Province, Western India: Implications on Thermal Modeling and Seismogenesis

AbstractWe have investigated 78 core samples of basaltic trap and Neoarchean basement in the laboratory from a 981 m deep scientific borehole KBH‐05, in the Koyna−Warna seismic zone, in order to characterize their thermal and physical properties and present a probable crustal thermal model. The Koyna−Warna, located in the Deccan Volcanic Province (DVP), is globally one of the most prominent Reservoir Triggered Seismicity (RTS) regions. Thermal conductivity, density, and porosity vary widely (1.0–1.7 Wm−1K−1, 2,400–3,000 kg m−3, 0.2%–10%) for the basalt due to their lithological heterogeneities, i.e., massive/amygdaloidal/vesicular. In comparison, the basement, which dominantly consists of gneiss/migmatite gneiss (granodiorite to tonalite to quartz monzodiorite in composition) and amphibolite, have shown a wider range in thermal conductivity (2.2–3.4 Wm−1K−1) but constricted range in density and porosity (2,600–2,800 kg m−3, 0.01%–0.15%). Based on gamma and sonic logs, as well as radioelements (Th, U, K), the 499 m thick basaltic trap can be divided into two thick layers (325, 174 m) and five sub‐layers, which can be correlated with different basaltic formations. Similarly, the underlying basement can also be divided into two layers (93, 384 m). The upper basement layer has two times higher concentrations of Th and U than the lower layer, with heat production of 2.0, 0.8 μWm−3. Further, the study provides a robust temperature estimate of 165°C–250°C at 10 km depth, considerably higher than reported earlier for the DVP, and reveals that in addition to thermal parameters, RTS also plays an important role in the seismogenesis of the region.