Fault Plane Solutions Of Earthquakes Research Articles

Complex Multi‐Fault Dynamics in Sikkim Himalaya: New Insights From Local Earthquake Analysis

AbstractAnomalously overturned thrust faults, lineaments and segmentation causing cross‐cutting basement structures characterize the tectonic setting of Sikkim Himalaya. However, its seismotectonics is poorly constrained along with the speculated northward extension of the Dhubri‐Chungthang Fault Zone (DCFZ) causing segmentation. Here, we utilize the precise location of newly acquired local earthquake data and fault plane solutions using full‐waveform moment tensor inversion to better constrain seismically active zones. Transtensional shearing along the Main Himalayan Thrust in central Sikkim is possibly incited by fluid‐rich upper‐crust. Cessation of the mapped 20 km wide mid‐crustal seismogenic zone of DCFZ at Chungthang and its northward discontinuation into the Higher Himalayas is confirmed by the striking variation in focal mechanisms. Earthquakes along imbricated segments in the lower‐crust originate possibly in response to crustal shortening. Extensional shearing along the Moho triggers seismicity to the northwest of Sikkim. Such complex tectonic dynamics instigating persistent seismicity indicates high potential for future great earthquakes in Sikkim Himalaya.

Effect of permanent stress on the time-dependent earthquake probability: implication for seismic hazard assessment in the African continent

SUMMARY Earthquake hazard assessment is the first step towards implementing prevention, preparedness and response or faster recovery actions to reduce the risk of seismic disasters. In this paper, we present a comprehensive study on present-day seismicity in terms of the estimated waiting time and conditional probability in Africa by 2022–2072 using four competing distribution models (Brownian passage time (BPT), gamma, lognormal and Weibull). We also investigate how much Coulomb stress change $\ ( {\Delta {\rm CFF}} )$ induced by previous earthquakes that occurred in neighbouring active sources should revise the probability of occurrence at the location of the next events. We analyse large earthquakes with moment magnitude, ${M}_{\rm w} \ge 6.0$, collating data from the Global Centroid Moment Tensor and from several published literature that list fault plane solutions of large African earthquakes since 1900. We assume that the data set is stationary and consists of independent events. First, the model's parameters are estimated and the results of the statistical analysis of the interevent times show clear evidence of quasi-periodic recurrence behaviour for large earthquakes ${M}_{\rm w} \ge $ 6.0 in different seismotectonic regions in Africa. Next, a comparison among the distribution models is performed with the aim of selecting the most suitable one. The results in terms of the maximum likelihood criterion and its extension (Akaike Information Criterion) indicate that, in general, the BPT and Weibull models had similar fits to each other, and performed slightly better than gamma and lognormal models. Then, we use Gaussian random distributions to treat parameter uncertainties (e.g. aperiodicity, maximum expected magnitude, slip rate and mean recurrence time) of the distribution models associated with each seismotectonic region. From repeated Monte Carlo draws, we assess uncertainties of the 50-yr conditional probability values for the next earthquake obtained from two distribution models (BPT and BPT + $\Delta {\rm CFF}$) related to the 50th percentile. The results of the BPT distribution indicate very high chances of future earthquakes in the study region where the conditional probability of a large earthquake reaches 99.5, 95.6, 83.1 and 82.2 per cent for the western branch of the East African Rift System (EARS), northwest Africa, the Afar region, and the eastern branch of EARS, respectively. Taking into account the effect of stress change from interacting sources (BPT + $\Delta {\rm CFF}$), these probabilities are slightly modified to 99.8, 98.4, 89.9 and 87.3 per cent for the western branch of EARS, northwest Africa, the Afar region and the eastern branch of EARS, respectively. These marginal increases suggest that the estimated effect of the earthquake interaction introduced by the coseismic slip of previous earthquakes on neighbouring active sources is minor if compared with the uncertainties affecting the renewal models used for the basic time-dependent conditional probability assessment.

Paleoseismological features and tectonic setting of the Fethiye-Burdur fault zone (SW Turkey)

The Fethiye?Burdur fault zone is not a single line. Still, it consists of discontinuous northeasterly trending fault segments developed parallel to each other between the Gulf of Fethiye and Burdur Lake. The NE?trending and leftlateral oblique?slip Fethiye?Burdur fault zone bounds the Isparta Angle to the west and is probably a continuation of the Pliny fault zone of the Hellenic arc. Year 1914, 1957, and 1971 earthquakes occurred in the Fethiye?Burdur fault zone, with magnitudes of 7.1, 7.0, and 6.1, respectively. The epicenter distribution of the earthquakes of the last century indicates the continuation of the Fethiye? Burdur fault zone under the Gulf of Fethiye, probably to Rhodos Island. According to GPS measurements, the Fethiye?Burdur fault zone is the southern limit of the Aegean extensional region. The Aegean region is characterized by coherent motion toward the SW at 30 mm/yr relative to the Eurasian plate. Field observations and fault plane solutions of recent earthquakes in and around the Burdur Lake region reflect normal and left?lateral oblique faulting considering the NE extension of the fault zone. According to the proposed kinematic model, the different seismotectonic behaviors of the Fethiye?Burdur fault zone originate in the rigid influence of Ye?ilova peridotite massif of the Lycian nappes that occur to the SW of Burdur Lake. The kinematic interactions between the mainly NE?trending enechelon fault constituents of the Fethiye?Burdur fault zone result in different seismotectonic characteristics. Consequently, a new kinematic model is obtained for Burdur Lake and its surroundings.

Seismic reflectivity, fracturing and stress field data from the FFC-1 exploratory geothermal project in SW Skåne, Sweden

Enhanced geothermal systems (EGS) are a potential heat source in many parts of the world, even in locations where the temperature gradient is relatively low. We present here an integrated study of reflection seismic data, borehole logs and seismicity analysis performed in conjunction with a geothermal exploratory project operated by E.ON in Malmö, Sweden. In 2020, the pre-existing 2.1 km deep FFC-1 borehole through the sedimentary cover was deepened into the crystalline basement to about 3.1 km vertical depth. Combined interpretation of the reflection seismic data and geophysical wireline logs show that most of the reflectivity in the Precambrian basement is likely generated by lenses of mafic amphibolite embedded in a felsic gneissic matrix. The general structural bedding and foliation is gently dipping to sub-horizontal, similar to other locations in southwest Sweden. Fracture frequency is relatively high in the crystalline rock mass, with heavy fracturing in the uppermost part of the crystalline basement, obscuring a clear reflection from the top of the Precambrian. Highly fractured and hydraulically conductive intervals are also found between 2,562 and 2,695 m based on a temperature drop and the interpretation of the geophysical data. Open fractures, both natural and induced, have a clear N–S orientation, contrasting with the expected NW–SE direction based on the orientation of the Sorgenfrei-Tornquist Zone and earthquake fault plane solutions to the north. This difference may be partly explained by local variations in the stress field near the FFC-1 borehole and vairations in the stress field with depth. Despite this, the data from the FFC-1 well provide novel and unique information on the complex physical state of the crystalline basement on the margin of the Fennoscandian Shield, which further addresses the need for obtaining in-situ stress data to fully understand the local stress field prior to any stimulation. A temperature of 84°C measured at 3 km depth indicates that a desired EGS temperature of 120–140°C may be reached at 5–6 km depth, assuming a temperature gradient of about 20°C. If the relatively high fracture frequency and occurrence of fracture zones down to 3.1 km are also present at these target depths, then the FFC-1 location may be suitable for heat extraction if the rock mass is properly characterized before stimulation.

Seismic hazard potential of Kachchh paleo-rift basin of Indian craton from seismic and geodetic strain rates

The Kachchh paleo-rift basin of western India which recently experienced Mw 7.6, 26th January 2001 Bhuj earthquake, is one of the most active and vulnerable intra-cratonic zones in the world categorized as seismic zone V, i.e. having potential to produce earthquakes of intensity IX or higher. We analysed the geodetic and seismic principal strain rates to reveal the seismic hazard potential of the Kachchh paleo-rift basin. The geodetic strain rates are calculated from the reported surface motion in the region using a modified least-square inversion scheme that utilizes distance and spatial coverage dependent weight factor on the uncertainty in the plate motion to get a reliable estimate of principal strain rates and their orientations at regular grid nodes. An optimal weighting factor yields a mean rate of compression of ~ − 16 ± 6 nano strain/yr towards north-northeast in the region. The seismic strain rates are estimated based on fault plane solutions of the significant past earthquakes (Mw ≥ 2.5) of the region using the Kostrov formulations. The seismic strain rate and stress inversion estimate indicate that the region released elastic strain at an average rate of ~ − 85 nano strain/yr towards north-south in the past 350 years. A composite analysis of the geodetic and seismic strain rates indicates 900–3000 years of recurrence interval of major (Mw 7.8–8) earthquakes in the region. Our analysis indicates a high rate of strain build-up on the reactivated major faults in the region that have the potential to generate devastating (Mw ≥6) earthquakes.

A study of Palnadu earthquake sequence in the Cuddapah basin and its tectonic implications

On January 26, 2020, a moderate-sized earthquake (ML ~ 4.6) located in the Palnadu sub-basin of Cuddapah basin was felt both in the states of Telangana and Andhra Pradesh. The earthquakes prior and after the ML 4.6 are located close to the thrust and along the periphery of the backwaters of the Pulichintala Reservoir. About 965 earthquakes in the magnitude range of 0.1–4.6 ML have been located by seismological network. The epicentral parameters obtained from double difference technique illuminated a steep seismogenic structure extending up to 8 km depth using a minimum 1-D velocity model. The b-value estimate is 0.82 for a completeness magnitude of Mc 1.8 and could be associated with the intraplate event having a longer recurrence time. The focal mechanism solution obtained from waveform inversion reveals a pure double-couple mechanism of a strike-slip motion with a reverse component on a N–S trending focal plane. The spatial distribution of the earthquake sequence and fault plane solution of the main event are close to a thrust which is the most likely source region of the main-shock.

Tectonic stress regime and stress patterns from the inversion of earthquake focal mechanisms in NW Himalaya and surrounding regions

Because the impact of earthquake effects can extend over great distances from their origin, stress field inversion was performed for 440 earthquake focal mechanisms from the northwest Himalayas and surrounding regions compiled from the data bulletins of international seismological institutions. Earthquakes between November 1976 and April 2019 in the depth range of 10–258 km with moment magnitudes between 4.6 and 7.9 were selected. High-quality solutions were inverted to locate the best fitting stress tensor. Most of the earthquake fault plane solutions indicated thrust faulting, confirming northward underthrusting of the Indian plate along the Main Boundary Thrust and the Main Central Thrust systems and eastward underthrusting along the Burmese Arc. The focal mechanisms indicated right-lateral motion along the Karakoram Fault and left-lateral motion along the Kirthar–Sulaiman Range, which agrees with the expected sense of the lateral mass movement of the continental collision model. The present-day stress regimes obtained from the earthquake focal mechanism inversions indicated a predominantly compressional stress regime represented by NNE–SSW trending normal fault mechanisms in northwest India and Nepal and NNW–SSE trending normal fault mechanisms in Pakistan and Hindukush. These are consistent with the direction of the ongoing India–Eurasia plate collision and the extensional stress of WNW–ESE trending thrust faulting in the Xizang and Kashmir regions. These tectonic regimes connected with the major tectonic affecting the Arabian Peninsula. Accordingly, it is highly recommended to assess the earthquake hazards of the major cities in the eastern countries of Arabian Peninsula as Kuwait, Saudi Arabia, United Arab Emirates and Oman.

Rate of seismic deformation in the Gulf of Aqaba inferred from moment-tensor summation

This study aims to quantify the rate of coseismic deformation in the Gulf of Aqaba. Earthquake catalogue, Gutenberg-Richter relationship and fault plane solutions were integrated to measure the size and shape of deformation using the moment-tensor summation technique. First, the Gutenberg-Richter relationship was established using seismicity data from the period of 1964–2019. Then, the moment-tensor summation based on 44 focal mechanism solutions was used to calculate the shape of deformation. The eigenvalues of moment-tensor reflect the diversity of focal mechanism solutions that alternate from normal to strike-slip fault styles in the deformation zone. The analysis reveals a dominant shear deformation in the Gulf of Aqaba that extends in a direction of N42.2°E at a rate of 2.6 ± 0.04 mm yr−1and shortens in the direction of N305.2°E at a rate of 2.0 ± 0.02 mm yr−1. These results suggest that the active deformation occurring in the Gulf of Aqaba is due to the relative tectonic movements between the Arabian and African plates, as well as Sinai subplate.

Focal mechanisms in Romania: statistical features representative for earthquake-prone areas and spatial correlations with tectonic provinces

Fault plane solutions for earthquakes recorded in Romania (1929–2012) are analysed on three depth levels: crust (0–50 km), upper (50–110 km) and lower segment (110–201 km). For the Vrancea intermediate-depth source reverse faulting is predominant. However, local-scale variations occur at the upper and lower limits of the active volume: normal faulting (upper side) and strike-slip with normal faulting (lower side). These edge effects are probably caused by the interaction of cold descending lithosphere with hot surrounding asthenosphere acting there. Fault plane solutions of crustal earthquakes reflect complicated patterns associated to local stress sources perturbing the regional field. One important result of our analysis is the delimitation of specific active alignments in North Dobrogea Orogen, Bârlad Depression, Danubian and Banat zones, while seismicity is diffuse and close to random distribution in the other seismogenic zones. The polar diagrams for azimuthal and dip angle distributions and the ternary diagrams for P, T and B axes show prevalence of reverse faulting in Vrancea intermediate-depth source, strike-slip in combination with normal faulting in South Carpathians and Banat region and a deficit of strike-slip faulting south-east of Carpathians. Lack of strike-slip component makes us believe that the deformation field is controlled in the Carpathians Foredeep not by transcurrent deformation along the major faults crossing the region, but rather by subsidence and folding processes as stress release mechanisms in the crust in response to the intense tectonic processes beneath Vrancea region.

On the determination of fault plane solutions of weak crustal earthquakes of Vrancea region (Romania)

An inversion scheme using only few good quality high-frequency local waveforms was previously applied to estimate the source mechanism for a pilot set of crustal earthquakes with local magnitudes less than 4, from the bend of the South-eastern Carpathians. Taking advantage of the improved velocity and attenuation models recently determined for the study region, we reevaluate the focal mechanism of a couple of low-magnitude events (local magnitude ≥ 3), and assess the uncertainty of the fault plane solutions. The results evidence the increased reliability of the retrieved mechanisms and emphasize the capability of the approach to provide satisfactorily constrained fault plane solutions for the weak-to-moderate earthquakes in the study region.

Propagation and termination of a strike slip fault in an extensional domain: The westward growth of the North Anatolian Fault into the Aegean Sea

Geometry and kinematics of the active submarine faults over the North Aegean Trough, a complex and highly deforming boundary zone between the Eurasian and the Anatolian/Aegean micro-plates, were examined to investigate how a strike slip motion propagates through an extensional domain. The bathymetric and seismic reflection data analysis show, that the investigated area is affected from east to west, by three sets of active faults striking ENE-WSW, NE-SW and WNW-ESE. The ENE-WSW and NE-SW striking faults are right lateral strike slip faults and are considered strands of the northern branch of the North Anatolian Fault into the north Aegean Sea. The former set bounds the Saros basin and the latter set occupies the eastern and central part of the Sporades Basin in the eastern and western parts of the North Aegean Trough, respectively. The WNW-ESE striking faults are normal low angle faults and develop at the western parts of the Sporades Basin. It is suggested that the strike-slip fault system advances westwards and when it reaches the 23.8°E meridian, its right lateral motion is transferred to the WNW-ESE normal faults system, forming a 70 km wide normal faulting zone through which the shear is distributed. The interaction between the strike slip and normal faults probably started with the arrival of the northern branch of the North Anatolian Fault into the western part of the Sporades Basin since 430 ka BP (Marine Isotopic Stage, 12). The described fault architecture and their spatial distribution in the North Aegean Trough are consistent with: (i) the earthquake fault plane solutions and (ii) the stress and strain pattern prevailing across the area. Furthermore, all these three provide evidence that the northern branch of the North Anatolian Fault terminates at the western end of the NAT and does not continue to central Greece.

Crustal structure beneath Tien Shan orogenic belt and its adjacent regions from multi-scale seismic data

As one of the world’s most active intracontinental mountain belts, Tien Shan has posed questions for researchers regarding the formation of different tectonic units and active shallow seismicity. Here, we used a huge data set comprising of 7094 earthquakes from local, regional and teleseismic seismic stations. We used waveform modeling and multi-scale double-difference earthquake relocation technique to better constrain the source parameters of the earthquakes. The new set of events provided us with better initial earthquake locations for further tomographic investigation. We found that reverse-faulting earthquakes dominate the whole study area while the fault plane solutions for earthquakes beneath the northwestern Tarim Basin and the Main Pamir Thrust are diverse. There is a low-velocity anomaly beneath Bashkaingdy at depth of 80 km, and high-velocity anomalies beneath central Tien Shan at shallower depths. These observations are the keys to understand the mechanism of Tien Shan’s formation because of Tarim Basin northward and Kazakh Shield’s southward subduction in the south and north respectively. Velocities beneath western Tien Shan are relatively high. We thus infer that the Western Tien Shan is relatively less deformed than the eastern Tien Shan primarily due to a relatively brittle mantle.

The relation between fault movement potential and seismic activity of major faults in Northwestern Vietnam

The relation between fault movement potential and seismic activity of major faults in Northwestern Vietnam

FAULTING DEFORMATION OF THE MESOHELLENIC TROUGH IN THE KASTORIA-NESTORION REGION (WESTERN MACEDONIA, GREECE)

The Kastoria-Nestorion region, which belongs to the Tertiary MesoHellenic Trough (MHT), is a low relief NW-SE trending intermountainous basin filled with Tertiary molasse-type sedimentary rocks and nowadays drained by the Aliakmnonas River and its tributaries. In the present work, the large fault zones in the region and the general fault pattern are defined, mapped and described with the aid of satellite images. In addition, a large number of fault-slip data from the mesoscale exposed faults has been recorded, in order to better understand the faulting geometry and kinematics of the region. The stress-inversion analysis of these fault-slip data in comparison with earthquake faultplane solution information permits us to define the stress regimes imposed to the region from the Late Tertiary up to the present and to correlate them with the late orogenic and post-orogenic deformation of the Hellenic orogen. In particular, five stress regimes have been defined from which the former two (D1 and D2) are related to the late collisional processes between the Apulia and Eurasia plates, the next two events (D3 and D4) are related to the present-day Hellenic subduction zone, whereas the last D5 event which is the active deformation of the region appears as an intra-continental or intra-plate deformation more related with the Adria-Eurasia ongoing convergence rather with the Hellenic subduction zone.

Focal Mechanisms of Mw 6.3 Aftershocks from Waveform Inversions, Phayao Fault Zone, Northern Thailand

The focal mechanisms of Mw 6.3 aftershocks, Chiang Rai Province, Northern Thailand, were determined by using a multistation waveform inversion. Three aftershocks were selected and their waveforms were inverted for moment tensor calculation. Waveform inversions were derived from three broadband stations with three components and epicentral distances less than 250 km after all seismic stations were considered. The deviatoric moment tensor inversion was used for focal mechanism calculations. Band-pass filtering in the range of 0.03–0.15 Hz was selected for reducing low- and high-frequency noise. Source positions were created by using a single-source inversion and a grid-search method computed to optimize the waveform match. The results showed stable moment tensors and fault geometries with the southwest azimuth in the northern part of the Payao Fault Zone (PFZ) with depths shallower than 10 km. Left-lateral strike-slip with a reverse component was detected. The tectonics of the PFZ is constrained by fault-plane solutions of earthquakes. WSW directional strikes are observed in the northern part of the PFZ.

Structural development of northwest Saudi Arabia using aeromagnetic and seismological data

High resolution aeromagnetic and seismological data constrained by field-based structural investigations have been used to map and delineate the structural elements that affected and shaped the Midyan area in the northwest part of Saudi Arabia. The area was divided into four major domains defined by NNE, NNW, NW and ENE trending faults identified by trends, patterns and intensity of magnetic anomalies. The ENE trending left-lateral strike-slip faults intersected by NNE trending faults are the predominant tectonic features in the Gulf of Aqaba coastal area and stop at the boundary of a central domain characterized by complexity in the pattern and intensity of magnetic anomalies, that may be attributed to heterogeneity of basement rocks containing complex igneous rock suites including diorite, gabbro, ultramafic and alkali granitic rocks. This domain is characterized by the presence of narrow linear magnetic anomalies that extend for kilometers in an NNW direction, indicating dikes intruded through NNW trending faults. These dikes become WNW-oriented near their northern termination by transfer of movement to WNW-oriented faults marking the northern termination of the Red Sea rift. It is believed that this fault zone is still experiencing neotectonic activity, as evident from recorded seismicity. The aeromagnetic structural results coincide with fault plane solutions for the largest earthquakes, confirming aeromagnetic interpreted trends and illustrating mixed mechanisms between extensional and strike-slip faulting. Thus the study area displays different mechanisms associated with different tectonic trends which show clearly in the structural patterns of the area.

Joint analysis of geodetic and earthquake fault-plane solution data to constrain magmatic sources: A case study from Kīlauea Volcano

A joint analysis of geodetic and seismic datasets from Kīlauea Volcano during a period of magmatic unrest in 2006 demonstrates the effectiveness of this combination for testing and constraining models of magma dynamics for a complex, multi-source system. At the end of 2003, Kīlauea's summit began a four-year-long period of inflation due to a surge in magma supply to the volcano. In 2006, for the first time since 1982, Kīlauea's Southwest Rift Zone (SWRZ) also experienced inflation. To investigate the characteristics of active magma sources and the nature of their interactions with faults in the SWRZ during 2006, we integrate, through Coulomb stress modeling, contemporary geodetic data from InSAR and GPS with a new catalogue of double-couple fault-plane solutions for volcano-tectonic earthquakes. We define two periods of inflation during 2006 based on the rate of deformation measured in daily GPS data, spanning February to 15 March 2006 (Period 1) and 16 March to 30 September 2006 (Period 2). InSAR data for these two periods are inverted to determine the position, change in size, and shape of inflation sources in each period. Our new models are consistent with microseismic activity from each period. They suggest that, during Period 1, deformation in the SWRZ can be explained by pressurization of magma in a spherical reservoir beneath the south caldera, and that, during Period 2, magma was also aseismically intruded farther to the southwest into the SWRZ along a sub-horizontal plane. Our Coulomb stress analysis shows that the microseismicity recorded in the SWRZ is induced by overpressurization of the south caldera reservoir, and not by magma intrusion into the SWRZ. This study highlights the importance of a joint analysis of independent geophysical datasets to fully constrain the nature of magma accumulation.

FOCAL MECHANISMS OF EARTHQUAKES AND STRESS FIELD OF THE CRUST IN MONGOLIA AND ITS SURROUNDINGS

We have compiled and analyzed earthquake focal solutions for the territory of Mongolia and its surroundings in order to reveal a spatial variability of stress orientation and stress regimes of the crust. According to the stress inversion results, the SHmax is turning from W-E in the eastern Mongolia to SW-NE in the Gobi Altay and the central Mongolia, and then to S-N in the western part of the region. Comparison with data derived from GPS measurements shows that directions of the strain axes revealed by the geodetic and seismological observations are generally consistent. A contradiction is found for the Bolnai zone where results of GPS estimation indicate the predominance of extension (in the SE-NW direction), whereas earthquake data for the longer period of seismic observations reveal compression. Compression in this zone is mainly due to the Tsetserleg-Bolnai earthquakes contribution; however, a part of the recent data on focal mechanisms fits an extensional stress field with the NNW orientated extension axis. These data are in accordance with some published works which suggest a transtensive field from some structural geology studies in the eastern part of the Bolnai zone. The paper is supplemented with a list of M≥4.5 earthquake fault plane solutions and unpublished focal mechanisms for some M≤4.5 earthquakes of the northern Mongolia and the southern Baikal region.

Seismicity and stress inversion study in the Kangra–Chamba region of north-west Himalaya

The seismotectonic of the Kangra–Chamba region, the source zone of the 1905 great Kangra earthquake (M 8.0), has been studied analysing about 350 local earthquakes recorded by broadband seismological network under operation in NW Himalaya. The study reveals that the seismic activity is concentrated to the north of surface trace of the Main Boundary Thrust, Panjal Thrust and Chamba Thrust. The hypocenters are confined within 40 km depth with higher concentration in the uppermost crust. Fault plane solutions of 41 selected local earthquakes are examined. The fault plane solutions for shallow earthquakes (<10 km) predominantly shows thrust mechanisms, whereas earthquakes below ~10 km depth shows normal faulting mechanism. The stress tensor study reveals maximum compressional stress (σ 1) with trends 29°N and plunges at 6°, and the minimum stress (σ 3) trends at 134°N and plunges at 67° in the upper 10-km crust favouring thrust mechanism of the earthquakes with a NE-directed regional compressive stress. In the deeper part, σ 1 trends at 244°N and plunges at 59° while σ 3 shows extension trending at 9°N with a plunge of 20° giving evidence of extensional tectonics in the central part of the study area. The NE-dipping thrust sheets existing in the region as well as SW-dipping Chenab Normal Fault merges with the detachment that creates a complex tectonic interaction which is responsible for generating normal faulting earthquakes below the detachment plane.

Seismicity and Seismotectonics of Jeddah-Makkah Region, West-Central Saudi Arabia

Jeddah-Makkah regionis have been suffering from earthquake crisis where some moderate to destructive earthquakes have been recorded. These earthquake activities are oriented along major faults or clustered in certain spots. Moreover, these earthquake events have annual recurrence periods, so the identification of these seismogenic source zones is of utmost importance for mapping the most hazardous localities which should be avoided in the future urban planning. Historical and instrumental earthquakes have been collected from national and international data centers and unified in catalogue. The existence of microearthquakes inland suggests that there is a significant level of tectonic activity at away from the axial trough of the Red Sea. Then, seismogenic source zones have been defined depending on the major tectonic trends; distribution of earthquake epicenters, seismicity rate (a & b-values) and fault plane solution of major earthquakes. It is concluded that Jeddah-Makkah region is affected by the outlined five seismogenic source zones; three of these zones aligned of the main Red Sea axial trough (southwestern Jeddah, western Jeddah, and northwestern Jeddah zones), while the other two zones are located in the land area of the region (Thewal-Rabegh and Jeddah-Makkah zones). These inland zones correlated well with the main trends of major tectonics which refleet the reactivation of tectonic movements along these fault trends. The Red Sea zones are in agreement with the main path of the axial trough. The range of b-value in these identified zones is 0.65 to 1.03 through these identified zones. The area characterized by higher b-values could be indicative of a relative low stress regime which was a result of resulting from the stress release by the earthquakes. Whereas, the areas of lower b-values can be considered as an evidence of a relatively higher stress regime associated with a dominantly extensional stresses. Based on aforementioned, the region is suffering from different stress level accumulations which, in turn, cause earthquakes with different magnitudes. Accordingly, deployment of local seismograph network through Jeddah-Makkah region is highly recommended. These results will support, to a great extent, seismic hazard assessment and risk mitigation of the region.