Strike-slip Faulting Mechanisms Research Articles

Spatial variation of the source mechanisms in the Egyptian continental margin

The spatial variability of the focal mechanism solutions in the NW Egypt Cyrenaica and Levant-north Egypt domains of the Egyptian Continental margin is investigated based on the polarity of the P-wave first motions of eleven earthquakes with local magnitudes ML ranging from 4.2 to 6.2. The identified focal mechanism solutions in the Levant-north Egypt domain are evaluated in two sub-provinces with different geological settings. The first sub-province shows strike-slip faulting mechanisms with right-lateral motion along the NE-trending Rosetta-Qattara fault zone, while the NW-trending Temsah fault zone produces left-lateral strike-slip motion. The inland extension of the Rosetta-Qattara fault zone towards the Western Desert reflects the same sense of motion. Strike-slip faulting with thrust components dominates the focal mechanisms in the second sub-province but with a different stress orientation. A single thrust faulting mechanism event which occurred on the periphery of the continental shelf shows the same common orientation of the stress field at the Hellenic Arc. This type of mechanism in this locality can be explained by the mean of compressional stress transfer towards the coast generated by the accelerated southwest retreat of the Hellenic Trench. The complex regional stress along the Cyprian and Hellenic arcs, the current local structure features, and the dominant orientation of the pre-existing fault are the factors that contribute to the changeable stress pattern field in the Levant-north Egypt domain. A single event in the second domain, the NW Egypt Cyrenaica, is a strike-slip with a right-lateral sense of motion on a WNW-oriented plane, consistent with motion along the WNW tectonic fault which separates the continental shelf from the oceanic crust.

Cianjur M5.6 Earthquake Aftershock Survey (CEAS) Using ITB Temporary Seismographs

Western Java, part of the Sunda Arc, is an active tectonic region with high seismicity. The tectonic activity has produced active faults which are located near densely populated areas. On November 21, 2022, a significant earthquake rocked the Cianjur region, West Java, causing severe damage and loss of life. In this study we acquired aftershock data which was generated from the Cianjur M5.6 earthquake. Twenty stations consisting of 19 Smartsolo IGU-16HR 3C 5 Hz Seismometer (short-period type) and 1 Guralp 6TD 30s to 100 Hz Seismometer (broadband type) were deployed to monitor aftershocks from 22 November to 23 December 2022. The seismograph stations were deployed covering the earthquake source area and tectonic features in the Cianjur region. The results show that the aftershock events is clearly recorded at 20 stations with an average difference between the arrival times of the S waves and P waves (Ts-Tp) of 2 seconds. A total of ±500 earthquake events were obtained consisting of ±4000 P wave phases and ±3000 S wave phases with a Vp/Vs ratio value of 1.72 and RMS error less than 0.1 second. The Cianjur M5.6 earthquake was classified as a shallow crustal seismicity with a strike slip fault mechanism, hypocenters were generally distributed at a depth of 3-10 km.

Seismic Load Design of Rukoh Dam Suppletion Tunnel in Pidie Regency, Aceh, Indonesia, Based on Deterministic and Probabilistic Seismic Hazard Analyses

The Rukoh Dam suppletion tunnel construction site in Pidie Regency, Aceh province, is located 5.6 kilometers from the Seulimeum-South segment of the active Sumatran fault. The active fault features a strike-slip fault mechanism with a slip rate of 7.0 millimeters per year and a maximum earthquake-recorded magnitude of 6.9 Mw. Considering that the construction site is located less than 10 kilometers from the active fault, which may cause earthquakes with a magnitude of at least 6 Mw, the SNI 1729:2019, which describes the procedures for planning earthquake resistance for buildings and non-building structures, requires the seismic load to be designed based on the site-specific responses analysis. Determination of the seismic load using a deterministic seismic hazard analysis was the objective of this study. The seismic load based on the probabilistic seismic hazard analysis by Pusat Studi Gempa Nasional (PusGen) in 2017 is also compared. The 2017 Indonesian Earthquake Source and Hazard Map were used to find out about the fault mechanism, slip rate, deterministic magnitude, orientation, and coordinates of traces. The deterministic seismic hazard analysis was conducted using EZ-FRISK 8.07 software and attenuation equations with shallow crustal earthquake sources for fault and shallow background earthquake sources, i.e., the Boore-Atkinson NGA 2014, the Campbell-Bozorgnia NGA 2014, and the Chiou-Youngs NGA 2014 equations. The deterministic analysis based on those attenuation equations predicted the active fault to result in a peak ground acceleration (PGA) value of 0.66 g at the construction site of the Rukoh Dam suppletion tunnel. In comparison, the probabilistic hazard analysis conducted by Pusat Studi Gempa Nasional (PusGen) in 2017 indicated a peak ground acceleration (PGA) of 0.537 g. Therefore, the seismic load is recommended for the structural designs of the embankment dam and the other infrastructures, particularly the suppletion tunnel.

Comparison of Deformation Vectors Due to Earthquake in Subduction Zone and Sumatran Fault for Each Phase of Earthquake Cycle

This study compares the deformation in West Sumatra due to the earthquakes in the subduction zone and the Sumatran Fault. The Mw6.0 Mentawai earthquake 2019 with a thrust fault mechanism and the Mw5.4 South Solok earthquake 2019 with a strike-slip fault mechanism were used as case studies for the subduction zone and Sumatran Fault, respectively. The deformation was observed using 12 SuGAr (Sumatra GPS Array) and 8 InaCORS (Indonesian Continuously Operating Reference Station) stations, which were processed using GAMIT/GLOBK software. There are differences in the deformation vectors of the two earthquakes. The Mentawai earthquake experienced larger energy accumulation than the South Solok earthquake. The coseismic phase of the Mentawai earthquake experienced the largest horizontal shift at the SLBU station, which was 15.48 mm in the direction of S29.96W, while the South Solok earthquake is found to horizontally shift the CSDH station at the size of 5.75 mm towards S11.45E. The postseismic phase of the Mentawai earthquake lasted 60 days, longer than the South Solok earthquake (20 days). The difference in deformation characteristic between these two earthquakes found in this study will be valuable information in modeling earthquakes in Sumatra.

Deformation analysis of 2020 mw 5.7 Karlıova, Turkey, earthquake using DInSAR method with different incidence angle SAR data

Earthquakes, one of the biggest natural disasters, affect the regions to which it occurs to a high degree. Determining the mechanisms of the earthquake that occurred in the light of this information is very important for both the region and the subsequent earthquakes. Interferometric Synthetic Aperture Radar (InSAR) is a commonly used method for determining co-seismic surface deformations. Especially in areas where there is no Global Navigation Satellite System (GNSS) receiver and teleseismic data source, results can be obtained quickly and effectively with InSAR. In this study, the co-seismic surface deformation caused by the Mw 5.7 earthquake in Karliova (Bingol), Turkey on June 14, 2020, was determined with Sentinel-1 SLC images with different incidence angles. Since Karliova is located in the region where North Anatolian Fault (NAF) and East Anatolian Fault (EAF) lines intersect, the information given by the earthquakes in this region is very important and carefully monitored. As the InSAR technique has not been applied to earthquakes that have Mw <6 in this region, two ascending and two descending orbits were used in the study to ensure results. In the study, the amount of displacement in the line of sight (LOS) direction was determined approximately 25–30 cm. In addition, 4 different horizontal and vertical results were obtained with the combination of ascending and descending orbits. Maximum horizontal displacement created by the earthquake was obtained as 13 cm to the East, and 15 cm to the West. Moreover, in the vertical direction, uplift up to 10 cm in the northern part of the fault and subsidence up to 10 cm in the southern part of the fault were determined. According to these results, the earthquake was determined as right strike slip fault mechanism compatible with NAF tectonic structure.

Seismogenic Source Model of the 2019, Mw 5.9, East-Azerbaijan Earthquake (NW Iran) through the Inversion of Sentinel-1 DInSAR Measurements

In this work, we investigate the Mw 5.9 earthquake occurred on 7 November 2019 in the East-Azerbaijan region, in northwestern Iran, which is inserted in the tectonic framework of the East-Azerbaijan Plateau, a complex mountain belt that contains internal major fold-and-thrust belts. We first analyze the Differential Synthetic Aperture Radar Interferometry (DInSAR) measurements obtained by processing the data collected by the Sentinel-1 constellation along ascending and descending orbits; then, we invert the achieved results through analytical modelling, in order to better constrain the geometry and characteristics of the seismogenic source. The retrieved fault model shows a rather shallow seismic structure, with a center depth at about 3 km, approximately NE–SW-striking and southeast-dipping, characterized by a left-lateral strike-slip fault mechanism (strike = 29.17°, dip = 79.29°, rake = −4.94°) and by a maximum slip of 0.80 m. By comparing the inferred fault with the already published geological structures, the retrieved solution reveals a minor fault not reported in the geological maps available in the open literature, whose kinematics is compatible with that of the surrounding structures, with the local and regional stress states and with the performed field observations. Moreover, by taking into account the surrounding geological structures reported in literature, we also use the retrieved fault model to calculate the Coulomb Failure Function at the nearby receiver faults. We show that this event may have encouraged, with a positive loading, the activation of the considered receiver faults. This is also confirmed by the distribution of the aftershocks that occurred near the considered surrounding structures. The analysis of the seismic events nucleated along the left-lateral strike-slip minor faults of the East-Azerbaijan Plateau, such as the one analyzed in this work, is essential to improve our knowledge on the seismic hazard estimation in northwestern Iran.

Spatial variation in the present-day stress field and tectonic regime of Northeast Tibet from moment tensor solutions of local earthquake data

SUMMARYFocal mechanism solutions and their predicted stress pattern can be used to investigate tectonic deformation in seismically active zones and contribute to understanding and constraining the kinematic patterns of the outward growth and uplift of the Tibetan Plateau. Herein, we determined the focal mechanisms of 398 earthquakes in Northeast Tibet recorded by the China National Seismic Network (CNSN) by using the cut-and-paste method. The results show that the earthquakes predominately exhibited thrust and strike-slip faulting mechanisms with very few normal events. We then combined the derived focal mechanisms with global centroid moment tensor (GCMT) catalogue solutions and previously published solutions to predict the regional distribution of the stress field through a damped linear inversion. The inversion results show that most of region is dominated by a thrust faulting regime. From the southern East Kunlun fault in the west to the northern Qilian Mountains along the Altyn Tagh fault (ATF), the maximum compression axis rotates slightly clockwise; farther to the south of the Haiyuan fault in the east, there is an evident clockwise rotation of the maximum compression axis, especially at the eastern end of the Haiyuan fault. In the Qilian Mountains, the axis of the compressive stress orientation approximately trends NE–SW, which does not markedly differ from the direction of India–Eurasia convergence, emphasizing the importance of the compressive stress in reflecting the remote effects of this continental collision. The overall spatial pattern of the principal stress axes is closely consistent with the GPS-derived horizontal surface velocity. A comparison of the stress and strain rate fields demonstrated that the orientations of the crustal stress axes and the surface strain axes were almost identical, which indicates that a diffuse model is more suitable for describing the tectonic characteristics of Northeast Tibet. Additionally, the compressive stress orientation rotated to ENE–WSW in the northern Qilian Mountains along the ATF and to ENE–WSW or E–W along the eastern part of the Haiyuan fault and its adjacent area to the south, highlighting the occurrence of strain partitioning along large left-lateral strike-slip faults or the lateral variation of crustal strength across these faults. Combining geodetic, geological and seismological results, we suggest that a hybrid model incorporating both the diffuse model associated with shortening and thickening of the upper crust and the asthenospheric flow model accounting for the low-velocity zone in the middle-lower crust may reflect the primary mode of crustal deformation in Northeast Tibet.

RELOKASI HIPOSENTER GEMPABUMI DONGGALA TAHUN 2018 MENGGUNAKAN METODE HYPORELOCATE

Pada Jumat, 28 September 2018, pukul 17.02.44 WIB terjadi gempabumi yang berpotensi tsunami di daerah Donggala. BMKG mengeluarkan parameter gempabumi dengan magnitudo 7.7, lokasi 0.18 LS dan 119.85BT atau 26 km dari Utara Donggala. Gempabumi dengan kedalaman 10 km tersebut terjadi dengan mekanisme sesar mendatar akibat aktivitas sesar Palu-Koro dan diikuti oleh ratusan gempabumi susulan. Sesar Palu-Koro melintang dari Selat Makasar sampai pantai utara Teluk Bone memotong jantung kota Palu hingga ke sungai Lariang, Donggala. Studi ini akan merelokasi hiposenter gempabumi Donggala beserta gempabumi pendahulu dan susulannya menggunakan metode hypoRelocate. Relokasi ini menggunakan waktu tempuh mutlak dari 599 gempabumi, beda waktu tempuh relatif dari 2.492 pasangan gempabumi dan waktu dari 11 tumpukan coda cross-correlogram untuk selanjutnya diinversi dengan simulated annealing. Metode ini memiliki kelebihan berupa berupa pengurangan ketergantungan hasil relokasi pada lokasi absolut katalog dengan menyisir waktu tempuh mutlak, pembatasan jarak relatif antargempabumi dengan menggunakan perbedaan waktu tempuh dan koreksi jarak antargempabumi berdasarkan tumpukan coda cross-correlogram. Hasil relokasi menunjukan episenter gempabumi lebih mengumpul atau terfokus daripada posisi awal dan membentuk kluster yang berwujud sebuah bidang dengan luas sekitar 250 km × 85 km. Sebagian besar episenter setelah direlokasi bergeser dengan jarak 15‒20 km dan perubahan kedalaman gempabumi dominan kurang dari 10 km. Arah pergeseran episenter tersebar merata secara acak namun arah barat lebih sedikit dominan. Relokasi menggunakan hypoRelocate menunjukkan peningkatan kualitas yang cukup baik dengan turunnya nilai total residual setelah relokasi. Hasil validasi relokasi hypoRelocate dengan peta citra satelit ALOS-2 mode ScanSAR dan ALOS-2 PALSAR-2 menunjukan bahwa hasil relokasi berada pada bidang sesar akibat gempabumi.

Intraplate stress field in South America from earthquake focal mechanisms

We present an updated compilation of earthquake focal mechanisms in Brazil together with focal mechanisms from the sub-Andean region (mainly from global CMT catalogs). All earthquakes in the sub-Andean region show reverse (majority) or strike-slip faulting mechanisms. Focal mechanisms in Brazil show reverse, strike-slip and normal faulting. Focal mechanisms of nearby earthquakes in the same tectonic environment were grouped and inverted for the stress tensor. In the sub-Andean region, stresses are compressional, as expected, with the principal major compression (S1) roughly E-W, on average. A slight rotation of S1 can be observed and is controlled by the orientation of the Andean plateau. In the sub-Andean region, the intermediate principal stress (S2) is also compressional (i.e., larger than the lithostatic pressure, Sv), a feature that is not always reproduced in numerical models published in the literature. In mid-plate South America stresses seem to vary in nature and orientation. In SE Brazil and the Chaco-Pantanal basins, S1 tends to be oriented roughly E-W with S2 approximately equal to S3. This stress pattern changes to purely compressional (both SHmax and Shmin larger than Sv) in the São Francisco craton. A rotation of SHmax from E-W to SE-NW is suggested towards the Amazon region. Along the Atlantic margin, the regional stresses are very much affected by coastal effects (due to continent/ocean spreading stresses as well as flexural effects from sediment load at the continental margin). This coastal effect tends to make SHmax parallel to the coastline and Shmin (usually S3) perpendicular to the coastline. Few breakout data and in-situ measurements are available in Brazil and are generally consistent with the pattern derived from the earthquake focal mechanisms. Although numerical models of global lithospheric stresses tend to reproduce the main large-scale features in most mid-plate areas, the S1 rotation from ∼E-W in SE Brazil to SE-NW in the Amazon region are not well explained by the current numerical models. This means that the observed stress pattern in mid-plate South America should provide new insights into upper mantle dynamics, distinct from current global convection models.

3D displacement field retrieved by integrating Sentinel-1 InSAR and GPS data: the 2014 South Napa earthquake

In this study the integration of Sentinel-1 InSAR (Interferometric Synthetic Aperture Radar) and GPS (Global Positioning System) data was performed to estimate the three components of the ground deformation field due to the Mw 6.0 earthquake occurred on August 24th, 2014, in the Napa Valley, California, USA. The SAR data were acquired by the Sentinel-1 satellite on August 7th and 31st respectively. In addition, the GPS observations acquired during the whole month of August were analyzed. These data were obtained from the Bay Area Regional Deformation Network, the UNAVCO and the Crustal Dynamics Data Information System online archives. The data integration was realized by using a Bayesian statistical approach searching for the optimal estimation of the three deformation components. The experimental results show large displacements caused by the earthquake characterized by a predominantly NW-SE strike-slip fault mechanism.

Probabilistic Risk Assessment: Piping Fragility due to Earthquake Fault Mechanisms

A lifeline system, serving as an energy-supply system, is an essential component of urban infrastructure. In a hospital, for example, the piping system supplies elements essential for hospital operations, such as water and fire-suppression foam. Such nonstructural components, especially piping systems and their subcomponents, must remain operational and functional during earthquake-induced fires. But the behavior of piping systems as subjected to seismic ground motions is very complex, owing particularly to the nonlinearity affected by the existence of many connections such as T-joints and elbows. The present study carried out a probabilistic risk assessment on a hospital fire-protection piping system’s acceleration-sensitive 2-inch T-joint sprinkler components under seismic ground motions. Specifically, the system’s seismic capacity, using an experimental-test-based nonlinear finite element (FE) model, was evaluated for the probability of failure under different earthquake-fault mechanisms including normal fault, reverse fault, strike-slip fault, and near-source ground motions. It was observed that the probabilistic failure of the T-joint of the fire-protection piping system varied significantly according to the fault mechanisms. The normal-fault mechanism led to a higher probability of system failure at locations 1 and 2. The strike-slip fault mechanism, contrastingly, affected the lowest fragility of the piping system at a higher PGA.

Assessment the Effective Ground Motion Parameters on Seismic Performance of R/C Buildings using Artificial Neural Network

Building damage level due to earthquake is widely related to the features of the record which consist of many parameters. Although it is difficult to realize the ground motion parameters that have high influence on building performance, the vital parameters that may cause building damage may be considered as PGA, PGV, PGD, PGA/PGV, PGA/PGD, PGV/PGD, frequency content, effective time duration, fault line distance of the earthquake. In this study, these parameters were selected in order to determine more effective parameter on the building performance. For this aim, a model of Artificial Neural Network (ANN) algorithm was used as an efficient tool consisting of the obtained results of nonlinear time history analysis of samples. The 200 records, produced by strike-slip fault mechanism, were selected for the soil type C (Z3) according to the Turkish Earthquake Code [1]. A six story R/C frame building, with three various spans were analyzed via IDARC-2D software. The Park and Ang damage index was used in order to evaluate the vulnerability of buildings. The results showed that the ANN can be able to determine the effective parameters of ground motions with sufficient correlation. Also the most and least significant parameters of earthquake are discussed based on the results of the analysis.

Assessment the Effective Ground Motion Parameters on Seismic Performance of R/C Buildings Using Artificial Neural Network

Building damage level due to earthquake is widely related to the features of the record which consist of many parameters. Although it is difficult to realize the ground motion parameters that have high influence on building performance, the vital parameters that may cause building damage may be considered as PGA, PGV, PGD, PGA/PGV, PGA/PGD, PGV/PGD, frequency content, effective time duration, fault line distance of the earthquake. In this study, these parameters were selected in order to determine more effective parameter on the building performance. For this aim, a model of Artificial Neural Network (ANN) algorithm was used as an efficient tool consisting of the obtained results of nonlinear time history analysis of samples. The 200 records, produced by strike-slip fault mechanism, were selected for the soil type C (Z3) according to the Turkish Earthquake Code [1]. A six story R/C frame building, with three various spans were analyzed via IDARC-2D software. The Park and Ang damage index was used in order to evaluate the vulnerability of buildings. The results showed that the ANN can be able to determine the effective parameters of ground motions with sufficient correlation. Also the most and least significant parameters of earthquake are discussed based on the results of the analysis.

Tectonic structures across the East African Rift based on the source parameters of the 20 May 1990 M7.2 Sudan earthquake

Earthquakes in Kenya are common along the Kenya Rift Valley because of the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy, which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the M w = 7.2 southern Sudan earthquake of 20 May 1990 and aftershocks of M w = 6.5 and 7.1 on 24 May 1990, the 1937 M s = 6.1 earthquake north of Lake Turkana close to the Kenya-Ethiopian border, and the 1913 M s = 6.0 Turkana earthquake, among others. Source parameters of the 20 May 1990 southern Sudan earthquake show that this earthquake consists of only one event on a fault having strike, dip, and rake of 315°, 84°, and −3°. The fault plane is characterized by a left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment M o = 7.65 × 1019 Nm, and moment magnitude, M w = 7.19 (≅7.2). The fault rupture started 15 s earlier and lasted for 17 s along a fault plane having dimensions of ≅60 km × 40 km. The average fault dislocation is 1.1 m, and the stress drop, ∆σ, is 1.63 MPa. The distribution of historical earthquakes (M w ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW–SE alignment of epicenters is characterized by earthquakes of local magnitude M l ≤ 4.0, except the 1928 Subukia earthquake (M s = 6.9) in central Kenya. This NW–SE alignment of epicenters is consistent with the trend of the Aswa-Nyangia Fault Zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that the NW–SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.

The 1951 Mw 5.2 and Mw 5.3 Jaen, Southern Spain, Earthquake Doublet Revisited

We collected analog seismogram recordings and seismic bulletins for two moderate magnitude earthquakes in the province of Jaen, southern Spain, on 10 March and 19 May 1951, and the series of aftershocks. Seismograms from the two main events reveal striking similarity, pointing to nearby locations and similar source mechanisms. This casts a shadow on the quality of preserved phase readings and macroseismic data, which suggests a distance of several tens of kilometers between both mainshocks and aftershocks. A critical review of available phase readings permitted us to detect several misinterpretations in the original bulletins and to obtain better constrained hypocenter relocations—about 10 km apart—for the two mainshocks, as well as location estimates for 20 aftershocks. The recording of the 1951 Jaen earthquake doublet at a network of common stations allows a straightforward quality control of our digitized seismograms by waveform comparison. We estimate faulting parameters of the two mainshocks from regional moment tensor inversion, obtaining moment magnitude M w 5.2 and M w 5.3, respectively, depth of 20 km, and strike-slip faulting mechanisms with minor normal slip component and northeast–southwest oriented T axes. Deconvolution of body waveforms from an M w 4.4 aftershock yields simple triangular source time functions for both main events, with durations close to 1 sec. While several previous studies had difficulties in characterizing these earthquakes, partially describing them as unusual intermediate deep focus events, we propose simple shear faulting sources in the middle crust and faulting geometries consistent with the regional seismotectonic framework.

Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions

SUMMARY The south Caspian Basin is a relatively aseismic block within the Alpine-Himalayan Belt, but is surrounded by zones of high seismicity. We used the focal mechanisms of 16 earthquakes whose source parameters we determined from inversion of body waves and the mechanisms of 15 other earthquakes to determine the style of faulting in the seismic belts surrounding the south Caspian Basin. Earthquakes beneath the Talesh Mountains of north-west Iran and immediately off-shore in the south-west Caspian Sea have shallow thrust mechanisms, showing that the continental crust of north-west Iran is overthrusting the ‘oceanic-like’ crust of the south Caspian Basin. Earthquakes south of the Caspian Sea in northern Iran show a mixture of focal mechanisms. Both high-angle reverse faulting and left-lateral strike-slip faulting mechanisms are observed in the high Alborz Mountains. Farther south, on the edge of the central Iran plateau, oblique left-lateral reverse mechanisms are observed. It appears that the NE direction of shortening between central Iran and the south Caspian Basin is partitioned into pure left-lateral strike-slip and thrusting in the WNW-trending high Alborz, but is accommodated by oblique faulting in lower elevations. Earthquakes in the Kopet Dag Mountains east of the Caspian Sea also show a mixture of high-angle reverse and strike-slip faulting mechanisms and may be another example of the partitioning of oblique slip into strike slip and thrust motion. Normal faulting mechanisms at centroid depths of 35–50 km dominate in the belt of seismicity that extends across the central Caspian Sea. The significance of the normal faulting earthquakes is enigmatic. It is improbable that these events represent the motion between the southern Caspian Basin and Eurasia for they imply a sense of motion that is incompatible with the observed topography and folding in the sediments. Two shallow earthquakes at about 12 km depth in this belt, one a small event and the other a large second subevent of a multiple earthquake, have thrusting mechanisms suggesting that shortening occurs as the continental crust of the northern Caspian Sea is thrust over the ‘oceanic-like’ crust of the southern Caspian Basin. Shortening is also suggested by the orientations of folds in the sedimentary cover south of the central Caspian Sea seismic belt. We suggest that this shortening does indeed represent a NNE motion of the Caspian Sea relative to Eurasia, but that the motion is slow and has not produced many earthquakes. The deeper, normal-faulting events may be related to bending or down-dip extension of the incipient subducted slab. If the motion of the Caspian Sea relative to Eurasia is indeed slow, then the motion in the Alborz between central Iran and the south Caspian Basin will be almost the same as that between Iran and Eurasia, as has previously been assumed. The combined effect of the overthrusting of the south Caspian Basin by the Talesh-Alborz Mountains in the south, and by the continental crust of the northern Caspian Sea in the north will lead to the eventual destruction of the south Caspian Basin and the possible formation of an intermediate-depth, dipping seismic zone within the continental interior, similar to that presently observed in the Hindu Kush.

Comment on “The July 9 and 23, 1905, Mongolian earthquakes, a surface-wave investigation” by Emile Okal

We present an updated compilation of earthquake focal mechanisms in Brazil together with focal mechanisms from the sub-Andean region (mainly from global CMT catalogs). All earthquakes in the sub-Andean region show reverse (majority) or strike-slip faulting mechanisms. Focal mechanisms in Brazil show reverse, strike-slip and normal faulting. Focal mechanisms of nearby earthquakes in the same tectonic environment were grouped and inverted for the stress tensor. In the sub-Andean region, stresses are compressional, as expected, with the principal major compression (S1) roughly E-W, on average. A slight rotation of S1 can be observed and is controlled by the orientation of the Andean plateau. In the sub-Andean region, the intermediate principal stress (S2) is also compressional (i.e., larger than the lithostatic pressure, Sv), a feature that is not always reproduced in numerical models published in the literature. In mid-plate South America stresses seem to vary in nature and orientation. In SE Brazil and the Chaco-Pantanal basins, S1 tends to be oriented roughly E-W with S2 approximately equal to S3. This stress pattern changes to purely compressional (both SHmax and Shmin larger than Sv) in the São Francisco craton. A rotation of SHmax from E-W to SE-NW is suggested towards the Amazon region. Along the Atlantic margin, the regional stresses are very much affected by coastal effects (due to continent/ocean spreading stresses as well as flexural effects from sediment load at the continental margin). This coastal effect tends to make SHmax parallel to the coastline and Shmin (usually S3) perpendicular to the coastline. Few breakout data and in-situ measurements are available in Brazil and are generally consistent with the pattern derived from the earthquake focal mechanisms. Although numerical models of global lithospheric stresses tend to reproduce the main large-scale features in most mid-plate areas, the S1 rotation from ∼E-W in SE Brazil to SE-NW in the Amazon region are not well explained by the current numerical models. This means that the observed stress pattern in mid-plate South America should provide new insights into upper mantle dynamics, distinct from current global convection models.