Left-lateral Strike-slip Motion Research Articles

Microseismicity distribution in the southern Dead Sea basin and its implications on the structure of the basin

While the Dead Sea basin has been studied for a long time, the available knowledge about the detailed seismicity distribution in the area, as well as the deeper structure of the basin, is limited. Therefore, within the framework of the international project DESIRE (DEad Sea Integrated REsearch project), a dense temporary local seismological network was operated in the southern Dead Sea area. We use 530 local earthquakes, having all together 26 730 P- and S-arrival times for a simultaneous inversion of 1-D velocity models, station corrections and precise earthquake locations. Jackknife tests suggest an accuracy of the derived hypocentre locations of about 1 km. Thus, the result is the first clear image of the absolute distribution of the microseismicity of the area, especially in depth. The seismicity is concentrated in the upper crust down to 20 km depth while the lower limit of the seismicity is reached at 31 km depth. The seismic events at the eastern boundary fault (EBF) in the southern part of the study area represent the northward transform motion of the Arabian Plate along the Dead Sea Transform. North of the Boqeq fault the seismic activity represents the transfer of the motion in the pull-apart basin from the eastern to the western boundary. We find that from the surface downward the seismic events are tracing the boundary faults of the basin. The western boundary is mapped down to 12 km depth while the EBF reaches about 17 km depth, forming an asymmetric basin. One fifth of the data set is related to a specific cluster in time and space, which occurred in 2007 February at the western border fault. This cluster is aligned vertically, that is, it is perpendicular to the direction of the dominating left-lateral strike-slip movement at the main transform fault.

Analysis of Palaeogene strike-slip tectonics along the southern East Greenland margin (Sødalen area)

This paper describes structural data collected during field work in southern East Greenland, a region characterised by a complex tectonic history. Here, 3D photogeology based on aerial and oblique photographs using high-resolution photogrammetry of a 150 km2 area in Sødalen in southern East Greenland shows ESE–WNW-trending faults cross-cutting Paleocene rift structures and flexure-related normal faults. The kinematic analysis highlights oblique and left-lateral strike-slip movements along faults oriented 120°. Strike-slip and dip-slip kinematic indicators on the walls of the chilled contacts between alkaline E–W-oriented dykes and the volcanic host rocks suggest that the faults and dykes formed at the same time, or maybe the faults were re-activated at a later stage. Palaeostress analysis, performed by inversion of fault-slip data, shows the presence of three different tectonic events. Coupling the 3D photogeological tool with structural analysis at key localities is a fundamental way to understand better the tectonic history of such a large area.

Widespread Cretaceous inversion event in northern Spain: evidence from subsurface and palaeomagnetic data

Abstract: In this paper we present geological evidence of a mid-Cretaceous inversion event in the Polientes basin (northern Iberian plate) based on geological data. Analysis of seismic profiles across the basin indicates that this tectonic event resulted in the uplift of the basin depocentre and the formation of an upright gentle anticline before Cenomanian times. The presence of a pre-Alpine, syntectonic remagnetization in the Jurassic and Lower Cretaceous rocks of the Polientes basin allows us to define the geometry of the basin at the time between this early inversion event and the main Alpine compressional period. The early inversion in the Polientes basin is consistent with a transpressional tectonic setting, contemporaneous with the left-lateral strike-slip movement of Iberia with respect to Europe. This study shows the successful use of two approaches to unravel the geodynamic scenario of the northern Iberian plate during the Cretaceous, which was later obliterated during the Tertiary Pyrenean orogeny.

Strain data recorded in Xinjiang and their bearing on Tianshan’s uplift/shortening and Tarim basin’s rotation

Based on the continuous strain data recorded in Xinjiang since 1985, we discuss the mechanisms of Tianshan’s uplift and Tarim basin’s clockwise rotation. The results indicate; 1) The principal – compression directions in Tianshan are nearly NS, and their intersection angles with regional structures and mountains are nearly perpendicular, which is in accordance with Tianshan’s uplift and crustal shortening. 2) The principal compressions around Tarim basin tend to facilitate the regional faults’ left-lateral strike-slip movements and the basin’s clockwise rotation. These phenomena of uplift/shortening and rotation are fundamentally the results of India plate’s northward push on Euro-Asia plate, and the associated Pamir arc’s rapid northward movement and regional blocks’ interaction.

Co-seismic strike-slip surface rupture and displacement produced by the 2010 M W 6.9 Yushu earthquake, China, and implications for Tibetan tectonics

The magnitude (M W) 6.9 (M s 7.1) Yushu earthquake occurred on 14 April 2010 in the Yushu area, central Tibetan Plateau, causing widespread damage in high mountain regions. Field investigations reveal that the earthquake produced a 33-km-long surface rupture zone, with dominantly left-lateral strike-slip along the Yushu Fault of the pre-existing strike-slip Ganzi-Yushu Fault Zone. The co-seismic surface ruptures are characterized by discontinuous shear faults, right-stepping en echelon tensional cracks, and left-stepping mole track structures that indicate a left-lateral strike-slip shear sense for the seismic fault. Field measurements indicate co-seismic left-lateral strike-slip displacements of approximately 0.3–3.2 m (typically 1–2 m), accompanied by a minor vertical component of <0.6 m. The present results show that (i) the Yushu earthquake occurred upon the pre-existing active Ganzi-Yushu Fault Zone, which controlled the spatial distribution of co-seismic surface ruptures and displacements; (ii) the left-lateral strike-slip motion indicates that the Ganzi-Yushu Fault Zone partitions deformation into eastward extrusion of and northeastward shortening the central Tibetan Plateau to accommodate the continuing penetration of the Indian plate into the Eurasian plate.

Left-lateral active deformation along the Mosha–North Tehran fault system (Iran): Morphotectonics and paleoseismological investigations

The Mosha and North Tehran faults correspond to the nearest seismic sources for the northern part of the Tehran megacity. The present-day structural relationships and the kinematics of these two faults, especially at their junction in Lavasanat region, is still a matter of debate. In this paper, we present the results of a morphotectonic analysis (aerial photos and field investigations) within the central part of the Mosha and eastern part of the North Tehran faults between the Mosha valley and Tehran City. Our investigations show that, generally, the traces of activity do not follow the older traces corresponding to previous long-term dip–slip thrusting movements. The recent faulting mainly occurs on new traces trending E–W to ENE–WSW affecting Quaternary features (streams, ridges, risers, and young glacial markers) and cutting straight through the topography. Often defining en-echelon patterns (right- and left-stepping), these new traces correspond to steep faults with either north- or south-dipping directions, along which clear evidences for left-lateral strike–slip motion are found. At their junction zone, the two sinistral faults display a left-stepping en-echelon pattern defining a positive flower structure system clearly visible near Ira village. Further west, the left-lateral strike–slip motion is transferred along the ENE–WSW trending Niavaran fault and other faults. The cumulative offsets associated with this left-lateral deformation is small compared with the topography associated with the previous Late Tertiary thrusting motion, showing that it corresponds to a recent change of kinematics.

The Rudbār Mw 7.3 earthquake of 1990 June 20; seismotectonics, coseismic and geomorphic displacements, and historic earthquakes of the western ‘High-Alborz’, Iran

SUMMARY The Rudbār earthquake of 1990 June 20, the first large-magnitude earthquake with 80 km left-lateral strike-slip motion in the western ‘High-Alborz’ fold-thrust mountain belt, was one of the largest, and most destructive, earthquakes to have occurred in Iran during the instrumental period. We bring together new and existing data on macroseismic effects, the rupture characteristics of the mainshock, field data, and the distribution of aftershocks, to provide a better description of the earthquake source, its surface ruptures, and active tectonic characteristics of the western ‘High-Alborz’. The Rudbār earthquake is one of three large magnitude events to have occurred in this part of the Alborz during recorded history. The damage distribution of the 1485 August 15 Upper Polrud earthquake suggests the east–west Kelishom left-lateral fault, which is situated east of the Rudbār earthquake fault, as a possible source. The 1608 April 20 Alamutrud earthquake may have occurred on the Alamutrud fault farther east. Analysis of satellite imagery suggests that total left-lateral displacements on the Rudbār fault are a maximum of ∼1 km. Apparent left-lateral river displacements of ∼200 m on the Kashachāl fault and up to ∼1.5 km of the Kelishom fault, which are situated at the eastern end of the Rudbār earthquake fault, also appear to indicate rather small cumulative displacements. Given the relatively small displacements, the presently active left-lateral strike-slip faults of the western High-Alborz fold-thrust belt, may be younger than onset of deformation within the Alborz Mountains as a whole.

Strain analysis and stratigraphic status of Nongkhya, Sumer and Mawmaram conglomerates of Shillong basin, Meghalaya, India

Shillong basin, one of the Purana basins of the Indian peninsula is situated in the central and eastern parts of the Shillong plateau of NE India. Metasedimentary and metavolcanic rocks of the basin are of Mesoproterozoic age and lithostratigraphically belong to Shillong Group (erstwhile Shillong series) comprising Lower Metapelitic Formation (LMF) and Upper Quartzitic Formation (UQF). A long, persistent, faulted and tectonically attenuated conglomerate known as Nongkhya-Sumer-Mawmaram-conglomerate separates these two formations. In the present work, quantitative strain analyses of the pebbles of Sumer, Nongkhya and Mawmaram conglomerates of Ri-Bhoi and West Khasi Hills districts of Meghalaya are carried out using manual and computerized programmes. Eight different techniques for intrapebble, interpebble and bulk rock strain estimation are applied and results are compared systematically. Sumer and Mawmaram conglomerates bear the testimony of broadly flattening type of deformation (0 < k < 1) while Nongkhya conglomerate shows constriction type (1 < k < α). The change in strain ellipsoid shape from Nongkhya to Mawmaram area is accompanied by a change of fabric from S < L to L < S tectonites. Affinity of rotational strain is more in Nongkhya conglomerate as compared to Sumer and Mawmaram conglomerates. The compactness of pebbles is high in case of Nongkhya conglomerate and low to moderate in Sumer and Mawmaram conglomerates indicating high strain in Nongkhya conglomerate (northeastern part of Shillong basin) relative to Sumer and Mawmaram conglomerates (southwestern part of Shillong basin). Thus strain magnitude increases from SW to NE direction of the Shillong basin.The tectonostratigraphic status of these conglomerates suggest that the Sumer and Mawmaram conglomerates were initially a part of one conglomerate horizon of interformation type between LMF and UQF of the Shillong Group. With progressive deformation, the northeastern part of the Sumer conglomerate suffered tectonic attenuation and separation and eventually thrusted over the Basement Gneissic Group (BGG) as a tectonic mélange. This sector of the conglomerate is known as Nongkhya conglomerate. The tectonic configuration of the Nongkhya conglomerate is the effect of right and left lateral strike slip movement of Sumer conglomerate at Sumer and Adabasti points, respectively. This is a positive signature of post D 3 deformation on the Sumer conglomerate. The regional sigmoidal pattern of the interformational conglomerate broadly correlate with the Tyrsad-Barapani Shear Zone (TBSZ) of sinistral nature.

The Scotia–Antarctica plate boundary from 35°W to 45°W

A compilation of available multichannel seismic profiles acquired along the southern margin of the Scotia Sea east of the South Orkney microcontinent has allowed identifying and mapping the main morphological and structural features of the central segment of the Scotia–Antarctica plate boundary. This margin is composed by several bathymetric highs of variable size and uncertain crustal nature, separated by deep troughs and restricted oceanic basins. Some of these troughs represent pull-apart basins. Three main segments oriented WNW–ESE (the western sector), ENE–WSW (the central sector, here named Bruce Deep), and NE–SW (the eastern sector), have been described. These segments are separated by NNW–SSE-trending release zones, disposed in an en-echelon geometry, which represent mostly strike–slip faults. The western segment corresponds to the northern margin of the South Orkney microcontinent, where a subduction zone seems to be present, even if its present-day activity is unclear. The segment further to the east corresponds to an ENE-oriented basin (Bruce Deep), which separates the Bruce Bank from the eastern promontory of the South Orkney continental platform. To the south of the Bruce Deep, a wide deformation zone with N-verging folds and thrusts (here named Jane Thrust Belt), has been identified from seismic data. The eastern segment of the plate boundary is structurally the less constrained, and may be composed by a series of tectonic lineaments of different lengths. From the Bruce Bank to the east, focal mechanisms maintain a prevalent left-lateral strike–slip motion combined with an extensional component. In this sector, earthquakes are located in a 150 km wide area and on a local scale are difficult to follow unambiguously at the plate boundary. Lithologic analyses on dredged material recovered along a flank of one of the morphological relieves present south of the Discovery Bank to 35°W (here collectively named Irizar Highs), yielded a dominant granitic composition. A similar composition characterizes the rocks collected in the southern flank of the south-easternmost Jane Bank. This suggests a continental crust nature for these bathymetric highs, now dispersed along this sector of the Scotia–Antarctica plate boundary. We propose here a tectonic evolution for this margin, dominated since the Early Miocene by the northward subduction of the Weddell Sea oceanic crust. The development of a dextral, en-echelon transform fault system facilitated the process of fragmentation and dispersion of the crustal blocks, dismembered the subduction zone, and possibly inverted the direction of convergence: Therefore, the Scotia plate would subduct beneath the Antarctic plate, in the western sector, and Weddell Sea would subduct beneath Scotia plate, in the eastern sector. Finally, the activation of left-lateral transtensional strike–slip lineaments generated narrow pull-apart basins in the fore-arc sectors of the convergent zones.

The Bayamo Earthquake (Cuba) of the 18 October 1551

Using contemporary and original documents from the Archivo General de Indias it has been possible to complete the data for the 18 October 1551 earthquake in Cuba. The seism took place at midday, approximately. It had foreshocks and aftershocks. In Bayamo, 7 inhabitants were injured, and the town was severely affected. Maximum seismic intensity was IX degrees on the MSK scale, and the area of perceptibility is estimated at 40,000 km2. Liquefaction processes and soil type in Bayamo contributed to the damage. This locality is in the Eastern region of the island, and continues to suffer the most and the strongest seismic events. The epicenter was in the southern marine area of the western segment of Oriente trough (19.6 N 77.8 W, h = 15 km, Ms = 6.6), where there is a crossing of faults, and neotectonics and focal mechanisms are affected by transtension, although the Bartlett-Cayman region’s tendency to left-lateral strike-slip movement is maintained, in the Caribbean and North American plate boundary zone.

Source parameters of the M 6.5 Skyros Island (North Aegean Sea) earthquake of July 26, 2001

Teleseismic body wave modelling, time domain moment tensor inversion of regional waveforms and spectral analysis of the far-field P-wave pulses are used to derive the source parameters of the July 26, 2001 Skyros earthquake (M 6.5). Its epicentre is located south of the Sporades Islands in the North Aegean Sea (Greece). Previous focal mechanism solutions indicate motion on strike-slip faults. The time domain moment tensor inversion is applied for the first time to the regional waveforms of the recently established broadband network in Greece. Its application gave results which are highly consistent with teleseismic waveform modelling. The results of this study, in combination with the distribution of aftershocks, indicate left-lateral strike slip motion on a NW-SE striking fault with parameters: fault plane (strike = 151°, dip = 83°, rake = 7°) and auxiliary plane (strike = 60°, dip = 84°, rake = 173°), depth 12 km and M 0 = 5.98e18 Num. Moreover, the time domain moment tensor inversion technique yielded a pure double couple source with negligible CLVD. The spectral analysis of the far-field P-wave pulses resulted in a fault length L ~ 32 km, stress drop 6m ~ 9 bars and average displacement u ~ 30 cm. These values are in very good agreement with those estimated from empirical scaling relations applicable to the Aegean area.

Seismicity and Crustal Structure of the Polochic-Motagua Fault System Area (Guatemala)

Research Article| November 01, 2009 Seismicity and Crustal Structure of the Polochic-Motagua Fault System Area (Guatemala) A. Franco; A. Franco Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 1Laboratoire de Géologie, Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS), Paris, France Search for other works by this author on: GSW Google Scholar E. Molina; E. Molina Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 2Instituto Nacional de Sismología, Vulcanología, Meterología e hidrología, Guatemala Ciudad, Guatemala Search for other works by this author on: GSW Google Scholar H. Lyon-Caen; H. Lyon-Caen Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 1Laboratoire de Géologie, Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS), Paris, France Search for other works by this author on: GSW Google Scholar J. Vergne; J. Vergne Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 1Laboratoire de Géologie, Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS), Paris, France 3Institut de Physique du Globe de Strasbourg, CNRS, Strasbourg, France Search for other works by this author on: GSW Google Scholar T. Monfret; T. Monfret Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 4Geosciences Azur, University of Nice, Sophia Antipolis, CNRS, Institut de Recherche pour le Developpement, Nice, France Search for other works by this author on: GSW Google Scholar A. Nercessian; A. Nercessian Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 5Institut de Physique du Globe de Paris, CNRS, France Search for other works by this author on: GSW Google Scholar S. Cortez; S. Cortez Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 6Universidad de Coban, Coban, Guatemala Search for other works by this author on: GSW Google Scholar O. Flores; O. Flores Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 7Centro de Estudios Superiores de Energí a y Minas-Universidad de San Carlos de Guatemala, Guatemala Ciudad, Guatemala Search for other works by this author on: GSW Google Scholar D. Monterosso; D. Monterosso Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 8Coordinadora Nacional para la Reducción de Desastres, Guatemala Ciudad, Guatemala Search for other works by this author on: GSW Google Scholar J. Requenna J. Requenna Laboratoire de Géologie Ecole Normale Supérieure de Paris 24, rue Lhomond 75231 Paris cedex 05 France franco@geologie.ens.fr (A. F.) 6Universidad de Coban, Coban, Guatemala Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2009) 80 (6): 977–984. https://doi.org/10.1785/gssrl.80.6.977 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. Franco, E. Molina, H. Lyon-Caen, J. Vergne, T. Monfret, A. Nercessian, S. Cortez, O. Flores, D. Monterosso, J. Requenna; Seismicity and Crustal Structure of the Polochic-Motagua Fault System Area (Guatemala). Seismological Research Letters 2009;; 80 (6): 977–984. doi: https://doi.org/10.1785/gssrl.80.6.977 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search We report results from a six-month seismological experiment in the area of the eastern Polochic-Motagua fault system (Guatemala) designed to both characterize the present seismicity and bring some constraints on the lithospheric structure. The seismic activity occurs in the upper 15 km of the crust, on the Polochic and the Motagua faults as well as in a NS-trending graben south of the Motagua fault and within the active folds north of the Polochic fault. From receiver function analysis the Moho discontinuity is found at about 35 km depth north of the Polochic fault and south of the Motagua fault, while... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Finding Faults in the Charleston Area, South Carolina: 1. Seismological Data

Macroscopic observations following the 1886 Charleston, South Carolina, earthquake and analyses of instrumentally recorded seismicity between 1974 and 2004 suggest the presence of two or more active faults. In order to more clearly define the active faults and determine their seismotectonic framework, instrumentally located hypocenters were relocated using the double-difference algorithm HypoDD. The revised hypocentral locations were associated with different faults based on the first motions recorded at different locations. The result is a plausible framework that shows several important changes from earlier interpretations. This framework defines a localized stressed volume, which consists of the ∼50-km-long ∼N30°E striking, NW dipping Woodstock fault associated with oblique right-lateral strike-slip motion with a ∼6-km-long antidilatational left step near Middleton Place. Three ∼NW-SE striking reverse faults, two NE dipping and one SW dipping, were recognized within this left step; of these, the NE dipping Sawmill Branch fault zone lying between Middleton Place and Summerville is the most active. Minor activity was observed on the NE dipping Lincolnville and the SW dipping Charleston faults. The southernmost Sawmill Branch fault zone also shows evidence of left-lateral strike-slip motion. The ∼N55°W trending Ashley River fault lying between Middleton Place and the Magnolia Plantation appears to be currently inactive.

Determination of neotectonic features of the Karasu Basin (SE Turkey) and their relationship with Quaternary volcanic activity using Landsat ETM+ imagery

The junction between the East Anatolian and the Dead Sea fault zones lies close to an unstable F: Transform fault (FFF) triple junction where the African, Eurasian and Arabian plates meet in south-eastern Turkey. The Karasu Basin is an ephemeral rifted structure located close to the junction of these plate boundaries and is expressed by a range of tectono-morphological features. This study uses remote sensing to define tectonic structures and discriminate volcanic rocks linked to rifting within the basin using Landsat Enhanced Thematic Mapper Plus (ETM+) imagery. Prior to advanced image processing stages, images were corrected to eliminate atmospheric scattering effects and determine the best band combination for multi-spectral processing techniques based on statistical methods. Following pre-processing stages, edge detection filters were applied to derive tectonic structures defining the basin. The Brovey transformation, a statistical data merging method, was used to combine reflective multi-spectral bands with the Landsat ETM+ panchromatic band. This method fuses higher spatial data with data of lower spatial value. The western margin of the Karasu Basin is defined by the left-lateral Amanos Fault Zone with a contemporary motion (c. –0.4 mm a−1) probably accommodating most of the left-lateral strike-slip motion between Arabian and African plates on the northern continuation of the Dead Sea Fault Zone. The rift zone has been the site of extensive recent volcanism concentrated within the Brunhes Chron (<0.78 Ma) and linked to fault block rotations between the intracontinental master faults. The boundaries of this activity are defined here using a regolith mapping technique.

The Seismicity along the Dead Sea Fault during the Last 60,000 Years

Abstract Evidence for unchanging slip rate and a Gutenberg–Richter relation for earthquake distribution along the Dead Sea fault during the past 60,000 yr are presented. The evidence comes from three different segments, approximately 100 km apart, and from three different timescales: prehistoric–paleoseismic, historical, and modern (instrumental) records. The paleoseismic data are based on two different methods. In the southern Arava Valley and the northern Jordan Valley segments, the amount of normal displacement along several faults is used, while in the Dead Sea basin the appearance of brecciated beds, which are considered as seismites, is used. We found that for the southern Arava Valley segment a constant dip-slip rate of 0.5 mm/yr can explain the cumulative normal slip during the past 45,000 yr. This suggests that normal faulting is only ∼10% of the total left-lateral strike-slip motion. We also found that for all three segments, the paleoseismic and historical records of strong earthquakes lie on the linear extrapolation of the frequency–magnitude relation of the instrumental record. The calculated b-values for all three segments are between 0.85 and 1, similar to other major strike-slip faults in the world. It is concluded that the Gutenberg–Richter distribution is a stable mode in the tectonic setting of the Dead Sea fault during the past 60,000 yr.

Microseismic evidence of slip partitioning for the Rudbar-Tarom earthquake (Ms7.7) of 1990 June 20 in NW Iran

SUMMARY The focal mechanism of the destructive earthquake at Rudbar in northern Iran on 1990 June 20 was an unexpected left-lateral strike-slip motion on a previously unknown fault, the Baklor–Kabateh–Zard-Goli fault, within a complex system of reverse faults. We analysed microseismicity recorded by a dense local network deployed for 7 weeks, 8 yr after the main shock, to help evaluate the subsurface geometry and kinematics of the active fault system. In the west, the 1998 microseismicity seems to be related to the reverse Manjil. In the centre, we observe both reverse faulting associated to the Manjil fault and strike-slip faulting associated with de Zard-Goli rupture whose dip progressively changes from 45 ◦ N to vertical. In the east, the reverse faulting is located south of the Manjil fault and strike-slip faulting to the vertical Zard-Goli fault. We suggest that because the only stable geometry that accommodates large partitioning motion prevents the strike-slip fault to intersect the reverse fault, the active reverse fault jumps southwards beneath the Shahrud fault.

Iterative de-convolution of the local waveforms: Characterization of the seismic sources in Kachchh, India

The deviatoric and double couple (DC) constrained moment tensor inversions of multiple point sources (10–20 s) for regional (or local) earthquakes, developed by Zahradnik et al. (2005), has been applied on the data of nine significant Bhuj aftershocks of M w4.4– M w5.6 recorded at three-component 5–15 accelerograph and 5–11 seismograph stations (epicentral distances < 130 km). The deviatoric moment tensor solutions of events on the north Wagad fault (NWF) in the 15–29 km depth range reveal a systematic depth-wise variation in the faulting patterns. At shallow depth (~ 15 km), they suggest a left lateral strike-slip movement with a minor reverse component along a south dipping plane (~ 61°), whereas, at 18–22 km depth range they change to pure reverse movement on a preferred south dipping plane (10–54°) and finally they change to the normal movement with minor strike-slip (S-S) component at deeper (25–29 km) depth range. The deviatoric MT solution of one event on the south Wagad fault (SWF) suggests a reverse movement with a minor S-S component on a 35° southeast dipping plane at 24 km depth. The deviatoric MT solutions for two events on the Gedi fault reveal a reverse movement with a minor left-lateral strike-slip component on an E–W trending and south dipping (40–61°) plane at 3–4 km depth. Whereas, one event on the Island belt fault (IBF) suggests a right lateral strike slip movement with a normal component on an almost vertical (~ 79°) plane at 29 km depth. The deviatoric moment tensor solutions of all the nine events show a larger (94–99%) double-couple (DC) component at shallow (3–15 km) depth range suggesting domination of brittle failure in the upper crust beneath the Kachchh region. However, the deeper events show larger non-DC (i.e. compensated linear vector dipole, CLVD) component suggesting increase in deviation from the double-couple (DC) solution in the lower crust (15–30 km depth range). This increase in non-DC component could be attributed to the source complexity in the lower crust. Further, the increase in normal component on NWF as well as IBF in the 25–30 km depth range can be explained in terms of presence of aqueous fluid in the lower crust as revealed by the tomographic studies.

Coseismic growth of sedimentary basins along the Yammouneh strike-slip fault (Lebanon)

SUMMARY The left-lateral Yammouneh fault (YF) is the main active branch of the Dead Sea Transform Fault (DSTF) within the Lebanese restraining bend. Despite the overall transpressional setting, a series of sedimentary basins have developed along the trace of the YF. Consequently, palaeoseismic studies within these basins provide an opportunity to study the processes of coseismic growth of the basins, as well as elucidate earthquake behaviour of the fault, in general. Geodetic measurements of contemporary fault slip within the Lebanese restraining bend indicate that the YF accommodates most of the expected left-lateral strike-slip motion, despite the apparent lack of present-day seismicity. We studied the YF, using combined investigations of remote imagery, geomorphology and palaeoseismology. The active fault trace along a 51 km strip was delineated as relatively young surface ruptures and fault scarps that affect Holocene deposits with intermittent offset geomorphic markers. Seven closed basins that occur along-strike of the YF, were found to be related to faulting, with at least three of them displaying evident pull-apart settings. We concentrated our work on the rhombohedral Yammouneh basin, an actively evolving example of pull-apart basins, which is presently obliquely cut by the active fault, with an apparently young age of 1.4 ± 0.3 Myr. 3-D correlation and analysis of palaeoseismic investigations exposed a composite shear zone with a total subsidence that exceeds 1.6 m over the past 4000–4400 yr. Stratigraphic and geochronological constraints suggest the occurrences of at least five large faulting events during that period. By correlating the stratigraphy and ages of this trench with a previously published study located nearby, a combined palaeoseismic history for the past five events is constructed. This suggests a mean recurrence period of 1020–1175 yr for large earthquakes along this section of the YF. Our results suggest a subsidence rate due to faulting of about 0.36–0.40 mm yr −1 within the Yammouneh pull-apart basin. The geometrical relationship between the active fault trace and the older fault traces suggests that the basin may have evolved in a two stage process in which the original releasing fault bend developed a cross-cutting fault, accommodating most of the subsequent strike-slip displacement. The role of the YF in controlling basin evolution is further supported by the correspondence of the active fault with the boundary of the present depocentre. Our results highlight the interrelation between active strike-slip faulting and the coseismic incremental evolution of fault-related sedimentary basins, which may provide a conceptual model with broader implications on analogous tectonic systems worldwide.

Complicated phenomena of the Beichuan-Yingxiu surface fracture zone during Wenchuan Ms8.0 earthquake in Sichuan Province, China

After Wenchuan Ms8.0 earthquake, the surface fractures have been investigated in Beichuan City and Yingxiu Town. It was found that the Beichuan-Yingxiu earthquake fracture zone trends northeastward, dipping to northwest. The earthquake fracture zone is mainly characterized by thrust faulting with small amounts of strike-slip movement, and demonstrating various complexities for different areas. The phenomena related to dextral strike-slip displacement are found in Beichuan City and those related to left-lateral strike-slip movement in Yingxiu Town. The compressive shortening of the two surface fracture zones in Beichuan City is 2.8–3.9 m and the left-lateral strike-slip displacement of the surface fractures in Yingxiu Town is 0.52 m.

Late Cenozoic to recent transtensional deformation across the Southern part of the Gaoligong shear zone between the Indian plate and SE margin of the Tibetan plateau and its tectonic origin

Within the southeastern part of the Tibetan plateau the Gaoligong Shan shear zone forms the boundary between the Tengchong and Baoshan blocks and is one of the main Cenozoic strike–slip fault systems of the eastern Himalayan syntaxis. The mountain ranges rise to a maximum elevation of 3623 m, with a width of 20 km and a length of 400 km. The core of the shear zone consists of a narrow belt of mylonitic rocks with right-lateral strike–slip fabrics formed during the northward movement of India relative to Eurasia in Oligocene and Miocene time. To the south the mountains decreases in elevation and end against the Wanding fault. The latter is one of a series of NE–SW striking, active left-lateral strike–slip faults within the southeastern part of the Tibetan plateau. The cause and timing for the change in elevation of the Gaoligong Shan is unknown, but based on our recent structural and fission-track geochronological work, the decrease in elevation is the result of east-west extension across the southern part of the Gaoligong shear zone inferred from the late Cenozoic and active normal faults along its eastern and western boundaries. Southward the eastern boundary fault system merges with a series of splays of the Wanding strike–slip fault, while the western boundary fault system is bent into parallelism with the Wanding fault. This geometric relationship suggests that the normal faulting along the eastern and western boundary of the Gaoligong shear zone was caused by transfer of the NE–SW left-lateral movement along the Wanding fault into E–W extension on faults to the northeast. The resultant normal faulting caused the lowering of the topography in the southern part of the shear zone. Fission track data from 11 samples from the mylonitic rocks in the footwall of the Ruili oblique slip fault, (left-lateral thrust slip) yielded ages between 8.4 and 0.9 Ma, coeval with volcanism within the Jietou and Mangbang basins. The left-lateral strike–slip movement along the Wanding and Ruili faults is interpreted to have accommodated differential clockwise rotation of the Gaoligong shear zone and its adjacent tectonic units during late Cenozoic to Recent time, during the northward movement of the Indian plate with respect to the southeastern margin of the Tibetan plateau along the right-lateral Sagaing fault.