Pure Left-lateral Strike-slip Research Articles

Structure and kinematics of active faulting in the northern domain of Western and Central Alborz, Iran and interpretation in terms of tectonic evolution of the region

Field evidence, seismicity, and geodetic data are combined to define the tectonic evolution along the northern domain of the Western and Central Alborz ranges, which have experienced significant earthquakes. One of our main focuses is to present the geometric-kinematic characteristics of active fault planes in the North Alborz fault zone and its subsidiary faults as a basis for seismic hazard assessment. To evaluate the seismotectonics in the region, three active segments of the seismic zone are identified. These segments primarily exhibit reverse mechanisms and left-lateral strike-slip components and act as the boundaries of neotectonic stress domains around the transition zone between Alborz and the South Caspian Basin. The spatial distribution of the strike-slip and dip-slip faulting over the Western and Central Alborz range supports our understanding of the geodynamic processes of the region. Our map of local active faults reveals five main systems with N–S, NE–SW, ENE–WSW, E–W, and NW–SE directions. Field studies indicate that strike-slip movements predominate and are consistent with a ∼WNW-ESE-oriented left-lateral wrench system. Pure left-lateral strike-slip faults trending E-W, extensional faults trending NE–SW, and compressional faults trending NW-SE are identified. Regional field investigations reveal spatial variations in the fault zone width of the northern domain of the Western and Central Alborz. The findings demonstrate that fault zone structures and their spatial distribution along the width of the area are strongly affected by the strain partitioning process in the transpressional zone. A spectacular well-exposed transpressional zone from the Western and Central Alborz region is described in detail, where outer domains are faults with a dominant reverse mechanism, and the inner domain is a wrench zone that includes faults with a dominant strike-slip mechanism. The seismic activity in the Northern Alborz region is mainly concentrated along the North Alborz fault zone and its subsidiary fault planes, of which some are identified for the first time in this research. Significant seismic events have occurred at locations where these newly identified fault planes interact. The maximum values of the geodetic strain axes in Western and Central Alborz are consistent with the area of high seismicity.

Surface deformations of 24 January 2020 Sivrice (Elazığ)–Doğanyol (Malatya) earthquake (Mw = 6.8) along the Pütürge segment of the East Anatolian Fault Zone and its comparison with Turkey’s 100-year-surface ruptures

On Friday, January 24, 2020 at 20.55:11 local time (17:55 UTC), an earthquake with a magnitude of Mw = 6.8 has occurred in Sivrice district of Elazig (Eastern Turkey). Focal mechanism solution is consistent with pure left-lateral strike-slip faulting; the location of the epicenter and fault mechanism suggest deformation along the Puturge segment of the East Anatolian Fault Zone. A 10-day fieldwork was carried out along the Puturge segment to study surface deformation; the geometry of the surface rupture and other seismic geomorphological structures were mapped and studied in detail. The field data are also correlated with satellite images. This paper, therefore, presents classification of seismic geomorphological structures and discuss intimate relationship between fault geometry and stress field in the region. Seismic geomorphological deformation and related features of the Sivrice (Elazig) earthquake are observed in the area between Gezin (Elazig) and Ormanici (Puturge) villages; they are classified into two as seismotectonic and seismo-gravitational features. Field observations confirm that seismo-gravitational structures develop along both Gezin-Sivrice–Doganbagi and Doganbagi–Cevrimtas–Ilincak–Koldere–Ormanici sections of the Puturge segment, while surface rupture is mapped as seismotectonic structure only along the Doganbagi–Cevrimtas–Ilincak–Koldere–Ormanici section. Small-scale landslides, rock falls, feather cracks along asphaltic roads, and laterally discontinues ground failure-related features are common seismo-gravitational structures that developed along the fault zone. In addition, small-scale lateral spreading and liquefaction structures are common especially in areas where fault-perpendicular streams meet the Karakaya Dam reservoir. The surface rupture is mapped as stepping and overlapping en echelon fractures along elongated pressure ridges between Cevrimtas and Doganbag villages, to northwest of Ilincak village, along 1.5-km-long pressure ridge between Topalusagi and Doganyol, across the elongated hill that developed on an alluvial fan to the northwest of Doganyol and in the area between Koldere and Ormanici villages. Surface fractures deforming the pressure ridges are all aligned parallel to the long axes of the ridges and display reverse components that give rise to small-scale pop-up structures. Interferometric SAR (DInSAR) studies indicate a 10-cm uplift in the northwestern block of the fault and a 6-cm subsidence in the southeast block. The difference in vertical movements between two blocks of the fault is interpreted to suggest that at least 30-km-long section of the Puturge segment in the area between southwest of Sivrice and Puturge is broken during the main shock. Although the focal mechanism solution of the main shock gives pure left-lateral strike-slip faulting, there is no significant left-lateral displacement observed during the fieldwork. This can be explained by the following: (1) left-lateral strike-slip displacement was not able to reach the surface; (2) left-lateral torque movement of the fault around a vertical axis during the earthquake, (3) restraining bend nature of the Puturge segment, or (4) the presence of Puturge metamorphics along the fault strike. It is also important to note that along most part of the Puturge segment where surface rupture is observed, talus, colluvial or alluvial fan sediments are exposed; unconsolidated and/or poorly consolidated nature of these sediments may also be counted as one of the main reason for not observing horizontal displacement on the surface. When we compare these surface deformations with the surface ruptures that occurred in the last 100 years in Turkey, we suggest that the formation of the surface deformations is variable depending on: (1) the fault type and the state of regional stress, (2) the magnitude of the earthquake, (3) the duration time of the earthquake and (4) the geomorphologic feature of landscape in relation to the lithologic and structural features of the rock units along the active fault zone.

Revisiting the Laalam (Eastern Algeria) March 20, 2006 (Mw 5.1) Earthquake and its Seismotectonic Implication

Hypocenters of main shock and aftershocks of the March 20, 2006 Laalam earthquake are relocated using HypoDD double-difference technique. We combined accelerogram and seismogram data of the National Center of Applied Research in Earthquake Engineering (CGS). Among about 191 aftershocks, recorded at least by 4 stations, 141 aftershocks of Md 1.2–2.7 were relocated using HypoDD. The obtained swarm of epicenters occupying a crustal volume of 5 km × 3 km × 5 km and the focal mechanism corresponds to an unknown pure left lateral strike slip, trending N174°E. We were able to calculate focal mechanisms for only seven aftershocks with more than seven polarity readings, which give a P axis oriented NNW–SSE. The waveform inversion also provided values of Mw 5.1, M0 5.9 10−16 Nm and the depth 4.9 km. The dislocation and the stress drop were estimated to 90 cm and 16.5 bars, respectively.

The Beni Haoua, Algeria, Mw 4.9 earthquake: source parameters, engineering, and seismotectonic implications

A moderate Mw 4.9 earthquake struck the Beni Haoua (Algeria) coastal area on April 25, 2012. The mainshock was largely recorded by the accelerograph network of the Centre National de Recherche Appliquee en Genie Parasismique (CGS). The same day the earthquake occurred, eight mobile short period stations were deployed through the epicentral area. In this study, we use accelerogram and seismogram data recorded by these two networks. We combined the focal mechanism built from the first motion of P waves and from waveform inversion, and the distribution of aftershocks to well constrain the source parameters. The mainshock is located with a shallow focal depth, ∼9 km, and the focal mechanism shows a nearly pure left lateral strike slip motion, with total seismic moment of 2.8 × 1016 N.m (Mw = 4.9). The aftershocks mainly cluster on a narrow NS strip, starting at the coast up to 3–4 km inland. This cluster, almost vertical, is concentrated between 6 and 10 km depth. The second part of this work concerns the damage distribution and estimated intensity in the epicentral area. The damage distribution is discussed in connection with the observed maximum strong motion. The acceleration response spectrum with 5 % damping of the mainshock and aftershocks give the maximum amplitude in high frequency which directly affects the performance of the high-frequency structures. Finally, we tie this earthquake with the seismotectonic of the region, leading to conclude that it occurred on a N–S transform zone between two major compressional fault zones oriented NE–SW.

Geomorphic and structural variations along the Doruneh Fault System (central Iran)

This paper focuses on the analysis of geomorphic, structural, and behavioral characteristics along the Doruneh Fault System (DFS), east of longitude 56°45′E. Detailed geomorphic and structural analyses of different scale satellite images and digital topographic data, accompanied with field surveys allowed us to establish a fault segmentation model in which three discrete fault zones have been recognized: (1) the western fault zone (WFZ) characterized by reverse left‐lateral mechanism with left‐handed step‐over geometry, (2) the central fault zone (CFZ) which is pure left‐lateral strike‐slip and comprises nearly parallel faults, and (3) the eastern fault zone (EFZ) that is a trailing imbricate fan fault‐termination characterized by reverse faulting and fault‐related folding. Each fault zone shows discrete geometry and kinematics implying that deformation is not uniformly accommodated along the DFS. We propose a new kinematic model to explain how the DFS accommodate the Arabia‐Eurasia convergence normal to the overall fault orientation. According to this model, the DFS takes up the northward motion between central Iran–Lut block relative to Eurasia by a complex kinematics varying from pure reverse to pure left‐lateral strike‐slip faulting. The kinematics of the WFZ and EFZ corresponds to the direction of the NE‐trending regional compression. While, the partitioning of slip into strike‐slip and reverse component of faulting on parallel faults (strain partitioning) allows the CFZ to remain pure left‐lateral strike‐slip. Such a model propose a way to explain how large strike‐slip faults such as the DFS accommodate tectonic block motions perpendicular to strike of the faults.

Slip rate and slip magnitudes of past earthquakes along the Bogd left-lateral strike-slip fault (Mongolia)

We carried out morphotectonic studies along the left-lateral strike-slip Bogd Fault, the principal structure involved in the Gobi-Altay earthquake of 1957 December 4 (published magnitudes range from 7.8 to 8.3). The Bogd Fault is 260 km long and can be subdivided into five main geometric segments, based on variation in strike direction. West to East these segments are, respectively: the West Ih Bogd (WIB), The North Ih Bogd (NIB), the West Ih Bogd (WIB), the West Baga Bogd (WBB) and the East Baga Bogd (EBB) segments. Morphological analysis of offset streams, ridges and alluvial fans-particularly well preserved in the arid environment of the Gobi region-allows evaluation of late Quaternary slip rates along the different faults segments. In this paper, we measure slip rates over the past 200 ka at four sites distributed across the three western segments of the Bogd Fault. Our results show that the left-lateral slip rate is similar to 1 mm yr(-1) along the WIB and EIB segments and similar to 0.5 mm yr(-1) along the NIB segment. These variations are consistent with the restraining bend geometry of the Bogd Fault.;Our study also provides additional estimates of the horizontal offset associated with the 1957 earthquake along the western part of the Bogd rupture, complementing previously published studies. We show that the mean horizontal offset associated with the 1957 earthquake decreases progressively from 5.2 m in the west to 2.0 m in the east, reflecting the progressive change of kinematic style from pure left-lateral strike-slip faulting to left-lateral-reverse faulting. Along the three western segments, we measure cumulative displacements that are multiples of the 1957 coseismic offset, which may be consistent with a characteristic slip. Moreover, using these data, we re-estimate the moment magnitude of the Gobi-Altay earthquake at M-w 7.78-7.95.;Combining our slip rate estimates and the slip distribution per event we also determined a mean recurrence interval of similar to 2500-5200 yr for past earthquakes along the different segments of the western Bogd Fault. This suggests that the three western segments of the Bogd Fault and the Gurvan Bulag thrust fault (a reverse fault bounding the southern side of the Ih Bogd range that ruptured during the 1957 earthquake) have similar average recurrence times, and therefore may have ruptured together in previous earthquakes as they did in 1957. These results suggest that the western part of the Bogd Fault system, including the Gurvan Bulag thrust fault, usually behaves in a 'characteristic earthquake' mode.

Estimation of coseismic deformation and a fault model of the 2010 Yushu earthquake using PALSAR interferometry data

We present a map of the coseismic displacement field and slip distributions resulting from the Yushu earthquake on 14 April 2010 in Qinghai, China. The wide coverage of ScanSAR data helps in increasing the opportunity of interferometric synthetic aperture radar (InSAR) observations of a specific location on Earth, estimating surface slip, and constraining slip distribution on the fault plane. We increased InSAR sensitivity to the horizontal and vertical surface displacements by combining ascending and descending interferograms. We find that the maximum left-lateral surface slip is 166 cm at ~ 9.7 km WNW of Yushu. The end-to-end length of surface and subsurface faults is about 73 km, and the estimated lengths of the two surface fault lines are 30 km and ~ 9 km. Slip distribution on a fault plane inverted from InSAR data shows an almost pure left-lateral strike-slip, with two slip peaks near the epicentres and Yushu and a maximum slip of ~ 2.6 m. No postseismic deformation is observed southeast of the source region; however, we detected a significant postseismic displacement northwest of the source region. The deformation area is located a few kilometres northwest of the coseismic displacement area observed around Longbao Lake, suggesting that the coseismic slip and the postseismic slips are spatially isolated.

GPS constraints on Africa (Nubia) and Arabia plate motions

SUMMARY We use continuously recording GPS (CGPS) and survey-mode GPS (SGPS) observations to determine Euler vectors for relative motion of the African (Nubian), Arabian and Eurasian plates. We present a well-constrained Eurasia‐Nubia Euler vector derived from 23 IGS sites in Europe and four CGPS and three SGPS sites on the Nubian Plate (−0.95 ± 4.8 ◦ N, −21.8 ± 4.3 ◦ E, 0.06 ± 0.005 ◦ Myr −1 ). We see no significant (> 1m m yr −1 ) internal deformation of the Nubian Plate. The GPS Nubian‐Eurasian Euler vector differs significantly from NUVEL-1A (21.0 ± 4.2 ◦ N, −20.6 ± 0.6 ◦ E, 0.12 ± 0.015 ◦ Myr −1 ), implying more westward motion of Africa relative to Eurasia and slower convergence in the eastern Mediterranean. The Arabia‐ Eurasia and Arabia‐Nubia GPS Euler vectors are less well determined, based on only one CGPS and three SGPS sites on the Arabian Plate. The preliminary Arabia‐Eurasia and Arabia‐ Nubia Euler vectors are 27.4 ± 1.0 ◦ N, 18.4 ± 2.5 ◦ E, 0.40 ± 0.04 ◦ Myr −1 , and 30.5 ± 1.0 ◦ N, 25.7 ± 2.3 ◦ E, 0.37 ± 0.04 ◦ Myr −1 , respectively. The GPS Arabia‐Nubia Euler vector differs significantly from NUVEL-1A (24.1 ± 1.7 ◦ N, 24.0 ± 3.5 ◦ E, 0.40 ± 0.05 ◦ Myr −1 ), but is statistically consistent at the 95 per cent confidence level with the revised Euler vector reported by Chu & Gordon based on a re-evaluation of magnetic anomalies in the Red Sea (31.5 ± 1.2 ◦ N, 23.0 ± 2.7 ◦ E, 0.40 ± 0.05 ◦ Myr −1 ). The motion implied in the Gulf of Aqaba and on the Dead Sea fault (DSF) by the new GPS Nubia‐Arabia Euler vector (i.e. ignoring possible Sinai block motion and possible internal plate deformation) grades from pure left lateral strike-slip in the Gulf and on the southern DSF with increasing compression on the central and northern DSF with relative motion increasing from 5.6 to 7.5 mm yr −1 (± 1m m yr −1 ) from south to north. Along the northern DSF (i.e. north of the Lebanon restraining bend) motion is partitioned between 6 ± 1m m yr −1 left-lateral motion parallel to the fault trace and 4 ± 1m m yr −1 faultnormal compression. Relative motions on other plate boundaries (including the Anatolian and Aegean microplates) derived from the GPS Euler vectors agree qualitatively with the sense of motion indicated by focal mechanisms for large crustal earthquakes (M > 6). Where data are available on fault-slip rates on plate bounding faults (North Anatolian fault, East Anatolian fault, Dead Sea fault, Red Sea rift), they are generally lower than, but not significantly different from, the full plate motion estimates suggesting that the majority of relative plate motion is accommodated on these structures.

Source mechanism of the 22/11/1995 Gulf of Aqaba earthquake and its aftershock sequence

The Gulf of Aqaba earthquake occurred on 22/11/1995 with M W = 7.2 and was the largest event to occur along the Dead Sea Transform in at least a century. We determined the source mechanism of the event based on the inversion of the P and SH waveforms observed by teleseismic stations. Our solution consists of 2 similar subevents, where the first exhibits a left-lateral strike-slip motion with a normal component and the second subevent shows an almost pure left-lateral strike-slip motion along the gulf major fault system. The total seismic moment is 7.7 × 1026 dyne-cm, with source duration of 15 seconds. The aftershock sequence was recorded by the regional broad band stations BGIO and JER, of the Israel Seismic Network, and KEG, of the MEDNET regional network for earthquakeswith M W > 4.0. Aftershock epicenter seems to cluster mainly in the Eilat basin and the Aragonese basin. The dominant mechanism indicates left-lateral strike slip motion along the gulf trend, similar to the mainshock.

Strike-slip faulting on the northern margin of the South China Sea: evidence from two earthquakes offshore of Hainan Island, China, in December 1969

In December 1969, two earthquakes ( m b = 4.7and4.8) occurred in a rift basin offshore of Hainan Island, China, in a passive continental margin environment. By modeling short-period P and long-period SH waveforms recorded at WWSSN stations, we have determined the focal mechanisms and source parameters of these two events to be: strike 45 ± 7°, dip 78 ± 4°, rake −6 ± 7°, focal depth 8 ± 3 km, seismic moment (1.73 ± 0.4) × 10 23 dyne cm and stress drop 56 ± 6 bar for the 17 December event; and strike 36 ± 6°, dip 60 ± 4°, rake −2 ± 7°, focal depth 8 ± 3 km, seismic moment (1.43 ± 0.13) × 10 23 dyne cm and stress drop 47 ± 6 bar for the 20 December event. Results of relative location computed from differential arrival times indicate that the 20 December event is slightly deeper than the 17 December event but within the resolution limit of the waveform constraint. Analysis of the local tectonic setting, geometry of isoseismals, aftershock distribution and fault-plane solutions indicate that the 17 December event probably ruptured along a NE-SW-striking fault on the northern margin of the Songtao uplift in the Qiongdongnan basin. The 20 December event probably ruptured along another major NE-SW-striking fault on the northern margin of the Qiongdongnan basin. The fault motions of both earthquakes were almost pure left-lateral strike slip. P and T axes show that the epicentral area is subject to nearly horizontal N-S-directed compression and E-W-directed extension, consistent with previously observed stress field in the area. The two earthquakes represent reactivation of pre-existing zones of weakness in the basin by the present-day tectonic stress field. A review of earthquakes in the region indicates that strike-slip motion is characteristic of the seismotectonics of the northern margin of the South China Sea.

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.

Source geometry and slip distribution of the April 21, 1935 Hsinchu-Taichung, Taiwan earthquake

The disastrous Hsinchu-Taichung earthquake (magnitude M GR = 7.1) occurred on April 21, 1935. The earthquake was known to be associated with two faults: the Tuntzuchiao oblique right-lateral strike-slip fault and the Shihtan reverse fault. Preliminary studies for this earthquake have been done. At present, however, the source geometry and slip distribution are still not clear. In this study, the geodetic observations of coseismic ground deformation for this earthquake are modeled using the finite-element method to infer its source parameters and to deduce the mechanism of its fault rupture. We determined the length, width, and offset of the fault, the seismic moment, and the stress drop of this shock series and proposed that the Hsinchu-Taichung earthquake sequence was actually associated with three main faults—the Shihtan fault, the Tuntzuchiao fault and the fault of the July 17 aftershock. The Shihtan fault is a high-angle reverse fault of a curved plane geometry, with a dip angle decreasing continuously with depth. The major fault slips occurred in the central part of the fault plane. The fault length, width and average slip are 25 km, 11 km and 1.94 m, respectively. The Tuntzuchiao fault is a high-angle, right-lateral, strike-slip fault dipping southeast. The fault length, width and average slip are 15 km, 6.2 km and 1.9 m, respectively. The July 17 aftershock was associated with a pure left-lateral strike-slip fault, dipping at 60° toward southwest having a fault length of 12.5 km, a width of 5.8 km, and a uniform slip of 1.0 m. The estimated seismic moments of these faults are 3.2 × 10 26 dyne cm for the Shihtan fault, 1.1 × 10 26 dyne cm for the Tuntzuchiao fault, and 4.4 × 10 25 dyne cm for the July 17 aftershock. The stress drops are 162 bar for the Tuntzuchiao fault, 90 bar for the Shihtan fault, and 88 bar for the July 17 aftershock. Our results indicated that the Hsinchu-Taichung earthquake is a special case of complex fault geometry and heterogeneous slip distribution. Furthermore, from the fault rupture pattern and regional stress direction, we deduced that the motions on these faults were consistent with those under the regional tectonic stress in the SE-NW direction, and the ruptures of the Tuntzuchiao fault and these aftershocks along the Houlung River, including the July 17 aftershock, seem to indicate conjugate faulting.

Seismotectonics and present-day relative plate motions in the Tonga-Lau and Kermadec-Havre region

An updated compilation of shallow seismicity in the Tonga-Kermadec arc and back-arc region, including especially 304 focal mechanisms from centroid moment tensor solutions files, provides new insights on the present-day tectonics and allows the relative motions between the Pacific, Indo-Australian and Tonga-Kermadec plates to be quantified. Azimuths of slip vectors along the trench change at 19°S. In the southern domain (35-19°S) the mean azimuth ranges from N280°E to N285°E, while in the northern domain it trends N276–277°E. This 19°S boundary also exits in the back-arc domain. South of 19°S, from 35°S to 20°S the direction of back-arc extension is respectively N135°E, N122°E and N111°E within the southern Havre trough, northern Havre trough and southern Lau basin. In the southern Lau basin the N111°E opening direction is compatible with the trend of the newly mapped Valu Fa ridge. North of 19°S, in the northern Lau basin (16–18°S), a N93°E trending crustal extension occurs along the N135°E Peggy ridge and along an inferred N5–10°E spreading zone. Between these two extensional features a fourth plate, the northern Lau microplate, is present. At 18–19°S a ridge-ridge-ridge triple junction, accommodated by N95°-115°E right-lateral strike-slip fracture zones, is proposed at the complex transitional zone between the southern and the northern parts of the Lau basin. The Fiji-North Tonga fracture zone which bounds northward the Lau back-arc basin is composed of two segments: a N95°E eastern segment extending from the northern end of the Tonga trench to 178°30′W, and a N75°E western segment lying north of the Fiji platform. A ridge-fracture-fracture triple junction is inferred at 178°30′W–15°45′S between the northern end of the Peggy ridge and the two segments of the Fiji-North Tonga fracture zone. Using the model RM-2 of Minster and Jordan (1978) and the directions of back-arc spreading and convergence at the trench, relative plate motions are estimated. The extension rates in the back-arc domain increase northward, and are respectively 0.8, 2.1 and 8 cm/y at 33°S, 28°S and 24°S. Farther north parallelism between directions of different motions does not allow the calculation of any accurate result. Therefore north of 24°S we used a spreading rate of 8 cm/y which corresponds to the 24°S result and also to the value deduced from magnetic anomalies. The consumption rates along the Tonga-Kermadec trench are respectively 7.2, 9.6, 16.4, 17.1 and 17.8 cm/y at 33°S, 28°S, 24°S, 20°S and 17°S. Both the back-arc opening rate and the consumption rate of the Pacific plate sharply increase between the northern Havre trough and the southern Lau basin. This limit coincides with the Louisville ridge-trench junction. Along the N95°E eastern segment of the Fiji-North Tonga fracture zone, a pure left-lateral strike-slip motion, parallel to the N93°E opening tectonics within the northern Lau basin occurs. In contrast along the N75°E western segment of the Fiji-North Tonga fracture zone, a left-lateral strike-slip motion of 7 cm/y should be accompanied by a N135°E extensional motion of 3.5 cm/y.

Source processes of large earthquakes along the Xianshuihe fault in southwestern China

abstractThe Xianshuihe fault is one of the most active faults in southwestern China. Recently, three large earthquakes occurred along it in 1967 (Ms = 6.1), 1973 (Ms = 7.5), and 1981 (Ms = 6.8).The 1981 event occurred near the central portion of the fault zone. Modeling of the body and surface waves indicates pure left-lateral strike-slip motion on a vertical fault striking N40°W consistent with the surface trend of the Xianshuihe fault. Two major ruptures are suggested for this source, with a total moment of 1.3 ×1026 dyne-cm. The 1973 event occurred about 65 km northwest of the 1981 event and ruptured about 90 km bilaterally along the fault. The body-wave synthetics indicate three main ruptures during faulting within 43 sec, with a total moment of 1.8 ×1027 dyne-cm. The mechanisms are similar to the 1981 event, and the average slip is determined to be 3.8 m. The largest aftershock (Ms = 5.9) occurred 1 day after the main event with a normal-fault mechanism striking almost perpendicular to the surface breakage. The 1967 event occurred at the northwestern end of the fault zone, with a strike of N65°E. It had a nearly normalfault mechanism with a seismic moment of 4.5 ×1025 dyne-cm. The largest aftershock (Ms = 5.1) occurred 7 hr later with a similar focal mechanism.The primary faulting along the Xianshuihe fault is left-lateral strike-slip, but the normal faulting with strike direction about perpendicular to the Xianshuihe fault trace is common, especially in the northwestern segment. The faulting pattern in this region is consistent with the regional stress field caused by the India-Tibet collision. The normal event which is not on the major fault seems to have more frequent foreshocks and aftershocks than those on the main fault.

Teleseismic analysis of the 1980 Mammoth Lakes earthquake sequence

abstractThe source mechanisms of the three largest events of the 1980 Mammoth Lakes earthquake sequence have been determined using surface waves recorded on the global digital seismograph network and the long-period body waves recorded on the WWSSN network. Although the fault-plane solutions from local data (Cramer and Toppozada, 1980; Ryall and Ryall, 1981) suggest nearly pure left-lateral strike-slip on north-south planes, the teleseismic waveforms require a mechanism with oblique slip. The first event (25 May 1980, 16h 33m 44s) has a mechanism with a strike of N12°E, dip of 50°E, and a rake of −35°. The second event (27 May 19h 44m 51s) has a mechanism with a strike of N15°E, dip of 50°, and a slip of −11°. The third event (27 May, 14h 50m 57s) has a mechanism with a strike of N22°E, dip of 50°, and a rake of −28°. The first event is the largest and has a moment of 2.9 × 1025 dyne-cm. The second and third events have moments of 1.3 and 1.1 × 1025 dyne-cm, respectively. The body- and surface-wave moments for the first and third events agree closely while for the second event the body-wave moment (approximately 0.6 × 1025 dyne-cm) is almost a factor of 3 smaller than the surface-wave moment. The principal axes of extension of all three events is in the approximate direction of N65°E which agrees with the structural trends apparent along the eastern front of the Sierra Nevada.

Focal mechanism of earthquakes in Persia, Turkey, West Pakistan, and Afghanistan and Plate Tectonics of the Middle East

Focal mechanism solutions are determined for 23 earthquakes that occurred from 1965 to 1970 in Persia, Turkey, Pakistan, and Afghanistan. These mechanisms were obtained from the long-period first-motion P waves recorded by the World-Wide Standardized Seismograph Network stations. Mechanism solutions for the earthquakes in the folded foothills of the Zagros are predominantly thrust faulting, but depending on the assumed crustal velocity, many have strike-slip components. Cross sections of the seismic zone of the folded foothills of the Zagros show that the earthquakes are confined to a possible lithospheric slab nearly 60 km thick dipping 10° to 20° toward the north. For the earthquakes with focal depth of about 200 km, in the Hindu Kush in Afghanistan, mechanism solutions are characterized by a nearly horizontal compressional axis and a steeper extensional axis. The seismic zone appears as a contorted slab-like structure 20 to 30 km in width. This lithospheric slab may be a remnant of the Tethys Sea floor. Consistent right-lateral strike-slip mechanism solutions are found for earthquakes on the Anatolian fault in Turkey. For earthquakes on the Ferdows fault in eastern Persia, mechanism solutions vary from pure left-lateral strike-slip faulting to pure thrust faulting. Mechanism solutions for earthquakes on the Kirthar-Sulaiman shear zone in West Pakistan show consistent left-lateral strike-slip motion on a fault plane that strikes north-south. This is in agreement with the northward movement of India with respect to Afghanistan. On the basis of seismic and geological evidence, the entire Middle East is divided into eight plates which are moving northward, at different rates, with respect to Eurasia. The differential motion between various plates can explain the sense of offset and other geological features.