Earthquake source mechanisms from body-waveform inversion and intraplate tectonics in the northern Indian Ocean

Double-couple point-source parameters for 11 of the largest intraplate earthquakes in the northern Indian Ocean during the last 20 y were determined from a formal inversion of long-period P and SH waveforms. Nine of the events have centroid depths at least 17 km below the seafloor, well into the upper mantle; two have centroid depths as great as 39 km. Using the source mechanisms of these earthquakes, we distinguish two major intraplate tectonic provinces in the northern Indian Ocean. To the west of the Ninetyeast Ridge, in the southern Bay of Bengal, intraplate earthquakes have thrust-faulting mechanisms with P axes oriented N-S. The centroid depths of these earthquakes range from 27 to 39 km below the seafloor. Lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle, while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding. In the immediate vicinity of the Ninetyeast Ridge, earthquakes display strike-slip mechanisms with left-lateral motion on planes parallel to the ridge. This type of faulting occurs from at least 10°S to the northern end of the Ninetyeast Ridge near 10°N, where the ridge meets the Sunda Arc. Seismic activity diminishes to the east of the Ninetyeast Ridge, but is also characterized by strike-slip faulting. Despite these variations in deformational style, the inferred orientation of greatest compressive stress in the northern Indian Ocean displays a consistent long-wavelength pattern over a large portion of the Indian plate, varying smoothly from nearly N-S in the Bay of Bengal to NW-SE in the northeastern Indian Ocean. This plate-wide stress pattern and the high level of intraplate seismicity in the northern Indian Ocean are likely the results of substantial resistance, along the Himalayan continental collision zone, to the continued northward motion of the western portion of the Indian plate. Oceanic intraplate earthquakes in other regions, where the level of deviatoric stress associated with the long-wavelength part of the stress field is likely to be smaller, need not be comparably reliable indicators of the plate-wide stress field.     

Historical seismicity near Chagos: a complex deformation zone in the equatorial Indian Ocean

Over the last twenty years, Chagos Bank has a seismicity rate disproportionate to its supposed intraplate location. Earthquake relocation also shows a high seismicity rate in pre-WWSSN time (1912–1963), with seven events located off of the Central Indian Ridge, including large events in 1912 (M = 6.8) and 1944 (M = 7.2). This study uses the moment variance technique, a systematic search for the mechanism which best fits P, PP, SH, Love and Rayleigh amplitudes, to determine the focal mechanisms of two pre-WWSSN earthquakes. A test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available. Moment variance analysis shows a thrust faulting mechanism for the 1944 event, northeast of Chagos Bank near the Chagos-Laccadive ridge, and a strike-slip focal mechanism for a smaller 1957 event west of Chagos Bank. The 1944 event, one of the largest oceanic “intraplate” earthquakes known (moment 1.4 × 10²⁷ dyne-cm), indicates that the Chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought, but rather regional tectonic deformation extending northeast of Chagos Bank and including thrust, normal and strike-slip events. This seismicity and previously studied seismicity near the Ninetyeast Ridge and Central Indian Basin suggest a broad zone of deformation stretching across the equatorial Indian Ocean. This zone contains all known magnitude seven oceanic “intraplate” earthquakes not associated with subduction zones or continental margins, suggesting that elsewhere such extensive deformation occurs only along plate boundaries. This study proposes that a slow, diffuse plate boundary extends east from the Central Indian Ridge to the Ninetyeast Ridge and north to the Sumatra Trench. A recent plate motion study confirms this boundary and suggests that it separates the Australian plate from a single Indo-Arabian plate.     

Is there a Caroline plate?

We examine available marine geophysical and seismological data from the Caroline Sea region and conclude that a separate Caroline plate currently exists. The Caroline plate is moving relative to the Pacific plate on its northern and eastern boundaries, the Philippine plate on its western margin, and the Indian and smaller plates along its southern side in New Guinea and the Bismarck Sea area. The southern Yap Trench, the Palau Trench, and an accreting plate boundary within the Ayu Trough manifested as an axial rift valley comprise the Caroline-Philippine plate boundary. On the basis of sediment thickness and subsidence of basement away from the rift, we estimate that the Ayu Trough started to open during the Miocene. The northern section of Pacific-Caroline plate boundary coincides with the Sorol Trough which exhibits both strike-slip and extensional characteristics. The southeastern section of this boundary occurs along the Mussau Trench where Caroline plate underthrusts the Pacific plate. The section of plate boundary between the Sorol Trough and Mussau Trench is characterized by highly unusual deformational tectonics. Convergence between the Pacific and Caroline plates is apparently accommodated here by overthrusting of small slivers of sea floor towards the northeast. The intensity of deformation appears to increase southward towards the Mussau Trench. Our calculated instantaneous angular rotation vector for the Pacific-Caroline plates predicts that convergence rates increase uniformly south along the overthrust and underthrust sections of plate boundary. The transition in tectonic style from overthrusting to underthrusting occurs between 3° and 4°N.     

Tectonic structure and petrology of the Antarctic plate boundary near the Bouvet triple junction

An en echelon suite of four fracture zones, trending approximately N40°E, has been discovered during a survey of the Southwest Indian Ocean Ridge between Bouvet Island and 14°E. The largest of these fracture zones, the Islas Orcadas and Shaka, are less than 30 km wide, have more than 3 km of vertical relief, and are respectively 100 and 200 km in length. The morphology of these and the Bouvet and Prince Edward fracture zones have been used to compute a pole for the relative motion between Africa and Antarctica. This pole, at 4°S and 32°W, is within the range of previously computed pole positions.Ridge basalts were dredged at three separate locations: at the Conrad fracture zone near 55°40′S and 3°51′W, at the Islas Orcadas fracture zone near 54°5′S and 6°4′E, and at the ridge crest near 11°E. In addition, samples from a probable upper mantle intrusion were recovered from one wall of the Islas Orcadas fracture zone. The opposite wall was very different consisting entirely of normal mid-ocean ridge basalt.     

Geochronology of basalts from the ninetyeast ridge and continental dispersion in the eastern Indian Ocean

Conventional K-Ar and ⁴⁰Ar/³⁹Ar total-fusion and incremental-heating ages from basalts recovered from the Ninetyeast Ridge confirm earlier indication from paleontology of basal sediments that basement crystallization ages increase systematically from south to north. A rate of migration of volcanism of 9.4 ± 0.3 cm/yr best fits the age-distance relationship determined from these basalts. The geometry, distribution of ages, paleomagnetism and geochemistry are most compatible with an origin of the Ninetyeast Ridge by northward movement of the Indian plate over a hotspot near Heard Island in the southern Indian Ocean, from mid-Cretaceous to Early Oligocene time. The Ninetyeast Ridge and nearly parallel Chagos-Laccadive Ridge provide an absolute frame of reference for the reconstruction of the eastern Indian Ocean.     

Eastward subduction of the Indian plate beneath the Indo-Myanmese arc revealed by teleseismic P-wave tomography

The deep structure of the eastward-subducting Indian plate can provide new information on the dynamics of the India-Eurasia collision. We collected and processed waveform data from temporary seismic arrays (networks) on the eastern Tibetan Plateau, seismic arrays in Northeast India and Myanmar, and permanent stations of the China Digital Seismic Network in Tibet, Gansu, Qinghai, Yunnan, and Sichuan. We combined these data with phase reports from observation stations of the International Seismological Center on the Indian plate and selected 124,808 high-quality P-wave relative travel-time residuals. Next, we used these data to invert the 3-D P-wave velocity structure of the upper mantle to a depth of 800 km beneath the eastern segment of the arcuate Himalayan orogen, at the southeastern margin of the Tibetan Plateau. The results reveal a high-angle, easterly dipping subducting plate extending more than 200 km beneath the Indo-Myanmese arc. The plate breaks off at roughly 96°E; its fragments have passed through the 410-km discontinuity (D410) into the mantle transition zone (MTZ). The MTZ beneath the Tengchong volcanic area contains a high-velocity anomaly, which does not exceed the Red River fault to the east. No other large-scale continuous subducted plates were observed in the MTZ. However, a horizontally spreading high-velocity anomaly was identified on the D410 in some regions. The anomaly may represent the negatively buoyant 90°E Ridge plate or a thickened and delaminated lithospheric block experiencing collision and compression at the southeastern margin of the Tibetan Plateau. The Tengchong volcano may originate from the mantle upwelling through the slab window formed by the break-off of the subducting Indian continental plate and oceanic plate in the upper mantle. Low-velocity upper mantle materials on the west side of the Indo-Myanmese arc may have supplemented materials to the Tengchong volcano.     

New constraints on the tectonic evolution of the eastern Indian Ocean

From marine magnetic anomaly studies, a fossil spreading ridge is identified beneath the Nicobar Fan in the northwestern Wharton Basin. Several north-south-trending transform faults offset this ridge left-laterally east of the 86°E transform fault. Our findings show that this ridge, which was part of the plate boundary between the Indian and Australian plates, ceased its spreading shortly after formation of magnetic anomaly 20 (～ 45.6m.y. B.P.). Since the breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P., we suggest that the Indian, Australian, and Antarctic plates were moving relative to one another from about 90 to 45 m.y. B.P. A triple junction would have existed in the southeastern Indian Ocean during that period of time. At anomaly 19 time (～ 45m.y. B.P.), the junction became inactive, and Australia and India became a single plate. The northwest-southeast-trending Southeast Indian Ridge was formed by connecting the India-Antarctica spreading center with the Australia-Antarctica spreading center. Its activity has continued to the present time.     

The Mid-Atlantic Ridge at 33°N: the Hayes Fracture Zone

A geophysical study was conducted over the Mid-Atlantic Ridge between 32–39°N and 30–40°W. A particularly deep fracture was observed which offset the ridge crest 110 km in the vicinity of 33°N. A pole of relative motion between the North American and African plates was deduced from this fracture zone as being at 63.1°N, 17°W.     

板块运动与地震各向异性及应力场的相关性统计分析

利用收集到的各种来源共计7 959组的地震各向异性观测数据和21 750组应力场数据,结合板块绝对运动模型计算给出的各板块的运动规律,分别统计分析了板块运动与地震各向异性及应力场的相关性,并对板块运动对地震各向异性及应力场特征产生的影响进行了分析． 统计结果表明,阿拉伯、 加勒比、 胡安德富卡、 北美、 纳兹卡、 太平洋和南美板块上地震各向异性与板块运动均具有较好的相关性,而非洲、 南极洲、 澳大利亚、 欧亚、 印度和菲律宾板块上二者的相关性则相对较差. 讨论分析发现,板块运动拖动软流圈流动、 橄榄岩晶格优选方位、 化石各向异性和地幔流动或岩石圈流动等因素均在一定程度上控制并影响着地震各向异性与板块运动的一致性． 而板块基底拖曳力、 洋脊推力、 浮力作用和碰撞及俯冲作用等多种因素共同制约了板块运动与应力场的相关性,使得非洲、 可可斯、 欧亚、 胡安德富卡、 北美、 纳兹卡、 菲律宾和南美板块上二者的相关性较好,其它板块上其相关性则较差． 对于俯冲带地区,由于俯冲机制的复杂性和软流圈、 岩石圈地幔流动方向的不确定性,其板块运动与地震各向异性及应力场的相关性图像表现复杂,需要结合具体的俯冲带构造进行近一步研究．      

Plate kinematics: The Americas,East Africa,and the rest of the world

Euler vectors (relative angular velocity vectors) have been determined for twelve major plates by global inversion of carefully selected sea-floor spreading rates, transform fault trends, and earthquake slip vectors. The rate information comes from marine magnetic anomalies less than 5 m.y. old, so the motions are valid for post-Miocene times. Plate motions in a mean hotspot frame of reference have also been determined, and statistical confidence limits for all the Euler vectors estimated. Among the consequences of the global motion model is the conclusion that fast-spreading ridges (separation rates greater than 3 cm/yr) have plate motion nearly perpendicular to the strike of the ridge and magnetic anomalies. Four more slowly separating ridges have an average obliquity of spreading of almost 20°.For several plate boundaries, results that differ from previous studies are in agreement with geological evidence. The North and South American plates converge slowly about a pole east of the Antilles and near the Mid-Atlantic Ridge. The results for Africa versus Somalia imply slow east-west extension on the East African Rift Valleys. The pole for motion of Eurasia relative to North America is located near Sakhalin, in accordance with evidence from Siberia and Sakhalin.     

Global tectonics and the plate motion obtained from the ITRF97 station velocity vectors

~~Global tectonics and the plate motion obtained from the ITRF97 station velocity vectors@马宗晋 @任金卫 @张进~~     

Early Tertiary rupture of the Pacific plate: 1700 km of dextral offset along the Emperor trough-Line Islands lineament

Between 67 and ～40 Ma ago a northwest-southeast-trending fracture system over 8000 km long split the Pacific plate and accumulated at least 1700 km of dextral offset between the east and west portions. This system, here named the Emperor fracture zone (EFZ) system, consisted of several segments, one along the present trace of the Emperor trough and another along the Line Islands, joined by short spreading ridges. The EFZ terminated at its northern end against the Kula-Pacific ridge, and at its southern end in a ridge-transform system, called the Emperor spreading system, which extended to the west, north of Australia.The finite angular velocity vector describing the relative motion between the East and West Pacific plates is ～0.6°/Ma about a pole at 36°N, 70°W. This vector, added to the known Early Tertiary motion of the Pacific plate with respect to the global hotspot reference frame, accounts in large part for the NNW trend of the Emperor seamount chain relative to the WNW Hawaiian trend, without violation of the integrity of the Antarctic plate. The Meiji-Emperor and Emperor-Hawaiian bends date, respectively, the initiation (～67 Ma ago) and cessation (～40 Ma ago) of seafloor spreading on the Emperor spreading system.The postulated Early Tertiary relative motion along the EFZ between the East and West Pacific plates explains (1) the present misalignment of the two sets of magnetic bights of the Pacific, (2) the abrupt truncation of eastern Pacific bathymetric lineaments against the Emperor trough and Line Islands, (3) the contrast in paleolatitude between the eastern and western Pacific as indicated by paleomagnetic and sedimentologic studies, and (4) the anomalous gravity signature of the central Hawaiian ridge that indicates that the ridge loaded thin hot lithosphere.     

Relative Motion Between the Rivera and North American Plates Determined from the Slip Directions of Earthquakes

So far, the direction and rate of relative motion between the Rivera and the North American plates (RIV-NAM) has been determined by the combination of two Euler poles: Rivera (RIV), with respect to Pacific (PAC), and PAC with respect to North America. Here, we estimate the relative motion of this plate pair (RIV-NAM) assuming that the horizontal projection of the direction of slip of the earthquakes occurring on the RIV-NAM boundaries reflect their relative plate motion. A catalog of earthquakes for which focal mechanisms are reported since 1976 is used in the analysis. Earthquakes were considered in the three segments of the RIV-NAM plate boundary: the subduction zone of the Rivera plate beneath the Jalisco block, the Tres Marias Escarpment and the events associated with the Tamayo Fracture Zone. The best fitting Euler pole is determined using a grid search of 64 potential poles. The slip direction predicted for each grid point is compared to the slip direction of the focal mechanisms of the earthquakes on the plate boundary. The best fitting Euler pole, determined in a root mean square sense (RMS), is located at 21.8°N, 107.6°W. A rate of rotation of 5.3°/year is estimated assuming the seismic earthquake cycle of the 1932 and 1995 great earthquakes represents a lower bound of the rate of plate motion in the subduction zone. The best fitting Euler pole shows that the subduction of the Rivera plate takes place in a direction perpendicular to the trench with a relative velocity of 4.3 cm/year, offshore Manzanillo. The rate of relative motion RIV-NAM decreases from SE to NW. North of approximately 21°N, the subduction of the Rivera plate becomes oblique to the trench and the relative velocity between the two plates decreases to an average of 1.9 cm/year. This slow rate of convergence may explain the rapid decrease of seismicity in the trench and the apparent absence of large earthquakes in this region. In the Tres Marias Escarpment, our best-fitting pole suggests that subduction stops, giving way to high-angle reverse faulting perpendicular to the Tres Marias Escarpment, in agreement with the reverse faulting earthquakes occurring here. To the north of 22.5°N, the slip predicted by the best-fitting pole suggests right-lateral faulting in a direction parallel to the Tamayo Fracture Zone, at a very low velocity (0.5–1.0 cm/year). The best fitting Euler pole determined here lies very close to the RIV-NAM plate boundary in the vicinity of the Tamayo Fracture Zone. This location of our best fitting Euler pole explains the low relative plate velocity, the relatively low level of seismic activity and the presence of a broad zone of deformation that accommodates the RIV-NAM motion.     

Determination of Euler parameters of Philippine Sea plate and the inferences

Euler vectors of 12 plates, including Philippine Sea plate (PH), relative to a randomly fixed Pacific plate(PA) were determined by inverting the 1122 data from NUVEL-1 global plate motion model, earthquake slip vectors along Philippine Sea plate boundary, and GPS observed velocities. Euler vectors of Philippine Sea plate relative to adjacent plates are also gained. Our results are well consistent with observed data and can satisfy the geological and geophysical constraints along the Caroline(CR)-PH and PA-CR boundaries. Deformation of Philippine Sea plate is also discussed by using the plate motion Euler parameters.     

Recent Geodetic Results in the Azores Triple Junction Region

—GPS (Global Positioning System) observations started to be carried out in the Azores region under the scope of the TANGO (TransAtlantic Network for Geodesy and Oceanography) project in 1988. The measurements carried out between 1993 and 2000 (five campaigns) on nine GPS sites (one per island) were reprocessed using two state–of–the-art software packages. Different methodologies were applied to compute each campaign solution and the derived velocity field. The velocity fields, including the motions of two permanent stations, recently installed in the Azores, were computed within the most recent geodetic reference frame, ITRF2000 (International Terrestrial Reference Frame, solution 2000). They are compared with the motions of the stable rigid tectonic plates using as reference DEOS2k, a global tectonic model developed using geodetic data. The relative motions between the Western and Central groups of islands yield to evaluate the opening rate of the Mid-Atlantic Ridge (boundary between the North American plate and the Eurasian and African plates). Concerning the boundary between the Eurasian and African plates, the motion of the TANGO sites in the Central and Eastern groups clearly identifies the transition pattern between those two plates. Two of the sites are considered to be located in the stable part of these plates, whereas the remaining five are within the deformation region of the Eurasia-Africa boundary. The conclusions are analyzed in view of the different deformation models, derived from geodynamic or geophysical data that have been proposed for the region.     

Tectonics of the Nazca-Antarctic plate boundary

We have constructed a new bathymetric chart of part of the Chile transform system, based mainly on an R/V “Endeavor” survey from 100°W to its intersection with the East Ridge of the Juan Fernandez microplate at 34°30′S, 109°15′W. A generally continuous lineated trend can be followed through the entire region, with the transform valley being relatively narrow and well-defined from 109°W to approximately 104°30′W. The fracture zone then widens to the east, with at least two probable en echelon offsets to the south at 104° and 102°W. Six new strike-slip mechanisms along the Chile Transform and one normal fault mechanism near the northern end of the Chile Rise, inverted together with other plate motion data from the eastern portion of the boundary, produce a new best fit Euler pole for the Nazca-Antarctic plate pair, providing tighter constraints on the relative plate motions.     

Seismicity along the Azores-Gibraltar region and global plate kinematics

Seismicity along the western part of the Eurasia–Nubia plate boundary displays very complex patterns. The average motion is transtensional in the Azores, dextral along the Gloria transform zone and convergent between the SW Portuguese Atlantic margin and the Ibero-Maghrebian zone. To constrain the factors controlling the seismicity, we provide a new seismotectonic synthesis using several earthquakes. We show that the study area can be divided into six different regions, with each characterised by a coherent seismicity pattern. The total seismic moment tensor and the average slip velocities are provided for each region. To determine the spatial distribution of the seismicity, we computed the slip vector for each earthquake based on its focal mechanism and compared it to the relative velocity between the Eurasian and Nubian plates, deduced from global kinematic models. Despite local departures in the Alboran Sea and near the Mid-Atlantic Ridge, we found a good correlation between these two independent vector sets. Quantitatively, the slip velocities display a linear, non-affine correlation with the norms of the relative kinematic velocities. The norms of the slip velocities also seem to depend on the tectonic regime and on the morphology of the plate boundary.     

America-Eurasia plate boundary in eastern Asia and the opening of marginal basins

The opening of the Arctic Ocean during the past 55 Ma resulted in relative rotation of America with respect to Eurasia about a pole located in eastern Siberia, near the plate boundary. The extensional plate boundary enters deep inside the Eurasian continent up to the rotation pole. On the opposite side of the pole, on the Pacific side of the plate boundary, compressive tectonics are recorded along Sakhalin and Hokkaido. From the Oligocene to Middle Miocene, the relative movement was accommodated by strike-slip motion along Sakhalin and Hokkaido although the rotation pole was not located at a significatively different position from now. In this paper we explain this by independent motion of the southernmost tip of the American plate towards the Pacific margin which behaves as a free boundary. This oceanward motion resulted in an extension of the American plate giving rise to the wedge structure of the Okhotsk Sea. The Japan Sea opened as a pull-apart basin along the strike-slip boundary; finally the increasing extension in the Okhotsk Sea led to the opening of the oceanic Kuril Basin.     

Episodic tectonic plate reorganizations driven by mantle convection

Periods of relatively uniform plate motion were interrupted several times throughout the Cenozoic and Mesozoic by rapid plate reorganization events [R. Hey, Geol. Soc. Am. Bull. 88 (1977) 1404-1420; P.A. Rona, E.S. Richardson, Earth Planet. Sci. Lett. 40 (1978) 1-11; D.C. Engebretson, A. Cox, R.G. Gordon, Geol. Soc. Am. Spec. Pap. 206 (1985); R.G. Gordon, D.M. Jurdy, J. Geophys. Res. 91 (1986) 12389-12406; D.A. Clague, G.B. Dalrymple, US Geol. Surv. Prof. Pap. 1350 (1987) 5-54; J.M. Stock, P. Molnar, Nature 325 (1987) 495-499; C. Lithgow-Bertelloni, M.A. Richards, Geophys. Res. Lett. 22 (1995) 1317-1320; M.A. Richards, C. Lithgow-Bertelloni, Earth Planet. Sci. Lett. 137 (1996) 19-27; C. Lithgow-Bertelloni, M.A. Richards, Rev. Geophys. 36 (1998) 27-78]. It has been proposed that changes in plate boundary forces are responsible for these events [M.A. Richards, C. Lithgow-Bertelloni, Earth Planet. Sci. Lett. 137 (1996) 19-27; C. Lithgow-Bertelloni, M.A. Richards, Rev. Geophys. 36 (1998) 27-78]. We present an alternative hypothesis: convection-driven plate motions are intrinsically unstable due to a buoyant instability that develops as a result of the influence of plates on an internally heated mantle. This instability, which has not been described before, is responsible for episodic reorganizations of plate motion. Numerical mantle convection experiments demonstrate that high-Rayleigh number convection with internal heating and surface plates is sufficient to induce plate reorganization events, changes in plate boundary forces, or plate geometry, are not required.     

Glacial isostasy and plate motion

The influence of glacial-isostatic adjustment (GIA) on the motion of tectonic plates is usually neglected. Employing a recently developed numerical approach, we examine the effect of glacial loading on the motion of the Earth’s tectonic plates where we consider an elastic lithosphere of laterally variable strength and the plates losely connected by low viscous zones. The aim of this paper is to elucidate the physical processes which control the GIA-induced horizontal motion and to assess the impact of finite plate-boundary zones. We show that the present-day motion of tectonic plates induced by GIA is at, or above, the order of accuracy of the plate motions determined by very precise GPS observations. Therefore, its contribution should be considered when interpreting the mechanism controlling plate motion.