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1.
The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension.  相似文献   

2.
Hou  Jundong  Lv  Jun  Chen  Xin  Yu  Shiwei 《Natural Hazards》2015,77(1):97-107
The Andaman–Sumatra is one of the seismically active subduction zones and experienced three largest earthquakes in the recent past and rupturing more than 1,600-km-long portion of the plate boundary. The seismicity analysis of these large earthquakes source region (5°S–15°N latitude and 90°E–103°E longitude) has been carried out by several researchers and quantified the spatial and temporal variation of b-value which is a proxy to differential stress conditions and fractal dimension which is an indicator of material heterogeneity and strength. The results of all these studies clearly bring out the low b-value and low fractal dimension corresponding to locales were sizable magnitude earthquakes have occurred. Further locales of high stress regions are identified.  相似文献   

3.
Although subduction zones around the world are known to be the source of earthquakes and/or tsunamis, not all segments of these plate boundaries generate destructive earthquakes and catastrophic tsunamis. Costa Rica, in Central America, has subduction zones on both the Pacific and the Caribbean coasts and, even though large earthquakes (Mw = 7.4–7.8) occur in these convergent margins, they do not produce destructive tsunamis. The reason for this is that the seismogenic zones of the segments of the subduction zones that produce large earthquakes in Costa Rica are located beneath land (Nicoya peninsula, Osa peninsula and south of Limón) and not off shore as in most subduction zones around the world. To illustrate this particularity of Costa Rican subduction zones, we show in this work the case for the largest rupture area in Costa Rica (under the Nicoya peninsula), capable of producing Mw ～ 7.8 earthquakes, but the tsunamis it triggers are small and present little potential for damage even to the largest port city in Costa Rica.The Nicoya seismic gap, in NW Costa Rica, has passed its ～50-year interseismic period and therefore a large earthquake will have to occur there in the near future. The last large earthquake, in 1950 generated a tsunami which slightly affected the southwest coast of the Nicoya Peninsula. We present here a simulation to study the possible consequences that a tsunami generated by the next Nicoya earthquake could have for the city of Puntarenas. Puntarenas has a population of approximately eleven thousand people and is located on a 7.5 km long sand bar with a maximum height of 2 m above the mean sea level. This condition makes Puntarenas vulnerable to tsunamis.  相似文献   

4.
《Gondwana Research》2010,17(3-4):370-400
A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. In this article, recent studies on the subduction of the Philippine Sea and Pacific plates beneath the Japanese Islands and the mechanism of earthquake and magma generation associated with plate subduction are reviewed. Seismic tomographic studies have shown that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the Izu Peninsula as suggested in the past. The contact of the Philippine Sea plate with the Pacific plate subducting below has been found to cause anomalously deep interplate and intraslab earthquake activity in Kanto. Detailed waveform inversion studies have revealed that the asperity model is applicable to interplate earthquakes. Analyses of dense seismic and GPS network data have confirmed the existence of episodic slow slip accompanied in many instances by low-frequency tremors/earthquakes on the plate interface, which are inferred to play an important role in stress loading at asperities. High-resolution studies of the spatial variation of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes. Seismic tomography studies have shown that water released by dehydration of the slab and secondary convection in the mantle wedge, mechanically induced by slab subduction, are responsible for magma generation in the Japanese islands. Water of slab origin is also inferred to be responsible for large anelastic local deformation of the arc crust leading to inland crustal earthquakes that return the arc crust to a state of spatially uniform deformation.  相似文献   

5.
In mainland China and its surroundings the large-scale distribution of great, shallow, intraplate earthquakes shows that there are four main areas of high intraplate seismicity which are (a) the Northern China Seismic Area in east China (30°–42°N); (b) the Southeast-coast Seismic Area in eastern China (19°–25°N); (c) the North-south trending Seismic Area in western China and its surroundings (Burma–China–Mongolia); (d) the Central Asian Seismic Area in west China and its surroundings (Pamirs–Tianshan Mts–Baikal). These four intraplate seismic areas are approximately perpendicular to those sections of the Eurasian plate boundary where the Eurasian plate has a strong seismic coupling with the North American–Pacific Ocean–Philippine Sea plates, and with the Indian plate. The large-scale uneven distribution of intraplate seismicity in China and its surroundings may be controlled by heterogeneity in the stress state on different sections of the plate boundary.  相似文献   

6.
At the continental margin of north Costa Rica and Nicaragua, the strongly hydrated Cocos Plate subducts beneath the Caribbean Plate. From the downgoing Cocos plate fluids are released through extensional fractures in the overriding plate. At the seafloor, they form fluid seeps, mounds and other types of fluid expulsion. Using an offshore temporary seismic network, we investigated seismicity possibly related to these processes and observed several swarms of earthquakes located on the continental slope trenchward of the seismogenic zone of S Nicaragua. The seismicity occurred within the downgoing plate, near the plate interface and in the overriding plate. We interpret these swarm events as an expression of pore pressure propagation under critical stress conditions driven by fluid release from the downgoing plate. In order to estimate hydraulic diffusivity and permeability values, we applied a theory developed for injection test interpretation to the spatio-temporal development of the swarms. The resulting diffusivity and permeability values are in the ranges of 28–305 m²/s and 3.2 × 10?14 m²–35.1 × 10?14 m², respectively, applying to the continental and oceanic crust near the plate interface. These values are somewhat larger than observed in drill logs on the margin wedge off north Costa Rica, but of comparable magnitude to values estimated for the Antofagasta 1995 earthquake aftershock sequence.  相似文献   

7.
The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab.  相似文献   

8.
The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.  相似文献   

9.
Indian Ocean subduction zone is one of the most active plate margins of the globe as evident from its vast record of great magnitude earthquake and tsunami events. We use Bouguer admittance (Morlet isostatic response function) in Sumatra-Java subduction zones comprising both the subduction and over-riding plates to determine the lithospheric mechanical strength variations. We determine effective elastic thickness (T e ) for five oceanic windows (size 990 × 990 km2) by analyzing the admittance using Bouguer gravity and bathymetry data. The results show bimodal T e values < 20 km for Sumatra and 20−40 km for Java. The lower bimodal values obtained for Sumatra appears to correlate well with the zones of historical seismicity. This is in sharp contrast with Java subduction zone, which shows higher T e values (20–40 km) and apparently associated with low magnitude earthquakes. We suggest a strong and wide interseismic coupling for Sumatra between the subducting and over-riding plates, and deeper mantle contributing to low strength, shallow focus — high magnitude seismicity and vice versa for Java, leading to their seismogenic zonation.  相似文献   

10.
Data from the nation-wide GPS continuous tracking network that has been operated by the Geographical Survey Institute of Japan since April 1996 were used to study crustal deformation in the Japanese Islands. We first extracted site coordinate from daily SINEX files for the period from April 1, 1996 to February 24, 2001. Since raw time series of station coordinates include coseismic and postseismic displacements as well as seasonal variation, we model each time series as a combination of linear and trigonometric functions and jumps for episodic events. Estimated velocities were converted into a kinematic reference frame [Heki, K., 1996. Horizontal and vertical crustal movements from three-dimensional very long baseline interferometry kinematic reference frame: implication for reversal timescale revision. J. Geophys. Res., 101: 3187–3198.] to discuss the crustal deformation relative to the stable interior of the Eurasian plate. A Least-Squares Prediction technique has been used to segregate the signal and noise in horizontal as well as vertical velocities. Estimated horizontal signals (horizontal displacement rates) were then differentiated in space to calculate principal components of strain. Dilatations, maximum shear strains, and principal axes of strain clearly portray tectonic environments of the Japanese Islands. On the other hand, the interseismic vertical deformation field of the Japanese islands is derived for the same GPS data interval. The GPS vertical velocities are combined with 31 year tide gage records to estimate absolute vertical velocity. The results of vertical deformation show that (1) the existence of clear uplift of about 6 mm/yr in Shikoku and Kii Peninsula, whereas pattern of subsidence is observed in the coast of Kyushu district. This might reflect strong coupling between the Philippine Sea plate and overriding plate at the Nankai Trough and weak coupling off Kyushu, (2) no clear vertical deformation pattern exists along the Pacific coast of northeastern Japan. This might be due to the long distance between the plate boundary (Japan trench) and overriding plate where GPS sites are located, (3) significant uplift is observed in the southwestern part of Hokkaido and in northeastern Tohoku along the Japan Sea coast. This is possibly due to the viscoelastic rebound of the 1983 Japan Sea (Mw 7.7) and the 1993 Hokkaido–Nansei–Oki (Mw 7.8) earthquakes and/or associated with distributed compression of incipient subduction there. We then estimate the elastic deformation of the Japanese Islands caused by interseismic loading of the Pacific and Philippine Sea subduction plates. The elastic models account for most of the observed horizontal velocity field if the subduction movement of the Philippine Sea Plate is 100% locked and if that of the Pacific Plate is 70% locked. However, the best fit for vertical velocity ranges from 80% to 100% coupling factor in southwestern Japan and only 50% in northeastern Japan. Since horizontal data does not permit the separation of rigid plate motion and interplate coupling because horizontal velocities include both contributions, we used the vertical velocities to discriminate between them. So, we can say there is strong interplate coupling (80%–100%) over the Nankaido subduction zone, whereas it is about 50% only over the Kurile–Japan trench.  相似文献   

11.
The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone.  相似文献   

12.
The distribution of relocated seismicity and the evolving shape of fracture zones through time in the oceanic crust of the Australian Plate adjacent to the Australia:Pacific plate boundary south of New Zealand are used to constrain the deformation of this region of the Australian Plate, here called the Puysegur Block. Relocated seismicity reveals a broad distribution of earthquakes in the Puysegur Block on both inter- and intraplate structures, including two great (M8+) earthquakes in the region over the past twenty years, one of which occurred over 130 km from the plate boundary. Plate reconstructions from the Late Oligocene through Early–Mid Miocene allow us to determine the undeformed shape of fracture zones in the Puysegur Block, formed during the Paleogene when the plate boundary was dominantly a divergent mid-ocean ridge system. Comparing these reconstructions to the present-day shape of the fracture zones allows us to map the deformation that has occurred within the Puysegur Block since the fracture zones formed. These two sets of independent observations delineate a broad zone of deformation extending ~ 150 km into the plate interior from the Macquarie Ridge Complex, the modern plate boundary structure through the region. The persistence of this deformation through time indicates a link with the evolution of the plate boundary over the past ~ 25 Ma from divergence to translation and subduction of the Australian Plate further north at the Puysegur Trench. We infer that this deformation may be a result of stresses in the Puysegur Block resulting from the impingement of the subducting plate on the thickened lithosphere of southern New Zealand. Such a collision may resist subduction, and if resistance remains substantial, further deformation internal to the Puysegur Block may lead to a southward migration of the Australia:Pacific subduction interface and the capturing of this section of lithosphere onto the Pacific Plate.  相似文献   

13.
The Burmese Arc seismic activity is not uniform for its ∼ 1100 km length; only the Northern Burmese Arc (NBA) is intensely active. Six large earthquakes in the magnitude range 6.1–7.4 have originated from the NBA Benioff zone between 1954–2011, within an area of 200 × 300 km2 where the Indian plate subducts eastward to depths beyond 200 km below the Burma plate. An analysis on seismogenesis of this interplate region suggests that while the subducting lithosphere is characterized by profuse seismicity, seismicity in the overriding plate is rather few. Large earthquakes occurring in the overriding plate are associated with the backarc Shan-Sagaing Fault (SSF) further east. The forecasting performance of the Benioff zone earthquakes in NBA as forerunner is analysed here by: (i) spatial earthquake clustering, (ii) seismic cycles and their temporal quiescence and (iii) the characteristic temporal b-value changes. Three such clusters (C1–C3) are identified from NBA Benioff Zones I & II that are capable of generating earthquakes in the magnitude ranges of 7.38 to 7.93. Seismic cycles evidenced for the Zone I displayed distinct quiescence (Q1, Q2 and Q3) prior to the 6th August 1988 (M 6.6) earthquake. Similar cycles were used to forecast an earthquake (Dasgupta et al. 2010) to come from the Zone I (cluster C1); which, actually struck on 4 February 2011 (M 6.3). The preparatory activity for an event has already been set in the Zone II and we speculate its occurrence as a large event (M > 6.0) possibly within the year 2012, somewhere close to cluster C3. Temporal analysis of b-value indicates a rise before an ensuing large earthquake.  相似文献   

14.
The alkaline volcanic rocks of the 1.8–0.9 Ma Auca Mahuida and post-mid-Pliocene Rio Colorado backarc volcanic fields east of the Andean Southern Volcanic Zone at ~37°–38°S have pronounced intraplate-like chemical signatures with some striking similarities to oceanic DM-EM1-like lavas of the south Atlantic Tristan da Cunha type. These backarc lavas are considered to have formed as a series of mantle batches typified by 4–7 % melting, with decompression melting initiating in a garnet-bearing mantle above a steepening subduction zone, and final equilibration occurring near the base of a ~65- to 70-km-thick lithosphere at temperatures of ~1,350–1,380 °C. Evolved Auca Mahuida mugearite to trachytic samples are best explained by crystal fractionation with limited mixing of partial melts of recently underplated basalts, in line with isotopic signatures that preclude significant radiogenic contamination in a preexisting refractory crust. Higher Ba/La and subtly higher La/Ta ratios than in nearby ~24–20 Ma primitive basalts or oceanic (OIB) lavas are attributed to the residual effects of slab fluids introduced during a shallow subduction episode recorded in the arc-like chemistry of the adjacent 7–4 Ma Chachahuén volcanic complex. Positive Sr, K and Ba spikes on mantle-normalized patterns of both primitive Auca Mahuida and ~24–20 Ma basalts, like those in EM-like OIB basalts, are attributed to mixing of continental lithosphere into the asthenosphere. In Patagonia, this mixing is suggested to have peaked as the South America continent accommodated to major late Oligocene plate convergence changes, as similar Sr, K and Ba spikes and DM-EM1 signatures are absent in ~50–30 Ma backarc lavas north of 51°S, and all of those south of 51°S. Introduction of an EM1-like component associated with lateral mantle flow of a Tristan da Cunha source is largely precluded by its Cretaceous age and distance to Patagonia.  相似文献   

15.
Determining factors that limit coseismic rupture is important to evaluate the hazard of powerful subduction zone earthquakes such as the 2011 Tohoku‐Oki event (Mw = 9.0). In 1960 (Mw = 9.5) and 2010 (Mw = 8.8), Chile was hit by such powerful earthquakes, the boundary of which was the site of a giant submarine slope failure with chaotic debris subducted to seismogenic zone depth. Here, a continuous décollement is absent, whereas away from the slope failure, a continuous décollement is seismically imaged. We infer that underthrusting of inhomogeneous slide deposits prevents the development of a décollement, and thus the formation of a thin continuous slip zone necessary for earthquake rupture propagation. Thus, coseismic rupture during the 1960 and 2010 earthquakes seems to be limited by underthrusted upper plate mass‐wasting deposits. More generally, our results suggest that upper plate dynamics and resulting surface processes can play a key role for determining rupture size of subduction zone earthquakes.  相似文献   

16.
Ten new focal mechanisms are derived for earthquakes in southern Central America and its adjacent regions. These are combined with a study of seismicity and data of previous workers to delineate the position and nature of the plate boundaries in this complex region.The Middle America subduction zone may be divided into four or five distinct seismic segments. The plate boundary between North America and the Caribbean near the trench might be located more towards the south than previously suspected. Subduction has basically stopped south of the underthrusting Cocos Ridge. There is not much evidence for a seismically active strike-slip fault south of Panama, but its existence cannot be ruled out. More activity reveals the zone north of Panama which is identified as a subduction zone with normal fault events. Shallow seismicity induced by the interaction of the Nazca plate extends from the Colombia-Panama border south along the Pacific coast to meet a high-angle continental thrust fault system. Subduction with a pronounced slab starts only south of that point near a hot region which offsets the seismic trend at the trench. The Carnegie Ridge and/or the change of direction of subduction in Ecuador produce a highly active zone of seismicity mainly at the depth of 200 km. The area in the Pacific displays a termination of activity at a propagating rift west of the Galapagos Islands. The main eastern boundary of the Cocos plate, the Panama Fracture Zone, is offset towards the west at the southern end of the Malpelo Ridge. Its northern end consists of two active branches as defined by large earthquakes. A strike-slip mechanism near the southeastern flank of the Cocos Ridge was previously believed to be the site of an extended fracture zone. This paper proposes submarine volcanic activity as an alternative explanation.  相似文献   

17.
Meschede  Zweigel  Frisch  & Völker 《地学学报》1999,11(4):141-148
The convergent plate margin off the Osa peninsula in southern Costa Rica is characterized by the indentation of the Cocos ridge at 4–5 Ma. The indentation causes the uplift of the Osa mélange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone. We present evidence that, similar to the Nicoya segment of the Costa Rica convergent margin, subduction erosion rather than accretion has been the dominant process along the plate boundary. The composition of the Osa mélange is dominated by tectonized material of the upper-plate Nicoya ophiolite complex (basalt, radiolarite, limestone). Strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes, which partly experienced temperatures > 200°C. We thus interpret the Osa mélange to be a product of subduction erosion at the base of the outer arc wedge structure.  相似文献   

18.
The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.  相似文献   

19.
Earthquake hypocenters and travel time residuals have been analysed to constrain the geometry and physical state of the subducted Indian plate in the Indo-Burmese convergence zone. A critical analysis of earthquake hypocenters reveals the existence of a non-uniform Benioff zone, progressively shortening from north to south. The deepest level of seismicity is observed beneath the Naga hills (160 km) followed by that under the Chin hills (120 km) and Arakan-Yoma ranges (80 km). The region seems to be devoid of moderate sized shallow (< 40 km) earthquakes. Differential travel time residuals from pairs of shallow and intermediate depth earthquakes recorded at teleseismic distances show significantly faster travel time (up to l.2s) in the north-northeast and south-southwest azimuths, whilst slower arrivals (1.2 to 1.5 s) are recorded in the transverse direction. This observation points to the presence of a high velocity slab possibly linked to the subduction of the Indian oceanic lithosphere.  相似文献   

20.
A tomographic travel-time inversion has been applied to trace the subducted slab of the South China Sea (SCS) beneath the Manila Trench. The dataset, taken from the International Seismological Centre (1960–2008), is composed of 13,087 P-wave arrival times from 1401 regional earthquakes and 8834 from 1350 teleseismic events. The results image the different morphology of the subducted SCS slab as a high-velocity zone. The subducting angle of the slab varies along the trench: at 16° N and 16.5° N, the slab dips at a low angle (24° ~ 32°) for 20–250 km depth and at a moderate angle (50°) for ~250–400 km depth. At 17° N, the slab dips at a low angle (32°) to near 400 km depth, and at 17.5° N and 18° N the slabs are near vertical from 70 ~ 700 km depth, while at 20° N the high-velocity anomalies exhibit features from horizontal abruptly to near vertical, extending to 500 km depth. The dramatic steepening of the slab between 17° N and 17.5° N may indicate a slab tear, which is coincident with the axis of a fossil ridge within the SCS slab at around 17° N. In addition, low-velocity zones in the three profiles above 300 km depth may represent the formation of the slab window, induced by ridge subduction and slab tear, initiating upward mantle flow and resulting in the partial melting of the edge of the slab. The slab tear could explain the volcanic gap and geochemical difference between the extinct Miocene and Quaternary volcanoes in the Luzon Arc, the much higher heat flow around the fossil ridge, and the distribution of most of the adakites and the related porphyry Cu-Au deposits in the Luzon area. Based on the geometry and morphology of the subducted slab and certain assumptions, we calculate the initial time of ridge subduction, which implies that ridge subduction and slab tear possibly started at ~8 Ma.  相似文献   

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