From subduction to transform motion: a seabeam survey of the Hellenic trench system

Preliminary results of a multi-narrow beam survey of the Hellenic trench system, in the Eastern Mediterranean, are presented. The southwestern Ionian branch is divided in small basins, partly filled with Pleistocene sediments. The morphology suggests that the basins are deformed by a compressional stress acting roughly perpendicularly to the trench along N50°E. This direction is the direction of the regional slip vector of the shallow thrust-type earthquakes. The structure of the southeastern Pliny-Strabo branch is quite different. Narrow en-e´chelon slots, oriented N40°E, have been mapped within the main troughs oriented N60°E. The regional earthquake slip vector is also oriented along N40°E. We conclude that the Hellenic trench system is an active subduction system, dominated by thrust along the Ionian branch and by transform motion along the Pliny-Strabo branch.     

Imaging the Seismic Crustal Structure of the Western Mexican Margin between 19°N and 21°N

Three thousand kilometres of multichannel (MCS) and wide-angle seismic profiles, gravity and magnetic, multibeam bathymetry and backscatter data were recorded in the offshore area of the west coast of Mexico and the Gulf of California during the spring 1996 (CORTES survey). The seismic images obtained off Puerto Vallarta, Mexico, in the Jalisco subduction zone extend from the oceanic domain up to the continental shelf, and significantly improve the knowledge of the internal crustal structure of the subduction zone between the Rivera and North American (NA) Plates. Analyzing the crustal images, we differentiate: (1) An oceanic domain with an important variation in sediment thickness ranging from 2.5 to 1 km southwards; (2) an accretionary prism comprised of highly deformed sediments, extending for a maximum width of 15 km; (3) a deformed forearc basin domain which is 25 km wide in the northern section, and is not seen towards the south where the continental slope connects directly with the accretionary prism and trench, thus suggesting a different deformational process; and (4) a continental domain consisting of a continental slope and a mid slope terrace, with a bottom simulating reflector (BSR) identified in the first second of the MCS profiles. The existence of a developed accretionary prism suggests a subduction–accretion type tectonic regime. Detailed analysis of the seismic reflection data in the oceanic domain reveals high amplitude reflections at around 6 s [two way travel time (twtt)] that clearly define the subduction plane. At 2 s (twtt) depth we identify a strong reflection which we interpret as the Moho discontinuity. We have measured a mean dip angle of 7° ± 1° at the subduction zone where the Rivera Plate begins to subduct, with the dip angle gently increasing towards the south. The oceanic crust has a mean crustal thickness of 6.0–6.5 km. We also find evidence indicating that the Rivera Plate possibly subducts at very low angles beneath the Tres Marias Islands.     

Slip partitioning along major convergent plate boundaries

Along plate boundaries characterized by oblique convergence, earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors. Consequently, relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back-arc regions. The deformation behind the trench may take the form of strike-slip motion, back-arc extension, or some combination of both. We observe from our analysis of the Harvard Moment Tensor Catalog that convergent arcs characterized by back-arc spreading, specifically the Marianas and New Hebrides, are characterized by a large degree of slip partitioning. However, the observed rates, directions, and location of back-arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs, implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate. In the case of the Sumatran arc, where partitioning is accommodated by strike-slip faulting in the overriding plate, the degree of partitioning is similar to that observed along the Marianas, but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors. In the case of the Alaskan-Aleutian arc, where back-arc spreading is also absent, the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence. If partitioning in the Alaskan-Aleutian arc is accommodated by strike-slip faulting within the upper plate, the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either (1) the ratio of the depth extent of strike-slip faults behind the trenchZ _s to the subduction thrustZ _t increases westward along the arc, (2) the dip of the subduction thrust increases westward along the arc, or (3) the strength of the subduction thrust decreases westward along the arc.     

Moment Tensor Solutions of Earthquakes in the Indian Ocean from Surface Waves and their Tectonic Implications

—Rayleigh and Love waves generated by sixteen earthquakes which occurred in the Indian Ocean and were recorded at 13 WWSSN stations of Asia, Africa and Australia are used to determine the moment tensor solution of these earthquakes. A combination of thrust and strike-slip faulting is obtained for earthquakes occurring in the Bay of Bengal. Thrust, strike slip or normal faulting (or either of the combination) is obtained for earthquakes occurring in the Arabian Sea and the Indian Ocean. The resultant compressive and tensional stress directions are estimated from more than 300 centroid moment tensor (CMT) solution of earthquakes occurring in different parts of the Indian Ocean. The resultant compressive stress directions are changing from north-south to east-west and the resultant tensional stress directions from east-west to north-south in different parts of the Indian Ocean. The results infer the counterclockwise movement of the region (0°–33°S and 64°E–94°E), stretching from the Rodriguez triple junction to the intense deformation zone of the central Indian Ocean and the formation of a new subduction zone (island arc) beneath the intense deformation zone of the central Indian Ocean and another at the southern part of the central Indian basin. The compressive stress direction is along the ridge axis and the extensional stress manifests across the ridge axis. The north-south to northeast-south west compression and east-west to northwest-southeast extension in the Indian Ocean suggest the northward underthrusting of the Indian plate beneath the Eurasian plate and the subduction beneath the Sunda arc region in the eastern part. The focal depth of earthquakes is estimated to be shallow, varying from 4 to 20 km and increasing gradually in the age of the oceanic lithosphere with the focal depth of earthquakes in the Indian Ocean.     

Oblique subduction of a Newtonian fluid slab

A Newtonian fluid model is proposed to describe the oblique subduction of a planar 2-D slab. The slab is assumed to subduct in response to the ridge push force exerted along the trench, the slab pull force at the downdip of the slab, the gravitational body force within the slab, and the frictional resistance force at the upper surface of the slab. Because the slab motion along strike is being resisted by the frictional resistance at the interplate coupling area while the slab motion along the trench normal is being maintained by the gravitational pulling, the slab turns gradually toward the trench normal direction as it subducts. This model offers an alternative explanation for earthquake slip partitioning, the observation that the earthquake slip vectors deflect away from the relative plate motion direction toward the trench normal direction along most of the oblique subduction zones worldwide. Numerical models suggest that slip partitioning caused by slab deformation could be as much as 30% at 100 km downdip of the slab. The slab viscosity, the plate coupling width, the interplate resistance coefficient, the slab pull force, and the gravitational body force are all important in determining the geometry of the slab subduction.     

Signatures of downgoing plate-buoyancy driven subduction in Cenozoic plate motions

The dynamics of plate tectonics are strongly related to those of subduction. To obtain a better understanding of the driving forces of subduction, we compare relations between Cenozoic subduction motions at major trenches with the trends expected for the simplest form of subduction. i.e., free subduction, driven solely by the buoyancy of the downgoing plate. In models with an Earth-like plate stiffness (corresponding to a plate–mantle viscosity contrast of 2–3 orders of magnitude), free plates subduct by a combination of downgoing plate motion and trench retreat, while the slab is draped and folded on top of the upper-lower mantle viscosity transition. In these models, the slabs sink according to their Stokes’ velocities. Observed downgoing-plate motion–plate-age trends are compatible with >80% of the Cenozoic slabs sinking according to their upper-mantle Stokes’ velocity, i.e., subducting-plate motion is largely driven by upper-mantle slab pull. Only in a few cases, do young plates move at velocities that require a higher driving force (possibly supplied by lower-mantle–slab induced flow). About 80% of the Cenozoic trenches retreat, with retreat accounting for about 10% of the total convergence. The few advancing trench sections are likely affected by regional factors. The low trench motions are likely encouraged by low asthenospheric drag (equivalent to that for effective asthenospheric viscosity 2–3 orders below the upper-mantle average), and low lithospheric strength (effective bending viscosity ～2 orders of magnitude above the upper-mantle average). Total Cenozoic trench motions are often very oblique to the direction of downgoing-plate motion (mean angle of 73°). This indicates that other forces than slab buoyancy exert the main control on upper-plate/trench motion. However, the component of trench retreat in the direction of downgoing plate motion (≈ slab pull) correlates with downgoing-plate motion, and this component of retreat generally does not exceed the amount expected for free buoyancy-driven subduction. High present-day slab dips (on average about 70°) are compatible with largely upper-mantle slab-pull driven subduction of relatively weak plates, where motion partitioning and slab geometry adjust to external constraints/forces on trench motion.     

Oblique Plate Convergence in the Indo-Burma (Myanmar) Subduction Region

— The Indo-Burma (Myanmar) subduction boundary is highly oblique to the direction of relative velocity of the Indian tectonic plate with respect to the Eurasian plate. The area includes features of active subduction zones such as a Wadati-Benioff zone of earthquakes, a magmatic arc, thrust and fold belts. It also has features of oblique subduction such as: an arc-parallel strike-slip fault (Sagaing Fault) that takes up a large fraction of the northward component of motion and a buttress (the Mishmi block) that resists the motion of the fore-arc sliver. In this paper, I have examined the seismicity, slip vectors and principal axes of the focal mechanisms of the earthquakes to look for features of active subduction zones and for evidence of slip partitioning as observed in other subduction zones. The data set consists of Harvard CMT solutions of 89 earthquakes (1977–1999 with 4.8≦̸Mw≦̸7.2 and depths between 3–140 km). Most of these events are shallow and intermediate depth events occurring within the Indian plate subducting eastward beneath the Indo-Burman ranges. Some shallow events within the fore-arc region have arc-parallel Paxes, reflecting buttressing of the fore-arc sliver at its leading edge. Some of the shallowest events have nearly E-W oriented P axes which might account for recent folding and thrusting. Examination of earthquake slip vectors in this region shows that the slip vector azimuths of earthquakes in the region between 20°–26°N are rotated towards the trench normal, which is an indication of partial partitioning of the oblique convergence. It is seen that all aspects of seismicity, including the paucity of shallow underthrusting earthquakes and the orientation of P axes, are consistent with oblique convergence. The conclusions of this paper are consistent with recent geological studies and interpretations such as the coexistence of eastward subduction, volcanic activity and transcurrent movement through mid-Miocene to Quaternary period.     

Study on the sources of some deep focus earthquakes in Northwest Pacific region

The sources for thirteen deep focus earthquakes ofm _b≥5.5 andh>400 km in Northwest Pacific region were studied using waveform fitting and shear fracture source model. The source parameters were obtained as follows: focal depth, faulting plane, slip direction, rupture velocity, rupture length, rupture direction and scalar moment tensor. It was found that all these earthquake sources can be interpreted as shear faulting and have simple source time functions. The strike direction of faulting plane for most deep focus earthquakes coincides with that of the subduction zone, especially in the deep part of the subduction zone, it results in the tendency of reducing the dip angle of the subduction zone. The multiple point source model was also used to study the source process. The waveform fitting is better than the shear fracture model, but the general rupture direction which coincides with that from unilateral shear source model can not be obtained from the multiple point source model. This study is supported by the National Science Foundation of China and the Chinese Joint Seismological Science Foundation.     

The Mid-Rivera-Transform Discordance: Morphology and Tectonic Development

To better define the morphotectonic elements and tectonic development of the Mid-Rivera-Transform Discordance, multibeam bathymetric, seafloor backscatter, multichannel seismic reflection and total field marine magnetic data were collected along the entire Rivera Transform west of 107°W during the BART and FAMEX campaigns of the N.O. L??Atalante conducted in 2002. These data show that, although the transform tectonized zone of the Rivera Transform west of 107°30??W is a single continuous morphologic basin, this basin consists of two distinct morphotectonic domains: an eastern domain which morphologically is a deep rhombochasm within which organized seafloor spreading has occurred, and a western ??leaky transform?? domain. These new data, in conjunction with the results of previous studies, support the idea that the Rivera-Pacific Euler pole is migrating southward towards the eastern half of the Rivera Transform, and further indicate a recent (<0.14?Ma), and most likely ongoing, clockwise reorganization of the principle transform displacement zones of the Rivera Transform west of 108°W. We propose that the Mid-Rivera-Transform Discordance owes its origin to this eastward progressing, clockwise reorganization of the transform segments that is occurring in response to recent changes in Rivera-Pacific relative plate motion.     

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

Fault plane solutions for earthquakes in the central Hellenic arc are analysed to determine the deformation and stress regimes in the Hellenic subduction zone in the vicinity of Crete. Fault mechanisms for earthquakes recorded by various networks or contained in global catalogues are collected. In addition, 34 fault plane solutions are determined for events recorded by our own local temporary network on central Crete in 2000–2001. The entire data set of 264 source mechanisms is examined for types of faulting and spatial clustering of mechanisms. Eight regions with significantly varying characteristic types of faulting are identified of which the upper (Aegean) plate includes four. Three regions contain interplate seismicity along the Hellenic arc from west to east and all events below are identified to occur within the subducting African lithosphere. We perform stress tensor inversion to each of the subsets in order to determine the stress field. Results indicate a uniform N-NNE direction of relative plate motion between the Ionian Sea and Rhodes resulting in orthogonal convergence in the western forearc and oblique (40–50^∘) subduction in the eastern forearc. There, the plate boundary migrates towards the SE resulting in left-lateral strike-slip faulting that extends to onshore Eastern Crete. N110^∘E trending normal faulting in the Aegean plate at this part is in accordance with this model. Along-arc extension is observed on Western Crete. Fault plane solutions for earthquakes within the dipping African lithosphere indicate that slab pull is the dominant force within the subduction process and responsible for the roll-back of the Hellenic subduction zone.     

Paleotectonic structures and their influence on recent seismo-tectonics in the south Kuril subduction zone

Abstract Bathymetric data from south of Hokkaido obtained during a cruise of R/V Hakuho-Maru are summarized, and their correlation with earthquake occurrence is discussed. There are structural lineations on the seaward slope of the Kuril Trench, oblique to the Kuril Trench axis and parallel to the magnetic lineations in the Pacific plate. The structural lineations comprise horst-grabens generated by normal faulting. This suggests that Cretaceous tectonic structures originating at the spreading centre affect present seismotectonics around the trench axis. The structural-magnetic relation is compared to the case of the Japan Trench. North-east of the surveyed area, there are two major fracture zones (Nosappu Fracture Zone and Iturup Fracture Zone) that divide the oceanic plate into three segments. If the fracture zones (FZ) and the zone of paleo-mechanical weakness, represented by magnetic lineations, can control the direction of normal faults at a trench, the extent of the resulting topographic roughness on the seaward slope of the trench would be different across an FZ because of the differences in ages. By studying recent large earthquakes occurring in the south Kuril region, it is shown that several main-aftershock distributions for large earthquakes in this region are bounded by the Nosappu FZ and the Iturup FZ. Two models (Barrier model and Rebound model) are presented to interpret earthquake occurrence near the south Kuril Islands. The Barrier model explains seismic boundaries seen in several examples for earthquake occurrence in the south Kuril regions. The fracture zone forming the boundary of two segments with different magnetic lineations is also the boundary of two different normal fault systems on their ocean bottom, and the difference in sea-bottom roughness between two normal fault systems should affect the seismic coupling at a plate interface. Due to the difference of seismic coupling, earthquake occurrence is controlled by an FZ and then the FZ acts as a seismic boundary (Barrier model). Existing normal faults created by plate bending of subducting oceanic plate should rebound after its subduction (Rebound model). This rebound of normal faults may cause intraplate earthquakes with a high-angle reverse-fault mechanism such as the 1994 Shikotan Earthquake. The energy released by an intraplate earthquake generated by normal-fault rebounding is not directly related to that of interplate earthquakes such as low-angle thrust earthquakes. It is a reason why large earthquakes occurred in the same region during a relatively short period.     

日本海沟俯冲带MW9.0地震震源区应力场演化分析

于2011年3月11日发生在日本东北部的M_W9.0级逆冲型板间地震是日本有地震记录以来震级最大的一次地震.本研究基于NIED F-net矩张量解目录中的震源机制解,选取两个长轴相互垂直的矩形区域进行应力场2D反演,获取了日本海沟俯冲带地区应力场的空间及时间分布图像.结果表明：主震前,俯冲带地区应力状态在空间上大体趋于一致,即应力轴（P轴、σ₁轴及S_Hmax轴）系统性地倾向板块汇聚方向,P轴、σ₁轴倾角整体偏缓（<30°）,且远离震源区及日本海沟东侧区域内的应力轴倾角普遍大于主震震源区内应力轴倾角;主震前,受2003年5月26日在宫城县北部发生的M_W7.0地震影响,位于M_W9.0地震震源区西北侧的应力场出现明显扰动,σ₁轴倾向顺时针偏转150°~180°,并于之后大体恢复至震前状态,同期其他地区没有明显变化,这种情况可能和主震断层局部（深部）的前兆性滑动有关;主震后,距离震源区较远处应力场变化不大,主震震源区内应力场发生显著改变,P轴及σ₁轴均以大角度（>60°）倾伏于板块汇聚方向,S_Hmax轴顺时针偏转60°~90°且在日本海沟附近普遍平行于海沟轴.这项研究以时空图像的方式展示了大地震前应力场变化的特点,反映了大地震孕震过程中构造与地震的相互作用,对于理解大地震孕震过程有重要意义.     

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.     

Erratum

The Philippine Sea at 5 m.y. B.P has been reconstructed by the following process. Firstly, it was rotated rigidly relative to the Eurasian plate around the pole of rotation at 45.5°N, 150. 2°E with a rotation angle of 6.0° for the past 5 m.y. Secondly, the evolution and deformation along the plate boundaries were incorporated in the rigid rotation. This reconstruction suggests: (1) the Izu Peninsula, which was originally a volcanic island of the Izu-Bonin Arc, collided with central Honshu in a west-northwest direction a few million years B.P.; (2) a TTT(a)-type triple junction east of Honshu has migrated west-northwestward relative to the Eurasian plate; and (3) the subduction zone of the Pacific plate, beneath the central part of the Mariana Arc, has remained fixed relative to Eurasia. Westward motion of the Philippine Sea plate and subduction beneath the eastern Eurasian margin resulted in the opening of the Marian Trough.     

Fault tectonic analysis of Kii peninsula,Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

The southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.     

福建街面水库现代构造应力场分析

福建街面水库将于2006年底下闸蓄水,通过收集库区的地震震源机制解资料和本次工作得到的地震震源机制解结果,以及利用中小地震求解的综合震源机制解,本文分析了街面水库库区现代构造应力场特征:最大主压应力P轴为NW—SE方向,P轴倾角为23°;主张应力T轴为NE—SW方向,T轴倾角为3°。在这样的应力场中,主要以发生走滑型断裂运动性质的地震为主,同时还具有发生正断层运动性质的地震。     

Faulting Processes of Historic (1917–1962) M = 6.0 Earthquakes Along the North-central Caribbean Margin

—The plate boundary along the north-central Caribbean margin is geologically complex. Our understanding of this complexity is hampered by the fact that plate motions are relatively slow (1 to 2 cm/yr), so that recent seismicity often does not provide a complete picture of tectonic deformation. Studies of the faulting processes of instrumentally recorded earthquakes occurring prior to 1962 thus provide important information regarding the nature and rate of seismic deformation within the region, and are essential for a comprehensive assessment of seismic hazard. We have conducted body waveform modeling studies of eight earthquakes which occurred along the north-central Caribbean plate margin, extending from southeastern Cuba to the Swan Island fracture zone (75 to 83°W). None of these earthquakes has been previously studied and several occurred in regions where no recent (post-1962) seismicity has been recorded. The plate margin in the western portion of our study area is characterized by a transform fault-spreading center system. In the central and eastern portions of our study area the plate margin is a complex, diffuse region of deformation that couples transform motion in the Cayman trough to subduction along the Lesser Antilles arc. Our results show that the western portion of the study area has only experienced large strike-slip earthquakes. Off southeastern Cuba two earthquakes appear to have occurred on high angle, northward dipping, reverse faults with south to southeastward directed slip vectors. An earthquake in northern Jamaica in 1957 shows pure strike-slip faulting, most likely along an east-west trending fault. Finally, an unusual sequence of events located in the Pedro Bank region ~70 km southwest of Jamaica has a mainshock with a reverse-oblique mechanism, suggesting continuity of the plate interface stress field well south of the northern Caribbean margin.     

Morphology and magnetic survey of the Rivera-Cocos plate boundary of Colima,Mexico

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of Rivera-Cocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during campaigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3°N) of the present day Rivera Transform at about Chron 2A₃ (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3°N and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3°N the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.     

Seismicity and tectonics of the far Western Aleutian Islands

Study of focal mechanisms of earthquakes in the Near and Komandorsky Islands indicate that there are several distinct zones of tectonic activity. South of the Near Islands, normal faulting occurs in the trench east of 172°E and low-angle thrusting dominates the Aleutian ridge. Mechanisms indicate underthrusting as far west as Mednyy Island with strike-slip faulting restricted to the south and west of Beringa Island. A zone of northeast striking left-lateral faulting near 1645.°E is proposed to separate the Aleutian Ridge from Kamchatka Peninsula. This motion, as well as faulting north of the Komandorsky Islands, may be related to the existance of a buffer plate comprising the Aleutian Ridge in the Komandorsky Islands. Active subduction terminates near 173°E and the faulting north of the Komandorsky Islands may, in part, be due to the bouyancy of a remnant slab. Depth phase modelling indicates bulletin-reported depths are overestimated due to a misidentification of depth phases.     

千岛—鄂霍次克海地区的地震分布、震源机制及应力状态

利用1971年1月至1982年12月的地震资料,研究了千岛岛弧地区的地震分布及震源机制解,进而讨论了贝尼奥夫带的形态及应力状态。地震分布于沿海沟展布的NE向的弧形带上,除地壳内地震外,形成明显的贝尼奥夫带,贝尼奥夫带最深达619公里,两侧较浅,少于200公里,倾向近于NW55°,倾角为45°。地壳内的压应力轴位于NW方向,且接近于水平,反映了太平洋板块的挤压;俯冲带上应力轴随深度变化:114公里以上的T轴沿俯冲方向,114公里至175公里震源机制解分为两组,T轴沿俯冲方向和P轴沿俯冲方向;320公里至440公里范围内P轴有接近俯冲方向的趋势,但较为分散;515公里以下P轴相当集中,且沿俯冲方向。本文对这种应力分布的成因进行了讨论