Seismic activity associated with the subducting motion of the Philippine Sea plate beneath the Kanto district, Japan

The Pacific plate and the Philippine Sea plate overlap and subduct underneath the Kanto region, central Japan, causing complex seismic activities in the upper mantle. In this research, we used a map selection tool with a graphic display to create a data set for earthquakes caused by the subducting motion of the Philippine Sea plate that are easily determined. As a result, we determined that there are at least four earthquake groups present in the upper mantle above the Pacific plate. Major seismic activity (Group 1) has been observed throughout the Kanto region and is considered to originate in the uppermost part of mantle in the subducted Philippine Sea plate, judging from the formation of the focal region and comparison with the 3D structure of seismic velocity. The focal mechanism of these earthquakes is characterized by the down-dip compression. A second earthquake layer characterized by down-dip extension (Group 2), below the earthquakes in this group, is also noted. The focal region for those earthquakes is considered to be located at the lower part of the slab mantle, and the Pacific plate located directly below is considered to influence the activity. Earthquakes located at the shallowest part (Group 3) form a few clusters distributed directly above the Group 1 focal region. Judging from the characteristics of later phases in these earthquakes and comparing against the 3D structure of seismic velocity, the focal regions for the earthquakes are considered to be located near the upper surface of the slab. Another earthquake group (Group 4) originates further below Group 2; it is difficult to consider these earthquakes within a single slab. The seismic activities representing the upper area of the Philippine Sea plate are Group 3. This paper proposes a slab geometry model that is substantially different from conventional models by strictly differentiating the groups.     

Seismic velocity discontinuities in the crust and uppermost mantle beneath the Kanto district, central Japan, identified from receiver function imaging and repeating earthquake activity

We constructed vertical cross-sections of depth-converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the Kanto district, central Japan. Repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting Philippine Sea plate and the subducting Pacific plate. As a result, we present images of some major seismic discontinuities. The upper boundary of the Pacific plate dips to the northwest in northern Kanto and to the west–southwest in southern Kanto with some undulations. On the other hand, the upper boundary of the Philippine Sea plate as a whole dips to the northwest. However, it is concave to the northeast in the southern Boso peninsula. We suggest that the low-velocity mantle wedge may be indicated on the top of both subducting plates. Plate thickness gradually decreases to the northeast. The northeastern end of the Philippine Sea plate is interpreted to be at depths of 45–90 km. The Moho discontinuity in the overriding plate is deeper than 25 km in the northern Kanto. It contacts the subducting Philippine Sea plate in the southwestern part near 35.8°N.     

Configuration of the subducting Philippine Sea slab in the eastern part of southwestern Japan with seismic array and Hi-net data

It is important to know the shape of a subducting slab in order to understand the mechanisms of inter-plate earthquakes and the process of subduction. Seismicity data and converted phases have been used to detect plate boundaries. The configuration of the Philippine Sea slab has been obtained at the western part of southwestern Japan. At the eastern part of southwestern Japan, however, the configuration of the Philippine Sea slab has not yet been confirmed. A spatially high-density seismic network makes it possible to detect the boundaries of the Philippine Sea slab. We used a spatially high-density temporal seismic array in the area. The configuration of the Philippine Sea plate is obtained at the eastern part of southwestern Japan using the temporal seismic array and permanent seismic network data and comparing the seismic structure obtained from the results of refraction surveys. The configuration of the Philippine Sea plate obtained by this study does not bend sharply compared to previous models obtained from receiver function analyses. We delineated the upper boundary of the slab to a depth of about 45 km. The weak image of the boundary, which corresponds to the upper mantle reflector beneath the source area of the 2000 Western Tottori earthquake, was detected using the spatially dense array.     

Configuration of subducting Philippine Sea plate and crustal structure in the central Japan region

A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3–5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0–6.4 and 6.6–6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20–35 km. The dip angle of the Philippine Sea plate was estimated to be 26° for a depth range of about 20–26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26–33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20–26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes.     

Tidal triggering of earthquakes in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, Japan

We found a characteristic space–time pattern of the tidal triggering effect on earthquake occurrence in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, central Japan, where a large interplate earthquake may be impending. We measured the correlation between the Earth tide and earthquake occurrence using microearthquakes that took place in the Philippine Sea plate for about two decades. For each event, we assigned the tidal phase angle at the origin time by theoretically calculating the tidal shear stress on the fault plane. Based on the distribution of the tidal phase angles, we statistically tested whether they concentrate near some particular angle or not by using Schuster's test. In this test, the result is evaluated by p-value, which represents the significance level to reject the null hypothesis that earthquakes occur randomly irrespective of the tidal phase angle. As a result of analysis, no correlation was found for the data set including all the earthquakes. However, we found a systematic pattern in the temporal variation of the tidal effect; the p-value significantly decreased preceding the occurrence of M ≥ 4.5 earthquakes, and it recovered a high level afterwards. We note that those M ≥ 4.5 earthquakes were considerably larger than the normal background seismicity in the study area. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak at the phase angle where the tidal shear stress is at its maximum to accelerate the fault slip. This indicates that the observed small p-value is a physical consequence of the tidal effect. We also found a distinctive feature in the spatial distribution of p-values. The small p-values appeared just beneath the strongly coupled portion of the plate interface, as inferred from the seismicity rate change in the past few years.     

Is northern Honshu a microplate?

Seismic slip vectors along the Japan Trench, the eastern margin of the Japan Sea and the Sagami Trough are compared with global relative plate motions (RM2, Minster and Jordan, 1978) to test a new hypothesis that northern Honshu, Japan, is part of the North American plate. This hypothesis also claims that the eastern margin of the Japan Sea is a nascent convergent plate boundary (Kobayashi, 1983; Nakamura, 1983).Seismic slip vectors along the Japan Trench are more parallel to the direction of the Pacific-North American relative motion than that of the Pacific-Eurasian relative motion. However, the difference in calculated relative motions is too small avoid to the possibility that a systematic bias in seismic slip vectors due to anomalous velocity structure beneath island arcs causes this apparent coincidence. Seismic slip vectors and rates of shortening along the eastern margin of the Japan Sea for the past 400 years are also consistent with the relative motion between the North American and Eurasian plates calculated there. Seismic slip vectors and horizontal crustal strain patterns revealed by geodetic surveys in south Kanto, beneath which the Philippine Sea plate is subducting, indicate two major directions; one is the relative motion between the North American and Philippine Sea plates, and the other that between the Eurasian and Philippine Sea plates.One possible interpretation of this is that the eastern margin of the Japan Sea may be in an embryonic stage of plate convergence and the jump of the North American-Eurasian plate boundary from Sakhalin-central Hokkaido to the eastern margin of the Japan Sea has not yet been accomplished. In this case northern Honshu is a microplate which does not have a driving force itself and its motion is affected by the surrounding major plates, behaving as part of either the Eurasian or North American plate. Another possibility is that the seismic slip vectors and crustal deformations in south Kanto do not correctly represent the relative motion between plates but represent the stresses due to non-rigid behaviors of part of northern Honshu.     

Plate subduction,and generation of earthquakes and magmas in Japan as inferred from seismic observations: An overview

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.     

Complex slab structure and arc magmatism beneath the Japanese Islands

A dense nationwide seismic network recently constructed in Japan has resulted in the production of a large amount of high-quality data that have enabled the high-resolution imaging of deep seismic structures in the Japanese subduction zone. Seismic tomography, precise locations of earthquakes, and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath Japan, revealing that the subducting Philippine Sea slab underneath southwestern Japan has an undulatory configuration down to a depth of 60–200 km, and is continuous from Kanto to Kyushu without disruption or splitting, even within areas north of the Izu Peninsula. Analysis of the geometry of the Pacific and Philippine Sea slabs identified a broad contact zone beneath the Kanto Plain that causes anomalously deep interplate and intraslab earthquake activity. Seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the Philippine Sea slab beneath the Japan Sea north of Chugoku and Kyushu, and beneath the East China Sea west of Kyushu down to a depth of ∼450 km. Seismic tomography also allowed the identification of an inclined sheet-like seismic low-velocity zone in the mantle wedge beneath Tohoku. A recent seismic tomography work further revealed clear images of similar inclined low-velocity zones in the mantle wedge for almost all other areas of Japan. The presence of the inclined low-velocity zones in the mantle wedge across the entirety of Japan suggests that it is a common feature to all subduction zones. These low-velocity zones may correspond to the upwelling flow portion of subduction-induced convection systems. These upwelling flows reach the Moho directly beneath active volcanic areas, suggesting a link between volcanism and upwelling.     

Aseismic deep subduction of the Philippine Sea plate and slab window

We have made great efforts to collect and combine a large number of high-quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both South Korea and West Japan. This is the first time that a large number of Korean and Japanese seismic data sets are analyzed jointly. As a result, a high-resolution 3-D P-wave velocity model down to 700-km depth is determined, which clearly shows that the Philippine Sea (PHS) plate has subducted aseismically down to ∼460 km depth under the Japan Sea, Tsushima Strait and East China Sea. The aseismic PHS slab is visible in two areas: one is under the Japan Sea off western Honshu, and the other is under East China Sea off western Kyushu. However, the aseismic PHS slab is not visible between the two areas, where a slab window has formed. The slab window is located beneath the center of the present study region where many teleseismic rays crisscross. Detailed synthetic tests were conducted, which indicate that both the aseismic PHS slab and the slab window are robust features. Using the teleseismic data recorded by the Japanese stations alone, the aseismic PHS slab and the slab window were also revealed (Zhao et al., 2012), though the ray paths in the Japanese data set crisscross less well offshore. The slab window may be caused by the subducted Kyushu-Palau Ridge and Kinan Seamount Chain where the PHS slab may be segmented. Hot mantle upwelling is revealed in the big mantle wedge above the Pacific slab under the present study region, which may have facilitated the formation of the PHS slab window. These novel findings may shed new light on the subduction history of the PHS plate and the dynamic evolution of the Japan subduction zone.     

Migration of crustal deformation

Observations on the migration rates of crustal deformation, as recently discovered in several tectonic areas, such as the south Kanto and central Tohoku districts, Japan and the West Cordillera Mts., Peru, has opened up a new opportunity for the study of crustal dynamics. Briefly, these examples from coastal areas are characterized by migration landwards with a velocity of about 10–100 km/yr. This agrees well with the velocity of mIgration of seismicity as previously known. Dispersion and dissipation of the deformation waveform are also noted as characteristics.Simple extrapolation of the migration path back toward the ocean may locate a possible origin of the event. In the case of the south Kanto district, for example, the deformation front seems to have originated in the early 1950s from the vicinity of the junction of the Japan and Izu—Mariana trenches. The deformation front in the central Tohoku district, on the other hand, is thought to have originated in the northern part of the Japan Trench in the late 1960s. One may suppose that either a repeated irregular aseismic plate motion generates the deformation events, or that it results from a periodic seismic slip at a plate boundary. In the latter case, the 1953 Boso-oki and the 1968 Tokachi-oki earthquakes might be suspected of generating the deformation fronts in the south Kanto and central Tohoku districts respectively.As Scholz speculated, the migration of a deformation front might trigger earthquakes, if it hits areas of high seismic potential. Studies of migration events can contribute significantly to earthquake prediction studies.     

Inhomogeneous distribution of deep slow earthquake activity along the strike of the subducting Philippine Sea Plate

The spatial distribution of deep slow earthquake activity along the strike of the subducting Philippine Sea Plate in southwest Japan is investigated. These events usually occur simultaneously between the megathrust seismogenic zone and the deeper free-slip zone on the plate interface at depths of about 30 km. Deep low-frequency tremors are weak prolonged vibrations with dominant frequencies of 1.5–5 Hz, whereas low-frequency earthquakes correspond to isolated pulses included within the tremors. Deep very-low-frequency earthquakes have long-period (20 s) seismic signals, and short-term slow-slip events are crustal deformations lasting for several days. Slow earthquake activity is not spatially homogeneous but is separated into segments some of which are bounded by gaps in activity. The spatial distribution of each phase of slow earthquake activity is usually coincident, although there are some inconsistencies. Very-low-frequency earthquakes occur mainly at edges of segments. Low-frequency earthquakes corresponding to tremors of relatively large amplitude are concentrated at spots where tremors are densely distributed within segments. The separation of segments by gaps suggests large differences in stick-slip and stable sliding caused by frictional properties of the plate interface. Within each segment, variations in the spatial distribution of slow earthquakes reflected inhomogeneities corresponding to the characteristic scales of events.     

Source mechanisms and tectonic significance of historical earthquakes along the nankai trough,Japan

Two recent and three historical earthquakes which occurred along the Nankai trough, marking the northern plate boundary between the Philippine Sea and the Asian Plate, are studied mainly on the basis of the data of crustal deformations and tsunami waves. These earthquakes are the 1946 Nankaido, the 1944 Tonankai, the 1854 Ansei I, II and the 1707 Hoei earthquakes. They are all interpreted as low-angle thrust faults at the plate boundary, with the oceanic side underthrusting northwestward against southwestern Japan. The fault parameters of the historical earthquakes are assumed here to be common to those of the recent two earthquakes, except for the magnitude of dislocation.The entire fault region, which extends for 530 km from western Shikoku Island in the west to the Tokai district in the east, is divided into four fault planes, which are denoted the planes A, B, C and D, from west to east, respectively. Then, the five earthquakes may be attributed to the planes A, B, C and D, in the following manner: the Nankaido earthquake, A + B; the Tonankai earthquake, C; the Ansei II earthquake, A + B; the Ansei I earthquake, C + D; and the Hoei earthquake, A + B + C + D.The latest cycle of earthquake migration seems incomplete as proved by the recent inactivity in D. Consequently, the future major earthquake next to occur is expected there, off the Tokai district. Eight further ancient earthquakes from A.D. 684 to 1605 are also discussed. Taking the results of the foregoing studies into consideration, their sequence is well interpreted by the four migration cycles. Topographical data, tilt of coastal terraces and location of hinge lines, prove that the thrusting has continued all along the extension of the Nankai trough for at least 300,000 years.     

Intra-plate seismicity and inter-plate earthquakes: Historical activity in Southwest Japan

Over the last four hundred years the spatial variation of intra-plate seismicity in Southwest Japan correlates well with the occurrence of great inter-plate earthquakes. For fifty years before an inter-plate earthquake the intra-plate seismic activity is highest along a belt inland. For ten years afterwards it falls off in this belt, but rises on both sides along the Philippine Sea and Japan Sea coasts. Then it becomes low and remains low throughout the whole region until fifty to thirty years before the next inter-plate event, as shown by Utsu in 1974. An intermittent underthrusting drag exerted by the Philippine Sea plate seems to control the intra-plate seismicity, which partly takes up the relative plate motion as internal deformation. When a great inter-plate earthquake occurs, tectonic stress is released and seismic activity falls off in the central belt. The breaking of the plate boundary temporarily weakens the coupling between the two plates along the shallower part of the interface, which gently dips toward the Japan Sea coast. The decoupling causes stress concentration in the deeper part and results in increased seismic activity along the Japan Sea coast. The activity along the Philippine Sea coast may be interpreted as aftershock activity.     

中国岩石圈应力场与构造运动区域特征

笔者系统分析了1918—2005年间中国大陆及其周缘发生的3130个中、强地震的震源机制解,根据其特征进行了岩石圈应力场构造分区,首次得到区域应力场的压应力轴和张应力轴空间分布的统计数字结果。在此基础上研究了应力场的区域特征、探讨了其动力学来源以及构造运动特征。总体结果表明,中国大陆及其周缘岩石圈应力场和构造运动可以归结为印度洋板块、太平洋板块、菲律宾海板块与欧亚板块之间相对运动,以及大陆板内区域块体之间的相互作用的结果。印度洋板块向欧亚板块的碰撞挤压运动所产生的强烈的挤压应力,控制了喜马拉雅、青藏高原、中国西部乃至延伸到天山及其以北的广大地区。在青藏高原周缘地区和中国西部的大范围内,压应力P轴水平分量方位位于20～40°,形成了近NE方向的挤压应力场。大量逆断层型强震集中发生在青藏高原的南、北和西部周缘地区,以及天山等地区。而多数正断层型地震集中发生在青藏高原中部高海拔的地区,断层位错的水平分量位于近东西方向。表明青藏高原周缘区域发生南北向强烈挤压短缩的同时,中部高海拔地区存在着明显的近东西向的扩张运动。中国东部的华北地区受到太平洋板块向欧亚板块俯冲挤压的同时,又受到从贝加尔湖经过大华北直到琉球海沟的广阔地域里存在着的统一的、方位为170°的引张应力场的控制。华北地区大地震的震源机制解均反映出该区地震的发生大体为NEE向挤压应力和NNW向张应力的共同作用结果。台湾纵谷断层是菲律宾海板块与欧亚板块之间碰撞挤压边界。来自北西向运动的菲律宾海板块构造应力控制了从台湾纵谷、华南块体,直到中国南北地震带南段东部地域的应力场。地震的震源机制结果还表明,将中国大陆分成东、西两部分的中国南北地震带是印度洋板块、菲律宾海板块与太平洋板块在中国大陆内部影响控制范围的分界线。     

Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography

We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-V_P/V_S zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-V_P/V_S zone; however, V_P/V_S at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-V_P/V_S zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-V_P/V_S zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity.     

Inhomogeneous distribution of deep slow earthquake activity along the strike of the subducting Philippine Sea Plate

The spatial distribution of deep slow earthquake activity along the strike of the subducting Philippine Sea Plate in southwest Japan is investigated. These events usually occur simultaneously between the megathrust seismogenic zone and the deeper free-slip zone on the plate interface at depths of about 30 km. Deep low-frequency tremors are weak prolonged vibrations with dominant frequencies of 1.5–5 Hz, whereas low-frequency earthquakes correspond to isolated pulses included within the tremors. Deep very-low-frequency earthquakes have long-period (20 s) seismic signals, and short-term slow-slip events are crustal deformations lasting for several days. Slow earthquake activity is not spatially homogeneous but is separated into segments some of which are bounded by gaps in activity. The spatial distribution of each phase of slow earthquake activity is usually coincident, although there are some inconsistencies. Very-low-frequency earthquakes occur mainly at edges of segments. Low-frequency earthquakes corresponding to tremors of relatively large amplitude are concentrated at spots where tremors are densely distributed within segments. The separation of segments by gaps suggests large differences in stick-slip and stable sliding caused by frictional properties of the plate interface. Within each segment, variations in the spatial distribution of slow earthquakes reflected inhomogeneities corresponding to the characteristic scales of events.     

Kinematics of Convergence and Deformation in Luzon Island and Adjacent Sea Areas:2-D Finite-Element Simulation

The Luzon Island is a volcanic arc sandwiched by the eastward subducting South China Sea and the northwestward subducting Philippine Sea plate.Through experiments of plane-stress,elastic,and 2-dimensional finite-element modeling,we evaluated the relationship between plate kinematics and present-day deformation of Luzon Island and adjacent sea areas.The concept of coupling rate was applied to define the boundary velocities along the subduction zones.The distribution of velocity fields calculated in our models was compared with the velocity field revealed by recent geodetic (GPS) observations.The best model was obtained that accounts for the observed velocity field within the limits of acceptable mechanical parameters and reasonable boundary conditions.Sensitivity of the selection of parameters and boundary conditions were evaluated.The model is sensitive to the direction of convergence between the South China Sea and the Philippine Sea plates,and to different coupling rates in the Manila trench,Philippine trench and eastern Luzon trough.We suggest that a change of±15° of the di rection of motion of the Philippine Sea plate can induce important changes in the distribution of the computed displacement trajectories,and the movement of the Philippine Sea plate toward azimuth330° best explains the velocity pattern observed in Luzon Island.In addition,through sensitivity analysis we conclude that the coupling rate in the Manila trench is much smaller compared with the rates in the eastern Luzon trough and the Philippine trench.This indicates that a significant part of momentum of the Philippine Sea plate motion has been absorbed by the Manila trench;whereas,a part of the momentum has been transmitted into Luzon Island through the eastern Luzon trough and the Philippine trench.     

发展中的板块边界:天山-贝加尔活动构造带

亚洲内陆的强地震密集地发生在天山-贝加尔一线,但该处并不存在一条连续的大断裂,学术界对这个问题的认识长期相左。文中分析了这条地震带的时空分布、分区特点、应力状态和活动周期,计算了欧亚大陆的布格重力异常场、均衡重力异常场,反演了上地幔的密度分布和剪切波速分布。发现在这个部位的70~250km的深部有一条北东向的密度、速度陡变带,它是新生代的冷地幔和热地幔的交界带,与浅部构造存在立交关系,对亚洲大陆的现今构造运动和应力场具有重要的控制作用。这个带的地震不同于传统意义上的板缘地震和板内地震,是一种因为深浅构造不同而造成的结构性地震,性质上为大陆内缘地震。文中还就深浅构造的空间立交关系、时间镜像关系进行了讨论,指出在南北地震带和伊朗东侧地震带的立交结构也与上地幔构造有关。天山-贝加尔活动构造带是正在发展中的板块边界,是大陆内部的一个典型构造,北侧为稳定的俄罗斯-西伯利亚次板块,南侧为活动的中国-东南亚次板块。     

中国东部大陆边缘晚新生代构造演化及板块相互作用过程重建

晚新生代中国东部大陆边缘的构造活动主要集中于东海东缘。中新世以来菲律宾海板块俯冲、冲绳海槽弧后张裂、台湾弧-陆碰撞等一系列重大构造过程,塑造了现今琉球沟-弧-盆体系、台湾碰撞造山带和南海东北部的构造-地貌格局。本文基于对重磁和多道地震资料的解译,并结合前人研究成果,恢复了冲绳海槽构造演化史,阐明了冲绳海槽弧后张裂和台湾弧-陆碰撞之间的关系。在此基础上,重建了中新世以来欧亚板块、菲律宾海板块、南海板块之间的相互作用过程模型。本研究有助于进一步理解板块汇聚背景下东亚大陆边缘深部动力-热力过程对浅部构造格局变迁的制约和影响。     

Seismic coupling and uncoupling at subduction zones

Seismic coupling has been used as a qualitative measure of the “interaction” between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane (asperities are strong regions that resist the motion between the two plates). The zones with the largest earthquakes have very large asperities, while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. While the variation in asperity size is used to establish a connection between earthquake size and tectonic stress, it also implies that plate age and rate affect the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the “preferred trajectory” (i.e. the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity). Indirect influences are many, including oceanic plate topography and the amount of subducted sediments.All subduction zones are apparently uncoupled below a depth of about 40 km, and we propose that the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change should start at a depth of 30–35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change.