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The relatively simple subducting slab geometry in the Izu-Bonin region provides a valuable opportunity to study the multiplicity of the 660-km discontinuity and the related response of the subducting slab on the discontinuity. Vertical short-period recordings of deep events with simple direct P phases beneath the Izu-Bonin region were retrieved from two seismic networks in the western USA and were used to study the structure of the 660-km discontinuity. After careful selection and pre-processing, 23 events from the networks, forming 32 pairs of event-network records, were processed. Related vespagrams were produced using the N-th root slant stack method for detecting weak down-going SdP phases that were inverted to the related conversion points. From depth histograms and the spatial distribution of the conversion points, there were three clear interfaces at depths of 670, 710 and 730 km. These interfaces were depressed approximately 20–30 km in the northern region. In the southern region, only two layers were identified in the depth histograms, and no obvious layered structure could be observed from the distribution of the conversion points.

The studies on the structure and physical properties of the mantle by Chinese geophysicists from 2003 to 2007 are reviewed in this report. It mainly contains studies on the seismic velocity structure of the mantle, anisotropy of the mantle, mantle discontinuities, mantle convection and the physical properties of mantle. The review concerns mainly the contents, the methods used and the results of the studies. It can be found that new progress in the study on the structure and physical properties of mantle has been made in the last four years in China. In some preexistent areas much progress has been made, advanced methods have been adopted, extensive international co-operation has been conducted in many ways, and the scope of the co-operation has gradually expanded. Moreover, some new fields appear as well.

An empirical formula for the fracture strength of the principal rock type in the lithosphere is obtained based on the experimental data from previous studies, in which the effects of the confining pressure, size of the rock sample, temperature, strain rate and the pore pressure are taken into account, the empirical formulae for the effects of them are also presented. By comparing the frictional strength to the fracture strength calculated using the new empirical formula, it is shown that frictional sliding is dominant in the upper crust but brittle fracture is dominant in the lower part of the crust and the lithosphere beneath the crust. Therefore the fracture mechanism must be taken into account in the study of the rheological structure of the lithosphere. The empirical formula for the fracture strength is applied to study the rheological structure of the lithosphere in the Ordos block. Brittle regime in the rheological structure can be divided into two sub-regions, in one of which brittle fracture and in the other frictional sliding are dominant, respectively, unlike previous conventional studies in which frictional sliding is assumed to be the only factor; the magnitude of the rheological strength of the lithosphere calculated by the empirical formula is also lower than that obtained in previous conventional studies.

Effects of temperature and strain rate on fracture strength of rocks are studied based on available experiment data, and a new empirical formula for the fracture strength of some typical rocks in the lithosphere is obtained. The effects of the confining pressure, size of rock samples, temperature and strain rate are taken into account in this formula so that it can further reveal the actual fracture states in the lithosphere. Application of the new empirical formula to the Ordos block shows that the conventional rheological model overestimates the rheological strength and misinterpret rheological deformation mechanisms in the lithosphere. The brittle regime in the rheological structure calculated from the new formula can be divided into two sub-regions in which frictional sliding or brittle fracture is dominant, respectively. The magnitude of the rheological strength is decreased and the area of the brittle regime is enlarged when more representative rocks and effect of the strain rate are taken into account.

We review the existing three constraints on the rheological structure of the lithosphere, namely, constant strain rate, constant tectonic force and the strain-rate-dependent force. We think that they can not describe the realistic distribution of the strain rate in the lithosphere. In this paper a new constraint on the rheological structure of the lithosphere using strain rates obtained from GPS data is presented. The calculation for the rheological structure of the lithosphere in North China shows that this constraint can overcome the difficulties faced by the three constraints mentioned above, and more reliable rheological structure of the lithosphere can be obtained under this new constraint. With this structure the relationship between the geotectonics and rheology can be revealed better, and influences of other state or material parameters on the rheological structure can also be reflected to some extent.

The SdP, pdP and sdP phases are picked up with the Nth root slant stack method from the digital waveform data recorded by the networks and arrays in USA, Germany and Switzerland for the earthquakes occurring beneath Izu-Bonin and Japan Sea. The mantle discontinuities and the effects of subducting slab on the 660 km and 410 km discontinuities are studied. It is found that there are mantle discontinuities existing at the depths of 170, 220, 300, 410, 660, 850 and 1150 km. Beneath Izu-Bonin, the 410 km discontinuity is elevated, while the 660 km discontinuity is depressed; for both discontinuities, there are regionalized differences. Beneath Japan Sea, however, there is no depth variation of the 410 km discontinuity, and the 660 km discontinuity is depressed without obvious effect of the subducting slab.

Summary We determine Euler vectors for 12 plates, including the Philippine Sea plate (PH), relative to the fixed Pacific plate (PA) by inverting the earthquake slip vectors along the boundaries of the Philippine Sea plate, GPS observed velocities, and 1122 data from the NUVEL-1 and the NUVEL-1A global plate motion model, respectively. This analysis thus also yields Euler vectors for the Philippine Sea plate relative to adjacent plates. Our results are consistent with observed data and can satisfy the geological and geophysical constraints along the Caroline (CR)–PH and PA–CR boundaries. The results also give insight into internal deformation of the Philippine Sea plate. The area enclosed by the Ryukyu Trench–Nankai Trough, Izu–Bonin Trench and GPS stations S102, S063 and Okino Torishima moves uniformly as a rigid plate, but the areas near the Philippine Trench, Mariana Trough and Yap–Palau Trench have obvious deformation.

The method of N-th root slant stack for studying of mantle discontinuities was discussed. The characteristics of the N-th root slant stack method were shown by stacking synthetic seismograms with different values of N. It was shown that the N-th root slant stack method could increase the SNR (signal-to-noise ratio), make the signal clearer, but distort the shape of the signal. The selection of N-ths depends on the SNR, noise distribution of data and the number of stations. The effects of inclination of the discontinuity on the differences of the slowness and travel time between converted phases and direct P phase were studied. It was shown that the correcting these effects is necessary for picking out the signals and calculating the conversion depths of secondary seismic phases.

The present boundaries between the Philippine Sea (PH) plate and its surrounding plates are determined based on the evolution history of the Philippine Sea and the earthquake distribution around the Philippine Sea. The deformation inside the Philippine Sea plate and along the boundary of PH-Eurasia (EU) plate is studied by comparing the observisional data of geophysics, geology, and GPS with the theoretically calculated results for the Euler parameters of the Philippine Sea plate. The interaction between the Philippine Sea plate and the Eurasian plate is analyzed using the geophysical , geological and GPS data. It is suggested that the interaction is different in different sections of the PH-EU boundary. The PH plate produces strong compression to the Eurasian plate along the Nankai trough. There is no compression along the Ryukyu trench and probably there is an extension caused by the secondary mantle flow. The collision of two plates at the Taiwan area causes strong compression to southeast part of Chinese continent. There is a deformation belt in the Philippine islands between the Philippine Sea plate and the Eurasian plate, which makes interaction between the two plates be weakened and there is no influence on the southern China Sea from the Philippine Sea plate.

The thermal structures of lithosphere in North China(105°E – 124°E, 30°N – 41°N) were calculated using two kinds of average heat flow on 1° × 1° Latitude-Longitude grid combining with four heat production models which were obtained from observed heat production and from the inversion of Curie depth distribution. By comparison and analysing the thermal structures obtained from different models, the most reasonable model was obtained. The temperature at Moho-discontinuity is about from 450°C to 750°C. The thickness of thermal lithophere is from 60km to 180km. It is found that big fault systems are related to thiner part of the lithosphere and the earthquakes of MS >6.5 occurred near the area of thiner part of the lithosphere and the area in which the temperature of the Moho-discontinuity is higher.

We calculate thermal and phase structures of subducting slabs for different subducting velocities by a modified coupling code of the kinetic phase-transformation equations and the heat-diffusion equation with latent-heat release. Whereafter, we estimate their rheology structures based on the thermal and phase structures from the mineral physical point of view. At shallow depth, the upper layer has a high effective viscosity greater than 10 34 Pa·s; while the lower layer has a relatively low effective viscosity, which is greater than 10 26 Pa·s nevertheless. The effective viscosities below the kinetic phase boundary of olivine to wadsleyite decrease obviously, and reach a minimum of 10 22 Pa·s. Small areas with higher effective viscosities exist above the depth of about 700 km in subducting slabs, which are produced by lower temperatures that are related with endothermic phase transformation of spinel to perovskite and magnesiowustite. The 1% and 99% isograds of spinel proportion delineate tortuous belts with low effective viscosities, which would affect the geodynamic behavior of subducting slabs.

Digital broadband seismograms recorded from 1988 to 2000 at the stations of MDJ and HIA were used to study the mantle discontinuities and the influence of subducting slab under the Japan Sea on the 660km discontinuity beneath the two stations by using P-SV converted phases. In order to increase the reliability of results, the rectilinearitypolarization filter analysis and modified N-th root slant stack methods were used to pick out P-SV converted phases. Appropriate smoothing and constraint of cubical angle were applied to the filtering; the modification of focal depths and correction for the inclination of discontinuities were used in the stacking. It was found that there are mantle discontinuities at the depths of 660, 410, 220 and 520km under the stations of MDJ and HIA. Besides that, the radial structures of the mantle are complicated and there are still some relatively strong discontinuities near the depths of 140, 350, 570, 740 and 1080km. It is shown that the structure between 660km and 840km under MDJ station is more complicated than that under HIA station. It may show the influence of subducting slab on the 660km discontinuity beneath MDJ station.

We calculated the negative buoyancy of subducting lithospheres based on numerical simulations of the temperature fields by means of 2-D FEM. The values of the negative buoyancy increase with depth in depths of h 740 km. On the other hand, there is a decrease of negative buoyancy in depths of 660 to 740 km, because there is phase transformation from ringwoodite to perovskite, which prevents slab penetrating into the lower mantle. Metastable olivine changes the distribution of negative buoyancy between depths of 400 to 660 km. Especially, it drastically altered the increasing trend of negative buoyancy with depth, leading to a net decrease of negative buoyancy below the 400 km depth. The existence of metastable olivine reduces the increased value of negative buoyancy with depth, which would make the penetration through 660 km discontinuity more difficult. Moreover, it would greatly change the pattern of stress distribution in and around the area of metastable olivine.

Results of numerical simulation show that positive velocity anomalies exist in most parts of subduction zones. These values reach their maxima at approximately 400 and 550 km depths, which are consistent with the wave velocity structures determined by seismic tomography. Though resolution is low, seismic tomography shows the basic features of P-wave velocity variations in subduction zones. Data from regional stations indicate that P-wave velocity anomalies can be positive or negative in the range from -10% to 9%. Low velocity anomalies exist at the depth of about 700 km, and metastable olivine leads to formation of low velocity regions that decline along the dip directions of subduction zones. Low resolution of seismic tomography is an obstacle to show detailed P-wave velocity structure of subduction zones. To investigate fine structures of subduction zones, we should use high-resolution data obtained from regional networks.

Following a quasi-dynamic scheme proposed by Minear and Toksoz (1970), thermal structures of subduction zones for different models by finite element method (FEM) were calculated. Density distribution and P-wave anomaly of subduction zones were calculated at the same time. Comparing with seismological evidences and results of laboratories, it is proposed that earthquakes occurred below 400 km depth are probably controlled by anti-crack mechanism.

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.

The distribution of earthquakes from 1973 to 1982 in the South China Sea, Indochina and their vicinity was studied using the data from I. S. C. It was found that the earthquakes are mainly concentrated along the boundaries of plates. Beside, some of shallow eartqhuakes are distributed in west part of Burma and the boundary between Burma and China, a few of earthquakes occurred in South China Sea. The features of Benioff zone along the boundaries between India plate, Philippine Sea plate and Eurasia plate were studied. The plate do not coupled well under the Java trench and the Philippine trench. The subducted India plate under Burmese range, Andaman—Nicobar arc moves NNE. The fault plane solutions of earthquakes were studied using the first motions of P wave. The stress state on subduction zones and within the area were deduced from the fault plane solutions and the fault movement. It was found that the direction of principal compression axis of stress is in the NNE in west part of Burma, in S—N in south and middle part of Bruma and Thailand, and in NNE or S—N in the South China Sea. It was also found that the stress state has close relation with the interaction of plates.

The source parameters of the Bohai Sea earthquake, July 18, 1969 and Yongshan, Yunnan earthquake, May 11, 1974 were determined by full — wave theory synthetic seismograms of teleseismic P waves. P+pP+sP wereform were calculated with WKBJ approximation and real integral paths. One — dimensional unilateral, finite propagation source was also considered. By trail — and — error in comparing the theoretical seismograms with the observational ones of WWSSN stations, the source parameters were obtained as follow: for Bohai earthquake, φ=195°, δ=85°, λ=65°,Mo=0.9×1019Nm,L=59.9km.VR=3.5km/s, ∧R=160°; for Yongshan earthquake, φ=240°, δ=80°, ∧=150°,Mo=1.3×1018Nm,L=48.8km,VR=3km/s, ∧R=−10°, where φ is strike, δ dip angle, λ slip angle,Mo seismic moment,L rupture length,VR rupture propagation speed. As III type fractures the faulting propagated along the fault planes, and ∧R is the angle from the strike to the propagation direction. Yongshan earthquake showed complexity in its focal process, having four sub—ruptures during the first 60 seconds.

[1] We study the discontinuities beneath Tonga subduction zone using the deep earthquakes recorded by the Pacific Northwest Seismic Network (PNSN). The multiple discontinuities around 660 km beneath Tonga subduction zone are found using converted phase SdP. The amplitude ratios of the converted phase with the direct P wave (Ac/Ap) are also determined. The largest number of robust converted phases with large Ac/Ap comes from the depth range of 660–690 km, its peak is at 680 km; the second one appears in the depth range of 700–745 km, its peak is at 740 km. The two discontinuities are obviously depressed by the subduction slab. According to experimental and numerical studies, a possible explanation of the observation is that the first discontinuity is formed by the transition of γ-spinel to perovskite and magnesiowustite and the second is formed by the transition of ilmenite to perovskite.

[1] A modified coupling scheme is proposed to calculate the thermal structures and the kinetics of olivine to wadsleyite phase transformation in subduction zones, which assures a high computational precision. The phase and thermal structures of some subduction zones in the western Pacific region are calculated. Computational results show that most subduction zones don't have large wedges of metastable olivine, which lets the mechanisms of transformational faulting have difficulty explaining deep seismicity.

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