Velocity properties of the lithosphere in the ocean-continent transition zone in the Kamchatka region from seismic tomography data

Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.     

The Campania-Lucania (southern Italy) earthquake of 23 November 1980

We present the main seismological results of our study of the Campania-Lucania earthquake of 23 November 1980. A complete set of far field and local data has been analysed. From long-period body waves data we determine the fault plane solution (φ₁ = 140°,δ₁ = 60°,φ₂ = 75°,δ₂ = 54°), a depth of 15 km and calculate a seismic moment of 6 × 10²⁵ dyne cm and a source duration of 6 s. From data of a local network deployed immediately after the event we determine aftershock locations: they are aligned in a direction NW-SE that fit extremely well with the focal solution determined above. We can choose as fault plane the plane striking 140° and dipping at 60° and the event is a normal event with a large component of left-lateral strike slip. The source area evaluated from this aftershock distribution 14 km × 40 km is quite suitable for an earthquake of a seismic moment of 6 × 10²⁵ dyne cm.     

利用sPn震相测定芦山MS7.0级地震余震的震源深度

利用南北地震带南段密集流动地震台阵的观测数据,采用波形互相关方法拾取Pn波走时,应用滑动时窗相关法识别sPn震相,通过sPn与Pn震相之间的走时差测定了芦山地震序列中28个M_L4.0级以上余震的震源深度.结果表明,震源深度集中在10~20 km范围内,垂直余震带的北西-南东向震源深度剖面揭示,余震分布表现出西深东浅的特点,倾角大约为39°.这些余震在空间上具有较好的线性分布特征,推测可能发生在与主震有关的破裂面上或邻近位置,由此推测主震的破裂面倾角大约为39°.根据余震的空间分布特征,认为芦山地震的发震断层并非双石-大川断裂,可能是其东侧的一条隐伏断层.     

永清MS4.3地震和廊坊MS3.0地震的发震构造研究

基于首都圈数字地震台网的宽频带资料,首先采用CAP方法确定了永清M_S4.3地震和廊坊M_S3.0地震的震源机制解:永清地震节面Ⅰ的走向、倾角和滑动角分别为52°,62°和?140°,节面Ⅱ的走向、倾角和滑动角分别为300°,55°和?35°;廊坊地震节面I的走向、倾角和滑动角分别为48°,57°和?147°,节面Ⅱ的走向、倾角和滑动角分别为299°,63°和?38°。两次地震的震源机制解较为一致,推测它们可能具有相同的发震断层。利用近震转换波获得两次地震的震源深度,分别为19 km和13 km。利用双差法对两次地震的主余震进行重新定位,结果显示:两个地震序列的震中均呈NE向分布,余震震源深度均浅于主震震源深度,震源深度分别集中在17—20 km和12—13 km范围内,两个序列的短轴剖面揭示了震源分布均呈现倾向SE,倾角陡立的特点。将地震序列的分布与震源机制解的结果进行对比,认为两个序列的水平展布方向与其对应的主震震源机制解中节面Ⅰ的走向比较接近,深度分布的高倾角特征也与节面Ⅰ比较相似,因此认为发震断层面均为节面Ⅰ。通过将震源机制解中节面Ⅰ的参数和地震序列的分布与区域活动断层的产状性质进行比较,取得了一些关于发震构造和地震成因的重要认识:① 永清M_S4.3地震和廊坊M_S3.0地震的发震构造不是上地壳的先存正断裂?河西务断裂,不排除与中下地壳的新生构造或深大断裂有关;② 永清、廊坊地震发生在13—19 km深度上,结合地壳结构、断裂构造以及区域流变结构等资料,推测该深度范围可能是廊固凹陷的壳内脆性?韧性转换区域,是地震孕育和发生的有利构造部位。      

Studies of mantle structure of U.S.S.R. territory on long-range seismic profiles

Long-range seismic sounding carried out during the last few years on the territory of the U.S.S.R. has shown a basic inhomogeneity of the uppermost mantle, as well as evidence of regularities in the distribution of its seismic parameters. The following data were used: times and apparent velocities of P- and S-waves for investigation of mantle velocities, converted waves for seismic discontinuity model studies and wave attenuation for Q-factor estimation. Strong regularities were distinguished in the distribution of average seismic velocities for the uppermost mantle, in their dependence on the age and type of geostructure and on their position relative to the central part of the continent. Old platforms and the inner part of the continent are marked by velocities under the Mohorovi?i? discontinuity of more than 8.2–8.3 km s^?1, young platforms and outer parts of the continent by 8.0–8.2 km s^?1, and orogenic and rift zones by 7.8–8.0 km s^?1. The difference becomes more pronounced at a depth of about 100–200 km: for the old platform mantle velocities of 8.5–8.6 km s^?1 are typical; beneath the orogenic and rift areas, inversion zones with velocities less than 7.8 km s^?1 are observed.The converted waves show fine inhomogeneities of the crust and uppermost mantle, the presence of many discontinuities with positive and negative changes of velocity, and anisotropy of seismic waves in some of the layers. Wave attenuation allowed the determination of the Q-factor in the mantle. It varied from one region to another but a close relation between Q and P-wave velocity is the main cause of its variation.     

The study of the radiation characteristics and propagation of seismic waves in the North Caucasus by modeling the accelerograms of the recorded earthquakes

For evaluating the parameters of the vibrations of the Earth’s surface in the case of strong earthquakes, which are possible in the future, the regular patterns of the emission and propagation of seismic waves in the North Caucasus regions are investigated. The regional parameters of emission and propagation of seismic waves are evaluated by solution of the inverse problems of stochastic modeling of the accelerograms of the earthquakes, recorded by the seismic station in Sochi. The horizontal components of the strongest earthquakes (M _w ～ 3.9?5.6), that occurred in 2002–2006 within a radius of ～300 km from the seismic station, with source depths up to 60 km are modeled. For calculations of accelerograms, estimates of the quality are used, obtained earlier for this region in the form: Q(f) ～ 80 ～ f ^0.9. The parameter settings are carried out, which determine the shapes of the source spectra, the amplification of the seismic waves in the Earth’s crust, the weakening of the waves at high frequencies (κ), the parameters that determine the shape and duration of accelerograms, etc. Sufficiently good agreement of the calculated and recorded accelerograms is obtained, the regional characteristics of emission and propagation of seismic waves, which can be used for prediction of the parameters of strong motions in the North Caucasus, are evaluated; however, in the future these characteristics should be studied in more detail.     

Focal Depths of Earthquakes in the Crimea–Black Sea Region

The problem of determining focal depths of earthquakes in the Crimea–Black Sea region is considered. Based on the results of interannual studies, it is found that the focal depths of Crimean earthquakes are mainly crustal, with maximum values of up to 60 km. Some recent publications, however, have described deep-focus earthquakes with depths of up to 300 km which were “revealed” in the Crimean region. In this respect, there arose the need to study such a large difference in estimated focal depths. Convincing examples show that the sensational “revelation” of deep earthquakes in Crimea was caused by incorrect processing of the experimental data, in particular, due to (1) a sharp distortion in the recorded arrival times of body waves, (2) exclusion of data from stations nearest to a source, (3) unreasonable arbitrary selection of data from seismic stations, and (4) dropping of data from the worldwide seismological network, including those on deep seismic phases. Thus, the conclusions about the presence of deep mantle earthquakes in Crimea are erroneous. We have redetermined the parameters of hypocenters and verified that the focal depths of earthquakes in the Crimea–Black Sea region are no more than 60 km. Based on these data, we analyze the features of the spatial distribution of focal depths to show that earthquake sources are grouped along conduits that dip southeastward, from the continental part of Crimea toward the Black Sea Basin, in the case of grouping of sources in the Alushta–Yalta and Sevastopol areas. The seismic focal layer of the Kerch–Anapa area dips northeastward, from the Black Sea beneath the North Caucasus.     

Seismic moment tensor inversion using 3D velocity model and its application to the 2013 Lushan earthquake sequence

Source inversion of small-magnitude events such as aftershocks or mine collapses requires use of relatively high frequency seismic waveforms which are strongly affected by small-scale heterogeneities in the crust. In this study, we developed a new inversion method called gCAP3D for determining general moment tensor of a seismic source using Green's functions of 3D models. It inherits the advantageous features of the “Cut-and-Paste” (CAP) method to break a full seismogram into the Pnl and surface-wave segments and to allow time shift between observed and predicted waveforms. It uses grid search for 5 source parameters (relative strengths of the isotropic and compensated-linear-vector-dipole components and the strike, dip, and rake of the double-couple component) that minimize the waveform misfit. The scalar moment is estimated using the ratio of L₂ norms of the data and synthetics. Focal depth can also be determined by repeating the inversion at different depths. We applied gCAP3D to the 2013 M_s 7.0 Lushan earthquake and its aftershocks using a 3D crustal-upper mantle velocity model derived from ambient noise tomography in the region. We first relocated the events using the double-difference method. We then used the finite-differences method and reciprocity principle to calculate Green's functions of the 3D model for 20 permanent broadband seismic stations within 200 km from the source region. We obtained moment tensors of the mainshock and 74 aftershocks ranging from M_w 5.2 to 3.4. The results show that the Lushan earthquake is a reverse faulting at a depth of 13–15 km on a plane dipping 40–47° to N46° W. Most of the aftershocks occurred off the main rupture plane and have similar focal mechanisms to the mainshock's, except in the proximity of the mainshock where the aftershocks' focal mechanisms display some variations.     

Nemuro-Oki earthquake of June 17, 1973: A lithospheric rebound at the upper half of the interface

P-wave first motions, radiation patterns and amplitudes of long-period surface waves, relocated aftershock distributions, leveling and tsunami data indicate that the 1973 Nemuro-Oki earthquake is caused by a low-angle thrust-faulting, representing a rebound at the upper 50 km of the interface between the continental and oceanic lithospheres. Rebound, most likely aseismic, at depths below 50 km, is suggested to take place in the near future from a comparison of recent geologic crustal deformation with pre-seismic and co-seismic data. The estimated seismic moment is about $\text{1}\text{3}\text{–}\text{1}\text{4}$ of that for the neighboring great earthquakes. The macro-seismic data suggest that the 1973 earthquake is smaller than the 1894 Nemuro-Oki earthquake, the last great earthquake in this region.The 1973 earthquake had been predicted on the basis of a seismic gap. Although the prediction was successful as to the location and nature of the faulting and partly as to the occurrence time, it is smaller than the predicted one. A part of the seismic gap may still remain. The difference between the observed seismic slip (1.6 m) and that predicted on the basis of the pre-seismic crustal deformation (3.0 m) indicates either (1) the 1973 earthquake relieved only a part of the strain accumulated in the upper 50 km, or (2) a significant amount of aseismic slip took place on the seismic fault and completely relieved the accumulated strain in the focal region of the 1973 earthquake. If the former is the case, the remaining strain, not only in the focal region, but also in the remaining seismic gap adjoining it, may be relieved in a larger earthquake in the future.The source parameters obtained are as follows: fault plane, dip direction = N40°W, dip angle = 27°; seismic moment = 6.7 · 10²⁷ dyn cm; average slip dislocation, 1.6 m in N63°W direction; stress drop = 35 bars. In these calculations, the fault dimension and the rigidity are assumed to be 100 · 60 km² and 7.0 · 10¹¹ dyn/cm², respectively.     

Earthquakes Along the Passive Margin of Greenland: Evidence for Postglacial Rebound Control

—?An intriguing observation in Greenland is a clear spatial correlation between seismicity and deglaciated areas along passive continental margins, a piece of evidence for earthquake triggering due to postglacial rebound. Another piece of evidence for induced seismicity due to deglaciation derives from earthquake source mechanisms. Sparse, low magnitude seismicity has made it difficult to determine focal mechanisms from Greenland earthquakes. On the basis of two normal faulting events along deglaciated margins and from the spatial distribution of epicenters, earlier investigators suggested that the earthquakes of Greenland are due to postglacial rebound. This interpretation is tested here by using more recent data. Broadband waveforms of teleseismic P waves from the August 10, 1993 (m _b = 5.4) and October 14, 1998 (m _b = 5.1) earthquakes have been inverted for moment tensors and source parameters. Both mechanisms indicate normal faulting with small strike-slip components: the 1993 event, strike = 348.9°, dip = 41.0°, rake =?56.3°, focal depth = 11?km, seismic moment = 1.03?×?10²⁴ dyne-cm, and M _w = 5.3; the 1998 event, strike = 61.6°, dip = 58.0°, rake =?95.5°, focal depth = 5?km, seismic moment = 5.72?×?10²³ dyne-cm, and M _w = 5.1. These and the two prior events support the theory that the shallow part of the lithosphere beneath the deglaciated margins is under horizontal extension. The observed stress field can be explained as flexural stresses due to removal of ice loads and surface loads by glacial erosion. These local extensional stresses are further enhanced by the spreading stress of continental crust and reactivate preexisting faults. Earthquake characteristics observed from Greenland suggest that the dominant seismogenic stresses are from postglacial rebound and spreading of the continental lithosphere.     

Shear wave velocity models retrieved using Rg wave dispersion data in shallow crust in some regions of southern Ontario, Canada

Many crucial tasks in seismology, such as locating seismic events and estimating focal mechanisms, need crustal velocity models. The velocity models of shallow structures are particularly important in the simulation of ground motions. In southern Ontario, Canada, many small shallow earthquakes occur, generating high-frequency Rayleigh (Rg) waves that are sensitive to shallow structures. In this research, the dispersion of Rg waves was used to obtain shear-wave velocities in the top few kilometers of the crust in the Georgian Bay, Sudbury, and Thunder Bay areas of southern Ontario. Several shallow velocity models were obtained based on the dispersion of recorded Rg waves. The Rg waves generated by an m _N 3.0 natural earthquake on the northern shore of Georgian Bay were used to obtain velocity models for the area of an earthquake swarm in 2007. The Rg waves generated by a mining induced event in the Sudbury area in 2005 were used to retrieve velocity models between Georgian Bay and the Ottawa River. The Rg waves generated by the largest event in a natural earthquake swarm near Thunder Bay in 2008 were used to obtain a velocity model in that swarm area. The basic feature of all the investigated models is that there is a top low-velocity layer with a thickness of about 0.5 km. The seismic velocities changed mainly within the top 2 km, where small earthquakes often occur.     

The April–June 2007 Trichonis Lake earthquake swarm (W. Greece): New implications toward the causative fault zone

On 10 April 2007, three moderate earthquakes with M_w = 4.9–5.1 occurred in the vicinity of Trichonis Lake (W. Greece). A local network composed of 12 three-component digital seismographs was installed in the epicentral area and recorded more than 1600 events. The double-difference algorithm HYPODD, incorporating both catalog and waveform cross-correlation differential travel-time data, was applied for the successful relocation of 1490 earthquakes. The latter led to the distinction of a main NW-SE trending and NE-dipping zone, as well as of three neighboring faults; a conjugate NW-SE striking and SW-dipping marginal fault mapped along the northeastern flanks of the lake; a E-W trending and south-dipping low-angle normal fault, possibly related to the major Agrinio Fault Zone (AFZ), parallel to the northern bank of the lake; a NE-SW striking and NW-dipping normal fault, likely related to a segment of the active Evinos fault, located south of the lake. Calculation of the Coulomb stress induced by the combination of the 1975 M_w = 6.0 event and the three largest events of 10 April 2007 on the inferred structures, reveals that most of the seismicity lies within the “stress-loaded” region, except for the westernmost activity, which probably belongs to the deep part of the AFZ. A total of 178 reliable focal mechanisms were determined by regional and local body-wave modeling (5 largest events) and P-wave first motion polarity data. The types of the obtained focal mechanisms are predominantly normal and strike-slip, however, numerous earthquakes were found to exhibit reverse faulting. Inversion of focal mechanism data showed that the prevailing principal horizontal component σ₃ is quite homogeneous throughout the activated area with a roughly NW-SE trend, parallel to the strike of the Hellenides. On the contrary, the compressional field σ₁ appears in two patterns: NE-SW trending onshore and NW-SE trending beneath the lake. This apparent rotation of σ₁ by 90° reveals a complex system enclosed by the suggested NW-SE trending antithetic faults in depths between 7 and 9 km. The calculated stress ratios beneath the lake imply that vertical forces are close to the overburden pressure. The overall inferred stress pattern is rather linked to topographic variations, locally imposing increase or decrease of the vertical forces. The presence of the water in the lake possibly plays an additional important role, penetrating through the bedrock, reducing the friction coefficient, while the pore pressure and, consequently, the effective stress increase. Thus, shearing along mature fractures is enhanced, likely yielding the observed diversity.     

Studies on the June 23, 2010 north Ottawa M W 5.2 earthquake and vicinity seismicity

The basic parameters for the earthquake with a moment magnitude (M _W) of 5.2 on the 23rd of June 2010 have been investigated. The earthquake occurred on a hidden fault in the northwest direction about 60?km north-northeast of Ottawa in the Western Quebec Seismic Zone (WQSZ) and had a focal depth of about 21?km. The focal mechanism was a thrust type with strike in the northwest direction and dipping in the northeast direction. The relative relocations of seven larger aftershocks show that the source rupture area was about 6?km². The b value of the aftershock sequence was 0.8?C1.0, and the decay rate of the aftershocks was faster than normal cases. The dominant seismogenic depths are about 12 to 22?km in most parts of the WQSZ, while the seismogenic depth along the Ottawa?CBonnechere Graben can be as deep as 28?km. Based on the seismic activity in the WQSZ and vicinity since 1961, it seems that the periods of moderate earthquakes are about 6?C10?years.     

A determination of source properties of large intraplate earthquakes in Alaska

Historically, large and potentially hazardous earthquakes have occurred within the interior of Alaska. However, most have not been adequately studied using modern methods of waveform modeling. The 22 July 1937, 16 October 1947, and 7 April 1958 earthquakes are three of the largest events known to have occurred within central Alaska (M _s =7.3,M _s =7.2 andM _s =7.3, respectively). We analyzed teleseismic body waves to gain information about the focal parameters of these events. In order to deconvolve the source time functions from teleseismic records, we first attempted to improve upon the published focal mechanisms for each event. Synthetic seismograms were computed for different source parameters, using the reflectivity method. A search was completed which compared the hand-digitized data with a suite of synthetic traces covering the complete parameter space of strike, dip, and slip direction. In this way, the focal mechanism showing the maximum correlation between the observed and calculated traces was found. Source time functions, i.e., the moment release as a function of time, were then deconvolved from teleseismic records for the three historical earthquakes, using the focal mechanisms which best fit the data. From these deconvolutions, we also recovered the depth of the events and their seismic moments. The earthquakes were all found to have a shallow foci, with depths of less than 10 km.The 1937 earthquake occurred within a northeast-southwest band of seismicity termed the Salcha seismic zone (SSZ). We confirm the previously published focal mechanism, indicating strike-slip faulting, with one focal plane parallel to the SSZ which was interpreted as the fault plane. Assuming a unilateral fault model and a reasonable rupture velocity of between 2 and 3 km/s, the 21 second rupture duration for this event indicates that all of the 65 km long SSZ may have ruptured during this event. The 1947 event, located to the south of the northwest-southeast trending Fairbanks seismic zone, was found to have a duration of about 11 seconds, thus indicating a rupture length of up to 30 km. The rupture duration of the 1958 earthquake, which occurred near the town of Huslia, approximately 400 km ENE of Fairbanks, was found to be about 9 seconds. This gives a rupture length consistent with the observed damage, an area of 16 km by 64 km.     

2010年以来四川地区中强地震震源机制反演及深度确定

2008年5月12日四川龙门山断裂带发生了汶川8.0级地震,之后四川境内发生了两次7.0级地震(其中一个是芦山地震),为了研究汶川地震之后龙门山断裂带及周边区域的地震活动性,本研究收集了国家地震台网和四川区域地震台网2010年1月1日—2017年12月31日四川地区发生的17次M≥5.0地震以及120多次5.0>M≥4.0地震的波形资料,利用波形拟合法反演了震源机制解及区域应力场.反演结果显示,位于龙门山断裂带上的地震,震源机制以逆冲型为主,鲜水河断裂带地震震源机制以走滑型为主,而川滇块体西南部的理塘断裂、金沙江断裂附近,震源机制解以正断层为主.根据震源机制解反演得到的龙门山地区、鲜水河地区的主压应力场方向为WNW、近EW向.川滇块体的巴塘、理塘等地区,其主压应力轴方向为12°左右,接近SN向,且仰角接近40°左右.本研究利用面波振幅谱特征对震源深度进行了精确定位,定位结果与中国地震台网中心(CENC),美国地震调查局(USGS),国际地震中心(ISC)等机构地震目录进行了对比.结果显示,四川地区强震震源深度主要分布在20km以上的中上地壳.龙门山地区震源优势分布在10～20km,鲜水河断裂地震震源深度在10km左右,川滇块体西南部的理塘断裂,巴塘断裂,金沙江断裂等地区,震源深度一般在5～10km范围.     

The earthquake of July 22, 2011 (<Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript> = 4.5) in a low-seismicity area of the Argun region

The paper considers the Argun earthquake of July 22, 2011 (M _w = 4.5), which occurred in the Argun River valley in a low-seismicity territory in China. The focal parameters of the earthquake (depth of the hypocenter, moment magnitude, scalar seismic moment, and focal mechanism) were determined by calculating the seismic moment tensor from the amplitude spectra of surface waves and the data on the signs of the first arrivals of body waves at regional stations. The solution of the focal mechanism makes it possible to assume a relationship between the earthquake focus and a fault with a northeastern strike bordering the southeastern side of the Argun Basin (in Chinese territory). The Argun earthquake was felt in Russia with an intensity of II–III to V at the epicentral distances up to 255 km. The intensity of shaking did not exceed values suggested by new GSZ-2012 and GSZ-2014 seismic zoning maps of Russian territory. Nevertheless, the question on the possible occurrence of stronger earthquakes in the studied region remains open.     

Focal depths and mechanisms of Tohoku-Oki aftershocks from teleseismic P wave modeling

Aftershocks of the 2011 Tohoku-Oki great earthquake have a wide range of focal depths and fault plane mechanisms. We constrain the focal depths and focal mechanisms of 69 aftershocks with M _w > 5.4 by modeling the waveforms of teleseismic P and its trailing near-surface reflections pP and sP. We find that the “thrust events” are within 10 km from the plate interface. The dip angles of these thrust events increase with depth from ~5° to ~25°. The “non-thrust events” vary from 60 km above to 40 km below the plate interface. Normal and strike-slip events within the overriding plate point to redistribution of stress following the primary great earthquake; however, due to the spatially variable stress change in the Tohoku-Oki earthquake, an understanding of how the mainshock affected the stresses that led to the aftershocks requires accurate knowledge of the aftershock location.     

Earthquake doublets in the Solomon Islands

Large, shallow, thrust earthquakes in the Solomon Islands region tend to occur in closely related pairs. Two recent sequences are July 14, 1971 (M_S = 7.9) and July 26, 1971 M(_S = 7.9) and 14^h37^m, July 20, 1975 (M_S = 7.9) and 19^h54^m, July 20, 1975 (M_S = 7.7). The mechanism of these seismic doublets has important bearing on the triggering mechanism of earthquakes in subduction zones. Detailed analysis of the seismic body waves and surface waves were performed on the 1971, 1974, and 1975 doublets, providing a better understanding of: (1) the mechanics of seismic triggering, (2) the state of stress on the fault plane, and (3) the nature of subduction between the Pacific and Indian plates. The results indicate that although the geometry of the subduction zone in the Solomon Islands is complicated by the presence of several sub-plates, the slip direction of the Indian plate with respect to the Pacific plate is relatively uniform over the entire region. The large seismic moments of the 1971 sequence (1.2 · 10²⁸ and 1.8 · 10²⁸ dyne cm) indicate that these events directly represent the underthrusting of the Indian and Solomon plates beneath the Pacific plate. The body waves from these doublets, recorded on the WWSSN long-period seismograms, are remarkably impulsive and simple compared with those from events of comparable seismic moment in other subduction zones. In addition, the source dimensions of the body waves are 30–70 km in length, substantially smaller than the overall rupture surfaces radiating the surface waves which are 100–300 km in length. These facts suggest the existence of relatively large, isolated high-stress zones on the fault plane. This type of stress distribution is distinct from other regions which have more heterogeneous stress distribution on the fault plane, and this is proposed as the principal characteristic of this region responsible for the occurrence of the doublets and for the apparent efficiency of triggering in the Solomon trench. Prior to the 1971 sequence, similar sequences have occurred in the same area in 1919–1920 and 1945–1946. From the amount of slip (1.3 m) determined for the 1971 sequence and the apparent recurrence interval of 25 years, a seismic slip rate of 5 cm yr^?1 is determined. This value is a significant portion of the convergence rate between the Indian and Pacific plates indicating that the plate motion here is taken up largely by seismic slip.     

Upper mantle structure beneath central Western Europe from data on phase velocities of surface waves

The deep structure of the upper mantle is determined from data on phase velocities of Love and Rayleigh waves measured by a differential method on traces between two stations in central Western Europe. One-dimensional velocity structures are first constructed from data of each pair of stations, after which two-dimensional distributions of SH and SV velocities are calculated by the method of two-dimensional tomography from S wave velocities at fixed depths. The results are presented in the form of 2-D vertical structures of the average S wave velocity (S = (SV + SH)/2) constructed along profiles crossing the region in directions of the best resolution. The main structural features are a higher velocity zone at depths of 60–80 km in the area (48°–50°N, 9°–11°E) and a lower velocity zone in the western part of the region at depths of 100–150 km, probably extending farther beyond the studied area.     

Seismic relocation,focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu MS7.1 earthquake and its aftershocks

The 2010 Yushu M_S7.1 earthquake occurred in Ganzi-Yushu fault, which is the south boundary of Bayan Har block. In this study, by using double difference algorithm, the locations of mainshock (33.13°N, 96.59°E, focal depth 10.22 km) and more than 600 aftershocks were obtained. The focal mechanisms of the mainshock and some aftershocks with M_S>3.5 were estimated by jointly using broadband velocity waveforms from Global Seismic Network (GSN) and Qinghai Seismic Network as well. The focal mechanisms and relocation show that the strike of the fault plane is about 125° (WNW-ESE), and the mainshock is left-laterally strikeslip. The parameters of shear-wave splitting were obtained at seismic stations of YUS and L6304 by systematic analysis method of shear-wave splitting (SAM) method. Based on the parameters of shear-wave splitting and focal mechanism, the characteristics of stress field in seismic source zone were analyzed. The directions of polarization at stations YUS and L6304 are different. It is concluded that after the mainshock and the M_S6.3 aftershock on April 14, the stress-field was changed.