Ambiguous Moment Tensors and Radiation Patterns in Anisotropic Media with Applications to the Modeling of Earthquake Mechanisms in W-Bohemia

Anisotropic material properties are usually neglected during inversions for source parameters of earthquakes. In general anisotropic media, however, moment tensors for pure-shear sources can exhibit significant non-double-couple components. Such effects may be erroneously interpreted as an indication for volumetric changes at the source. Here we investigate effects of anisotropy on seismic moment tensors and radiation patterns for pure-shear and tensile-type sources. Anisotropy can significantly influence the interpretation of the source mechanisms. For example, the orientation of the slip within the fault plane may affect the total seismic moment. Also, moment tensors due to pure-shear and tensile faulting can have similar characteristics depending on the orientation of the elastic tensor. Furthermore, the tensile nature of an earthquake can be obscured by near-source anisotropic properties. As an application, we consider effects of inhomogeneous anisotropic properties on the seismic moment tensor and the radiation patterns of a selected type of micro-earthquakes observed in W-Bohemia. The combined effects of near-source and along-path anisotropy cause characteristic amplitude distortions of the P, S1 and S2 waves. However, the modeling suggests that neither homogeneous nor inhomogeneous anisotropic properties alone can explain the observed large non-double-couple components.The results also indicate that a correct analysis of the source mechanism, in principle, is achievable by application of anisotropic moment tensor inversion.     

2013年前郭MS5.8震群矩张量研究

2013年吉林前郭M_S5.8震群为爆发性震群,目前余震活动仍然在持续.基于吉林、辽宁、黑龙江、内蒙古四省地震台网记录的前郭震群波形资料,利用波形信噪比、震源类型、台站及速度模型组合的指标选择最佳的反演方案,应用矩张量的三种反演模式,对序列中5个M_S≥5.0地震进行矩张量反演研究,获得了全矩张量、偏矩张量和纯双力偶的矩张量.使用F-test对地震的三种模式的矩张量反演结果进行显著性检验来确定最佳反演模式.结果显示,5个地震的最优矩张量解均为全矩张量模式反演获得的结果,其双力偶分量仅有20%~65%,矩心深度位于地下3~4 km处,地震在Hudson震源类型图上的投影远离双力偶震源类型区域.这些结果表明,震源类型并非典型的构造地震,推断前郭地震可能是与人类活动有关的诱发地震.     

On the retrieval of moment tensors from borehole data

The complete moment tensors of seismic sources in homogeneous or vertically inhomogeneous isotropic structures cannot be retrieved using receivers deployed in one vertical borehole. The complete moment tensors can be retrieved from amplitudes of P‐waves, provided that receivers are deployed in at least three boreholes. Using amplitudes of P‐ and S‐waves, two boreholes are, in principle, sufficient. Similar rules also apply to transversely isotropic media with a vertical axis of symmetry. In the case of limited observations, the inversion can be stabilized by imposing the zero‐trace constraint on the moment tensors. However, this constraint is valid only if applied to observations of shear faulting on planar faults in isotropic media, which produces double‐couple mechanisms. For shear faulting on non‐planar faults, for tensile faulting, and for shear faulting in anisotropic media, the zero‐trace constraint is no longer valid and can distort the retrieved moment tensor and bias the fault‐plane solution. Numerical modelling simulating the inversion of the double‐couple mechanism from real data reveals that the errors in the double‐couple and non‐double‐couple percentages of the moment tensors rapidly decrease with increase in the number of boreholes used. For noisy P‐ and S‐wave amplitudes with noise of 15% of the top amplitude at each channel and for a velocity model biased by 10%, the errors in the double‐couple percentage attain 25, 13 and 6% when inverting for the double‐couple mechanism from one, two and three boreholes.     

2016年印尼苏门答腊岛海域MW7.8地震震源运动学特征

根据中国和全球地震台网记录的波形记录,采用W震相矩张量反演、反投影分析及有限断层模型反演方法,研究了2016年3月2日印尼7.8级地震破裂过程,分析讨论印尼地震震源运动学特征.结果表明：此地震为一次对称的双侧破裂走滑型事件,北北东─南南西向的断层节面（走向5°/倾角85°）为发震断层面.标量地震矩约6.19×10²⁰ Nm,矩震级为7.79,最大的滑动量约11 m,位于破裂起始点北东,沿着断层走向约30 km处.破裂平均速度2.0~2.2 km·s^-1,破裂持续时间35 s,破裂在5~25 s内释放的能量,约占总能量的97%.最终形成了总长度90 km左右的断层.印尼地震具有破裂持续时间短、破裂速度慢、高滑动能量带相对集中等显著特点.本研究对进一步增进海洋岩石圈地震的震源特性认识有重要参考意义.     

Waveform inversion and analysis of an unusual earthquake swarm in the Boshan mining area,Shandong Province,China

Moment tensor solutions were retrieved for the earthquake swarm that occurred during November and December 2010 in the Boshan mining area, Shandong Province, China. The results showed that the double-couple components in the source mechanisms were higher at the beginning of the swarm and consisted mainly of shear faulting controlled by tectonic stress. The subsequent events had significant non-double-couple components, indicating tensile faulting. The double-couple components predominately presented as normal faulting and the P axes were orientated almost vertically. The slip vectors of the swarm events were relatively stable. With reference to the tectonic features near the epicenter, we concluded that the swarm was a result of subordinate fault motion related to the Wangmu Mountain fault and that high-pressure pore fluids played a crucial role in the activity of the earthquake swarm.     

The 15 August 2007 Peru Earthquake and Tsunami: Influence of the Source Characteristics on the Tsunami Heights

The tsunami caused by the 2007 Peru earthquake (M_w 8.0) provoked less damage than by the seismic shaking itself (numerous casualties due to the earthquake in the vicinity of Pisco). However, it propagated across the Pacific Ocean and small waves were observed on one tide gauge in Taiohae Bay (Nuku Hiva, Marquesas, French Polynesia). We invert seismological data to recover the rupture pattern in two steps. The first step uses surface waves to find a solution for the moment tensor, and the second step uses body waves to compute the slip distribution in the source area. We find the slip distribution to consist of two main slip patches in the source area. The inversion of surface waves yields a scalar moment of 8.9 10²⁰ Nm, and body-wave inversion gives 1.4 10²¹ Nm. The inversion of tsunami data recorded on a single deep ocean sensor also can be used to compute a fault slip pattern (yielding a scalar moment of 1.1 10²¹ Nm). We then use these different sources to model the tsunami propagation across the Pacific Ocean, especially towards Nuku Hiva. While the source model taken from the body-wave inversion yields computed tsunami waves systematically too low with respect to observations (on the central Pacific Ocean DART buoy as on the Polynesian tide gauge), the source model established from the surface-wave inversion is more efficient to fit the observations, confirming that the tsunami is sensitive to the low frequency component of the source. Finally we also discuss the modeling of the late tsunami arrivals in Taiohae Bay using several friction coefficients for the sea bottom.     

用应变地震观测求解震源矩张量的基本原理

震源机制解,即对地震矩张量的推断,对于地震研究具有至关重要的意义.应变地震观测是张量观测,与摆式地震仪的位移矢量观测不同,可以为地震研究提供新的数据源.本文讨论用应变地震观测求解震源矩张量的基本原理.在距离震源足够远的地方,地震波可以看成平面波,其性质决定于震源矩张量.假设平面地震波的应变张量可以由震源矩张量通过坐标变换计算得到,就可以通过观测应变地震波求解震源机制.这个假设至少对于双力偶震源机制是成立的.由此可以证明,在理想的无限介质中,只要有两个以上不同地点的应变地震波观测,就可以解出震源矩张量.这为解决震源机制问题提供了新的方法.目前的地震矩张量求解方法需要两方面的条件:或者需要很多观测点(例如体波反演),或者需要长周期地震波资料(例如面波反演).这些方法只适用于分析比较大的地震.对于小震,因为通常其震中周围不会有足够多的摆式地震仪观测点观测到其地震波,而地震波周期又短,难以利用传统方法给出可靠的震源机制解,所以只需少数观测点就能求解震源矩张量的新方法特别有意义.用应变地震观测求解震源机制,可以给出更为精确的结果.     

Use of long-period surface waves for rapid determination of earthquake-source parameters

A method for rapid retrieval of earthquake-source parameters from long-period surface waves is developed. With this method, the fault geometry and seismic moment can be determined immediately after the surface wave records have been retrieved. Hence, it may be utilized for warning of tsunamis in real time. The surface wave spectra are inverted to produce either a seismic moment tensor (linear) or a fault model (nonlinear). The method has been tested by using the IDA (International Deployment of Accelerographs) records. With these records the method works well for the events larger than M_s = 6, and is useful for investigating the nature of slow earthquakes.For events deeper than 30 km, all of the five moment tensor elements can be determined. For very shallow events (d ? 30 km) the inversion becomes ill-conditioned and two of the five source moment tensor elements become unresolvable. This difficulty is circumvented by a two-step inversion. In the first step, the unresolvable elements are constrained to be zero to yield a first approximation. In the second step, additional geological and geophysical data are incorporated to improve the first approximation. The effect of the source finiteness is also included.     

The 2011 Lorca seismic series: Temporal evolution,faulting parameters and hypocentral relocation

The Lorca 2011 seismic series was recorded by an unprecedented set of high quality on scale broadband seismograms and strong motion accelerograms. The waveforms from permanent and temporary broadband seismic networks deployed in the region by different institutions allowed to invert regional moment tensor for the fore, main and largest aftershock of the complete seismic sequence. Using double-difference algorithm we have performed a precise relocation of the seismic series, where body wave travel times from strong ground motion accelerograms were included in the data set. Regional moment tensor inversion for the three main events show similar oblique-reverse faulting regime with a northeast-southwest fault orientation. The scalar seismic moment, moment magnitude and focal depth retrieved from the inversion yield the following values for each event: \(\hbox {Mo}=6.5\times 10^{16}\) Nm (Mw = 5.2) for the mainshock, \(\hbox {Mo}= 9.6 \times 10^{15}\) Nm (Mw = 4.6) for the foreshock and \(\hbox {Mo}=7.3\times 10^{14}\) Nm (Mw = 3.9) for the large aftershock. The centroid depths range between 4 and 6 km. The double-difference relocation of the seismic series shows significant epicentral differences with the preliminary routine location. The epicentral solutions given by this relocation show a seismic sequence distributed following a NE–SW strike, subparallel to the Alhama de Murcia fault and compatible with the faulting parameters inverted from the moment tensor analysis. The hypocenters of the series generate a subvertical trend in depth distribution, being concentrated between 2 and 6 km. The depth distribution of the main events, which range from 4.6 to 5.5 km, is in good relationship with the faulting and depth parameters deduced from the moment tensor inversion technique. The regional moment tensor solutions for the three largest earthquakes, the epicentral distribution and the focal depths show good relationship with the surface geometry and tectonic regime of the Alhama de Murcia fault. The stress drop deduced for the mainshock gives a value ranging between 58 and 85 bars, which does not support the idea of a high stress drop release as a main factor contributing to the high ground acceleration recorded at Lorca. The PGA values observed at Lorca, which contributed to the high damage independently of structural deficiencies, could be generated mainly by shallowness and proximity to the seismic source together with a directivity effect in the seismic radiation.     

Microseismic Full Waveform Modeling in Anisotropic Media with Moment Tensor Implementation

Seismic anisotropy which is common in shale and fractured rocks will cause travel-time and amplitude discrepancy in different propagation directions. For microseismic monitoring which is often implemented in shale or fractured rocks, seismic anisotropy needs to be carefully accounted for in source location and mechanism determination. We have developed an efficient finite-difference full waveform modeling tool with an arbitrary moment tensor source. The modeling tool is suitable for simulating wave propagation in anisotropic media for microseismic monitoring. As both dislocation and non-double-couple source are often observed in microseismic monitoring, an arbitrary moment tensor source is implemented in our forward modeling tool. The increments of shear stress are equally distributed on the staggered grid to implement an accurate and symmetric moment tensor source. Our modeling tool provides an efficient way to obtain the Green’s function in anisotropic media, which is the key of anisotropic moment tensor inversion and source mechanism characterization in microseismic monitoring. In our research, wavefields in anisotropic media have been carefully simulated and analyzed in both surface array and downhole array. The variation characteristics of travel-time and amplitude of direct P- and S-wave in vertical transverse isotropic media and horizontal transverse isotropic media are distinct, thus providing a feasible way to distinguish and identify the anisotropic type of the subsurface. Analyzing the travel-times and amplitudes of the microseismic data is a feasible way to estimate the orientation and density of the induced cracks in hydraulic fracturing. Our anisotropic modeling tool can be used to generate and analyze microseismic full wavefield with full moment tensor source in anisotropic media, which can help promote the anisotropic interpretation and inversion of field data.     

Spatial and temporal rupture process of the January 26, 2001, Gujarat, India, M_S=7.8 earthquake

Introduction An earthquake of MS=7.8 occurred near the Gujarat of India on January 26, 2001, which was one of the most deadly earthquakes since there was the record in the Indian history (Bendick, et al, 2001; Gupta, et al, 2001). The USGS of USA determined the origin time of the earthquake to be 3h16min41s (UTC), and the epicenter location to be 70.32篍, 23.40篘. Shortly after the earthquake, the moment tensor solutions or focal mechanisms and other related parameters were offered by s…     

汶川MS8.0级地震发生背景与过程的研究

本文首先阐明汶川M_S8.0级地震发生在由区域布格重力异常和地震震中分布所确定的武都—松潘—茂汶—汶川—泸定地震带上.汶川地震所在地段是地震前兆和中小地震（M≤7.0）的空白区,震前出现明显的孕震空区,M_S8.0级地震发生在空区周围区域中小地震活动峰值之后的减少段里.地震的破裂超出孕震空区范围,空区内、外余震活动呈现出不同的衰减特征,依此将余震活动分为WS和NE两个区段.地震破裂过程、4级以上余震矩张量及震区应力场反演和余震应力降的测定结果表明,两个区域的位错、余震机制解和应力降及最大主应力的方〖JP2〗向等明显有别.根据这些特征和地震应力触发的研究,推测NE段地震的发生可能是〖JP〗由WS段主破裂的发生所触发.     

Implications of stress-drop models of earthquakes for the inversion of stress drop from seismic observations

We discuss the inversion of stress drops from seismic observations on the basis of crack or stress-drop models of earthquake mechanism. Since a formal inverse problem cannot be posed at present we discuss implications of solutions to direct problems. We first discuss the static approximations used to obtain stress drop from seismic moment and source dimensions. We show that the geometrical effects are quite significant if only one source dimension has been retrieved from seismic observations. The effect of variable stress drop is discussed and we show that the inverted stress drop is not a simple average of the actual stress drops on the fault. We discuss the energy release during faulting and show that the apparent stress has a complicated relation to the stress drop on the fault. We also show that the static stress drops obtained by seismologists are a lower bound to the actual dynamic stress drops on the fault. This may in part explain disagreements with laboratory results. Finally, we discuss the inversion of source dimensions from the far-field radiation. We analyse two extreme, simple dynamical source models, a circular fault and a rectangular fault and show that geometry has a much more pronounced effect on radiation than is usually acknowledged.     

Source properties of the 1997–98 Central Italy earthquake sequence from inversion of long-period and broad-band seismograms

We study source properties of the main earthquakes of the 1997–98 Umbria-Marche (central Italy) sequence by analysis of regional-distanceand teleseismic long period and broadband seismograms recorded by MedNet and IRIS/GSN stations. We use a modified Harvardcentroid-moment tensor (CMT) algorithm to allow inversion of long period waveforms, primarily Rayleigh and Love waves, for small earthquakes (4.2 M_W 5.5) at local to regional distances (<15^°). For the seven largest earthquakes (M_W>5.2) moment tensors derived from local and regional data agree well with those determined using teleseismic waveforms and standard methods of analysis. We also determine moment tensors for a foreshock and 12 other aftershocks, that were too small for global analysis. Focal depth and rupture propagation are analyzed for three largest shocks by inversion of teleseismic broadband body waves. The earthquakes are generally located at shallow depth (5 km or shallower) and are characterized by normal faulting mechanisms, with a NE-SW tension axis. The presumed principal fault plane dips at a shallow angle towards the SW. Only one of the events analyzed has an entirely different faulting geometry, indicating instead right-lateral strike-slip motion on a plane approximately E-W, or left-lateral faulting on a N-S plane. The other significant exception to the regular pattern of mechanisms is represented by the March 26, 1998, event, located at 51 km depth. Its connection with the shallow earthquake sequence is unclear and intriguing. The time evolution of the seismic sequence is unusual,with the mainshock accounting for only approximately 50% of the total moment release. The broadband teleseismic waveforms of the main, September 26, 09:40, earthquake are very complicated for the size of the event and suggest a complex rupture. In our favored source model, rupture initiated at 5 km depth, propagated updip and was followed, 3 seconds later, by a shallower subevent with a slightly rotated mechanism.     

Spatial and temporal rupture process of the January 26, 2001, Gujarat, India, M S=7.8 earthquake

The source parameters, such as moment tensor, focal mechanism, source time function (STF) and temporal-spatial rupture process, were obtained for the January 26, 2001, India, M _S=7.8 earthquake by inverting waveform data of 27 GDSN stations with epicentral distances less than 90°. Firstly, combining the moment tensor inversion, the spatial distribution of intensity, disaster and aftershocks and the orientation of the fault where the earthquake lies, the strike, dip and rake of the seismogenic fault were determined to be 92°, 58° and 62°, respectively. That is, this earthquake was a mainly thrust faulting with the strike of near west-east and the dipping direction to south. The seismic moment released was 3.5×10²⁰ Nm, accordingly, the moment magnitude M _W was calculated to be 7.6. And then, 27 P-STFs, 22 S-STFs and the averaged STFs of them were determined respectively using the technique of spectra division in frequency domain and the synthetic seismogram as Green’s functions. The analysis of the STFs suggested that the earthquake was a continuous event with the duration time of 19 s, starting rapidly and ending slowly. Finally, the temporal-spatial distribution of the slip on the fault plane was imaged from the obtained P-STFs and S-STFs using an time domain inversion technique. The maximum slip amplitude on the fault plane was about 7 m. The maximum stress drop was 30 MPa, and the average one over the whole rupture area was 7 MPa. The rupture area was about 85 km long in the strike direction and about 60 km wide in the down-dip direction, which, equally, was 51 km deep in the depth direction. The rupture propagated 50 km eastwards and 35 km westwards. The main portion of the rupture area, which has the slip amplitude greater than 0.5 m, was of the shape of an ellipse, its major axis oriented in the slip direction of the fault, which indicated that the rupture propagation direction was in accordance with the fault slip direction. This phenomenon is popular for strike-slip faulting, but rather rare for thrust faulting. The eastern portion of the rupture area above the initiation point was larger than the western portion below the initiation point, which was indicative of the asymmetrical rupture. In other words, the rupturing was kind of unilateral from west to east and from down to up. From the snapshots of the slip-rate variation with time and space, the slip rate reached the largest at the 4th second, that was 0.2 m/s, and the rupture in this period occurred only around the initiation point. At the 6th second, the rupture around the initiation point nearly stopped, and started moving outwards. The velocity of the westward rupture was smaller than that of the eastward rupture. Such rupture behavior like a circle mostly stopped near the 15th second. After the 16th second, only some patches of rupture distributed in the outer region. From the snapshots of the slip variation with time and space, the rupture started at the initiation point and propagated outwards. The main rupture on the area with the slip amplitude greater than 5 m extended unilaterally from west to east and from down to up between the 6th and the 10th seconds, and the western segment extended a bit westwards and downwards between the 11th and the 13th seconds. The whole process lasted about 19 s. The rupture velocity over the whole rupture process was estimated to be 3.3 km/s. Foundation item: 973 Project (G1998040705) from Ministry of Science and Technology, P. R. China, and the National Science Foundation of China under grant No.49904004. Contribution No. 02FE2026, Institute of Geophysics, China Seismological Bureau.     

The determination of the best fitting and the extreme source models by a global inversion method

In this paper, a global inversion method is developed for seismic moment tensor inversion by using the body wave forms. The algorithm depends on neither the selection of starting model nor the forms of objective function and constraints. When the error function, measure of the difference between the observed and synthetic waveforms, is chosen as the objective function, the best fitting source model is found; when a certain combination in seismic moment tensor elements is selected as the objective function and the values of error function are constrained in a suitable bound, the extreme source models can be produced by minimizing or maximizing this combination. By changing the form of the combination of moment tensor elements, a variety of different source characteristics can be considered. Therefore the extreme solution provides an estimation of the uncertainty in the best fitting source model. The seismic waveform data was used to evaluate the effectiveness of this algorithm. This research was supported by the National Natural Science Foundation of China.     

THE SOURCE PARAMETERS AND RUPTURE PROCESS OF THE MW7.0 EARTHQUAKE IN ALASKA,USA ON DECEMBER 1, 2018

A magnitude M_W7.0 earthquake struck north of Anchorage, Alaska, USA on 1 December 2018. This earthquake occurred in the Alaska-Aleutian subduction zone, on a fault within the subducting Pacific slab rather than on the shallower boundary between the Pacific and North American plates. In order to better understand the earthquake source characteristics and slip distribution of source rupture process as well as to explore the effect of tectonic environment on dynamic triggering of earthquake, the faulting geometry, slip distribution, seismic moment, source time function are estimated from broadband waveforms downloaded from IRIS Data Management Center. We use the regional broadband waveforms to infer the source parameters with ISOLA package and the teleseismic body wave recorded by stations of the Global Seismic Network is employed to conduct slip distribution inversion with iterative deconvolution method. The focal mechanism solution indicates that the Alaska earthquake occurred as the result of tensile-type normal faulting, the estimated centroid depth from waveform inversion shows that the earthquake occurred at the depth of 56.5km, and the centroid location is 10km far away in northeast direction relative to the location of initial epicenter. We use the aftershock distribution to constrain the fault-plane strike of a normal fault to set up the finite fault model, the finite fault inversion shows that the earthquake slip distribution is concentrated mainly on a rectangular area with 30km×20km, and the maximum slip is up to 3.6m. In addition, the slip distribution shows an asymmetrical distribution and the range of possible rupture direction, the direction of rupture extends to the northeast direction, which is same as that of aftershock distribution for a period of ten days after the mainshock. It is interesting to note that a seismic gap appears in the southwest of the seismogenic fault, we initially determined that the earthquake was a typical normal fault-type earthquake that occurred in the back-arc extensional environment of the subduction collision zone between the Pacific plate and the North American plate, this earthquake was not related to tectonic movement of faults near the Earth's surface. Due to the influence of high temperature and pressure during the subduction of the Pacific plate toward to the north, the subduction angle of the Pacific plate becomes steep, causing consequently the backward bending deformation, thus forming to a tensile environment at the trailing edge of the collision zone and generating the M_W7.0 earthquake in Alaska.     

Seismic Moment Tensor Resolution on a Local Scale: Simulated Rockburst and Mine-induced Seismic Events in the Kopanang Gold Mine, South Africa

Seismic records contain information about the effect of the source as well as the effect of wave propagation through the rock mass. The effect of wave propagation is usually not well known as only simplified models of geological structures are available. Therefore, the information about the source retrieved by inverting seismograms may include errors due to incomplete knowledge of the rock mass along the propagation path, which in turn cause a distortion in the calculated moment tensor (MT). The distortion of the MT on a local scale was observed by inverting records of a simulated rockburst conducted at the Kopanang gold mine in South Africa. A dominant isotropic component of the explosive characteristics was found from the inversion. The deviatoric components retrieved from the blast are spurious. A test of their stability indicated that they are not significant, assuming an uncertainty above 5% for velocities and 10% for attenuation within the homogeneous model available for the mine. Thus, the retrieval of the MT from records of local networks in mines using a homogeneous model of the rock mass seems to be feasible. However, the homogeneous model of the rock mass can only be applied to close stations, within a few kilometers of the source. The seismic records from distant stations were too complex to be modelled by a homogeneous rock mass. Records of six mine-induced seismic events recorded at the Kopanang gold mine were also inverted. A vertical linear dipole along the pressure (P) axis was found for three of the events, suggesting a pillar burst. The mechanism of two events contains an isotropic implosion together with a nearly vertical dip-slip, and seems to indicate a combination of a cavity collapse with a down dip-slip along a nearly vertical fault. One event corresponds to a dipole along the tensional (T) axis. However, it is vertical, thus its association with tensile faulting of the hangingwall is uncertain.     

A new technique for moment tensor inversion with applications to the 2008 Wenchuan <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>8.0 earthquake sequence

A M_S8.0 earthquake occurred in Wenchuan County, Sichuan Province, China, on May 12, 2008, and subsequently, numerous aftershocks followed. We obtained the moment tensor solutions and source time functions (STFs) for the Wenchuan earthquake and its seven larger aftershocks (M_S5.0～6.0) by a new technique of moment tensor inversion using the broadband and long-period seismic waveform data from the Global Seismic Network (GSN). Firstly, the theoretical background and technical flow of the new technique was briefly introduced, and an aftershock of the Wenchuan earthquake sequence was employed to illustrate the real procedure for inverting the moment tensor; secondly, the moment tensor solutions and STFs of the eight events, including the main shock, were presented, and finally, the interpretation of the results was made. The agreement of our results with the GCMT results indicates the new approach is efficient and feasible. By using this approach, not only the moment tensor solution can be obtained but also the STF can be retrieved; the inverted STFs indicate that the source rupture process may be complicated even for the moderate earthquakes. The inverted focal mechanisms of the Wenchuan earthquake sequence show that the most of the aftershocks occurred in the main faults of the Longmenshan fault zone with predominantly thrustingwith minor right-lateral strike-slip component, but some of them may have occurred in the subfaults with strike-slip faulting in the vicinity of the main faults.