Earthquake Science

Published by Springer Verlag
Online ISSN: 1867-8777
Print ISSN: 1674-4519
Publications
We use interferometric synthetic aperture radar (InSAR) and broadband seismic waveform data to estimate a source model of the 11th July, 2004 M W6.2 Zhongba earthquake, Tibet of China. This event occurred within the seismically active zone of southwestern Tibetan Plateau where the east-west extension of the upper crust is observed. Because of limitations in one pair of InSAR data available, there are trade-offs among centroid depth, rupture area and amount of slip. Available seismic data tightly constrain the focal mechanism and centroid depth of the earthquake but not the horizontal location. Together, two complementary data sets can be used to identify the actual fault plane, better constrain the slip model and event location. We first use regional seismic waveform to estimate point source mechanism, then InSAR data is used to obtain better location. Finally, a joint inversion of teleseismic P-waves and InSAR data is performed to obtain a distributed model. Our preferred point source mechanism indicates a seismic moment of ∼2.2×1018 N·m (M W6.2), a fault plane solution of 171° (342°)/42°(48°)/−83°(−97°), corresponding to strike/dip/rake, and a depth of 11 km. The fault plane with strike of 171° and dip of 42° is identified as the ruptured fault with the aid of InSAR data. The preferred source model features compact area of slips between depth of 5–11 km and 10 km along strike with maximum slip amplitude of about 1.5 m.
 
Micro-aftershocks with magnitude range of 1.5–4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5–4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0–5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of <5 km, the b-values are very small (∼0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ∼5–11 km, where most aftershocks concentrated, the b-values become as high as ∼0.9–1.0. At the depth below ∼11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.
 
(a) The locations and mechanism obtained from different datasets and methods. (b) The stations location and the 20 s Rayleigh wave group velocity in East Asia. GCMT: global CMT solution, Li InSAR: result of Li et al. (2008), Zhang InSAR: location provided by Zhang et al. (2002), Gao aftershock: location provided by Gao et al. (2002). 
(a) The record section of 1998 Zhangbei earthquake. (b) The NCFs between IC/IRIS stations and ZHB station. The solid line denotes the segments used in differential time measurements in following. 
(a) The location result without NCF calibration. The optimal location with minimum misfit is marked with “1D” square. The contour lines denote the distance far from the InSAR location with interval of 5 km. (b) The location result with NCF calibration. Both (a) and (b) use the same color bar of misfit function. 
Because ambient seismic noise provides estimated Green’s function (EGF) between two sites with high accuracy, Rayleigh wave propagation along the path connecting the two sites is well resolved. Therefore, earthquakes which are close to one seismic station can be well located with calibration extracting from EGF. We test two algorithms in locating the 1998 Zhangbei earthquake, one algorithm is waveform-based, and the other is traveltime-based. We first compute EGF between station ZHB (a station about 40 km away from the epicenter) and five IC/IRIS stations. With the waveform-based approach, we calculate 1D synthetic single-force Green’s functions between ZHB and other four stations, and obtain traveltime corrections by correlating synthetic Green’s functions with EGFs in period band of 10–30 s. Then we locate the earthquake by minimizing the differential travel times between observed earthquake waveform and the 1D synthetic earthquake waveforms computed with focal mechanism provided by Global CMT after traveltime correction from EGFs. This waveform-based approach yields a location which error is about 13 km away from the location observed with InSAR. With the traveltime-based approach, we begin with measuring group velocity from EGFs as well as group arrival time on observed earthquake waveforms, and then locate the earthquake by minimizing the difference between observed group arrival time and arrival time measured on EGFs. This traveltime-based approach yields accuracy of 3 km, Therefore it is feasible to achieve GT5 (ground truth location with accuracy 5 km) with ambient seismic noises. The less accuracy of the waveform-based approach was mainly caused by uncertainty of focal mechanism. Key wordsambient seismic noise–estimated Green’s function–ground truth location–Rayleigh wave
 
Based on the measurement of the arrival time of maxima magnitude from band-pass filtering signals which were determined using a new Morlet wavelet multiple-filter method, we develop a method for measuring intrinsic and attenuative dispersion of the first cycle direct P-wave. We determine relative group delays of spectral components of direct P-waves for 984 ray paths from SML and ALS stations of the Taiwan Central Weather Bureau Seismic Network (CWBSN). Using continuous relaxation model, we deduce a new transfer function that relates intrinsic dispersion to attenuation. Based on the genetic algorithm (GA), we put forward a new inversion procedure for determining which is defined the flat part of quality factor Q(ω) spectrum, τ 1 and τ 2 parameters. The results indicate that (1) The distribution of Q m values versus epicentral distance and depth show that Q m values linearly increase with increasing of epicentral distance and depth, and Q m values is clearly independent of earthquakes magnitude; (2) In the different depth ranges, Q m residual show no correlation with variations in epicentral distance. Some significant changes of Q m residual with time is likely caused by pre-seismic stress accumulation, and associated with fluid-filled higher density fractures rock volume in the source area of 1999 Chi-Chi Taiwan earthquake. We confirm that Q m residual with time anomaly appears about 2.5 years before the Chi-Chi earthquake; (3) A comparison of Q m residual for different depth range between SML and ALS stations show that the level of stress has vertical and lateral difference; (4) The area near observation station with both anomalously increasing and decreasing averaged Q m residual is likely an unstable environment for future strong earthquake occurrence. This study demonstrates the capability of direct P-waves dispersion for monitoring attenuation characteristics and its state changes of anelastic medium of the Earth at short propagation distance using seismograms recorded from very small events.
 
The September 21, 1999, Jiji (Chi-Chi) M W7.6 earthquake is the strongest event occurred since 1900 in Taiwan of China. It is located in the middle segment of the western seismic zone of Taiwan. Based on several versions of China earthquake catalogue this study found that a seismic gap of M≥5 earthquakes appeared, in and around the epicenter region, 24 years before and lasted up to the mainshock occurrence. This study also noticed that there existed a lager seismically quiet region of M≥4 earthquakes, which lasted for about 2.5 years before the mainshock occurrence. The spatial variation pattern of regional seismicity before the mainshock seems to match with its coseismic source rupture process. The mentioned seismicity gap and seismic quiescence might be an indication of the preparation process of the Jiji strong earthquake. Key wordsTaiwan-Jiji (Chi-Chi) earthquake-seismicity gap-seismic quiescence CLC numberP315.5
 
Structure Parameters Used in This Calculation
In this study, we preliminarily investigated the dynamic rupture process of the 1999 Chi-Chi, Taiwan, earthquake by using an extended boundary integral equation method, in which the effect of ground surface can be exactly included. Parameters for numerical modeling were carefully assigned based on previous studies. Numerical results indicated that, although many simplifications are assumed, such as the fault plane is planar and all heterogeneities are neglected, distribution of slip is still consistent roughly with the results of kinematic inversion, implying that for earthquakes in which ruptures run up directly to the ground surface, the dynamic processes are controlled by geometry of the fault to a great extent. By taking the common feature inferred by various kinematic inversion studies as a restriction, we found that the critical slip-weakening distance D c should locate in a narrow region [60 cm, 70 cm], and supershear rupture might occur during this earthquake, if the initial shear stress before the mainshock is close to the local shear strength.
 
This paper presents 2.5D scattering of incident plane SH waves by a canyon in layered half-space by the indirect boundary element method (IBEM). The free field response is carried out to give the displacements and stresses on the line which forms boundary of the canyon. The fictitious uniform moving loads are applied to the same line to calculate the Green’s functions for the displacements and stresses. The amplitudes of the loads are determined by the boundary conditions. The displacements due to the free field and from the fictitious uniform moving loads have to be added to obtain the whole motion. The numerical results are carried out for the cases of a canyon in homogenous and in one layer over bedrock. The results show that the 2.5D wave scattering problem is essentially different from the 2D case, and there exist distinct differences between the wave amplification by a canyon in layered half-space and that in homogeneous half-space. The reasons for the distinct difference are explored, and the effects of the thickness and stiffness of the layer on the amplification are discussed.
 
Recently, effects of Earth’s curvature and radial heterogeneity on coseismic deformations are often investigated based on the 2004 Sumatra earthquake. However, such effects are strongly related to earthquake types. As a low dip angle event, the 2004 Sumatra earthquake is not a good seismic case for such a topic since the effects for moderate dip angle events are much bigger. In this study, the half-space and spherical dislocation theories are used, respectively, to calculate coseismic displacements caused by the 2008 Wenchuan earthquake and the 2004 Sumatra earthquake. Effects of Earth’s curvature and stratification are investigated through the discrepancies of results calculated using the two dislocation theories. Results show that the effects of Earth’s curvature and stratification for the 2008 Wenchuan earthquake are much larger than those for the 2004 Sumatra earthquake. Ignoring the effects will cause errors up to 100%–200% in far field displacements for a moderate dip angle event like the 2008 Wenchuan earthquake. Such great effects are much bigger than those conclusions of previous studies. Besides, comparison with observations verifies that spherical dislocation theories yield better results than half-space ones in far fields. Key wordscoseismic displacement-dislocation theory-Earth model CLC numberP315.2
 
To better understand repeatability of strong earthquakes in previously ruptured zones during one seismogenic period, we studied the rupture zones of the doublet of M6 earthquakes in Zhongba region of southcentral Tibet, China, in 11 July 2004 and 7 April 2005, respectively. We focused on the overlapping degree of two strong quakes’ aftershock areas one week after the mainshocks by using the SQH station in China Seismic Network and a 68-stations temporary broadband seismic array, a part of the international HI-CLIMB project. About 115 local earthquakes were recorded in one week after the mainquakes, and we located these earthquakes by master event relative location (MERL) method. We also used this method to relocate 31 other M3.7+ earthquakes from 1 July 2004 to 1 July 2005. Meanwhile, we studied two mainshocks’ coseismic ruptures with satellite interferometric synthetic aperture radar (InSAR). Our results show that the ruptured zones of the two earthquakes do not overlapp substantially, either from early aftershock data or from InSAR inversions. Key wordsrupture zone overlapping–Zhongba earthquake sequences–earthquake doublets–master event relative location–InSAR
 
The data of ionospheric perturbations observed on DEMETER before the 2007 Pu’er earthquake are analyzed. The three-component plasma (ions, electrons and heavy ions) is studied in the fluid concept. The linear dispersion relation for ion-acoustic wave is found in the presence of heavy ions. The nonlinear dynamics is studied for arbitrary amplitude of the wave. The Sagdeev potential is calculated, which shows that solitary structure exists for Mach number within a range defined by the presence of heavy ions. The developed ion-acoustic solitons may be used as precursor for earthquake prediction.
 
The paper introduces firstly the seismic loss assessment method based on macro-economic indicators and new vulnerability models determined by the data from the on-site damage and loss survey to earthquakes occurred in China during the last two decades. The fast assessment for the 2008 Wenchuan earthquake with M S8.0 is given based on an empirical intensity attenuation relationship. Compared with the assessment based on the practical seismic intensity map of the event according to the on-site investigation, the result demonstrates the usability of the seismic vulnerability models introduced in the paper. In addition, it is indicated that the main uncertainty of losses in the fast loss assessment comes from the uncertainty of the estimation of seismic ground motion.
 
This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30–40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. Fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. Fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drilled very close to the Hongkou outcrop. The presence of carbon and graphite, observed next to the slipping-zone, may affect the mechanical properties of the fault and also provide useful information about coseismic chemical changes. Key wordsWenchuan earthquake–Longmenshan fault system–Beichuan fault–fault rock–fault mechanics–fault-zone structure–amorphous carbon–graphite
 
The paper discusses quantitatively the influence of the Yutian M S7.4 earthquake of March 21, 2008 and Wuqia M S6.9 earthquake of October 5, 2008 on regional seismicity in Xinjiang, and explains primarily the possible reason of earthquake activity feature in Xinjiang after the Yutian M S7.4 earthquake by analyzing the static Coulomb failure stress change produced by the Yutian M S7.4 earthquake and Wuqia M S6.9 earthquake, and the seismicity feature of M S≥3 earthquakes in the positive Coulomb stress change region of Kashi-Wuqia joint region, the central segment of Tianshan Mountain and Kalpin block. The result shows that the Yutian M S7.4 earthquake of March 21, 2008, may encourage the Wuqia M S6.9 earthquake of October 5, 2008, and the Yutian M S7.4 earthquake and Wuqia M S6.9 earthquake may change the seismicity state in the central segment of Tianshan Mountain, Kalpin block and Kashi-Wuqia joint region, and encourage the subsequent M S≥3 earthquakes.
 
High-velocity friction experiments were conducted on clayey fault gouge collected from Hongkou outcrop of Beichuan fault, located at the southwestern part of Longmenshan fault system that caused the disastrous 2008 Wenchuan earthquake. The ultimate purpose of this study is to reproduce this earthquake by modeling based on measured frictional properties. Dry gouge of about 1 mm in thickness was deformed dry at slip rates of 0.01 to 1.3 m/s and at normal stresses of 0.61 to 3.04 MPa, using a rotary-shear high-velocity frictional testing machine. The gouge displays slip weakening behavior as initial peak friction decays towards steady-state values after a given displacement. Both peak friction and steady-state friction remain high at slow slip rates are examined and gouge only exhibits dramatic weakening at high slip rates, with steady-state friction coefficient values of about 0.1 to 0.2. Specific fracture energy ranges from 1 to 4 MN/m in our results and this is of the same order as seismically determined values. Low friction coefficients measured on experimental faults are in broad agreement with lack of thermal anomaly observed from temperature measurements in WFSD-1 drill hole (Wenchuan Earthquake Fault Scientific Drilling Project), which can be explained by even smaller friction coefficient for the Wenchuan earthquake fault. High-velocity friction experiments with pore water needs to be done to see if even smaller friction is attained or not. Shiny slickenside surfaces form at high slip rates, but not at slow slip rates. Slip zone with slickenside surface changes its color to dark brown and forms duplex-like microstructures, which are similar to those microstructures found in the fault gouges from the Hongkou outcrop. Detailed comparisons between experimentally deformed gouge samples and WFSD drill cores in the future will reveal how much we could reproduce the dynamic weakening processes in operation in fault zones during Wenchuan earthquake at present.
 
Location of Zipingpu reservoir and the seismicity pattern around the reservoir area. The red cross indicates the epicenter of Wenchuan earthquake reported by USGS. The yellow cross indicates the epicenter of Wenchuan earthquake reported by CEA.
Simulated change in effective Coulomb stress due to the load from the Zipingpu Reservoir. (a) Ge et al. (2009)'s result; (b) The result with Ge et al. (2009)'s model , but the dip is set to be 35° at the depth 14−15 km; (c) The result with Ge et al. (2009)'s model, but shift the reservoir 2.5 km left to let the reservoir cross the rupturing fault YB. The yellow cross indicates the epicenter of Wenchuan earthquake reported by CEA.
Simulated change in effective Coulomb stress on the rupturing fault YB with the 3-D model. (a) Induced Coulomb stress change on the fault on the day before Wenchuan earthquake occurrence. The red cross indicates the location of Wenchuan earthquake reported by USGS. The yellow cross indicates the location of Wenchuan earthquake reported by CEA; (b) Water level variation on time and Coulomb stress variations with time at the initial rupturing area. The origin of time is the reservoir impoundment time (September 30, 2005). Zipingpu reservoir impoundment. The institute did not presented any reports on micro-seismicity abnormally increase around Zipingpu reservoir region, either. Huang (2008) has already reached a similar conclusion. He adopted RTL (Region-Time-Length) method to quantitatively analyze the characteristics of the seismicity changes prior to the Wenchuan earthquake occurrence and showed a seismic quiescence anomaly appeared during 2006−2007, which again suggests the reservoir impoundment did not trigger a regional micro-seismicity
Some recent publications presented a result suggesting that Zipingpu reservoir hastened the occurrence of the 2008 M S8.0 Wenchuan earthquake by tens to hundreds of years. Their researches calculated the Coulomb stress change induced by Zipingpu reservoir on the rupturing fault of Wenchuan earthquake. Their results, however, are critically dependent upon the 3-D event location, reservoir location, and the fault plane orientation. We repeated Ge et al.’s work in this paper and found that an improper dip angle parameter of their 2-D fault model might lead to a wrong conclusion. Both the modeling results based on the 2-D model and 3-D model with proper fault parameters will show Coulomb stress changes alone were neither large enough nor had the correct orientation to affect the occurrence of Wenchuan earthquake, which supports our recent argument based on the local seismicity analysis and the induced Coulomb stress change calculation with a 3-D model. Key wordsWenchuan earthquake-reservoir induced seismicity-Coulomb stress CLC numberP315.72+7
 
The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults’ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.
 
Ionospheric TEC (total electron content) time series are derived from GPS measurements at 13 stations around the epicenter of the 2008 Wenchuan earthquake. Defining anomaly bounds for a sliding window by quartile and 2-standard deviation of TEC values, this paper analyzed the characteristics of ionospheric changes before and after the destructive event. The Neyman-Pearson signal detection method is employed to compute the probabilities of TEC abnormalities. Result shows that one week before the Wenchuan earthquake, ionospheric TEC over the epicenter and its vicinities displays obvious abnormal disturbances, most of which are positive anomalies. The largest TEC abnormal changes appeared on May 9, three days prior to the seismic event. Signal detection shows that the largest possibility of TEC abnormity on May 9 is 50.74%, indicating that ionospheric abnormities three days before the main shock are likely related to the preparation process of the M S8.0 Wenchuan earthquake.
 
The rupture process of the May 12, 2008 M S8.0 Wenchuan earthquake was very complex. To study the rupture zones generated by this earthquake, four dense temporary seismic arrays across the two surface breaking traces of the main-shock were deployed in July and recorded a great amount of aftershocks. This paper focuses on the data interpretation of two arrays across the central main fault, the northern array line 1 and southern array line 3. The fault zone trapped waves recorded by the two arrays were used to study the structure of the central main fault and the difference between the northern and southern portions. The results show that the widths of the rupture zone are about 170–200 m and 200–230 m for northern and southern portions respectively. And the corresponding dip angles are 80° and 70°. The seismic velocity inside the fracture zone is about one half of the host rock. By comparison, the northern portion of the rupture zone is slightly narrower and steeper than the southern portion. Besides these differences, one more interesting and important difference is the positions of the rupture zone with respect to surface breaking traces. At the northern portion, the rupture zone is centered at the surface breaking trace, while at the southern portion it is not but is shifted to the northwest. This difference reflects the difference of rupture behaviors between two portions of the central main fault. The width of the rupture zone is smaller than that of M8.1 Kunlun earthquake though these two earthquakes have almost the same magnitudes. Multiple ruptures may be one factor to cause the narrower rupture zone.
 
Significant postseismic deformation of the 2008 M W7.9 Wenchuan earthquake has been observed from GPS data of the first 14 days after the earthquake. The possible mechanisms for the rapid postseismic deformation are assumed to be afterslip on the earthquake rupture plane and viscoelastic relaxation of coseismiclly stress change in the lower crust or upper mantle. We firstly use the constrained least squares method to find an afterslip model which can fit the GPS data best. The afterslip model can explain near-field data very well but shows considerable discrepancies in fitting far-field data. To estimate the effect due to the viscoelastic relaxation in the lower crust, we then ignore the contribution from the afterslip and attempt to invert the viscosity structure beneath the Longmenshan fault where the Wenchuan earthquake occurred from the postseismic deformation data. For this purpose, we use a viscoelastic model with a 2D geometry based on the geological and seismological observations and the coseismic slip distribution derived from the coseismic GPS and InSAR data. By means of a grid search we find that the optimum viscosity is 9×1018 Pa·s for the middle-lower crust in the Chengdu Basin, 4×1017 Pa·s for the middle-lower crust in the Chuanxi Plateau and 7×1017 Pa·s for the low velocity zone in the Chuanxi plateau. The viscoelastic model explains the postseismic deformation observed in the far-field satisfactorily, but it is considerably worse than the afterslip model in fitting the near-fault data. It suggests therefore a hybrid model including both afterslip and relaxation effects. Since the viscoelastic model produces mainly the far-field surface deformation and has fewer degree of freedoms (three viscosity parameters) than the afterslip model with a huge number of source parameters, we fix the viscositiy structure as obtained before but redetermine the afterslip distribution using the residual data from the viscoelastic modeling. The redetermined afterslip distribution becomes physically more reasonable; it is more localized and exhibits a pattern spatially complementary with the coseismic rupture distribution. We conclude that the aseismic fault slip is responsible for the near-fault postseismic deformation, whereas the viscoelastic stress relaxation might be the major cause for the far-field postseismic deformation.
 
Geographic map of the Longmenshan and western Sichuan basin. The epicenter (white dot) and focal mechanism of the 2008 M 7.9 Wenchuan earthquake are shown together with the three major faults that makes up the Longmenshan fault zone. The regional seismic network of the China Earthquake Administration and local seismic network for reservoir monitoring are shown by red and orange triangles, respectively. Inset in the right bottom corner shows surface motion (in unit of mm/a) of the Indian plate and different blocks within the Tibetan plateau relative to the stable Siberian craton. Inset in the upper left corner shows a schematic cross section of the Longmenshan fault zone. Dot indicates the hypocenter of the M 7.9 earthquake.  
(a) An example of seismograms from the sequence S05 recorded at the YZP station. Each trace was normalized by its maximum amplitude. The lowest panel shows the overlap of all the seismograms. (b) Map view of the relative locations of the 13 events in the S05 multiplet. Three events (#4, #6, #9) have little to no overlap with the other events, and were therefore eliminated from the repeating earthquake sequence. (c) Cumulative slips calculated from the repeating earthquake sequence S05, which have 10 events. Event 1 occurs at time zero and is not shown here.  
Slip rates estimated from the 11 repeating earthquake sequence are shown in a depth section together with inverted coseismic slip of Ji and Hayes (2008). Amplitude of coseismic slip is shown on the right side of the map. Crosses indicate the locations of the 11 sequences with sizes proportional to the estimated slip rates in unit of mm/a.  
Repeating microearthquakes were identified along the edge of the rupture area of the 2008 M W7.9 Wenchuan earthquake. Slip rates at depths derived from seismic moments and recurrence intervals are found to be systematically larger than those observed at surface. This large deep slip rate may explain the odds about the occurrence of this unanticipated event. Our observations here suggested that seismic hazard could be underestimated if surface measurements alone are employed. Key wordsdeep slip rate–repeating microearthquake–Wenchuan earthquake
 
Based on the finite element numerical algorithm, the coseismic displacements of the Wenchuan M S8.0 earthquake are calculated with the rupture slip vectors derived by Ji and Hayes as well as Nishimura and Yaji. Except in a narrow strip around the rupture zone, the coseismic displacements are consistent with those from GPS observation and InSAR interpretation. Numerical results show that rupture slip vectors and elastic properties have profound influences on the surface coseismic deformation. Results from models with different elastic parameters indicate that: ① in homogeneous elastic medium, the surface displacements are weakly dependent on Poisson’s ratio and independent of the elastic modulus; ② in horizontally homogeneous medium with a weak zone at its middle, the thickness of the weak zone plays a significant role on calculating the surface displacements; ③ in horizontally and vertically heterogeneous medium, the surface displacements depend on both Poisson’s ratio and elastic modulus. Calculations of coseismic deformation should take account of the spatial variation of the elastic properties. The misfit of the numerical results with that from the GPS observations in the narrow strip around the rupture zone suggests that a much more complicated rupture model of the Wenchuan earthquake needs to be established in future study.
 
A M W6.4 earthquake occurred in L’Aquila, central Italy at 1:32:42 (UTC), April 6, 2009. We quickly obtained the moment tensor solution of the earthquake by inverting the P waveforms of broadband recordings from the global seismographic network (GSN) stations using the quick technique of moment tensor inversion, and further inferred that the nodal plane of strike 132°, dip 53° and rake −103° is the seismogenic fault.
 
The pioneer study of simulating the wave field in media with irregular interface belongs to Aki and Larner. Since that many numerical methods on the subject have been developed, such as pure numerical techniques, ray method and boundary method. The boundary method based on boundary integral equation is a semi-analytical method which is suitable to modeling wave field induced by irregular border. According to the property of the applied Green’s function the boundary methods can be sorted into space domain boundary method and wavenumber domain boundary method. For both of them it is necessary to solve a large equation, which means much computation is needed. Thus, it is difficult for the boundary methods to be applied in simulating wave field with high frequency or in large range. To develop a new method with less computation is meaningful. For this purpose, localized boundary integral equation, i.e., discrete wavenumber method is proposed. It is rooted in the Bouchon-Campillo method, an important wavenumber domain boundary method. Firstly the force on interface is separated into two parts: one is on flat part and the other on irregular part of the interface. Then Fourier transform is applied to identify their relation, the unknown distributes only on irregular part. Consequently computation efficiency is dramatically improved. Importantly its accuracy is the same as that of Bouchon-Campillo. Key wordsBouchon-Campillo-irregular interface topography-Fourier transform- loBIE-DWM CLC numberP315.9
 
We propose a novel seismic tomography method, Source Side Seismic Tomography (3STomo), which is designed particularly to image the subsurface structure beneath seismically active regions. Unlike the teleseismic tomography, in which the data are relative traveltime residuals between closely spaced stations for each teleseismic event, 3STomo uses relative traveltime shifts between earthquakes within the study region for each distant station. Given the relatively evener distribution of global seismic stations, this method has unique advantages for imaging the structure beneath regions that have numerous earthquakes but lack of dense seismic stations, for example, some subduction zones and spreading ridges in the ocean. In addition, 3STomo has potentially better vertical resolution at shallow depths than the traditional teleseismic tomography. The effect of the inaccurate source parameters on its resolution can be minimized by using depth phases and the technique of joint source and structure inversion. Numerical experiments and application to Luzon Island, Philippines show that 3STomo can be a valuable tool to investigate the subsurface structure beneath some areas where the traditional method cannot be applied to, or at least it can be used as a complementary component of conventional teleseismic tomography to obtain better back-azimuth coverage and achieve higher resolution at shallow depths in the inversion. Key wordsseismic tomography-teleseismic tomography-subduction zone-lithosphere-Luzon Island CLC numberP315.2
 
Map showing the stations of the national and regional networks operated by the China Earthquake Administration (solid triangles). Also shown is the focal mechanism of two deep earthquakes used in this study (beach balls). Contours of upper boundary of the subducting Pacific slab are outlined by dashed gray lines.
Schematic ray paths of the multiple arrivals (a) and the calculated travel times of these arrivals (b). x denotes the distance, which also have the same meaning in Figures 4, 6, 7. A 520 km deep source and the IASP91 model were used in calculating the travel times. The direct AB phase, the reflected BC phase, and the refracted CD phase are shown in solid, dotted, and dashed lines, respectively.
Comparison between observed and synthetic waveforms from the model with a depressed 660-km discontinuity (a) and IASP91 standard model (b).
We examined the spatial variation of velocity structures around the 660-km discontinuity at the western Pacific subduction zones by waveform modeling of triplication data. Data from two deep earthquakes beneath Izu-Bonin and Northeast China are used. Both events were well recorded by a dense broadband seismic network in China (CEArray). The two events are located at approximately the same distance to the CEArray, yet significant differences are observed in their records: (1) the direct arrivals traveling above the 660-km discontinuity (AB branch) are seen in a different distance extent: ∼29° for the NE China event, ∼23° for Izu-Bonin event; (2) the direct (AB) and the refracted waves at the 660-km (CD branch) cross over at 19.5° and 17° for the NE China and the Izu-Bonin event, respectively. The best fitting model for the NE China event has a broad 660-km discontinuity and a constant high velocity layer upon it; while the Izu-Bonin model differs from the standard IASP91 model only with a high velocity layer above the 660-km discontinuity. Variations in velocity models can be roughly explained by subduction geometry. Key wordsP-wave triplication–660-km discontinuity–western Pacific subduction zone
 
P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. No fast (slab) velocity anomalies was not find beneath the 660-km discontinuity through tomography besides a stagnant slab within the transition zone. Slow P-wave velocity anomalies are present at depths of 100–250 km below the active volcanic arc and East Asia. The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China. We suggested that active mantle convection might be operating within this horizontally expanded “mantle wedge” above both the active subducting slabs and the stagnant flat slabs beneath much of the North China plain. Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection. Cold or thermal anomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle. Key wordstomography-stagnant slab-mantle wedge CLC numberP315.2
 
This is the first of two papers that describes a regional tomography investigation, which combines P-wave arrival times of both regional and teleseismic earthquakes to obtain 3D mantle structures of East Asia up to 1 000 km depth. The most important findings of this tomography study are reported in this paper as follows. (1) No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660 km discontinuity; instead the subducted oceanic slabs become flattened and stagnant within the transition zone. (2) The high velocity anomalies in the transition zone extend up to 1 500 km to the westward of the active trenches, which is a unique feature in the worldwide subduction systems. (3) Slow P-wave velocity anomalies are visible up to ∼250 km underneath most of the East Asia on the east of 115°E, similar to the area of the stagnant slabs. These observations have important implications for the geodynamic process at depths beneath the East Asia, which might in turn control the widespread Cenozoic volcanism and associated extensional tectonics seen at the Earth’s surface. Key wordstomography-stagnant slab-mantle wedge CLC numberP315.2
 
According to the least square criterion of minimizing the misfit between modeled and observed data, this paper provides a preconditioned gradient method to invert the visco-acoustic velocity structure on the basis of using sparse matrix LU factorization technique to directly solve the visco-acoustic wave forward problem in space-frequency domain. Numerical results obtained in an inclusion model inversion and a layered homogeneous model inversion demonstrate that different scale media have their own frequency responses, and the strategy of using low-frequency inverted result as the starting model in the high-frequency inversion can greatly reduce the non-uniqueness of their solutions. It can also be observed in the experiments that the fast convergence of the algorithm can be achieved by using diagonal elements of Hessian matrix as the preconditioned operator, which fully incorporates the advantage of quadratic convergence of Gauss-Newton method.
 
The Tibetan plateau as one of the youngest orogen on the Earth was considered as the result of continent-continent collision between the Eurasian and Indian plates. The thickness and structure of the crust beneath Tibetan plateau is essential to understand deformation behavior of the plateau. Active-source seismic profiling is most available geophysical method for imaging the structure of the continental crust. The results from more than 25 active-sources seismic profiles carried out in the past twenty years were reviewed in this article. A preliminary cross crustal pattern of the Tibetan Plateau was presented and discussed. The Moho discontinuity buries at the range of 60–80 km on average and have steep ramps located roughly beneath the sutures that are compatible with the successive stacking/accretion of the former Cenozoic blocks northeastward. The deepest Moho (near 80 km) appears closely near IYS and the crustal scale thrust system beneath southern margin of Tibetan plateau suggests strong dependence on collision and non-distributed deformation there. However, the ∼20 km order of Moho offsets hardly reappears in the inline section across northern Tibetan plateau. Without a universally accepted, convincing dynamic explanation model accommodated the all of the facts seen in controlled seismic sections, but vertical thickening and northeastern shorten of the crust is quite evident and interpretable to a certain extent as the result of continent-continent collision. Simultaneously, weak geophysical signature of the BNS suggests that convergence has been accommodated perhaps partially through pure-shear thickening accompanied by removal of lower crustal material by lateral escape. Recent years the result of Moho with ∼7 km offset and long extend in south-dip angle beneath the east Kunlun orogen and a grand thrust fault at the northern margin of Qilian orogen has attract more attention to action from the northern blocks. The broad lower-velocity area in the upper-middle crust of the Lhasa block was once considered as resulted from partially melted rocks. However the low normal ν P/ν S ratio and the Moho stepwise rise fail to support significant partial melting in the middle-lower crust of the central-northern Tibetan plateau. Furthermore, the lower-velocity of crust occasionally disappears, and/or local thinned exhibits their non-stationary spatial distribution.
 
The transverse density structure of the crust and mantle beneath the Dagze-Deqen-Domar profile using a joint gravity-seismic inversion technique was studied in order to obtain the Moho and the asthenospheric configuration beneath the profile and understand the deep dynamics mechanism of the Yadong-Gulu rift. The mutual technique is a good tool for fitting gravity anomaly by polygon within space sections, which is a method of qualitative and half-quantitative analysis. The deep reflection seismic profile along the Yadong-Gulu rift shows that there is a low velocity zone under the rift. The top boundary of the asthenosphere protrudes upward, like an anisomerous anticline. The smallest depth of the asthenosphere is 110 km and it increases rapidly southward and slowly northward. The research results show that the rigid Indian lithosphere, the weak lower crust and anisotropic mantle of Tibet make it possible that the Indian lithospheric mantle subducted far away from south to north and the Tibet mantle detached from the crust.
 
This paper studies the relations between the great Wenchuan earthquake and the active-quiet periodic characteristics of strong earthquakes, the rhythmic feature of great earthquakes, and the grouped spatial distribution of M S8.0 earthquakes in Chinese mainland. We also studied the relation between the Wenchuan earthquake and the stepwise migration characteristics of M S≥7.0 earthquakes on the North-South seismic belt, the features of the energy releasing acceleration in the active crustal blocks related to the Wenchuan earthquake and the relation between the Wenchuan earthquake and the so called second-arc fault zone. The results can be summarized as follows: ➀ the occurrence of the Wenchuan earthquake was consistent with the active-quiet periodic characteristics of strong earthquakes; ➁ its occurrence is consistent with the features of grouped occurrence of M S8.0 earthquakes and follows the 25 years rhythm (each circulation experiences the same time) of great earthquakes; ➂ the Wenchuan M S8.0 earthquake follows the well known stepwise migration feature of strong earthquakes on the North-South seismic belt; ➃ the location where the Wenchuan M S8.0 earthquake took place has an obvious consistency with the temporal and spatial characteristic of grouped activity of M S≥7.0 strong earthquakes on the second-arc fault zone; ➄ the second-arc fault zone is not only the lower boundary for earthquakes with more than 30 km focal depth, but also looks like a lower boundary for deep substance movement; and ➅ there are obvious seismic accelerations nearby the Qaidam and Qiangtang active crustal blocks (the northern and southern neighbors of the Bayan Har active block, respectively), which agrees with the GPS observation data.
 
The instruments for Wenchuan aftershock observation 
Mobile stations in Wenchuan earthquake
In this paper, the mobile strong ground motion observation for the destructive earthquake is introduced. Considering the characteristics and its spatial distributions of aftershock, 59 strong ground motion instruments were installed along the Longmenshan fault area, and more than 2 000 records have been accumulated. It shows that it is necessary to perform the mobile strong ground motion observation after the destructive earthquake, and the precious collected data could be applied for further research.
 
Coseismic water level changes which may have been induced by the Wenchuan M S8.0 earthquake and its 15 larger aftershocks (M S≥5.4) have been observed at Tangshan well. We analyze the correlation between coseismic parameters (maximum amplitude, duration, coseismic step and the time when the coseismic reach its maximum amplitude) and earthquake parameters (magnitude, well-epicenter distance and depth), and then compare the time when the coseismic oscillation reaches its maximum amplitude with the seismogram from Douhe seismic station which is about 16.3 km away from Tangshan well. The analysis indicates that magnitude is the main factor influencing the induced coseismic water level changes, and that the well-epicenter distance and depth have less influence. M S magnitude has the strongest correlation with the coseismic water level changes comparing to M W and M L magnitudes. There exists strong correlation between the maximum amplitude, step size and the oscillation duration. The water level oscillation and step are both caused by dynamic strain sourcing from seismic waves. Most of the times when the oscillations reach their maximum amplitudes are between S and Rayleigh waves. The coseismic water level changes are due to the co-effect of seismic waves and hydro-geological environments.
 
Based on Gutenberg-Richter’s relation, Bath’s law, Omori’s law and Well’s relation of rupture scale, this paper forecasts the temporal decay, total number, possible area and greatest magnitude of strong aftershocks (greater than or equal to M6.0) of the M S8.0 Wenchuan earthquake by using the magnitude and statistical parameters of earthquakes in California area of USA. The number of strong aftershocks, the parameters of Gutenberg-Richter’s relation and the modified form of Omori’s law are validated based on the relocation data of aftershock sequence of the M S8.0 Wenchuan earthquake. Moreover, the spatio-temporal characteristics and wave energy release of the strong aftershocks (M≥6.0) are analyzed. The result shows that strong aftershocks may occur at the end of local drop and sharp drop on the wave energy release curve. Key wordsWenchuan earthquake-strong aftershock-wave energy release-statistic of aftershocks CLC numberP315.3
 
Distribution of epicenters of earthquake sequences before relocation and seismic stations in Longmenshan region. Faults are based on Deng et al. (1994). The position of the main shock is indicated by blue circle. The red dots represent aftershock sequence. Triangles represent temporary seismic stations and the squares are fixed seismic stations. F31. Dida-Longdong fault; F32. Wenchuan- Maoxian fault; F33. Qingchuan-Pingwu fault; F21. Yanjing-Wulong fault; F22. Beichuan-Yingxiu fault; F23. Chaba-Linansi fault; F11. Dachuan-Shuangshi fault; F12. Guanxian-Jiangyou fault; F13. Jiangyou- Guangyou fault; F4. Miyaluo fault.  
Topographic map of Longmenshan region and the relocation of Wenchuan earthquake sequence. (a) Distribution of all earthquake sequences. (b) Distribution of the main shock and 5.0≤M ≤6.9 aftershocks. Blue circle is the position of the main shock. Yellow circle represent the position of 6.0≤M ≤6.9 aftershock. Green circle represent the position of 5.0≤M ≤5.9 aftershock. Red circle represent the position of 2.0≤M ≤4.9.  
The imbricate thrust structure inferred from this study (a), Chen et al. (2009)(b), Zhu et al. (2008) (c) and Huang et al. (2008) (d).  
From 14:28 (GMT+8) on May 12th, 2008, the origin time of M S 8.0 Wenchuan earthquake, to December 31th, 2008, more than 10 000 aftershocks (M≥2.0) had been recorded by the seismic networks in Sichuan and surrounding areas. Using double difference algorithm, the main shock and more than 7 000 aftershocks were relocated. The aftershocks distribute about 350 km long. The depths of aftershocks are mainly between 10 km and 20 km. The average depth of aftershocks is about 13 km after relocation. In the southwest, the distribution of aftershocks is along the back-range fault, the central-range fault and the front-range fault of Longmenshan faults. In the middle, the distribution of aftershocks is along the central-range fault. In the north, aftershocks are relocated along the Qingchuan-Pingwu fault. Relocations suggest that the back-range fault mainly induced and controlled the aftershock occurrence in the northern section of aftershocks sequence. The M S 8.0 main shock is between central-range and front-range of Longmenshan faults and is near the shear plane of the fault bottom. From the depth distribution of aftershock sequence, it suggests that these three faults show imbricate thrust structure.
 
Diffraction of plane P waves around an alluvial valley of arbitrary shape in poroelastic half-space is investigated by using an indirect boundary integral equation method. Based on the Green’s functions of line source in poroelastic half-space, the scattered waves are constructed using the fictitious wave sources close to the interface of the valley and the density of fictitious wave sources are determined by boundary conditions. The precision of the method is verified by the satisfaction extent of boundary conditions, and the comparison between the degenerated solutions and available results in single-phase case. Finally, the nature of diffraction of plane P waves around an alluvial valley in poroelastic half-space is investigated in detail through numerical examples.
 
In the present analysis on the relationships among the depth of lithosphere brittle fracture, seismotectonics and geothermal anomalous active in Tibetan plateau were investigated using the seismic dada from ISC and Chinese seismic net and geothermal data. The results suggest that the region of anomalously geothermal activity almost coincides with that of the normal faulting type earthquake. The geothermal anomaly activity region coincides spatially with that of the events deeper than 60 km as well as. The normal faulting earthquakes may be mainly tectonic activity regimes until 110 km deep in the thermal anomaly region. The strike directions of events are likely the N-S direction, coinciding with the strike of the thermal anomaly active belts. The earthquakes align along the normal faults and faulted-depression zone with the N-S direction. The thermal anomaly activity also distributes along the faulted-depression zone. Many events deeper than 60 km exist in the anomalously geothermal activity region in the plateau. Events extend to bottom of the lithosphere of 110 km from the surface, like columnar seismic crowd. The lithosphere extends along the E-W direction due to the E-W extensional stress in the central and southern Tibetan plateau, altitude of the plateau. The tensional stress in the E-W results in the lithosphere fractures and the normal faults striking N-S direction, grabens and faulted-depression zones. Thermal material from the asthenosphere wells upward to the surface along deep seismic fractures and faults through the thick crust. The anomalously thermal activities are attributable to the upwelling thermal material from the mantle in the altitude of Tibetan plateau.
 
We discuss two array-based tomography methods, ambient noise tomography (ANT) and two-planewave earthquake tomography (TPWT), which are capable of taking advantage of emerging large-scale broadband seismic arrays to generate high resolution phase velocity maps, but in complementary period band: ANT at 8–40 s and TPWT at 25–100 s period. Combining these two methods generates surface wave dispersion maps from 8 to 100 s periods, which can be used to construct a 3D v S model from the surface to ∼200 km depth. As an illustration, we apply the two methods to the USArray/Transportable Array. We process seismic noise data from over 1 500 stations obtained from 2005 through 2009 to produce Rayleigh wave phase velocity maps from 8 to 40 s period, and also perform TPWT using ∼450 teleseismic earthquakes to obtain phase velocity maps between 25 and 100 s period. Combining dispersion maps from ANT and TPWT, we construct a 3D v S model from the surface to a depth of 160 km in the western and central USA. These surface wave tomography methods can also be applied to other rapidly growing seismic networks such as those in China.
 
The method of extracting Green’s function between stations from cross correlation has proven to be effective theoretically and experimentally. It has been widely applied to surface wave tomography of the crust and upmost mantle. However, there are still controversies about why this method works. Snieder employed stationary phase approximation in evaluating contribution to cross correlation function from scatterers in the whole space, and concluded that it is the constructive interference of waves emitted by the scatterers near the receiver line that leads to the emergence of Green’s function. His derivation demonstrates that cross correlation function is just the convolution of noise power spectrum and the Green’s function. However, his derivation ignores influence from the two stationary points at infinities, therefore it may fail when attenuation is absent. In order to obtain accurate noise-correlation function due to scatters over the whole space, we compute the total contribution with numerical integration in polar coordinates. Our numerical computation of cross correlation function indicates that the incomplete stationary phase approximation introduces remarkable errors to the cross correlation function, in both amplitude and phase, when the frequency is low with reasonable quality factor Q. Our results argue that the distance between stations has to be beyond several wavelengths in order to reduce the influence of this inaccuracy on the applications of ambient noise method, and only the station pairs whose distances are above several (>5) wavelengths can be used. Key wordsambient seismic noise-stationary phase approximation-Green’s function CLC numberP315.01
 
We present a new approach to polarization analysis of seismic noise recorded by three-component seismometers. It is based on statistical analysis of frequency-dependent particle motion properties determined from a large number of time windows via eigenanalysis of the 3-by-3, Hermitian, spectral covariance matrix. We applied the algorithm to continuous data recorded in 2009 by the seismic station SLM, located in central North America. A rich variety of noise sources was observed. At low frequencies (<0.05 Hz) we observed a tilt-related signal that showed some elliptical motion in the horizontal plane. In the microseism band of 0.05–0.25 Hz, we observed Rayleigh energy arriving from the northeast, but with three distinct peaks instead of the classic single and double frequency peaks. At intermediate frequencies of 0.5–2.0 Hz, the noise was dominated by non-fundamental-mode Rayleigh energy, most likely P and Lg waves. At the highest frequencies (>3 Hz), Rayleigh-type energy was again dominant in the form of Rg waves created by nearby cultural activities. Analysis of the time dependence of noise power shows that a frequency range of at least 0.02–1.0 Hz (much larger than the microseism band) is sensitive to annual, meteorologically induced sources of noise. Key wordsseismic noise-polarization analysis-central North America CLC numberP315.3
 
(a) Plot of dispersion measurements for different periods used in the inversion; (b) Ray path coverage at period T =15 s; (c) Ray path coverage at T =30 s.
Estimated Love wave group velocity maps at periods of 8 (a), 15 (b), 24 (c) and 30 s (d). Period is indicated in the lower left corner of each map.
We estimate Love wave empirical Green’s functions from cross-correlations of ambient seismic noise to study the crust and uppermost mantle structure in Italy. Transverse-component ambient noise data from October 2005 through March 2007 recorded at 114 seismic stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broadband network, the Mediterranean Very Broadband Seismographic Network (MedNet) and the Austrian Central Institute for Meteorology and Geodynamics (ZAMG) yield more than 2 000 Love wave group velocity measurements using the multiple-filter analysis technique. In the short period band (5–20 s), the cross-correlations show clearly one-sided asymmetric feature due to non-uniform noise distribution and high local activities, and in the long period band (>20 s) this feature becomes weak owing to more diffusive noise distribution. Based on these measurements, Love wave group velocity dispersion maps in the 8–34 s period band are constructed, then the SH wave velocity structures from the Love wave dispersions are inverted. The final results obtained from Love wave data are overall in good agreement with those from Rayleigh waves. Both Love and Rayleigh wave inversions all reveal that the Po plain basin is resolved with low velocity at shallow depth, and the Tyrrhenian sea is characterized with higher velocity below 8 km due to its thin oceanic crust. Key wordsambient noise-Love wave-tomography-crustal structure-Italy CLC numberP315.3
 
Ray path coverages (top) and resolution maps (bottom) at periods (T ) of 16 and 30 s. Resolution is presented in units of km and is defined as twice the standard deviation of a 2-D Gaussian fit to the resolution surface at each geographic node (e.g., Barmin et al., 2001).
Velocity maps at depths of 5 (a), 10 (b), 20 (c), 30 (d), 40 (e), and 50 km (f) respectively. Shear wave velocity are plotted as perturbations relative to the average value at each depth.
We apply ambient noise tomography to significant seismic data resources in a region including the northeastern Tibetan plateau, the Ordos block and the Sichuan basin. The seismic data come from about 160 stations of the provincial broadband digital seismograph networks of China. Ambient noise cross-correlations are performed on the data recorded between 2007 and 2009 and high quality inter-station Rayleigh phase velocity dispersion curves are obtained between periods of 6 s to 35 s. Resulting Rayleigh wave phase velocity maps possess a lateral resolution between 100 km and 200 km. The phase velocities at short periods (<20 s) are lower in the Sichuan basin, the northwest segment of the Ordos block and the Weihe graben, and outline sedimentary deposits. At intermediate and long periods (>25 s), strong high velocity anomalies are observed within the Ordos block and the Sichuan basin and low phase velocities are imaged in the northeastern Tibetan plateau, reflecting the variation of crustal thickness from the Tibetan plateau to the neighboring regions in the east. Crustal and uppermost mantle shear wave velocities vary strongly between the Tibetan plateau, the Sichuan basin and the Ordos block. The Ordos block and the Sichuan basin are dominated by high shear wave velocities in the crust and uppermost mantle. There is a triangle-shaped low velocity zone located in the northeastern Tibetan plateau, whose width narrows towards the eastern margin of the plateau. No low velocity zone is apparent beneath the Qinling orogen, suggesting that mass may not be able to flow eastward through the boundary between the Ordos block and the Sichuan basin in the crust and uppermost mantle. Key wordsphase velocity-Ordos block-ambient noise tomography-crustal structure CLC numberP315.2
 
We collected continuous noise waveform data from January 2007 to February 2008 recorded by 190 broadband and 10 very broadband stations of the North China Seismic Array. The study region is divided into grid with interval 0.25°×0.25°, and group velocity distribution maps between 4 s and 30 s are obtained using ambient noise tomography method. The lateral resolution is estimated to be 20–50 km for most of the study area. We construct a 3-D S wave velocity model by inverting the pure path dispersion curve at each grid using a genetic algorithm with smoothing constraint. The crustal structure observed in the model includes sedimentary basins such as North China basin, Yanqing-Huailai basin and Datong basin. A well-defined low velocity zone is observed in the Beijing-Tianjin-Tangshan region in 22–30 km depth range, which may be related to the upwelling of hot mantle material. The high velocity zone near Datong, Shuozhou and Qingshuihe within the depth range of 1–23 km reveals stable characteristics of Ordos block. The Taihangshan front fault extends to 12 km depth at least. Key wordsseismic noise-surface wave tomography-velocity structure-genetic algorithm-North China CLC numberP315.2
 
Long-time cross correlation of ambient noise has been proved as a powerful tool to extract Green’s function between two receivers. The study of composition of ambient noise is important for a better understanding of this method. Previous studies confirm that ambient noise in the long period (3 s and longer) mostly consists of surface wave, and 0.25–2.5 s noise consists more of body waves. In this paper, we perform cross correlation processing at much higher frequency (30–70 Hz) using ambient noise recorded by a small aperture array. No surface waves emerge from noise correlation function (NCF), but weak P waves emerge. The absence of surface wave in NCF is not due to high attenuation since surface waves are strong from active source, therefore probably the high ambient noise mostly consists of body wave and lacks surface wave. Origin of such high frequency body waves in ambient noise remains to be studied. Key wordsambient noise-cross correlation-Green’s function-body wave-high frequency CLC numberP315.3
 
We have developed a new stacking technique in ambient noise tomography to obtain high-quality dispersion curves of Rayleigh waves. This technique is used to stack the vertical components of the Estimated Green Functions (EGFs) obtained respectively from cross correlation of the ambient noise data recorded by a remote seismic station and one of the short distance seismic stations of a seismic array. It is based on a phase-matched filter and is implemented by a four-step iterative process: signal compression, stacking, signal extraction and signal decompression. The iterative process ends and gives the dispersion curve of Rayleigh wave when the predicted one and the processing result converge. We have tested the method using the vertical components of synthetic Rayleigh wave records. Results show that this new stacking method is stable and it can improve the quality of dispersion curves. In addition, we have applied this method to real data. We see that the results given by our new technique are obviously better than the ones employing the traditional method which is a three-step process: signal compression, signal extraction and signal decompression. In conclusion, the new method proposed in this paper can improve the signal to noise ratio of EGFs, and can therefore potentially improve the resolution of ambient noise tomography. Key wordsseismotectonics-continental dynamics-ambient noise tomography-array technique-stacking CLC numberP315.3+1
 
Vertical cross sections of the shear wave velocity model. White lines represent the crustal thickness from the Crust2.0 model (http://igppweb.ucsd.edu/~gabi/rem.html). Locations of profiles AA′, BB′ and CC′ in first row are shown in Figure 1. The center and bottom rows are along different latitude (from left to right, 30°N, 31°N, and 34°N) and longitude (from left to right, 86°E, 95°E, and 101°E), respectively. Images are clipped outside the resolution boundary as shown in Figures 4 and 5. Mid-lower crustal low velocity layers at different regions in the Tibetan plateau are seen to be diverse in geometry, depth distribution, and in the intensity of the velocity reduction.  
Averaged shear wave velocity profiles for the major blocks in Figure 8.  
Resolution test of low velocity regions in the crust and uppermost mantle. The input is a uniform model with two low velocity layers underneath Tibet region (latitude 27°N to 38°N, longitude 80°E to 102°E). One layer is in the mid-lower crust (25 km to 45 km depth) and one in the upper mantle (100 km to 140 km depth), with velocity reductions of 10%. The top row shows cross sections of input (left) and retrieved models (right two) along latitudes 30°N and 35°N, respectively. The bottom row shows input (left) and retrieved models (right two) along longitudes 90°E and 95°E, respectively. The horizontal axes in the top and bottom rows are longitude and latitude, respectively.  
We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green’s functions obtained from seismic ambient noise cross-correlation. The data we use are from the China National Seismic Network, global and regional networks and PASSCAL stations in the region. We first acquire cross-correlation seismograms between all possible station pairs. We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis method from 8 s to 60 s. After that, Rayleigh wave group and phase velocity dispersion maps on 1° by 1° spatial grids are obtained at different periods. Finally, we invert these maps for the 3-D shear wave velocity structure of the crust and upper mantle beneath China at each grid node. The inversion results show large-scale structures that correlate well with surface geology. Near the surface, velocities in major basins are anomalously slow, consistent with the thick sediments. East-west contrasts are striking in Moho depth. There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau (TP) and Tianshan (Junggar, Tarim, Ordos, and Sichuan). These strong blocks, therefore, appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape. In northwest TP in Qiangtang, slow anomalies extend from the crust to the mantle lithosphere. Meanwhile, widespread, a prominent low-velocity zone is observed in the middle crust beneath most of the central, eastern and southeastern Tibetan plateau, consistent with a weak (and perhaps mobile) middle crust. Key wordsambient noise-surface wave-tomography-crust and upper mantle-China CLC numberP315.3
 
Not too long ago, seismic imaging of the Earth’s interior relied almost exclusively on the illumination from energetic sources, such as earthquakes or artificial sources. However, recent theoretical and laboratory studies have shown that the impulse response (Green’s function) of a structure between two receivers can be obtained from the cross-correlation of “random” noise wavefields recorded at the two receivers. The basic idea is that a random field contains coherent signals traveling between the receivers, which can be stacked and amplified while all other arrivals are canceled out in the cross-correlation. The idea has now found rapid applications in seismology. In particular, surface waves have been found to be most easily retrievable from the cross-correlations of the ambient seismic noise in the Earth. The new area of research is expanding rapidly and has quickly become an important branch of seismological research. This field, which I use a general term ambient noise seismology here, is known as Green’s function retrieval and is often called seismic interferometry in the exploration geophysics community (because of the cross-correlation involved). Major developments include new theories, new data processing techniques, imaging of Earth structures from ambient noise tomography, monitoring of subsurface processes, and characteristics of ambient noise sources. The special issue contains 13 papers on this timely topic from researchers from China, France, Germany, and the U.S.. We hope that the collection can serve as a small window into this exciting field for the vast readership of the Chinese seismological community, while promoting scientific exchanges and communication between researchers in China and western countries. For readers who are interested in learning more about the ∗ Received 22 September 2010; accepted in revised form 23 September 2010;
 
The relative amplitude method (RAM) is more suitable for source inversion of low magnitude earthquakes because it avoids the modeling of short-period waveforms. We introduced an improved relative amplitude method (IRAM) which is more robust in practical cases. The IRAM uses a certain function to quantify the fitness between the observed and the predicted relative amplitudes among direct P wave, surface reflected pP and sP waves for a given focal mechanism. Using the IRAM, we got the fault-plane solutions of two earthquakes of m b4.9 and m b3.8, occurred in Issyk-Kul lake, Kyrgyzstan. For the larger event, its fault-plane solutions are consistent with the Harvard’s CMT solutions. As to the smaller one, the strikes of the solution are consistent with those of the main faults near the epicenter. The synthetic long period waveforms and the predicted P wave first motions of the solutions are consistent with observations at some of regional stations. Finally, we demonstrated that fault-solutions cannot interpret the characteristics of teleseismic P waveforms of the underground nuclear explosion detonated in Democratic People’s Republic of Korea (DPRK) on October 9, 2006.
 
We have updated the lateral variations of the quality factor Q 0 (Q at 1 Hz) beneath the crust of North China using M L amplitude tomography with near three times data. The data were selected from the Annual Bulletin of Chinese Earthquakes (ABCE) in 1985–2009, including 26 283 M L amplitude readings from 4 204 events recorded by 38 stations. The result is similar with previous research but has higher resolution. Estimated Q 0 values are consistent with tectonic and topographic structure in North China. Q 0 is low in the active tectonic regions having many faults, such as Bohai bay, North China basin, the Shanxi and Yinchuan grabens, while it is high in the stable Ordos craton. Q 0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, and Subei-Huanghai Sea basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinling and Dabie mountains, Luxi and Jiaoliao uplifts. Quality factor estimates are also consistent with Pn and Sn velocity patterns. High velocity values in general correspond with high Q 0 and vice versa. This coincides with a common temperature influence in the crust and uppermost mantle.
 
Two rupture zones of Wenchuan earthquake have been successfully located by an offset-tracking procedure of ENVISAT ASAR amplitude images. Accuracy of offset-tracking strongly depends on the quality of image coregistration. In order to remove noise caused by coregistration, a combined filter method is proposed to remove noise of estimated offsets. In addition, Kriging interpolation is applied to smooth the azimuth displacement offset map. The displacement offset maps of azimuth and range show that about 245 km and 60 km in length along Beichuan-Yingxiu fault and Guanxian-Jiangyou fault were ruptured by Wenchuan earthquake, respectively. The result of offset-tracking is validated by field observations of China Earthquake Administration. Key wordsWenchuan earthquake-offset-tracking-ENVISAT ASAR-InSAR CLC numberP315.5
 
Top-cited authors
Sergey Alexander Pulinets
  • Space Research Institute
Patrick Timothy Taylor
Dmitry Davidenko
  • RSC Energia
Dimitar Ouzounov
  • Chapman University
Menas Kafatos
  • Chapman University