An insight into crack density,saturation rate,and porosity model of the 2001 Bhuj earthquake in the stable continental region of western India

The 2001 Bhuj earthquake (Mw 7.6) source zone is examined in the light of crack density (ε), saturation rate (ξ) and porosity parameter (ψ) using new data set derived from a large aftershock sequence recorded by the Gujarat seismic network (GSNet) during November, 2006–December, 2009. Processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset. Pattern of crustal heterogeneities associated with high-ε, high-ξ and high-ψ anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by Mishra and Zhao (2003). The anomalous zone is found extended distinctly by 50–60 km in the lateral direction, indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 Bhuj earthquake in the source area. It is inferred that the presence of a fluid-filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone. Anomalous pattern of high-ε with wider distribution of high-ξ indicates the existence of micro-cracks in the lower crust, while high-ψ suggests the cementation of cracks through permeation of residual magma/metamorphic fluids into the hypocenter zone. The results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences.     

Seismic imaging,crustal stress and GPS data analyses: Implications for the generation of the 2008 Wenchuan Earthquake (M7.9), China

Seismic imaging together with global positioning system (GPS) and crustal stress data analyses show that the Mw7.9 2008 Wenchuan earthquake occurred within a distinct area of high crustal stress (～ 17.5 MPa) and high Poisson's ratio (7–10%) anomalies centered on the Longmen-Shan (Shan means Mountain in Chinese) tectonic fault belt. Low P-wave and S-wave velocities in the southwest (SW) segment contrast with high-velocity anomalies in the central portion (CP) and northeast (NE) segment within the uppermost ～ 15 km depths along the tectonic fault belt, though a presumably ductile zone with low-velocity anomalies separates the CP and NE segment. The rupture initiated near the southwestern end of the CP at a zone of high Poisson's ratio (σ) which extends down into the lower crust. These low-velocity and high-σ anomalies immediately below the source hypocenter, together with the high crustal stress, indicate the presence of high-pressure fluids from the lower crust, which might have reduced the mechanical strength of the fractured rock matrix and triggered the earthquake. Our study suggests that the structural heterogeneity and high crustal stress played an important role in the nucleation of the Wenchuan earthquake and its rupture process.     

Seismogenesis of clustered seismicity beneath the Kangra–Chamba sector of northwest Himalaya: Constraints from 3D local earthquake tomography

To investigate subsurface structure and seismogenic layers, 3D velocity inversion was carried out in the source zone of 1905 Kangra earthquake (M8.0) in the northwestern Himalaya. P-wave and S-wave phase data of 159 earthquakes recorded by a network of 21 stations were used for this purpose. Inverted velocity tomograms up to a depth range of 18 km show significant variations of 14% in Vp and Vs and 6% in the Vp/Vs across the major tectonic zones in the region. Synthesis of seismicity pattern, velocity structure, distinctive focal mechanisms coupled with nature of stress distribution allows mapping of three different source regions that control regional seismotectonics. Accumulating strains are partly consumed by sliding of Chamba Nappe to the southwest through reverse-fault movements along Chamba/Panjal/Main Boundary Thrusts. This coupled with normal-fault type displacements along Chenab Normal Fault in the north account for low magnitude widespread seismicity in upper 8–10 km of the crust. At intermediate depths from 8 to 15 km, adjusting to residual compressive stresses, the detachment or lower end of the MBT slips to produce thrust dominated seismicity. Nucleation of secondary stresses in local NE–SW oriented structure interacts in complex manner with regional stresses to generate normal type earthquakes below the plane of detachment and therefore three seismic regimes at different depths produce intense seismicity in a block of 30 × 30 km² centered NE to the epicenter of Kangra earthquake.     

Crustal structure in an onshore–offshore transitional zone near Hong Kong,northern South China Sea

To study the crustal structure beneath the onshore–offshore transitional zone, a wide-angle onshore–offshore seismic experiment was carried out in northern South China Sea near Hong Kong, using large volume airgun sources at sea and seismic stations on land. The crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the Dangan Islands based on the characteristics of Pg and Multiple Pg, and the crustal structure beneath the sedimentary layer is relatively simple. The Moho depth is about 25–28 km along the profile and the P-wave velocity increases gradually with depth. The velocities in the upper crust range from 5.5 to 6.4 km/s, while that in the lower crust is 6.4–6.9 km/s. It also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75–90 km distance, which suggests that the Littoral Fault Zone (LFZ) exists beneath the onshore–offshore transitional zone. The magnetism anomalies, bouguer gravity anomalies and active seismic zone along the coastline imply the LFZ is a main tectonic fault in the onshore–offshore area. Combined with two previously published profiles in the continental South China (L–G profile) and in the northern margin of South China Sea (OBS1993) respectively, we constructed a land-sea super cross-section about 1000 km long. The results show the onshore–offshore transitional zone is a border separating the unstretched and the stretched continental crust. The low velocity layer (LVL) in the middle crust was imaged along L–G profile. However, the high velocity layer (HVL) in the lower crust was detected along OBS1993. By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the LFZ could be the boundary fault between them.     

Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins

We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (P_n and S_n). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (P_cc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average P_n velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.     

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38 km thick crust beneath the Shillong Plateau which increases to ∼37–38 km beneath the Brahmaputra valley and ∼46–48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8 km along with a low velocity zone within ∼8–13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18 km and low velocity zone within ∼18–36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30⁰ toward northwest.     

Surface deformation related to the 2008 Wenchuan earthquake,and mountain building of the Longmen Shan,eastern Tibetan Plateau

The 12 May 2008 M_s 8.0 Wenchuan earthquake, China, was one of largest continental thrusting events worldwide. Based on interpretations of post-earthquake high-resolution remote sensing images and field surveys, we investigated the geometry, geomorphology, and kinematics of co-seismic surface ruptures, as well as seismic and geologic hazards along the Longmen Shan fold-and-thrust belt. Our results indicate that the Wenchuan earthquake occurred along the NE–SW-trending Yingxiu–Beichuan and Guanxian–Anxian faults in the Longmen Shan fold-and-thrust belt. The main surface rupture zones along the Yingxiu–Beichuan and Guanxian–Anxian fault zones are approximately 235 and 72 km in length, respectively. These sub-parallel ruptures may merge at depth. The Yingxiu–Donghekou surface rupture zone can be divided into four segments separated by discontinuities that appear as step-overs or bends in map view. Surface deformation is characterized by oblique reverse faulting with a maximum vertical displacement of approximately 10 m in areas around Beichuan County. Earthquake-related disasters (e.g., landslides) are linearly distributed along the surface rupture zones and associated river valleys.The Wenchuan earthquake provides new insights into the nature of mountain building within the Longmen Shan, eastern Tibetan Plateau. The total crustal shortening accommodated by this great earthquake was as much as 8.5 m, with a maximum vertical uplift of approximately 10 m. The present results suggest that ongoing mountain building of the Longmen Shan is driven mainly by crustal shortening and uplift related to repeated large seismic events such as the 2008 Wenchuan earthquake. Furthermore, rapid erosion within the Longmen Shan fold-and-thrust belt occurs along deep valleys and rupture zones following the occurrence of large-scale landslides triggered by earthquakes. Consequently, we suggest that crustal shortening related to repeated great seismic events, together with isostatic rebound induced by rapid erosion-related unloading, is a key component of the geodynamics that drive ongoing mountain building on the eastern Tibetan Plateau.     

Crustal and lithospheric thinning beneath the seismogenic Kachchh rift zone,Gujarat (India): Its implications toward the generation of the 2001 Bhuj earthquake sequence

A high-resolution passive seismic experiment in the Kachchh rift zone of the western India has produced an excellent dataset of several thousands teleseismic events. From this network, 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions (for the 30–310° back-azimuth ranges), which show a positive phase at 4.5–6.1 s delay time and a strong negative phase at 8.0–11.0 s. These phases have been modeled by a velocity increase at Moho (i.e. 34–43 km) and a velocity decrease at 62–92 km depth. The estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3–7 km in crustal thickness and 6–14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un-rifted parts of the Kachchh rift zone. On an average, the Kachchh region is characterized by a thin lithosphere of 75.9 ± 5.9 km. The marked velocity decrease associated with the lithosphere–asthenoshere boundary (LAB), observed over an area of 120 km × 80 km, and the isotropic study of xenoliths from Kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of CO₂ rich lherzolite beneath the Kachchh seismic zone that might have caused by rifting episode (at 88 Ma) and the associated Deccan thermal-plume interaction (at 65 Ma) episodes. Thus, the coincidence of the area of the major aftershock activity and the Moho as well as asthenospheric upwarping beneath the central Kachchh rift zone suggests that these pockets of CO₂-rich lherzolite partial melts could perhaps provide a high input of volatiles containing CO₂ into the lower crust, which might contribute significantly in the seismo-genesis of continued aftershock activity in the region. It is also inferred that large stresses in the denser and stronger lower crust (at 14–34 km depths) induced by ongoing Banni upliftment, crustal intrusive, marked lateral variation in crustal thickness and related sub-crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the Kachchh seismic zone.     

Deep electrical structure over the igneous arc of the Indo Burman Orogen in Sagaing province,Myanmar from magnetotelluric studies

Magnetotelluric studies over the igneous arc of the Indo Burman range in the Sagaing province of Myanmar have delineated the high resistivity Indian plate subducting westwards beneath the Burmese block to depths of 30 km and beyond. The thick moderately resistive (20–100 Ω m) layer overlying the subducting Indian plate may be due to the low resistivity sediments. The entire region is covered with prominent sedimentary layer with a conductance varying between 20 and 3000 S showing a general increase from the east to west, suggesting that their thickness increases toward the west. The large unsystematic variations in the conductance are indicative of the widely varying depositional environments and also possible vertical block movements during the course of their deposition. A west dipping low resistivity zone to the east of Burmese block seems to demarcate its eastern limit, suggesting the possibility of a hitherto unknown deep seated fault, which is also supported by the several earthquake foci located over this zone. The nature of the crustal movements over this fault is not immediately apparent. Possibility exists that the Sagaing fault is an en echelon fault and the present feature observed here is a part of this en echelon fault. The possibility of channel flows of the weakened rocks in the deep crust observed in the vicinity of the eastern Himalayan syntaxis may also cause such low resistivity zones.     

40Ar/39Ar age pattern associated with differential uplift along the Eastern Highlands shear zone,Cape Breton Island,Canadian Appalachians

The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende ⁴⁰Ar/³⁹Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The ⁴⁰Ar/³⁹Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The ⁴⁰Ar/³⁹Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.     

Crustal heterogeneities beneath the 2011 Talala,Saurashtra earthquake,Gujarat, India source zone: Seismological evidence for neo-tectonics

During the 1st decade of the 21st century, the study area of Talala, Saurashtra of western India witnessed three damaging earthquakes of moderate magnitude, year 2007 [Mw 5.0; Mw 4.8] and in the year 2011 [Mw 5.1] that generated public panic in the region. The last damaging moderate earthquake of the 20th October 2011 in Talala region (21.09°N;70.45°E), located at about 200 km south to the devastating 2001 Bhuj (23.412°N, 70.232°E) mainshock (Mw 7.6), jolted the entire Saurashtra region of Gujarat. A long series of aftershocks followed hereafter, recorded at nine seismograph/accelerograph stations. Hypocenters of aftershocks were relocated accurately using absolute and relative travel time (double-difference) method. In this study, we, for the first time, determined 3-D tomographic images of the upper crust beneath the 2011 Talala earthquake source zone by inverting about 1135 P and 1125 S wave arrival time data. Estimates of seismic velocities (Vp, Vs) and Poisson’s ratio (σ) structures offer a reliable interpretation of crustal heterogeneities and their bearing on geneses of moderate earthquakes and their aftershock sequences beneath the source zone. It is found that the 2011 Talala mainshock hypocenter depth (6 km) is located near the boundary of the low and high velocity (Vp, Vs) and the source zone is associated with low-σ anomalies guarded by the prominent high-σ anomalies along the active fault zone having strike-slip motion beneath the earthquake source zone. The pattern of distribution of (Vp, Vs, σ) and its association with occurrences of aftershocks provide seismological evidence for the neo-tectonics in the region having left lateral strike-slip motion of the fault.     

An attempt to detect temporal variations of crustal structure in the source area of the 2006 Wen-An earthquake in North China

Lei et al. (2008) revealed a low-velocity (low-V) anomaly in the lower crust under the source area of the 4 July 2006 Wen-An earthquake (M 5.1) in North China. In this work we tried to investigate the temporal variations of the crustal structure by using a number of P and PmP (Moho reflected) wave arrival times recorded by 107 digital seismic stations from earthquakes that occurred separately in 2002, 2003, 2004, and 2005–2006 to determine P-wave velocity structures in and around the source area of the Wen-An earthquake in different periods. Our results show that tomographic images inferred from the data sets in different years are all dominated by a low-V anomaly in the lower crust under the Wen-An source area. However, there exist some differences in the P-wave velocity image in the Wen-An source area inferred from the P and P + PmP data sets, suggesting that the PmP data have improved the tomographic images in the middle and lower crust, but the results from the 2005–2006 P and P + PmP data sets all show a relatively lager increase of the low-V anomaly under the Wen-An source area in the amplitude and extent as compared with those from the 2003 and 2004 P and P + PmP data sets. Incorporating the previous results, if this low-V anomaly may indicate the existence of fluids, then our results suggest that the occurrence of the Wen-An earthquake is not only related to the long-term influence of fluids that decrease the effective normal stress on the fault plane, but also closely associated with the drastic increase of such influence. However, this study is just an experimental work and the results are still preliminary because the resolution scale of the present tomographic model is much larger than the Wen-An source area and our extensive tests show that different samplings of seismic rays from different data sets have affected the details of the tomographic images, suggesting that the present sparse data coverage can hardly detect reliably any temporal variations of the velocity anomalies in the Wen-An source area. In future studies it is necessary to improve the resolution of crustal tomography to the size of the rupture zone and utilize identical seismic ray paths from the same pairs of sources and receivers in order to detect any temporal variations of the crust structure in the source area of a large earthquake.     

Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent

We here present the results of the inverse modeling of crustal S-phases recorded from a 400-km-long seismic profile, with azimuth nearly N30W, from Lianxian, near Hunan Province, to Gangkou Island, near Guangzhou City, Guangdong Province, in the southern margin of South China continent. The finding in this case is that many shot gathers provided by this wide-angle seismic experiment show relatively strong reflected and refracted S-phases, in particular some crustal refractions (Sg waves) and Moho reflections (SmS waves or simply Sm waves). The P-wave velocity structure of the crust and uppermost mantle was already obtained through the interpretation of vertical-component shot gathers. Now, with constraints introduced by the P-wave velocity architecture and after picking up S-wave traveltime data on the seismograms, we have obtained the S-velocity model of the crust by adjusting these traveltimes but keeping the geometry of the crustal reflectors. Our results demonstrate: (1) the average crustal S-velocity is about 3.64 km/s to the northwest of the Wuchuan-Sihui fault, and 3.62 km/s to the southeast of this fault; (2) relatively constant S-velocity of about 3.42 km/s for the upper crust, 3.55 km/s for the middle crust and laterally varying shear velocity around 3.82 km/s for the lower crust; (3) correspondingly, Vp/Vs ratio is 1.73 for the upper crust, 1.71 for the middle crust and 1.74 for the lower crust. Both shear velocities and Vp/Vs ratio correlate well with the major active faults that break the study area, and show significant changes especially in the upper crust. High Poisson’s ratio (1.8) is observed at shallow depth beneath the Minzhong depression to the southeast of the Wuchuan-Sihui fault and the Huiyuan depression in the southern margin of South China continent. In contrast, a very low Vp/Vs ratio (1.68) is observed between 8 and 14 km depth beneath Huiyuan. At deeper depth, a high Vp/Vs ratio (1.76) is observed in the lower crust beneath the Minzhong depression.     

Precursors and fluid flows in the case of the 1996, ML = 5.2 Saint-Paul-de-Fenouillet earthquake (Pyrenees,France): A complete pre-, co- and post-seismic scenario

Earthquake precursors are now regularly described but often detected only after a major or moderate seismic event. Presence and influence of fluids in the seismogenic processes are often observed at the time of earthquake studies. Even today, the understanding of the physical processes involved in the source region is a real challenge for seismic hazard assessment. Here, the aftershock sequence of the M_L = 5.2, 1996 Saint-Paul-de-Fenouillet (Eastern Pyrenees, France) earthquake is first re-examined with P-wave cross-correlations, resulting in extracting three multiplets and in determining new locations. Multiplets and spatio-temporal distribution analysis of the aftershocks allow for quantifying the hydraulic diffusivity D at a maximum value of 5 m²/s and the permeability K at 10^? 15 m² in the upper Pyrenean crust. Second, a model is established in order to explain the hydrogeochemical transient anomalies, which occurred during the 15 day-period preceding the 1996 earthquake. These anomalies consist on a temporal and spatial sequence of gas emissions in the epicentral area and on chloride and lead concentration variations in a bottled mineral water 25 km north to the main shock epicenter. The proposed model processed in a standard elastic half-space, consists of creep on a low-angle crustal normal-fault, generating volumetric strain field changes over a distance of 25 km from the epicentral area. This model is able to constrain not only the mechanisms and the locations of the geochemical anomalies, but also their timing and probable casual links to the triggering of the impending major event. Also, the active extension proposed here is compatible with seismological observations in the Pyrenees. Thus, the possibility of such creep, which can be considered as a slow-slip event, is discussed in the Pyrenean tectonic and geological context. The model is discussed and compared to previous proposed models on precursor processes of earthquakes, especially concerning the preparation zone concept. Finally, a complete seismic scenario over the period beginning 15 days before the quake and ending 5 days after is proposed and discussed.     

Genetic waveform modeling for the crustal structure in Northeast Japan

We propose a genetic algorithm (GA) search procedure for waveform modeling of local crustal earthquakes for optimal one-dimensional (1-D) crustal velocity model. Both waveforms and travel-time data are used for the structure determination. The use of travel times in model evaluation improves the waveform modeling performance in the sense of computation speed and accuracy. We applied this method to broadband waveforms of a local crustal earthquake (M 4.2) in Northeast Japan. P-wave velocities of the crustal model are found to be 4.95 ± 0.30, 5.9 ± 0.02, and 6.51 ± 0.20 km/s for a surface layer, upper crust and lower crust, respectively. The surface layer thickness and the Conrad and Moho depths are found to be 3.01 ± 0.8, 17.77 ± 0.4 and 34.59 ± 1.0 km, respectively. For epicentral distances <200 km, our synthetic waveforms match the observed ones generally well. Early arrivals are mainly observed at stations near the Pacific coast in the forearc area having a thinner crust. In contrast, delayed arrivals appear at stations near the volcanic front and back-arc areas where low-velocity anomalies exist due to the effect of the Pacific slab dehydration and the hot upwelling flows in the mantle wedge. In general, our results agree well with the main tectonic setting of the study area, which confirms the reliability of the proposed approach. Despite a 1-D velocity model is too simple to represent the complex crustal structure, it is still required for the conventional routine analysis of seismology, such as earthquake location and source parameter studies. The current approach is considered as a step toward the genetic full waveform modeling for the 3-D velocity model estimation.     

3D modeling of earthquake cycles of the Xianshuihe fault,southwestern China

We perform 3D modeling of earthquake generation of the Xianshuihe fault, southwestern China, which is a highly active strike-slip fault with a length of about 350 km, in order to understand earthquake cycles and segmentations for a long-term forecasting and earthquake nucleation process for a short-term forecasting. Historical earthquake data over the last 300 years indicates repeated periods of seismic activity, and migration of large earthquake along the fault during active seismic periods. To develop the 3D model of earthquake cycles along the Xianshuihe fault, we use a rate- and state-dependent friction law. After analyzing the result, we find that the earthquakes occur in the reoccurrence intervals of 400–500 years. Simulation result of slip velocity distribution along the fault at the depth of 10 km during 2694 years along the Xianshuihe fault indicates that since the third earthquake cycle, the fault has been divided into 3 parts. Some earthquake ruptures terminate at the bending part of the fault line, which may means the shape of the fault line controls how earthquake ruptures. The change of slip velocity and displacement at 10 km depth is more tremendous than the change of the shallow and deep part of the fault and the largest slip velocity occurs at the depth of 10 km which is the exact depth of the seismic zone where fast rupture occurs.     

Seismic velocity and Poisson’s ratio tomography of the crust beneath East Anatolia

Eastern Anatolia is a region in the early stages of continent–continent collision and so provides a unique opportunity to study the early development of continental plateau. Located within the Alpine–Himalayan fold-thrust fault belt, the Anatolian plateau is geologically very complex, with over half of the surface area covered with late Cenozoic volcanics of diverse composition. The plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike-slip motion. In this study, we determine 3-D tomographic images of the crust beneath eastern Anatolia by inverting a large number of arrival time data of P- and S-waves. From the obtained P- and S-wave velocity models, we estimated the Poisson’s ratio structures for a more reliable interpretation of the obtained velocity anomalies. Our tomographic results are generally consistent with the major tectonic features of the region. High P- and S-wave velocity anomalies are recognized near the surface, while at deeper crustal layers, low seismic wave velocities are widely distributed. Poisson’s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure. The seismic activity is intense along highly heterogeneous zones and is closely associated with pre-existing faults in the central and western parts of the study area. Results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km. The low-velocity/high Poisson’s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong Sn attenuation, low Pn and Sn velocity, and the absence of mantle lid, implying the presence of partial melt in the uppermost mantle.     

Crustal structure of the Paleozoic Kunlun orogeny from an active-source seismic profile between Moba and Guide in East Tibet, China

We herein present a new seismic refraction/wide-angle reflection profile that crosses the Songpan–Ganzi terrane, the Animaqing suture zone and the eastern Kunlun mountains (comprised of the South Kunlun and Middle Kunlun blocks separated by the Middle Kunlun fault). The profile is 380 km long and extends from Moba to Guide in eastern Tibet. The crustal thickness is about 62 km under the Songpan–Ganzi terrane, 62–64 km under the South Kunlun, and 60 km under the Middle Kunlun block. The Songpan–Ganzi flysch seems to be present up to a depth of 15 km south of the Animaqing suture zone, and up to a depth of 10 km in the Middle Kunlun block, with thicknesses elsewhere that depend on assumptions about the likely lithologies. The profile exhibits clear lateral variations both in the upper and lower crust, which are indicative of different crustal blocks juxtaposed by the Kunlun fault system. Whether or not the Songpan–Ganzi flysch was originally deposited on oceanic crust, at the longitude of our profile (100°E) it is now underlain by continental crust, and the presence of continental crust beneath the Songpan–Ganzi terrane and of a continental arc under the South Kunlun block suggest Paleozoic continent–continent arc collision in the eastern Kunlun Mountains. Comparison of crustal velocity columns from all wide-angle seismic profiles across the eastern Kunlun mountains indicates a remarkable west-to-east change in the Moho topography across the Kunlun fault system (15–20 km Moho step at 95°E, but only 2–5 km along our profile at 100°E). Lower-crustal thickness of the Kunlun terranes is rather uniform, about 35 km, from 80°–95°E, which suggests that similar thrust-thickening processes have played a role where the Qaidam Basin abuts the Kunlun fault, but thins to 20–25 km at 100°E, east of the Qaidam Basin. The increased crustal thickness from 93° to 98°E compared to that at 100°E may be due to the differences in the thickness of the crust of the two plates before their collision, and/or largely achieved by thickening of the lower crust, perhaps indicating a crustal flow mechanism operating more strongly in the western region.     

Source characteristics of the Yutian earthquake in 2008 from inversion of the co-seismic deformation field mapped by InSAR

On 21 March 2008, an Ms7.3 earthquake occurred at Yutian County, Xinjiang Uygur Autonomous Region, which is in the same year as 2008 Mw 7.9 Wenchuan earthquake. These two earthquakes both took place in the Bayar Har block, while Yutian earthquake is located in the west edge and Wenchuan earthquake is in the east. The research on source characteristics of Yutian earthquake can serve to better understand Wenchuan earthquake mechanism. We attempt to reveal the features of the causative fault of Yutian shock and its co-seismic deformation field by a sensitivity-based iterative fitting (SBIF) method. Our work is based on analysis and interpretation to high-resolution satellite (Quickbird) images as well as D-InSAR data from the satellite Envisat ASAR, in conjunction with the analysis of seismicity, focal mechanism solutions and active tectonics in this region. The result shows that the 22 km long, nearly NS trending surface rupture zone by this event lies on a range-front alluvial platform in the Qira County. It is characterized by distinct linear traces and a simple structure with 1–3 m-wide individual seams and maximum 6.5 m width of a collapse fracture. Along the rupture zone are seen many secondary fractures and fault-bounded blocks by collapse, exhibiting remarkable extension. The co-seismic deformation affected a big range 100 km × 40 km. D-InSAR analysis indicates that the interferometric deformation field is dominated by extensional faulting with a small strike-slip component. Along the causative fault, the western wall fell down and the eastern wall, that is the active unit, rose up, both with westerly vergence. The maximum subsidence displacement is ～2.6 m in the LOS, and the maximum uplift is 1.2 m. The maximum relative vertical dislocation reaches 4.1 m, which is 10 km distant from the starting rupture point to south. The 42 km-long seismogenic fault in the subsurface extends in NS direction as an arc, and it dipping angle changes from 70° near the surface to 52° at depth ～10 km. The slip on the fault plane is concentrated in the depth range 0–8 km, forming a belt of length 30 km along strike on the fault plane. There are three areas of concentrating slip, in which the largest slip is 10.5 m located at the area 10 km distant from the initial point of the rupture.     

Source complexity of the 4 March 2010 Jiashian,Taiwan, Earthquake determined by joint inversion of teleseismic and near field data

The Jiashian earthquake (M_L 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a M_w 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake.