U23B-0049
Spectra Analysis of the Wenchuan (8.0) Great Earthquake and its aftershocks
Data recorded at 130 global broadband seismic stations have been used for spectra analysis for the 12 May 2008 Wenchuan Ms=8.0 great earthquake and its aftershocks. The conventional amplitude spectrum for a ω-2 source model can provide independent information for the rupture process: the low-frequency amplitude gives the earthquake moment; value of the corner frequency constrains the scale of the rupture surface; and the azimuth variation of both wavelet length and corner frequency due to the Doppler effect shows the rupture direction. A non-linear least square method is adopted in a computer program to obtain the corner frequency automatically. Then more than 130 stations with high qualified data are selected manually. Our analyses show that the rupture propagation of the main earthquake is unidirectional at an azimuth of 64°, the area of rupture is about 5500 square of kilometers. Other important source parameters such as the magnitude and the stress drop can also be constrained by this method. Several aftershocks were also analyzed that they show no obvious azimuth variation, indicating a symmetric rupture propagation. Finally this highly automatic method can give very quick results for the source model with a reasonable accuracy. Getting these information about the source of a major earthquake in time should be important for predicting aftershocks and planning rescue operations.
U23B-0050
Numerical Study on the Seismic Wave Propagation Excited by Wenchuan earthquake
We Applied parallel numerical simulation method to investigate the ground wave propagation from Wenchuan Mw8.0 earthquake, occurred in China's Sichuan Province (31.49 N, 104.11E) at 14:28 local time (06:28 GMT), May 12, 2008. The ground motions were so strong and destructive that all the neighboring provinces and major cities felt the strong ground waves, as far as Beijing and Shanghai. The governing equations are based on the elastic propagation theory. The spectral element method and high performance parallel computing were incorporated to simulate the seismic wave propagation processes excited by this big event. Our numerical model is accordance with ak135 model, with the consideration of the Earth's ellipticity, topography and attenuation. Comprehensive numerical simulation results showed the processes of seismic wave propagations at difference stages. Further detailed analysis on recorded data with different epicenters and different orientations were carried out. Synthetic seismogram is in excellent agreement with theoretical travel time. The data of P wave is also consistent with seismic data from IRIS. The numerical simulation results clearly show that the Wenchuan earthquake is thrust and right lateral strike-slip earthquake; the major fault system is NE-SW orientated, while the seismic fracture propagating to the north-east. Our simulation results agree well with the recorded information, and consistent with the theoretical estimation.
U23B-0051
Modeling of the 3d Wave Propagation of the Sichuan Ms 7.9 Earthquake of 12 05 08
The seismic potential of southern China is manifested in the Western Sichuan Plateau (eastern Tibetan Plateau) by among others, the seismic activity of the Longmenshan fault. The seismicity observed on the latter includes recent historical events with magnitudes up to 6.5, and the one of the 12 05 2008 Ms 7.9 (Mo 1.15 1028 dyne/cm) thrust mechanism, the so-called Sichuan earthquake. Herewith, a hybrid procedure, combining long period (obtained a recently optimized 3D seismic wave propagation parallel finite difference code) and high frequency (by using the empirical Green fuction approach) simulations was used to obtain 3D synthetics seismograms for the mentioned Sichuan earthquake. The modeling included the USGS web- site, 40 x 315 km2 kinematic description of the earthquake's rupture. The comparisons between observed and synthetic seismograms for stations sites of the Seismological Network of China, such as CD2 (Chengdu), GYA, LHZ and TIY at about 90, 500, 600 and 1200 Km from the epicenter of the Sichuan event, respectively, are satisfactory. Assuming rock sites, maximum synthetic accelerations, velocities and displacements, and permanent displacements of: 1.70 m/s2 , 1.90 m/s, 3.30 m, and 1.25 m; were obtained for Beichuan and 0.625 m/s2 , 0.092 m/s, 0.10 m, and 0.0 m, were computed for Chengdu, which partially explains the modification of the topography, and the extensive damage observed on the infrastructure and towns located on top of the Sichuan earthquake rupture zone, as well as the slight damage observed at Chengdu located at an epicentral distance of 90 km.
U23B-0052
Anisotropic Tomographic Inversion of the P-Travel Times in the Northwestern Sichuan, China
The velocity and anisotropy image of the crust and upper mantle nearby the eastern Tibetan edge is important for the Tibetan plateau dynamics. The Wenchuan earthquake occurred on May 12, 2008 makes this more interesting. Up to now, however, the resolution of the former results about this area is still poor. The dense movable seismic array deployed recently in the western Sichuan (100°~ 105°E, 26°~ 32°N) provides a good opportunity to insight into the crust and upper mantle structure with high-resolution. In this presentation, the P-travel time tomographic technique has been extended to the case of anisotropy. In our method, under assumption of weak anisotropy, the P-travel times are calculated in terms of the fast marching method. The anisotropic inversion of the P-travel times in the northwestern Sichuan (100°~ 105°E, 29°~ 33°N) is performed by using this technique. Our results show: 1) The lithosphere beneath the Sichuan Basin has a high-velocity structure, but a low-velocity layer exists in the upper mantle at depth range of 150-00 km. The lithosphere underneath the Tibetan Plateau has a lower P-wave velocity than that of the Sichuan Basin, but we did not find intracontinental subduction. Especially, according to the crustal velocity structure, the crust in the Longmen Shan fault belt can be separated into two parts bounded with the Wenchuan earthquake region; both of them have high- velocity perturbation up to 3%. Beneath the intersection of the Longmen Shan and Xianshui He faults, the lithosphere structure is very complicated, suggesting that the large deformation was taking place in this area. 2) In the eastern Tibetan Plateau, the fast-direction of the P-waves is toward the NE direction, which is consistent with the GPS observations. However, the fast-direction is toward the SE in the Sichuan basin. Nearby the Longmen Shan faults belt, it is perpendicular to the faults at the north segment and parallel to the faults at the southern segment, suggesting that these two segments could have completely different deformation manner and dynamical mechanism.
U23B-0053
Surface Rupture along the Beichuan Segment of the 12 May 2008 Wenchuan Earthquake, Sichuan, China
Hazards associated with large magnitude earthquakes along reverse faults are poorly constrained, only a few earthquakes greater than magnitude 7.5 along continental reverse faults have been well-documented. On 12 May 2008 the Ms 8.0 Wenchuan, western China, earthquake struck Longmen Shan at the eastern margin of the Tibetan plateau, produced surface rupture as much as 200 km long. It was the largest thrust-slip earthquake in China in historical record and has offered an unusual opportunity to characterize the behavior of an active thrust fault. We have conducted detailed field mapping of the surface rupture near Beichuan town,where we found that ~19 km long surface rupture can be divided into two distinct domains. To the south of town, the southern segment extended northeastward for a distance of more than 14 km, and typically consisted of a single major scarp with or without multiple minor scarps that collectively exhibited a northwest-side-up vertical separation of up to ~5 m. Outcrop measurements indicated that the surface rupture as nearly pure thrust slip in most places, except in some short strike-slip transfer sub-segments. The trace of the fault rupture is highly sinuous, which probably indicates a shallow dip of fault. To the north of town, a spectacular, linear trend of bedrock fault striking N40-50°E for a distance of more than 5.3 km, cuts westward dipping hill slopes. The steeper fault dip-angles, clear striations, the strike- perpendicular components of dip-slip, and the associated offset geomorphic structures along the fault imply a pattern of oblique thrust faulting, with right-lateral strike-slip components. The measured vertical displacement, right-lateral slip, shortening, and net slip of the rupture are ~4.5 m, ~3.4 m, ~3.7 m, and ~4.9 m, respectively. One key feature of this segment is the stratigraphy-controlled slip surface: the fault slip plane generally follows the Paleozoic black shale. Mapped surface rupture trace is largely coincident with the surface traces of the older structures, the Leigu tear fault and the Beichuan reverse fault formed during the Late Triassic Indosinian Orogeny [e.g. Burchfiel et al., 1995, 2008]. Geomorphic features indicate that these faults have generated large earthquakes during the last several thousand years. However, two segments are different in terms of paleoearthquakes, active geomorphic expression, and long-term structural development. Further detailed seismotectonic studies are needed for mitigation against the seismic hazards in the active fold-belt of Longmen Shan.
U23B-0054
Co-seismic ruptures of the 12 May, 2008, Mw 8.0 Wenchuan earthquake, Sichuan: EW crustal shortening on oblique, parallel thrusts along the eastern edge of Tibet
The Mw8.0, Wenchuan earthquake, which devastated the mountainous, western rim of the Sichuan basin in central China on May 12th, 2008 produced a surface rupture at least 225 km-long, with oblique thrust/dextral slip and maximum slip of 8-10m. It thus ranks as one of the world¡¯s largest continental mega- thrust events in the last 150 years. Field investigation shows clear surface breaks along two of the main branches of the NE-trending Longmen Shan thrust fault system. The principal rupture, on the NW-dipping Beichuan fault, displays near equal amounts of NW hanging-wall up ¡¡ìC thrust and right-lateral slip. Basin-ward of this rupture, another continuous surface break is observed for over 60 km on the parallel, shallower-NW-dipping Pengguan fault. Slip on this latter fault was pure thrusting, with a maximum scarp height of ~ 2.8 m. This is one of the very few reported instance of co-seismic surface rupture on parallel thrusts. The long rupture,large-offsets, and distributed surface breaks that characterize this out-of-sequence event clearly attest to regional, EW-directed, present-day crustal shortening along the NE-SW trending, eastern margin of Tibet. It calls for a re-evaluation of tectonic models anticipating little or no active shortening of the upper crust along that edge of the plateau, and highlights the need for a re-assessment of seismic hazard along potentially under-rated active faults across the densely-populated western Sichuan basin and mountains.
U23B-0055
Co-seismic thrusting slip and shortening structures produced by the 2008 M?? 7.9 Wenchuan earthquake, China
The magnitude (Mw) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the Longmen Shan region, the transition zone between the Tibetan Plateau and the Sichuan Basin, China, resulting in extensive damage throughout central and western China. Official estimates of casualties released by the Chinese Government as of 31 August 2008 include 69,197 confirmed deaths, 374,176 injured, and 18,209 missing persons. To understand the seismic faulting mechanism and surface deformation features associated with the earthquake, including rupture length and slip distribution, our survey group traveled to the epicentral area 2 days after the earthquake and undertook 10 days of fieldwork, collecting primary data related to rupture structures and slip distribution. Based on this preliminary survey, we carried out additional detailed fieldwork on the co-seismic surface rupture over the past 3 months. Here we report the main results of our field investigations and discuss the co-seismic rupturing mechanism, including its implications for seismotectonics in the eastern marginal zone of the Tibetan Plateau. Field data reveal that the Wenchuan earthquake produced a 285-km-long surface rupture zone along the Longmen Shan Thrust Belt upon the eastern margin of the Tibetan Plateau, with a dominantly thrusting slip accompanied by a right-lateral component in the northern segment of the rupture zone, and a left-lateral component in the southernmost segment. The co- seismic ruptures mainly occurred along the pre-existing Yingxiu-Beichuan, Guanxian-Anxian, and Qingchuan faults, which are the main faults of the Longmen Shan Thrust Belt. The displacements measured in the field are approximately 0.5-5 m in the vertical, generally 1-3 m, accompanied by an average left-lateral component of <2 m in the 60-km-long southernmost segment of the rupture zone, and an average right-lateral component of <1 m in the 100-km-long central-northern segment. The rupture length and maximum displacement are the largest among intracontinental thrust-type earthquakes reported to date. Our results document and confirm that i) the Wenchuan earthquake occurred on the pre-existing active faults of the Longmen Shan Thrust Belt, which controlled the spatial distribution of co-seismic surface rupture and displacement and the rupture processes of the earthquake; ii) the long rupture length and large thrusting slip resulted from compressive stress associated with eastward extrusion of the Tibet Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate; and iii) present-day shortening strain upon the eastern margin of the Tibetan Plateau is mostly released by seismic slip along thrust faults within the Longmen Shan Thrust Belt.
U23B-0056
Coseismic Faults and Crust Deformation Accompanied the 2008 Wenchuan Earthquake, China by Field Investigation and InSAR Interferogram
The devastated Mw 7.9 Wenchuan earthquake occurred along the steep eastern margin of the Tibetan
plateau in Sichuan, China, on 12 May 2008. Over 86,592 people were dead or missing, 374159 injured, and
more than 4.8 million homeless. The ruptures possibly occurred over a length of 285 km along the northeast
striking Longmen Shan (LMS) thrust belt. In order to study the oversized fault ruptures, existing active faults
related and relationships with the damages caused, we conducted field investigations during 4-15 June and
3-9 October 2008, covered about 140km length of LMS faults, including Beichuan(BC), Anxian(AC), Mianzhu,
Shifang, Pengzhou, Dujiangyan, Yingxiu (YX) and Wenchuan.
On the field investigation we found coseismic surface faults along several profiles perpendicular to the LMS
faults. The coseismic surface faults we discovered were at Leigu(L), Hanwang(H), Yinghua(Y), Bailu(BL),
Xiaoyudong(X), and Baiyunding (BYD). Of them the maximum vertical displacement reached 4.6m at L,
Beichuan County. The uplifting displacements dominated in the southwestern section of the rupture.
Moreover, the northwest-striking left-lateral fault was found with horizontal displacement of 2.8m, and vertical
of 1.5m as well, at X, Pengzhou City. The left-lateral fault, inversely under-controlled movement of right-
lateral fault in the area, showed the complexity of the fault movements.
The field results showed the coseismic surface ruptures locally while the overall faults movements and Crust
deformation could be understood by the Interferometric SAR(InSAR) technique (NIED, 2008) using data from
the Phased Array L-band SAR sensor (PALSAR) equipped on Advanced Land Observing Satellite (ALOS).
The larger deformation zones detected by InSAR interferogram occurred with a width of ~30 km in
southwestern section, and of ~10km in northeastern section of LMS faults. In the southwestern section, the
deformation zone occurred mostly within the existing active faults zones: Guanxian-Anxian faults F1 that
eastbound the Sichuan basin, Wenchuan-Maoxian fault F3 that westbound the Tibetan plateau, and Yingxiu-
Beichuan faults F2 that located in the between. From X to AC in the middle section of LMS faults, the
deformation zone occurred from F1 to F2 and even over F2. In the northeastern section, it almost fit the fault
F2 from BC to Pingtong(PT), Nanba(NB), Shiba(SB) and Qingchuan (QC) in a narrow zone.
The coseismic faults were confirmed by both field investigation and InSAR interferometry along the following
segments:
Segment 1: from/through BYD, YX to X along Yingxiu-Beichuan faults F2; Segment 2: changed direction at X
as a corner to N60E, went along the N20W Xiaoyudong fault until BL as another corner; Segment 3:
from/through BL, H along Guanxian-Anxian faults F1 until AC(Angchang); Segment 4: changed direction at
AC as a corner to N00E, went along the Leigu fault through L, to BC; Segment 5: from/through BC, PT, NB
and SB to QC along Yingxiu-Beichuan faults F2. Among these segments, the Segment 1 and Segment 4
supposedly played important roles for triggering or transferring the ruptured faults F1 to the ruptured faults
F2.
The InSAR interferogram from X to BC showed the grey belt, whose phase incoherence demonstrated strong
earth-crust deformation, but it was difficult to identify whether the coseismic faults occurred or not. On the
other hand, no coseismic faults from X to BC are reported, where road was/is blocked in the mountain area.
http://www.bosai.go.jp/library/k_news_img/k_news164.pdf
U23B-0057
Coseismic ground displacements from sub-pixel correlation for the 2008 Wenchuan Earthquake, Sichuan, China
On May 12, 2008, a Mw 7.9 earthquake occurred in the Longmen Shan mountains with an epicenter near Wenchuan in Sichuan Province of southwestern China. This event caused more than 69,000 fatalities and huge economic losses. The tectonic context of the epicentral area is characterized by NW-SE trending and east-vergent folds and thrust faults which also topographically form the boundary zone between Tibetan plateau and the Sichuan basin. Large earthquakes in continental compressional settings are uncommon and each such event offers a rare opportunity to understand how fold-and-thrust belts develop and a deeper understanding of their seismic hazard potential. In that regard, detail mapping of fault ruptures and measurements of fault slip are required. This information is extremely difficult to gather in the field because well-defined linear markers and preexisting horizontal surface are rare in mountainous area and near surface deformation is in fact composed of contributions from internal faulting and folding. We use an alternative approach to measure co-seismic surface deformation from the correlation of satellite images using the COSI- corr software. Sub-pixel correlation was conducted on SPOT, FORMOSAT II and amplitude of ALOS and ENVISAT SAR images to obtain coseismic surface deformation. Our results provide details of surface displacement, showing thrust component, generally <10 meters, is dominant along the southern segment. To realize the long-term fault kinematics a comparison is further made between co-seismic deformation and long-term geomorphic deformation recorded in the landforms.
U23B-0058
Active faults, surface ruptures, and occurrence of landslides associated with the May 12 2008 Wenchuan earthquake, western Sichuan, China
The Ms 7.9 Wenchuan earthquake occurred at a depth of about 12 km on the NNE-striking Yingxiu-Beichuan fault. This is one of the three west-dipping, active reverse fault zones in the easternmost Longmen Shan thrust belt in western Sichuan; the other two are the west-dipping Jiangyu-Dujiangyan fault zone (also known as the Anxian-Guanxian fault) to the southeast and the Wenchuan-Qingchuan fault system to the northwest. The Wenchuan-Qingchuan fault zone consists of several faults exhibiting variable older-over-younger (Proterozoic-Silurian rocks over Devonian-Triassic strata) and younger-over-older (Proterozoic-Devonian strata over Precambrian basement) relationships. The above field relationships suggest that the fault zone had experienced a complex deformation history characterized by both normal faulting and out-of-sequence thrusting. The Yingxiu-Beichuan fault zone comprises a zone of imbricate thrusts that place Precambrian to Devonian rocks over Triassic strata. Finally, the Jiangyu-Dujiangyan fault zone, a range-bounding structure, places Triassic rocks over Jurassic strata that are in turn overlain by a sequence of east-dipping Cretaceous strata extending into the Sichuan basin. Near Yingxiu, the main strand of the Yingxiu-Beichuan fault dips about 50 degrees and places the Precambrian Pengguan metamorphic complex over Devonian strata. At Hangwang, the Jiangyou-Dujiangyan fault dips about 40 degrees and places Early to Middle Triassic strata over Jurassic sediments. Because the three fault zones all lie in pre-Cenozoic strata and the eastern Longmen Shan had experienced a strong contractional event in the early Jurassic as expressed by the occurrence of numerous thrust klippes in the footwall of the Yingxiu-Beichuan thrust zone, the exact magnitude of Cenozoioc shortening is currently unknown. Surface fractures associated with the Wenchuan earthquake are expressed as P shears (N20-30E), R shears (N60W), conjugate R shears (N10W), and extensional T fractures (N80E). They occur mainly along the 400-km long central segment of the Yingxiu- Beichuan fault zone, but are also present along the northernmost Wenchuan-Qingchuan fault zone and southernmost Jiangyu-Dujiangyan fault zone. The above observation indicates a left-step en echelon pattern of surface rupture across the eastern Longmen Shan thrust belt during the Wenchuan earthquake. Striations on the fault zones together with the surface fracture patterns indicate that all three reverse fault zones have a right-slip component. Specifically, co-seismic motion on the Yingxiu-Beichuan fault north of Beichuan city is dominantly right-reverse slip whereas north of Beichuan city is mainly right-slip. Landslides are closely associated with the occurrence of surface fractures during the Wenchuan earthquake. Motion on most of the landslide occurred through the reactivation of the older landslides.
U23B-0059
Tectonic Background of the 2008 Sichuan, China, Earthquake as Seen From GPS Data
We discuss the tectonic background of the 2008 Sichuan, China, Earthquake based on the GPS data in China. There are four subjects to be discussed: (1) Deformation field of China continent based on plate tectonic framework, (2) Eastward extrusion of China tectonic block and the Sichuan-Yunnan tectonic belt, (3) Block convergence at the Longmenshan fault, and (4) Stress change and risk assessment on the Xianshuihe fault due to the earthquake. First, we combined available velocity fields in and around China continent to draw a unified velocity map in Eastern Asia. Then we divided the China continent into blocks. We employed a rigid block and fault deficit algorithm to thus assumed block model to estimate relative block velocities and couplings between blocks. Here we assumed that the block corresponding to the Eurasian plate is fixed and the Indo-Australian plate is converging toward the China continent with a velocity of 37mm/yr. The estimated block motions demonstrate the eastward extrusion of blocks and clockwise rotation in the Sichuan-Yunnan Tectonic block. Our model suggests that the Logmenshan fault is converging with significant right-lateral offsets. The relative velocity across the fault is from 2.4mm/yr to 6.4mm/yr from west to east. Given that the offset due to the earthquake was about 6m, the average repeat time on the fault would be from 1000yr to 3000yr. Finally, we evaluated the Coulomb Failure Stress change around the source region based on the slip model derived from the seismic wave form inversion analysis (Hikima et al., 2008). The targeting model was set as the Xianshuihe fault, where many historical earthquakes have occurred. Results suggest that the stress in the fault segment of the most imminent area nearby Bamei decreased due to the earthquake.
U23B-0060
Analytical and finite element modeling of the 2008 Sichuan earthquake from DInSAR data
The magnitude Mw = 8.0 earthquake that struck China's Sichuan region on 12 May 2008 has been imaged by X, C, and L-band SAR satellites (ASI's Cosmo-Skymed, ESA's ENVISAT, and NASDA's ALOS, respectively), allowing to attempt the recovering of the ground deformation associated to the fault dislocation. We analyzed all the available Cosmo stripmap images (3-m resolution), some ENVISAT frames, and more than sixty ALOS-PALSAR scenes. The X and C band interferograms show good coherence only in the lowland, near the city of Chengdu, but the coherence decreases dramatically in the strong relief areas. Moreover their spatial coverage is limited with respect to the large extent of the fault (~ 300 km long). The best coverage is provided by PALSAR interferograms which maintain very good coherence also in the mountain areas, thanks to the long wavelength of the L band (23.8 cm). We calculated ~ 45 differential PALSAR interferograms across time spans of a few months, obtaining a good view of the co-seismic surface deformation along the fault. We observe over 2.5 m of Line of Sight ground displacement. We use the displacement field to infer the geometry and the slip distribution of the seismogenic fault, by means of a linear and non-linear inversion of an analytic elastic source; an additional modelling is then performed by means of a finite element approach.
U23B-0061
The Application of InSAR to the 2008 Mw 7.9 Wenchuan Earthquake, China
On 12 May 2008 a Mw 7.9 earthquake occurred near Wenchuan in Sichuan Province, China. More than 69,000 people were killed and over 4.8 million became homeless. Based on initial seismological reports, the earthquake was generated by slip one or more thrust faults within the Longmen Shan mountain belt, where oblique crustal shortening is taking place the Tibetan Plateau and the Sichuan Basin. In initial results, we are studying the displacements using in SAR techniques on several data types. One descending ASAR image before the earthquake and two after acquired by the satellite ENVISAT, European Space Agency, we are able to obtain two coseismic and one postseismic interferograms. Because of temporal decorrelation, large baseline for each coseismic pair, denser vegetation, and steep relief, it is lacking of reliable phase information on the Longmen Shan mountain belt, which is regarded as the hanging wall of this thrust event. To its east in the Sichuan basin, more than 50 cm subsidence in the direction of line of sight can be observed coseismically. This observed coseismic deformation in the footwall clearly appears as half-circular fringes. Its western boundary enable us to identify the surface rupture that is roughly located along the previously reported Beichuan fault. The postseismic interferogram shows phase differences between two acquisitions, one month after the main shock (16 June – 21 July). Phase signals could be investigated in both of the Longmen Shan mountain belt and the Sichuan Basin with rather small phase differences. However, this task is significantly perturbed by orbit errors and atmospheric effects; therefore, not much information of the surface displacement can be reliably retrieved. In the future we will add one more orbit located in further northeastern Longmen Shan to completely trace the NE-SW surface ruptures of the Wenchuan earthquake. Furthermore, the combined result from InSAR and sub-pixel comparison technique can further allow us construct a surface displacement field for the entire region and assist the modeling of fault kinematics to understand the seismogenic fault system of the Wenchuan earthquake.
U23B-0062
Parallel Simulation of Stress Evolution of Wenchuan 8.0 Earthquake
We used parallel finite element modeling to simulate time-dependent Coulomb stress migration following the 12 May 2008 Wenchuan earthquake (Mw 8.0) in Sichuan, China. The model domain is 650x650x100 km (E100-106, N27-33); we used more than 2.6 million unstructured meshes to represent realistic fault systems in the region and three-dimensional variations of lithospheric structure and rheology. We calculated day-by- day Coulomb stress migration within two months after the mainshock. The predicted spatiotemporal Coulomb stress changes and migration are consistent with the occurrence of aftershocks in the Longmenshan seismic zone. Further analysis shows that aftershock strain decayed fast and converged locally, so the Wenchuan earthquake is unlikely to trigger big earthquakes in neighboring regions. This type of numerical simulations can provide real-time assessment of earthquake hazard in the aftermath of a large earthquake. The numerical capacity of this model also permits its application to study short- and long-term seismic risks in other regions.
U23B-0063
Stress evolution and fault interactions before and after the 2008 Mw 7.9 Wenchuan earthquake
The 12 May 2008 Wenchuan earthquake (Mw 7.9) ruptured more than 300 km of the Longmen Shan fault, killing ~90,000 people and devastating many cities in the Sichuan province, China. Here we explore stress evolution and fault interactions before and after the Wenchuan earthquake in eastern Tibetan Plateau and western Sichuan, where the Indo-Asian collision is accommodated by a number of major fault systems including the Longman Shan fault. Using a 3-D viscoelasto-plastic finite element model loaded by the observed crustal motion on the boundaries and by the topographic changes, we derived a steady state regional stress field, and then calculated coseismic and postseismic Coulomb stress changes associated with the Wenchuan earthquake. Model results show ~0.1-0.4 bar increase of the Coulomb stress on the eastern Xianshuihe fault and eastern Kunlun Fault, and ~1-10 bar increase on the southern segment of the Longmen Shan fault. We also calculated the Coulomb stress change from numerous large historic earthquakes in the region since 1893. The collective effects of these events differ significantly from that due to the Wenchuan earthquake alone - they show more stress increase on the eastern Kunlun Fault, the southern Longmen Shan fault, and the Anninghe fault, but considerable stress decrease on the eastern Xianshuihe fault. Finally, we explored the dynamic interactions among the faults and found that interseismic locking on the Xianshuihe fault can increase Coulomb stress on the Longmen Shan fault at a rate up to ~50 Pa/yr. The Xianshuihe fault has been locked since 1981, which may have added ~0.01 bar on the Longmen Shan fault by 2008.
U23B-0064
Changes of Groundwater Level in Taiwan Following the 2008 Wenchuan Earthquake
The 2008 Wenchuan earthquake occurred at the border of the Indo-Australian Plate and the Eurasian Plate. Following the earthquake, persistent changes of groundwater level were observed in more than 60 monitoring wells in Taiwan located at the boundary between the Eurasian plate and the Philippine Sea plate. In the past, oscillatory changes were observed in Taiwan after many distant earthquakes, including the 2004 South Asia earthquake. It was the first time that both oscillatory changes and persistent changes were observed due to a distant earthquake occurred 1900 km away. The monitoring well stations in Taiwan were installed primarily in the coastal plain. Most of groundwater level data were recorded every hour, but some were recorded every two minutes or every second. The coseismic change following the Wenchuan earthquake ranges from a fall of 40 cm to a rise of 23 cm. At some stations, both coseismic rise and coseismic fall were observed in wells at different depths. Coseismic changes in some wells have never been induced by local large earthquakes before. Based on the focal mechanism of the Wenchuan earthquake, Taiwan is located at the extension quadrangle. While 70% of the changes were coseismic falls, coseismic rises were observed in more than 20 wells, particularly along the belt adjacent to the hills. The underlying mechanism of the coseismic changes warrants further investigations.
U23B-0065
Wenchuan earthquake enhanced crustal permeability in Taiwan
Surface wavetrain from the 2008 M7.9 Wenchuan earthquake generated water-level changes in wells in Taiwan, some 1900 km away from the epicenter. The water level changes, recorded every second, are compared side-by-side with broadband recordings of the surface wavetrain (courtesy of Drs. Wen-Tzong Liang and Rong-Yuh Chen) to decipher the effect of the seismic waves on far field groundwater. Water level in one well oscillated with an amplitude <2 cm on the arrival of the wavetrain and then decline gradually and monotonically over a period of ~15 minutes after the passing of the wavetrain, with a total decease of ~14 cm. The oscillations of the water level reflect a pore-pressure response in the aquifer to the passing wavetrain; but the later monotonic decline of water level that occurred after the passing of the wavetrain cannot be a direct response to seismic waves, but may have been triggered by the small pore- pressure oscillations in the aquifer. We hypothesize that the surface wavetrain may have unclogged some pre-existing fractures in the shallow crust, enhanced the crustal permeability and connected the aquifer to a low-pressure reservoir. The study suggest that great earthquakes can enhance shallow crust permeability over a large region several thousands km from the epicenter.
U23B-0066
Initial report on the northeastern Tibetan plateau seismic experiment and study of the May 12, 2008 Wenchuan earthquake
Within three weeks after the May 12, 2008 Wenchuan earthquake, we installed 26 broadband seismic stations in an approximately 300-km-by-600-km array west of the earthquake fault zone. The timing of the fieldwork coincided with a pre-scheduled seismic experiment in the northeast Tibetan plateau to study the growth and rise of the northeastern Tibetan plateau and its interaction with the surrounding tectonic units. The data collected in the experiment are important and timely to the study of the Wenchuan earthquake and its aftershocks. This initial report presents the experiment setting, initial data collected, and results of preliminary data analysis in the spirit of promoting data exchange and collaborative study of the Wenchuan earthquake and the northeastern Tibetan plateau. The project is sponsored by the U.S. National Science Foundation, Chinese Academy of Geological Sciences, and China University of Geosciences (Beijing).
U23B-0067
Landslides and Earthquake Lakes from the Wenchuan, China Earthquake – Can it Happen in the U.S.?
The May 12, 2008 M7.9 Wenchuan, China earthquake destroyed five million homes and schools, causing over 87,650 deaths. Landslides, a secondary effect of the shaking, caused much of the devastation. Debris flows buried homes, rock falls crushed cars, and landslides dammed rivers. Blocked roads greatly impeded emergency access, delaying response. Our August 2008 field experience in the affected area reminded us that the western United States faces serious risks posed by earthquake-induced landslides. The topography of the western U.S. is less extreme than that near Wenchuan, but earthquakes may still cause devastating landslides, damming rivers and blocking access to affected areas. After the Wenchuan earthquake, lakes rapidly rose behind landslide dams, threatening millions of lives. One landslide above Beichuan City created Tangjiashan Lake, a massive body of water upstream of Mianyang, an area with 5.2 million people, 30,000 of whom were killed in the quake. Potential failure of the landslide dam put thousands more people at risk from catastrophic flooding. In 1959, the M7.4 Hebgen Lake earthquake in Montana caused a large landslide, which killed 19 people and dammed the Madison River. The Army Corps excavated sluices to keep the dam from failing catastrophically. The Hebgen Lake earthquake ultimately caused 28 deaths, mostly from landslides, but the affected region was sparsely populated. Slopes prone to strong earthquake shaking and landslides in California, Washington, and Oregon have much larger populations at risk. Landslide hazards continue after the earthquake due to the effect strong shaking has on hillslopes, particularly when subjected to subsequent rain. These hazards must be taken into account. Once a landslide blocks a river, rapid and thoughtful action is needed. The Chinese government quickly and safely mitigated landslide dams that posed the greatest risk to people downstream. It took expert geotechnical advice, the speed and resources of the army, and some luck. It would pay to learn from their success.
U23B-0068
A few Issues about Wenchuan earthquake of 2008, Sichuan, China
A devastating earthquake of magnitude 8.0, named Wenchuan earthquake, struck China's Sichuan province on May 12, 2008. The rupture occurred over a length of about 300 km along the northeast-striking Longmen Shan thrust belt. The earthquake resulted in huge casualties and property losses at Longmen Shan region of Sichuan Province, the Southern Shaanxi and Southern Gansu, more than 80,000 people were killed. Longmen Shan is the convergent margin of the eastern Tibetan plateau and Yangtze block. However the area has numerous geological features not typical of active convergent mountain belts. After the earthquake, lots of opinions have been expressed. Basing on field investigation and analysis, we put forward the following opinions: (1) the causative structure of the earthquake may be Pengguan fault, not the Beichuan-Yingxiu fault (central fault)£»(2) the uplift of the Longmen Shan and the earthquakes in Longmen Shan may be controlled by the bilateral stresses from the Songpan -Ganzi block on the west side and Sichuan basin on the east side; (3) by calculating the balance between the accumulation and dissipation of the compressed strain energy, we may make out an estimation of the rhythm of strong Earthquake in Longmen Shan; (4) By analyzing the pressure data measured at oil or gas producing wells, we may monitor the changes of the earth stress in deep; (5) Earthquakes in Longmen Shan may be favorable to the forming of the fractured reservoir and hydrocarbon accumulation in Western Sichuan depression. The work was supported by NSFC under grant No.40739907
U23B-0069
Study on Features of Secondary Disasters in the Worst-hit Areas of Wenchuan Earthquake
The Wenchuan earthquake on May 12, 2008 has caused heavy casualties and property losses. The violent earthquake and numerous intense aftershocks also induced secondary natural disasters in large area, which seriously threaten the regional eco-environment and human settlement environment. Based on the field investigation of some disaster area and analysis on RS data collected after the earthquake, the authors present the following opinions on features of secondary disaster caused by the Wenchuan earthquake. The Wenchuan earthquake occurred at the Longmen Mountain fault zone, which starts from east side of the Jiajin Mountain in Tianquan, Ya'an, and extends northeastward connecting with the Daba Mountain fault zone, approximately 500 km long and 70 km wide. This fault zone is composed of the front mountain the fault, central fracture and the rear mountain fault. Primary disaster area is mostly in the central fault zone. Large geomorphological unit of the disaster area is located at the transitional zone between the upmost ladder and middle ladder of landform in China. Primary disaster area is in the high mountain and middle mountain areas at edge of the Sichuan Basin, where valleys are deeply incised by streams and the mountains are very high with very steep slopes, greater than 25 degrees at most sites. The worst-hit area is of subtropical mountainous humid region under monsoon climate. Tendency of the mountain range is in a northeast-southwest orientation, so it is windward to southwestern monsoon, and has plenteous precipitation. Annual precipitation in Dujiangyan is 1178mm, and 1280mm in Beichuan, which are the center of opulent rainfall region in China. It provides conditions for incompact materials to form debris flow after the earthquake. The strong earthquake altered the landscape in disaster area violently. Under the action of seismic force and gravity, a great quantity of materials loosened in the earthquake slide downwards along steep slopes, thus secondary disasters such as collapse and landslides happen. Secondary disasters are concentrated along Longmen Mountain fault zone and two banks of valleys. For instance, 109 clusters of collapses, 98 clusters of landslides and 12 sites of debris flow develop along 138 km valley of Dujiangyan¨CMaoxian section in the mainstream of Minjiang River, totally 219 sites. Secondary disasters are mainly collapse, landslide and debris flow. The earthquake stimulated tens of thousands of these disasters in the disaster area. Lakes formed by blocked rivers by substance of landslide or collapsed in rivers. More than 30 of the larger ones were investigated.
U23B-0070
The Wenchuan, China M8.0 Earthquake: A Lesson and Implication for Seismic Hazard Mitigation
The Wenchuan, China M8.0 earthquake caused great damage and huge casualty. 69,197 people were killed, 374,176 people were injured, and 18,341 people are still missing. The estimated direct economic loss is about 126 billion U.S. dollar. The Wenchuan earthquake again demonstrated that earthquake does not kill people, but the built environments and induced hazards, landslides in particular, do. Therefore, it is critical to strengthen the built environments, such buildings and bridges, and to mitigate the induced hazards in order to avoid such disaster. As a part of the so-called North-South Seismic Zone in China, the Wenchuan earthquake occurred along the Longmen Shan thrust belt which forms a boundary between the Qinghai-Tibet Plateau and the Sichuan basin, and there is a long history (~4,000 years) of seismicity in the area. The historical records show that the area experienced high intensity (i.e., greater than IX) in the past several thousand years. In other words, the area is well-known to have high seismic hazard because of its tectonic setting and seismicity. However, only intensity VII (0.1 to 0.15g PGA) has been considered for seismic design for the built environments in the area. This was one of the main reasons that so many building collapses, particularly the school buildings, during the Wenchuan earthquake. It is clear that the seismic design (i.e., the design ground motion or intensity) is not adequate in the Wenchuan earthquake stricken area. A lesson can be learned from the Wenchuan earthquake on the seismic hazard and risk assessment. A lesson can also be learned from this earthquake on seismic hazard mitigation and/or seismic risk reduction.