U22B-01 INVITED
Wenchuan Ms8.0 Earthquake: The Crustal Velocity Structure and Stress Field From The Western-Sichuan Seismic Array Observations
The Wenchuan Ms 8.0 earthquake of May 12, 2008, occurred on the Longmenshan faults along the eastern margin of the Tibetan plateau. Up to now, however, we have only a poor understanding of the crustal structure beneath the earthquake and adjacent region for lack of the high-resolution seismic image study. In October of 2006, we deployed a movable seismic array composed of 297 broadband stations in the western Sichuan (100° ~ 105° E, 26° ~ 32° N), which covers the most of the Longmenshan fault zone as well as the Songpan-Ganzi block and Sichuan basin. Totally, 690 teleseismic events (Mb>5.0, 30° ≤ Δ ≤ 90°) have been recorded by this array, until June of 2008. The part of these data are used for investigating the 3D S-wave velocity structure of the crust and upper mantle down to 100km in the range of 100° ~ 105° E, 29° ~ 32° N by using the receiver function methods. Also we analysis the S-wave splitting of the local events, including the Wenchuan earthquake sequence, recorded by our movable seismic array. Our results show: 1)The Longmenshan fault is a clear tectonic boundary of the crust. The crustal thickness is about 60km in the Songpan-Ganzi block, and it varies from 40km to 54km beneath the Sichuan basin and along the Longmenshan faults. Especially, the crust beneath the Sichuan basin nearby the Longmenshan faults is obviously thickened, and inside of this fault zone, the crustal thickness, bounded with 31.2° N, is increased abruptly from 40 ~ 46km to 50km more along the north-south direction. The crustal thickness reaches to 52km in the Wenchuan earthquake source region. 2)The S-wave velocity reaches to 4.0km/s below the depth of 20km in the crust of the Sichuan basin, and the crustal velocity structure of the Songpan-Ganzi block is laterally blocked. The Xianshuihe fault is one of boundaries between the blocks. Beneath the Wenchuan earthquake and its adjacent region, the crust has a high-velocity structure with ~ 4.0 km/s in depth of 14 ~ 20km and the middle-lower crust has a low- velocity layer with 3.3 ~ 3.6km/s. These manifest that the reason why the Wenchuan earthquake released vast energy could be this event occurred within the high-velocity medium of the upper crust and the low-velocity media around and below the source provide a relaxation-boundary for the reverse thrust of the upper crust under the eastward movement of the Tibetan plateau against the hard lower crust beneath the Sichuan basin. 3)The polarization analysis of the S-wave splitting of the local events manifests that the fast-wave directions are perpendicular to the Longmenshan faults in the Songpan-Ganzi block and parallel with the faults in the Sichuan basin. Moreover, inside of the Longmenshan fault zone, the fast-wave directions are perpendicular to the faults in the south part, and it becomes parallel with the faults in the north part. These observations manifest that the stress field is completely different in the north and south part of the Longmenshan fault zone. This could be able to interprete the difference of the focal mechanism between the main shock and aftershocks along the Longmenshan faults.
U22B-02
Three-dimensional imaging of Longmenshan fault using aftershocks of Wenchuan 8.0 earthquake, Sichuan, China
We present a three-dimensional (3D) compressional wave speed (Vp), shear wave speed (Vs) and Vp/Vs model for the Longmenshan fault region, Sichuan China, using aftershocks associated with the 2008 Wenchuan 8.0 earthquake. The velocity and ratio models are obtained by a new version of the double- difference seismic tomography method (Zhang, 2003) to simultaneously solve for Vp, Vs, Vp/Vs and event locations. The data used in inversion include 73,013 P arrival times, 62,287 S arrival times and 61,823 S-P travel times recorded by 63 stations on both permanent network and temporary stations in a region 400 km northeast¡§Csouthwest by 200 km northwest¡§Csoutheast. The tomographic results show that: (1) in southern segment of Longmenshan fault, the fault is a clear boundary in velocity and Vp/Vs ratio. High P and S velocity and low Vp/Vs ratio exist in western region to the fault, while low velocity and high Vp/Vs ratio in eastern region to the fault, and the Longmenshan fault extends to near 30 km in depth; (2) in northern segment, the general pattern looks like the southern segment, but the Longmenshan fault cuts off a small block with high velocity and low Vp/Vs ratio from western region to the fault with low Vp/Vs ratio; (3) the earthquakes relocated by the joint tomographic inversion collapse to a thin line along the Longmenshan fault, consistent with ground surface ruptures.
U22B-03
Coseismic Rupture Distribution of the 2008 Wenchuan, China Earthquake Inferred Using GPS and InSAR Measurements
We use GPS and InSAR data collected at the vicinity of the Wenchuan earthquake to invert for its coseismic slip distribution. More than 100 GPS coseismic displacement vectors are derived using data collected before and after the quake mainly from the Crustal Motion Observation Network of China within a 200-km distance from the seismogenic fault. The ALOS and Envisat SAR data are used to derive the interference scenes covering a 320-km stretch of the Longmen Shan fault ruptured during the quake. We device a fault model whose dip angle varies continuously and is inverted along with fault rupture distribution iteratively using the least-squares method. Our result shows that from southwest to northeast along the ruptured Beichuan- Yingxiu fault the fault geometry changes progressively from shallow southwest dipping to near vertical, and the rupture changes from predominantly thrust to right lateral faulting. The Guanxian-Jingyou fault which is in parallel and about 13-15 km southeast of the Beichuan-Yingxiu fault also had a segment ruptured for ~70 km long during the quake. This fault dips shallowly to the northwest, suggesting that the two ruptured faults may converge into the same ramp at depth. Fault rupture on the Beichuan-Yingxiu fault peaks at two places, one is near Yingxiu with about 5.8 m maximum reverse faulting, and the other near Beichuan with a maximum of about 5.0 m and 4.8 m reverse and dextral faulting. Both places happen to have suffered extreme structural damages and greatest losses in human life. Our estimated total seismic moment release is 1.06×1021 N-m, equivalent to an earthquake magnitude Mw 8.0.
U22B-04
Kinematic Coseismic Slip Model for the 12 May 2008 Wenchuan-Beichuan Mw 7.9 Earthquake in Sichuan, China from Joint Inversion of ALOS, Envisat and Teleseismic Data*
The Mw 7.9 earthquake struck Sichuan province on 12 May 2008 causing catastrophic damage over a large area including the county seats of Wenchuan and Beichuan. We use pixel-offset analysis of amplitude images for ALOS PALSAR and Envisat ASAR coseismic pairs to map the major surface ruptures. The largest amount of displacement (2--8 m of oblique right-lateral slip) occurred along the Beichuan fault and an extension to the north for a total distance of about 215 km. A second major rupture occurred on the Hanwang fault (a section of the Pengguan fault), beneath an anticline in the Sichuan basin about 10 km to the SE of the Beichuan fault, with nearly pure thrust slip. A third short, but intense, rupture strikes NW through the town of Xiaoyudong and transfers slip to another thrust about 5 km SE of the main rupture that ruptured only about 6 km parallel to the Beichuan fault. This small, probably shallow, block moved at least 5 m in the ALOS line of sight. Kinematic slip models for the earthquake have been estimated using a joint inversion of teleseismic data with the six ascending-track PALSAR and three descending-track ASAR strip-map interferograms. The models show that the rupture initiated with a small, 3-second pulse of emergent displacement followed by 15 seconds of moderate moment release with nearly pure thrust motion. More rapid moment release then started about 20 km from the hypocenter at shallower depths near the SW end of the major surface ruptures on the Beichuan fault. Another major patch of slip, coherent with the pixel-offset and InSAR analysis, is identified 130 km away from the epicenter, near the town of Beichuan. While the southern part of the rupture had primarily thrust motion, slip rotated to right-lateral as the rupture propagated the NE. *Part of this research was performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration.
U22B-05
Co-seismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China
Field investigations show that the Mw 7.9 Wenchuan earthquake on 12th May 2008 ruptured two NW-dipping imbricate reverse faults along the Longmenshan thrust fault zone at the eastern margin of the Tibetan plateau. This earthquake produced a 240-km-long surface rupture zone along the Beichuan fault characterized by right-lateral oblique faulting and a 72-km-long surface rupture zone along the Pengguan fault characterized by dip-slip reverse faulting. Maximum vertical and horizontal dispacements of 6.2 m and 4.9 m, respectively, were observed along the Beichuan fault, whereas a maximum vertical displacement of 3.5 m occurred along the Pengguan fault. This co-seismic surface rupture pattern, involving multiple structures, is among the most complicated of recent great earthquakes and its surface rupture length is the longest among the co-seismic surface rupture zones for reverse faulting events ever reported in the intraplate setting. A 3D model for the rupture geometry shows crustal shortening to be the dominant process along the Longmenshan thrust fault zone to accommodate long-term deformation. Thus, oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of Tibet Plateau accommodates, in part, the continuing penetration of the Indian plate into the Eurasian plate, and this extrusion is transformed at the eastern margin of the Tibetan Plateau into crustal thickening and shortening that is responsible for the growth of high topography in the region.
U22B-06
Precursory Signals Using Satellite and Ground data Associated with the Wenchuan Earthquake of May 12, 2008
The recent studies using satellite and ground data have shown existence of strong coupling between lithosphere-atmosphere-ionosphere. Multi satellite sensor and ground observation data are analyzed soon after the Wenchuan earthquake of magnitude 7.9 on Richter scale occurred on May 12, 2008 which was felt by millions of people living in a number of countries. This earthquake took 60000 lives and made millions of people homeless, damaging small and large infrastructure, and huge area was affected by the surface manifestations. The surface manifestations have further given water rise in lakes which have threatened millions of people. Detailed analysis of multi sensor satellite and ground observation data show pronounced changes showing complementary nature prior and after the main earthquake event. The multi parameters related to land, atmosphere and ionosphere showing existence of strong coupling between land, atmosphere and ionosphere. A possible mechanism about the coupling will be presented that show a great potential of combined multi satellite sensor and ground data for earthquake precursory studies.
U22B-07
Identification of Seismic Asperities of the 2008 Wenchuan Earthquake Through Geomagnetic Anomalies
May 12, 2008 Wenchuan earthquake was one of the largest earthquakes in the past 100 years, which occurred inland of continental region. We apply the waveform inversion by Kikuchi and Kanamori (1991, 2003) to obtain slip distribution in the source fault at the mainshock. We use 47 broadband seismograms of IRIS GSN and IFREE OHP seismic stations with epicentral distance between 30 and 100 degrees. The results show two significant asperities along the fault. We compare the earthquake rupture model with the geomagnetic anomalies originated from magnetized rocks in the crust because the geomagnetic anomalies of the crust are considered to relate to the temperature of crustal rocks. We use the NGDC-720 model, which expands the Earth's magnetic field of crustal origin by the spherical harmonic degree from 16 to 720. We convert the field values into the Keyhole Markup Language (KML) format so that we can easily view the field on Google Earth (Nagao et al., 2008). By using KML format, which is based on XML standard, it is possible to display any type of Earth science data on the surface of the globe as far as they have geographical location as attributes (Yamagishi et al., 2006, 2008). Here we show that the locations of these seismic asperities coincide with those of geomagnetic anomalies originated from Earth's crust. The geomagnetic anomalies can be interpreted as depth variation of the Curie point temperature inside crust, above which the magnetites lose their magnetic property (about 580 degree centigrade). The Curie point depth distribution can be obtained through the spectral analysis of geomagnetic anomalies within the crust and used to infer the temperature distribution inside the crust. We follow basically the procedure of Spector and Grant (1970). Depth variation of the Curie point suggests that these seismic asperities coincide with the region where the Curie point is deep and the temperature of the fault corresponds to the condition of frictional instability. This result suggests that the location of seismic asperity along the active fault inside crust may be identified through geomagnetic measurements, which is suitable to remote regions like fault area of this event.
U22B-08
Variations of shear wave splitting in the 2008 Wenchuan earthquake region
In this work, variation of shear wave splitting in the 2008 Wenchuan earthquake sequence was studied. By analyzing S-wave particle motion of local events in the shear wave window, the polarization directions of the faster shear wave and the delay times between the faster and the slower shear waves were derived from seismic recordings at the stations near the fault zones. The shear wave splitting results of seven stations in the area of Longmenshan fault zone reveal spatial variation of the polarization directions of the fast shear wave. The directions at stations in the southeastern side of the Longmenshan fault zone (in the Sichuan Basin area) are in the NE direction, while the direction at station PWU (in the Plateau), which is in the northwestern side of the faults, is in the EW direction. Systematic changes of the time delays between two split shear waves were also observed. At station L5501 in the southern end of the aftershock zone, the delay times of the slower shear wave decrease systematically after the main shock. After the main shock, the delay times at station PWU were larger than those before the earthquake. Seismic shear wave splitting is caused mostly by stress-aligned microcracks in rock below the stations. The results demonstrate changes of local stress field during the main-shock and the aftershocks. The stress in the southern part of Wenchuan seismogenic zone was released by the main-shock and the aftershocks. The crustal stresses transferred to the northeastern part of the zone, resulting in stress increase at station PWU after the main-shock.