Earthquake near Wenchuan,West Sichuan, China

2008 May 12 06:28:01 UTC; Magnitude 7.9


On Monday 12 May 2008, an earthquake of magnitude 7.9 struck northwestern Sichuan province of China. It is the most devastating earthquake in China in more than three decades. As of June 1, 69,016 deaths have been confirmed, 368,565 people have been injured, and many more have lost their homes. Our thoughts are with the people in Sichuan, China.

For many years, scientists at MIT's Department of Earth, Atmospheric, and Planetary Sciences have been involved in research of Tibet and SW China. With this web site we wish to share results of that research insofar it relates to the recent seismic events.


1) GPS measurements (Yellow arrows) in West Sichuan, China; Epicenters from USGS; aerial photograph from Google Earth:

MIT scientists were involved in the first GPS measurements in this area, with involvement continuing until recently. The field shown is a combination of data acquired by the Institute of Geology and Mineral Resources, Chengdu (Chen Ziliang, Liu Yuping, Zhang Xuanyang, and Zhao Jixiang), in collaboration with MIT (Clark Burchfiel, Robert King, and Leigh Royden), the Lehigh-led collaboration for the eastern syntaxis (4 Chinese and 7 US authors, including Robert King, Einat Lev, and Rob van der Hilst), and published velocities from China Earthquake Authority acquisitions (worked on by MIT graduate student Jiangning Lu). Velocities are plotted relative to Chengdu in the South China reference frame. Uncertainties are < 2 mm/yr. A scale is given in the lower-right corner. Also shown are USGS epicenter locations for M > 5 earthquakes through May 15, 2008 and their moment tensor mechanism for the main event.

 

2) Large scale structure from travel time tomography:

For tomographic imaging Chang Li (MIT, Ph.D. 2007) used data from a global catalog, from national and provincial networks in China, and from a temporary seismograph array operated by MIT and CIGMR (Chengdu Institute of Geology and Mineral Resources) from Sept 2003 to Oct 2004 (Li et al., 2008). Black lines depict the major fault zones in the region. The focal mechanism -- determined by the US Geological Survey's National Earthquake Information Center (USGS) -- indicates that the main shock in Wenchuan County involved thrusting due to compression in NW-SE direction. The aftershocks (green circles) occurred along the Longmen Shan thrust belt. The background events (purple dots) are from the EHB catalog (1964~2007).

The Longmen Shan marks the tectonic contact between the Sichuan Basin to the East and the mountains of western Sichuan and the eastern Tibetan plateau to the West (Burchfiel et al., 1995), see below. Marked by fast P -wave propagation to at least 250 km depth, the low-elevation and topographically flat Sichuan Basin appears to be a deeply-rooted, mechanically strong unit underlain by craton-like lithosphere that has resisted Mesozoic and Cenozoic deformations that affected the surrounding regions. The slow seismic wave propagation west of the Longmen Shan fault zone suggests that the mechanical strength is here much lower than beneath the Sichuan Basin (Royden et al., 2008). The earthquakes reflect tectonic stresses resulting from the relative motion between these tectonic units. Geological structures along the Longmen Shan suggest a total displacements of tens of kilometers since the Late Cenozoic (Burchfiel et al., 1995) and GPS measurements constrain active rates at a few mm/yr (Meade, 2007; Royden et al., 2008).

3) Longmen Shan Fault zone:

EAPS scientists have been investigating the geology of the Longmen Shan region for many years. Burchfiel et al. (1995) illustrate that the tectonic evolution of the Longmen Shan is complex and still only moderately understood. The Longmen Shan marks not only the present boundary between the high topography of the Tibetan Plateau to the west and the relatively undeformed Sichuan Basin to the east, but this region also marks the limit of deformation during the Mesozoic Indosinian orogeny. During the Late Triassic to Early Jurassic, a sequence of continental margin sediments and flysch were highly deformed and thrust eastward onto the rocks of the Yangtze craton while the Sichuan Basin was accumulating clastic sediments as a foredeep basin. The structures of the Longmen Shan region primarily reflect this Mesozoic deformation; Cenozoic faults and folds tend to parallel and often reactivate Mesozoic structures. Cenozoic deformation in the Longmen Shan is difficult to constrain, but there is evidence for right-lateral strike-slip, thrusting, and normal faulting on several different structures. The fault that appears to have broken in the earthquake is at or very near the boundary between the Precambrian rocks of the Pengguan Massif and the Mesozoic foredeep sediments of the Sichuan Basin . The fault has a history of mostly right-lateral strike-slip and a smaller amount of thrust motion (Densmore et al., 2007).

4) Structure of the crust from surface wave array tomography:

Huajian Yao (MIT, Ph.D. 2009) inferred spatial variations in shear wavespeed in the upper-, middle-, and lower crust of West Sichuan province using waveform data (Green's functions) obtained from interferometry (long-term cross-correlation). This "ambient noise tomography" allows high resolution imaging of the shallow subsurface beneath dense seismograph arrays. The white contour lines in (c) depict the depth to the crust-mantle boundary; the Longmen Shan marks the (rapid) transition from thick (60 km +) crust beneath the Tibetan Plateau to continental crust with normal thickness (around 40 km) beneath the Sichuan Basin.

These results show that strong lateral gradients in wavespeed in the middle and lower crust often coincide with major faults (black lines). Understanding the relationship between seismically slow (and, presumably, mechanically weak) layers and crust-cutting faults is important for our understanding of the tectonic deformation and the seismicity in the region (Yao et al. 2008; Royden et al., 2008).

The Wenchuan earthquake of 12 May 2008 occurred just outside the footprint of the MIT-CIGMR array, where the spatial resolution of the geological structures is relatively low. In close collaboration with colleagues at the Chinese Earthquake Administration, in Beijing and in Chengdu, our research aims to improve our understanding of the dynamics and seismo-tectonics in this area of high seismic risk.

 

5) Acknowledgments:

MIT research of the structure and geological evolution of the Tibetan Plateau and SW China has been supported by the National Science Foundation (Earth Sciences, Continental Dynamics Program).

6) References Cited:

Chen Z., B.C. Burchfiel, Y. Liu, R.W. King, L.H. Royden, W.Tang, E. Wang, J. Zhao, and X. Zhang. Global Positioning System measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation , J. Geophys. Res. 105 16215-16228 (2000). (PDF)

Shen Z., J. Lu, M. Wang, R. Burgmann, Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau, J. Geophys. Res., 110, 1-17 (2005).

Sol S., Meltzer A., Burgmann R., van der Hilst R.D., King R., Chen Z., Koons P., Lev E., Liu Y., Zeitler P.K., Zhang X., Zhang J., Zurek B., Geodynamics of southeastern Tibet from seismic anisotropy and geodesy . Geology, 35, 563-566, doi: 10.1130/G23408A1 (2007). (PDF)

Densmore A.L., M.A. Ellis, Y. Li, R. Zhou, G.S. Hancock, and N. Richardson, Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau, Tectonics, 26, TC4005, doi:10.1029/2006TC001987 (2007).

B.C. Burchfiel, Z. Chen, Y. Liu, L.H. Royden, Tectonics of the Longmen Shan and adjacent regoins, central China, International Geological Review, 37(8), edited by W.G. Ernst, B.J. Skinner, L.A. Taylor (1995).

C. Li, R.D. van der Hilst, A.S. Meltzer, and E.R. Engdahl, Subduction of the Indian lithosphere beneath the Tibetan Plateau and Burma, revised for: Earth and Planetary Science Letters (2008).

L.H. Royden, B.C. Burchfiel, and R.D. van der Hilst, The geological evolution of the Tibetan Plateau, revised for: Science (2008)

H. Yao, C. Beghein, and R.D. van der Hilst, Surface-wave array tomography in SE Tibet from ambient seismic noise and two-station analysis: II - crustal and upper mantle structure, Geophysical Journal International, vol. 173, 205-219 (2008). (PDF)

B.J. Meade, Present-day kinematics at the India-Asia collision zone, Geology, 35, 81-84, doi:10.1130/G22942A.1 (2007).

 

7) Further information:

For more information about the Weichuan earthquake and its aftershocks, please check the following web sites:

http://earthquake.usgs.gov/
http://earthquake.usgs.gov/eqcenter/recenteqsww/Maps/10/105_30.php
http://neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_cmt.html


For more information about the geodynamic studies, please contact Leigh Royden or Bradford Hager (EAPS-MIT).

For more information about the geological studies, please contact B. Clark Burchfiel (EAPS-MIT).

For more information about the tomographic studies, please contact Robert D. van der Hilst (EAPS-MIT).