(January 26, 2009) Christian Klose, a geophysical hazards research scientist from Columbia University in New York, says geophysical data suggests that the Zipingpu dam reservoir — just a few kilometers from the epicenter of China’s great quake of 2008 — likely triggered the deadly quake and explains how it happened.

According to Klose’s presentation to last month’s American Geophysical Union meeting in San Francisco, “Several geophysical observations suggest that this M7.9 earthquake was triggered by local and abnormal mass imbalances on the surface of the Earth’s crust. These observations include (1) elastostatic response of the crust to the mass changes (2) slip distribution of the main rupture, and (3) aftershock distribution.” [see Abstract (PDF)].

Now, Science, the magazine of the American Association for the Advancement of Science, is covering the issue and explains Klose’s theory: “the added weight both eased the squeeze on the fault, weakening it, and increased the stress tending to rupture the fault. The effect was 25 times that of a year’s worth of natural stress loading from tectonic motions … When the fault did finally rupture, it moved just the way the reservoir loading had encouraged it to…”[full story].

Abstract
The 2008 M7.9 Wenchuan earthquake – Result of Local and Abnormal Mass Imbalances?
By Klose, C D
ck2204@columbia.edu
Columbia University, School of Engineering and Applied Science, 351 Mudd Building, New York, NY 10027, United States

AB: The May 12, 2008 M7.9 Wenchuan earthquake occurred along the Longmen Shan margin of the eastern Tibetan plateau in the Sichuan province of the People’s Republic of China. A complex and NNW dipping reverse fault system including the Beichuan fault ruptured 250-300 km parallel to the Longmen Shan thrust belt. This region has been tectonically loaded for >10kyr. It has low deformation rates of less than 1.0±1.0 mm yr-1 resulting in no major seismic activity during the Quaternary period. Several geophysical observations suggest that this M7.9 earthquake was triggered by local and abnormal mass imbalances on the surface of the Earth’s crust. These observations include (1) elastostatic response of the crust to the mass changes (2) slip distribution of the main rupture, and (3) aftershock distribution. Initially, approximately 2 years prior the nucleation of the mainshock, at least 320 million tonnes of water accumulated within the upper Min river valley. It enters the Chengdu plain of the Sichuan basin, a stable continental region (SCR). The water volume amplified the strain energy on the Earth’s crust. Shear stresses increased by >1kPa on the Beichuan fault at the nucleation point in about 20km depth. Normal stresses decreased by <-4kPa and weakened the fault strength. Pore pressure increases might have additionally destabilized the fault locally due to pore pressure diffusion. This effect, however, might be minor in 20km depth, because of low lateral fracture connectivity and permeability between the area of water accumulation and the Beichuan fault. Overall, the stress alterations within a 120±70km2 large area resulted in the Beichuan fault coming closer to failure. Such an area ruptured would account for a M7.2±0.1 earthquake assuming only 10 MPa stress drop. Secondly, a reverse fault focal mechanism dominated, in particular, during the first 50 seconds of the main M7.9 rupture. The Beichuan fault slipped up to 7m upward peaking at shallow depth (<7km) (Nishimura and Yagi 2008). A third reason is that the aftershock distribution (M>3) along the ruptured fault zone pointed toward a very shallow seismicity in the uppermost part of crust. Data show that about 87% of the total seismic moment has been released in the upper third part of the crust (<10km), 1% in the middle third (10-20km), and 12% in the lower third (>20km). Such a bimodal depth distribution with an aseismic mid crust is typical for earthquakes in stable continental regions (Klose and Seeber 2007, SRL 78:554-562). Thus, high spatiotemporal correlatives can be observed at shallow depth between both the seismic moment release of the aftershocks and the rupture slip distribution of the mainshock. This indicates that Mohr-Coulomb failure stress states were much higher in the uppermost part of crust than near a possible nucleation point of the mainshock in 19km depth. In conclusion, the ensemble of geophysical observations suggests that the root cause of triggering the M7.9 Wenchuan earthquake may have stemmed from local and rapid mass changes on the surface.
UR: http://www.geol.tsukuba.ac.jp/~nisimura/20080512/ [PDF]
AGU Fall Meeting

Science and American Geophysical Union, Three Gorges Probe, January 26, 2009