Seismic properties of the Longmen Shan complex: Implications for the moment magnitude of the great 2008 Wenchuan earthquake in China

摘要

The 12 May 2008 Wenchuan earthquake is the largest active tectonic event reported to date in Sichuan (China). We have experimentally calibrated, up to 800MPa, seismic and elastic properties of 12 representative samples from the Longmen Shan complex in which this great earthquake took place and its coseismic ruptures nucleated and propagated. Most of the samples show little V _p or V _s anisotropy at pressures above the microcrack-closure pressure (P _c=200-300MPa), and so the variation of anisotropy with pressure provides important hints for the preferred orientation of microcracks in the nonlinear poroelastic regime below P _c. Geothermal and rheological profiles indicate that the focal depth (~19km) corresponds to the base of the schizosphere, below which the Longmen Shan complex switches from the brittle to ductile behavior. The investigation reveals that the crust of the Longmen Shan range consists of 4 layers from the surface to the Moho: Layer 1: V _p<4.88km/s (0-3km thick, sedimentary rocks such as limestone, sandstone, conglomerate, and mudstone); Layer 2: V _p=5.95-6.25km/s (25-28km thick, felsic rocks); Layer 3: V _p=6.55km/s (10km thick, 67.5% felsic and 32.5% mafic rocks); and Layer 4: V _p=6.90km/s (8km thick, 20.0% felsic and 80.0% mafic rocks). The average V _p/V _s ratio of 1.71 or Poisson's ratio of 0.24 calculated for the whole crust is consistent with the results measured using teleseismic receiver function techniques. This study also offers necessary information for broadband simulations of strong ground motions in the assessment and forecast of earthquake hazards in the region. Furthermore, the study, which yields a moment magnitude of 7.9-8.0 given the variation in the dip of the coseismic ruptures and the uncertainty in the depth to which the coseismic rupture may propagate downwards below the depth of the mainshock hypocenter, presents the first accurate quantification of the 2008 Wenchuan earthquake's size.