The 2008 Wenchuan earthquake occurred on imbricate, oblique, steeply dipping, slowly slipping, listric-reverse faults. Measurements of coseismic slip, the distribution of aftershocks, and fault-plane solution of the mainshock all confirm this style of deformation and indicate cascading earthquake rupture of multiple segments, each with coseismic slip occurring in the shallow crust above a depth range of 10 to 12 km. Interactions among three geological units-eastern Tibet, the Longmen Shan, and the Sichuan basin-caused slow strain accumulation in the Longmen Shan so that measurable preearthquake slip was minor. Coseismic deformation, however, took place mostly within the interseismically locked Longmen Shan fault zone. The earthquake may have initiated from slip on a fault plane dipping 30-40 • northwest in a depth range from 15 to 20 km and triggered oblique slip on the high-angle faults at depths shallower than 15 km to form the great Wenchuan earthquake.
International audienceWe used continuous recordings in Sichuan, China to track the temporal change of the seismic wave speed at a regional scale, for 2 years including the Wenchuan Mw 7.9 earthquake. The data are recorded by a temporary network of 156 broad‐band seismographs in a region that covers the southern 2/3 of the fault activated during the earthquake. A doublet analysis applied on the codas of seismic noise cross correlation functions is used to detect temporal velocity changes. We found clear evidence that the seismic velocity drops by up to 0.08% in the fault region just after the earthquake with fluctuations within 0.02% before the earthquake. We compared the measurements in different sub‐arrays to get a spatial distribution of the velocity changes. This distribution is consistent with the volumetric strain change during the Wenchuan earthquake and shows that the co‐seismic velocity change is not controlled by the response of sediments
We track the temporal evolution of seismic wave speed to detect changes in material properties at depth, driven by deformation associated with the 2008 Mw 7.9 Wenchuan earthquake. We analyze ambient noise correlation functions to monitor seismic wave speed variations. The data were continuously recorded during 2 years by 114 broadband stations in a region that covers the southern two thirds of the ruptured fault. We perform the analysis in the 12‐to‐20‐s period band. By comparison with measurements in the band 1‐to‐3 s, we show that the seismic velocity changes cannot be explained by a shallow perturbation but are related to deformation at depth in the crust. The spatial and temporal characteristics of these variations can thus be viewed as reflecting the middle crust behavior. In particular, the results suggest that the deformation in the middle crust is different beneath Tibet and the Sichuan basin.
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