Present Deep Deformation along the Longmenshan Fault by Seismic Data and Implications for the Tectonic Context of the Wenchuan Earthquake

One of the causes for the Longmenshan fault to be neglected before the May 12, 2008, Wenchuan, MS 8.0 earthquake was that geological evidences and GPS measurements all indicated that this fault is the one with extremely slow deformation rate. Suspecting that whether this conclusion holds for the real situation at depth because geological estimation deals with a very long time scale, and GPS measurement deals with the horizontal deformation at the surface of the Earth only within the recent decade, we tried to take the approach similar to Kostrov method to investigate the deep deformation using data of microseismicity since the recent 3 decades. We used local earthquake catalogues complete down to ML 2.5 to calculate the Benioff strain along the Longmenshan fault and the distribution of a-value in the Gutenberg-Richter frequency-magnitude relation. We considered the nearby Xianshuihe, Anninghe, and Zemuhe fault as a comparison. What is observed is that at the time scale different from geological process and GPS-observed deformation, for the depths different from the surface of ground, considering both horizontal and vertical deformation, and comparing with its neighboring fault systems, Longmenshan fault is indeed not as 'quiet' as traditionally assessed. At shorter time scale, some anomalous pattern of seismicity can be observed retrospectively before the Wenchuan earthquake, but they are hard to use as an indication of the approaching of this catastrophic event. Implication of the observations to the study of the Wenchuan earthquake is discussed.