Seismic imaging under the 2013 Ms 7.0 Lushan Earthquake, China

On 20 April 2013, a large earthquake (Ms 7.0) occurred at southern end of the Longmen-Shan fault zone. More than 200 people were killed and about 14,000 people were hurt by the earthquake. The earthquake occurred with some distinct features: 1) The hypocenter of the Lushan earthquake is located close to the devastating 2008 M 8.0 Wenchuan earthquake occurred at the Longmen-Shan fault zone; 2) The time scale of the earthquake generation is not more than five years after the M 8.0 earthquake; 3) The magnitude of the Lushan earthquake is as large as 7.0 within such a close time of the Wenchuan earthquake; 4) The hypocenter of the Lushan earthquake seems to be located at the southern part of the Longmen-Shan faut zone with a 70-km distance away from the Wenchuan source. These features of the Lushan earthquake leads to a number of researcher wonder of its nucleation mechanism, rupture process and the correlation of the wenchuan earthquakes. Global seismic waveform data analyzing shows where the rupture initiated and how it expanded for the 2013 Ms 7.0 Lushan earthquake. Our seismic imaging and crustal stress analyzing indicates that the hypocenter of the Lushan earthquake occurred at a strong high-velocity (Vp, Vs) and low-Poisson's ratio zone with high crustal stress. Similarly, high-velocity (Vp, Vs) and low-Poisson's ratio with high crustal stress is revealed under the 2008 Wenchuan earthquake (Ms 8.0) source area. However, a sharp contrast gap zone with low-velocity, high-Poisson's ratio anomalies is clearly imaged under the conjunction area between the two earthquake sources. We suggest that the strong structural variation and high crustal stress together with high coseismic stress by the Wenchuan Earthquake triggered the 2013 Lushan Earthquake (Ms 7.0) and controlling its rupture process. We believe that the rapid seismic imaging together with the crustal stress analysis could help to understand the Lushan earthquake generation and to evaluate the possibility of producing large aftershocks at the gap zone along the Longmen-Shan fault belt. Tectonic crustal stress data analyzing provide important insights into relative plate motion, mountain building and genesis of larger crustal earthquakes underneath tectonically active regions. Most of the researcher suppose that the Lushan earthquake might be initiated by the high coseismic stress and be one of the aftershocks of the Wenchuan earthquakes due to the close distance of the hypocenters. Teleseismic waveform inversion indicates 60-90 cm coseismic slip with 50 km lateral expandation and an extend depth of 30 km along the Longmen-Shan fault zone. A coseismic gap zone with 30-40 km horizontal space is clearly revealed between the 2013 Lushan and 2008 Wenchuan earthquakes, which leads to many geologists and geophysists put their eyes on it. Some of them consider that there is high risk of large aftershock occurring at the gap zone whilst the others do not think so. Geological field works imply no obvious surface coseismic slip is observed in the source area as well as the gap zone. We consider that the detail seismic structures in the source area and its surrounding regions are necessary for the investigation of the earthquake generation and rupture process as well as the possibility of earthquake occurrence at the gap zone.