Mechanism and Prediction of the Intraplate Earthquakes:An Example from Wenchuan Earthquake in Sichuan Province

The Wenchuan earthquake is a typical intraplate earthquake.Continental intraplate earthquakes are mainly disseminated in the Cenozoic thick-crust orogenic belt or plateau,and distribute in the middle crust as shallow source earthquake in the section.The active faults in continental basin-mountain system exhibit regular pattern,such as reverse or thrust faults on the basin-mountain extrusion boundary,strike-slip faults on the basin-mountain transform boundary,or normal and detachment faults in the extensional orogenic zone interior.While the tectonophysic process of continental intraplate earthquakes is generally controlled by the lamination and rheology of continental crust.Continental lower crust provides heat energy for earthquake,and thermal softening and thermal melting media in lower crust occur slow ductile flow as causative structure.The ductile-brittle transition zone in middle crust appears heat-stress transformation,and accumulate stress and strain.The brittle seismogenic faults in upper crust result from stress and strain accumulation in middle crust caused by lower crustal flow.Seismic source locates at the connection spot between the ductile-brittle shear zone in middle crust and brittle active faults in upper crust,while the focus depth in the Qinghai-Tibet plateau is mostly ound from 12 to 35 km.Earthquake predictioni is a world scientific problem.However,new theory of continental dynamics and intraplate earthquake based on scientific data has probabilities for short-term and imminent prediction.If lower crustal flow causes seismic source of middle crust and brittle fracture of upper crust,the brittle faults of upper crust can release thermal fluid and gas from deep crust,which results in the geothermal anomaly,hydrologic anomaly and atmosphere anomaly in the local areas.It is suggested that thermal anomaly and ingredient variation of underground water,ground water,and atmosphere are systematically monitored along active fault zone and adjacent areas,and activity of crust and probability of earthquake can be appraised according to above thermal anomaly and relative data such as thickness of lower crustal flow layer,stress-strain change,the gravity anomaly,magnetism anomaly and terrestrial electricity anomaly.The Wenchuan earthquake of May 12th,2008 on the tectonic margin between the Longmenshan orogenic belt and Sichuan Basin had a magnitude of 8.0 Ms,with a depth of about 12 km on the Yingxiu-Beichuan fault.A 350 km-long surface rupture zone exhibits right-lateral and left-step features,related to the west-dip Wenchuan-Maoxian-Qingchuan fault,Yingxiu-Beichuan fault and the Jiangyou-Dujiangyan fault shown dextral strike-slip and thrust.The brittle active faults in the upper crust,which control surface rupture zones and landslides,were produced by slowly ductile flow of lower crust with the stress and strain accumulation at the ductile-brittle shear zone of the middle crust.The brttle seismogenic faults may release some heat energy from the lower crust and cause the local thermal anomaly of underground water,ground water and atmosphere.