Correlation of major earthquake sequences on the northern and eastern boundaries of the Bayan Har block, and its relation to the 2008 Wenchuan earthquake

The 2008 Wenchuan earthquake occurred on the eastern boundary of the Bayan Har block.To understand better the regional dynamic environment and its relation to the earthquake, this research studies the kinematics and deformation of the fault systems of the northern and eastern boundaries of the block,as well as the correlation of occurrences of major earthquake sequences on the two boundaries,based on analyzing data from active tectonics,focal mechanism,GPS station velocity,earthquake rupture distribution and seismicity of historical major earthquakes.The result shows that,owing to the resistance of the South China block,the east-to southeast ward "extruding" movement of the Bayan Har block,which is manifested by the active left-lateral strike-slip faulting along the northern boundary of the block,is transformed into thrust faulting and lateral shortening with strike-slip faulting on the eastern boundary of the block.This suggests that the "extruding" movement of the block has been "loading on" the eastern boundary remarkably.Such loading is also presented in the correlation of occurrences of major earthquake sequences on the two fault systems of the northern and eastern boundaries of the block.From the middle to late 19th century to 2001,a sequence of 8 major events ruptured about2/3 of the whole length of the northern boundary of the Bayan Har block.At the same time, the sequence has the characteristics of accelerating strain release and gradually shortening intervals between events,suggesting that the movement of the block has been speeding up since 1902 at the latest.As a response to this speeding-up movement,a sequence of 3 major events have occurred on the eastern boundary of the block since about 1933,which has also the characteristics of accelerating strain release and gradually shortening intervals between events. The 2008 Wenchuan earthquake is the most-recent event in the response sequence.The factor that the occurrence of the response sequence of major earthquakes on the eastern boundary is delayed for decades at least behind the sequence on the northern boundary would be associated with the non-rigid motion of the block.In addition,it is found that the event s order-number versus time relationships of the two sequences of major earthquakes are helpful in analyzing the mid-and long-term seismic potentials for the northern and eastern boundaries of the Bayan Har block.