Passive Imaging From Ambient Seismic And Its Application In The Northeastern Margin Of The Tibet Plateau

The northeastern margin of the Tibet Plateau is located in the junction region ofQinghai Xizang block, Erdos block and Alashan block. India plate impinges upon theTibet Plateau, Erdos block and Alashan block at a rate of5cm/year. Under the intenseinteraction between blocks, many active blocks are formed covering both the internaland the boundary of the plates. Because of the interaction between active blocks,which in turn result into strong tectonic activity on their boundaries, a series of activefaults are formed, including the main fracture such as Altun fault and Haiyuan faultand Kunlun fault, where the main strong earthquakes are located. The region haswitnessed three earthquakes over the history; those are Yinchuan Ping Luo, NingxiaMs8earthquake in1739, the Haiyuan Ms8.5earthquake in1920and the Gulang Ms8earthquake in1927. Complex tectonic environment and intense seismic activitymake the northeastern edge of the Tibetan Plateau become a hot area of the seismicstudy.Because the traditional body wave and surface wave tomography method areinfluenced by the location of the earthquake, earthquake location accuracy and thestation distribution, the resolution of the velocity structure obtained is often low,especially for shallow crust; and the receiver function method can only get the crustand upper mantle structure just below the station location. The cross-correlationmethod of ambient seismic noise emerged in recent years is able to extract the surfacewave empirical Green function; ambient seismic noise tomography can overcome thedisadvantages mentioned above and by which high resolution information on thevelocity structure for shallow crust is obtainable. Therefore, the empirical Green’sfunction is obtained by using the interstation Cross-correlation functions of ambientseismic noise of continuous vertical component of motion of the72digital stationslocated in the northeastern margin of the Tibetan Plateau from June,2008to Dec,2009; the Rayleigh surface wave group velocity dispersion curves of9periods,covering from8Sec. to44Sec., were obtained by the ambient seismic noise method.The results show that, the group velocity of short period was affected by the deposition. There are low velocity anomaly in the Yinchuan basin, Erdos block andthe Western and Eastern Alashan basin, while the eastern of Tibet Plateau has highvelocity distribution, suggesting that the Haiyuan fault belt is the transition zone ofthe velocity; The group velocity distribution with the periods longer than the30sindicates that the Tibet Plateau has low velocity anomaly, the northern QilianMountains fault, as the boundary of velocity, north of which performance has highvelocity, while south has low velocity anomaly; TheTibet Plateau crust’s low velocityanomalies may be related with softening medium of the lower crustal.Meanwhile, we analysis the characteristic of the ambient seismic noise，Thedirection of the azimuth for the strongest noise energy between5and10s is found tobe very stable in time covering105°165°, and the main source region may be in thePacific. At the same time, the primary (10-20s) microseism has a very clear seasonalvariability, whose azimuth is165°210°in the Summer and moves to300°350°and165°210°in winter. This suggests that the noise source may be coming from theeffect of the Indian Ocean during the summer and the effect of the Pacific and thenorthern Atlantic during the winter. During the study on the underground mediumwave velocity change over time, the necessary consideration includes not only theseasonal variations which may result from the characteristics of strong seasonalvariations and direction dominance of energy distribution presented by the noisesource of The northeastern margin of the Tibet Plateau, but also the azimuthorientation of ray, especially when using station ray time shift method to study on themedium velocity variation.Finally, using the dynamic ambient seismic noise tomography method,exploratory research on the velocity variation before and after the earthquake wasdone; the results indicated that, before the earthquake in the vicinity of the focal area,there would be a certain velocity anomalies. As an attempt, more earthquake casesneed to be studied.