Spatiotemporal Characteristics Of Velocity Changes Associated With Tectonic Deformation And Strong Earthquakes In North-South Seismic Belt In China

Noise-based seismic monitoring is an important technique to track the stress in the subsurface,which contributes to understand the dynamics of the seismicity.Due to the collision of the Indian continent with Eurasian Plates,the crustal deformation is strong.Monitoring of the stress buildup and release is helpful to probe earthquake nucleation processes and occurrences.In this region,several large earthquakes have occurred,such as 2008 Wenchuan earthquake,and 2013 Lushan earthquake,and 2014 Ludian earthquake,and 2017 Jiuzhaigou earthquake.Every earthquake causes large fatalities and enormous economic damage.Since the slip rates are very small across the Longmen Shan fault,the researchers did not anticipate the Wenchuan earthquake to occur in this region.Based on the questions above,we measured the changes of the properties of the medium based on the ambient noise interferometry.We measured the seismic velocity changes to track the stress evolution.The main results and conclusions of the dissertation are listed below:(1)we cross correlated the continuous vertical records to obtain the cross correlation functions,and compared the cross correlations at different periods to measure the velocity changes.In the entire north-south seismic belt,we observed clear seasonal velocity changes.The seasonal velocity changes exhibit different characteristics in different regions.In the eastern margin of the Tibet Plateau,the seasonality is stronger,in contrast,the seasonality is weak in the Sichuan basin.Moreover,the seasonality is also weak in South China.In Guangdong,the seasonality is weakest.(2)The seasonality rapidly varies in strength,amplitude,phase,periods,and location.But for the large scales,the seasonal deformation is consistent with the tectonic deformation.The stronger the seasonality,the greater the tectonic deformations is,and vice versa.(3)The phases of the seasonality changes well match the peak and through of the precipitation,and the seasonal velocity changes at different depths are generally synchronized for most paths.Based on the key information,we conclude the loading of the precipitation is main mechanism of the seasonal velocity changes.The pore pressure change is a secondary effect.(4)Although the precipitation causes the seasonal velocity changes,the key controlling factor of the seasonal deformation is not the precipitation,but the tectonic deformation.In Guangdong,the precipitation is strongest,but the seasonal velocity changes are weakest in this region.In contrast,in the north segment of the north-south seismic belt,the seasonal velocity changes are very significant,but the precipitation is very small.(5)We observed significant coseismic velocity drop associated with the Wenchuan earthquake.The spatiotemporal patterns of the velocity changes are well consistent with the Wenchuan earthquake.(6)From the station pairs with the weakest seasonality,we found a velocity drop delayed by about 1.5 years.Moreover,we observed postseismic velocity change recovery at greater depth.We conclude that the delay of the maximum velocity drop is governed by the aftershock slips.The viscoelastic relaxation of the stress at depth causes the postseismic recovery process.