Seismic study of the crust and upper mantle structure in eastern Asia using ambient noise correlation and earthquake data
Seismic methods using natural earthquakes are commonly applied to investigate the velocity structures of the crust and upper mantle. Among all these methods, seismic tomography is one of the most powerful tools to image the earth’s structure. Recent theoretical and experimental studies suggest that the media impulse response (Green’s function) of surface wave between two seismic stations can be retrieved from the cross correlation of diffusive wave field. This method provides additional data coverage to the study region and has been quickly used in various seismic studies. In this dissertation, I develop several tomography methods using traditional earthquake data as well as surface wave data from ambient noise correlation and apply them in eastern Asia. A joint inversion scheme using first and secondary Pg arrivals and Pn arrivals is developed to determine the crustal and uppermost mantle P wave velocity and the depth of Moho discontinuity. This method addresses the trade-off issue between the media velocity and a discontinuity location. I apply this method to the eastern and southern margin of Tibetan Plateau, where the Moho depth changes rapidly. Significant features are revealed from tomography results, in consistence of regional geological settings, including slow upper crust but fast lower crust under Sichuan Basin, rapid Moho variation, and slow Pn in the plateau and fast Pn in the borderland separated by major boundaries and faults. The mid-lower crust in the plateau is relatively slow compared to the average model and Sichuan Basin and Poisson’s ratio is anomalous (exceeding 0.3), which is consistent with weak mid-lower crust in the plateau. The crust and upper mantle shear wave velocity in China and its surrounding region is obtained from regional surface wave tomography. By combining traditional earthquake surface wave data with ambient noise correlation data, I am able to extend the Rayleigh wave group velocity period from 8 s to 120 s. Together with Rayleigh phase velocity from 8 s to 70 s derived from ambient noise data, shear velocity down to 150 km is inverted in the study area. Striking crust and upper mantle structures are well resolved, including major basins, Moho depth variation, mantle velocity contrast between eastern and western North China craton, widespread low-velocity zone in mid-crust in much of the Tibetan Plateau, and clear velocity contrasts of the mantle lithosphere between north and southern Tibet with significant E-W variations. iii To overcome the depth-velocity trade-off of teleseismic P wave receiver function and the insensitivity to velocity contrast in surface wave inversion, a nonlinear searching scheme using Neighborhood Algorithm is developed to jointly determine shear velocity and Moho depth using these two very different data sets. This method is applied to Hi-CLIMB seismic stations in Tibetan Plateau. A high resolution 2D shear velocity image is derived from joint inversion. The results reveal the detailed shape of mid crust low velocity zone that exists underneath most part of Tibet and the Moho variations between Lhasa block and Qiangtang block. A zone of interrupted Moho between northern Lhasa block and Bangong-Nujiang suture is imaged. The results demonstrate the complexity of the crust and upper mantle structure under Tibetan plateau and agree with previous studies. Because the process of ambient noise correlation is completely repeatable, ambient noise correlation data can be used to monitor the media velocity changes. By examine ambient noise data at different time periods between the same station pairs deployed in Malaysia, Indonesia and Thailand for over four years, significant time shifts of Rayleigh wave signals are observed after three major Sumatra earthquakes in 2004, 2005, and 2007, respectively. In addition to the time shift after those earthquakes, we observed a plausible precursor signal one month before 2004 event. The time shifts appear to be frequency dependent. The absence of corresponding time shift in Love wave suggests the signal is not from clock error. The observations are interpreted as stress changes and subsequent relaxation in upper-mid crust in the immediate vicinity of the rupture as well as the broad area near the fault zone.
Seismic tomography, Joint inversion, Surface wave, Ambient noise correlation, Temporal changes, Tibetan Plateau
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign