Multi-Scale Finite-Frequency Travel-time TomographyApplied to Imaging 3-D Velocity Structure of theUpper Mantle beneath the Southwest United States

• Main Authors: Yi-Fan Yin, 尹一帆
• Other Authors: 洪淑蕙
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/77651579917800258948

碩士 === 國立臺灣大學 === 地質科學研究所 === 97 === Seismic tomography has played a key component in unraveling the deep processes that caused the surface morphology and rift magmatism in the southwest United States. Earlier study used teleseismic body-wave arrivals recorded by the La Ristra experiment, a dense broadband array of 950-km in length deployed during 1999-2001 and run through the Great Plains, the Rio Grande Rift (RGR), and the Colorado Plateau, to construct a 2-D tomographic image of the upper mantle structure beneath this linear array. However, because of the inevitable smoothing and damping imposed in the inversion, the resulting velocity contrast is too weak to explain distinct P and S waveform changes across the array. In this study, all the available data from the La Ristra, nearby arrays, and newly deployed USArray are utilized for the determination of frequency-dependent travel-time shifts by inter-station cross correlation of waveforms at both high- (0.3-2 Hz for P and 0.1-0.5 Hz for S) and low-frequencies (0.03-0.125 Hz for P and 0.03-0.1 Hz for S). Different from the previous models that rely on classical ray theory and regular grid parameterization, the new tomographic images are built based on state-of-the-art 3-D, finite-frequency sensitivity kernels that relate observed travel-time data to velocity heterogeneity and a wavelet-based, multi-scale parameterization that enables to yield robust structures with spatially-varying resolutions subject to data sampling. The resulting P and S models reveal a prominent smile-shaped region of low wave speed anomalies encircling the Colorado Plateau and confined in the uppermost 250 km depth. The lowest velocity anomalies are highly correlated with the late Cenozoic volcanic fields but do not underlie the rift center. At depths greater than 250 km, the variations of P and S velocity structures appear less coherent with the surface geological features. A very slow-velocity core inside the Colorado Plateau extending from ~300 km to greater depths may provide a plausible explanation for its elevated topography.