Browsing by Author "Yao, Yao 1989-"
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Item RAYLEIGH WAVE TOMOGRAPHY OF TEXAS FROM AMBIENT SEISMIC NOISE(2013-05) Yao, Yao 1989-; Li, Aibing; Wang, Guoquan; Bird, DaleTexas has been one of the leading states in petroleum production since the discovery of the Spindletop oil field in 1901. Despite its huge economic value of petroleum and numerous geophysical explorations in several oil fields, the crustal structure of the Texas is not well studied. This thesis aims to investigate crustal structure in central and eastern Texas using seismic ambient noise data recorded at 87 broad-band stations from the Transportable Array of the USArray network between March 2010 and February 2011. Seismic observations based on cross-correlations of long ambient noise sequences between pairs of stations are used to obtain phase velocities at periods from 6 s to 40 s. Phase velocity maps show that positive anomaly area coincides with Laurentia craton crust and the negative anomaly area coincides with continental margin crust. The boundary between positive and negative anomaly is perfectly consistent with the Ouachita belt. From the inversion of phase velocity results, we construct 1-D and 3-D shear-wave model with four crust layers and one mantle layer. In the 3-D shear-wave velocity model, the high velocity province is imaged in central and northwestern Texas with the highest velocity beneath the Llano Uplift. This pattern extends across the whole crustal depth, reflecting cold cratonic crust in general and igneous influence at the Llano Uplift. The lowest velocity appears in northeastern and southeastern Texas at shallow crust, correlating with thick sediment layers. In deep crust and upper mantle, the lowest velocity is confined in southeastern Texas at the continental margin. Ouachita orogen could have brought old oceanic crust that contained large amount of water to the lower crust and upper mantle.Item Seismic Velocity and Radial Anisotropy in the Crust and Upper Mantle of the South Central United States from Surface Wave Tomography(2017-12) Yao, Yao 1989-; Li, Aibing; Murphy, Michael A.; Hall, Stuart A.; Li, LunThe goal of this dissertation is to improve the resolution of crust and mantle structure in the South-Central U.S. so as to better understand lithospheric evolution and modification through the last Wilson cycle. Rayleigh and Love wave phase velocities at the periods of 6-167 s were obtained from ambient noise and earthquake data recorded at 207 broadband stations of the USArray Transportable Array. 3-D shear-wave velocity and radial anisotropy models were derived from the phase velocities for the Gulf Coast region from Texas to Alabama. In the shallow crust, low velocity appears in the coastal plain to the south of the Ouachita front due to thick sediments. In the middle and lower crust, the Ouachita Belt in Texas is characterized by a seaward-dipping high-velocity band, which is not observed in the east Ouachita, indicating structural variation along the orogeny. The crust of the Texas Ouachita Belt is also much thicker than that in the east, consistent with the notion that the Ouachita was formed by a hard collision in Texas and a soft collision in Arkansas and Mississippi. Strong positive radial anisotropy (Vsh > Vsv) is largely confined in the coastal plain in the lower crust, limiting the Mesozoic crustal extension to the Ouachita and Appalachian front. Velocity variations in the mantle do not follow geological boundaries on the surface. A high-velocity lithosphere to ~75 km exists across the entire region except at a few local uplifts (Uvalde, Sabine, and Benton), the Arkansas River Valley, and the Mississippi Valley Graben. A low-velocity layer with positive radial anisotropy pervasively appears below 75 km while high velocity continues to great depths in the southern Appalachians and northwest Texas, where anisotropy is weak or slightly negative (Vsv > Vsh). Significantly low velocities are imaged in southernmost Texas and at the Uvalde, Monroe, and Jackson uplifts, where Cretaceous igneous rocks are found. These slow anomalies correlate with high surface heat flow and strong positive radial anisotropy, indicating hot and weak mantle blocks. The southernmost anomaly aligns with a high-gravity lineation in the Gulf of Mexico and probably played an important role in opening the gulf during the Mesozoic.