Browsing by Subject "Crustal structure"
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Item A seismologic study of the Three-Gorges Reservoir (TGR) region, China(2012-08) Zou, Zhihui; Zhou, Hua-wei; Nagihara, Seiichi; Gurrola, HaroldFor many years there have been a number of geologic investigations have been conduct in the Three Gorges Reservoir (TGR) region of the Yangtze River that provided useful information to hydro-power generation, engineering construction, flooding control and geohazard mitigation. However, there has not yet been a complete knowledge on the deep lithospheric structure, especially its seismic properties of the TGR region. The water impoundment of the 660-kilometer-long TGR makes the safety and geohazard mitigation a top concern in the TGR region and the entire central China. To address such concern, a new study of the lithospheric structure and seismotectonics of the TGR region has been carry out. Previous seismic studies mainly used the data acquired by the local permanent networks and focused on the event location and 1-D seismic velocity model building. However, the coverage of the local network cannot offer the sufficient coverage for the microearthquake monitoring and is not dense enough for reflection array analysis. In addition, due to the complex tectonic history, seismic velocity of this region is heterogeneous on surface and has high gradient at near surface. The 1-D velocity model doesn’t provide sufficient accuracy for event location and local structural studies. Here I established a hybrid event location method based on the combination of traditional location method and the modified master station method to overcome the velocity-depth ambiguity, and used a newly inverted 3-D velocity model to re-locate the earthquakes in the TGR region. The relocated hypocenters match with the subsurface features. To address the poor coverage of the local permanent network I deployed temporary dense networks of geophones in TGR region in the summer from 2008 to 2011. During the period of observation a number of microearthquakes less than ML0.1, which were missed by the local permanent network, were recorded. To overcome the limitation of frequency band we established the power spectral density ratio (PSDR) method to estimate the recoverable frequencies. By applying the PSDR method, different types of earthquakes, including an Mb 5.9 event with a 93-degree offset, are recognized from the geophone data. Application of the reverse time imaging method to the data reveal multiple-earthquake phenomenon at two nearby faults. Application of the ray-tracing modeling and virtual source imaging method to the acquired data revealed the layer structure of the crust and upper mantle. The imaged shallow basins matches well with the surface geological feature and have the bottoms at 5 km depth. An upper mantle reflector at around 50 km depth may have the relation with the complex tectonic history of the TGR region.Item The Northeastern Gulf of Mexico : volcanic or passive margin? : seismic implications of the Gulf of Mexico Basin opening project(2013-05) Duncan, Mark Hamilton; Christeson, Gail L.; Van Avendonk, Harm J. A.The Gulf of Mexico Basin Opening project (GUMBO) is a study of the lithological composition and structural evolution of the Gulf of Mexico (GoM) that uses Ocean Bottom Seismometer (OBS) data from four transects in the Northern GoM. I examine 39 OBS shot records in the easternmost transect for shear wave arrivals and pick shear wave travel times from the 11 usable records. I then carry out a tomographic inversion of seismic refraction travel times. I use the resulting shear-wave velocity model in conjunction with a previously constructed P-wave model to examine the relationship between Vp and Vs. I compare velocities in the sediment and basement with empirical velocities from previous studies for the purpose of constraining lithological composition below the transect and make an interpretation of the structural evolution of the eastern GoM. The seismic velocities for crust landward of the Florida Escarpment are consistent with normal continental crust. Seaward of the Escarpment, velocities in the upper oceanic crust are anomalously high (Vp = 6.5 – 7 km/sec; Vs = 4.0 – 4.6 km/sec). A possible explanation for this observation is that GoM basalt formation consisted of basaltic sheet flows, forming oceanic crust that does not contain the vesicularity and lower seismic velocities found in typical pillow basalts. Increased magnesium and iron content could also account for these high velocities. Seismic refraction and reflection data provide a means of investigating the nature of the Moho in the northeastern GoM. I use a finite difference method to generate synthetic record sections for data from eight instruments that are part of the two easternmost GUMBO seismic lines (lines 3 & 4). I then vary the thickness of the Moho in these synthetic models and compare the results with the original receiver gather to examine the effects this variability has on amplitudes. The data from the instruments chosen for these two lines are representative of continental and transitional crust. The finite difference models indicate that the Moho beneath GUMBO 3 is ~1500 m thick based on the onset and amplitudes of PmP arrivals. All five instruments display consistent results. The instruments along GUMBO 4 suggest a Moho almost twice as thick as GUMBO 3 on the landward end of the transect that grades into a Moho of similar thickness (1750 m) in the deep water GoM. The three instruments used to model the Moho in this area show that the Moho ranges from ~1750 to 3500 m in thickness. The sharper boundary beneath continental crust in GUMBO Line 3 supports other evidence that suggests magmatic underplating and volcanism in the northern GoM during the mid-Jurassic. The thicker Moho seen on the landward end of GUMBO Line 4 that is overlain by continental crust was likely unaffected by GoM rifting. Therefore, the Moho beneath the Florida Platform might be as old as the Suwannee Terrane, and complex Moho structure is not uncommon for ancient continental crust.