ALBERT

Description: The southern United States experienced extensional tectonics during the early Mesozoic development of the Gulf of Mexico and subsequent accumulation of up to 12 km of marine sediment. Few seismic stations have operated in the Gulf Coastal Plain for extended periods of time, and data for constraining ground-motion prediction models are sparse. Previous studies found that the region differs from other parts of eastern North America in terms of Lg attenuation. The Earthscope Transportable Array was in the south-central United States in 2010–2012, during a time of increased seismicity. That circumstance provided data for investigating Lg propagation. Sixteen earthquakes with magnitudes in the 3.2–5.6 range occurred in Oklahoma, Arkansas, and Texas and were recorded to distances of 1000 km. Stations in the coastal plain of Texas, Arkansas, Louisiana, and Mississippi exhibit strong attenuation of the Lg phase, compared with stations located to the north in the Ouachita orogenic belt and cratonic platform. Q associated with the crustal waveguide underlying the Gulf Coastal Plain was estimated as Q =365 f 0.62 . However, Lg blockage due to crustal thinning and anelastic attenuation due to the presence of several kilometers of sedimentary deposits occurs near the Gulf Coast. The average 0 value south of latitude 33° N is 96±10 ms. A remarkable linear correlation exists between the attenuation parameter 0 and the thickness of post-Jurassic sediments in the study region, such that stations near the coast of Texas, Louisiana, and Mississippi, which overlie ~12 km of sediments, exhibit values of 0 ~160 ms. This strong spatial dependence of attenuation has important implications and should be incorporated in the development of ground-motion prediction models using the approach suggested here. Online Material: Figures showing maps of regression residuals and receiver terms for 12 frequency bands.

Description: The southern United States experienced extensional tectonics during the early Mesozoic development of the Gulf of Mexico and subsequent accumulation of up to 12 km of marine sediment. Few seismic stations have operated in the Gulf Coastal Plain for extended periods of time, and data for constraining ground-motion prediction models are sparse. Previous studies found that the region differs from other parts of eastern North America in terms of Lg attenuation. The Earthscope Transportable Array was in the south-central United States in 2010–2012, during a time of increased seismicity. That circumstance provided data for investigating Lg propagation. Sixteen earthquakes with magnitudes in the 3.2–5.6 range occurred in Oklahoma, Arkansas, and Texas and were recorded to distances of 1000 km. Stations in the coastal plain of Texas, Arkansas, Louisiana, and Mississippi exhibit strong attenuation of the Lg phase, compared with stations located to the north in the Ouachita orogenic belt and cratonic platform. Q associated with the crustal waveguide underlying the Gulf Coastal Plain was estimated as Q =365 f 0.62 . However, Lg blockage due to crustal thinning and anelastic attenuation due to the presence of several kilometers of sedimentary deposits occurs near the Gulf Coast. The average 0 value south of latitude 33° N is 96±10 ms. A remarkable linear correlation exists between the attenuation parameter 0 and the thickness of post-Jurassic sediments in the study region, such that stations near the coast of Texas, Louisiana, and Mississippi, which overlie ~12 km of sediments, exhibit values of 0 ~160 ms. This strong spatial dependence of attenuation has important implications and should be incorporated in the development of ground-motion prediction models using the approach suggested here. Online Material: Figures showing maps of regression residuals and receiver terms for 12 frequency bands.