HR: 0800h
AN: S41B-0565    [Abstracts]
TI: Lg Amplitude Tomography in the Western United States
AU: * Phillips, W S
EM: wsp@lanl.gov
AF: Los Alamos National Laboratory, MS F665, Los Alamos, NM 87545, United States
AU: Stead, R J
EM: stead@lanl.gov
AF: Los Alamos National Laboratory, MS F665, Los Alamos, NM 87545, United States
AB: Lg Q can be used to isolate source effects, and determine magnitude (mbLg), and patterns have been shown to correlate with deformation age, geothermal circulation, and possibly partial melt within the crust. Tomographic studies of Lg Q are typically performed using a few stations and many earthquakes, the density of earthquakes providing spatial resolution. The unprecedented breadth and density of the USArray should allow more balanced data sets, and better resolution in aseismic regions. To test this, we processed amplitude data from 515 events recorded by the broad-band vertical channels of 395 stations within the initial footprint of the USArray. Events were obtained from PDE monthly and weekly catalogs, and were chosen to maximize station coverage, including consideration of the USArray deployment schedule, and a magnitude-distance criteria, selecting the potentially best recorded event per quarter-degree block. The IRIS DMC provided waveform data and instrument response information. We measured RMS amplitudes in overlapping octave width bands between 0.5 and 8 Hz, for windows defined by group velocities 3.6 to 3.0 km/s. Over 30,000 amplitudes passed a signal-to- noise threshold of 2. We then solved for laterally varying crustal Q, as well as site and source terms. Results for the 1-Hz band (0.75 – 1.5 Hz) show dramatic variations in Q ranging from below 100 to over 600. Low Q is observed throughout the Pacific region, including Coast and Cascade ranges. Lowest Q is found at the Geysers and in the Salton Trough. A large inland region of low Q extends from the Yellowstone area, through the Snake River Plain, to the Great Salt Lake and Sevier basins. High Q crust is observed for the Colorado Plateau, and the Columbia Basin. Smaller islands of high Q include the Sierra Nevada, higher altitude regions of Nevada, such as the Toiyabe, Toquima, Monitor, Hot Creek, Snake, Wilson Creek and Jarbidge ranges, the Bitterroot Range, the Snoqualmie Batholith, the central Willamette Valley, Antelope Valley in the western Mojave, and the Sierra Juarez. Feature outlines as small as 50 km appear to be resolved via visual correlation with topography; however, resolution calculations remain to be performed. Residuals average 0.16 log10 amplitude units. This is higher than for comparable studies in Asia, perhaps due to source radiation effects for the shorter USArray paths.
DE: 7205 Continental crust (1219)
DE: 7219 Seismic monitoring and test-ban treaty verification
DE: 7270 Tomography (6982, 8180)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting