1. Building a Las Vegas Seismic Model
In building a model for computation of 2-sec-period F-D synthetic seismograms, we create a 300x200x40-km 3-d grid, in the red box, to contain Las Vegas Valley, sources at Little Skull Mtn. and the Nuclear Test Site, and many surrounding basins as well. Regional geological and geophysical compilations define most of the model. For Las Vegas Valley, in the white box, Wanda Taylor of UNLV and Jeff Wagoner of LLNL have built a detailed stratigraphic and geotechnical model of the basin from Langenheim’s detailed USGS gravity model, 1145 water well logs, and 79 geotechnical measurements.
J. Louie 9/5/2005

2. Model Rendered as Amplification Map
Geology, Basin Depth, Geotech, Geophysical data into ModelAssembler
Las Vegas
Basin
Little
Skull Mtn.
Deep
Volcanic
Rifts
This ModelAssembler ( output shows the predicted effects of the shallow shear-velocity model, together with the amplifications of basin depths (as regressed by Ned Field for LA). Both alluvial and volcanic basins are included; volcanic rifts north of LSM reach depths of 9 km, according to gravity analyses by the USGS (Jachens, Moering, Langenheim, Blakely). LVV basin depths are from Langenheim; shallow Vs is from the stratigraphic model of Taylor and Wagoner.
J. Louie 9/5/2005

3. Max. Ground Motion Computed– 0.5 Hz
E3D elastic finite-difference solution, by Shawn Larsen, LLNL
Las Vegas
Basin
Little
Skull Mtn.
Deep
Volcanic
Rifts
Maximum horizontal motion patterns from an LSM-type earthquake, in a 2-sec period E3D computation on a desktop. Note the streaky pattern of higher amplitudes across the Las Vegas Basin.
J. Louie 9/5/2005

4. LVV GM Pattern Computed at 0.5 Hz
Raw maximum horiz. motion is “speckly” (left)
Smooth to clarify streaks (arrows, right); geological input to grid is smooth
Maximum horizontal motion patterns within Las Vegas Valley (LVV) from an LSM-type earthquake, in a 2-sec period E3D computation on a desktop. Note the streaky pattern of higher amplitudes across the Las Vegas Basin.
J. Louie 9/5/2005

5. LVV GM Pattern Origin in Basin Depth?
GM pattern in LVV (left) does not resemble basin-depth map (right)
Some GM amplification at steep walls of basin on north and east (circled)
J. Louie 9/5/2005

6. LVV GM Pattern Origin in Basin Edges?
Complexities in the basin edges– re-entrants of shallow basins into the bedrock (circles)– concentrate conversion of Rayleigh-wave energy in the bedrock to Love-wave energy in the re-entrants, which channels down the small valleys and “beams” across the main basin.
This effect is highly source, path, and basin dependent.
The re-entrant basins need only be ~100 m deep.
The predicted amplification map on the right (computed with Field’s [2001] regression values for Los Angeles) shows both the very shallow basin-edge geometry as a purple-to-red boundary, and the deep basin geometry with the greatest depths (2-5 km) in yellow.
Inspiration from Bill Stephenson, GNS
Soil Dynam. Earthquake Eng. 25 (2005)
J. Louie 9/5/2005