The Collaboratory for Computational Geosciences at the College of Science, University of Nevada, is modeling synthetic earthquake motions through complex geological structures. The simulations are teaching us what we need to know in order to accurately anticipate the ground shaking and other effects of likely earthquakes.
This narrated video presents two scenarios for the same M7.5 earthquake affecting Las Vegas. In the first, the epicenter of the earthquake is at the southeast end of the fault near the resort of Furnace Creek, Calif., and the fault rupture propagates northwest, away from the city. In the second scenario, the epicenter of the earthquake is at the northwest end of the fault, at Ubehebe Crater, and the rupture is toward Las Vegas to the southeast.
The one-minute video presents the the geologic and geotechnical data that went into the model, shows the wave propagation from both rupture scenarios, and compares the peak ground shaking from each scenario in Las Vegas. The direction of the fault rupture and the location of the earthquake's epicenter turn out to be the most important effects on the level of ground-shaking hazard in Las Vegas. To see additional earthquake scenarios affecting Nevada cities, please visit crack.seismo.unr.edu/ma .
The computations were set up and run by undergraduate Geological Sciences and Engineering students in the fall Geophysics and Geodynamics course at the Univ. of Nevada, Reno. Student Liz Lennox set up the Death Valley scenarios. John Louie, the course instructor, prepared the animations and narrated the video.
The narrated video includes the sound of the synthetic seismic waves, sped up by a factor of ten and used to modulate pink noise, for clarity. The noise modulation makes the waves sound a little like ocean waves lapping at a beach. On the left channel is the sound of a station in Amargosa Valley, close to Death Valley. On the right channel is a station in Las Vegas Valley, at the Community College of Southern Nevada, Cheyenne Campus. Being closer to the earthquake, you will hear sounds on the left channel earlier than on the right. Additional sound effects were added for illustration.
The wave-propagation animations each last 12 seconds, and they are sped up by a factor of ten over the 120 sec of wave propagation that is simulated. The the initially coherent earthquake energy soon converts to drawn-out horizontal vibrations of energy trapped in the soft sedimentary basins that are sprinkled through this region like the holes in Swiss cheese- each basin rings like a gong. This trapped energy has the highest amplitude and presents the greatest shaking hazard to Las Vegas. Though the trapped energy looks like noise, these synthetics have clean viscoelastic wave propagation with no noise or stochastic effects added.
The animations showing the wave propagation illustrate the trapping and amplification well. As in the graphic on the right, each frame of the movies present a map of 3-component ground motions for the region on the map. The movie frames are 281 km wide from NW to SE and 251 km high from SW to NE. The three primary computer display colors of red, green, and blue (RGB) are used to represent the three directional components of ground vibration X, Y, and Z, respectively. Each color is given an intensity related to the intensity of shaking motion in the respective direction. Where there is no color, and you can just see the gray shaded-relief of the basin model, there is very little ground motion; red is motion in the X direction (East, horizontal on the screen); green is motion in Y (North, vertical on the screen); and blue is motion in Z (in and out of the screen). Where shaking directions combine, the colors combine according to the rules of colored light- yellow indicates combined horizontal motion (relative to the ground) of X and Y, adding red and green light, so could be north-south or east-west. White color, adding red, green, and blue all together, indicates high-intensity shaking on all components, including up and down. With these colors, P waves will be mostly blue, S waves red, green, or yellow; and the Rayleigh surface wave is identifiable by having blue up-down motion between the red, green, or yellow radial motions (elliptical particle motions).
The wave propagation movies were created with the help of the software listed on the Creating Wave-Propagation Movies page.