1. Using Google Earth to Explore Strain Rate Models of Southern California
Glenn A. Richard - Mineral Physics Institute, Stony Brook University, Stony Brook, NY
A series of strain rate models for the Transverse Ranges of southern California were developed based on Quaternary fault slip data and geodetic data from high precision GPS stations in southern California. Pacific-North America velocity boundary conditions are applied for all models. Topography changes are calculated using the model dilatation rates, which predict crustal thickness changes under the assumption of Airy isostasy and a specified rate of crustal volume loss through erosion. The models were designed to produce graphical and numerical output representing the configuration of the region from 3 million years ago to 3 million years into the future at intervals of 50 thousand years. Using a North American reference frame, graphical output for the topography and faults and numerical output for locations of faults and points on the crust marked by the locations on cities were used to create data in KML format that can be used in Google Earth to represent time intervals of 50 thousand years. As markers familiar to students, the cities provide a geographic context that can be used to quantify crustal movement, using the Google Earth ruler tool. By comparing distances that markers for selected cities have moved in various parts of the region, students discover that the greatest amount of crustal deformation has occurred in the vicinity of the boundary between the North American and Pacific plates. Students can also identify areas of compression or extension by finding pairs of city markers that have converged or diverged, respectively, over time. The Google Earth layers also reveal that faults that are not parallel to the plate boundary have tended to rotate clockwise due to the right lateral motion along the plate boundary zone. KML TimeSpan markup was added to two versions of the model, enabling the layers to be displayed in an automatic sequenced loop for a movie effect. The data is also available as QuickTime (.mov) and Graphics Interchange Format (.gif) animations and in ESRI shapefile format.
Present-day configuration of cities, faults, and topography. The organization of the strain model data is listed in the places pane to the left of the Google Earth 3-D viewer. Data for cities, faults, and topography are stored in separate folders. In turn, the data is organized into time frames. Each time frame can be expanded to list the names of individual cities. On each level, checkboxes can be used to control the visibility of the objects.
The visibilities of the topography overlays are controlled by radio buttons, enabling one overlay to be visible at a time. A slider controls the transparency of the overlays, enabling the underlying Google Earth aerial imagery to be viewed.
This is a screen capture of a frame from a gif animation of model shear strain rates (for pure strike-slip shear) from 3 million years ago to present. The time bar shows the age of the model values. These shear strain rates have been concentrated in regions where strike-slip related strains are highest. Right-lateral strike slip faults are thin red lines, left-lateral faults are thicker red lines, and thrust faults are thick red lines with ‘teeth’.
Using the Google Earth ruler to determine the length of the Garlock Fault from its eastern end to Tehachapi ( kilometers), and the displacement of the fault trace that has occurred between 1 million years ago and now in the vicinity of Tehachapi (8.79 kilometers).
Map overlays for southern California topography and faults 2.5 million years ago, 1 million years ago, and at present, displayed on Google Earth, using a North American reference frame.
Points marked by cities in the basin and range area to the east in Nevada have moved at relatively low rates, according to the model. In the case of Pahramp, Nevada, the distance is 0.56 kilometers over the past 1 million years.
Using the Google Earth ruler to compare the distance between points marked by Watsonville now and 1 million years ago (36.26 kilometers) to the distance between points marked by Hollister, farther to the east, now and 1 million years ago (16.23 kilometers).
Points marked by cities have moved by differing amounts depending on relation to the plate boundaries. Faults on the North American Plate that are nearly perpendicular to the boundary have exhibited clockwise motion.