Strain Rate Models for Southern California
Southern California is one of the most tectonically active regions of the United
States as a result of interaction between the North American plate and the Pacific
Plate. Currently, the region is undergoing large-scale right-lateral strike-slip
motion between the North American and Pacific Plates. The resulting deformation
is complex, characterized by areas of uplift, large strike-slip faults, and
local areas of subsidence.
Using Google Earth to Explore Strain Rate Models of Southern California
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.
Beth Ann Bell's abstract on Quantification
of Past Tectonic Rates in Southern California
Strain Rate Model Movies
How the Models and Movies Were Generated
