INFERRING SEGMENT STRENGTH CONTRASTS AND BOUNDARIES ALONG LOW FRICTION FAULTS USING SURFACE OFFSET DATA, WITH AN EXAMPLE FROM THE CHOLAME-CARRIZO SEGMENT BOUNDARY ALONG THE SAN ANDRES FAULT, SOUTHERN CALIFORNIA
HILLEY, George E, Department of
Geology, Arizona State University, Tempe, AZ, 85287, email: firstname.lastname@example.org;
ARROWSMITH, J Ramon, Department of Geology, Arizona State University, Tempe, AZ85287, email: email@example.com; STONE, Elizabeth M., Department of Geology, Arizona State University,
Tempe, AZ, 85287, email: firstname.lastname@example.org
Rupture segmentation arises from changes in fault geometry and strength. We use
boundary element models of
frictionless strike-slip fault segments to quantify
how fault geometry and strength change earthquake surface offset distributions.
Using these relationships between fault geometry, strength, and surface offsets
, we can infer fault strength from the surface offsets in cases where the fault
geometry can be independently constrained.
This paper includes normalized plots of the surface offset distribution expected
from rupture along low-friction fault segments with strength contrasts of 1/4
, 1/3, 1/2, 1, 2, 3, and 4 for a range of
fault segment geometries. These plots may be used with offset data
strength of two coplanar, adjacent fault segments. This analysis is applied to
the Cholame and Carrizo segments of the
San Andreas Fault. The available surface offset data suggest that the offset in
fault deepens; in addition, the observed offset gradient at the segment boundary
requires a 2/3 to 1/4 strength ratio of the
Cholame to the Carrizo segment.
Using this Website
This website contains the analysis of the offset data, microseismicity, and
the relative strengths of the Cholame and
Carrizo segments of the San Andreas Fault. Many workers who are referenced
below have contributed
to the database of offset data and microseismicity. For some background
on the Cholame segment of the San Andreas Fault, take a look at Elizabeth
Stone's Cholame Web Page which is a compilation of figures and data
presented at the Fall 1998 Meeting of the American Geophysical Union.
This paper will be published in the June, 2001 issue of the Bulletin of the Seismological Society
of America. I am currently checking the copyright restrictions, but hope to have a PDF version available for those wishing to download the article.
As part of this project, we have created a series of normallized plots for
use by paleoseismologists working on faults that they may believe to be
low friction faults. In order to determine the appropriate fault geometry
parameters to use, use the microseismicity to determine 1) the length of
each of the two segments under consideration (c and (a-c) in the figure
below), 2) the depth of the deeper of
the two segments (d in the figure below), 3) the depth increase of the shallow
segment to the next (e in the figure below). Next, compute the following
ratios: 1) a/c, 2) d/c, 3) e/d. These three parameters define the fault
geometry with which you are working. Note that all of the diagrams are
reverable (about a vertical mirroring line), so if the deeper of your two
segments lies to the left on the figure below, just create a mirror image
of any of the figures.
Once the fault segment geometries are defined, the offset data needs to be
normalized. Divide all offsets by the maximum offset to obtain a
normalized offset plot. Then, divide the distance along the fault at
which each point was measured by (c) in the above diagram. This should
produce a dimensionless plot (on both axes) of your distance vs. offset
data. The axes of the plot are (distance/c) for the horizontal axis and
(offset/maximum offset) for the vertical axis.
Using the ratios that determine the fault geometry, work down the following
tree to find a graph of normalized offset vs. normalized distance for
different strength ratios. Overlay your normalized plot onto these plots
to estimate a range of strength ratios possible for the fault segements
under consideration. All figures are available in both JPEG and postscript
format, so overlaying data is possible withing both raster graphics
software packages (such as Adobe Photoshop) and illustration packages (such
as Canvas and Adobe Illustrator).
Fault Geometry Parameters:
First, click the d/c value which best represents your fault geometry:
d/c = 1
d/c = 1/2
d/c = 1/3
d/c = 1/4
d/c = 1/8
Pages maintained by George Hilley, last modified June 6, 2001.