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: hilley@asu.edu; ARROWSMITH, J Ramon, Department of Geology, Arizona State University, Tempe, AZ85287, email: ramon.arrowsmith@asu.edu; STONE, Elizabeth M., Department of Geology, Arizona State University, Tempe, AZ, 85287, email: emstone@asu.edu



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 to constrain the 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 creases where the 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.



explanation.jpg


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:



Step 1

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.