San Andreas fault and eastern California shear zone
description
Transcript of San Andreas fault and eastern California shear zone
Rick BennettDept of GeosciencesUniversity of Arizona
with contributions from Josh Spinler, Noah Fay, Sigrún Hreinsdóttir, and Megan Anderson
Spatial versus temporal variation in San Andreas fault and eastern California shear zone slip rates
San Andreas fault and eastern California shear zone
Southern California slip rates
Conclusions (preliminary)
•SAF slip rate varies appreciably along strike
•Along strike variability reconciles (all?) differences among geological and geodetic estimates
•Slip rate variation facilitated by transrotation across the ETR
The Joshua Tree GPS network
San BernardinoSan BernardinoGPS network GPS network (CSUSB and (CSUSB and
UofA)UofA)
Joshua TreeJoshua TreeGPS network GPS network
(UofA)(UofA)
Joshua Tree GPS network
Joshua Tree GPS network
West
East
Southern California velocity field
UofASCEC v3
– Observed – Predicted
Strain rate field, So Cal
– Observed – Predicted
Strain rate field, ETR
“belt” of high strain rate
Seismicity and faulting
– Observed – Predicted
Previous models
Johnson et al., 1994
Also see Bennett et al., 1996; Meade and Hager, 2005; Becker et al., 2005
San Andreas fault and eastern California shear zone
SmallSmallOffset (meters)Offset (meters)
LargeLargeOffset (km’s)Offset (km’s)
Respect the mapped geology
Rationale: If perfectly good faults already exist (e.g., Pinto Mt, Blue Cut, others), why create new ones?
Can we reject this hypothesis? That is, is the observed strain rate field really inconsistent with strain accumulation on these mapped faults?
What is the role of the Pinto Mountain and Blue Cut faults?
Block model for interseismic strain accumulation
A block model for deformation: crustal velocity w.r.t. SBR
A block model for deformation: rigid block motion
Rotating JT Block
SBRfixed
Translatingblocks
CW
A block model for deformation: strain accumulation due to locking
Residual velocities
Residual velocities
Large SJFZLarge SJFZresiduals residuals
Small JTSmall JTresidualsresiduals
Kinematic results
55
-3-7-2-5
5512 -1 -9-6
-9
23 2
JT rotationJT rotation
14º/Myr14º/Myr
clockwiseclockwise
R-L strike slip (mm/yr)Shortening (mm/yr)
Kinematic results
• Block rotation: Paleomag 30-40º sometime in past 10 Myr (Carter et al. 1987). If 14º/Myr => rotating since ~2-3 Ma.
• San Andreas fault slip rate ranges between 5 to 23 mm/yr. ~12 mm/yr just north of Biskra Palms. In general agreement with geology.
• Pinto Mt and Blue cut left-extensional. Rates as high as 3-6 mm/yr. Total offset 10-20 km (?) consistent with age of 2-3 Ma.
Strain rates reconsidered
Observed
Modeled
Strain rates reconsidered
Observed
Modeled at GPS points
only
Conclusions
Geodesy consistent with the known geology; no Geodesy consistent with the known geology; no “new” faults required“new” faults required
Slip rates on ETR faults are importantSlip rates on ETR faults are important
San Andreas slip rate varies appreciably along San Andreas slip rate varies appreciably along strikestrike
Block rotation rates are fastBlock rotation rates are fast
Dangerous to base geometry of fault models on Dangerous to base geometry of fault models on strain rate inference alonestrain rate inference alone
Respect the mapped geologyRespect the mapped geology
Future work
Continue GPS measurements!!Continue GPS measurements!!
Improve model fault geometry!!Improve model fault geometry!!
Model elastic strain associated with dip-slip Model elastic strain associated with dip-slip motion!!motion!!
Thoroughly test alternative models!!Thoroughly test alternative models!!
Future work
APPLES AND ORANGES ?
• Geology sees actual fault displacement
• Geodesy (primarily) sees transient strain accumulation
What is “present-day” slip rate? Zero!
Conceptual issue
– Observed – PredictedNon-uniqueness
– Observed – PredictedNon-uniqueness
– Observed – PredictedNon-uniqueness
Fialko, 2006
– Observed – PredictedNon-uniqueness
Conceptual issues
Friedrich et al., JGR, 2003; Weldon et al., GSA Today, 2004
Conceptual issues
Variable fault loading rate?
Friedrich et al., JGR, 2003; Weldon et al., GSA Today, 2004
Conceptual issues
Constant loading rate?
Friedrich et al., JGR, 2003; Weldon et al., GSA Today, 2004
Conceptual issues
Weldon et al., 2004
Reconciliation of different slip rates
Bennett et al., 2004
– Observed – Predicted
Recent hypothesis (weak version): co-varying clusters?
See also Press and Allen, 1995
Dolan et al., 2007
– Observed – Predicted
Nur et al., 1993
Du and Aydin, 1996
Li and Liu, 2006
Recent hypothesis for present-day behavior: new fault forming