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