Variations in aseismic slip along the creeping sectionof the San Andreas Fault from space-geodetic data

Frederique Rolandone*, Ingrid Johanson**, Eric Fielding***, Roland Bürgmann**,

*UPMC, Paris, **Dept. of Earth and Planetary Science, UC Berkeley, ***JPL, Pasadena

GAMIT/GLOBK processing0.15 micro-strain/yr

In central California most of the relative motion between the Pacific and North American plates isaccommodated by strike slip along the San Andreas fault system. Along the central San Andreas fault(CSAF), from San Juan Bautista to Parkfield, almost all the slip along the CSAF in the brittle upper crustis accommodated aseismically. We use GPS, InSAR and trilateration data to resolve both thedistribution of aseismic slip along the CSAF, and the deformation across adjacent, secondary faultstructures. Since 2003, we conducted several GPS surveys along the CSAF. The dense GPSmeasurements, along with data from permanent GPS stations in the area, allow us to constrain theregional strain distribution and contributions from adjacent faults. InSAR is well suited to monitoringdetails of the shallow slip along the CSAF. However low interferometric correlation and atmosphericsignal make it difficult to measure the surface deformation. To resolve aseismic slip at the spatial andtemporal resolution we seek integrated use of GPS, InSAR, surface creep, and strain data.

GPS data

InSAR data

BaVu , Matt d’Alessio (UCB)SCEC CMM3.1SIO, SCIGN

Continuous GPS (University of Wisconsin)

Our new campaign measurements Creepmeters

NA fixed

Burford & Harsh (1980)Lisowski & Prescott (1981)

EDM and GPS data

ERS-1 & 2 satellitesTrack 27, Frame 2871

=> The stacked interferogram showsa sharp discontinuity in the phasecoinciding with the mapped trace ofthe fault. 10 mm/yr in LOS is about32 mm/yr of right lateral motion.

Distributed slip inversion along the CSAF

Kinematics of aseismic slip along the CSAF

Deformation adjacent to the CSAF

Deformation in the southern part of the CSAFrelated to the Parkfield earthquake Mw6.0, 9/28/2004

ObservedPredicted

=> Very little strain on the crustal blocks adjacent to the fault (less than 2 mm/yr of contraction).

=> Geodetic data indicates a slip rate of 32 mm/yr and shallower creep of 29 mm/yr for the CSAF

(Agnew, 2004)

Aseismic slip is highly time variable and responds to subtle regional stresschanges. Transient slip variations in response to regional deformation events?

Both ENVISAT interferograms show offseton creeping section NW of creepmeter XSC1.

(Rosen et al., 1998)

=> No extra slip at time of the earthquake but sliprate increase at end of Oct. 2004, consistent withInSAR analysis.

=> East component offset across SAF increeping section is about 20mm, or about28mm parallel to fault.

LOS

mm

6/23/04-12/15/04

XSC1

mm

9/14/04-11/23/04

=> GPS results are very coherent withprevious EDM measurements. A constantrate fits the data over a 40 year span.5 surveys

Swath profile at azimuth 47o fromdifference of the 2 interferograms.

km