SCEC Award Number 12219 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title High-resolution geodetic imaging of damage zones of major seismogenic faults in Southern California
Investigator(s)
Name Organization
Yuri Fialko University of California, San Diego
Other Participants Valerie Sahakian, graduate student
SCEC Priorities 1d, 4a SCEC Groups Geodesy
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 We used high resolution InSAR and GPS measurements of crustal motion across the Southern San Andreas Fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths. Geodetically measured strain rates are asymmetric with respect to the mapped traces of both the Southern San Andreas and San Jacinto faults. Two possibilities have been proposed to explain this observation: large contrasts in crustal rigidity across the faults, or an alternate fault geometry such as a dipping San Andreas fault or a blind segment of the San Jacinto fault \citep{fi06a}. We evaluated these possibilities using a 2D elastic half space model accounting for heterogeneous elastic structure computed from the SCEC crustal velocity model CVM-H 6.3 \citep{suess&shaw03,plesch09}, and several fault geometries at depth suggested by seismic observations. We inverted the geodetic data using a Monte-Carlo sampling algorithm, allowing us to quantify uncertainties in the model parameters. The results demonstrate that variations in elastic properties of the crust constrained by seismic tomography have only a minor effect on the inferred slip rates in this area, and cannot explain the observed strain rate asymmetry. However, small changes in the position of faults at depth are shown to produce a significant asymmetry in the strain rate pattern. This effect may explain the large variability in slip rates reported by previous studies. Our preferred model includes a Southern San Andreas fault dipping to the Northeast at 60 degrees, and two active branches of the Southern San Jacinto fault zone, the Coyote Creek fault and the Clark fault with a blind southern continuation into the Borrego badlands. The best-fitting models suggest a nearly equal partitioning of slip between the San Andreas and San Jacinto fault zones, with slip rates of 18 $\pm$ 2 mm/yr for each. These slip rates are in good agreement with geologic measurements representing average slip rates over the last $10^4 - 10^6$ years. In a related study, we investigated interseismic deformation along the central section of the North Anatolian fault (NAF), a mature strike-slip fault in Turkey that shares a number of similarities with the San Andreas fault in California. We generated maps of satellite line-of-sight (LOS) velocity using five ascending ALOS tracks and one descending ENVISAT track covering the NAF between 31.2-34.3 degree East. The LOS velocity reveals discontinuities of up to $\sim$5 mm/year across the Ismetpasa segment of the NAF, implying surface creep at a rate of $\sim$9 mm/year; this is a large fraction of the inferred slip rate of the NAF (21-25 mm/year). The lateral extent of significant surface creep is about 75 km. We model the inferred surface velocity and shallow fault creep using numerical simulations of spontaneous earthquake sequences that incorporate laboratory-derived rate and state friction. Our results indicate that frictional behavior in the Ismetpasa segment is velocity strengthening at shallow depths and transitions to velocity weakening at a depth of 4-6 km. The inferred depth extent of shallow fault creep is 5.5-7 km, suggesting that the deeper locked portion of the partially creeping segment is characterized by a higher stressing rate, smaller events, and shorter recurrence interval. We also reproduce surface velocity in a locked segment of the NAF by fault models with velocity-weakening conditions at shallow depth. Our results imply that frictional behavior in a shallow portion of major active faults with little or no shallow creep is mostly velocity weakening.
Intellectual Merit Fundamentally, geodetic measurements of strain provide an indirect measure of fault slip rates, which must be inferred through a model. Because modeling assumptions such as material rheology, fault geometry, or variations in material properties are determined prior to the conduction of an inversion, their impact on the result is not reflected in the formal error statistics. Our results show that models incorporating heterogeneous material properties constrained by tomography do not produce a significant change in the inferred fault parameters, indicating that this is not a likely source of disagreement between earlier reported models. This result is not surprising, given that the tomographic model we use (SCEC CVM-H 6.3) shows only a modest rigidity contrast of a factor of ~1.5 across the SAF; as previous studies hve shown, a much larger contrast would be needed to produce a measurable effect in the geodetic data.

In contrast, here we have shown that minor changes in the assumed location of the shear dislocation below a fault can significantly impact the inferred slip rates and locking depths. Introducing a strain asymmetry across the surface trace of the SAF by allowing the fault to dip at 60 deg. to the northeast can significantly alter the inferred locking depths for both faults, and shift as much as 6 mm/yr of inferred slip to the SJF. The magnitude of this effect is more than three times greater than the formal uncertainties computed from our Bayesian inversion procedure, highlighting the need for better constraints on the fault geometry at depth when attempting to make robust inferences about slip rates from geodetic data.
Broader Impacts This project provided training and support for one graduate student
(Lindsey) and one postdoc (Kaneko). The PI (Fialko) used results of
this study in two graduate classes taught at SIO.
Exemplary Figure Figure 1 or 2
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