Long-term fault slip rates, distributed deformation rates, and forecast of seismicity in the western United States from joint fitting of community geologic, geodetic, and stress direction data sets
Peter BirdPublished November 11, 2009, SCEC Contribution #1251
The long-term-average velocity field of the western United States and adjacent offshore regions is computed with a kinematic finite-element code. Community datasets used include fault traces and geologic offset rates, geodetic velocities, principal stress directions, and Euler poles; each datum is qualified by a meaningful formal uncertainty. There is an irreducible minimum amount of distributed anelastic deformation, which accommodates 1/3 of Pacific-North America relative motion in California. Much of this is probably due to slip on faults not included in the model. All datasets are fit at a common RMS level of 1.8 datum standard deviations. Experiments with alternate weights, fault sets, and Euler poles define a suite of acceptable community models. In a pseudo-prospective test of these community models, fault offset rates are compared to 126 additional published rates not used in the computation: 44% are consistent; another 48% have discrepancies under 1 mm/a, and 8% have larger discrepancies indicating remaining problems. Updated models are then computed including these additional data. Novel predictions include: dextral slip at 2~3 mm/a in the Brothers fault zone, two alternative solutions for the Mendocino triple junction, slower slip on some trains of the San Andreas fault than in recent hazard models, and clockwise rotation of some small domains in the Eastern California shear zone and eastern Transverse Ranges. The preferred kinematic model is converted to a long-term seismicity forecast by assigning each fault and finite element the conversion parameters (coupled thickness, corner magnitude, and spectral slope) previously determined for the most comparable type of plate boundary. This long-term seismicity forecast is retrospectively compared to instrumental seismicity, which was not used in its construction. The western U.S. has been below its long-term-average seismicity during 1977-2008, primarily because of (temporary) inactivity of the Cascadia subduction zone and San Andreas fault system.
Citation
Bird, P. (2009). Long-term fault slip rates, distributed deformation rates, and forecast of seismicity in the western United States from joint fitting of community geologic, geodetic, and stress direction data sets. Journal of Geophysical Research, 114(B11), B11403. doi: 10.1029/2009JB006317.
