How do “ghost transients” from past earthquakes affect GPS slip rate estimates on southern California faults?

Elizabeth H. Hearn, Fred F. Pollitz, & Wayne R. Thatcher

Published April 2013, SCEC Contribution #1666

In this study, we investigate the extent to which viscoelastic velocity perturbations (or “ghost transients”) from individual fault segments can affect elastic block model-based inferences of fault slip rates from GPS velocity fields.  We focus on the southern California GPS velocity field (CMM4.0, Shen et al., 2011), exploring the effects of known, large earthquakes for two end-member rheological structures. Our approach is to compute, at each GPS site, the velocity perturbation relative to a cycle average for earthquake cycles on particular fault segments. We then correct the SCEC CMM4 velocity field for this perturbation and invert the corrected field for fault slip rates. We find that if asthenosphere viscosities are low enough (3 x 10^18 Pa s) the current GPS velocity field is significantly perturbed by viscoelastic earthquake cycle effects on the San Andreas Fault segment that last ruptured in 1857 (Mw = 7.9). That is, current strain rates around this part of the SAF are lower than their average values. Correcting the GPS velocity field for this perturbation adds up to about 5 mm/a to the SAF slip rate along the Mojave and San Bernardino segments, consistent with Johnson et al. (2007) and Chuang and Johnson (2011). The GPS velocity perturbations due to large earthquakes on the Garlock Fault (most recently, events in the early 1600’s) and the White Wolf Fault (most recently, the Mw = 7.3 1952 Kern County earthquake) are smaller and do not influence block-model inverted fault slip rates. This suggests that either the large discrepancy between geodetic and geologic slip rates for the Garlock Fault is not due to a ghost transient, or that unmodeled transients (e.g. from the Mojave earthquakes of 1992 and 1999) may influence the SCEC CMM4 velocity field.

Hearn, E. H., Pollitz, F. F., & Thatcher, W. R. (2013). How do “ghost transients” from past earthquakes affect GPS slip rate estimates on southern California faults?. Geochemistry, Geophysics, Geosystems, 14(4), 828-838. doi: 10.1002/ggge.20080.