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

Abstract

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, 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 CMM4.0 velocity field for this perturbation and invert the corrected field for fault slip rates. We find that if asthenosphere viscosities are low (3 × 1018 Pa s), the current GPS velocity field is significantly perturbed by viscoelastic earthquake cycle effects associated with the San Andreas Fault segment that last ruptured in 1857 (Mw = 7.9). Correcting the GPS velocity field for this perturbation (or “ghost transient”) adds about 5 mm/a to the SAF slip rate along the Mojave and San Bernardino segments. The GPS velocity perturbations due to large earthquakes on the Garlock Fault (most recently, events in the early 1600s) 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 un‐modeled transients from recent Mojave earthquakes may influence the GPS velocity field.

Recovery of secular deformation field of Mojave Shear Zone in Southern California from historical terrestrial and GPS measurements

“…The pre‐Landers historical geodetic data favor San Bernardino SAF slip rates near the lower bound in our elastic block motion models. The estimated slip rates may be lower than the long‐term slip rate due to the effect of enduring postseismic transients of the 1857 M w 7.9 Great Fort Tejon earthquake [ Hearn et al ., ].…”

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

“…Again, as with timing since the last earthquake, ideal experiments would be those where all data were compiled on exactly the same fault segment. The introduction of finite-length faults to the two-layer Maxwell model may improve the ability to fit geodetic data (e.g., Hearn et al, 2013) as compared with the infinitely long 2D fault models considered here. However, relaxation times are the same in both 2D and 3D cases, and thus so are the velocity decay rates throughout the earthquake cycle.…”

Kinematically consistent models of viscoelastic stress evolution

“…In three‐dimensional kinematically consistent viscoelastic block models [ Smith and Sandwell , ; Hilley et al ., ; Chuang and Johnson , ; Hearn et al ., ; Sato and Matsu'ura , ; P. R. DeVries et al, submitted manuscript, 2016], interseismic velocities v I are modeled as the sum of five components: v B , the velocities due to rigid block motion; v SD , the velocities due to the accumulated slip deficit; , the velocities due to internal strain of blocks; v VE , the viscoelastic effect of the most recent earthquake; and , the mean velocity throughout the earthquake cycle. …”

“…Here we present a kinematically consistent model of the viscoelastic perturbation to the stress in the crust over an arbitrary number of periodic earthquake cycles [ Pollitz and Schwartz , ], based on a viscoelastic block model framework [e.g., Sato and Matsu'ura , ; Hilley et al ., ; Chuang and Johnson , ; Hearn et al ., ; Tong et al ., ; P. R. DeVries et al, submitted manuscript, 2016]. A kinematically consistent framework for viscoelastic earthquake cycle stress was previously applied to probabilistic seismic hazard estimation in the San Francisco Bay Area [ Pollitz and Schwartz , ].…”