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

Abstract

Characterizing surface deformation throughout a full earthquake cycle is a challenge due to the lack of high-resolution geodetic observations of duration comparable to that of characteristic earthquake recurrence intervals (250-10,000 years). Here we approach this problem by comparing long-term geologic slip rates with geodetically derived fault slip rates by sampling only a short fraction (0.001%-0.1%) of a complete earthquake cycle along 15 continental strike-slip faults. Geodetic observations provide snapshots of surface deformation from different times through the earthquake cycle. The timing of the last earthquake on many of these faults is poorly known, and may vary greatly from fault to fault. Assuming that the underlying mechanics of the seismic cycle are similar for all faults, geodetic observations from different faults may be interpreted as samples over a significantly larger fraction of the earthquake cycle than could be obtained from the geodetic record along any one fault alone. As an ensemble, we find that geologically and geodetically inferred slip rates agree well with a linear relation of 0:94 0:09. To simultaneously explain both the ensemble agreement between geologic and geodetic slip-rate estimates with observations of rapid postseismic deformation, we consider the predictions from simple two-layer earthquake-cycle models with both Maxwell and Burgers viscoelastic rheologies. We find that a two-layer Burgers model, with two relaxation timescales, is consistent with observations of deformation throughout the earthquake cycle, whereas the widely used two-layer Maxwell model with a single relaxation timescale, is not, suggesting that the earthquake cycle is effectively characterized by a largely stressrecoverable rapid postseismic stage and a much more slowly varying interseismic stage.

Afterslip following the 2007 Mw 8.4 Bengkulu earthquake in Sumatra loaded the 2010 Mw 7.8 Mentawai tsunami earthquake rupture zone

“…Previous researchers investigating postseismic deformation following the Sumatran earthquakes estimated mantle viscosities in this region between 10 17 to 10 19 Pa s [ Lubis et al , ; Panet et al , ; Pollitz and Banerjee , ]. The value of the mantle viscosity can depend on the time window of geodetic observations examined; examining geodetic observations late in the interseismic period of the earthquake cycle can yield mantle viscosities on average 1 order of magnitude higher than geodetic observations early in the interseismic period of the earthquake cycle [ Meade et al , ]. In addition, some studies of early stage postseismic deformation assume predominantly viscoelastic deformation and do not account for afterslip, employing instead transient rheologies to explain different time‐dependent decay patterns of geodetic time series [e.g., Panet et al , ].…”

Stress‐driven relaxation of heterogeneous upper mantle and time‐dependent afterslip following the 2011 Tohoku earthquake

“…Viscous relaxation by power law flow was found to match the observed rapid decay of postseismic deformation observed following the 1992 Landers and 1999 Hector Mine earthquakes [ Freed and Bürgmann , ] and the 2002 Denali earthquake [ Freed et al ., ]. A linear biviscous Burgers body rheology, possibly representing transient creep following a stress step, has also been found to capture the rapidly decaying deformation following these and other strike‐slip earthquakes [e.g., Pollitz , , ; Hearn et al ., ] and has been used to model time‐dependent relaxation following recent subduction zone events [e.g., Pollitz et al ., ; Hoechner et al ., ; Hu and Wang , ; Meade et al ., ; Mikhailov et al ., ; Trubienko et al ., ; Broerse et al ., ; Wiseman et al ., ]. Freed et al .…”

The role of space-based observation in understanding and responding to active tectonics and earthquakes

“…Second, the slip rates of major faults derived from simple steady-state geodetic models have been shown to agree within error with those estimated from dating geological offsets100101. This suggests that strain rates are more or less steady for most of the earthquake cycle.…”

“…If all faults behave in a similar way, we can use observations from different faults, each at different points of the cycle, to infer the time-dependent behaviour of an individual fault. This approach has been used to establish a few key patterns of behaviour that any successful model must be able to reproduce11100. First, and with remarkably few exceptions, the deformation between earthquakes at major faults is focused around the fault (for example, Fig.…”