Geodetically Inferred Locking State of the Cascadia Megathrust Based on a Viscoelastic Earth Model

Abstract

In a viscoelastic Earth, stresses slowly built up due to fault locking are relaxed concurrently during the entire interseismic period. This interseismic stress relaxation causes crustal deformation much farther away from the locked fault than can be explained using elastic models that neglect the relaxation. Here we develop a viscoelastic geodetic inversion model to address this problem at Cascadia. We invert ~500 horizontal velocity vectors based on continuous and campaign geodetic measurements over the past two decades. Ambiguities arising from long‐term rotation of upper‐plate crustal blocks are addressed by test‐correcting the geodetic velocities with two different block‐motion models. Fault back slip (i.e., slip deficit) Green's functions are derived using a Maxwell viscoelastic finite element model with realistic subduction zone structure and megathrust geometry. The preferred model features a narrow and shallow megathrust locked zone, consistent with earlier thermorheological reasoning. For an elastic model to fit the data to the same fidelity, megathrust locking has to extend to much greater depths. However, even with the viscoelastic model, the land‐based geodetic data still cannot resolve whether there is some creep (incomplete locking) in the shallowest part of the megathrust far offshore. Neither can the land data fully resolve along‐strike variations of the locking state. These ambiguities can be resolved only when adequate seafloor geodetic data are obtained.

Geodetic Evidence of Time‐Dependent Viscoelastic Interseismic Deformation Driven by Megathrust Locking in the Southwest Japan Subduction Zone

“…To explain this widespread viscoelastic signal, elastic Earth models need to either deepen the downdip extent of the locked zone to a physically unreasonable depth or attribute the far-field signals to long-term geological deformation (Li et al, 2015). In the Peru-North Chile and Cascadia subduction zones, where the last great-to giant-magnitude (>M8.5) megathrust earthquakes occurred centuries ago, the interseismic viscoelastic relaxation has been found to be nearly in a steady state (i.e., the deformation rate does not change significantly with time), reflecting the deformation pattern in the late-stage interseismic period (Li et al, 2015(Li et al, , 2018). Yet, the temporal variability in surface deformation during the early-stage (when the postseismic transient has almost vanished) to late-stage (when the next event is about to occur) interseismic period is largely unexplored in these and other subduction zones with proper geodetic constraints.…”

Constraining Interseismic Deformation of the Cascadia Subduction Zone: New Insights From Estimates of Vertical Land Motion Over Different Timescales

“…Numerous studies have examined geophysical and geological data with the intent of better understanding the underlying processes and/or constraining models of these processes, with the majority of studies focusing on the signal associated with tectonics (Burgette et al, 2009;Dragert et al, 1994;Hyndman & Wang, 1995;Krogstad et al, 2016;Leonard et al, 2010;Li et al, 2018;Smith-Konter et al, 2014;Wang, 2007;Wang et al, 2003). Since the Global Navigation Satellite Systems, in particular, the U.S.…”

Co-location of the downdip end of seismic locking and the continental shelf break

“…In Cascadia, the extent of high coupling is somewhat shallower with a viscoelastic model (S. Li et al, 2018) but not significantly different (Figure 1). However the uplift hinge line modelled by S. Li et al (2018) lies closer to the coastline than predictions of elastic models for the same margin.…”

“…In Cascadia, the extent of high coupling is somewhat shallower with a viscoelastic model (S. Li et al, 2018) but not significantly different (Figure 1). However the uplift hinge line modelled by S. Li et al (2018) lies closer to the coastline than predictions of elastic models for the same margin. Yet, regardless of the inversion method employed, the lack of submarine geodetic data will affect the modelled location of the interseismic downdip end of high coupling and the position of the modelled uplift hinge line (S. Li et al, 2018).…”