Probabilistic estimation of rheological properties in subduction zones using sequences of earthquakes and aseismic slip
Abstract
Constraining the effective rheology of major faults contributes to improving our understanding of the physics of plate boundary deformation. Geodetic observations over the earthquake cycle are often used to estimate key rheological parameters, assuming specific laboratory-informed classes of viscous or frictional rheological models. However, differentiating between various rheological model classes using only observations of a single earthquake (coseismic and postseismic deformation) is difficult—especially in the presence of coarse spatiotemporal sampling, inherent observational noise, and non-uniqueness of the inverted properties. In this study, we present a framework to estimate key rheological parameters of a subduction zone plate interface using simulations of sequences of earthquakes and aseismic slip, constrained by pre- and postseismic surface displacement timeseries. Our simplified forward model consists of a two-dimensional subduction zone, represented by a discretized planar fault or narrow shear zone, divided into a locked, shallow region ("asperity") experiencing periodically imposed coseismic events, and a stress-driven creeping section governed by power-law viscoelasticity or rate-dependent friction. Our inverse model fits the rheological parameters of the interface to surface displacement timeseries in a Bayesian probabilistic way. We validate that our proposed framework can successfully recover depth-dependent profiles of effective viscosity using a synthetic dataset of pre- and postseismic observations. Our first set of numerical experiments show that our framework is only mildly sensitive to uncertainties in the rupture history or assumed coseismic slip, making it robust enough to be applied to real observations of subduction zones. Our second set of tests considers the similarities of surface displacement timeseries between synthetic models that model the plate interface either as a shear zone described by power-law viscosity, or a surface described by rate-dependent friction. Here, we find that the ability to fit surface observations using functional or mechanical models assuming frictional behavior does not constitute sufficient evidence to actually infer frictional behavior at depth, as the surface expressions are virtually indistinguishable from deformation generated from models with depth-variable power-law viscous behavior. Based on our numerical experiments, we conclude that studies that aim to infer the mechanical behavior and rheological properties at depth in subduction zones should consider the surface expression from time periods representative of the entire seismic cycle.
Copyright and License
© The Author(s) 2024. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Acknowledgement
We thank Lijun Zhu for his work extending the AlTar software (https://altar.readthedocs.io/en/cuda/) for nonlinear forward models, as well as providing guidance on using and modifying the software.
Funding
This work has been partially supported through a collaboration with the King Abdulaziz City for Science and Technology (KACST). R. M. acknowledges funding from the Texaco Prize Postdoctoral Fellowship. This work was done as an outside activity and not in R. M.’s capacity as an employee of the Jet Propulsion Laboratory, California Institute of Technology.
Contributions
All authors contributed to the conceptualization of this study. T. K. performed the formal analysis, development of the methodology and software, visualizations, and writing of the original draft. R. M. supported the development of the methodology and software. R. M. and M. S. provided valuable supervision and reviewed the manuscript. M. S. supplied the necessary funding and computational resources.
Data Availability
The code to generate and visualize the synthetic data used in the simulations in this study (i.e., the forward model) is publicly available under the GPL−3.0 license at https://github.com/tobiscode/seqeas-public. The inversion is performed using the AlTar framework, and the integration is published at https://github.com/lijun99/altar.
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Additional details
- King Abdulaziz City for Science and Technology
- California Institute of Technology
- Texaco Prize Postdoctoral Fellowship
- Accepted
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2024-12-11Accepted
- Available
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2025-01-13Published online
- Caltech groups
- Division of Geological and Planetary Sciences, Seismological Laboratory
- Publication Status
- Published