Forces in the Earth's crust lead to ongoing deformation, and in most places, crustal deformation results in destructive earthquakes. Crustal forces and deformation are mathematically represented by stress and strain tensors, respectively. Surface deformation is routinely measured through geodetic techniques and gives insight into the crustal strain field. On the other hand, measuring stress directly is difficult and costly. Hence, the crustal stress field is poorly known compared to strain. In the case of earthquakes, strain around faults that have not recently ruptured indicates the rates that those faults are being loaded, while strain during earthquakes yields information on how the faults slipped at depth. It is crucial to note, however, that earthquakes are inherently stress phenomena, with faults continually loaded until the built-up stress on the faults overcomes their strength. Therefore, while monitoring surface strain provides some information on earthquake potential, insight into seismogenic stresses can advance estimations of earthquake hazard. Over the past several decades, progress has been made in quantifying the orientations of the stresses that led to significant earthquakes; however, quantification of the magnitudes of the stresses that caused those earthquakes has been elusive. In places of high topographic relief, topography itself results in significant stresses on faults. Although, topographic fault stress is only one part of the total stress budget, quantifying it allows the magnitudes of stresses that led to significant earthquakes to be constrained. In addition to an understanding of earthquake processes, this research will contribute to a broader understanding of active tectonics and crustal deformation.
In this project, the researcher will constrain the orientations and magnitudes of seismogenic stresses that are consistent with recent moderate to large, continental earthquakes. The researcher seeks to answer three primary questions: 1) What are the magnitudes and orientations of tectonic stress in the crust? 2) How do seismogenic stresses compare to coseismic stress changes? 3) Are topographic stresses correlated to coseismic slip? Answering these questions relies on knowing both the topographic and tectonic stress tensor fields. Topographic stresses are the heterogeneous component of stress that perturb a laterally invariant lithostatic stress in regions of topography. Topographic stresses can be quite heterogeneous across faults, adding shear stresses >10 MPa and normal stresses >50 MPa onto a fault. Correlations between variation of topographic fault stresses and coseismic slip across faults, suggest that the topographic stress field modulates rupture patterns. In this study, a radial basis function, finite difference (RBF-FD) method will be developed to calculate topographic stresses. The RBF-FD method allows for heterogeneous elastic properties and densities, holds for high topographic gradient, and computes stresses throughout the subsurface. The study will use Bayesian methods to estimate the tensorial tectonic stresses that when added to the topographic stresses are consistent with known fault slip. To further constrain stress, focal mechanisms and geologic observations nearby to the earthquakes will be included. The researcher will further analyze the estimated stresses, including investigating correlations between topographic stress and coseismic slip, constraining mechanical fault parameters, and comparing inferred stress in different tectonic regimes. Constraints on seismogenic stresses have the potential to yield substantial insight into issues of earthquake mechanics and active tectonics.
