Integrating Rupture and Tsunami Modeling to Study Large Subduction Zone Earthquakes

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Date

2024-08-07

Authors

Small, David

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Publisher

University of Oregon

Abstract

This dissertation focuses on studying past and future large subduction zone earthquakes through creating and utilizing a stochastic slip rupture modeling technique in combination with deformation and tsunami modeling. Here, I created a method for incorporating fault zone specific characteristics, like interseismic coupling, into the stochastic slip rupture modeling approach. With this method, output rupture models are informed as to where slip may be more likely to occur based on the specific pattern of coupling. I use the Cascadia subduction zone as a case study to then show how including coupling for rupture generation can impact the resultant tsunami hazards for an area when compared to the traditional approach for stochastic modeling. I find that imposing coupling into the workflow creates noticeable effects on the coastal tsunami hazards.Next, I validate this approach for earthquake modeling in its ability to model M9+ earthquakes. Prior to this, stochastic slip rupture modeling had not been validated in its ability to create “realistic” slip distributions for such large magnitude events. Here, I compare the dissimilarity of a suite of slip distributions created to previously published finite fault models for 4 recent and historic M9+ earthquakes. These stochastic models are completely blind to the events we compare them to, however, the stochastic slip modeling approach is able to produce ruptures that have similar slip distributions, and therefore “realistic”, to all 4 earthquakes. Additionally, when coupling models are available, the amount and similarity of “realistic” slip distributions can increase. However, the converse could also be true depending on the a priori assumption of coupling included, so a warning should be heeded. Then, with the validated method for producing coupling informed stochastic slip models, I constrain potential slip distributions for the last great Cascadia earthquake. Unlike any previous attempt, here I incorporate 3 different paleoseismic proxies associated with the event and compare them to results from tsunami and coastal deformation modeling. I specifically compare previously estimated tsunami arrival heights in Japan, subsidence estimates across the coastal Pacific Northwest, and locations of onshore tsunami sand deposits at 7 coastal marsh and lacustrine environments to those produced by the synthetic rupture models. By utilizing all 3 constraints, I find 7 unique but similar, wall-to-wall heterogeneous slip distributions. I also find that sequences of 3 or 4 closely time (years to a few decades) events can satisfy all 3 constraints equally well. While both modes of failure fit the constraints, I favor the full margin events because adequate sequences require specific extreme tidal conditions to inundate many coastal sites and their fits to the geologic observations are weaker. This dissertation includes previously published and unpublished co- authored material.

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Keywords

Cascadia, Paleoseismology, Rupture Modeling, Subduction Zones, Tsunami Hazards, Tsunami Modeling

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