Deep Coseismic Slip in the Cascadia Megathrust Can Be Consistent With Coastal Subsidence

At subduction zones, the down‐dip limit of slip represents how deep an earthquake can rupture. For hazards it is important ‐ it controls the intensity of shaking and the pattern of coseismic uplift and subsidence. In the Cascadia Subduction Zone, because no large magnitude events have been observed...

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Bibliographic Details
Published in:Geophysical research letters 2022-02, Vol.49 (3), p.n/a
Main Authors: Melgar, Diego, Sahakian, Valerie J., Thomas, Amanda M.
Format: Article
Language:English
Subjects:
Earthquakes
Geological hazards
Geological samples
Seismic activity
Shaking
Slip
Subduction
Subduction (geology)
Subduction zones
Subsidence
Uplift
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Summary:At subduction zones, the down‐dip limit of slip represents how deep an earthquake can rupture. For hazards it is important ‐ it controls the intensity of shaking and the pattern of coseismic uplift and subsidence. In the Cascadia Subduction Zone, because no large magnitude events have been observed in instrumental times, the limit is inferred from geological estimates of coastal subsidence during previous earthquakes; it is typically assumed to coincide approximately with the coastline. This is at odds with geodetic coupling models as it leaves residual slip deficits unaccommodated on a large swath of the megathrust. Here we will show that ruptures can penetrate deeper into the megathrust and still produce coastal subsidence provided slip decreases with depth. We will discuss the impacts of this on expected shaking intensities. Plain Language Summary We know that large earthquakes at the Cascadia subduction zone, in the Pacific Northwest of the U.S., are possible. In order to understand how strong shaking will be when this occurs and how the coast will subside, we need to know how deep into the Earth the fault can move. The deeper this limit is, the stronger the shaking will probably be. In Cascadia, because we have not observed a large event in the last few centuries, it is difficult to determine this limit. The common wisdom, from analysis of geological samples, is that this limit is fairly shallow. However, if this were true, it would lead to some contradictions with other behaviors observed in Cascadia. Here we show that it is possible for the limit to be deeper so long as the next earthquake has certain properties. This deeper limit resolves the contradictions but it also implies that shaking in the next big earthquake can be significantly stronger than previously anticipated. Key Points Deep coseismic slip is possible in Cascadia if slip tapers with depth Slip can extend into the slow‐slip zone Deeper slip can produce overall higher intensity ground motions in some regions
ISSN:0094-8276
1944-8007
DOI:10.1029/2021GL097404