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ABSTRACT A major earthquake ruptured the Cascadia subduction zone (CSZ) on 26 January 1700. Key paleoseismic evidence associated with this event include tsunami deposits, stratigraphic evidence of coastal coseismic subsidence, written Japanese records of a tsunami unaccompanied by earthquake shaking, and margin-wide turbidites found offshore and in lacustrine environments. Despite this wealth of independent clues, important details about this event remain unresolved. Dating uncertainties do not conclusively establish whether the proxies are from one earthquake or a sequence of them, and we have limited knowledge of the likely slip distributions of the event or events. Here, we use a catalog of 37,500 candidate synthetic ruptures between Mw 7.8 and 9.2 and simulate their resulting coseismic deformation and tsunami inundation. Each model is then compared against estimated Japan tsunami arrivals, regional coastal subsidence records, and local paleotsunami deposits mapped at six different coastal marshes and one coastal lake along the CSZ. We find that seven full-margin ruptures with a median magnitude of Mw 9.1 satisfy all three constraints. We favor one Mw 9.11 model that best matches all site paleoseismic observations and suggests that the Cascadia megathrust slipped up to ∼30 m and must have shallow geodetic coupling. We also find that some sequences composed of three or four ruptures can still satisfy the observations, yet no sequences of two ruptures can. Sequences are differentiated into three groups based on whether they contain a mainshock rupture located in the south (>44° N) or further north. All sequences contain unruptured portions of the megathrust and most contain mainshocks with peak slip above 40 m. The fit of the geologic evidence from sequences is poor in comparison to single-event models. Therefore, sequences are generally less favored compared to full-margin events.
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Relationships Among Forearc Structure, Fault Slip, and Earthquake Magnitude: Numerical Simulations With Applications to the Central Chilean Margin
Abstract Two adjacent segments of the Chile margin exhibit significant differences in earthquake magnitude and rupture extents during the 1960 Valdivia and 2010 Maule earthquakes. We use the discrete element method (DEM) to simulate the upper plate as having an inner and outer wedge defined by different frictional domains along the décollement. We find that outer wedge width strongly influences coseismic slip distributions. We use the published peak slip magnitudes to pick best fit slip distributions and compare our models to geophysical constraints on outer wedge widths for the margins. We obtain reasonable fits to published slip distributions for the 2010 Maule rupture. Our best‐fit slip distribution for the 1960 Valdivia earthquake suggests that peak slip occurred close to the trench, differing from published models but being supported by new seismic interpretations along this margin. Finally, we also demonstrate that frictional conditions beneath the outer wedge can affect the coseismic slip distributions.
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- Award ID(s):
- 1723249
- PAR ID:
- 10359900
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 48
- Issue:
- 13
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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