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

Abstract

The Cascadia Subduction Zone (Figure 1, CSZ) is a 1,200 km plate boundary that extends from the Mendocino Triple Junction in Northern California to the tectonically complex region surrounding the Explorer Plate offshore of Northern Vancouver Island (Braunmiller & Nábělek, 2002;Savard et al., 2020). It accommodates roughly 4 cm of annual convergence between the Juan de Fuca and North American plates (DeMets et al., 2010). It is well established that the CSZ hosts magnitude 9+ earthquakes and is the dominant source of seismic hazard in the region (Petersen et al., 2020).The most recent great earthquake occurred in 1700, this is supported by a variety of geologic observations summarized in Walton & Staisch, et al. (2021). Across the margin there are instances of buried marsh soils and sand over mud contacts interpreted to represent sudden coseismic subsidence and deposition soon after caused by a tsunami. Microfossil analyses at these coastal marshes allow quantification of the subsidence (Figure 2). Similarly, there is evidence of deep-sea turbidite in cores, these are inferred to be deposited simultaneously at locations across the plate boundary and are thought to be seismically-induced. Historical documents in Japan reveal that a tsunami occurred in Honshu at sites as far as ∼800 km apart without an accompanying earthquake. From these documents Atwater et al. (2005) and Satake et al. ( 2003) deduced the amplitudes of the tsunami there and hydrodynamic modeling found that an earthquake in the M8.7-M9.2 range rupturing most of the CSZ could simultaneously explain the North-American and Japanese observations. Despite this knowledge, quantifying seismic hazards has been challenging because recurrence intervals are long, and no large events have occurred in the instrumental record. Geodetic locking models (Li et al., 2018;Schmalzle et al., 2014) can provide estimates of fault moment rates but do not resolve the offshore region clearly. Paleoseismic inferences of magnitudes and recurrences of events (Goldfinger et al., 2012(Goldfinger et al., , 2017Nelson et al., 2021) can be used as well. This information can be combined with suitable ground motion models (GMMs; e.g., Abrahamson et al., 2016), which estimate the severity of ground motions for an earthquake as a function of magnitude, source dimensions, distance, and local geology to make inferences about likely shaking and its probability of occurring.

Cascadia Subduction Zone Fault Heterogeneities From Newly Detected Small Magnitude Earthquakes

“…For a majority of the margin, our seismogenic zone does not extend to the region of tremor epicenters, instead leaving a gap up to ∼57 km wide (Figure 4b). Similarly, Melgar et al (2022) show that the 1700 coseismic subsidence can be explained by ruptures extending inland, beneath the coast, but not by ruptures forced to extend to the top of the SSE zone. This gap between the seismogenic zone and the tremor zone is not unique to the CSZ (e.g., Husker et al, 2012;Obara, 2011).…”

“…Similarly, Melgar et al. (2022) show that the 1700 coseismic subsidence can be explained by ruptures extending inland, beneath the coast, but not by ruptures forced to extend to the top of the SSE zone. This gap between the seismogenic zone and the tremor zone is not unique to the CSZ (e.g., Husker et al., 2012; Obara, 2011).…”

A pressure solution flow law for the seismogenic zone: Application to Cascadia

“…8A ) ( 70 , 71 ). Cascadia is also an active margin with inversions of geodetic data ( 72 , 73 ) that can be compared with the predictions of the slip rate deficit calculated using the pressure solution flow law. Thermokinematic models for Cascadia ( 57 ), combined with estimates of the plate boundary geometry ( 74 ), predict that the 400°C isotherm resides at a depth of 30 km at a distance 170 km from the trench ( Fig.…”

Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking

“…Continuously advancing observations and modeling studies have led to remarkable progress in understanding the Cascadia megathrust and the processes of great subduction earthquakes (Walton et al., 2021; K. Wang & Tréhu, 2016). These observational and modeling advances include long‐term seismological and geodetic monitoring (e.g., Bartlow, 2020; Dragert et al., 2001; McCaffrey et al., 2013; McCrory et al., 2012; Rogers & Dragert, 2003; Stone et al., 2018; Toomey et al., 2014), lithospheric scale geophysical imaging (e.g., Audet & Schaeffer, 2018; Carbotte et al., 2022; Davis & Hyndman, 1989; Han et al., 2017; Nedimović et al., 2003; Yuan et al., 1994), paleo‐seismology and ‐tsunami studies of historical ruptures (e.g., Atwater et al., 1995; Goldfinger et al., 2012; Priest et al., 2010; P. L. Wang et al., 2013), laboratory studies of fault‐rock frictional properties (e.g., Ikari et al., 2009; Stanislowski et al., 2022), and modeling of the seismogenic extent constrained by thermal and/or deformation observations (e.g., Hyndman & Wang, 1993; Melgar et al., 2022; Schmalzle et al., 2014; K. Wang et al., 2003). Nonetheless, existing models of the Cascadia megathrust locking distribution are all constrained solely by land‐based GNSS measurements (Li et al., 2018; Pollitz & Evans, 2017; Schmalzle et al., 2014; K. Wang et al., 2003).…”

“…These observational and modeling advances include long-term seismological and geodetic monitoring (e.g., Bartlow, 2020;Dragert et al, 2001;McCaffrey et al, 2013;McCrory et al, 2012;Rogers & Dragert, 2003;Stone et al, 2018;Toomey et al, 2014), lithospheric scale geophysical imaging (e.g., Audet & Schaeffer, 2018;Carbotte et al, 2022;Davis & Hyndman, 1989;Han et al, 2017;Nedimović et al, 2003;Yuan et al, 1994), paleo-seismology and -tsunami studies of historical ruptures (e.g., Atwater et al, 1995;Goldfinger et al, 2012;Priest et al, 2010;P. L. Wang et al, 2013), laboratory studies of fault-rock frictional properties (e.g., Ikari et al, 2009;Stanislowski et al, 2022), and modeling of the seismogenic extent constrained by thermal and/or deformation observations (e.g., Hyndman & Wang, 1993;Melgar et al, 2022;Schmalzle et al, 2014;K. Wang et al, 2003).…”