Impact of Down‐Dip Rupture Limit and High‐Stress Drop Subevents on Coseismic Land‐Level Change during Cascadia Megathrust Earthquakes

Abstract

Seismic hazard associated with Cascadia megathrust earthquakes is strongly dependent on the landward rupture extent and heterogeneous fault properties. We use 3D numerical simulations and a seismic velocity model for Cascadia to estimate coseismic deformation due to M 9–9.2 earthquake scenarios. Our earthquake source model is based on observations of the 2010 M 8.8 Maule and 2011 M 9.0 Tohoku earthquakes, which exhibited distinct strong motion‐generating subevents in the deep portion of the fault. We compare our estimates for land‐level change to paleoseismic estimates for coseismic coastal subsidence during the A.D. 1700 Cascadia earthquake. Results show that megathrust rupture extending to the 1 cm/yr locking contour provides a good match to geologic data. In addition, along‐strike variations in coastal subsidence can be matched by including low slip, strong motion‐generating subevents in the down‐dip region of the megathrust. This work demonstrates the potential to improve seismic hazard estimates for Cascadia earthquakes by comparing physics‐based earthquake simulations with geologic observations.

Minimal stratigraphic evidence for coseismic coastal subsidence during 2000 yr of megathrust earthquakes at the central Cascadia subduction zone

“…However, as with the preferred model of Wang et al (2013, their fig. 8), subsidence at the Siuslaw River estuary in the most consistent model presented by Wirth and Frankel (2019, their fig. 3c) is ~0.2 m.…”

“…Geophysical models of the rupture during the great earthquake of 1700 CE (discussed below) are consistent with our correlation in that they show coseismic subsidence of ~0.2 m near the Siuslaw River estuary (Wang et al, 2013, their fig. 8; Wirth and Frankel, 2019, their fig. 3c).…”

“…In a series of alternative models for the 1700 CE rupture using a more detailed three-dimensional, seismic-wave velocity model of the subduction zone, Wirth and Frankel (2019) distinguished between (1) large rupture patches of more uniform, low-to-moderate slip in the downdip region of the megathrust, including small, higher-slip subevents (earthquakes), which would have generated high-frequency energy (that strongly impacts buildings and infrastructure), and (2) broad, high-slip rupture patches that would generate low-frequency energy (most important for generating tsunamis). Within their series of magnitude 9.2 earthquake scenarios, Wirth and Frankel (2019) explained the apparent along-strike slip heterogeneity indicated by the large differences in coseismic subsidence at sites only tens of kilometers apart in central Oregon (Kemp et al, 2018) by inserting a high-frequency subevent (a magnitude 8.2 earthquake in the downdip part of the rupture) 70-150 km north of the Siuslaw River.…”

“…The series of great (moment magnitude >8) earthquakes early in the twenty-first century has resulted in greater appreciation for the variability of megathrust earthquake ruptures at subduction zones (Wang, 2007;Melnick et al, 2012;Wang and Tréhu, 2016;Bilek and Lay, 2018;Wang et al, 2018). Because such variability complicates the local as well as ocean-wide earthquake and tsunami hazard forecasts used to direct hazard mitigation, reconstructing the history of the greatest ruptures and their accompanying destructive tsunamis remains fundamental to hazard assessment (Mueller et al, 2015;Wirth and Frankel, 2019). Once used primarily to estimate the average recurrence of great earthquakes for entire subduction zones, the chief benefit of recently developed earthquake and tsunami histories is to limit increasingly complex models of megathrust rupture to what has happened in the past (e.g., Witter et al, 2012;Nelson, 2013;Wang et al, 2013;Moernaut et al, 2014;Shennan et al, 2016;Gao et al, 2018;Wirth and Frankel, 2019).…”

“…Because such variability complicates the local as well as ocean-wide earthquake and tsunami hazard forecasts used to direct hazard mitigation, reconstructing the history of the greatest ruptures and their accompanying destructive tsunamis remains fundamental to hazard assessment (Mueller et al, 2015;Wirth and Frankel, 2019). Once used primarily to estimate the average recurrence of great earthquakes for entire subduction zones, the chief benefit of recently developed earthquake and tsunami histories is to limit increasingly complex models of megathrust rupture to what has happened in the past (e.g., Witter et al, 2012;Nelson, 2013;Wang et al, 2013;Moernaut et al, 2014;Shennan et al, 2016;Gao et al, 2018;Wirth and Frankel, 2019). The most valuable histories-particularly in subduction zones that lack long historical records-include reconstructions that extend models based on instrumental measurements back in time through multiple cycles of great earthquakes (e.g., Ely et al, 2014;Shennan et al, 2014;Garrett et al, 2016;Hayward et al, 2015;Meltzner et al, 2015;Wesson et al, 2015;Milker et al, 2016;Pinegina et al, 2020).…”