Comparing felt intensity patterns for deep intraslab earthquakes in the Cascadia and Chilean subduction zones, offshore British Columbia, United States, and Chile

Abstract

In this study, we utilize US Geological Survey citizen science earthquake felt intensity data to investigate whether deep, intraslab earthquakes in the Chilean Subduction Zone show similar, 'felt intensity' distributions to events of the same magnitude and depths within the Cascadia Subduction Zone (Quitoriano & Wald, 2020; USGS Earthquake Hazards Program, 2020). In a companion article (Rutherford & Cassidy, 2022) we examine crustal earthquake intensity patterns for the Chile and Cascadia subduction zones. One goal of this comparison is to determine whether felt intensity information from several recent large (M8-8.8) subduction earthquakes in Chile can be applied to Cascadia (where no subduction earthquakes have been felt since 1700). This will provide a better understanding of shaking intensity patterns for future subduction earthquakes in Cascadia - critical information for scientists, engineers, and emergency management organizations. For this research, we utilized 20 years of catalogued 'Did You Feel It?' (DYFI) citizen science data from the US Geological Survey's (USGS) earthquake online catalogue, the ANSS Comprehensive Earthquake Catalog (ComCat) Documentation (USGS Earthquake Hazards Program, 2021). In total, we compared intensity patterns from five earthquakes in Cascadia (M4.8, M5.0. M6.7 & M6.8) to the intensity patterns from 157 earthquakes in Chile, with the same magnitudes as the Cascadia events (M4.8-M6.8). Our analysis involved plotting and fitting the Chile and Cascadia earthquakes' DYFI responses in order to compare the intensity patterns for the two subduction zones. Overall, we find good agreement between felt patterns in Chile and Cascadia. For example, all plots show the expected downward trend for intensity with distance; and there is generally a similar intensity clustering of responses around 50 to 300 km hypo-central distance. These results provide confidence that we can use Chilean intensity data for megathrust earthquakes in Cascadia.