Comparing felt intensity patterns for crustal 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 , crustal 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 intraslab deep earthquake intensity patterns for the Chile and Cascadia subduction zones. Building on from the intraslab companion article, the 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 would 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 cataloged Did-You-Feel-It (DYFI) citizen science data from the US Geological Survey's (USGS) earthquake online catalog, the ANSS Comprehensive Earthquake Catalog (ComCat) Documentation (USGS Earthquake Hazards Program, 2021). In total, we considered and compared intensity patterns for fourteen magnitudes from 30 earthquakes in Cascadia (ranging from magnitudes 4.5 to 7.2, the highest magnitude event in Cascadia zone) to the intensity patterns from 114 earthquakes in Chile, with the same magnitudes as the Cascadia events (M4.5-M7.2). Our analysis involved plotting and fitting the Chile and Cascadia earthquake DYFI responses 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. Even distribution with limited clustering is seen in all fourteen magnitudes, with slight intensity clustering of responses around the 30 to 600 km. This is slightly different from the intraslab pattern which demonstrated a distinct cluster at further distance from the hypocenter, e.g., cluster at 50 to 300 km. These results provide confidence that we can use Chilean intensity data for megathrust earthquakes in Cascadia.