Augmenting Onshore GNSS Displacements With Offshore Observations to Improve Slip Characterization for Cascadia Subduction Zone Earthquakes

Abstract

For the Cascadia subduction zone, Mw~8 megathrust earthquake hazard is of particular interest because uncertainties in the predicted tsunami size affect evacuation alerts. To reduce these uncertainties, we examine how augmenting the current Global Navigation Satellite Systems (GNSS) network in Cascadia with offshore stations improves static slip inversions for Mw~8 megathrust earthquakes at different rupture depths. We test two offshore coseismic data types: vertical-only bottom pressure sensors and pressure sensors combined with GNSS-Acoustic aided horizontal positions. We find that amphibious networks best constrain slip for a shallow earthquake compared to onshore-only networks when offshore stations are located above the rupture. However, inversions using vertical-only offshore data underestimate shallow slip and tsunami impact. Including offshore horizontal observations improves slip estimates, particularly maximum slip. This suggests that while real-time GNSS-Acoustic sensors may have a long development timeline, they will have more impact for static inversion-based tsunami early warning systems than bottom pressure sensors.