Tectonic Contributions to Regional Sea-level Projections: Global Probabilistic Vertical Land Motions Model at Subduction Zones

Vertical land motions (VLM) are large sources of uncertainties in regional relative sea-level projections. While there are many processes that contribute to VLM, VLM due to tectonics is a major cause of local spatial variability and uncertainty in relative sea-level. To address this uncertainty, we developed a model to estimate, globally, the probability of VLM due to large earthquakes that might occur in a 50-year period at subduction zones. To maintain global self-consistency, we only use globally available datasets as inputs, such as Slab2.0 fault geometries and earthquake rates from the Global Earthquake Activity Rate (GEAR) model, rather than concentrating on the specifics of each subduction zone. To estimate the VLM from earthquakes, we model coseismic and afterslip motion using an Okada elastic deformation model with elliptical rupture patches for coseismic slip, and afterslip surrounding the coseismic slip rupture area. The postseismic motion was modeled using parametric approximations of existing finite-element models. We ran a suite of simulations to find a distribution of displacements that might occur for locations around the global coastlines. We then extract the probability that each location will subside more than or equal to a certain amount. Based on our models, we show spatial variability in VLM due to tectonics at various locations near each subduction zone. There are locations that observe more subsidence than uplift and vice versa. Further, while some locations experience VLM changes in the order of millimeters to centimeters scale, there are also some locations that can expect changes in the order of meters in 50 years with a low probability. The VLM changes are non-negligible and should not be ignored for relative sea-level projections. To check that our modeling is realistic, we show that some events in our catalogue can reproduce both the coseismic and postseismic afterslip behavior of several recent large earthquakes - 2004 Mw 9.2 Sumatra-Andaman, 2010 Mw 8.8 Maule, and 2011 Mw 9.0 Tohoku-Oki events. With further improvements, our framework can be incorporated into sea-level projections that include both climate-related impacts and VLM, providing a critical next step for more accurate mitigation planning.