Combined earthquake and tsunami hazards

In current earthquake ground motion and tsunami hazard analysis, the effects of tsunamis and ground shaking are typically considered independent of each other. The same is true for tsunamis of different origin, such as earthquake induced tsunamis and landslide generated tsunamis. Yet all these processes are tied to each other, either causatively, or due to an elevated probability of the processes concurrently. For instance, a magnitude 7.4 aftershock struck the Tohoku region within half an hour after the main event (Mw 9) occurred, during which time tsunami inundation was in full force. Considering the effects of ground shaking and tsunami inundation independently might therefore not be sufficient for a proper engineering load analysis. A comprehensive earthquake catalog survey of the first few hours after recent megathrust events is currently under way to develop a statistical model that allows us to analyze the probability of strong ground shaking during the tsunami inundation that follows the main earthquake.

The second problem we study is the combined tsunami hazard from the earthquake source and earthquake-induced submarine landslides. Using a reciprocal Green's function technique, we can compute the effect of a combined earthquake and landslide source very efficiently at offshore locations. This allows us to address this issue probabilistically by integrating over a large set of combinations of earthquake sources and landslides. The timing and location of the landslides are not random, but follow a distribution that is based on simple models of ground motion arrivals, amplitude and duration. Currently, the uncertainties in the location, volume and triggering of the landslides are large, but the efficient computational model allows us to explore a large range of epistemic uncertainties to include in any probabilistic analysis.