RECENT COMPREHENSIVE TSUNAMI MODELING FOR COASTAL OREGON

Oregon tsunami hazard assessment emphasizes reduction of losses to Cascadia subduction zone (CSZ) sources, because of their high potential to cause loss of life. Paleoseismic evidence for recurrence of CSZ earthquakes on the order of ~250 years (Goldfinger et al., 2012) supports this emphasis. The assessment leaned heavily on paleotsunami and paleoseismic evidence, since there is no local historical data. Width and geometry of the seismogenic zone was determined from a best fit to geothermal, geodetic, seismic, and geologic data. Length of CSZ ruptures was estimated from extent of correlated turbidites deposited over the last 10,000 years. Relative size of CSZ earthquakes was determined from relative turbidite mass and time between turbidites, the latter serving as a proxy for potential slip deficit release. This approach is based on the observation of Goldfinger et al. (2012) that there is a rough correlation for 19 full-margin turbidites between mass and time to the next turbidite (i.e., a time-predictable model). Energy cycle models may offer improvement in explaining the data, but do not offer a prescriptive binning method, thus we retain the time -predictable model.

Witter et al. (2011) thus used turbidite follow times to sort the 19 full-margin events into slip deficit time release bins labeled with the “T-shirt” sizes Sm (300 yrs) , M (425-525 yrs), L (650-800 yrs), XL (1050-1200 yrs), and XXL (1200 yrs). Scenarios were weighted using a logic tree approach that accounted for splay versus no-splay faulting, megathrust ruptures extending nearly to the trench versus terminating at the Quaternary accretionary wedge, and tapering of slip from north to south for the M, L, and XL scenarios. Only full-margin ruptures were simulated, because most tsunami energy from elongate sources like the CSZ is radiated perpendicular to the long axis. This conclusion was verified by Priest et al. (2014) who found that CSZ tsunami height is quite small more than 70 km north of a segment rupture and that recurrence of CSZ tsunami deposits in Bradley Lake only equals recurrence of turbidite deposits for turbidites directly offshore of the lake. The result was 15 CSZ scenarios, 5 of which were chosen for depiction on statewide tsunami inundation maps. These 5 are the splay fault scenarios that produced the largest tsunamis for a given slip deficit time.