[1] The first probabilistic tsunami flooding maps have been developed. The methodology, called probabilistic tsunami hazard assessment (PTHA), integrates tsunami inundation modeling with methods of probabilistic seismic hazard assessment (PSHA). Application of the methodology to Seaside, Oregon, has yielded estimates of the spatial distribution of 100-and 500-year maximum tsunami amplitudes, i.e., amplitudes with 1% and 0.2% annual probability of exceedance. The 100-year tsunami is generated most frequently by far-field sources in the Alaska-Aleutian Subduction Zone and is characterized by maximum amplitudes that do not exceed 4m,with an inland extent of less than 500 m. In contrast, the 500-year tsunami is dominated by local sources in the Cascadia Subduction Zone and is characterized by maximum amplitudes in excess of 10 mand an inland extent of more than 1k m. The primary sources of uncertainty in these results include those associated with interevent time estimates, modeling of background sea level, and accounting for temporal changes in bathymetry and topography.N onetheless, PTHA represents an important contribution to tsunami hazard assessment techniques; viewed in the broader context of risk analysis, PTHA provides amethod for quantifying estimates of the likelihood and severity of the tsunami hazard, which can then be combined with vulnerability and exposure to yield estimates of tsunami risk.

From 22 to 0 Ma, σ3 in the Cascade arc rotated clockwise from approximately N–S to E–W. σ1 rotated from subhorizontal to vertical at about 7 Ma, producing an extensional stress regime at 7–0 Ma. Rotation of σ1 and σ3 was likely a response to decreasing influence of ENE to NE compression at the Juan de Fuca plate‐North American plate (JDFP‐NAP) boundary relative to N‐S compression and attendant continental extension produced at the Pacific plate (PP)‐North American plate boundary. Decreases in orthogonal convergence rate, convergence angle, and length of the convergent margin relative to the NAP‐PP transform boundary caused the stress rotations. Volcanic production decreased by a factor of 3 from the interval 35–17 Ma to 16.9–7.4 Ma, probably reflecting a decrease in convergence rate. Volcanic production increased at 7.4–0 Ma even though convergence rate continued to decrease. The extensional stress regime at 7–0 Ma promoted mafic volcanism that caused the increased volcanic production. Volcanic production is therefore a function of convergence rate and upper plate stress regime. The volcanic front migrated progressively eastward from 35 to 0 Ma as the volcanic belt narrowed. The narrowing was caused primarily by steepening slab dip at depths greater than 100 km. Eastward migration was likely caused by decreasing shallow (0–100 km) slab dip resulting from thinning of the NAP. Uplift of the Western Cascades province in the early Pliocene may have been caused by vigorous flow into the mantle wedge accommodating an increase of free rollback rate of the subducted plate at 4 Ma.

[1] We test hypothetical tsunami scenarios against a 4,600-year record of sandy deposits in a southern Oregon coastal lake that offer minimum inundation limits for prehistoric Cascadia tsunamis. Tsunami simulations constrain coseismic slip estimates for the southern Cascadia megathrust and contrast with slip deficits implied by earthquake recurrence intervals from turbidite paleoseismology. We model the tsunamigenic seafloor deformation using a three-dimensional elastic dislocation model and test three Cascadia earthquake rupture scenarios: slip partitioned to a splay fault; slip distributed symmetrically on the megathrust; and slip skewed seaward. Numerical tsunami simulations use the hydrodynamic finite element model, SELFE, that solves nonlinear shallow-water wave equations on unstructured grids. Our simulations of the 1700 Cascadia tsunami require >12-13 m of peak slip on the southern Cascadia megathrust offshore southern Oregon. The simulations account for tidal and shoreline variability and must crest the $6-m-high lake outlet to satisfy geological evidence of inundation. Accumulating this slip deficit requires ≥360-400 years at the plate convergence rate, exceeding the 330-year span of two earthquake cycles preceding 1700. Predecessors of the 1700 earthquake likely involved >8-9 m of coseismic slip accrued over >260 years. Simple slip budgets constrained by tsunami simulations allow an average of 5.2 m of slip per event for 11 additional earthquakes inferred from the southern Cascadia turbidite record. By comparison, slip deficits inferred from time intervals separating earthquake-triggered turbidites are poor predictors of coseismic slip because they meet geological constraints for only 4 out of 12 ($33%) Cascadia tsunamis.Citation: Witter, R. C., Y. Zhang, K. Wang, C. Goldfinger, G. R. Priest, and J. C. Allan (2012), Coseismic slip on the southern Cascadia megathrust implied by tsunami deposits in an Oregon lake and earthquake-triggered marine turbidites,

Characterizations of tsunami hazards along the Cascadia subduction zone hinge on uncertainties in megathrust rupture models used for simulating tsunami inundation. To explore these uncertainties, we constructed 15 megathrust earthquake scenarios using rupture models that supply the initial conditions for tsunami simulations at Bandon, Oregon. Tsunami inundation varies with the amount and distribution of fault slip assigned to rupture models, including models where slip is partitioned to a splay fault in the accretionary wedge and models that vary the updip limit of slip on a buried fault. Constraints on fault slip come from onshore and offshore paleoseismological evidence. We rank each rupture model using a logic tree that evaluates a model's consistency with geological and geophysical data. The scenarios provide inputs to a hydrodynamic model, SELFE, used to simulate tsunami generation, propagation, and inundation on unstructured grids with <5-15 m resolution in coastal areas. Tsunami simulations delineate the likelihood that Cascadia tsunamis will exceed mapped inundation lines. Maximum wave elevations at the shoreline varied from ~4 m to 25 m for earthquakes with 9-44 m slip and M w 8.7-9.2. Simulated tsunami inundation agrees with sparse deposits left by the A.D. 1700 and older tsunamis. Tsunami simulations for large (22-30 m slip) and medium (14-19 m slip) splay fault scenarios encompass 80%-95% of all inundation scenarios and provide reasonable guidelines for landuse planning and coastal development. The maximum tsunami inundation simulated for the greatest splay fault scenario (36-44 m slip) can help to guide development of local tsunami evacuation zones.

To explore the local tsunami hazard from the Cascadia subduction zone we (1) evaluate geologically reasonable variability of the earthquake rupture process, (2) specify 25 deterministic earthquake sources, and (3) use resulting vertical coseismic deformations for simulation of tsunami inundation at Cannon Beach, Oregon. Maximum runup was 9-30 m (NAVD88) from earthquakes with slip of *8-38 m and M w *8.3-9.4. Minimum subduction zone slip consistent with three tsunami deposits was 14-15 m. By assigning variable weights to the source scenarios using a logic tree, we derived percentile inundation lines that express the confidence level (percentage) that a Cascadia tsunami will not exceed the line. Ninety-nine percent of Cascadia tsunami variation is covered by runup B30 m and 90% B16 m with a ''preferred'' (highest weight) value of *10 m. A hypothetical maximum-considered distant tsunami had runup of *11 m, while the historical maximum was *6.5 m.