What If the Cascadia Megaquake Hit Tonight? We Ran 10,000 Simulations

In January 1700, a tsunami flooded villages in Japan. Nobody there felt an earthquake. The wave came from a fault off the coast of Oregon and Washington, and that fault has been locked ever since. We rebuilt it: the real sea floor, the real fault, a magnitude 9.0 rupture racing north at 2.8 km/s. Then we ran the tsunami over the real bathymetry, sent it across the Pacific, and broke the fault 10,000 more times to see which towns get hit hardest. CHAPTERS 0:00 January 26, 1700 0:56 The fault 1:53 The evidence: ghost forests and 10,000 years of mud 3:00 The rupture, minute by minute 4:23 The wave 5:16 Seaside, Oregon: 31 minutes 6:09 10,000 earthquakes 7:20 Across the Pacific to Japan 8:08 What it means 9:00 Which disaster next? KEY RESULTS (OUR MODEL) Full rupture M9.0: about 1,130 km of fault breaks in about 6 min 40 s. Portland shakes at 2:46, Seattle at 3:52. Sea floor rises up to 5.4 m; the coast drops 0.5-1.6 m. First wave over 1 m: Brookings 9 min, Coos Bay 12 min, Newport 21 min, Cannon Beach 30 min, Seaside 31 min, Ocean Shores 35 min. Peak in 30 m deep water at Seaside: 9.1 m at 43 min. Run-up on land is higher. 10,000 runs: southern Oregon and northern California get a wave over 1 m in about 3 of 4 runs; Washington in 46%. Seaside/Cannon Beach over 5 m in ~31% of runs, over 10 m in ~10%. Japan: first arrival at 9 h 15 min (records from 1700: about 10 h). HOW WE DID IT Fault: Cascadia trench traced from real bathymetry (AWS Terrain Tiles), 22 segments, Okada (1985) elastic dislocation, 11 degree dip, 130 km wide. Tsunami: linear shallow-water model on a 1/60 degree grid (near field) and 0.15 degree grid (Pacific), real bathymetry. 10,000-run Monte Carlo: full vs. southern-only ruptures in proportion to the turbidite record (Goldfinger et al. 2012), lognormal patchy slip. Ghost forests and the 1700 date: Atwater et al., The Orphan Tsunami of 1700 (USGS/UW Press); Satake et al. (1996). Odds: about 15% in 50 years for a full rupture (Pacific Northwest Seismic Network). Casualty scenario: FEMA Cascadia Rising (2016), Oregon + Washington. Refuge: Ocosta Elementary School, Westport WA (2016). This is a simplified physics model for education, not an official hazard map. For your evacuation route, check your state's tsunami maps. Which disaster should we run next? Tell us in the comments. #Cascadia #Tsunami #WhatIf