PREPARING FOR THE NEXT CASCADIA EARTHQUAKE AND TSUNAMI, SEASIDE, OREGON

The city most-exposed to Cascadia tsunamis in Oregon, Seaside is built on a 4000-yr-old, slowly prograding, sand and cobble dune-spit complex that is divided into three evacuation landforms of varying risk by two beach-parallel rivers and a marsh system. Ninety percent of the inundation area lies at elevations of 8 to 22 ft NGVD. Inundation could exceed 45 ft. The first wave may arrive in about 25 minutes. Some parts of town need 35 minutes to reach safety.

Cascadia 1700 AD tsunami deposits extend at least one mile inland from the shoreline. Tsunami surge corridors cross the city at large angles to the shoreline, most likely due to wave reflection off a nearby headland. Cobbles and boulders have been transported from abandoned berm ridges and deposited inland as gravel sheets interbedded within silty peat. The 1964 Alaskan tsunami ran up 18 ft on the ocean beach, 8.5 ft above prevailing tide in the estuary, left up to 14 inches of sand, and caused $250,000 in damage.

Assuming we are in a cluster of Cascadia quakes, the next event has a recurrence probability of up to 45 percent over the next 50 years. Mean recurrence within clusters averages around 340 years, or about 25 years from the present. Massive landslides will damage highways and bridges, isolating Seaside for at least two weeks from overland assistance. Damage to utilities will deny water, sewer, communications, electricity, and natural gas for months. Populations in summer range up to 35,000 and are as low as 6000 in winter. Without constant citizen education and investment into evacuation and survival infrastructure, fatality rates from drowning and hypothermia could range above 50 percent.

A citizen-sponsored strategic investment plan, tentatively approved by City government, proposes to fund outreach and grant-writing to replace seven unsafe critical traffic bridges; provide at least two pedestrian bridges in critical locations; support construction of vertical evacuation towers and battery-powered intersection lights; and store enough survival supplies for 20,000 refugees for 20 days by 2028. Estimated costs are $28 million. Funding may include higher motel room taxes, local sales tax, road levies, preparedness fees with water bills, and federal and state assistance. Relocating five schools to stable ground above tsunami run-up will cost about $100 million.