Latest tsunami evacuation map for Wailua to Keālia, showing the normal and extreme tsunami evacuation zones. Courtesy of Kaua‘i County Civil Defense Agency
I was challenged at a recent public meeting by a particularly inquisitive individual. He wanted to know why the Eastside’s tsunami evacuation zones were so large, especially when there is a reef extending out into the ocean that may act like as an protection. It got me thinking, how many others are similarly puzzled about how the evacuation maps are drawn?
It seemed like perfect timing for me to brush up on my tsunami modeling methodology, so I touched base with Dr. Kwok Fai Cheung from the University of Hawai‘i.
First he gave me a little history for perspective. In the last 100 years, five major tsunamis have inundated Hawai‘i. Up until 1963, the evacuation zone was everywhere below 50 feet in elevation. In the early days, scientists used historical data to determine the evacuation zone. After each tsunami event, experts went out in the field and surveyed things like debris and water marks to determine the maximum runup.
In 1963, evacuation maps started being published by the Hawai‘i State Civil Defense based on a geometric approach that took into account historical runup records as well as coastal terrain, e.g. topographical features such as valleys, dunes, reefs, etc.
It wasn’t until 1991 that the evacuation maps would be updated again, this time utilizing computer modeling of the five most destructive tsunamis to infer their inundation limits on the land. Since then, Cheung’s two-dimensional models have helped update the maps twice (2014 and 2016).
Kaua‘i’s 2014 update (the current tsunami evacuation zone) was based on the five worst 20th century tsunami, recalculated at high tide and run at different angles around the island. Cheung’s model reproduced the events from the source (e.g. earthquake) all the way across the ocean to calculate the inundation areas, taking into account numerous factors including water depth and bathymetry (e.g. ocean floor topography, reefs, etc.) — things that transform the tsunami as it travels. Once the model was tested and shown to match the historical runups calculated in the field, then the model was deemed accurate for all areas along the coastline.
The major difference with this year’s update is that it factors in a chance of an “extreme” tsunami, a 500- to 1,000-year event. After Japan’s Tohuko tsunami, a one in 1,000-year event which took everyone by surprise, scientists here decided to go back to the drawing board and model our worst case scenario. This means a mega earthquake coming from the nearest subduction zone in the Aleutian Islands in Alaska. Cheung worked with a seismologist to model 9.3 and 9.6 magnitude earthquakes and associated tsunami, calculated inundation areas, and delineated a second evacuation zone.