Geographic Research To Help Protect Lives From Future Tsunamis
By Nathan J. Wood
Figure 1. Maps of (A) Ocean Shores, Wash., and (B) Aberdeen, Wash., showing modeled pedestrian travel times to high ground out of tsunami-hazard zones, assuming a travel speed of 1.1 m/s. Tsunamis associated with a Cascadia subduction zone earthquake are predicted to arrive in these communities within 25 minutes after generation (modified from Wood and Schmidtlein, 2013).
In 2011, more than 15,000 people lost their lives from tsunamis associated with an Mw 9.0 earthquake off the coast of Japan. Although some may think tsunami disasters only occur in other countries (from dramatizations in movies like "The Impossible"), the reality is that many U.S. coastal communities are threatened by tsunami hazards. Low-lying areas along many parts of the U.S. coastline also could be inundated minutes after a local earthquake, such as off the coasts of the Pacific Northwest, Alaska, Puerto Rico, and American Samoa.
To protect lives from future tsunamis, local emergency managers need to know whether or not individuals in tsunami-hazard zones would have enough time to reach natural high ground before tsunami waves arrive. In areas where evacuations are possible, education and training can be provided to help at-risk individuals understand how to interpret signs of imminent tsunamis and to know what to do to reach high ground. In areas where successful evacuations are not realistic, vertical-evacuation strategies may be warranted, such as creating berms, raised platforms, or other structures to provide safe havens in the event of future tsunamis.
Determining if and where vertical-evacuation strategies may be warranted requires an interdisciplinary understanding of tsunami hazards, land cover conditions, and population distributions. To help frame evacuation challenges in response to future tsunamis, geographers with the USGS Western Geographic Science Center have been developing new ways of characterizing population exposure and evacuation potential by integrating demographic, economic, and hazard data to identify U.S. communities throughout the Pacific Ocean Basin that have high population exposure to tsunami hazards. This allows us to identify hot-spots of societal vulnerability to tsunamis in terms of high population densities, urban footprints, and certain sub-populations that may have difficulty responding to future events. Reports comparing community exposure to tsunami hazards have been completed for Oregon, Washington, Hawaii, and California.
Figure 2. Graph of the distribution of residents in Ocean Shores and Aberdeen (Wash.) as a function of modeled pedestrian travel time to safety (modified from Wood and Schmidtlein, 2013). Distance to high ground and wave-arrival times are related to a Cascadia subduction zone earthquake scenario, which is predicted to generate tsunami waves that strike the coast in 25 minutes.
To understand the potential for successful evacuations, we are modeling pedestrian travel times to safety by leveraging many of The National Map data theme products including orthoimagery, elevation, hydrography, boundaries, transportation, structures, and land cover (Figure 1). Maps are developed using various travel-speed assumptions to identify sites where an increase in travel speeds could save more lives. Maps of evacuation travel times are merged with demographic and economic data to estimate the distribution of at-risk residents, employees, dependent-care facilities, and public venues within communities as a function of travel time to safe locations (Figure 2). Recent modeling efforts suggest that certain communities in southwest Washington may have sufficient time to reach higher safe locations (such as Aberdeen, Wash.) and others may not (such as Ocean Shores, Wash.) before tsunami waves associated with a Cascadia subduction zone earthquake arrive (Figure 2). In areas where successful evacuations are unlikely, we also are modeling the impact of potential vertical-evacuation structures on evacuation travel times and population exposure.
To help frame the difficult decisions in siting vertical-evacuation strategies, we are developing a geospatial pedestrian-evacuation toolbox that leverages Esri's ArcMap platform. The tool is being developed to help automate data preprocessing and evacuation-modeling procedures, as well as streamline the integration of travel-time maps with demographic and economic data. Once completed, the ArcMap extension will enable researchers to work with emergency managers to quickly assess and track changes in population exposure with each potential vertical-evacuation site. Although we can't stop tsunamis from happening, USGS geographic research and geospatial data in The National Map can help emergency managers develop strategies to minimize life loss.
Wood, N. J., and Schmidtlein, M.C., 2013, Community variations in population exposure to near-field tsunami hazards as a function of pedestrian travel time to safety: Natural Hazards, v. 65, p. 1603-1628.