FY 2010

Tsunami hazard assessment for Guam

Uslu, B., V.V. Titov, M. Eble, and C. Chamberlin

NOAA OAR Special Report, Tsunami Hazard Assessment Special Series, Vol. 1, 186 pp (2010)

Executive Summary

Several Pacific Ocean Basin tsunamis occurred in the mid 20th century, causing destruction to United States coastal communities over a wide spatial scale. The destruction and unprecedented loss of life following the December 2004 Sumatra tsunami provided a reminder of the potential hazard posed to coastal communities and served as the catalyst for the formation of a partnership between the Pacific Risk Management ‘Ohana and the National Oceanic and Atmospheric Administration Center for Tsunami Research. The goal of this partnership is to conduct comprehensive tsunami hazard assessments for Pacific communities within the United States and her territories in support of hazard mitigation efforts that leverage multi-level agency collaboration for the benefit of the Pacific community ‘Ohana (or family). Of the specific communities identified for tsunami hazard assessment, the Island of Guam was chosen as the pilot study with the goal of identifying earthquake sources with the greatest potential to impact Guam, its population, and economy. The scope of this study includes five tsunami-vulnerable sites along the coastline of Guam for which 725 probable earthquake scenarios are considered.

Tsunami hazard assessments for each of five coastal communities on the Island of Guam—Tumon Bay, Agana Bay, Pago Bay, Apra Harbor, and Inarajan Bay—are conducted with both moderate (Mw 8.5) and Great Earthquake (Mw ≥ 9.0) scenarios. Wave amplitudes as high as 7.0 m at Tumon Bay are predicted, and at Pago and Inarajan bays the leading wave is predicted to arrive within 20 min with amplitudes as high as 15 m and 9 m, respectively, following a Great or worst-case earthquake. The Great Earthquake scenario is particularly important when considering far-field sources. Model results indicate that sources along the western Aleutians and Cascadia pose a significant risk to Guam due to favorable tsunami directivity from these regions to Guam. Although Great Earthquakes represent worst-case scenarios, occurrence of a moderate Pacific Basin earthquake is the most likely scenario. Tumon Bay, Apra Harbor, and Agana Bay are at significant risk from tsunamis generated by moderate earthquakes occurring along Ryukyu-Nankai source segments, while Manus and West Aleutian source segments pose a significant risk to both Pago Bay and Inarajan Bay in the event of a moderate earthquake. The greatest risk for all five communities, however, is posed by tsunamis generated along Mariana Trench and east Philippines sources, as identified from results of the 3625 optimized model runs. Overall, results show that Guam is at risk from tsunamis generated by earthquakes occurring in both near and far field due to Guam’s location relative to Pacific Basin subduction zones. Near-field earthquakes pose a time dependent problem for emergency managers due to Guam’s proximity to the seismic sources. The potential for local hazard from these sources with associated impact to population and marine structures is predicted to be significant.