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Guam Homeland Security briefs mayors on tsunami risks, siren limits and evacuation mapping
Summary
Guam Homeland Security reviewed tsunami threat areas, local warning signs, AHA siren operation limits and current inundation modeling; mayors raised concerns about siren reach, nighttime testing and evacuation logistics.
Stephen Cahill, Guam Tsunami Program manager at Guam Homeland Security Office of Civil Defense, told mayors that Guam faces tsunami threats from the Marianas Trench, the Nankai Trough (north), the Philippine Trench (west) and the New Guinea Trench (south). Cahill described three tsunami categories (local, regional and distant) and said a local tsunami originating in the Marianas Trench could deliver first waves in roughly 10 minutes after a large earthquake.
Cahill described the three natural warning signs of a local tsunami: a very long, strong and violent earthquake; a sudden, extreme rise or fall of the ocean (not normal low tide); and an audible ocean roar. He emphasized that local tsunamis may arrive before Pacific Tsunami Warning Center (PTWC) messages and urged residents in coastal evacuation zones to act immediately upon experiencing the signs.
On public-alert systems, Cahill explained that AHA sirens are activated by an encoder at Guam Homeland Security with battery backup; those signals ping repeaters and sirens run on rechargeable batteries. He said Guam currently lacks a wireless emergency alert (WEA) capability tied to cell carriers and that the agency is discussing mobile alerts, Ready Guam app push messages, and other options — but funding and phone-company cooperation are constraints.
Cahill reviewed recent projects: University of Hawaii NeoWave inundation modeling for southern villages, evacuation maps accessible on the Guam Homeland Security website (village maps and an address lookup), and coordinated tsunami evacuation drills (examples: school evacuations at Hagat'a— and vertical evacuation at Mount Carmel). He said NeoWave modeling continues for Tumon and Hagat'a— bays and will next close the gap toward Apra Harbor.
Mayors raised practical concerns: siren audibility in populated or noisy areas (for example, Tumon events), the value of nighttime testing to measure resident awareness, whether mobile sirens can be expanded, funding limits for siren and signage upgrades, evacuation transport capacity and the logistics of protecting landlocked low-lying communities quickly. Cahill said many of those issues are funding and coordination challenges: NOAA grant levels have been steady or reduced and larger changes require multiyear planning and interagency work.
Questions also touched on preparedness measures such as life jackets and the force of tsunami currents; Cahill cautioned that high-speed debris-laden currents reduce the effectiveness of personal flotation devices for survival in many scenarios and reiterated the recommended safe actions: move at least 100 feet inland or 50 feet up or to a tall concrete structure and await an official all-clear.
Ending
Cahill invited mayors to use the evacuation maps online, to convey natural-warning signs to residents and to coordinate drills. He said the agency will continue outreach and modeling and that mayors' questions will inform planning and funding requests.
