Dr Kwok told a briefing in Saipan that the trench and the island's shallow western shelf actually amplify tsunami energy rather than absorb it. Photo: Supplied / Mark Rabago
A leading ocean engineer says long-held public beliefs that the Mariana Trench or Saipan's coral reef would shield the Commonwealth of Northern Marianas (CNMI) from tsunamis are incorrect.
Dr Kwok Fai Cheung, a distinguished professor of ocean and resources engineering at the University of Hawai'i, says new modelling shows the CNMI could experience more severe impacts than Guam if a major event occurs.
He told a briefing in Saipan that the trench and the island's shallow western shelf actually amplify tsunami energy rather than absorb it.
"The Mariana Trench makes a tsunami more severe for the islands because the tsunami is generated near the deeper part of the trench," he said. "As it moves toward the island, the energy compresses and the wave height increases."
Dr Kwok appeared alongside longtime Pacific forecaster Chip Guard during a public presentation hosted by the CNMI Homeland Security and Emergency Management (HSEM) Office.
Both warned that a strong local earthquake - magnitude 8.3 or higher - occurring near the trench could send a 15-metre (50-foot) wave toward Saipan, Tinian, Rota, and the Northern Islands within 10 to 15 minutes.
Because of its broad, shallow shelf, Saipan's west coast could experience greater amplification and deeper inland flooding than neighboring Guam.
Guard was direct about the comparison. "If a big tsunami comes off that trench, the CNMI will get it worse than Guam. The bathymetry focuses energy right onto Saipan's west side," he said.
Dr Kwok added that Saipan's tourist district of Garapan and the school-lined corridor along Beach Road are especially vulnerable.
Dead tree at Micro beach in Garapan, Saipan, Northern Marianas. Photo: AFP / Michael Runkel
"On the west side, the slope is gentle and flat. A tsunami would reach those locations very quickly," he said.
Steeper cliffs on the east coast provide comparatively more elevation, though the entire island remains exposed.
Both scientists stressed that residents should not wait for official alerts.
"If the shaking is strong enough that you cannot stand, and it lasts for around 20 to 30 seconds, you must evacuate immediately to high ground-15 metres or more-without waiting for instructions," Dr Kwok said.
He also reiterated that Saipan's barrier reef provides no protection from tsunami waves.
While reefs can dissipate energy from storm waves with short periods, a tsunami behaves like "a fast-moving river for 10 to 15 minutes," he said, adding that "the reef directs that flow upward and amplifies the energy".
A narrow band of mangroves near the governor Eloy Inos Peace Park also offers little benefit, he added, as meaningful protection requires extensive coastal coverage.
Guard noted that major misconceptions about reefs and trenches were already disproven in 2009, when a destructive tsunami killed 34 people in American Samoa and 159 in Samoa.
During the presentation, Dr Kwok, who works under the US National Tsunami Hazard Mitigation Program, unveiled updated inundation models for Saipan using high-resolution LIDAR data and new "credible worst-case" earthquake scenarios.
The modelling shows:
- Water could travel well past Beach Road and into Lake Susupe.
- Flow depths of 3-6 metres (10-20 feet) are possible in Garapan and San Antonio.
- Several areas previously shown outside the hazard zone - including the Mariana Beach Resort - would flood under new projections.
He said older NOAA maps underestimated inundation because they relied on coarser elevation data and an arbitrary magnitude 9 scenario. He urged CNMI authorities to update evacuation maps, noting that FEMA's National Risk Index-which influences mitigation funding-depends on accurate exposure information.
The briefing also highlighted two major hazards:
1. Local tsunami (Mariana Trench, M8.3-8.5)
- Arrival time: 10-15 minutes
- No practical warning
- Largest inundation expected on Saipan's west coast
- Could strike CNMI harder than Guam due to local bathymetry and wave amplification
2. Distant tsunami (Nankai Trough, Japan, likely M8.7)
- Travel time: 2.5-3 hours
- First waves small; more damaging waves about an hour later
- Amplified by Saipan's broad western shelf
- Japan assigns a 60-94 percent chance of a full-rupture event within 30 years
Multiple public schools lie within the inundation zone.
Saipan Mayor's Office compliance officer Ray Dela Cruz said moving hundreds of students uphill within 10 minutes during a local event is "not realistic," urging the government to pursue vertical-evacuation plans with nearby hotels and multi-storey buildings.
The FY24 modelling covers Garapan to San Antonio, with FY25 work expanding north to Puerto Rico, into the harbour, and adding maritime hazard maps for vessels.
HSEM special adviser Clement Bermudez said stronger community education, more sirens, and clearer messaging are needed.
"It's not to scare but to accurately inform our people," he said.
"If it's a local tsunami, we've got less than 10 minutes to get out of that pavilion at Minatchom Atdao. If it's a distant one, well, you might have time to finish another case of beer-but you still need to get to high ground."
