Coastal Processes, Hazards, and Society

Hawaiian Landslides and Potential Megatsunami

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Hawaiian Landslides and Potential Megatsunami

Possibly the largest tsunamis ever experienced were triggered by massive landslides off the flanks of Hawaiian volcanoes. And these unstable areas will fail again in the future and trigger a massive tsunami that may devastate coastlines around the Pacific Ocean. It’s not a matter of if, it’s a matter of when.

Hawaiian volcanoes are some of the fastest-growing landforms in the world. The Pacific Plate is moving to the northwest over a massive plume of heat known as a “hot spot”. The plate is moving to the northwest at a rate of 10 cm per year and there is a clear age progression of volcanoes from older on Kauai the northwesternmost island to younger on the southeast flank of the Big Island, Hawaii. In fact, the active youngest flanks are those that have the potential to fail in the future, as we will see.

The evidence for landslides is super clear. Images made with sonar show pockmarked areas of seafloor littered with massive blocks of displaced material off the flanks of islands that show evidence for past failure.

The Nu’uanu slide lies off the northeast coast of the island of Oahu and is one of the largest landslides on Earth. The slide is 235 km wide and 35 km long and occurred 1 to 1.5 million years ago when nearly half of the Ko’olau volcano collapsed. One of the largest blocks in the slide is called the Tuscaloosa seamount which is 30 km long by 17 km wide and 2 km high! The remaining part of the caldera shows a steep fault escarpment where the failure occurred. The Wailua slide off the north coast of Molokai lies close to the Nu’uanu slide and occurred 1.4 million years ago when the East Molokai volcano collapsed. The slide is 195 km long and 40 wide. The volume of material generated by these two slides must have caused massive tsunamis, or megatsunami, which models suggest were up to 100 m high on the coasts of the Hawaiian islands! One possible piece of evidence for tsunami is found on the island of Lanai where blocks of coral are found 35 meters above sea level. Such corals could not have grown at these elevations and must have been delivered by tsunami. In all likelihood, tsunami generated by these events hit the west coast of the US and Canada but there is no known record in these locations.

Hawaiian volcanoes grow with lavas spreading out from the crater at rapid rates. GPS data show extremely fast motions of 10cm/year along the southeast flank of the Big Island of Hawaii, the youngest Hawaiian island. All eyes are on this margin where recent collapse formed the Hilina slide and current gradual movement is forming the Hilina slump. This coast is also where future flank collapse looks likely and arcuate cliffs of the Hilina Pali look like possible pre-collapse features.

This rapid motion has caused visible features in the Hawaiian landscape. The so-called Great Crack on the southwest rift zone of Kilauea is 6 miles long, 60 feet wide and 60 feet deep in places and is and highly visible in Google Earth. The crack is in no immediate danger of failing but is a reminder that the rapidly accreting Hawaiian islands pose a unique tsunami hazard in the Pacific Ocean.