The map showing the Aleutians with respect to Hawaiʻi. The red and yellow arcs indicate the sections of the Aleutian subduction zones considered in the probability analysis. Stars and dates indicate epicenters of prior 20th century great earthquakes (Mw > 8). (credit: Butler et al., 2016)

A team of researchers from the University of Hawaiʻi at Mānoa published a study this week that estimated the probability of a magnitude 9+ earthquake in the Aleutian Islands—an event with sufficient power to create a mega-tsunami especially threatening to Hawaiʻi. In the next 50 years, they report, there is a 9 percent chance of such an event. An earlier State of Hawaiʻi report (PDF) (Table 6.12) has estimated the damage from such an event would be nearly $40 billion, with more than 300,000 people affected.

Earth’s crust is composed of numerous rocky plates. An earthquake occurs when two sections of crust suddenly slip past one another. The surface where they slip is called the fault, and the system of faults comprises a subduction zone. Hawaiʻi is especially vulnerable to a tsunami created by an earthquake in the subduction zone of the Aleutian Islands.

Back to basics

“Necessity is the mother of invention,” said Rhett Butler, lead author and geophysicist at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). “Having no recorded history of mega tsunamis in Hawaiʻi, and given the tsunami threat to Hawaiʻi, we devised a model for magnitude 9 earthquake rates following upon the insightful work of David Burbidge and others.”

Butler and co-authors Neil Frazer (SOEST) and William Templeton (now at Portland State University) created a numerical model based only upon the basics of plate tectonics: fault length and plate convergence rate, handling uncertainties in the data with Bayesian techniques.

Using the past to inform the future

To validate this model, the researchers utilized recorded histories and seismic/tsunami evidence related to the 5 largest earthquakes (greater than magnitude 9) since 1900 (Tohoku, 2011; Sumatra-Andaman, 2004; Alaska, 1964; Chile, 1960 and Kamchatka, 1952).

“These five events represent half of the seismic energy that has been released globally since 1900,” said Butler. “The events differed in details, but all of them generated great tsunamis that caused enormous destruction.”

To further refine the probability estimates, they took into account past (prior to recorded history) tsunamis—evidence of which is preserved in geological layers in coastal sediments, volcanic tephras, and archeological sites.

“We were surprised and pleased to see how well the model actually fit the paleotsunami data,” said Butler.

Mitigating the risk

Using the probability of occurrence, the researchers were able to annualize the risk. They report the chance of a magnitude 9 earthquake in the greater Aleutians is 9 percent ± 3 percent in the next 50 years. Hence the risk is 9 percent of $40 billion, or $3.6 billion. Annualized, this risk is about $72 million per year. Considering a worst-case location for Hawaiʻi limited to the Eastern Aleutian Islands, the chances are about 3.5 percent in the next 50 years, or about $30 million annualized risk. In making decisions regarding mitigation against this $30-$72 million risk, the state can now prioritize this hazard with other threats and needs.

The team is now considering ways to extend the analysis to smaller earthquakes, magnitude 7–8, around the Pacific.

—By Marcie Grabowski

The only well-documented paleotsunami deposit in Hawaiʻi from the 16th century is on Kauaʻi. The Makauwahi sinkhole, on the side of a hardened sand dune, is viewed toward the southeast from an apparent altitude of 342 m. Inset photos show two of the wall edges, indicating the edges of the sinkhole. The east wall, left, is 7.2 m above mean sea level and about 100 m from the ocean. Note for scale the people in the right image. (photo credits: R. Butler, left, Gerard Fryer, right, GoogleMaps, background and figure from Butler et al., 2014)