Earthquakes in Chile and the Aleutian Islands of Alaska historically serve up the most dangerous tsunami threats to the Hawaiian Islands.
And while Wednesday’s quake off Chile had a very large magnitude — 8.3 — the unleashed energy is only one of the factors that determine whether ensuing seismic sea waves are destructive or deadly.
“For an earthquake so far away — for it to cause major problems in Hawaii, it has to be bigger than this,” said Gerard Fryer, a geophysicist with the Pacific Tsunami Warning Center, by telephone.
“It certainly produced a big tsunami along the coast of Chile. This is really the most troublesome size of earthquake for us (forecasters) because it might produce a big tsunami and it might not. It depends on the details of the rupture and whether it was in the deepest part of the trench, where it moves a lot of water. So these are the tricky ones to handle.
“But we think we got it right. We’re fairly confident in our models and we have had some confirming data from the deep-ocean gauges. We think the tsunami is going to max out between 2 and 3 feet high in Hawaii.”
As important as magnitude, scientists say, is the amount of seafloor displaced and in which direction.
Up and down movement, like dropping a rock in a water bucket, is most likely to send energy coursing outward through the sea. (In fact, an asteroid strike in the ocean is recognized as another potential source of tsunamis.)
The most powerful earthquake ever recorded, at magnitude-9.5, happened about 400 miles south of Wednesday’s epicenter. That quake, on May 22, 1960, killed an estimated 1,655 people in southern Chile and 61 in Hawaii, mostly in Hilo, where the run-up height reached 35 feet.
The town of Puerto Saavedra was destroyed by waves of up to 38 feet, which carried houses 2 miles inland, according to the U.S. Geological Survey.
A day later, waves hit northern Honshu in Japan, leaving 185 people dead or missing.
Yet the most deadly tsunami to hit Hawaii, on April 1, 1946, came from a much smaller quake, magnitude-8.1.
That quake, with an epicenter at Unimak Island in the Aleutians, generated a wave of higher than 100 feet that swept away the island’s Scotch Cap lighthouse and its five Coast Guard occupants. In Hilo, the resulting tsunami killed 159 people.
In general, quakes in the central Aleutians direct their energy right at Hawaii, and they are closer, so don’t need huge magnitudes to do damage.
The tectonic forces at work off Chile involve the Nazca Plate, which moves eastward and subducts or passes under the continent, creating the Andes Mountains and associated volcanism. Subduction typically creates just the sort of up-down motion that dangerous Pacific-wide tsunamis require — but not always, as was the case Wednesday. (By contrast, the San Andreas fault in California involves side-to-side tectonics, so quakes there rarely, if ever, generate tsunamis.)
The Nazca Plate ranges from Panama to the southern quarter of Chile, where it meets the Antarctic Plate. It moves about 3 inches a year in the south and 2.6 inches per year in the north and the angles of descent also vary along its length.
Since 1900, according to the U.S. Geological Survey, several magnitude-8 or larger earthquakes have occurred at this subduction zone, followed by destructive tsunamis. They include the 1960 quake; a magnitude-8.5 quake near Esmeraldas, Ecuador, in 1906; another the same size in 1922 near Coquimbo, Chile; a magnitude-8.4 quake in 2001 at Arequipa, Peru; a magnitude-8 quake in 2007 near Pisco, Peru; and a magnitude-8.8 quake near Maule, Chile, just north of the 1960 event.