Officials hoisted tsunami warnings in New Zealand, and cautioned that the Kermadec Islands would experience a dangerous three- to 10-foot tsunami. But the wave heights that actually occurred were fortunately far less damaging. There was also no Pacific-wide tsunami disaster of the sort that one might expect from a trio of such strong quakes.
In the end, the rise in water levels didn’t top a meter in most spots, and scenes of massive tidal waves engulfing coastal communities didn’t materialize.
“I think it was about what one would expect,” said Lucy Jones, a California-based seismologist and research associate at the Cal Tech Seismological Laboratory. “If you look at any [large] tsunamis, those usually come from magnitude 9s.”
Thursday’s biggest quake was a magnitude 8.1; each increase of one on the Moment Magnitude Scale signifies a tenfold jump in shaking. The quake occurred along a subduction zone known as the Kermadec Trench. A subduction zone marks the boundary at which one tectonic plate converges with and slips below another; in this case, it’s the Pacific Plate crunching up beneath the Indo-Australian Plate at the rate of up to 10 centimeters per year.
During thrust quakes, pent-up stress is released by a slip that can shake the sea floor vertically.
“The water gets lifted by the fault,” Jones said. “But with the travel time, geometric spreading, to have enough water involved to do damage on the other side of the ocean, all the big ones are magnitude 9s.”
She explained that, with lesser quakes, “you can get local tsunamis,” which still have a destructive potential given their “really fast currents” and the momentum stored in swiftly moving water.
Another reason the quake didn’t generate a more potent tsunami? The location of the greatest slip.
The quake’s hypocenter, the specific point in the Earth’s crust where a slip is first observed, was shallow — only about 13 miles below ground. At first, that, coupled with the thrusting mechanism of the slip, looked like the perfect recipe for a tsunami.
But it turns out the maximum slip, or greatest movement of earth, was much deeper below ground; that mitigated the effects on the sea floor, and cut back on the water displaced and subsequent amplitude of the tsunami.
“The hypocenter was shallow,” said Stephen Hicks, a seismologist at the Imperial College of London. “Think about it like tearing a piece of paper. The hypocenter … that’s [at the edge] where you start to tear it. But where you tear it most strongly was deeper, further away from the sea floor, [where you get the biggest] sudden offset.”
The initial uncertainty of not knowing at what depth the biggest slip occurred prompted officials to issue precautionary warnings urging action, as significant impacts would have resulted if the biggest slip — of more than 33 feet — was close to the surface.
If that slip happened closer to the sea floor, the impacts could have been much more severe.
“Going back, there was one in [a] similar location in 1947,” Hicks said. “There were two magnitude 7 earthquakes, so a similar size, but quite a damaging tsunami in the area.”
He said wave heights topped 30 feet — an order of magnitude greater than what transpired Thursday (Friday local time).
And another ingredient? The speed at which the stress is released. Jones differentiated between ordinary and “slow” fault ruptures.
“In 1946, there was an earthquake in Alaska. … It was a 7.1, and created tsunamis for a bunch of people in Hawaii,” Jones said. That’s highly atypical of a quake of that magnitude.
“Seismologists later learned it was a slow earthquake,” she explained, comparing the rupture of a fault to snapping one’s fingers. “The faster you do it, [the] sharper the sound.”
That means that, in a slow earthquake, a large displacement of the water column can occur — while generating a massive tsunami — even with comparatively tepid shaking. That meant the 1946 quake was able to yield a bigger tsunami while not racking up an overly impressive magnitude.
But how slow is a “slow” quake?
“Probably the average rupture velocity of an earthquake is 3.5 kilometers per second,” Hicks said. “A slow earthquake … 1.5 to 2 kilometers per second.”
Hicks noted that the 1947 quake that impacted New Zealand with a devastating tsunami was also a slow earthquake, accounting for why it had such a big impact despite a magnitude in the sevens.
It generally takes up to 30 minutes for data to become available regarding the speed of the slip and where, in comparison to the hypocenter, the greatest slip occurs. By then, warnings are usually issued, and it’s more practical to backpedal or cancel watches or warnings than issue them too late. That comes at the price of false alarms.
In other words, seismologists won’t know if it’s a fast or slow earthquake, and the depth at which the greatest slip occurred, until after warnings have been issued.
Jones also referred to a possible disconnect between the U.S. Pacific Tsunami Warning Centers, which are located in separate government agencies. Tsunami warnings are routinely issued for quakes of magnitude 7 or greater, even though sevens seldom produce far-reaching, damaging tsunamis.
The warning centers are part of the National Oceanic and Atmospheric Administration, under the Commerce Department, while the USGS is in the Interior Department. While the agencies work together, there can also be differences in how they approach such hazards.
“We once had a tsunami warning for magnitude 7.2 where within 15 minutes, seismologists knew it was a strike-slip earthquake,” Jones recalled. Strike-slip earthquakes occur when one piece of crust sideswipes another, and is generally unable to produce a tsunami.
“We were trying to call NOAA, and they weren’t allowed to call off [the] warning until they had a record on a buoy showing there was no tsunami,” Jones said. “So you had people at nursing facilities pushing patients up a hill. You get consequences with those wrong warnings, but NOAA suffers consequences for missing [an event].”
Jones knows it’s a Catch-22, though.
“The warning system is required to worry about the outliers,” Jones said. “But if we reach out to disaster communication specialists … [they emphasize] the need to worry about the outliers muddies the message of what’s likely going to happen.”