Laurie Dingler | Hidden earthquakes, sudden tsunamis in the Atlantic – The Times Standard

Last July and August were busy with earthquakes. An earthquake measuring 8.2 on the Richter scale occurred in Alaska on July 28, followed by a series of strong earthquakes in the South Atlantic Ocean two weeks later. The Atlantic earthquake deserved a brief mention in my column, but was dismantled by the more damaging M7.2 in Haiti two days later.

A research paper published in Geophysical Research Letters last week suggests that I should have cared more. “The 2021 South Sandwich Mw 8.2 earthquake: a slow event trapped between regular faults,” by Caltech seismologists take a closer look at the sequence and discoveries that, for some earthquakes, standard methods for identifying potential tsunami risks may not be enough.

The two American Tsunami Warning Centers are linked to a network of seismic stations from around the world. When an earthquake that may be large enough to constitute a tsunami hazard occurs, the centers begin to function. For earthquakes centered in North America, Hawaii, or the Caribbean, they are usually able to estimate location and magnitude and issue an assessment within five minutes. For earthquakes in other parts of the globe, it takes about twice as long.

On August 12, 2021 at 11:32 AM, a major earthquake occurred in the South Sandwich Islands region of the South Atlantic. Tsunami Warning Centers detected the earthquake and issued a tsunami statement – a 7.5 earthquake occurred, posing no tsunami danger because it was relatively deep (about 40 miles) and far from populated areas. Tsunami data is the most popular product issued by tsunami centers. It means an earthquake has occurred but is unlikely to produce a measurable tsunami.

I duly noted the 7.5 earthquake in my daily earthquake recording (707-826-6020) and that a small tsunami was detected at the Philippines tide gauges. The next day, the USGS revised its assessment. It was actually a double earthquake and a 7.5 followed by an 8.1 about three minutes later. By this time, any tsunami threat was long gone.

The February 8 paper and its publication of the tsunami heights recorded from this event points to a weakness in our current tsunami detection system and warrants a more comprehensive discussion. The Philippines was not the only place where a tsunami was recorded. Two days ago, the Pacific Tsunami Warning Center released a tsunami summary. It has been registered in more than 100 sites around the world. It was the first true global tsunami ever recorded in three oceans (Atlantic, Pacific, and Indian) since the Great Indian Ocean Tsunami in December 2004.

The largest water surge (4.2 feet from peak to bottom) was recorded in the British territory south of St. George’s Island, about 500 miles from the epicenter. The tide gauge is located in a sheltered inlet and is a poor representative of the size of a tsunami. A better sense of scale is scoring from the coast of South Africa 2,700 miles away where the elevation was just over 4 feet. The site was more than 2,700 miles from the epicenter. And yes, it was recorded in Crescent City with a height of 1.3 feet from peak to trough, the largest value recorded anywhere in the Pacific Ocean. Crescent City was about 9,000 miles from the epicenter.

The tsunami did no harm, but the workers at the Tsunami Warning Centers will be the first to tell you that it should have been handled better. If the earthquake had been given a magnitude of 8.1 initially, a threatening message would have been sent to countries in the South Atlantic and Indian Oceans, and they would have discussed whether to issue a warning to our coast. Why did she miss it?

Earthquakes can be sneaky, and like the December 20 earthquake, they’re hard to handle quickly when they’re close together. Time is of the essence for tsunami warning centers, and they don’t have to spend days or weeks analyzing complex seismographs before making a call about issuing an alert. But how could an 8-magnitude earthquake be missed?

Caltech seismologists describe 8.1 as a slow or silent earthquake. It did not produce the amplitudes of seismic waves that we normally see with typical large earthquakes. Most earthquakes are caused by cracks in rapidly growing rocks. The rupture begins at a point on a subsurface fault and continues to grow at a rate of a few miles per second, just below the speed of sound in the rock. This rapid propagation of the fault is like a sharp kick to the ground that produces a set of seismic waves that we can use to estimate the magnitude.

But not all earthquakes work this way. Some ruptures occur more slowly and produce much weaker seismic signals in the frequency range we usually look for. Often these earthquakes are much shallow, where the strength of the rocks is weaker, and it is easier to get a large fault with much less tremor. But just because the seismic signals are smaller, it doesn’t mean that there is no potential for a tsunami. These slow rips may be more effective at producing large tsunamis than conventional earthquakes of the same size.

We’ve known about the so-called tsunami for decades. In September 1992, the M7.7 earthquake occurred off the coast of Nicaragua. The earthquake was so weak that not many people could feel it. 116 people died in the ensuing tsunami. 7.8 in October 2010 in the Indonesian Mentawai Islands had a similar effect. The shaking was much weaker than the other recent earthquakes, but it caused a larger tsunami and killed more than 400.

The South Sandwich Islands earthquake assessment was more difficult because it was buried in the M7.5 earthquake signal less than three minutes ago. To make matters more complicated, a more natural tectonic rupture followed less than two minutes later. The largest earthquake occurred between two events with typical seismic signals.

This wasn’t much of a problem for the very remote South Sandwich Island region, but the August 12 earthquake and tsunami raised concerns about areas much closer to populated areas. Fortunately, there are other ways to assess the likelihood of a tsunami, but they require more tools in all areas where large earthquakes can occur.

Watch an animation of scientists at the NOAA Center for Tsunami Research at https://www.youtube.com/watch?v=PPylDTGuM0Y.

Laurie Dingler is Cal Poly Humboldt Professor Emeritus of Geology and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of Preparedness magazine “Living on Shaken Earth”? Leave a message at 707-826-6019 or email [email protected].

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