Twin earthquakes in Alaska exceed expectations

Last week’s 7.6-magnitude Alaska earthquake raises questions about the nature of aftershocks.

Written by Elizabeth Nadine, Ph.D., Associate Professor, University of Alaska Fairbanks

Quote: Nadine, E., 2020, Twin Alaska Earthquakes Exceeding Expectations, Tumbled, http://doi.org/10.32858/temblor.130

It’s hard to think of a 7.6 magnitude earthquake as an aftershock, but the earthquake that struck southern Alaska on October 19 seems to be exactly that. This came exactly about three months after an earthquake measuring 7.8 on the Richter scale, which had partially “closed” the so-called Schumagen gap, and was large enough to generate a tsunami warning in the area.

The epicenter of the 7.8-magnitude earthquake (orange star) and before was before October. 19 aftershocks (red circles) constitute an approximate ENE-WSW trend. The aftershocks of the 19 October earthquake (yellow circles and star) are falling along an almost vertical direction. Credit: Alaska Earthquake Center

An aftershock is any smaller earthquake that follows a major shock. These are distinguished from the tremors that come before the main event. Both are important for deciphering fault-zone dynamics capable of large, destructive earthquakes. However, aftershocks don’t always follow the rules that scientists put them down. One of these rules, called Båth’s Law, states that the largest aftershock of an earthquake is expected to be one less than the main shock.

“If you follow this rule, [the magnitude-7.6 on Oct. 19] “The aftershock is bigger than anyone expected,” says Natalia Robert, chief scientist at the Alaska Earthquake Center. “But it is still an aftershock.”

Some people may object to this definition, notes Michael West, director of the Alaska Earthquake Center. But he says, “This earthquake probably wouldn’t have happened [Oct. 19] If not for the earthquake that occurred in July. So, we call it aftershock. ”

Uniquely large aftershock

Seismic activity has remained steady in the area since the July 22 Simonov earthquake, with some 2,400 aftershocks above 1.6 on the Richter scale that filled an area 50 miles (80 km) wide between the main tremor and last week’s 7.6 magnitude aftershocks. The largest of them – so far – was 6.1 power just days after Simanov. All of the aftershocks fell within the 60 × 120 miles (100 × 200 km) area that erupted in July. The October 19 event was within the earthquake zone. In addition to the earthquake’s remarkably large strength, its location and sense of motion are also intriguing.

The main shock of July tore apart the plate interface between the Pacific and North America in a push. This recent aftershock was deeper – most likely in the middle of the thick Pacific plate sloping under North America. The “resolution” of the fault movement, called the focal mechanism, indicates that the level of the fault was pointing from north to south, perpendicular to the fault that ruptured in July.

Aftershocks do not match

Given how different the types of earthquakes are – one occurred on a thrust fault and the other had a slip motion – it’s worth asking if there has been any accumulation of the more recent event. “The thinking is that there is some delayed stress that this is causing,” says Robert. In other words, the July earthquake relieved the pressure on the particular fault which ruptured, but this event carried pressure on a different fault, which subsequently ruptured in a different way.

After a major earthquake, the United States Geological Survey (USGS) is building a model of how far the fault has slipped into the event, which could shed light on potential rupture areas in the future. In the case of the July earthquake – a thrust event – the majority of aftershocks were consistent with the thrust fault. The mismatched few did not raise any astonishment. But interestingly, the USGS model of this earthquake did not match the distribution of the aftershocks, says Robert. Last week’s size of 7.6 also does not match the model, which has left some open questions.

A particularly large aftershock is a topic of particular focus because it indicates that, contrary to mainstream thought, a single large earthquake does not necessarily mean that the pressure disappears from the area. This aftershock highlights the importance of understanding the entire network of faults in an area, and how stress can be transmitted from one to the other.

“We don’t know much about [last week’s quake] Now to provide any more specific explanations. We’d like to wait a bit to transfer more aftershocks to find out where they fell, ”says Robert. The October 19 earthquake created a well-defined aftershock region, whose locations will shed more light on the dynamics of what happened in this complex region. Fissures.

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