Laurie Dingler | The story of the global earthquake of 2021 – Times-Standard

The year 2021 was notable for the Western Hemisphere. The biggest earthquake this year occurred in Alaska and the deadliest in Haiti. There were some surprises and, unfortunately, they reaffirmed what we have known for many years – earthquake casualties are caused mainly by poorly built structures.

Large earthquakes have a magnitude of 8 or greater. The world had three in 2021. Its 8 releases a massive amount of energy – in the same order as the annual US energy consumption or a 5-megaton nuclear explosion. The technique of releasing the energy is different, the seismic energy mainly goes to the fault rupture and the heat generated along the fault zone and only a small fraction is released as the seismic waves we feel, but it’s still an impressive number.

62 Great earthquakes have occurred since 1950, less than one annually. 2021 Joins the Elite Club for only three years which totaled over two years. The M8.1 missile hit the Kermadec Islands in northern New Zealand in March, hit 8.2 south of the Alaskan Peninsula in late July, and 8.1 in the south Atlantic in August. These three earthquakes make 2021 the seventh in terms of annual seismic energy production since 1970. The good news is that they were too far from population centers to cause significant damage.

The July 29 M8.2 Chignik earthquake was the largest. It’s also the largest US earthquake since 1965, and the largest anywhere in the world since 2018. Its epicenter was in the Alaskan Aleutian subduction zone of southern Alaska in an area seismologists call the Shumagin Gap, where no major earthquakes have occurred. Several decades before 2020.

Seismic gaps are regions along active plate boundaries with notable absence of recent earthquakes, possibly ripe for major earthquake production. The Shumagin Gap is of particular concern to the west coast of the United States because a major earthquake can focus tsunami energy on us. The Shumagin region began to move in July 2020, when a 7.8-magnitude earthquake occurred near the eastern edge of the gap. Seismologists agreed that it may not have been large enough to fill the gap, and they speculated that earthquakes of similar or larger size were still possible.

In October 2020, the M7.6 earthquake occurred about 50 miles southwest of the first earthquake. It was a slip earthquake and not on the board boundary, so it didn’t solve the gap problem. This year’s M8.2 will likely be on the subduction front. It was felt strongly in the cities and villages of the peninsula and offshore islands. Minor damage was reported at Sand Point and Pereville. It was located in a large part of the state and as far away as Fairbanks, more than 700 miles away. It is still not clear whether the gap is now closed.

8.2 A tsunami warning has been issued for coastal areas from eastern Aleutians to southern Alaska. A modest tsunami (under a foot at Sand Point) was observed and detected as far away as Hawaii and the central California coast. The most interesting observations were inside Humboldt Bay where tsunami waves a few inches high lasted for more than a day. Fishing boats attempting to cross the strip at the entrance to Humboldt Bay noticed unusual cross currents at the time of the tsunami. The lesson is that even a very small tsunami adds to the potential danger at the difficult strip crossing.

An M8.1 rocket on March 4 in the Kermadec Islands also produced a tsunami and a series of tsunami warnings for New Zealand and the southern Pacific region. The lack of instrumentation in the source area made it difficult to assess the tsunami risk for several hours. It likely reached a height of eight feet or more on the uninhabited island of Raúl but elsewhere the elevations were modest.

We tend not to think of the Atlantic Ocean as a seismically active region, but rather with two earthquake hotspots. It was the third big earthquake this year (August 12, M8.1) in the South Sandwich Range as the South American plate descends under the Scotia Plate. The epicenter was far from the shores of Argentina near the uninhabited island of Montagu and no one reported how it felt.

August 8.1 also produced a tsunami, with a peak height of 2.5 feet on the island of South Georgia 470 miles away. Small tsunami signals have been detected in the Azores, Madagascar and Alaska making this the first tsunami observed in four oceans since 2004 after Indonesia’s M9.1. It’s a reminder that all oceans are connected and that tsunamis sometimes occur in the Atlantic.

The second hotspot in the Atlantic Ocean is the Caribbean, and it has resulted in the deadliest earthquake this year. On August 14, a 7.2-magnitude earthquake struck Haiti. Located on Haiti’s Tiburon Peninsula, it created a powerful shaking that affected more than a million people, many of whom lived in buildings not built to resist earthquakes. The death toll reached 2,248, making this earthquake the deadliest in the past three years.

The Caribbean is a complex geological region where the South American plate descends under the Caribbean plate along the arc of the Lesser Antilles. The 7.2 Haiti earthquake was associated with the shifting of the boundary region between the Caribbean Sea and the North American Plate. It was on the same fault that on January 13, 2010, the M7.0 earthquake killed at least 100,000 people. The lower death toll in 2021 is due to the fact that it was 60 miles west of the 2010 earthquake epicenter and away from the densely populated metropolitan area of ​​Port-au-Prince.

The Haiti earthquake caused more than 90% of earthquake deaths in 2021; Without it, this year would have had the fewest casualties since 1971. The year 2021 also illustrates the complexity of the sequences and fault relationships. The Alaskan earthquake is part of a chain that began 18 months ago, and it remains unclear if it is over. The Haiti earthquake was on the same fault system as the 2010 earthquake but was not directly adjacent. It leaves a gap of about 30 miles between the two slits – a spot that is perfectly capable of producing another M7 earthquake.

Laurie Dingler is Professor Emeritus of Geology at Humboldt State University, and an expert on 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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