Tsunamis and earthquake sequences: Cases of chain reactions

Published: 03/09/2021

Several massive earthquakes and three tsunamis in New Zealand and multiple quake sequences in Europe. How are they all connected?

A spectacle for geologists and seismologists worldwide, a horrific situation for those affected: In its early days, March 2021 delivers firework-like bunch of geological activity. Volcanoes become active, severe earthquakes shake numerous regions and one country experiences three tsunami warnings within just 12 hours. But, what does this activity mean and how is it all connected? An analysis.

What happened?

An intense swarm of more than 20,000 earthquakes which shook the Iceland’s capital Reykjavik has been active in Iceland for more than a week. With quakes up to magnitude 5.7, it is one of the strongest swarms there for decades. The cause of this swarm, as the authorities confirmed on March 1, is a magma intrusion. The first volcanic eruption in 800 years on the Reykjanes Peninsula could be imminent.

On March 3rd, a strong earthquake with magnitude 6.2 severely damaged more than 900 buildings in Central Greece. Numerous homes and churches collapsed near the city of Larissa. Several people were injured. Hundreds of aftershocks followed, but also a second strong earthquake on March 4th with magnitude 5.9, which exacerbated the damage. Recently, at least 400 people had to stay in emergency shelters. Previously, on March 1st, two people died in a quake measuring 5.1 in Colombia, but there was only minor property damage.

Then there was March 4th: Three severe earthquakes, three tsunamis: magnitude 7.2 off the North Island of New Zealand, magnitude 7.5 and 8.1 further north off the Kermadec Islands. The last two quakes were harmless as this region is almost uninhabited. The first earthquake off the North Island left only minor damage . However, the people along the coast of the North Island were alarmed three times that day because of tsunami warnings following each of these quakes. Fortunately, three warnings without any damaging tsunami following. But why were there so many major earthquakes within a few hours?

Chain reaction

On Iceland, an interplay of volcanic and tectonic processes led to the extensive and persistent swarm of earthquakes. The primary trigger was a magma intrusion below the Reykjanes Peninsula that has already begun in the previous year. But, during the latest sequence, the magma rose to higher layers of the earth's crust and is currently stuck only one kilometres below the surface. The subsequent rapid uplifts activated the fault zone, which then initially led to the "mainshock" of magnitude 5.7. At the same time, a creeping shear movement set in along the plate boundary running southwest-northeast, such that, starting from the magma body, the two plates shifted over a length of around 30 kilometres for several millimetres. This activated further earthquake hotspots.

This tectonic shift has already ended. Earthquakes and ground deformations continue as a direct result of the movement of magma, which may reach the surface and cause a volcanic eruption.

The events in Greece can be described as a doublet earthquake in two adjacent sections of a fault zone. It means that the first earthquake, which occurred on a previously unknown fault, immediately triggered the second larger earthquake on a segment further to the north west. This kind of chain reaction is not unusual and often observed, but still impossible to predict. Since the second fault section is located away from densely populated areas, this earthquake was less devastating. However, as both events triggered intense aftershock sequences, several of those smaller events contributed to the losses, including a shallow M5.1 quake which damaged multiple rural homes

Another, much larger chain reaction could have caused the three major earthquakes on March 4th in New Zealand. However, many questions remain open regarding these connections.

Just a few hours after the M7.2 off the coast of the North Island, the M7.5 followed near the Kermadec Islands, almost 1000 kilometres to the north. This is definitely too far for direct triggering, but the seismic waves from the first quake could have played a role here. These were relatively low-frequency, as the rupture process of the quake was complex, which among other observations led to an unusually large shaking area and also a small tsunami. It is possible that these complex seismic waves activated the subduction fault along the Kermadec arc and thus triggered the M7.5 event.

However, there are no doubts that the M7.5 was the cause of the subsequent mainshock. A classic example of a quake (afterwards classified as a foreshock) that triggered a larger one at an adjacent fault section. A similar process as in Greece almost at the same time, but with significantly larger events.

What is striking about the earthquakes along the Hikurangi-Kermadec subduction zone is that they “jumped” a huge gap between Raoul Islands and the North Island in which no very large earthquakes have occurred in recent times. This so-called seismic gap thought to be capable of a possible mega-quake in the range of up to magnitude 9 . But of course, it is not possible to say when this will happen. However, the recent earthquakes may have slightly increased the likelihood of it happening soon.

Three earthquake sequences, three different and independent processes with different levels of complexity, which show us again how different the interplay of individual tectonic processes can be and where there is still a lack of understanding. Scientists around the world, but especially in New Zealand, will chase answers for open questions in the coming months and study this seismically extremely active beginning of the month.

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