The conversation: after three major earthquakes off New Zealand, questions remain about how they might be linked

ANALYSIS:

A sequence of three major earthquakes offshore, including a magnitude 8.1 earthquake near the Kermadec Islands, triggered tsunami alerts and evacuations along New Zealand’s east coast this morning.

In the early afternoon, the National Emergency Management Agency (NEMA) lifted the evacuation order, but emphasized that people should stay away from beaches and shoreline.

All three earthquakes occurred along the Tonga Kermadec subduction zone, where the Pacific tectonic plate dips and then sinks below the Australian plate.

This subduction zone is the longest and deepest system on Earth. It extends from the north of the Eastern Cape, about 2,600 km northeast in an almost straight line to southern Samoa.

One of the questions seismologists around the world are trying to answer now is whether the three earthquakes were related and previous ruptures triggered the magnitude 8.1 earthquake.

Possible links between breakups

The Tonga Kermadec Subduction Zone ends northeast of the Eastern Cape, where it later becomes the Hikurangi Subduction Zone. The first 7.3 magnitude rupture occurred at 2.27 am, 174 km off the east coast, where the Hikurangi and Tonga Kermadec systems merge.

The US Geological Survey recorded this event at a depth of 21 km, not at a depth of 95 km as early reports in New Zealand suggested. This earthquake had an unusual mechanism: an element of lateral movement known as landslide.

The other two earthquakes occurred about 900 km to the north, but just west of the Tonga-Kermadec trench and at depths of about 56 km (for the 6.40 am and 7.4 magnitude event) and 20 km (for the 8.1 magnitude earthquake). at 8.28 am). These latter events had push or compression mechanisms, in which one body of rock is compressed against another, sliding up and over it during the earthquake.

This is what we might expect in a subduction zone where one tectonic plate slides under another and creates a collision, which in turn results in compression.

When the Pacific plate begins to slide under the Australian plate, it starts at a shallow angle and then turns along a curved path to finally drop at a very steep angle (60 degrees). But when it’s at a shallow level, it only sinks, say, 10-20 degrees and creates a lot of friction with the overlapping (Australian) plate. This is typically where these big earthquakes occur.

Activation sequence

Magnitude 8 earthquakes in these subduction zone environments are not unusual. Indeed, earthquakes up to magnitude 9 can occur, such as the 2011 Tōhoku earthquake in Japan, the underwater earthquake off Sumatra that triggered the 2004 Indian Ocean tsunami, and the 1964 Alaska earthquakes and 1960 Chile earthquakes.

The funny thing about this offshore sequence from New Zealand is whether and how the breaks are related to each other. Certainly, the first of the two subsequent earthquakes, located tens of kilometers from each other, can be considered as an anticipated earthquake, followed by the main one of magnitude 8.1. But was the first earthquake at 2.27am north of the East Cape related?

In general, seismologists consider that a distance of 1000 km is too much for even a 7.4 magnitude rupture to disturb the ground enough to trigger another. But there are increasing arguments that the Earth is critically stressed in setting the plate boundaries to such a level that just a small push can trigger another event.

After the 2004 Sumatra earthquake, scientists made a good argument that it triggered more earthquakes an hour later, some 7,000 miles away in Alaska. But in this case, they were smaller events after a major triggering earthquake.

It is also interesting that large earthquakes have occurred in the Kermadec Islands in the past. In 1976, a magnitude 7.7 event was followed 51 minutes later by a magnitude 8 event. This reflects what we saw today.

Both events in 1976 were thought to be thrust earthquakes like today’s. Then, in 1986, at a depth of 45 km, a magnitude 7.7 event showed a push-and-slide movement. The interpretation of this event was that it was not a plate interface event, but had occurred within the subducted and bent Pacific plate.

This could explain the second earthquake this morning, since its depth of 56 km seems to place it within the Pacific plate. We will have to wait until the depths and final mechanics are worked out.

This article has been republished from The Conversation under a Creative Commons license. Read the original article.