Regular earthquake hit the South island of New Zealand late last year, triggered a slow earthquake near the North island.

Move the earth’s crust in the zone of the Alpine fault and related earthquake led to the formation of the Southern Alps. The photo – mount cook, the highest point of New Zealand. (Photo: B. muirhead / Wikimedia.)

Earthquake in the South island (marked by a star) triggered slow earthquake in the area of the North island (marked with a red fill). (Illustration: The University of Texas at Austin Jackson School of Geosciences.)

The boundaries of tectonic plates. Transform green, red, divergent, pink – convergent (blue – subduction zone – a special case). (Image: Eric Gaba.)”

As is known, the earth crust is constantly changing under the pressure of the hot bowels of the earth, and the most active geological processes (including accompanied by earthquakes) are on the boundaries of tectonic plates. Events can evolve differently .

On the one hand, plates can simply slide parallel to each other (such boundaries are called transform), such as the North American and Pacific plates slide along the 800-mile fault San Andreas fault near San Francisco (one of the few examples of transform boundaries on land, in General they are typical of the ocean).

On the other hand, plates can go in different directions (the so-called divergent boundaries). On land in this case appear to be a rift valley (East African), and in ocean – mid-ocean ridges, of which hit the hot hydrothermal vents – the famous “black smokers”, and can form volcanic Islands (archipelagos like Hawaii and the Canary, or single, as Iceland).

Finally, the slab can run into each other, and then talk about convergent boundaries. If the continental crust, it becomes deformed and formed mountain ranges (like the Himalayas), and if the bark, at least one of the plates is oceanic, one plate descends beneath another and there is a chute (like the Mariana trench, where the Pacific plate goes under the Philippine).

Places where one tectonic plate dives under the other, called subduction zones. In these zones occurs 80% of the most powerful earthquakes (with a magnitude of more than six points), and that there can sometimes be mysterious, recently opened phenomenon called “slow earthquake.”

Unlike a normal earthquake, where pieces of the crust move relative to each other in seconds, slow lasts weeks and even months. The movement of the crust and the release of energy occurs so slowly that we did not feel, and to capture what is happening, we need modern sensitive seismographs. However, the changes that occur in earth crust, can match the ordinary earthquakes with great magnitudes on the Richter scale, which usually take hundreds of thousands of lives.

The first slow earthquake was clearly recorded in 2001 in the Cascade mountains near Vancouver (Canada). Since they were observed in Japan, New Zealand, Costa Rica and Mexico. Striking in their regularity: in the Cascade mountains of the slow earthquakes occur on average every 14 months (and lasts about 2 weeks), Mexico – every 4 years (last about 6 months). Slow earthquake, reported by researchers from the University of Texas at Austin, happened again in New Zealand.

In General, New Zealand – the most dangerous region in terms of seismic activity, no wonder she is part of the Pacific volcanic ring of fire. However, the origin of New Zealand, unlike in Iceland, no volcanic. Once these Islands were part of the ancient supercontinent of Gondwana. About 100 million years ago a large piece (Zealand) broke away from Gondwana and moved into the Pacific ocean, where it sank. Later part of the sunken continent rose out of the water on the border of the Australian and Pacific plates, forming New Zealand.

These two tectonic plates, which is now New Zealand, form the North and South of the subduction zone. From the North the Pacific plate dives under the Australian, while from the South the opposite is true – the Australian dives under the Pacific, and in the middle they slide relative to each other, forming the Alpine fault. Plate from time to time get stuck, accumulate tectonic stress, and then start driving. If the movement is sharp, there is an ordinary earthquake, but if plates moved slowly, then slow.

A slow earthquake was recorded this time, occurred near the Northern subduction zone, off the East coast of the North island. It lasted about one to two weeks and covered an area of over 15 thousand sq. km. But most importantly, it began immediately after the second strongest in the history of New Zealand “ordinary” earthquakes (magnitude 7.8 on the Richter scale) that erupted in November 2016 for hundreds of kilometers to the South, in the Northern part of the South island.

This is not the first case when the seismic waves cause perturbations are so remote and at the same time, so extended in space. His discovery made the researchers analyzing seismic data from sensors GeoNet; all the results are published in Nature Geoscience.

If we well understood, the relationship between slow and ordinary earthquakes, we could be much more efficient to predict seismic processes and to avoid tragedies and deaths. For example, a powerful 9-magnitude earthquake in Japan in March 2011, which caused a terrible accident at the NPP “Fukushima” was preceded by a slow earthquake in February of the same year. But until now our understanding of this relationship remains very vague, primarily because of the fact that not enough data has been accumulated. But because the new research here is welcome.