MIPT has determined the conditions for the occurrence of the strongest earthquakes

(ORDO NEWS) — The MIPT Laboratory for Geophysical Research of the Arctic and the Continental Margins of the World Ocean conducted an analysis of the seismogenic process of the Aleutian Island Arc, which runs from Kamchatka to Alaska.

It was found that it is the direction of the approach vector of the lithospheric plates, on the border of which the islands are located, that has a significant effect on these processes. The study will help predict future earthquakes, their strength and the most dangerous areas.

According to statistics, the average recurrence periods of the strongest earthquakes are about 100–1000 years at moving plate boundaries and 1000–10000 years in stable interior regions.

At the same time, studies confirm that the seismic process in each particular zone is fairly stationary in time. However, in practice, due to the action of various factors, noticeable deviations are possible, which was confirmed by MIPT researchers.

An example of such unevenness is the seismogenic activation of the Aleutian zone in the middle of the 20th century.

In the period from 1938 to 1965, a whole series of strong earthquakes occurred in the Aleutian island arc, the sources of which filled almost the entire frontal part. Of particular interest is the western half of the arc, since the strongest earthquakes of magnitude 8 occur here more often than in its eastern part.

MIPT has determined the conditions for the occurrence of the strongest earthquakes 1

The Aleutian Island Arc is located between the Bering Sea in the north and the North Pacific Basin in the south and has about 300 islands.

In geological terms, this territory is a zone of the so-called subduction at the boundary of lithospheric plates, along which the Pacific lithospheric plate is submerged , moving to the northeast, under the North American lithospheric plate. The process goes with a significant level of seismic activity.

Head of the Laboratory for Geophysical Research of the Arctic and Continental Margins of the World Ocean, MIPT, has developed a model that describes the cyclicity of earthquakes due to the movement of blocks in the subduction zone the so called key block model:

“The process of interaction between oceanic and continental lithospheric plates is never interrupted, the blocks accumulate stress, which is released during the strongest seismic events.

At the same time, unloading blocks are almost instantly displaced towards the ocean. However, during the fast seismic stage, only a partial release of accumulated stresses occurs.

MIPT has determined the conditions for the occurrence of the strongest earthquakes 2

The release of the remaining part of the elastic energy stored by the blocks occurs in the process of the final “straightening” of the system. The result of this process is the further movement of blocks into the ocean. This stage can last for several years, and its completion means the end of the past and the beginning of a new seismic cycle.”

The Aleutian arc stretches for 1740 kilometers, the islands are located on the North American plate, under which the Pacific plate is submerged, and the frontal part of the islands is a series of blocks separated by faults perpendicular to the axis of the island arc.

In the section, these lithospheric blocks (the so-called “keys”) are wedge -shaped and go deep to the top of the subducting plate.

The subducting plate exerts pressure on the blocks, leading to their compression and accumulation of elastic stresses. At some point, these stresses reach threshold values, which manifests itself in the form of strong earthquakes.

The most important feature of the Aleutian arc is that in its western part the direction of subduction occurs tangentially to the direction of the arc.

This leads not only to direct, but also to lateral compression (lateral pressure), which, in turn, gives a higher level of internal stresses and an earlier discharge of accumulated energy at the same time. As a result, the strongest earthquakes can occur here more often than in the zone where subduction occurs perpendicular to the direction of the arc.

Thus, as a mechanism explaining the increase in the frequency of earthquakes in the zone of the Aleutian island arc, it is proposed to consider a change in the direction of subduction from a trench almost perpendicular to the axis of the trough in the eastern part to a tangential trajectory in the west.

Another striking feature of seismic cycles in the western part of the Aleutian zone is the absence of recurring events in the same segment.

Within the framework of the key-block approach, the boundaries of the source zones do not have to be exactly the same. Thus, the focus of a new event may include neighboring blocks that did not move during the previous one.

At the same time, some blocks that were displaced during the previous seismic cycle may remain unaffected due to the fact that the stresses accumulated in them are insufficient for dynamic separation, which somewhat complicates the prediction of seismic activity in such regions.

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