The Pacific Plate is slowly sliding under the North American Plate, heading north and at a rate of 6.3 cm per year in the area recently affected by the earthquake. Called subduction, this process is capable of raising mountains or giving birth to volcanoes such as those in the Aleutian Islands. However, the subduction of our two tectonic plates does not go smoothly: as they progress, tension builds up along the fault and once the breaking point is reached, the earth suddenly tips and causes an earthquake.
This phenomenon is at the origin of the earthquake that occurred on the night of July 28 in Alaska and scientists believe that the fracture appeared in the subsurface at the junction between the Pacific and North American plates.
Given this tectonic clash, the earthquakes are not surprising in Alaska. On average, a tremor shakes the detectors of the Alaskan Earthquake Center every 15 minutes, or several tens of thousands of earthquakes per year.
It is also in Alaska that the last earthquake of higher intensity in the United States occurred, at the level of the Rat Islands in 1965 with a magnitude of 8.7. A year earlier, the region was hit by another major earthquake, this time of magnitude 9.2, the second most violent earthquake on Earth.
SEISMIC CURIOSITIES
The latest event to date is particularly intriguing in the eyes of scientists, because it occurred a few tens of kilometers from the two major earthquakes that shook the region in 2020: the first of magnitude 7.8, on July 22, and the second of magnitude 7.6, on October 19.
Although the difference between the numbers may seem small, it reflects a considerable difference in energy released. As Bohon explains to us using a gastronomic metaphor, if a spaghetti is the energy released by a magnitude 5 earthquake, then 900 spaghetti corresponds to magnitude 7 and 25,000 to magnitude 8.
The location of these three major events is unlikely to be a coincidence: if an earthquake is able to release voltage in a given area, it can transfer it to neighbors as well, increasing the risk of future events. jerks.
“Each earthquake increases the likelihood of another happening,” sums up Bohon. Wednesday’s earthquake occurred within 40 km of another 8.2 magnitude earthquake in the region in November 1938. Scientists have not yet completed their analysis, but it is possible that areas where appeared these two subsurface earthquakes overlap.
The focus of both of these earthquakes is near a fascinating area known as the Shumagin Gap, or Shumagin Islands Gap. A seismic gap is an area along the subduction zone that has not experienced an earthquake for a relatively long period of time. In our case, it has been almost a century since Shumagin’s Gap has shown itself to be rather peaceful. The July 2020 earthquake shattered part of that loophole, much to the surprise of scientists who believed the plates were sliding quietly against each other in this region, without building up enough voltage to trigger a major earthquake.
The July 28 earthquake does not appear to have reached the peaceful zone of the gap. However, he could tell us more about the underground forces at work. There are still many unknowns about the Shumagin Gap and what makes it so different from other areas of the Alaskan coastline.
“Is it because of the way the tension builds up?” Or to the properties of the rocks of the lacuna? Asks Bohon.
“The more earthquakes we have, the better we can study the behavior of the plates over time,” she adds.
TSUNAMI ALERT
The release of energy accumulated by tectonic plates has the potential to unleash another devastating force of nature: tsunamis. As the Pacific plate plunges beneath the Alaskan coast, the overlying North American plate compresses. In an earthquake, the release of the tension causes the top plate to suddenly relax like a spring, pushing the ocean nearby like the mechanism of a wave pool.
Immediately after the July 28 earthquake, alarm systems sent tsunami warnings to the Hawaiian Islands and an abnormally high amount of waves began to hit the nearby coastline, with no sign of a tsunami.
This is explained by the depth of the earthquake, the origin of which was more than 30 km underground. The most important movements along a plate occur at the level of the hypocenter, the underground starting point of the seismic rupture. If an earthquake strikes near the surface, the movements of the ocean are intense. On the other hand, if the hypocenter is deep, these movements lose in intensity as the earthquake approaches the surface.
At this stage, the risk of a tsunami has been ruled out and even if the region will still register some aftershocks, they will be weaker. The risk of a magnitude 7 or greater earthquake occurring in this part of Alaska in the next few weeks is less than 4%, according to data from the United States Institute of Geological Studies.
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