Deadly Earthquake Swarm Caused by Ancient Dead Volcano

A deadly earthquake swarm that has been rocking Japan for three years is possibly being caused by an ancient volcano.

An earthquake swarm refers to a sequence of seismic events occurring in a local area within a relatively short period.

A study published in June in the journal JGR Solid Earth analyzed what might be causing an "ongoing intense" earthquake swarm in the crust of Japan's Noto Peninsula.

These earthquakes have been occurring since 2019, and until now, remained a mystery to scientists. This is because quakes like this are often associated with volcanic activity—however there has been none in this area since the Middle Miocene period, which was around 15 million years ago, the study reported.

Yet, the earthquake swarms exhibit very similar patterns to those near volcanoes. This suggests that the earthquakes are actually being caused by magma being released by an ancient long-dead volcano.

"These findings were surprising in multiple ways," author of the study Keisuke Yoshida, told Newsweek. "We were first surprised by the clarity of the migrations of seismicity from deep to shallow through a complex fault network, which we believe represents the movement of fluids originating from the subducting plate."

There have been over a thousand earthquakes since this earthquake swarm began in 2019. In June 2022, there was a magnitude 5.4 quake, and in May this year, a magnitude 6.5 quake killed one person and injured dozens more.

The study notes that "significant crustal deformation" has been observed during the earthquake swarm.

Yoshida and co-workers studied three years' worth of seismic waves emanating from the swarm of over 10,000 earthquakes of magnitude 1 or bigger. They found that they all come from the crust, and traveled upwards through an existing, "complex network of faults" shallower than 12.4 miles.

They then found that the swarm revolved in a circular pattern "similar to the ring fault that forms above the magma reservoir," the study reported.

"The present results suggest that hidden magma-induced structures and fluids can generate earthquakes even in areas where no volcanic activity has been observed for over 10 million years," the study said.

"It was also a great surprise to find that the ring-shaped earthquake distribution at a depth of 18 km (11 miles), which is the starting point of the current swarm activity, is very similar to that often found in shallow areas near caldera volcanoes," Yoshida told Newsweek. "It was also surprising that a magnitude 6.5 earthquake occurred in the shallow extension of a fault of the swarm activity (just prior to the acceptance of the paper in May 2023). As a whole, seismicity that began deep in the crust reached near the surface in this aftershock activity."

Volcanoes go extinct when they are cut off from their lava supply and collapsed systems like this can occur when magma is forcefully expelled from a volcano during an eruption. This can cause the volcano to collapse, when the magma chamber empties.

"I believe that the example we have obtained of fluid movement from deep to shallow, progressing while causing earthquake swarms, is not limited to this case, but is occurring at various times in various places," Yoshida said.

"Fluids not only reduce fault strength and cause earthquakes to occur but also cause a seismic slip and affect the fault zone in various ways. I think it is necessary to understand such a whole picture during a fluid migration at depth," he said.

"Furthermore, a damaging earthquake of magnitude 6.5 occurred in the shallow extension of this swarm. We believe that more research is needed to obtain information on what was happening prior to the occurrence of such a major earthquake," Yoshida said.

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