The boom has been heard around the St. Louis area, but why? Earthquake experts weigh | metro

A small earthquake gently shook the St. Louis area on Friday night — an event that caused a stir, thanks in large part to the accompanying surge that residents widely reported across the region.

Some said the knock sounded like an explosion or thunder – only louder. Buzz began almost immediately after 5:30 p.m., when the quake struck, with its epicenter near Valley Park in southern St. Louis County. Online message boards, such as Nextdoor, were quickly overtaken by area users who asked almost the same question.

“What was that? The whole house shook at 5:30,” one user wrote. Several others described hearing deafening noises, and several people wrote that the sound forced them to check if their house had hit something.

Another person wrote: “I heard an explosion and the house was shaking.” “Almost as if it were underground.”

Bingo. But there is clearly more to it than that. The screeching noises persisted, especially as it seemed to defy the earthquake’s paltry 2.8 on the Richter scale – too low to cause damage.

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“It’s not uncommon for people to hear a bang or a bang,” Don Blackman, a geophysicist with the US Geological Survey’s National Earthquake Information Center in Colorado, said Monday.

He said the sounds could come from the fractured layers of rock in the Earth’s crust, after pressure builds up and gives way to movement.

“Not only does it generate waves in the ground, it can also emit a sound wave,” Blackman said. “When that happens and those rocks break, sometimes people hear it.”

He said the sounds were initially difficult for many people to identify.

“The way I heard it most often,” he said, “was, ‘It sounded like someone hit something,’” he said. “Just a bang. …” I thought a garbage truck came into the house, “or something like that.”

Other experts said Friday’s boom was likely more noticeable thanks to the quake’s relatively shallow depth, and proximity to locals in a densely populated area. US Geological Survey Report “Have You Felt It?” Within 24 hours of the earthquake, he said, more than 4,100 people across the region reported feeling it, with most responders from the zip code in Ballwin, near the epicenter.

“It’s the short distance and the fact that the earthquake happened over a very short period of time,” said Robert Hermann, professor emeritus of geophysics at Saint Louis University. “Suppose the waves in the Earth cannot be modified to propagate the signal.”

Hermann said the “slip-slip” earthquake occurred about 5 miles underground, where “pressure builds up in a horizontal direction and the fault moves horizontally.” He said the fault was small – less than the size of a football field – and that it only moved a fraction of an inch or so during Friday’s earthquake.

“Somewhere at this depth of the earth, there is some vulnerability,” Hermann said. “Something just broke.”

He said a similar earthquake occurred in roughly the same location in May 2008.

Experts said that earthquakes of this low type are not capable of causing damage. Generally, earthquakes need to register a magnitude of 4 or higher to push an image off a wall, and 5 or higher to threaten structural damage, according to US building codes, Blackman said. Earthquakes increase exponentially with each increasing number on the scale – which means that a 2.8 St. Louis earthquake had a fraction of the force needed to move household objects.

Any resident worried about the potential positive effects can breathe a sigh of relief. According to Hermann, such a small tremor would have no effect on earthquake insurance rates. The event also had no effect on the New Madrid Seismic Zone, which is centered in the Boothill region of southeastern Missouri — where some of the largest earthquakes recorded on the continent have been triggered in more than 200 years, he said, forcing current state officials to tackle potential contingency planning. Repeat scenarios.

Besides the loud bang, experts said earthquakes could trigger other interesting phenomena. For example, they can sometimes produce “earthquake lights,” thanks to the compression of quartz in the rocks – and they can create a flash of light visible at night.

“Some of the rocks can be squeezed to generate electricity,” Hermann said. “It doesn’t have to be something in common.”

Kim Bell of Post-Dispatch contributed to this report.

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