Alaskan Researchers Discover Innovative Method to Enhance Earthquake Magnitude Assessment

A new study reveals that sensors that detect changes in atmospheric pressure due to the shaking of the ground can also obtain data on large earthquakes and explosions that exceed the upper limit of many seismometers.

The sensors, which detect inaudible ultrasound transmitted through the air, could improve tsunami warnings and other emergency responses while also cutting costs.

Research by scientists from the University of Alaska’s Fairbanks Institute of Geophysics shows that ultrasonic sensors can improve size determination. Initial tsunami warning depends only on the estimated magnitude and location.

Ultrasonic sensors cost less than seismographs, are reliable and are found in large numbers in Alaska for other uses.

Ken MacPherson of the Geophysical Institute’s Wilson Alaska Technical Center said, “What we’ve done is use infrasound for a purpose for which it wasn’t really intended. We’ve found that it works well to provide complete data on strong earthquakes.”

These pressure-sensing ultrasonic instruments are generally used for non-seismic purposes such as detection of mining explosions or nuclear explosions. They also record landslides, volcanoes or meteorites entering the Earth’s atmosphere.

MacPherson details the use of infrasound sensors for seismology in a paper published April 21. Bulletin of the American Seismological Society.

MacPherson is an audiology research and operations scientist. Other participants from the Wilson Alaska Technical Center involved in the research include Director David Fee, data specialist Julian Coffey, and machine learning specialist Alex Wetsell, who now work in the private sector.

Infrasound sensors record changes in air pressure caused by infrasound waves, which are at a frequency lower than what humans can hear.

Ultrasonic sensors can record the full range of ground motion of an earthquake by detecting air pressure changes caused by the up and down movement of the ground during an earthquake.

The upward motion of the Earth compresses the air, which increases the air pressure like a piston does. The downward motion reduces pressure.

Pressure changes from even the largest earthquakes are well below the upper limit of ultrasonic sensors.

In contrast, seismometers, which record the actual motion of the Earth, have an upper limit, which means that sophisticated data can be absent for large earthquakes. They can also miss data for smaller earthquakes if they happen close to the seismometer.

Seismologists call this data loss “slashing.”

“If you turn the stereo up too high, you get terrible sound,” MacPherson said. “It means you’re out of the speaker’s dynamic range. That can happen to a seismograph.”

Seismologists can overcome the snip by deploying powerful motion detectors, which are different from ultrasonic sensors. These motion sensors will not go off during intense shaking but are expensive and not as accurate for smaller earthquakes. There are about 130 people around Alaska, mostly in urban areas and near known faults.

For example, MacPherson and colleagues compared infrasound data from the magnitude 7.1 Anchorage earthquake on November 30, 2018, with data from a seismometer. Both instruments were at the same location 18.6 miles from the epicenter.

“The seismometer’s recording of that earthquake went straight into the instrument’s dynamic range and got stuck,” MacPherson said. “So there is a loss of capacity information.”

The seismometer was one of several seismometers in the south-central Alaska region that lacked high-level data from this earthquake. The data from the ultrasound sensor is not clipped.

To verify the accuracy of the infrasound monitor’s cutting-edge data, MacPherson matched it with data from a powerful motion seismometer at the same location. match.

Ultrasonic sensors can also provide timely data just like seismographs. This is especially important if a tsunami is possible. The National Tsunami Warning Center has only four minutes to issue a warning of the time of an earthquake.

“If all nearby seismometers are out, and the tsunami warning center is trying to get an accurate tsunami warning magnitude, they can quickly calculate magnitudes from a nearby infrasound station that has been co-located with the seismometer,” Coffey said.

Alaska has about 150 ultrasonic sensors along with seismographs across the state.

Some of these were part of EarthScope movable array, a National Science Foundation-funded project to map Earth’s crust and upper mantle. The temporary group gradually moved across the country, reaching Alaska in 2014. Ninety-six of these stations are now part of the Alaska Seismic Center permanent monitoring network.

“We have this unique resource in Alaska, and we’re pushing science to get the most out of it as much as we can,” MacPherson said. “We’re looking forward to using it in new ways.”