`Infrasound’ array of instruments picks up meteor’s kiloton blast over the

A unique array of listening devices deployed by researchers at Scripps
Institution of Oceanography at the University of California, San Diego, is
one of the first stations in an important new global network that will
detect signals from events as diverse as secret nuclear weapons tests,
volcanic eruptions, and hurricanes in early formation.

One of the first significant signals received by the Scripps instruments
resulted from the April 23 explosion of a large meteor crashing into Earth’s
atmosphere. The meteor, reportedly 8- to 12-feet across, exploded with a
yield of a few thousand tons of TNT, nearly the force of the atomic bomb
dropped on Hiroshima.

The Scripps array consists of eight microbarometers spread across two
kilometers at the Cecil and Ida Green Pinon Flat Observatory, located in the
mountains south of Palm Springs, Calif. Each device is equipped with a noise
reduction system that filters unwanted energy from atmospheric turbulence
and increases sensitivity to signals at the “infrasonic” scale that fall
below the 20 hertz level of human hearing. The array records signals that
are too faint, and vary too slowly, to be detected by humans.

The array is one of the first in a planned network of 60 that will play a
vital role in efforts to monitor the globe for clandestine nuclear testing
blasts. The infrasonic network tracks the atmosphere as part of a network
that combines infrasonic signal tracking with seismic stations that pick up
signals from the solid earth, hydroacoustic stations that monitor energy in
the oceans, and a radionuclide network that checks the air for radioactive

“Infrasound energy tracking was big business in the 50s and 60s, when there
was a lot of nuclear testing in the atmosphere,” said Michael Hedlin,
associate researcher at the Cecil H. and Ida M. Green Institute of
Geophysics and Planetary Physics at Scripps, and, with Jon Berger, a lead
scientist in the Pinon Flat infrasound array development. “Interest in
infrasound decreased when nuclear testing moved underground. Now infrasound
monitoring has re-emerged in importance due to the number of countries that
may be capable of developing nuclear weapons. We need to monitor around the

Hedlin says an infrasonic network is capable of providing data not only from
nuclear blasts, but from a variety of natural phenomena that may become
useful in scientific research.

This was the case on April 23, when the large meteor crashed into the
atmosphere over the Pacific Ocean several hundred miles west of Baja

“If this rock had come into the atmosphere at a slightly different time, it
might have exploded not over the Pacific, but over a large metropolitan
area,” said Hedlin. “With this global listening network we can develop much
better statistics on large meteors and get a better idea of how often these
massive objects enter the atmosphere.”

Large explosions send part of their acoustic energy into the audible range,
but those signals dissipate rapidly. They also emit large amounts of energy
into the infrasonic range in signals that decay slowly across vast
distances. Thus the April 23 explosion was prominently featured 1,800
kilometers away on the Pinon Flat instruments. The signals were also
recorded approximately 11,000 kilometers away by an infrasound array in

In addition to meteors, infrasonic energy is generated by chemical
explosions, supersonic aircraft, tornadoes, landslides, earthquakes, and

“Our colleagues in Japan have learned that minor volcanic eruptions of magma
or gas might be missed seismically but produce strong acoustic signals,”
said Hedlin. “Seismic and infrasound data taken together give a much fuller
account of activity inside the volcano that might be indicative of an
impending, significant eruption.”

A new infrasonic array is set to be deployed in Cape Verde, a location off
the coast of Africa known as a nursery for brewing hurricanes. As the
hurricanes develop and emit infrasonic signals, Hedlin believes such data
might contribute to early detection.

“There is a lot going on in the atmosphere that we need to know more about.
The infrasound network will offer us an unprecedented opportunity to better
understand these phenomena on a global scale. We anticipate that this global
network of listening posts that monitors Earth’s fluid exterior shell where
we live will some day become as indispensable as the global seismic network
that monitors the Earth’s solid interior for seismic activity.”

Although the Scripps group provided the closest observations of the meteor,
the event was analyzed by a consortium of universities and laboratories. The
explosion was first noticed by a group at Los Alamos National Laboratory.
Early characterization of the event was done by the Los Alamos group, the
Center for Monitoring Research, the University of Hawaii, and the University
of Alaska.

The consortium is led by Henry Bass at the University of Mississippi.
Construction of the array was supported by the Defense Threat Reduction
Agency (DTRA), the Provisional Technical Secretariat (PTS) of the UN
Comprehensive Test Ban Treaty Office in Vienna, and the US Army Space and
Missile Defense Command (SMDC) University Research Initiative (URI).
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