Supersensitive ear in Panamá hears cars, earth’s hum

This equipment designed to detect tsunamis and earthquakes also detects background sounds including cars and the hum of the earth. Photo: Smithsonian Tropical Research Institute/J. McMillan

An unusual signal detected by the seismic monitoring station at the Smithsonian Tropical Research Institute’s research facility on Barro Colorado Island results from waves in Lake Gatun, the reservoir that forms the Panamá Canal channel, scientists report.

Understanding seismic background signals leads to improved earthquake and tsunami detection in the Caribbean region where 100 tsunamis have been reported in the past 500 years, they said.

As part of a $37.5 million U.S. presidential initiative to improve earthquake monitoring following the devastating tsunami in the Indian Ocean in 2004, a seismic sensor was installed on Barro Colorado Island in 2006.

The sensor is one of more than 150 sensors that comprise the U.S. Geological Survey’s Global Seismographic Network.

Barro Colorado Island is a hilltop that was isolated by the waters of the reservoir created when the Chagres River was dammed to form Lake Gatun, a critical part of the Panama Canal. The Barro Colorado seismic monitoring station is a collaboration between the U.S. Geological Survey, the U.S. National Oceanic and Atmospheric Administration, the University of Panama and the Smithsonian Tropical Research Institute.

Ultra-sensitive devices at the station pick up a large range of ground motion from felt earthquakes to nanometer-scale seismic background noise. The instruments at the station include very sensitive broadband seismometers used to detect distant earthquakes and low-gain accelerometers that measure ground movement and withstand violent local earthquakes and explosions.

The sensors detect signals from many different sources that include cars, boats and machinery operating up to several kilometers away. They also pick up the background hum of the Earth caused by ocean waves breaking on continental shelves around the world.

Scientists noticed that sensors on Barro Colorado recorded an intriguing wave pattern at an intermediate frequency. They suspected that this pattern could be caused by standing waves in Lake Gatun. Standing waves are common in enclosed bodies of water like lakes and harbors where waves moving in opposite directions interact. By installing a water-level detection meter along the shoreline, researchers confirmed that changes in the water level of the lake correspond to the unusual seismic signal.

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