Equipment designed to detect tsunamis and earthquakes also detects background sounds including cars and the hum of the earth. Credit: J. McMillan
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 Panama 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.
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.
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 STRI.
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
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.
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, also
known as “seiches,” 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.
is not the first report of seiches in Lake Gatun. Earlier reports
correlated the release of methane gasses in the sediments below the
canal to seiches and bottom currents in the lake. The Panama Canal
Authority provided data about the depth of the Canal channel and of Lake
Gatun that the authors used to model wave patterns in the lake.
traffic and wind speed correlate with the unusual wave pattern, which
was more common during the day than it was at night, but more
information is needed to confirm what is actually causing the waves.
report, published in the Journal of Geophysical Research, provides a
new method to quantify the impact of water movements as recorded by
land-based seismometers. A more exact understanding of the seismic
signals resulting from water movements will improve estimates of other
phenomena like tsunami impacts.