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 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. A seiche is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing the formation of the standing wave.
Seiches are often imperceptible to the naked eye, and observers in boats on the surface may not notice that a seiche is occurring due to the extremely long wavelengths. The effect is caused by resonances in a body of water that has been disturbed by one or more of a number of factors, most often meteorological effects (wind and atmospheric pressure variations), seismic activity or by tsunamis.
As part of a $37.5 million U.S.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 part of the Panama Canal.
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 have 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 (seiches) in Lake Gatun. 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.
This 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.
Boat 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.
This 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.
Earthquake-generated seiches can be observed thousands of miles away from the epicenter of a quake. Swimming pools are especially prone to seiches caused by earthquakes, as the ground tremors often match the resonant frequencies of small bodies of water. The 1994 Northridge earthquake in California caused swimming pools to overflow across southern California. The massive Good Friday Earthquake that hit Alaska in 1964 caused seiches in swimming pools as far away as Puerto Rico. The 2004 Indian Ocean earthquake caused seiches in standing water bodies in many Indian states as well as in Bangladesh, Nepal and northern Thailand.
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