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Ears Underwater: The Detection of Seafloor Earthquakes and Eruptions
From: Woods Hole Oceanographic Institution
| By:
Edwin Schiele |
EDITOR'S INTRODUCTION |
Hydrophones, originally employed by the United States Navy to detect enemy submarines, have given scientists ears on the ocean floor. Marine seismologists use these underwater microphones to detect and record T waves, seismic waves produced only by underwater earthquakes. With the data supplied by the hydrophones, Columbia University associate research scientist Maya Tolstoy and her colleagues are able to monitor earthquakes on the Mid-Atlantic Ridge and locate recent underwater volcanic eruptions in the Atlantic and Pacific Oceans. Their work has the potential to help scientists understand the geologic processes shaping the mid-ocean ridges and the impacts that earthquakes and eruptions have on the biological communities inhabiting the seafloor. |
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| Tolstoy on the deck of the R/V Melville in the Eastern Pacific. | |
 f a volcano erupts on the seafloor and nobody is around, does the eruption make a sound? For oceanographers, this is more than a philosophical problem. Earthquakes and eruptions take place all of the time along the vast mid-ocean ridge system. However, the chances that an oceanographer can be at the right place at the right time to witness an eruption, or even find evidence of a recent eruption, are remote. Now it is possible to hear seafloor eruptions without being there. Marine seismologist Maya Tolstoy and her colleagues are using underwater microphones called hydrophones to listen for earthquakes and eruptions and then pinpoint their locations. |
All earthquakes produce a series of waves that travel through the earth. Seismology is the study of earthquakes, and scientists call the waves caused by earthquakes seismic waves. Primary (P) waves are the fastest waves and move through solids and liquids. The slower, secondary (S) waves travel only through solid materials. Seismologists traditionally locate and study earthquakes by measuring the magnitude and timing of these two types of waves. Tolstoy and her colleagues, however, are taking advantage of third type of wave produced only by underwater earthquakes. These tertiary (T) waves can travel thousands of kilometers within a layer of the upper ocean called the SOFAR channel. Hydrophones deployed in the SOFAR channel can record even small earthquakes throughout the entire ocean. |
Studying waterborne seismic waves
In the early 1940s, Daniel Linehan reported a class of seismic waves that were observed only on coastal and island seismic stations. These are T-waves. T-waves are seismically generated acoustic waves that propagate over great distances in the ocean sound channel. The "T" refers to tertiary, since these water-borne seismic waves travel slower than solid-earth P (primary) and S (secondary) waves, and thus arrive third on seismograph records. |
The origin of T waves was first explained
 by Ivan Tolstoy (Maya's father) and Maurice Ewing in 1950. They recognized that the phase arrival times recorded on land seismometers were consistent with ocean sound velocities and occurred only with events that permitted ocean propagation paths. Seismic events on or below the seafloor generate energy that is transmitted into the ocean water column. Some of this energy gets trapped within the SOFAR channel and can be recorded by hydrophones suspended in the channel. |
The SOFAR channel is a zone of low acoustic velocity. The speed of sound depends on the temperature of the water,
 its salinity and the pressure (which is equivalent to depth below the sea surface). The speed of sound ranges between 1,400 and 1,570 m/sec (4,593 and 5,151 ft/sec). This is roughly 1.5 km/sec (just under 1 mile/sec), or about four times faster than sound travels through air. There is a sound-speed minimum in the depth range of between 700 m and 1,200 m below the sea surface. Sound waves that enter this region (SOFAR channel) bounce back and forth between the top and the bottom of the channel. Since the attenuation of seawater is very low, energy can travel very long distances, eventually coupling back into solid rock at the coastlines. |
Hydrophones as earthquake monitors
The United States Navy originally deployed hydrophones to detect enemy submarines. After the Cold War, the navy gave scientists access to their hydrophones. In addition, scientists from the National Oceanic and Atmospheric Administration (NOAA) set out additional arrays of hydrophones in the eastern Pacific and the North Atlantic. About once a year, scientists and technicians journey out to these hydrophones to download the data and change the batteries. |
Hydrophones are now used to record T waves from earthquakes generated throughout the ocean basins. In order to understand the nature of earthquakes at the Juan de Fuca spreading center, a highly active region in the Pacific ocean, off the shore of North America, NOAA's Pacific Marine Environmental Laboratory began monitoring the US Navy's SOSUS (Sound Surveillance System) hydrophones in the north Pacific on August 29, 1991. On June 26, 1993, recurrent, low-level (mb = 1.8-3.5) seismicity was recorded on the real-time system and was subsequently located on a small segment of the Juan de Fuca ridge, the CoAxial Segment. NOAA, Canadian and academic research vessels investigated the site of this first event. This effort was remarkably successful, and the nature and consequences of an episode of the spreading of molten rock on the seafloor was thoroughly studied for the first time. |
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| Tolstoy and colleagues test seismic monitoring equipment off the San Diego coast. | |
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In February, 1996, another magmatic event was detected along the northernmost segment of the Gorda Ridge, leading to another coordinated and successful investigative effort. Because the magnitude of the events was so small, none of them were detected by land-based seismometers in the Pacific Northwest. The ability to monitor seafloor spreading events using hydroacoustic methods is really enhancing our understanding of how mid-ocean ridges work. |
Tolstoy and other scientists are using these hydrophones to monitor earthquakes on the Mid-Atlantic Ridge and locate recent underwater volcanic eruptions in the Atlantic and Pacific Oceans. By zeroing in on the most seismically active areas, geologists will be able to better understand the processes shaping the mid-ocean ridges. This work also promises to help biologists who want to witness the impact of the eruptions on the biological communities on the ocean floor. For example, biologists will be able to study how organisms recolonize recent eruption sites. |
 Tracking down volcanic eruptions, however, requires more than just listening for seismic activity. One of Tolstoy's greatest challenges is figuring out whether the sounds that hydrophones have picked up are due to tectonic events (earthquakes caused by movement of plates) or magmatic events (volcanic eruptions). The interpretation is not always easy. One pattern Tolstoy looks for is whether or not the source of the earthquake changes over time. A tectonic earthquake and its aftershocks usually remain centered in one area, but earthquakes caused by an eruption often migrate due to the movement of the magma beneath the seafloor. |
In the spring of 2000, Tolstoy helped lead a cruise to the East Pacific Rise to examine several sites where, based on the hydrophone data, they thought eruptions had taken place. Although what they found was not always straightforward, they did see evidence of recent eruptions at a couple of sites that matched the timing of the signals that the hydrophones recorded. These findings will help Tolstoy and other seismologists refine the technique of using hydrophones to study seafloor eruptions and earthquakes. |
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