Magazine article Oceanus

Seismic Network Management Structure

Magazine article Oceanus

Seismic Network Management Structure

Article excerpt

Coordinating and maintaining the network of continuously operating, high quality digital seismometers and distributing the acquired data to investigators worldwide is a monumental task overseen by the Federation of Digital Seismic Networks (FDSN), an international committee. In the United States the development, deployment, and support of modern seismic instrumentation for research is overseen by Incorporated Research Institutions for Seismology (the IRIS consortium). IRIS has three major programs: the Global Seismic Network (GSN), the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), and the Data Management System (DMS). The Ocean Seismic Network (OSN) is an extension of the Global Seismic Network to the seafloor. Another international body, International Ocean Networks (ION), coordinates programs in various countries to establish permanent seafloor observatories. -Ralph Stephen

There are three major challenges in establishing a seafloor seismic observatory: installing the sensor to obtain comparable quality to land or island stations, providing power to the instrument, and retrieving the data.

A 'modern' seismic installation will faithfully measure accelerations of the solid earth in three dimensions (vertical, north-south, and east-west) down to the quietest earth motions over a broad band of frequencies. In the absence of earthquakes and such human activity as traffic, shipping, and explosion tests, the earth's surface vibrates in response to wind, ocean currents, and ocean waves. While the human ear responds to vibrations in air spanning frequencies from 20 hertz to 20 kilohertz, seismometers used to study whole earth structure respond to lower frequencies, from 0.002 hertz to 10 hertz. At 0.002 hertz the seismometer will rise and fall once in about 8 minutes.

Useful signals for tomographic studies of the earth have frequencies around 1 hertz. To get a feeling for the sensitivity of a modern seismometer, consider an earthquake and a receiving station that are 20deg apart (about 2,200 kilometers or the distance from Woods Hole to New Orleans). A very large earthquake, with a magnitude of about 8, will cause displacements at the seismometer of about 1 centimeter A small earthquake, with a magnitude of about 4, will cause displacements of about 10 millimicrons. (A millimicron is a billionth of a meter.) The seismometer will also be responding faithfully to true earth ambient noise with frequencies around 1 hertz and displacements less than 1 millimicron. These correspond to accelerations of about one-tenth of a billionth of the acceleration due to gravity. Directly above a large, shallow earthquake, the surface of the earth will be displaced over a meter, but a high quality seismometer can measure displacements as small as a few millimicrons. The ability to faithfully measure small variations occurring at the same time as large variations is called dynamic range. A modern digital seismometer uses 24 bits to represent a number and has a "nominal" dynamic range of 144 decibels. If the full dynamic range were available, this would correspond to displacements ranging from a millimicron to about a centimeter. It is rare for a seismic station to be directly over a large earthquake. Special "strong motion" sensors are used for the very large displacements expected in earthquake-prone areas such as Japan and California.

A seismometer in Montana can measure ground vibrations that are excited by storms over the North Pacific and North Atlantic Oceans. Wave interaction during the storm at sea creates low frequency "sound" that travels through the solid earth to stations deep within the continents.

Experience on land with Global Seismic Network stations indicates that the best results are obtained by placing the seismic sensor in a borehole at about 100 meters depth. Although expensive, this has two advantages. First, a sensor set firmly in bedrock moves faithfully with the true motion of the solid earth. …

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