Marine Seismology

By Purdy, G. M. | Oceanus, Winter 1992 | Go to article overview

Marine Seismology


Purdy, G. M., Oceanus


Seismology is the study of sound propagation in the earth. Research in continental seismology has been active since the late 1800s. When Croatian seismologist Andrija Mohorovicic published his landmark discovery of a shallow transition to a remarkably uniform layer of high sound (or seismic) velocities in 1909, he had, in fact, discovered the boundary between Earth's crust and upper mantle, which is now known as the Moho. But marine seismology did not become a significant endeavor until the 1930s. It then grew rapidly during World War II, fueled by the submarine-warfare-related need to better understand sound propagation in the oceans. Like almost all ocean sciences, then, seismology is a young and somewhat immature field: It is exciting, unpredictable--and fulfilling to the curious seeker of new truths about Planet Earth.

Understanding seismology requires knowledge of only the most basic laws of physics, but over the past 100 years it has provided fundamental insights into the structure of our planet, including:

* Earth has a liquid outer core surrounding a solid inner core;

* The outermost skin (or crust) upon which we live is about 30 kilometers thick beneath the continents, but only 6 to 8 kilometers thick beneath the deep oceans;

* The distribution of earthquakes around the globe (earthquakes are Earth's greatest sound generator) delineates narrow active zones that form boundaries between the rigid crustal plates (described by the plate tectonic paradigm); and

* The shelves beneath the shallow seas that bound the continents are formed of piles of sediment as much as 10 kilometers thick that have eroded from adjacent land. These are but a few of the important facts about Earth that have been revealed through seismology.

In mapping Earth's interior structure, seismologists identify changes in the sound velocity that can be related to types and physical properties of rocks, and they map major boundaries that are sufficiently abrupt to actually reflect sound energy back to the surface. A minimal physical basis for seismology can be provided by conveying two simple principles: how sound (or seismic) energy actually propagates in Earth, and the simple principles of reflection and refraction of that energy. Seismic energy propagates either through the body of a material as "body" waves, or along boundaries with water or air as "interface" waves. For simplicity, here we will consider only body waves. When a disturbance occurs in a medium (be it an earthquake, an explosion, or simply

hitting a table with your knuckles), energy propagates away in all directions in the form of particle vibrations in the medium. Particles vibrate either along the direction of energy propagation, when they are called compressional or pressure (P) waves, or perpendicular to that direction, when they are called transverse or shear (S) waves. Liquids cannot support shear, so shear waves occur only in solids. However, because it is possible to convert energy from P to S (and vice versa) at a liquid-solid boundary, shear waves remain important in ocean seismology even when man-made sound sources, which are frequently located within the water column, generate only compressional wave energy.

These waves of particle disturbances propagate radially from a source with characteristic wavelengths and frequencies. As they propagate, energy is lost to attenuation: The particle motions actually cause frictional heating (to an extremely small degree) within the medium. Attenuation is also caused by scattering from structural heterogeneities. Long wavelengths (with corresponding low frequencies) lose less energy to frictional heating and less to scattering because the long wavelengths do not "see" or sense small-scale structural changes. Therefore almost all Earth seismology employs very low-frequency sound. Studies in the uppermost 20 to 30 kilometers most commonly use energy in the 2- to 25-hertz band, which, given that the velocity of sound in crustal rocks varies between 2 and 7 kilometers, corresponds to wavelengths of 80 to 3,500 meters (using the simple relationship, velocity = frequency x wavelength). …

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