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Marine seismology.

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). Using such large wavelengths results in a view of Earth's interior that lacks detail. We use these frequencies not by choice, but because the physics of energy propagation in a solid requires them. The high-frequency energy that could contribute to a much more detailed picture of crust and upper-mantle structure is lost to attenuation: Nature guards its secrets well.

As energy travels through the crust, it is reflected from boundaries and refracted by velocity changes. Reflection is discussed at length by Detrick and Mutter in their description of multichannel seismics beginning on page 54. In its simplest form, refraction can be explained in terms of the same Snell's law that is most commonly applied to optics. As sound velocity increases with depth (as it most commonly does), energy is continuously refracted until it is turned back to the surface and recorded. Precise navigation and accurate timing are of paramount importance to the marine seismologist, because to determine sound velocities we need to know both the separation and the travel time between sources and receivers as exactly as possible.

One of the greatest challenges of marine seismology is developing instrumentation for acquiring the necessary data. Sound sources range from simple packages of explosives (weighing from a few pounds to a ton or more) to complex arrays of air guns. Earthquakes are tremendously powerful sound sources. They are rich in shear-wave energy and occur within the crust--but, of course, have the disadvantage of being unpredictable in time, and to a lesser extent, in space. Recording systems can be hydrophone arrays towed astern of research vessels, or instruments with internal recording devices that remain on the seafloor for extended periods of time. One of the latter is shown on page 63. It is one of 30 available to the US academic community and one of 15 operated by the Woods Hole Oceanographic Institution (WHOI) Ocean Bottom Seismometer Facility. This $85,000 instrument is designed to remain on the ocean floor for as long as two months to record the output from three seismometers and a hydrophone on an optical disk. The seismometers record ground motion in one vertical and two horizontal directions, and the hydrophone records pressure waves in the water column. An accurate, low-power clock keeps time to an accuracy of 10 to 20 milliseconds over a two-month period, and two acoustic releases (only one of which needs to operate) assure reliable recoveries.

Imaging the Earth

ln seismology, the fundamental unit of data is the seismogram. It is a time series, typically a few tens of seconds in length, that shows (in the case of hydrophone data) changes in pressure with time. To be useful, the various packets of energy that can be seen on the seismogram must be identified as refractions, reflections, P-waves or S-waves, etc., and there are many criteria to aid the seismologist in making these determinations. A typical modern experiment will involve several thousand of these seismograms. One of many interpretational approaches can then be taken to convert them into descriptive information about the earth. This information generally takes one of two forms: an image showing the location of reflecting boundaries, or a representation (frequently a contour map) of changes in velocity with depth.

The most basic information contained in a seismogram is the absolute arrival times of various packets of energy at a receiver. Combining this with the knowledge of the instant that the sound source was triggered provides the energy's travel time along its ray path from the source to the receiver. Travel times for many different paths, from a spatially distributed set of shots to a network of receivers, can provide a three-dimensional determination of the location and nature of velocity anomalies within the crust and upper mantle. Doug Toomey has described a particularly successful example of this travel-time tomography in revealing structural details of the magma chamber beneath the East Pacific Rise (see Oceanus, Winter 1991/92, page 92).

Another approach is to generate a direct image of the location and geometry of the primary boundaries within the crust using the seismograms themselves. Although this is a straightforward task when dealing with near-vertical reflections, recent work indicates that using high-angle reflections from interfaces can provide information not visible any other way. However, when using such energy it is important to be able to reconstruct the position of the reflections to their real location within Earth. This process, called "migration," when applied by Steve Holbrook (WHOI) to wide-angle reflection data collected from the US East Coast using ocean bottom hydrophone receivers and large air gun sources, has produced the first direct image of the thinning of Earth's crust across the transition from continent to ocean. Like most first measurements, this result is crude and imperfect, but it is an exciting example of what is possible. The limitation on image quality is not physics or geology, but the density of crustal sampling by various ray paths from the source to the receiver. If more receivers were available, the detail of this image could be improved substantially.

Listening to the Earth Move

There is more to marine seismology than mapping boundaries and velocity anomalies in the crust and upper mantle. The richest data source for active processes is microearthquakes. Every day there are thousands of microearthquakes beneath the ocean floor, and each one, if properly recorded, can tell us about the nature, depth, and location of the cracking and faulting that occurs in response to plate-motion stresses. Unlike the velocity information described above, which must be interpreted before it is useful, earthquake data provides direct observations of Earth's processes and responses. Ambiguities associated with the interpretation of velocity anomalies, that is, conversion of sound-velocity information into geological inference, are profound. Microearthquakes, however, do not lie.

One powerful approach is to combine inferences from velocity-anomaly data with observations of faulting and active tectonics provided by microearthquakes. One of the few areas where sufficient data is available to attempt this is illustrated on page 69. This cross section of the Mid-Atlantic Ridge (MAR) near latitude 26 |degrees~ N is from WHOI/MIT Joint Program student Laura Kong's Ph.D. thesis. It shows the relationship between microearthquake locations beneath the MAR median valley and a volume of lower-than-normal seismic velocities. Because increasing temperature is known to lower velocities in common oceanic rocks, one reasonable interpretation of this figure is that this low-velocity volume represents a zone of anomalously high temperatures associated with the magma-emplacement process at the ridge. The microearthquake locations provide important supporting data for this--very few earthquake foci are present within the high-temperature zone because, we infer, the hot rock is too weak to support the stresses required to generate an earthquake. Surrounding this volume, however, where hydrothermal circulation has cooled the crust to a brittle state, extensional faults penetrate to depths of 5 to 6 kilometers. Above the high-temperature volume, shallow events have contrasting mechanisms. They are associated with the magma-injection process itself, not with extension. By combining these two powerful approaches, we are able to build a thorough and complete picture of the processes occurring at considerable depths below the seafloor.

lt has been known for more than 15 years that many mid-ocean ridges are underlain by low-velocity zones. Velocities almost always increase with increasing depth below the seafloor, but in these instances, at depths between 2 to 5 kilometers, the velocity abruptly decreases with depth. On fast-spreading ridges like the East Pacific Rise, good evidence exists to prove that this phenomenon is associated with the presence of magma beneath the ridge crest. On slower spreading ridges, like the Mid-Atlantic Ridge, no steady-state magma body is thought to be present, so the velocity anomaly is interpreted to be a zone of substantially elevated temperatures (as described in the previous paragraph).

There appears to be a simple relationship between spreading rate and the depth to which hot rock or magma extends. On fast-spreading ridges where the magma heat supply is high, the velocity anomalies are shallow. On slow-spreading ridges, where the magma supply per unit of time must be substantially slower, the velocity (presumed to be temperature) anomaly is much deeper. This intriguing result, established from compilation of seismic data from many different mid-ocean ridge systems around the world, becomes even more significant when combined with the observation that the maximum depth of faulting beneath ridges (as revealed by large, globally recorded earthquakes) also increases with decreasing spreading rate. The seismic velocity results can now be interpreted in terms of the geological and physical processes that are occurring--and this is the important goal. The hypothesis is that a balance between heat supply (magma injection) and cooling rate (hydrothermal circulation) is being observed. If we accept that faults are important conduits for circulating cooling seawater deep into the crust, then in the figure opposite we see that on slow ridges, where the faults penetrate the deepest, the temperature anomaly is also at its deepest because heat is transported away by water circulating in the deep faults. On fast-spreading ridges the hot material can extend into the shallowest crust, because there are no deeply penetrating faults to transport the cooling water. This becomes a self-sustaining system, because cold, brittle rock is needed for faults to exist at all. On fast-spreading ridges that are kept hot by the high magma supply, faulting to significant depth cannot be initiated because hot, weak rock cannot support the necessary stresses.

Marine seismology has accomplished much in the last 20 years. We are beginning to understand the primary structural elements of the oceans. We know the basic architecture of oceanic lithosphere, and the primary structural characteristics of mid-ocean ridges, fracture zones, and continental margins. However, we know too little about their three-dimensional structure to adequately constrain process-oriented models for their formation and evolution, and our knowledge of structures within the upper mantle, where so much of our understanding of plate interactions must lie, is minimal. Sound arguments can be made that mapping Earth's deep structure is best carried out from the oceans. Large-scale experiments are easier to plan and perform when national borders, roads, and towns can be ignored. And the thin, young, homogeneous oceanic lithosphere can be likened to a clear pane of glass compared with the frosty cracked window that is the old, heterogeneous continental crust.

Innumerable fundamental questions concerning our planet's deep structure remain to be answered. We have the ideas, the knowledge, and the energy to answer these questions: We lack only opportunity. In striving to create these opportunities, we are building new national research programs that include the Ocean Seismic Network, which is dedicated to the emplacement of permanent broad-band seismographic stations on the ocean floor, and the Ridge Interdisciplinary Global Experiments Program (see Oceanus, Winter 1991/92), which has as a primary element mapping of the seismic structure of the upper mantle beneath a fast-spreading mid-ocean ridge.

Seismology in the oceans is a young endeavor that moves quickly and unpredictably. We can be sure that our view of Earth will be greatly changed a decade from now, and this new vision will have been shaped in large part by the contributions of marine seismology.

Mike Purdy used to be a Yorkshireman, who traveled to Woods Hole in 1974 while on his honeymoon in search of a peaceful life in a small coastal town. The hectic pace of Cambridge (where he gained his Ph.D. in marine geophysics) and London (BS in physics and MS in geophysics) had convinced him that in order to thrive he needed a quiet habitat without the pressures of these large chaotic centers of learning. He has never fully appreciated the magnitude of this fundamental error in judgment, from which he is slowly recovering only because of the help of four extraordinary individuals: Alix, Christopher, Pippa, and Catriona.
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Author:Purdy, G.M.
Date:Dec 22, 1992
Previous Article:New seismic images of the oceanic crust.
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