Velocity Structure Of The Earth

Knowledge of the internal layering of the Earth has been largely derived using the techniques of earthquake seismology. The shallower layers have been studied using local arrays of recorders, while the deeper layers have been investigated using global networks to detect seismic signals that have traversed the interior of the Earth.

The continental crust was discovered by Andrija MohoroviCiC from studies of the seismic waves generated by the Croatia earthquake of 1909 (Fig. 2.14). Within a range of about 200 km from the epicenter, the first seismic arrivals were P waves that traveled directly from the focus to the recorders with a velocity of 5.6 km s-1. This seismic phase was termed Pg. At greater ranges, however, P waves with the much higher velocity of 7.9 km s-1 became the first arrivals, termed the Pn phase. These data were interpreted by the standard techniques of refraction seismology, with Pn representing seismic waves that had been critically refracted at a velocity discontinuity at a depth of some 54 km. This discontinuity was subsequently named the Mohorovicic discontinuity, or Moho, and it marks the boundary between the crust and mantle. Subsequent work has demonstrated that the Moho is universally present beneath continents and marks an abrupt increase in seismic velocity to about 8 km s-1. Its geometry and reflective character are highly diverse and may include one or more sub-horizontal or dipping reflectors (Cook, 2002). Continental crust is, on average, some 40 km thick, but thins to less than 20 km beneath some tec-tonically active rifts (e.g. Sections 7.3, 7.8.1) and thickens to up to 80 km beneath young orogenic belts (e.g. Sections 10.2.4, 10.4.5) (Christensen & Mooney, 1995; Mooney et al., 1998).

A discontinuity within the continental crust was discovered by Conrad in 1925, using similar methods. As well as the phases Pg and Pn he noted the presence of an additional phase P* (Fig. 2.15) which he interpreted as the critically refracted arrival from an interface where the velocity increased from about 5.6 to 6.3 km s-1. This interface was subsequently named the Conrad discontinuity. Conrad's model was readily adopted by early petrologists who believed that two layers were necessarily present in the continental crust. The upper layer, rich in silicon and aluminum, was called the SIAL and was believed to be the source of granitic magmas, while the lower, silicon- and magnesium-rich layer or SIMA was believed to be the source of basaltic magmas. It is now known, however, that the upper crust has a composition more mafic than granite (Section 2.4.1), and that the majority of basaltic magmas originate in the mantle. Consequently, the petrological necessity of a two-layered crust no

Figure 2.14 Reduced time-distance relationship for direct waves (PJ and waves critically refracted at the Moho (PJ from an earthquake source.

Figure 2.14 Reduced time-distance relationship for direct waves (PJ and waves critically refracted at the Moho (PJ from an earthquake source.

Figure 2.15 Reduced time-distance relationship for direct waves (PJ, waves critically refracted at the Conrad discontinuity (P*) and waves critically refracted at the Moho (Pn) from an earthquake source.

longer exists and, where applicable, it is preferable to use the terms upper and lower crust. Unlike the Moho, the Conrad discontinuity is not always present within the continental crust, although the seismic velocity generally increases with depth.

In some regions the velocity structure of continental crust suggests a natural division into three layers. The velocity range of the middle crustal layer generally is taken to be 6.4-6.7 km s-1. The typical velocity range of the lower crust, where a middle crust is present, is 6.8-7.7 km s-1 (Mooney et al., 1998). Examples of the velocity structure of continental crust in a tectonically active rift, a rifted margin, and a young orogenic belt are shown in Figs 7.5, 7.32a, and 10.7, respectively.

The oceanic crust has principally been studied by explosion seismology. The Moho is always present and the thickness of much of the oceanic crust is remarkably constant at about 7 km irrespective of the depth of water above it. The internal layering of oceanic crust and its constancy over very wide areas will be discussed later (Section 2.4.4).

In studying the deeper layering of the Earth, seismic waves with much longer travel paths are employed. The velocity structure has been built up by recording the travel times of body waves over the full range of possible epicentral angles. By assuming that the Earth is radially symmetrical, it is possible to invert the travel time data to provide a model of the velocity structure. A modern determination of the velocity-depth curve (Kennett et al., 1995) for both P and S waves is shown in Fig. 2.16.

Velocities increase abruptly at the Moho in both continental and oceanic environments. A low velocity zone (LVZ) is present between about 100 and 300 km depth, although the depth to the upper boundary is very variable (Section 2.12). The LVZ appears to be universally present for S waves, but may be absent in certain regions for P waves, especially beneath ancient shield areas. Between 410 and 660 km velocity increases rapidly in a stepwise fashion within the mantle transition zone that separates the upper mantle from the lower mantle. Each velocity increment probably corresponds to a mineral phase change to a denser form at depth (Section 2.8.5). Both P and S velocities increase progressively in the lower mantle.

The Gutenberg discontinuity marks the core-mantle boundary at a depth of 2891 km, at which the velocity of P waves decreases abruptly. S waves are not transmitted through the outer core, which is consequently

2000

O 4000

6000

Lower mantle

Lower mantle

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O 4000

6000

Inner core

Figure 2.16 Seismic wave velocities as a function of depth in the Earth showing the major discontinuities. AK 135 Earth model specified by Kennett et al., 1995 (after Helffrich & Wood, 2001, with permission from Nature 412,501-7. Copyright © 2001 Macmillan Publishers Ltd.).

Inner core

Figure 2.16 Seismic wave velocities as a function of depth in the Earth showing the major discontinuities. AK 135 Earth model specified by Kennett et al., 1995 (after Helffrich & Wood, 2001, with permission from Nature 412,501-7. Copyright © 2001 Macmillan Publishers Ltd.).

believed to be in a fluid state. The geomagnetic field (Section 3.6.4) is believed to originate by the circulation of a good electrical conductor in this region. At a depth of 5150 km the P velocity increases abruptly and S waves are once again transmitted. This inner core is thus believed to be solid as a result of the enormous confining pressure. There appears to be no transition zone between inner and outer core, as was originally believed.

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  • hannele
    What is p wave speed in konrad discontinuity?
    2 years ago

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