Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 188

arrived at these shadow-zone seismological stations well
before any surface waves.
In the 1960s the last major interface within Earth to yield
its secret to seismologists was a subtle but profound change
in upper mantle mechanical strength, now known as the
Low Velocity Zone (LVZ). It defines the uncoupling
interface between strong lithospheric plate and weak
asthenospheric mantle; it is thus the fundamental dynamic
interface that enables plate tectonics to operate. The rigid
lithospheric plates (Section 5.2) slide around at velocities
of a few centimeters per year on the lubricating LVZ layer
because it contains a tiny but significant proportion of
molten rock. The LVZ was recognized because the partial
melt slightly slows down (by c .1 percent) the passage of
both P - and S -waves across it (see data in Box 4.2).
We must also mention the discovery, by a combination
of seismology and experimental rock physics, of two
discontinuities in the mantle (see Fig. 2.7) that owe their
origins to mineral phase changes . These are changes to the
arrangement of the atomic lattice (not chemical change),
involving a reorganization involving closer packing due to
the increasing pressure. For a very crude analogy think
about changing cubic to rhombic packing, as defined in
Section 4.11. The first of these phase changes occurs at
about 410 km depth in the mantle, where at c .14 GPa
pressure and temperature at c .1,700K, the common man-
tle Fe-Mg silicate mineral olivine (Sections 1.2 and 5.1)
changes to a more densely packed structure of the chemi-
cally equivalent mineral spinel ; the density increase is
6-7 percent. This causes both P - and S -wave velocities to
increase across the discontinuity. A larger and more impor-

tant density and velocity change occurs at 660 km depth at
c .23 GPa pressure and temperature c .1,900K, when the
spinel structure in turn transforms to the denser perovskite
phase. This discontinuity is taken as the boundary between
the upper and lower mantle. As we shall see in Section 5.2,
the discontinuity was once considered inviolate to the
downward passage of lithospheric slab; nowadays seismology
tells us that slabs may pass clean through to the core-mantle
boundary.

4.17.5 Earthquake seismology

The second major achievement of seismologists after
elucidation of the internal structure of a layered Earth has
been the theory of plate tectonics, though other disci-
plines, notably geomagnetism, contributed vital clues as to
the kinematics and physical processes involved. One seis-
mological clue came from the accurate determination of
the magnitude and geography of earthquakes as shown in
the map of Fig. 4.140 for over 30,000 earthquakes.
Intensity scales for earthquakes have been widely used and
these relate to the visible damage and environmental
effects felt by humans during the earthquake. The
Mercalli scale is one such intensity indicator. Using instru-
mental records the magnitude of any earthquake (Box 4.3)
must reflect the amplitude, A , of the seismic waves pro-
duced by it. Richter originally proposed the logarithmic
scale of earthquake magnitude, M L , that nowadays bears
his name: “on the Richter scale.” It must be one of the
most reported phrases in the human language! The earth-

Isolated events
Dense concentrations

Fig. 4.140 World seismology: Concentration of major earthquake epicenters (over 4.5 magnitude) for 14 years.