204 Encyclopedia of the Solar System
FIGURE 17 Three classical types of earthquakes (top) and the
distribution of the signs of the P-wave arrivals: (a) strike slip, (b)
thrust or reverse fault and (c) normal fault. The beachballs
represent the equal area projection of the signs of first motion of
the P-waves. The motion would be positive within the shaded
areas. The lines separating shaded areas with the unfilled ones
are called “nodal planes.”
auxiliary plane, indicated by a dashed line; a ground mo-
tion generated by a slip on the auxiliary plane (right lateral)
cannot be distinguished from that on the principal plane.
The bottom part of Figure 17a is a stereographic projection
of the sign of P-wave motion observed on the lower hemi-
sphere of the focal sphere (a mathematical abstraction in
which we encapsulate the point source in a small uniform
sphere). The plus sign corresponds to compressive arrivals
and minus sign to dilatational zones; quadrants with com-
pressive arrivals are shaded.
The top part of Figure 17b is a section in the vertical
plane. In this case, the block on the right moves upward on
a plane that dips at a 45◦angle with respect to the block
on the left; this mechanism is called thrust and is associ-
ated with compression in the horizontal plane and tension
in the vertical plane and corresponds to the convergence of
the material on both sides of the fault. Such processes are
responsible for mountain building. The shaded central re-
gion in the bottom part of Figure 17b, with the dilatational
arrivals on the sides, is characteristic of the thrust—reverse
faulting—events. Figure 17c illustrates the opposite mech-
anism, in which tension is horizontal and compression ver-
tical; this is called normal faulting and is associated with
extension, which can lead to the development of troughs or
basins.
The “beach-ball” diagrams are commonly used as a
graphic code to represent the tectonic forces. Some earth-
quakes are a combination of two different types of motion,
e.g., thrust and strike slip; in this case the point at which
the two planes intersect would be moved away from either
the rim or the center of the beach-ball diagram.
The size of the earthquake is measured by magnitude.
There are several different magnitude scales depending on
the type of a wave whose amplitude is being measured. In
general, magnitude is a linear function of the logarithm of
the amplitude; thus a unit magnitude increase corresponds
to a 10-fold increase in amplitude. Most commonly used
magnitudes are the body-wave magnitude, mb, and surface
wave magnitude, MS. The frequency of occurrence of earth-
quakes, i.e., a number of earthquakes per unit time (year)
above a certain magnitude M, satisfies the Gutenberg–
Richter law: log 10 N=a·M+b.The value of a is close
to−1, which means that there are, on average, 10 times
more earthquakes above magnitude 5 than above magni-
tude 6. A new magnitude, MW, based on the estimates of
the released seismic moment [shear modulus×fault area×
offset (slip) on the fault] is becoming increasingly popular;
it is more informative for very large earthquakes, for which
MSmay become saturated.
Figure 18 is a map of the principal tectonic plates, de-
fined in plate tectonic theory. The direction of the arrows
shows the relative motion of the plates; their length corre-
sponds to the rate of motion. At a plate boundary where
the blue arrows converge, we expect compression and,
therefore, thrust faulting; one of the plates is subducted:FIGURE 18 Principal tectonic plates and relative
plate motion rates. Red arrows signify spreading,
blue arrows–convergence, and green
arrows–strike-slip motion.