Page 107 - Global Tectonics

94 CHAPTER 5

events where the ridges are offset by transform faults pole. The pole and its antipole are the two unique
(Section 4.2.1). On land, shallow focus tensional events points on the surface of the Earth that do not move
are also associated with rifts, including the Basin and relative to either of the two plates. An important aspect
Range Province of the western USA (Section 7.3), the of relative plate motion is that the pole of any two

East African Rift system (Section 7.2), and the Baikal plates tends to remain fixed relative to them for long
Rift system. periods of time. Plate velocities are similarly constant
All intermediate and deep events are associated with for periods of several million years (Wilson, 1993).
destructive plate margins. The northern, eastern and There are three methods by which the pole of

western Pacific Ocean is ringed by a belt of earthquakes relative motion for two plates can be determined. The
which lie on planes, in places offset by transform faults, first, and most accurate, is based on the fact that for

dipping at an angle of about 45° beneath the neighboring true tangential motion to occur during the relative
plates. These planes of earthquake foci, known as Benioff movement of two plates, the transform faults along
(or Benioff Wadati) zones, are typically associated with their common boundary must follow the traces of
volcanic activity at the surface. The deepest events small circles centered upon the pole of relative motion
recorded lie at a depth of about 670 km. Collisional (McKenzie & Parker, 1967; Morgan, 1968). The pole
mountain belts such as the Alpine-Himalayan chain are of rotation of two plates can thus be determined by
similarly characterized by intermediate and deep focus constructing great circles at right angles to the trends
earthquakes although, since there is no longer a Benioff to transform faults affecting their common margin and
zone present in such regions, the seismic activity occurs noting their point of intersection. The most conve-
within a relatively broad belt (Fig. 10.17). Careful exam- nient type of plate margin to which to apply this tech-
ination of epicenter locations has revealed, however, that nique is the accretive type (Fig. 5.3), as ocean ridges
some of the shallow events lie on arcuate strike-slip fault
zones associated with the collisional event.
The intra-plate areas are relatively aseismic on this
timescale, although occasionally large magnitude earth-

quakes do occur. Although insignificant in their release
of seismic energy, intra-plate earthquakes are important
as they can indicate the nature and direction of stress
within plates (Section 12.7).

5.3 RELATIVE PLATE

MOTIONS

The present day motion of plates can now be measured
using the techniques of space geodesy (Section 5.8).
However, these techniques were only developed in the
1980s, and, ideally, measurements are required over a
period of 10–20 years (Gordon & Stein, 1992). Prior to
this relative plate motions, averaged over the past few
million years, were determined using geologic and geo-
physical data.
The motion of plates over the Earth’s surface can
be described by making use of Euler’s theorem (Section
3.2.1), which says that the relative motion between two Figure 5.3 Determination of the Euler pole for a
plates is uniquely defined by an angular separation spreading ridge from its offsetting transform faults that

about a pole of relative motion known as an Euler describe small circles with respect to the pole.