Geoscience Reference
In-Depth Information
solution
or a
focal mechanism
(discussed fully in Section 4.2.8). Fault-plane solutions
for earthquakes along a plate boundary can give the direction of relative motion between
the two plates. For example, earthquakes occurring on the transform fault between plates
AandBinFig. 2.8(b) would indicate that there is right lateral motion across the fault.
The location of the pole and the direction, though not the magnitude, of the motion can
thus be estimated.
4. Where plate boundaries cross land, surveys of displacements can be used (over large
distances and long periods of time) to determine the local relative motion. For example,
stream channels and even roads, field boundaries and buildings may be displaced.
5. Satellites have made it possible to measure instantaneous plate motions with some
accuracy. One method uses a satellite laser-ranging system (SLR) to determine dif-
ferences in distance between two sites on the Earth's surface over a period of years.
Another method, very-long-baseline interferometry (VLBI), uses quasars for the signal
source and terrestrial radio telescopes as the receivers. Again, the difference in distance
between two telescope sites is measured over a period of years. Worldwide, the rates of
plate motion determined by VLBI and SLR agree with geologically determined rates
to within 2%.
A third method of measuring plate motions utilizes the Global Positioning System
(GPS) which was developed to provide real-time navigation and positioning using satel-
lites. A worldwide network of GPS receivers with a precision suitable for geodynamics
has been established (1 cm in positioning and
<
10
−
3
arcsec in pole-position estimates).
It is called the International GPS Service for Geodynamics (IGS), and is a permanent
global network of receivers. Analysis of data from 1991-1996 shows that the agreement
of GPS velocities with the geologically determined velocities for all but a few locations
is to better than 95% confidence. This is another impressive corroboration of relative
plate motions -
the plates are continually in motion
.
An estimate of the present-day plate motions, NUVEL-1A, made by using 277
measurements of ridge spreading rate, 121 oceanic transform-fault azimuths and
724 earthquake slip vectors is given in Table 2.1.Figure 2.10 shows velocities
in southern California relative to North America as determined from geodetic
measurements (including GPS and VLBI) between 1972 and 1995. The mea-
sured velocity of motion across this boundary was 50 mm yr
−
1
, compared with
the 49 mm yr
−
1
predicted (Table 2.1). Thus again geological estimates based on
measurements with a timescale of a million years agree with measurements made
over a few years.
Although clear boundaries between rigid plates describe the relative motions
and structures well, there are a few boundaries for which the term '
diffuse plate
boundary
'isappropriate. The main examples are the North American and South
American plate boundary from the Mid-Atlantic Ridge to the Caribbean and the
boundary which subdivides the Indian plate.
It is important to realize that a rotation with a large angular velocity
does not
necessarily mean that the relative motion along the plate boundary is also large.
ω
