Apparent contradictions in the plate motion pattern

primary wave,Fig.2.14 Principle ofdirection of
fault-plane-solutions of an
propagation
earthquake hypocenter. The
fault plane of the earthquake(orthogonal to the paperplane) and an orthogonal virtual aiding plane define fourquadrants. First motions of
the primary waves oscillating
fault
in the propagation direction
planeand expressed by verticalmotions in the soil indicate
the sense of movement of
the blocks displaced during
the earthquake. Seismograms
subdivide two quadrants withcompressive and two quadrants
with dilatative first motion.

The orientation of planes of movement at plateboundaries can be deduced from earthquake
data. In the case of an earthquake triggered ata fault plane, the two blocks move by creating
an instantaneous oﬀset up to several meters.
This results in the generation of the two types ofseismic body waves. Primary (P-) waves oscillate
in the longitudinal direction of propagation. They
are faster than secondary (S-) waves that oscillate transversally. If all of the seismic data froma given earthquake collected around the Earthare put into a diagram, the quadrants and thetwo separating planes can be determined with
their spatial orientation (Fig. 2.14). In the twoquadrants that are in the direction of movement of eachblock, the first motion of the primary waves is away from theearthquake focus and an observer on the Earth’s surface firstreceives a push; the wave starts with a compressive movement (compressive first motion shown as black quadrants inFig. 2.14). First motion in the other two quadrants, shownin white, is in the opposite direction; it starts with a tension
and is dilatational. Each of these motions is registered bythe seismograph.
Using the principles discussed above, the quadrants andthe two separating planes can be determined with their
spatial orientation (Fig. 2.14). One of these planes represents
the slip plane generated by the earthquake; the other one isan aiding plane that has no use in nature. However, initially
it is not possible to decide which one of these two planes
was the slip plane. Commonly this can be deduced fromgeological observations if the approximate orientation of afracture zone is known. On the other hand, careful analysis ofseismic data generated by the aftershock activity following
every large earthquake provides the opportunity to identify

dilatational first motion

first motion ofEarth´s surface

Fault-plane-solutions of earthquakes

auxiliary plane

compressional first motion

the slip plane because of the shift of the seismic centers. If theslip plane is known, the sense of movement is easily detected(Fig. 2.15). Direction of movement in the slip plane is orthogonal
to the aiding plane. A process similar to that used for analyzing
P-waves can also be used for the analysis of S-waves that oscillate orthogonal to the direction of propagation.
Using this method, which is called fault-plane-solution, theorientation of a slip plane and the sense of movement can be
determined with high accuracy. Fault-plane-solutions allow fora reconstruction of plate boundaries and their movement patterns (Fig. 2.16). The analysis of earthquake first motion data
impressively confirmed the concept of three diﬀerent types ofplate boundaries (Sykes, 1967).
a) 80°W

70°W
South AmericanPlateb)

50°W

40°W

10°S

Arequipa
30°N

Fig. 2.15 Ambiguity of fault-plane-solutions illustrated by an earthquake that produces an overthrust. Illustration is a schematic verticalsection through the Earth.
earthquake wave

20°S

Nazca Platecompressional
first motion

20°N

30°S
dilatational
first motion

fault planea) Arequipa, Peru. June 23, 2001, magnitude 8.1

auxiliary plane

dilatational
first motion

compressional
first motion

Fig. 2.16 Examples of fault-plane-solutions of recent earthquakes at the three diﬀerent kinds of plateboundaries. Quadrants of the first motions of earthquake waves are shown in stereographic projectionsof the lower hemisphere. Earthquake (a) at a subduction zone (Peru), (b) at a mid-ocean ridge (Atlantic),and (c) at a transform fault (North Anatolian Fault).
Black quadrants – compressive first motion. Redline – active fault plane. Earthquake data are fromthe internet catalogue of the National Earthquake
Information Center (US Geological Survey).

North American
Plate

AfricanPlateb) Mid-Atlantic Ridge, 27.1°N, 44.3°W.
Dec. 13, 2001, magnitude 5.9
20°E
45°N

30°E
EurasianPlate

Black Sea

40°N
Anatolian Plate

35°Nc) Izmit, Turkey. Aug. 17, 1999, magnitude 7.4Licensed to jason patton<Jason-Patton@redwoods.edu>c)223