Geoscience Reference
In-Depth Information
(a)
D
A
E
F
C
B
(b)
Figure 4.23.
(a) A radar interferogram showing the change in distance from a
satellite orbiting at an altitude of 785 km to the ground surface resulting from the
Landers, California, earthquake. The two images used to construct the interferogram
were taken on 24 April and 7 August 1992, the earthquake occurred on 28 June 1992.
The
M
w
= 7.3, strike-slip earthquake ruptured 85 km along an almost vertical set of
faults. (b) A synthetic interferogram calculated with the fault modelled as eight
planar segments, which rupture from the surface to a depth of 15 km. Both images
are 90 km
-- +
110 km and one cycle of grey-scale represents a change in distance
of 28 mm. Colour version Plate 5. (Reproduced with permission from
Nature
Massonnet
et al.
,
Nature
,
364
, 138-42) Copyright 1993 Macmillan Magazines Ltd.)
×
Figure 4.24.
(a) A plane
view of an earthquake
strike-slip fault. A, B, C,
D, E and F are six
seismograph stations that
recorded the earthquake.
The first P-wave recorded
at stations A, C and E
would be compressional
(positive, up); the first
P-wave recorded at
stations B and F would be
dilatational (negative,
down); station D would
record no first P-wave
arrival. (b) The
distribution of polarity of
the first P-wave motion
falls into four quadrants.
The lobes indicate the
relative magnitude of the
first motion at any
location. The arrow
shows the magnitude at
location C.
orbiting at an altitude of 785 km. The interferogram (Fig. 4.23, Plate 5), with each
cycle of shading representing 2.8 cm of change in the ground-to-satellite distance,
contours the spatial extent of the ground deformation very clearly. There are at
least twenty fringes from the northern edge of the image to the cores of the lobes
along the faults, representing a difference in distance of 56 cm - this agrees
with the observed surface displacements. The area within 5-10 km of the rupture
zone shows no organized fringe pattern, the signal is incoherent. This region was
subject to large changes in satellite-to-ground distance, intense secondary fault-
ing and block rotation. Space-geodetic studies will clearly become an increasingly
powerful means for studying crustal deformation.
An earthquake occurring along the San Andreas Fault in California, U.S.A.,
is likely to have a different mechanism from one occurring on a subduction zone
beneath Japan. By studying the direction of movement, or
polarity
,ofthe first
seismic waves from an earthquake arriving at a number of seismograph stations
distributed over the Earth's surface, both the type of earthquake and the geometry
of the fault plane can usually be determined. To understand this, consider a simple
strike-slip fault as shown in Fig. 4.24. Imagine that the world is flat and that
seismograph stations A, B, C, D, E and F are located some distance from the fault
where the earthquake occurred.
