Geoscience Reference
In-Depth Information
(c)
(a)
δ
δ
(d)
(b)
δ
δ
Figure 4.22.
Fundamental types of faults and their various names. (a) A block model
of a fault before any motion has taken place;
δ
is the angle of dip. (b) A strike-slip
fault, transcurrent fault, lateral fault, tear fault, wrench fault;
δ
is often near 90
◦
. The
fault shown is left-handed or sinistral; a strike-slip fault with the opposite offset
would be called right-handed or dextral. (c) A normal fault, dip-slip fault, normal-slip
fault, tensional fault, gravity fault; generally, 45
◦
<δ<
90
◦
. (d) A thrust fault, dip-slip
fault, reverse-slip fault, reverse fault, compressional fault; often, 0
◦
<δ<
45
◦
. Arrows
show the slip vector, the relative motion between the two sides of the fault. The slip
vector always lies in the fault plane. (From B ath (1979).)
The arrival times of the waves at the stations can then be used to locate the source
(Section 4.2.1). In 1961, the United States Coast and Geodetic Survey established
the World-Wide Standardized Seismograph Network (WWSSN). More recently
anew Global Seismographic Network managed jointly by the United States
Geological Survey (USGS) and Incorporated Research Institutions for Seismol-
ogy (IRIS) has been established. Automatic digital recording by broad-band,
low-gain and short-period instruments ensures that vibrations of all frequencies
are recorded. Data are made available electronically to seismologists around the
world for use in determination of the Earth's structure, mitigation of seismic risk
and, importantly, as part of the international verification of the end of nuclear
testing. Figure 2.1 shows the 1978-1989 distribution of epicentres. It is clear that
seismicity accurately delineates many plate boundaries and that there are very
few intraplate events.
Satellites are greatly improving our knowledge of the Earth. Synthetic-aperture
radar interferometry (SAR) has provided a spectacular image of the displace-
ment resulting from the 1992 Landers, California, earthquake. This
M
W
=
7.3,
M
S
=
7.5, right-lateral strike-slip earthquake ruptured some 85 km along a fault
system, and resulted in a maximum slip of 6 m. By using two satellite radar images
of the region, one taken before the earthquake, the other after, it was possible to
calculate the small changes in the distance from the ground surface to the satellite
