Geoscience Reference
In-Depth Information
(with
M
0
in N m) and has the important advantage over the other magnitude scales
that it does not saturate towards the top end of the scale.
Afew examples of the magnitudes of some famous earthquakes are as follows:
the 'great' Lisbon earthquake of 1755, magnitude
8.7, caused a tsunami and
killed 60 000 people; the 1906 San Francisco, California, earthquake,
M
S
=
∼
7.8-
8.3 and
M
w
=
7.9 which ruptured about 300 km of the San Andreas Fault with
∼
6mofdisplacement; the 22 May 1960 Chile earthquake,
M
S
=
8.5 and
M
w
=
9.5 which ruptured about 1000 km along the Peru-Chile subduction zone
with
20-40 m of displacement and caused a tsunami up to 15 m high across
the Pacific; the 1963 earthquake in Skopje, Yugoslavia,
M
S
∼
=
6.0; the 1964
Alaska earthquake,
M
S
=
8.4 and
M
w
=
9.2; the 1971 San Fernando, California,
earthquake,
M
S
6.7; the 1976 Tangshan, China, earthquake,
which killed 650 000 people,
M
S
=
=
6.6 and
M
w
=
7.6; the 1985 Mexico earthquake had
M
S
=
8.1 and
M
w
=
8.0; estimates of the magnitude of the 1988 Armenian earthquake
range between 6.7 and 7.0; the 1989 Loma Prieta, California, earthquake had
M
w
=
6.9; and the 1995 Kobe, Japan earthquake which caused huge damage, left
300 000 homeless and killed 6300 people had a magnitude of 6.8. In 1999 the
North Anatolian fault in Turkey was the site of two major earthquakes: the Izmit
earthquake had
M
w
=
∼
7.4, caused surface faulting for
120 km and killed over
17 000 people; three months later the
M
w
=
7.2 Duzce earthquake extended the
fault rupture eastwards (see Fig. 10.20(b)). The 2001 Gujarat, India, earthquake,
which was a shallow, intraplate, thrust-faulting event with
M
w
7.7, caused
immense damage and loss of life (the previous earthquake in this region had
happened in 1819). The largest earthquake ever recorded in Japan occurred in
1933 and had
M
S
=
=
8.4.
The amount of damage done by an earthquake depends on factors such as
population density, soil conditions and local building standards in the epicentral
region, as well as on the focal depth, mechanism and magnitude of the earthquake.
In general, a shallow earthquake with an
M
S
of 6 is very destructive, one with
an
M
S
of 5 produces moderate damage, and earthquakes with magnitudes less
than about 3 are usually not felt by many people, though they will, of course,
be detected by local seismometers. The 1985 Mexico and 1989 Loma Prieta
earthquakes were particularly damaging because of the enhanced shaking caused
by shallow subsurface conditions-adrained lake-bed and young clayey-silt
deposits beneath landfill, respectively. Had the buildings been on older, more
solid rock, the damage would have been much less. The right-lateral strike-slip
Kobe earthquake was not expected, although there was evidence to indicate that
such earthquakes might occur in the complex fault system and regional stress field
of Southwest Japan.
Earthquake hazard
is an estimate of the potential occurrence
of damaging earthquakes. Figure 4.13, the earthquake-hazard map for the U.S.A.,
shows the estimate of the ground acceleration (as a percentage of gravity) that
has a 10% chance of being exceeded during the next fifty years. Plate 3 shows
the global earthquake hazard for a fifty-year window.
8.5 and
M
w
=
