Geoscience Reference
In-Depth Information
by earthquakes, with ones near Naples and Catania,
Sicily, in 1693, and at Messina on 29 December 1908,
taking between 60 000 and 150 000 lives each. The San
Francisco earthquake on 18 April 1906 was the worst
American earthquake, killing 498 people, with much of
the city being destroyed by ensuing fires. Most
recently, the 1976 Tangshan earthquake claimed
approximately 250 000 people, making it one of the
worst earthquakes on record.
While many earthquakes stand out historically, very
little research has been performed on the causes of
some of the largest such as the Calcutta earthquake. In
modern times, four to five events stand out because
of their magnitude and the nature of the disaster.
The 1906 San Francisco earthquake occurred in the
vulnerable San Andreas Fault area of California and
wiped out a major United States city in a country that
believed itself to be immune from such disasters. The
Tokyo earthquake of 1923 totally destroyed the largest
city in Japan. The Chilean earthquake of 1960 marked
a revival in large earthquake activity after a decade of
small events, and produced one of the largest wide-
spread
tsunami
events in the Pacific Ocean. The
Alaskan earthquake of 1964 took few lives, but literally
shook the globe like a bell. The 1976 Tangshan earth-
quake, notable for one of the most massive death tolls
in the twentieth century, struck an area not renowned
for seismic activity at a time when the Chinese
believed that they could predict major earthquakes.
The latter event is also notable for the cloak of secrecy
surrounding its aftermath. Not only did China refuse
international aid, but it also released very few details of
the event until five years afterwards. Two of these
events, the 1964 Alaskan earthquake and the 1923
Tokyo earthquake, are discussed in more detail
below.
7.2 on the M
s
scale occurred nearby. Other large
earthquakes had occurred in neighboring areas in
1937, 1943, 1947 and 1958. These earthquakes were
generated by the southward movement of Alaska over
the Pacific Plate at a shallow angle of 20 degrees.
The 1964 earthquake was one of the strongest to be
recorded since seismograph records were taken. Long
wave vibrations lasted 4-7 minutes. The shock waves
were so high in amplitude that seismographs ran off-
scale all over the world, thus preventing accurate
measurement of the earthquake intensity (reaching
8.6 and 9.2 on the M
s
and M
W
scales, respectively).
The earthquake rang the earth like a bell, and set up
seiching in the Great Lakes of America, 5000 km away.
Water levels fluctuated in wells in South Africa on the
other side of the globe. The ground motion was so
severe that the tops of trees were snapped off. Earth
displacements covered a distance of 800 km along the
Daniel fault system parallel to the Alaskan coastline.
Over 1200 aftershocks were recorded along this fault.
Tsunami exceeded 10 m in height along the Alaskan
coast and swept across the Pacific Ocean at intervals of
one hour. The west coast of North America as far south
as San Diego was damaged. In Crescent City, Cali-
fornia, over 30 city blocks were flooded, resulting in
$US7 million damage.
Maximum uplift and downthrow amounted to 14.4
and 3 m, respectively, the greatest deformation from an
earthquake yet measured. In some locations, individual
fault scarps measured 6 m in relief. Coastal and inland
cliffs in south Alaska were badly affected by landslides.
Land subsidence occurred in many places underlain by
clays, and in some areas liquefaction was noted (Fig-
ure 10.5). Liquefaction is a process whereby firm but
water-saturated material can be shaken and turned into
a liquid that then flows downslope under the effect
of gravity. About 60 per cent of the $US500 million
damage was caused by ground failures. Five landslides
caused $US50 million damage in Anchorage alone.
Most of the ground failure in this city occurred in the
Bootlegger Cove Clay, a glacial estuarine-marine
deposit with low shear strength, high water content
and high sensitivity to vibration. In the ports of
Seward, Whittier, and Valdez, the docks and ware-
houses sank into the sea because of flow failures in
marine sediments. Over the next nine hours, each of
these towns was overwhelmed by tsunami 7-10 m in
height.
Alaskan earthquake of 27 March
196
4
(Hansen, 1965; Whittow, 1980; Hays, 1981; Bolt, 1993)
The Alaskan earthquake struck on Good Friday when
schools and businesses were closed. It occurred in a
seismically active zone running westward along the
coast of Alaska (Figure 10.4), parallel to the Aleutian
Trench, and south of the Aleutian Islands arc. The
earthquake was only one in a series of earthquakes in
the area beginning 10 million years ago in the late
Pliocene
. In 1912 and 1934, earthquakes of magnitude
