Geoscience Reference
In-Depth Information
appearance. Today, Mexico City has grown to a
population in excess of 12 million. Two earthquakes
struck Mexico 400 km south-west of the city, offshore
from Rio Balsas, on 19-20 September 1985. The first
earthquake measured 8.1 on the M
s
scale, while the
second aftershock, 36 hours later, measured 7.5.
Jalisco, Michoacan, and Guerrero, the three coastal
states nearest the epicenter, suffered extensive
damage, with the loss of about 10 000 people. Damage
decreased sharply away from the epicenter. In Mexico
City, peak accelerations corresponded to a calculated
return period of 1 in 50 years. However, when the
shock waves traversed the lake beds underlying
Mexico City, they were amplified sixfold by resonance,
such that the underlying sediments shook like a bowl
of jelly under complex seiching. Surface gravity waves,
akin to waves on the surface of an ocean, crisscrossed
the basin for up to three minutes. These waves had
wavelengths of about 50 m, the same as the base
dimensions of many buildings, and caused the
destruction of 10 per cent of the buildings in the city
center, covering an area of 30 km
2
. Of the 400
multistorey buildings wrecked, all but 2 per cent were
between 6 and 18 stories high. Many had become
pliable with age and had developed a fundamental
resonance period of two seconds, the same as that
obtained for peak accelerations of surface seismic
waves. The buildings folded like a stack of cards. Close
to 20 000 people were killed in Mexico City and the
damage bill amounted to $US4.1 billion. The Mexican
earthquake illustrates a fundamental lack in our
knowledge regarding the nature of ground motion in
sedimentary basins containing high water content, or
alternating dry and wet sediment layers.
preceded by a down-drawing of water followed by a
rapid surge over the space of 30-120 minutes, the
name
tidal waves
has been used to describe such
phenomena. However tsunami have nothing to do
with tides and this usage is now discouraged. Tsunami
have a wavelength, a period, and a deep-water height;
and can undergo shoaling, refraction and diffraction.
Most tsunami originate from submarine seismic
disturbances. The displacement of the Earth's crust by
several metres during underwater earthquakes may
cover tens of thousands of square kilometres, and
impart tremendous potential energy to the overlying
water. Tsunami are rare events, in that not all sub-
marine earthquakes generate them. Between 1861 and
1948, only 124 tsunami were recorded from 15 000
earthquakes. Along the west coast of South America,
1098 offshore earthquakes have generated only 19
tsunami. This low frequency of occurrence may
simply reflect the fact that most tsunami are small in
amplitude and go unnoticed, or the fact that most
earthquake-induced tsunami require a shallow focus
seismic event greater than 6.5 on the M
s
scale.
Submarine earthquakes have the potential to
generate landslides along the steep continental slope
that flanks most coastlines. In addition, steep slopes
exist on the sides of ocean trenches and around the
thousands of ocean volcanoes, seamounts, atolls, and
guyots on the seabed. Because such events are difficult
to detect, submarine landslides are considered a minor
cause of tsunami. However, small earthquakes have
generated tsunami and one mechanism includes
subsequent tsunami landslides. A large submarine
landslide or even the coalescence of many smaller
slides has the potential to displace a large volume of
water. Geologically, submarine slides involving up to
twenty thousand cubic kilometres of material have
been mapped. Tsunami arising from these events
would be much larger than earthquake-induced waves.
Only in the last thirty years has coastal evidence for
these mega-tsunami been uncovered.
Tsunami can also have a volcanic origin. Of 92 doc-
umented cases of tsunami generated by volcanoes,
16.5 per cent resulted from tectonic earthquakes asso-
ciated with the eruption, 20 per cent from
pyroclastic
(ash) flows or surges hitting the ocean, and 14 per cent
from submarine eruptions. Only 7 per cent resulted
from the collapse of the volcano and subsequent
caldera
formation. Landslides or avalanches of cold
rock accounted for 5 per cent; avalanches of hot
TSUNAMI
Desc
ription
(Wiegel, 1964; Shepard, 1977; Myles, 1985;
Bryant, 2001)
Tsunami (both the singular and plural forms of the
word are the same) are water wave phenomena gener-
ated by the shock waves associated with seismic
activity, explosive volcanism, or submarine landslides.
These shock waves can be transmitted through oceans,
lakes, or reservoirs. The term tsunami is Japanese and
means harbor (
tsu
) wave (
nami
), because such waves
often develop as resonant phenomena in harbors after
offshore earthquakes. Because some tsunami are
