Environmental Engineering Reference
In-Depth Information
are never observed by ships at sea because in the open sea, wave amplitudes are only a
meter or so. They travel at great velocities, of the order of 700 km/h or more. The tsunami
caused by the 1960 Chilean quake ( M
8.4) reached Hawaii, a distance of 10,500 km, in a
little less than 15 h, and Japan, 17,000 km distant, in 22 h. (The velocity of water waves is
given approximately by the relationship v
gD , where g is the acceleration due to gravity
and D is the water depth. Wavelength
vT , where T is the
period. Tide gages around the Pacific showed the Chilean tsunami to have a period of
about 1 h; therefore, its wavelength was about 700 km.)
Coastal areas : The magnitude of a tsunami at its source is related to the earthquakes
magnitude. When it arrives at a coastline, the effect is influenced by offshore seafloor con-
ditions, wave direction, and coastline configuration. Wavelengths are accentuated in bays,
particularly where they have relatively shallow depths and topographic restrictions. The
wave funnels into the bay and builds to great heights. Containing tremendous energy, the
wavefront runs up onto the shore, at times reaching several kilometers inland. The crest is
followed by the trough during which there is a substantial drawdown of sea level, expos-
ing the seafloor well below the low tide level. After an interval of 30 min to an hour,
depending upon the wave period, the water rises and the second wave crest, often higher
than the first, strikes the beach. This sequence may continue for several hours, and the
third or fourth wave may sometimes be the highest. At Hilo, Hawaii, after the Chilean
event of 1960, the first wave reached 4 ft above mean sea level, the second 9 ft, and the
third, 33 ft. On Honshu and Hokkaido, Japan, the water rose 10 ft along the coast during
the Chilean tsunami.
λ
is given by the relationship
λ
Early Warning Services
After the very damaging 1946 tsunami, an early warning service was established by the
USGS and centered in Hawaii. When seismograph stations in Hawaii show a Pacific
Ocean focus earthquake, radio messages are sent to other Pacific seismograph stations
requesting data from which to determine the epicenter. Adequate time is available to com-
pute when tsunami waves might arrive and to so warn the public in coastal areas.
In general, a “watch” is initiated for magnitudes of 7.5 or greater. “Warnings” are issued
if tide gages detect a tsunami (Kerr, 1978). Unless waves strike the shores near the epicen-
ter, however, there is no way for people on distant shorelines to know if a tsunami has
been generated. Even though the Chilean earthquake caused tsunamis along the Chilean
coastline, many people in Hawaii chose to ignore the warning and not to move to higher
ground. Japanese officials similarly ignored the warning, since a Chilean earthquake had
never before caused a tsunami in Japan. The tsunami reached Hawaii within 1 min of the
predicted arrival time.
Hazard Prediction
When important structures or new communities are located along shorelines, the poten-
tial for the occurrence of tsunamis should be evaluated. Some procedures for evaluating
the tsunami hazard are given by Synolakis (2003).
Qualitatively there are several high-hazard conditions to evaluate:
Regional tsunami history and recurrence.
Near onshore earthquakes: recurrence and magnitude.
Coastline configuration: Irregular coastlines with long and narrow bays and
relatively shallow waters appear to be more susceptible than regular coastal
plains when exposed to tsunami waves generated by distant earthquakes of
large magnitudes.
Search WWH ::




Custom Search