Great Earthquake-Generated Events - Tsunami - page 110

the ocean above heaved in response, displacing 30 km 3 of
seawater that then propagated outwards along the 1200 km
distance of the fault line. The size of the earthquake defied
analysis for months because so much energy was concen-
trated in long seismic waves. In the end, the moment
magnitude, M w , was estimated at between 9.15 and 9.30,
equivalent to 1.1 9 10 18 J, more that the total energy
released by all earthquakes globally over the previous
10 years. The number of aftershocks greater than 5 in
magnitude was the greatest ever observed. The earthquake
was felt as far north as Bangladesh and as far west as the
Maldives. The whole surface of the earth moved vertically
at least one centimeter. The shift in mass slowed the Earth's
rotation by 2.7 m s. It was the second largest earthquake
ever recorded.
A disaster of global proportions was unwinding as the
deadliest tsunami in recorded history began to race across
the Andaman Sea and the Indian Ocean. The event is
definitive: it was the most devastating Sunday morning in
human history, the deadliest tsunami ever, and for the
Indian Ocean very unexpected. There was also one major
difference between large tsunami in the Indian Ocean over a
century ago and now. The increased population that had
spread onto coastal plains in Sumatra, Sri Lanka, India, and
Thailand since the Second World War were all vulnerable.
There was no warning for the thousands who would die over
the next 10 h, for the Indian Ocean had no tsunami warning
capability. Nor would a warning system have been that
effective because 73 % of the death toll occurred within
20-30 min of the earthquake. At Banda Aceh on the
northern tip of Sumatra, the sea receded 10 min after the
earthquake. Three minutes later the first wave measuring
5 m in height ran ashore. 5 minutes after this a second and
more devastating wave arrived with heights of 15-30 m.
Very few observations exist of the third and final wave,
because most people were dead.
For the first time, the height of a tsunami could be
measured using altimeters on satellites: TOPEX/Poseidon
and Jason operated jointly by NASA and the French space
agency (Fig. 6.11 c), CNES, the European Space Agency's
Envisat, and the U.S. Navy's Geosat Follow-On, that for-
tuitously passed above the wave front in the region of Sri
Lanka. The tsunami as it spread outwards throughout the
Indian Ocean had a height of 60 cm, exceeding the height
of the Chilean Tsunami described previously (National
Oceanic and Atmospheric Administration 2006 ). The
measurements backed up theory indicating that the tsunami
in 4,500 m depths at the time of sensing had a shallow
water wave speed of about 750 km h -1 . The length between
wave crests in the tsunami wave train, after correcting for
the speed of the satellite, was about 37 min. The wave took
10 h to cross the Indian Ocean (Fig. 6.11 a).

effect of the tsunami was minor. The wave averaged 0.4 m
high on tide gauges reaching a maximum elevation of 1.6 m
(Table 6.2 ). Little damage was reported. However, the
tsunami continued to be a major event along the coast of
South America. Here, the wave averaged 1.2 m in height,
reaching a maximum value of 4 m at Coquimbo, Chile.
Unlike the Chilean event 4 years previously, individual
Pacific islands were not badly affected by the Alaskan
Tsunami. A maximum wave height of only 0.6 m was
registered at the Galapagos Islands. Surprisingly, the Palmer
Peninsula in the Antarctic recorded a wave height of
0.4 m—a value greater than that recorded on most Pacific
Islands. South Pacific Islands appear to be immune from the
effects of tsunami generated by Alaskan earthquakes.

6.5 The Indian Ocean Tsunami December

26, 2004

The Indo-Australia Plate is moving northwards against the
southern extension of the Eurasian Plate at the rate of
37-57 mm yr -1 . However, no large earthquake with a
moment magnitude, M w , greater than 8.0 had occurred
along the boundary of these two plates for over a century
(National Geophysical Data Center 2006b ). Severe earth-
quakes had occurred in the region previously in 1797
(M w = 8.2), 1833 (M w = 9.0), 1861 (M w = 8.5), and 1881
(M w = 7.9) with some producing destructive local tsunami.
However, for over a century, the region was relatively
quiescent seismically. On December 23, 2004 at 14:59:03
(UTC) the whole Australian Plate began to move starting
with a magnitude 8.1 earthquake north of Macquarie Island
900 km southeast of Australia. Late Christmas Day,
Greenwich Mean Time, an experimental seismometer
located at the University of Wollongong, south of Sydney,
Australia, measured the passage of a very long wave
moving northwards through the Earth's crust. The Austra-
lian Plate was flexing. Two hours later at 7:58:47 AM local
time at Jakarta (58 min and 47 s past midnight UTC), on
December 26, 2004 (Boxing Day) the enormous pressures
that had built up over the past century began to rupture the
plate boundary 30 km below sea level at 3.3 N, 95.9 E,
about 160 km offshore of northern Sumatra (Ammon et al.
2005 ; Lay et al. 2005 ; Subarya et al. 2006 ). For 10 min, the
Indo-Australian Plate moved northwards slowly at first and
then accelerating to a speed of 2.8 km s -1 over a distance of
1200-1300 km (Bilham 2005 ). As the plates unzipped, the
Eurasian Plate jumped to the southwest. Between the
Andaman Islands and Nias Island, slip displacements
reached 15 m over a 600 km length of the plate boundary
centered on Banda Aceh, Indonesia. Vertical, upward dis-
placement of the sea floor off Sumatra reached 1-2 m and