Geoscience Reference
In-Depth Information
height of 42 m (Figure 11.8). The tsunami penetrated
5 km inland over low-lying areas. The largest wave
struck the town of Merak. Here, the 15 m tsunami rose
to 40 m because of the funnel-shaped nature of the
bay. The town of Anjer Lor was swamped by an 11 m
high wave, the town of Tjaringin by one 23 m in height,
and the towns of Kelimbang and Telok Betong were
each struck by a wave 22-24 m high. In the latter town,
the Dutch warship
Berouw
was carried 2 km inland
and left stranded 10 m above sea level. The highest
run-up reached 42 m above sea level at Merak in the
north-east corner of the Strait. Coral blocks weighing
up to 600 tonnes were moved onshore. Within the
Strait, eleven waves rolled in over the next fifteen
hours, while at Batavia (now Jakarta) fourteen consis-
tently spaced waves arrived over a period of thirty-six
hours. Between 5000 and 6000 boats were sunk in the
Strait. In total, 36 417 people died in major towns
and three hundred villages were destroyed because of
the tsunami.
Within four hours of the final eruption, a 4 m high
tsunami arrived at Northwest Cape, Western Australia,
2100 km away. The wave swept through gaps in the
Ningaloo Reef and penetrated 1 km inland over sand
dunes. Nine hours after the blast, three hundred river-
boats were swamped and sunk at Kolkata - formerly
Calcutta - on the Ganges River, 3800 km away. The
wave was measured around the Indian Ocean at Aden
on the tip of the Arabian Peninsula, Sri Lanka, Mahe in
the Seychelles Islands, and on the island of Mauritius.
The furthest this tsunami wave was observed was
8300 km away at Port Elizabeth, South Africa. Tsunami
waves were measured over the next thirty-seven hours
on tide gauges in the English Channel, the Pacific
Ocean, and in Lake Taupo in the center of the North
Island of New Zealand - where a 0.5 m oscillation in
lake level was observed. Around the Pacific Ocean, tide
gauges in Australia, Japan, San Francisco, and Kodiak
Island measured changes of 0.1 m up to twenty
hours after the eruption. Honolulu recorded higher
oscillations of 0.24
England when the main tsunami had effectively dissi-
pated its energy in the Indian Ocean. The generation
of tsunami in Sunda Strait and the Indian Ocean has
been attributed to four causes: lateral blast, collapse of
the caldera that formed on the north side of Krakatau
Island, pyroclastic flows and a submarine explosion.
Lateral blasting may have occurred to a small degree
on Krakatau during the fourth explosion; however, its
effect on tsunami generation is not known. During the
final explosion, Krakatau collapsed in on itself forming
a caldera about 270 m deep and with a volume of
11.5 km
3
. However, modelling indicates that this
mechanism underestimates tsunami wave heights by a
factor of three within Sunda Strait. Krakatau generated
massive pyroclastic flows. These flows probably gener-
ated the tsunami that preceded the final explosion.
At the time of the final eruption, ash was ejected into
the atmosphere towards the north-east. Theoretically,
a pyroclastic flow in this direction could have gener-
ated tsunami up to 10 m in height throughout the
strait; however, the mechanism does not account for
tsunami run-ups of more than 15 m in height in the
northern part of Sunda Strait. The pyroclastic flow now
appears to have sunk to the bottom of the ocean and
traveled 10-15 km along the seabed before depositing
two large islands of ash. The 40 m high run-up
measured near Merak to the north-east supports this
hypothesis. The tsunami's wave height corresponds
with the depth of water around Krakatau in this
direction. Water was simply expelled from the seabed
by the pyroclastic flows. As well, the fourth explosion
of Krakatau, at 9:58 am, more than likely produced a
submarine explosion as ocean water came in contact
with the magma chamber. A submarine explosion
could have generated tsunami 15 m high throughout
the Strait. If the explosion had a lateral component
northward, as indicated by the final configuration of
Krakatau Island, then this blast, in conjunction with
the pyroclastic flow, would account for the increase in
tsunami wave heights towards the northern entrance of
Sunda Strait.
m with a periodicity of thirty
minutes.
The tsunami in the Pacific have been attributed to
the atmospheric pressure wave, because many islands
effectively obscure the passage of any tsunami from the
Sunda Strait eastward. The atmospheric pressure wave
also accounts for seiching that occurred in Lake Taupo,
which is not connected to the ocean. Finally, it explains
the long waves observed along the coasts of France and
Mt P
elée (8 May 1902)
(Bolt et al., 1975; Whittow, 1980; Blong, 1984;
Scarth, 2002)
The year 1902 was not a good year for the residents
of the West Indies or the surrounding Caribbean
(Figure 11.9). The Pacific coast of Guatemala was
struck by a strong earthquake on 18 January and again
