Earthquake-Generated Tsunami - Tsunami - page 96

Fig. 5.13 Overwash splay of
sediment caused by the July 17,
1998 Tsunami at Arop, Papua
New Guinea. The splay is up to
two meters thick. Note the
number of trees left standing
despite the tsunami being over
15 m high and traveling at a
velocity of 15-20 m s -1 near the
shoreline. Source Dr. Bruce Jaffe
and Dr. Guy Gelfenbaum, U.S.
Geological Survey

bubbling water. The tsunami crest approached the coastline
at 100 km h -1 . The atmospheric pressure pulse preceding
this wave may have been sufficient to ignite this methane.
Certainly, the pulse was strong enough to flatten people to
the ground before the wave arrived. Those exposed to this
flaming wall of water would have been severely burnt before
being carried inland. The wave also deposited one of the
classic sedimentary signatures of tsunami by moving a
million cubic meters of sand onshore and spreading it as a
5-15 cm thick splay inland along the coast (Gelfenbaum and
Jaffe 1998 ). At Arop, the sand deposit was up to 2 m thick
and spread 60-675 m inland (Fig. 5.13 ). The basal contact
of the deposit was erosional. Mud rip-up clasts were evident
in places, indicating high-velocity turbulent flow. Grain size
decreased both upwards throughout the deposit and land-
ward—facts reflecting the reduced capacity of the flow to
carry sediment with time.
The Papua New Guinea event raises a conundrum in
ascribing tsunami to the occurrence of an antecedent earth-
quake no matter what its size. Secondary landslides are
associated with many of the tsunami generate by the largest
earthquakes. For example, many of the tsunami in Prince
William Sound following the Alaskan earthquake of March
27, 1964 were due to localized submarine landslides (Ka-
chadoorian 1965 ; McCulloch 1966 ). At present, slides are
perceived only as a minor contributor to tsunami. This view

may be neglectful given the fact that many tsunamigenic
earthquakes occur along the slopes of steep continental
shelves prone to topographic instability. For example, the
Tokyo earthquake of September 1, 1923 caused the seafloor
to drop in elevation up to 590 m (Shepard 1933 ). Under the
circumstances, it has been difficult to resolve what process
generated the PNG Tsunami which reached run-ups of over
15 m near Sissano Lagoon (Fig. 5.12 ). Tappin et al. ( 2001 )
convincingly show that a submarine slump was responsible
for this large tsunami. This underrated aspect of tsunami will
be discussed in depth in Chap. 7 .

References

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