Geoscience Reference
In-Depth Information
Fig. 5.12
Location and height
of Aitape, Papua New Guinea
Tsunami of July 17, 1998. Based
on Kawata et al. (
1999
) and
Tappin et al. (
1999
). Height bars
are scaled relative to each other
130°
135°
140°
145°
150°
0 °
Bismarck
Sea
Irian Java
-5 °
Papua New
Guinea
-10 °
Aftershock (7:09 PM)
Shelf slope
Serai
6:49 PM
Tumleo
Islands
Sissano
Sissano lagoon
Arop
Malol
Aitape
20 km
010
Earthquake epicentre
Height of tsunami
(= 5 m)
2-3 m. It then propagated northwards to Japan and Hawaii
where 10-20 cm oscillations were observed on tide gauges
about seven hours later. Over 2,200 people lost their lives.
Theory indicates that a 7.1 magnitude earthquake could
not have produced a tsunami higher than 2 m along this
coastline. While the event would appear to be a tsunami
earthquake, its moment magnitude, M
w
, was also too low,
having a value of 7.1—far less than that generated by tsunami
earthquakes. Submarine landslides have been suggested as a
cause for large tsunami following small tsunamigenic
earthquakes (Tappin et al.
2001
). This now appears to be the
main cause of this tragic event, and is supported by the fact
that there were only three closely spaced waves in the tsu-
nami wave train. Sea level withdrew from the coast, although
there is evidence that about 0.4 m of submergence took place
at the coastline. Eastwards the wave did not approach shore-
normal but travelled at an angle to the shore. This is unusual
for seismically generated waves originating beyond the
continental shelf, but fits dispersion away from the point
source of a submarine slump. Offshore mapping has now
delineated slump scars, fissures, and amphitheater structures
associated with rotational slides. Given the closeness of the
epicenter to shore, the tsunami should have arrived within
10 min of the earthquake. It took 10 min longer. Slumping
takes time after an earthquake to generate a tsunami, and this
fact can account for the delay. If a submarine landslide
generated the tsunami, theoretically 5 km
3
of material would
have had to be involved. Alternatively, numerous slides
could have coalesced instantaneously over an area of
1,000 km
2
. The most recent evidence indicates that a slump
with a volume of 6 km
3
triggered the tsunami. Topographic
focusing must have occurred to produce the tsunami flow
depths measured along the Aitape coast.
The wave was unusual because it was associated with fire,
bubbling water, foul-smelling air, and burning of bodies
(Hovland
1999
). Eyewitnesses reported that the crest of the
tsunami was like a wall of fire with sparkles flying off it. In
Chap. 1
, this sparkling was attributed to bioluminescence,
while the foul odor was linked to disturbance of meth-
ane-rich sediments in Sissano lagoon. The burnt bodies have
been ascribed to friction as people were dragged hundreds of
meters by the wave through debris and trees. These expla-
nations may not be correct. Subduction zones incorporate
organic material, which is converted to methane by anaer-
obic decomposition. The sudden withdrawal of 1-2 m depth
of water can cause degassing of these sediments, leading to
