Geoscience Reference
In-Depth Information
The enormity of the tsunami generated the most intense
research effort for such an event in history. Dozens of
experts took to the field to document the evidence, mea-
suring run-up at 965 sites throughout the northwest Indian
Ocean where the tsunami was most destructive. The tsu-
nami generated some of the greatest run-ups ever recorded
(Table
6.4
and Fig.
6.11
). At Banda Aceh, run-ups
exceeding 20 m were measured at ten sites (Choi et al.
2006
; National Geophysical Data Center
2006b
). The
greatest run-up reached 50.9 m at Labuhan. This height has
only been exceeded four times previously—Indonesia in
1674 (80 m), Kamchatka in 1737 (63 m), and Japan in 1741
(90 m) and in 1771 (85.4 m). Run-up heights approaching
20 m were recorded in Thailand. On the flat coastal plains
of Sri Lanka, and on the east coast of India, run-ups
reaching 11.3 and 11.5 m respectively swept up to a kilo-
meter inland. Here the second wave was the highest and
backwash was a major factor in the disaster as it scoured out
channels, and carried people and debris out to sea. Table
6.5
shows the greatest distance inland that the tsunami travelled
(Choi et al.
2006
; National Geophysical Data Center
2006b
). This amounted to 5 km in Banda Aceh close to the
fault line and up to 1.5 km in India 2,000 km away. In the
Maldives, the height of the tsunami exceeded 4.0 m. This
was high enough to overwash totally most islands, so that
the distance of flooding inland became irrelevant. Flow
velocities at Banda Aceh ranged between 5 and 8 m s
-1
.
The maximum velocity was estimated to be 16 m s
-1
.
The wave also carried significant amounts of sand and
mud inland from the shoreline plus thousands of small
boulders consisting of coral and cemented beach rock
(Nandasena et al.
2011
). In Sumatra, coastal retreat
amounting to tens of meters occurred with scouring of
sediment to depths of 0.5-1.0 m being common. In Banda
Aceh, a sand layer was laid down as a single massive unit
that was 0.7 m thick (United States Geological Survey
2005a
; Paris et al.
2007
)—thicker than anything previously
attributed to a tsunami (Fig.
6.12
a). The sands often con-
tained angular pebbles and soil rip-up clasts. While the
deposits appeared layered, no one-to-one relationship
between the number of layers and tsunami waves could be
consistently identified. The layering appeared to be a
function of variations in flow velocity as the wave moved
inland. Backwash also played a role in modifying and
redepositing sediment. In Sri Lanka, the sand layer was
0.37 m thick (Fig.
6.12
b). Here, the sand deposits could be
separated into distinct units representing the passage of
more than one wave (United States Geological Survey
2005b
). Grain size decreased upwards in each unit, a facet
typical of tsunami deposits. On the west coast of Thailand
and Malaysia where significant damaged also occurred,
sand layers were so thin and discontinuous that there will
probably be little permanent, sedimentological indication of
