Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 124

layer varied in thickness from 10 cm to over 80 cm depending upon the site.
The layers were typically marked by clast-supported gravel (i.e. gravel resting
against gravel without any intervening sediment such as sand or silt) at their
base merging upwards into coarse-grained, then fine-grained sand and silt at the
top of the sediment cycle. The sequence changed abruptly into the next overlying
layer starting with another unit of clast supported gravel. The same sequence
waspresent in the third or uppermost layer marking the inundation of the
coast by the third tsunami. A similar sequence of upward fining sediment cycles
was also observed by Shi et al .(1995)following the 1992 Flores Island, Indone-
sia tsunami. Four sediment cycles are present within coastal lake deposits in
eastern Norway marking the Storegga landslide-generated tsunami. Each of the
tsunami deposit layers is composed of coarse-grained sands and shell grading
upwards into finer-grained sediments and then plant fragments (Bondevik et al .,
1997).
The change in grain size within successive tsunami sediment cycles is likely
to be due to the decrease in energy of the wave as it passes over the coast. The
coarsest-grained sediments are transported as traction load being either rolled
or bounced along the bottom, whilst the finer-grained sediments are carried in
suspension. As the wave energy diminishes both at a location and as it travels
inland, the heaviest or coarsest-grained sediment is deposited first followed by
thelighter or finer-grained sediment. Such a scenario, however, assumes that
there is a variety of different sediment grain sizes available for transportation
by the tsunami. Dawson et al .(1996), in their study of the 1994 Java tsunami
deposit, found no variation in grain size with distance inland. They attributed
this to the uniformity of the sediment grain size available for transport at this
location.
Sediment cycles associated with multiple tsunami events are usually marked
by asignificant time gap or hiatus that can be identified either by geological
dating of the different layers, or the presence of a buried soil at the top of each
event cycle. The soil shows that the land surface following a particular tsunami
event had remained stable (not inundated or disturbed) so the upper layer of sed-
iments, at that time, was able to weather and experience pedogenesis (soil forma-
tion). Subsequent tsunami inundations then deposit sediments on top of the soil.
Separate tsunami events can also be distinguished by sharp erosional contacts
between layers. These contacts are abrupt and the base of the overlying sediment
unit can often contain 'rip-up' clasts of material derived from the ground surface
during passage of the tsunami. McSaveney et al .(2000) identified two tsunami
events prior to the 1998 event in the sediments near Sissano Lagoon, Papua
NewGuinea. They suggested that these events may have been associated with
theAD1907 and 1934 tsunamis at this location. The studies of Atwater (1987),