Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 119

thousands of kilometres across the North Pacific Ocean to strike Japan killing
many people. The origin of this tsunami in Japan had previously remained a
mystery for it wasnotassociated with any known earthquake felt in Japan.
The Japanese historical record was able to show that it had occurred on the
26th January 1700 (Satake et al ., 1996), a date that is bracketed by the chrono-
logical determinations for the northwest USA tsunami by Atwater. This study is
significant for it shows that palaeotsunamis can leave signatures (both historical
and presumably geological) of their inundation on coasts far removed from the
tsunami-generating source.
Sand sheets
Sand sheets extending inland from the shore are the most common
form of evidence used to identify palaeotsunami inundations. Tsunami laid sand
sheets vary in thickness from a few centimetres to approximately a metre. They
can be both continuous and discontinuous in their extent inland depending
upon variations in the onshore local topography, and they can extend inland
from tens of metres up to 2 km. Goff et al .(1998, 2001)present 16 diagnostic
criteria of tsunami laid sand sheets (Table 5.1).
There appear to be minor differences between sand sheets deposited by storm
surge and waves (Nanayama et al ., 2000). Both wave forms, however, can deposit
sand sheets extending hundreds of metres inland (see, for example, the sand
sheet deposited by Cyclone Vance in Chapter 4). Goff et al .(2004) compared a
tsunami and storm deposit on the North Island of New Zealand. They found
that the storm deposit was better sorted, coarser grained, didn't decrease in
thickness noticeably with distance inland and had a sharp lower boundary
whereas the tsunami deposit had an erosional lower contact. Another potential
difference between the two is the presence of sedimentary structures showing
backwash or the return flow of water to the sea in tsunami events, i.e. fore-
set beds showing both the onshore flow and backwash (Fig. 5.6). However, such
structures are by no means evident in all tsunami laid sand sheets, especially
where backwash did not return over the same area of beach or barrier that the
onshore flow transgressed. This was the case at Sissano Lagoon, PNG, where the
onshore flow swept across the beach and barrier and into the lagoon; the return
flow then travelled back to the sea through the mouth of the lagoon, not back
over the barrier. The other potential difference between the two types of sand
sheet is the presence, sometimes, of offshore or deeper water foraminifera and
diatoms in tsunami deposits. Tsunami can disturb sediments on the sea floor
at depths greater than that capable by wind-generated or storm waves because
tsunamis are shallow water waves for their entire existence. As a consequence,