Geoscience Reference
In-Depth Information
3
Signatures of Tsunami in the Coastal
Landscape
3.1
Introduction
Tsunami are high-magnitude phenomena that can achieve
flow velocities at shore of 15 m s
-1
or more. These flows
have the potential to leave many depositional and erosional
signatures near the coastline (Fig.
3.1
). Dawson et al. (
1991
),
Dawson and Shi (
2000
) have attempted to catalogue some of
these signatures, but ignored bedrock erosional and large-
scale landscape features. Figure
3.2
rectifies these deficien-
cies by encompassing most of the geomorphic signatures
found in the literature. The impact of paleo-tsunami can be
identified where these signatures have been preserved, singly
or more preferably, in combination. The depositional sig-
natures of tsunami can be further subdivided into sedimen-
tary deposits and geomorphic forms. The most commonly
recognized depositional signature is the occurrence of
anomalous sand sheets or lamina sandwiched in peats or
muds on coastal plains. The sedimentary deposits, except for
imbricated boulders, are less dramatic because they do not
form prominent features in the landscape. Without detailed
examination, they also could be attributed to other processes,
mainly storms. Many of the signatures, such as aligned stacks
of boulders, also reveal the direction of approach of a tsunami
to a coastline. In many cases, tsunami have approached at an
angle to the coast—a feature not commonly associated with
signatures define unique tsunami-dominated coastal land-
scapes. The signatures of tsunami will be described in this
chapter, while the formation of tsunami-generated land-
The signatures of tsunami were formulated from field
evidence linked to earthquake-generated tsunami around the
Pacific Rim and to the presence of tsunami identified along
a 400-km stretch of the south coast of New South Wales,
Australia (Fig.
3.3
a). Many of the features summarized in
Fig.
3.2
are dramatic and allude controversially to tsunami
Fig. 3.1
The remnant plug at the center of a vortex bored into granite
at the front of a cliff at Cape Woolamai, Phillip Island, Victoria,
Australia. The ridges in the foreground indicate that flow around the
plug was counterclockwise and consisted of a double helix. A
catastrophic tsunami wave produced the vortex as it washed along the
cliff. John Meier
events an order of magnitude larger than those normally
associated with earthquake-generated tsunami. These latter
types of signatures have since been used to identify the
presence of paleo-tsunami along other sections of the
Australian coastline, particularly in northeastern Queens-
land, northwest Australia, and on Lord Howe Island in the
Tasman Sea; in New Zealand; along the east coast of
Scotland; and in the Bristol Channel, UK. It is not intended
here to debate the merits of this evidence, as this has been
done in many peer-reviewed scientific papers. The signa-
tures of large tsunami are not related to storms. Submarine
landslides, asteroid impacts with the ocean, and the largest
earthquakes produce mega-tsunami features. This evidence
will be presented in the second half of this topic.