Geoscience Reference
In-Depth Information
moved the excessive amounts of sediment. These deltas now
lie abandoned in the lagoon.
Along the south Portuguese coast, the tsunami wave also
ran up valleys (Dawson et al. 1995 ). At Boca do Rio, the
tsunami laid down a dump deposit and a tapering sand layer.
The dump layer was deposited within 400 m of the coast by
the first wave in the tsunami wave train. It consists of a
chaotic mixture of muddy sand, cobbles, shell, sand-armored
mud balls, and the odd boulder up to 40 cm in diameter. The
shells incorporate the littoral bivalve Petricola lithophaga
and the subtidal sponge Cliona spp. The overlying sand layer
was laid down by subsequent waves. It consists of a
0.1-0.4 m thick unit sandwiched between silty clays. The
unit tapers landward over a distance of 1 km. The sand size
within the layer fines upwards from a coarse, gritty sand to a
silty or clayey, fine sand. The sequence is indicative of high-
energy flow that decreased landward. The layer also contains
clay 3-12 lm in size that originated from the weathered
substratum that was eroded by the passage of the tsunami.
Estuarine and intertidal shell species such as Mytilus edulis,
Scrobicularia plana, and Tellina tenius are present in the
lower part of the layer together with gravels and mud balls
0.5-5.0 cm in size. Foraminifera such as Elphidium crispum
and Quinqueloculina seminulum, both of which are found in
20-30 m depths of water, are present throughout the unit.
Dating of the sands indicates that they were deposited by the
Lisbon Tsunami. However two previous events around
2,440 ± 50 and 6,000-7,000 year ago cannot be excluded.
Platy boulders up to 7 m in length were transported by the
tsunami in southern Spain (Whelan and Kelletat 2005 ).
Theoretical flow depths of 14-16 m have been calculated
using these boulder dimensions.
Sediment signatures of tsunami were also deposited on
the Scilly Isles, 40 km southwest of Land's End, England
(Foster et al. 1991 ). Here, shallow lagoons backing wind-
swept dune fields and lying about a meter above the high-
tide limit were inundated by tsunami swash. The first wave
arrived at high tide and produced a 4.5 m high run-up. The
third and fourth waves were the largest. Coarse sandy layers
15-40 cm thick were deposited over sandy peats in three
lagoons. Over the last 250 years, peat has subsequently
covered these sands. Radiocarbon dating at the bottom of
this peat indicates that the sands were deposited around the
time of the Lisbon event.
Mountains. The most tsunamigenic section of coastline
occurs on the border between Peru and Chile. Since histor-
ical records began in 1562, there have been 230 tsunami
generated by earthquakes (Lockridge 1985 ). Five localities
have had ten or more tsunamigenic earthquakes over this
period within a 110 km radius of each other. Destructive
tsunami occur at roughly 30-50 year intervals. Three events
have had Pacific-wide impact: the events of August 13, 1868
and May 10, 1877, both near the town of Arica on the present
border between Peru and Chile, and the event of May 22,
1960. The Arica events regionally had maximum run-ups of
21 and 24 m respectively along the South American coast.
The 1868 Tsunami struck the town within half an hour of the
main shock. The sea rose initially 5 m and then withdrew,
leaving a 2 km wide strip of the seabed exposed. Several
minutes later, the main wave came in and swept across the
coastal plain (Fig. 2.11 ). Approximately 25,000 people lost
their lives in this region alone. The tsunami then swept the
Pacific Ocean with damage being reported in New Zealand,
Hawaii, and Japan. The 1877 event was just as large, if not
more widespread. Its run-up was 20 m high at Arica and
24 m high at Tocopilla, 600 km south of the epicenter. At
Cobija in northern Chile, the tsunami arrived 5 min after the
earthquake and reached 11.9 m above mean sea level,
destroying the town (Fig. 6.4 ). The tsunami also swept the
Pacific Ocean and had a particularly forceful impact on the
coast of New Zealand, where run-up of 6 m was reported. In
eastern Australia, the wave was responsible for the largest
tsunami, 1.07 m, recorded on the Sydney tide gauge.
The May 22, 1960 Tsunami was generated by the last of
over four dozen earthquakes occurring along 1,000 km of
fault line parallel to the Chilean coastline (Myles 1985 ;
Pararas-Carayannis 1998a ). The first earthquake began at
6:02 AM on Saturday, 21 May and destroyed the area around
Concepción. Large aftershocks continued until, at 3:11 PM
Sunday May 22nd, the largest earthquake with M s and M w
magnitudes of 8.9 and 9.5 respectively occurred with an
epicenter at 39.5 S, 74.5 W, and a focal depth of 33 km
(Fig. 6.5 ). Submarine uplift of 1 m and subsidence of 1.6 m
ensued along a 300 km stretch of coast. Subsidence extended
as far as 29 km inland with 13,000 km 2 of land sinking by
2-4 m. The length of rupturing occurred along 1000 km
parallel to the coastline. Many fisherman and their families
quickly put out to sea to escape the flooding that was to come.
Within 10-15 min, the sea quickly rushed in as a smooth
wave 4-5 m above normal tide level and just as quickly
raced back out to sea taking with it boats and flotsam. This
was only the harbinger of worse to come. Fifty minutes later,
the sea returned as a thunderous 8 m wall of green water
racing at 200 km hr -1 , drowning all those who had taken to
the sea. An hour later, an even higher 11 m wave came
ashore at about half the speed of its predecessor. This was
followed by a succession of waves that so obliterated coastal
6.3
Chile, May 22, 1960
Of the world's entire coastline, the west coast of South
America is one of the most prone to recurrent large tsunami.
Seismicity is linked to subduction of the Nazca Plate beneath
the South American Plate. Earthquake epicenters tend to
cluster along the coastline or at the base of the Andes
 
Search WWH ::




Custom Search