Geoscience Reference
In-Depth Information
6
Great Earthquake-Generated Events
6.1
Introduction
Lisbon, a city of 275,000 inhabitants situated 13 km
upstream on the Tagus River, was heavily damaged by the
earthquake, and consumed by fires (Reid
1914
; Myles
1985
;
Pararas-Carayannis
2013
). As the fires spread throughout the
city, survivors moved down to the city's docks. Some even
boarded boats moored in the Tagus River. Between 40 and
60 min after the earthquake, the water withdrew from the
harbor, and a few minutes later one of the most devastating
tsunami in history occurred as a 15 m high wall of water
swept up the river, over the docks, and into the city (Fig.
6.3
).
Just as violently, the backwash dragged bodies and debris
back out into the estuary. Two other waves subsequently
rolled into the city an hour apart. The tsunami also caused
widespread destruction along the coastline of Portugal,
where it swept inland up to 2.5 km. At Porto Novo, north of
Lisbon, run-up was 20 m high, while at Alvor and Sagres on
the southwest tip of Portugal it reached 30 m above sea level.
The tsunami also caused widespread devastation in
southwest Spain and western Morocco, as well as crossing
the Atlantic Ocean and sweeping islands in the Caribbean
5,700 km away. In southwest Spain, the tsunami caused
damage to Cádiz and Huelva, and travelled up the Gua-
dalquivir River as far as Seville. At Cádiz, the wave had a
run-up of 2.5 m (Blanc
2011
). At Gibraltar, the sea rose
suddenly by about 2 m; however, the wave's height rapidly
decreased as it travelled into the Mediterranean Sea. The
Moroccan coast from Tangier to Agadir was severely
affected by a wave about 2.7 m high. The waves swept into
towns, killing many people. The tsunami wave also swept
up the west coast of Europe into the North Sea, where it
caused great disturbance to local shipping as boats in har-
bors were pulled from their moorings. At Kinsdale, Ireland,
the tsunami was 1.7 m high. Waves 2-3 m high moved
through the English Channel on a high tide. The third and
fourth waves were the largest. Oscillations in sea level with
periods ranging from 10 to 20 min occurred over the next
5 h in places. At Plymouth, the tsunami tore up muds and
sandbanks at an alarming rate. The tsunami moved across
The purpose of this chapter is to define the benchmark of
run-up heights and geomorphic evidence produced by the
greatest tsunamigenic earthquakes in recorded history. This
is what we know for certain about the most common cause
of tsunami. Against this benchmark, the evidence for sub-
marine landslides and comet/asteroid impact with the ocean
assessed. Unfortunately, the largest of these earth-
quake-induced events has occurred recently and brings
home the point that tsunami research really hasn't decreased
the threat or minimized loss of life.
6.2
Lisbon, November 1, 1755
At 9:40 AM on November 1, 1755, All Saints' Day, one of
the largest earthquakes ever documented devastated south-
ern Portugal and northwest Africa. Backward ray tracing
simulations using the type of incompressible, shallow-water
center on the continental shelf less than 100 km southwest
of Lisbon (Fig.
6.2
) (Baptista et al.
1996
). This location lies
close to the boundary of the Azores-Gibraltar Plate, which
historically has given rise to many tsunamigenic earth-
quakes in the region (Moreira
1993
). Tsunami have been
generated beforehand in this region in 218/216 BC, 210 BC,
209 BC, 60 BC, AD 382, AD 881, AD 1531, AD 1731. The
earthquake had an estimated surface wave magnitude, M
s
,
of 9.0, lasted for 10 min, and consisted of three severe jolts.
The earthquakes may have generated submarine landslides
that contributed to the subsequent tsunami. Heavy loss of
life resulted in Lisbon and the Moroccan towns of Fez and
Mequinez. Seismic waves were felt throughout Western
Europe over an area of 2.5 9 10
6
km
2
(Reid
1914
). Sei-
ching occurred in ponds, canals, and lakes as far north as
Scotland, Sweden, and Finland (Mitchell
1760
).
