Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

and 18 cm. Although most of these chimneys lie
horizontally on the sea floor, about 220 chimneys have
been found protruding from muddy sediments in vertical
position. The higher chimneys concentrations are located
in the Cornide High. The fallen chimneys present a regular
spatial distribution in a NW‐SE direction. The basal
morphology of them shows common characteristics of an
angular breakage associated with flexo‐traction
processes, typical of slender structures. This character is
very significant because it prove that the chimneys were
broken at their base. This interpretation is contrasts with
the idea of some authors that propose the drop of the
chimneys due to topple by remobilization the sediments
inside of the chimneys with subsequent collapse (Díaz‐
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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del‐Río et al., 2003; Fernandez‐Puga, 2004). Moreover,
the preferential orientation of the fallen pieces points to
coseismic shake as the mechanisms driving the chimneys
rupture. During an earthquake chimneys may break, more
specifically by the vertical and horizontal acceleration of
the ground during the passage of seismic waves. The
horizontal component of the acceleration may be
responsible for part of the horizontal displacement. The
direction of the oscillation is parallel to the direction of
propagation of the seismic wave and produces strong
extensional stress at the bases of the chimneys. As a
result, the chimneys bend and fracture it along a
horizontal plane near its base and finally topple it.
Fig. 2: Bottom photographs of the carbonate chimneys taken a BENTHOS underwater camera along the crest of the Cornide High (T5), at
depths between 950 and 970 m. See Fig. 1B for location.
This work proposes the magnitude and location of the
fault that generated the earthquake that caused the
rupture of the carbonate chimneys located on the
Guadalquivir Diapiric Ridge and the Cadiz Contourite
Channel after they had been exhumed at the seafloor.
GEOLOGICAL SETTING
The Gulf of Cadiz is located in the southwestern part of
the Iberian Plate. This region straddles the E‐W trending
segment of the Eurasian‐Africa plate boundary that
extends from Azores to the Mediterranean Sea, between
the Gloria Fault and the western end of the Alpine
Mediterranean belt, the Gibraltar Arc. The diffuse nature
of this segment of the plate boundary is widely accepted
on the basis of the related seismicity that is characterized
by scattered shallow‐ and intermediate‐type earthquakes
(Buforn et al., 1995). Earthquake fault plane solutions
support the existence of a wide transpression zone
ascribed to the slow (2‐4 mm/year) oblique NW‐SE
convergence that initiated in the late Miocene. The plate
convergence is responsible for the reactivation of the
older rift faults, and a number of large, active, tectonic
structures have been detected along the continental
margin and within the oceanic domain (Zitellini et al.,
2001). The main faults that accommodate the NW‐SE
shortening in the Gulf of Cadiz are the Horseshoe Fault,
the Marquês de Pombal Fault, the Tagus Abyssal Plain
Fault and the Pereira de Sousa Fault on the western
margin (Zitellini et al., 1999; Terrinha et al., 2003; Gràcia
et al., 2003b; Zitellini et al., 2004; Cunha, 2006), and the
Guadalquivir Bank Fault on the southern margin (Gràcia et
al., 2003a). Since historical times a number of destructive
earthquakes/tsunamis has been reported to have
occurred in SW Iberia like the tsunami of 60‐63 B.C.
(Campos, 1991), which devastated the city of Cadiz, and
1531 and 1722 events that struck the coast of SW
Portugal. This area was also the source of the famous
1755 Lisbon Earthquake, the most terrifying cataclysm to
have occurred since historical times in Western Europe
with an estimated earthquake magnitude of 8.5‐8.7
(Martins and Mendes, 1990). The largest recent
earthquakes were in the Gorringe Bank (28 February
1969, Ms 7.9) and in the northern Gulf of Cadiz (15 March
1964, Ms 6.2).
MECHANICAL PROPERTIES OF CHIMNEYS
Several dredge hauls were also taken during the 2000 and
2001 Coornide de Saavedra cruises, mainly across the
Guadalquivir Diapiric Ridge and the Cadiz Contourite
Channel. Chimneys obtained from these sources have
been used to determinate mechanical properties of the
carbonate chimneys. Various laboratory tests have been
performed on the carbonate chimneys taken in the Gulf
of Cadiz in order to determine their mechanical