Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
PALEOSEISMOLOGY ALONG THE CARBONERAS FAULT: INTEGRATED
ONSHORE‐OFFSHORE EVIDENCE OF SEISMOGENIC ACTIVITY
X. Moreno (1,2), E. Gràcia (1), E. Masana (2), Á. Rodés (2), R. Bartolomé (1) and R. Pallàs (2)
(1) Unitat de Tecnologia Marina‐CSIC, Centre Mediterrani d’Investigacions Marines i Ambientals. 08003‐Barcelona, SPAIN.
(2) Dept. de Geodinàmica i Geofísica, Facultat de Geologia, Universitat de Barcelona. 08028‐Barcelona, SPAIN. Email:
xmoreno@cmima.csic.es
Abstract: The aim of this integrated onshore‐offshore study is to establish the seismic potential of the Carboneras Fault (CF) (Eastern Betics).
Onshore, the paleoseismological study was performed along La Serrata segment through geomorphological, microtopographic, trenching and
dating analysis. Offshore, the study was carried out based on high‐resolution geophysical data, coring and dating analysis. Seismic profiles
show a large variability of transpressive structures along the fault zone coinciding with onshore structures. Both, onshore and offshore studies
show faulted Quaternary layers and mass movement deposits related to paleoearthquakes suggesting that CF is seismogenic. Analysis of
trench walls show evidence of a minimum of six events since the Mid Pleistocene, a mean recurrence period of 20 ka and constrain the time of
the last earthquake between 772 AD and 889 AD.
Key words: Eastern Betics, Alboran Sea, active tectonics, slow faults
INTRODUCTION
Major cities from southeastern Iberian Peninsula were
historically damaged and destroyed by catastrophic
earthquakes, i.e. Almería 1487 (VIII), 1522 (IX), 1659 (VIII),
1804 (VIII) (Bousquet, 1979; Benito et al., 2006). But the
accuracy of historical epicenters location is too low to
establish a precise relationship between individual faults
and earthquakes.
Despite historical events, the seismogenic behaviour of
southern Iberian Peninsula faults, with shallow (2 to 4 km)
(Stich et al., 2006) low‐to‐moderate magnitude
instrumental seismicity, is poorly understood (e.g.
Masana et al., 2005), leading to an underestimation of the
seismic hazard. Furthermore, the continuation of faults
offshore and the marine seismogenic sources are poorly
known. The Carboneras Fault, a large fault zone with neat
morphologic expression, is a good opportunity to study
the response of Quaternary marine and terrestrial
deposits to the recent tectonic activity in south Spain.
SEISMOTECTONIC AND GEOMORPHOLOGIC SETTING OF
THE REGION
SE Spain hosts the slow convergent plate boundary
between Africa and Iberia (4‐5 mm/yr) which is
characterized by a 400 km wide zone of diffuse seismicity
(Argus et al., 1989; DeMets et al., 1990). In the Eastern
Betics, shortening is accommodated by a left‐lateral
strike‐slip fault system referred to as the Eastern Betics
Shear Zone (EBSZ) (Bousquet, 1979; Sanz‐De‐Galdeano,
1990). The EBSZ fault system extends southwards in the
Alboran Sea, changing its name to Trans‐Alboran Shear
Zone and connecting with the north‐African NE‐SW faults
(De Larouzière et al., 1988)
91
The Carboneras Fault with a length of almost 50 km
onshore and more than 100 km offshore is one of the
longest structures of the EBSZ (Gràcia et al., 2006).
Despite scarce seismicity associated to this fault, its
geomorphology reveals recent activity, suggesting long
recurrence (10 4 years) behavior as found in other faults
along the EBSZ (Masana et al., 2004).
OBJECTIVE AND METHODS
We present results of an integrated onshore‐offshore
paleoseismic study which aims to establish the seismic
potential of the Carboneras Fault.
Onshore, after identifying La Serrata as the segment of
the Carboneras Fault with stronger evidence of
Quaternary tectonic activity by geomorphological and
microtopographic analyses, a detailed paleoseismological
study was carried out in 5 sites (17 trenches) along the
NW boudanry of the range. Materials involved in the
deformation have been dated with different methods
(U/Th, TL, 14 C and 10 Be) depending on the nature and age
estimation of the sediments.
The IMPULS marine geophysical survey (RV Hespérides)
was carried out with the aim to characterize the geometry
and deep structure of the fault offshore based on swath‐
bathymetry, high‐resolution multichannel seismics and
very‐high‐resolution sub‐bottom profiler TOPAS. In
addition, marine sediment cores were acquired during
IMPULS and CARBMED cruises (M69/1, RV Meteor) to
sample and date ( 14 C) characteristic reflectors observed in
the TOPAS profiles.
This study attempts to assign the first paleoseismological
parameters (geometry of the fault, slip‐rates, recurrence
periods, elapsed time since the last earthquake and
maximum magnitude) of the Carboneras Fault and