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| Titel |
Identification of deep subaqueous co-seismic scarps through specific coeval sedimentation in Lesser Antilles: implication for seismic hazard |
| VerfasserIn |
C. Beck, J.-L. Reyss, F. Leclerc, E. Moreno, N. Feuillet, L. Barrier, F. Beauducel, G. Boudon, V. Clément, C. Deplus, N. Gallou, J.-F. Lebrun, A. Friant, A. Nercessian, M. Paterne, T. Pichot, C. Vidal |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Science ; 12, no. 5 ; Nr. 12, no. 5 (2012-05-29), S.1755-1767 |
| Datensatznummer |
250010837
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| Publikation (Nr.) |
 copernicus.org/nhess-12-1755-2012.pdf |
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| Zusammenfassung |
During the GWADASEIS cruise (Lesser Antilles volcanic arc, February–March
2009) a very high resolution (VHR) seismic-reflection survey was performed
in order to constrain Late Quaternary to Present faulting. The profiles we
obtained evidence frequent "ponding" of reworked sediments in the deepest
areas, similar to the deposition of Mediterranean "homogenites". These
bodies are acoustically transparent (few ms t.w.t. thick) and are often
deposited on the hanging walls of dominantly normal faults, at the base of
scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic
scarps between successive earthquakes, resulting in a flat and horizontal
sea floor through time. In a selected area (offshore Montserrat and Nevis
islands), piston coring (4 to 7 m long) was dedicated to a sedimentological
analysis of the most recent of these particular layers. It corresponds to
non-stratified homogenous calcareous silty sand (reworked calcareous
plankton and minor volcanoclastics). This layer can be up to 2 m thick,
and overlies fine-grained hemipelagites. The upper centimeters of the latter
represent the normal RedOx water/sediment interface. 210Pb and
137Cs activities lack in the massive sands, while a normal profile of
unsupported 210Pb decrease is observed in the hemipelagite below,
together with a 137Cs peak corresponding to the Atmospheric Nuclear
Experiments (1962). The RedOx level was thus capped by a recent
instantaneous major sedimentary event considered as post-1970 AD; candidate
seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one
(Mw = 6.3 and Mw = 7.4, respectively). This leads to consider that the syntectonic sedimentation
in this area is not continuous but results from accumulation of thick
homogenites deposited after the earthquakes (as observed in the following
weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of
such deposits suggests that, in the area of study, vertical throw likely
results from cumulated effects of separated earthquakes rather than from
aseismic creep. Examination of VHR profiles shows that all major co-seismic
offsets are recorded in the fault growth sequence and that co-seismic
offsets can be precisely estimated. By using a sedimentation rate deduced
from 210Pb decrease curve (0.5 mm yr−1) and taking into account minor
reworking events detected in cores, we show that the Redonda system may have
been responsible for five >M6 events during the last 34 000 yr.
The approach presented in this work differs from fault activity analyses
using displaced sets of isochronous surfaces and postulating co-seismic
offsets. Combining VHR seismic imagery and coring we can decipher co-seismic
vs. slow continuous displacement, and thus actually estimate the amplitude and
the time distribution of major co-seismic offsets. |
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