Identification of deep subaqueous co-seismic scarps through specific coeval sedimentation in Lesser Antilles: implication for seismic hazard

Abstract

Abstract. 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 finegrained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210 Pb and 137 Cs activities lack in the massive sands, while a normal profile of unsupported 210 Pb decrease is observed in the hemipelagite below, together with a 137 Cs 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 (M w = 6.3 and M w = 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 210 Pb 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 > M 6 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.

Estimation of successive coseismic vertical offsets using coeval sedimentary events – application to the southwestern limit of the Sea of Marmara's Central Basin (North Anatolian Fault)

“…As homogenite occurrences need the combination of gravity reworking (resuspension) and tsunami effect (especially in case of reflected tsunamis vs. seiche effect in closed basins), this process may also withdraw a bottom relief created simultaneously with the sedimentary reworking, such as the front of a landslide or a coseismic fault scarp. These direct rela- tionships (time and location) between a submarine coseismic scarp and synchronous specific layers have been proposed by Barnes and Pondard (2010) and Beck et al (2012), and have been used (1) to decipher coseismic rupturing from creeping, and (2) to estimate the vertical component of coseismic offsets. Concerning the Sea of Marmara's Central Basin, we have considered the pre-Holocene distribution of the different types of layers (Fig.…”

“…For the thickest ones (HmTu A, C, E, H, K) the homogeneous component accounts for about 90 % of the thickness increase in the deeper site. In the Lesser Antilles Arc, Beck et al (2012) described an active normal fault upon which the sea floor is maintained flat and horizontal, being each coseismic offset quite exactly compensated by a coeval silty-sandy homogenite ( Fig. 6 insert, case 2b).…”

“…-indirectly, (i) on the basis of correlations with reported seismic events (for historical seismicity) (e.g., Siegenthaler et al, 1987;Piper et al, 1992;Chapron et al, 1999;Goldfinger et al, 2007;Beck et al, 2012) and (ii) when detecting the same paleoevent in a large area independently from local setting (e.g., variable slope dip). This second approach is especially used for deep structures, as subduction, (e.g., Goldfinger et al, 2007;Gracia et al, 2010;Moernaut, 2011;Pouderoux et al, 2012) and also, in some cases, for surface-reaching major faults (e.g., Goldfinger et al, 2007);…”

“…This favorable setting recently allowed detailed analyses of fault activity (offsets, slip rates) through adjacent sedimentation (Carrillo et al, 2006(Carrillo et al, , 2008Bull et al, 2006;Barnes and Pondard, 2010;Beck et al, 2012). The here-presented work was dedicated to one of these cases: the deep part of the Sea of Marmara (northwestern Turkey) developed along the North Anatolian Fault.…”

“…As the here-used sedimentological tools and approaches have been previously published (Polonia et al, 2004;Sari and Çagatay, 2006;McHugh et al, 2006;Beck et al, , 2012Nakajima and Kanai, 2000;Uçarkuş, 2010;Drab et al, 2012;Eriş et al, 2012;Campos et al, 2013) only results (data and interpretations) implying paleoseismological aspects will be envisaged. Detailed sedimentological aspects may be consulted in the above-mentioned publications.…”