Lacustrine sedimentary records as complements to trench paleoseismic investigations: An example along the Boconó fault, north-western Venezuela

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Beck, Christian۱; Carrillo, Eduardo۲; Audemard, Franck۳
Holding Date 29 September 2008

The southwestern section of the south-caribbean active transform boundary - partly represented by the (seismogenic) Boconó Fault - has been investigated as it crosscuts the sedimentary fill of several high altitude lakes developed after the L.G.M. From detailed investigations based on field surveys (Los Zerpa paleolake) and coring (Mucubají lake) we could extract a 16 kyr paleo-environmental archive. Detailed sedimentological investigations (textural parameters, mineralogy, organic matter) and AMS14C dating permitted to disentangle, at different scales, the influence of climatic modifications (precipitations and temperature), and the impact of seismic activity. Concerning the latter, we could separate major sedimentary changes related to direct co-seismic displacement across the lakes, from sedimentary disturbances (or specific deposits) associated to regional shaking and not necessarily to local displacement of the Boconó fault itself.
As the two active traces were investigated through trenching at less than 10 km from the lacustrine fills, we could compare both series of results. Three situations were found, taking into account the dating error intervals: common events, older events (Late Glacial and pre-LateGlacial only present in lacustrine archive, younger events (Holocene) only present in trench catalogues. Thus, we tentatively consider that this combination of surveys both reinforce and complete the trenches results, leading to a better assessment of local to regional seismic risk. Sedimentological observations better assess the co-seismic origin of a disturbance, and their dating may be more direct and precise (e.g. final settling of re-suspended organic particles).
These investigations are part of a wider general project funded through FONACIT, CNRS-INSU, and ECOS grants, and concerning natural hazards assessments (earthquakes and related phenomena, debris flows) along the southern Caribbean.