Survey of liquefaction structures induced by recent moderate earthquakes

Abstract

In April–May 1989, the eastern coastlands of Falcon State (Northwestern Venezuela) were affected by a seismic swarm of over 2000 weak shallow events located on the continental platform. Nevertheless two moderate earthquakes (mb 5.7 and 5.0) occurred during this swarm, [anicking inhabitants and producing scarce structural damage but frequent fissures, fractures and wall falls of house masonry in the mesoseismal area. The most relevant geological features observed in the mesoseismal area, as a consequence of these main events, are related to liquefaction of saturated sand-layers and are concentrated in the flood plains and delta of the Tocuyo river, located less than 20 km away from the epicentral area. Both shocks triggered liquefaction with ground acceleration values ranging between 0.1 and 0.3 g. The FUNVISIS Earth Sciences Workgroup carried out a prospect-pit survey across these features (sand boils and vent-fractures) to have a better understanding of this phenomenon and to evaluate the liquefaction potential of sand layers in the mesoseismal area. The liquefied material came to the surface from subsurficial silty-sand layers deposited in fluvio-deltaic environments and located between 0.2 and 4.0 m in depth. The pathways used by liquefied sands correspond to pre-existing crab burrows and soil fractures.