Trench investigation on the main strand of the Boconó fault in its central section, at Mesa del Caballo, Mérida Andes, Venezuela

Abstract

The Mesa del Caballo trench assessment confirms the Holocene activity of the main strand of the Boconó fault at the Apartaderos pull-apart basin. Fifteen earthquakes, of which fourteen have been radiocarbon dated, have been recognized, spanning the last 20,500 yr. Recurrence intervals of these ≥7 magnitude events are variable. The dominant mode of recurrence is 400–450 yr, and the second one is 900 yr. Eventually some events are 1400–1800 yr apart. We suspect that our seismic record may be incomplete. This could be easily justified by several conditions: most of the earthquake recognitions is based on open-crack filling and they superpose spatially (eventually masking or destroying older fills), trenching may miss some events because the fault is made of en echelon Riedel shears, and a short return period may lead to faint differences between paleosoils few hundreds years of age apart. This trench also images an older activity of the fault, as evidenced by plentiful earthquake-triggered liquefaction features, as well as slumping and rotational sliding. By comparing paleoseismic results between the Morro de Los Hoyos and Mesa del Caballo trenches, it appears that both fault strands bounding the Apartaderos pull-apart basin move simultaneously. Besides, the main strand also coseismically slips twice in between those common events. In other words, the seismic scenario could be that the northern strand recurs every 1200–1350 yr while the southern does every 400– 450 yr. This is also in agreement with a respective slip share of 25 and 75% of the 9–10 mm/yr average slip of the Boconó fault in the Mérida Andes central sector.