Trench investigation along the Merida section of the Bocono fault (centralVenezuelan Andes), Venezuela

The Bocono fault is a major NE-SW-trending, dextral fault that extends for about 500 km along the backbone of the Venezuelan Andes. Several large hist orical earthquakes :in this region have been attributed to the Bocono fault , surd some of these have been recently associated with specific parts thro ugh paleoseismologic investigations. A new trench study has been performed, 60 km to the northeast of Merida ill the central Venezuelan Andes, where t he fault forms a releasing bend, comprising two conspicuous late Holocene f ault strands that are about 1 km apart. The southern and northern strands c arry about 70% and 30% (respectively) of the 7-10 mm/yr net slip rate measu red in this sector, which is based on a 40 vs. 85-100 m right-lateral offse t of the Late Pleistocene Los Zerpa moraines. A trench excavated on the nor thern strand of the fault (near Morros de los Hoyos, slightly northeast of Apartaderos) across a twin shutter ridge and related sag pond exposed two m ain fault zones cutting Late Pleistocene alluvial and Holocene peat deposit s. Each zone forms a shutter ridge with peat deposits ponded against the up lifted block. The paleoearthquake reconstruction derived from this trench a llows us to propose the occurrence of at least 6-8 earthquakes in the past 9000 yr, yielding a maximum average recurrence interval of about 1100-1500 yr. Based on the northern strands average slip rate (2.6 mm/yr), such an ea rthquake sequence should have accommodated about 23 m of slip since 9 ka, s uggesting that the maximum slip per event ranges between 3 and 4 m. No dire ct evidence for the large 1812 earthquake has been found in the trench, alt hough this earthquake may have ruptured this section of the fault. Further paleoseismic studies will investigate the possibility that this event occur red on the Bocono fault, but ruptured mainly its southern strand in this re gion. (C) 1999 Elsevier Science B.V. All rights reserved.