MULTIPLE TRENCH INVESTIGATION ON THE EL PILAR FAULT ACROSS THE SURFACE BREAK OF THE CARIACO 1997 EARTHQUAKE, NORTHEASTERN VENEZUELA

After the Cariaco July 09th, 1997 Ms 6.8 earthquake in northeastern Venezuela, a multiple paleoseimic trench assessment was undertaken on the newly ruptured portion of the dextral El Pilar fault. The surface rupture of that earthquake extended for 37 km from the seashore village of Villa Frontado to Río Casanay, along the onshore El Pilar fault section that runs between the gulfs of Cariaco and Paria (Sucre state). This investigation –among many others carried out aiming at better characterizing the main event and its surface rupture, the earthquake sequence itself and the associated site and induced effects-, intends to bring additional light on the past seismic history of that fault. For this, three backhoe-dug trenches were excavated between the towns of Cariaco and Río Casanay in early 1998, at the localities of Las Manoas, Carrizal de La Cruz and Guarapiche. This was complemented by the evaluation of an outcrop already in existence in a house backyard in Terranova (7 km west of Cariaco). The three trench sites exhibit very different sedimentary settings. Las Manoas site is an active papaya-cultivated sag-pond. The Carrizal de La Cruz site is an active alluvial terrace, slightly down-faulted with scarp facing the relief (against runoff). The third trench was cut at the foot of the northern slope of a pop-up, forming at a restraining overlap (as attested by the Cariaco 1997 rupture mapping). All trenches are about 20 m long and 3-4 m deep. The main outcomes of this assessment are: (1) over 10 earthquakes are common to the three trenches over a period of 5.6 ka, no matter what interpretation is made; (2) the latest five events, including the 1997 event, clearly seem to recur roughly every 300 years at least; (3) a minimum of 15 or 16 events can be deduced from colluvial wedges (stone lines) interfingered within the sag pond sequence at Las Manoas trench, averaging a repeat period of 350 years over the longer 5.6-ka timespan; (4) the predecessor of the 1997 event was the 1684 earthquake, whose chronicles could not allow before a more precise determination; and (5) it can not be ruled out that evidence of the 1974 event are present in the two westernmost trenches, but plastic-bag pieces in a filled open cracks at the Guarapiche trench do confirm that the surface rupture was partly surrounding, on the north at least, the Guarapiche push-up.