Paleoseismicity studies on the Oca-Ancon fault system, northwestern Venezuela

The E-W-trending, right-lateral Oca-Ancon fault system extends eastward for 650 km, from the Colombian Caribbean coast near the city of Santa Marta to the town of Boca de Area (located on the eastern coast of Falcon State, northwestern Venezuela), across the Goajira peninsula, the outlet of Lake Maracaibo, the coastal plains of Buchivacoa and the central Falcon range. This system is very conspicuous west of the city of Maracaibo where its single trace sharply truncates the northern ends of the Santa Marta block and the Perija range. Fault-related landforms along the system trace (e.g., displaced beach strandlines at Sinamaica and fault scarplets in the Buchivacoa plains) suggest that it may have been active during Holocene times. A first trenching attempt carried out north of Maracaibo (at Sinamaica) in 1969 only confirmed its Holocene tectonic activity. On the western coastal plains of Falcon State and east of Lake Maracaibo, the system is less simple as it is composed of two parallel fault strands but they are clearly evidenced by Holocene paleoseismic scarplets. Recently, this system has been paleoseismically evaluated by means of two trenches placed across each of the two parallel branches known as the Oca and the Ancon faults. Dimensions of these bulldozer-dug trenches are 80 m long and 8 m deep. Various lines of evidence obtained from these exploratory trenches indicate that: (a) three surface rupture events, dated slightly prior to 7755 +/- 320, 6240 +/- 39 and 1945 +/- 630 yr B.P., have occurred along the Oca fault; (b) one surface rupture event has occurred within the past 3125 +/- 185 yr on the Ancon fault; (c) the Holocene slip rate of the system is close to 2 mm/yr; and (d) either of the faults can generate a maximum event of magnitude 7.4-7.5. Recurrence of such earthquakes on the Ancon fault is close to 1900 years while it is about 4300 years on the Oca fault.