Large-magnitude late Holocene seismic activity in the Pereira-Armenia region, Colombia

Abstract

The Pereira-Armenia region, located west of the Colombian Central Cordillera, is crosscut by the Romeral fault system, which consists of an active north-south- trending, left-lateral, strike-slip fault system with a secondary thrust component in the Eje Cafetero zone (4°N-5°N). The terrain where the Liceo Taller San Miguel high school sits-9 km south of Pereira-is draped with an ~2-m-thick layer of volcanic ash younger than 30 k.y. in age. This locality has been affected by both N40°E- and E-W-trending faults that correspond to thrust faults or folds and normal rightlateral, strike-slip faults, respectively, in the tectonic model for the zone. Two kinds of strong fi eld evidence for the E-W faults were found at a site named Canchas: (1) the 50°N tilt of the late Quaternary interbedded sequence of volcanic ash and three paleosols, and (2) a vertical fault throw of ~1.70 m affecting the sequence (layers). A normal vertical throw of ~0.65 m at Parqueadero stands as a proof of the activity of the N40°E-trending faults. This latter faulting does not correspond with the stress tensor proposed for this region, and thus this deformation could be interpreted as being a consequence of fl exural slip induced by a NE-SW-striking blind thrust, where reverse faulting along bedding at depth is seen as normal faulting at the surface. Measured offsets could have generated seismic events of at least Mw 6.6 for the NE-trending fault that affected the paleosols and volcanic ash sequence at 13,150 ± 310 14C yr B.P., and a seismic event of Mw 6.9 for the E-W-trending fault that affected the paleosols and volcanic ash sequence at 19,710 ± 830 14C yr B.P. These two recently identifi edfaults are now named the Tribunas (NE-SW) and the Cestillal (E-W) faults. Up to now, the fault and its seismogenic potential determinations in this region have been based solely on morphologic evidence. The maximum seismic magnitude estimated for this region ranged from Mw 6.2 to Mw 6.6 for seismic sources 35 km away from the site. Seismic magnitudes like the one calculated in this work (Mw 6.9) were previously estimated only for source-site distances greater than 50 km. This work provides fi eld evidence that leads to a better understanding of the seismic activity of this region in the last 30 k.y. and confi rms the occurrence of local Mw >6.5 seismic events in this region. Although volcanic ash drapes and eventually hides the geomorphic evidence of active deformation, it turns out to be a perfect chronometer of a fault's activity whenever the deformation is revealed, as in this case. After the Armenia event of 1999, it is imperative to examine the seismic hazard assessments of this region in terms of local crustal seismicity. © 2011 The Geological Society of America. All rights reserved.