PALEOSEISMIC EVIDENCE AT LICEO TALLER SAN MIGUEL, PEREIRA-ARMENIA REGION, COLOMBIA - SOUTH AMERICA

The Pereira-Armenia region -located in the west flank of the Colombia Cordillera Central- is crosscut by the Romeral fault system, which is an active LLSS fault with a secondary thrust component in the Eje Cafetero zone (4-5º N latitude). The ground were sits the Liceo Taller San Miguel, located 9 km south of Pereira, is carpeted by over 2 m-thick volcanic ash falls of less than 30 ka. This locality has been affected by both N40ºE and E-W trending faults that correspond respectively to thrust faults or folds and normal-RLSS faults in the tectonic model for the zone. Specifically, field evidence of two types on the E-W faults were found at "Canchas": (1) the 50ºN tilt of the Late Quaternary interbedded sequence of volcanic ash falls and 3 paleo-soils and (2) an about 1.70 m vertical throw affecting the sequence. The activity of the N40ºE trending faults is attested by a normal offset of about 0.65 m at "Parqueadero". This latter faulting does not comply with the stress tensor proposed for this region; thus this deformation could be interpreted as being the consequence of flexural slip induced by a NE-SW striking blind thrust, where reverse faulting along bedding at depth turns into normal faulting at surface.

Such measured offsets can have generated an event of at least Mw 6.6 for the NE trending fault; and at least Mw 6.9 for the E-W trending fault. These two recently-identified faults are now named Puerto Samaria Fault (NE-SW) and Cestillal Fault (E-W). Up to now, fault and its seismogenic potential determinations in this region had been based solely on morphologic evidence. Maximum seismic magnitude estimated for this region was between 6.2 and 6.6 for seismic sources at 35 km away. Seismic magnitudes like the one calculated in this work (Mw 6.9) used to be estimated for source-site distances greater than 50 km. This work brings field evidence that leads to a better understanding of the seismic activity of this region in the last 30 ka and confirms that local Mw 6.5 seismic events have occurred in this region. Although volcanic ash falls smoothens and eventually hides the geomorphic evidence of active deformation, it is a perfect chronometer of its activity when active deformation is revealed, like in this case. After the Armenia 1999 event, it has become a must to revise the seismic hazard studies in this region in terms of local crustal seismicity.

Financed: Banco Republica,COLCIENCIAS,INQUA