The 19 January 1995 Tauramena (Colombia) earthquake: geometry and stress regime

Abstract The Tauramena (Colombia) earthquake, M w =6.5, occurred on January 19, 1995, in the Andean Eastern Cordillera foothill region, the so-called Piedemonte Llanero. The Harvard CMT focal mechanism indicates an almost pure reverse fault rupture. There was no surface faulting associated with this earthquake. This event was located at the northern tip of a zone, about 90 km in length, with relatively low microseismic activity along the central segment of the Piedemonte Llanero in Colombia. A field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity. More than 800 events were recorded during this period. A subset of the best located aftershocks (319 events) shows epicenters extended over an area of 800 km 2 , and suggests two antithetic planes on which most of the activity was concentrated. The main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the Colombian National Seismic Network and were relocated. They show an epicentral distribution similar to that of the subsequent events. Based on geologic information, aftershock locations, and focal mechanisms, we built a model for the Tauramena earthquake. Our results indicate that the Tauramena earthquake was produced by reverse faulting along a steep-dipping plane (dip ∼50°NW) associated with the Guaicaramo System. The fault plane cuts through basement rocks and folded Mesozoic and Cenozoic sedimentary rocks. Distribution of aftershocks in depth, suggesting the presence of splay reverse faults and a backthrust, is consistent with the hypothesis that the Guaicaramo Fault System was an old Mesozoic normal fault, reactivated during the Andean compression as a reverse fault. Stress tensor inversion of P-wave first motion polarities was performed, and focal mechanisms for the best recorded aftershocks were calculated. We found a well-defined sub-horizontal principal compression axis oriented in the NW–SE direction. The predominant reverse faulting for the Tauramena earthquake and the stability of σ 1 suggest that the tectonics in the central segment of the Eastern Cordillera Frontal Fault System is dominated by a compressive regime orthogonal to the cordillera, the controlling processes being probably the Caribbean subduction and the Choco Block collision.