The 2012 Magnitude 7.3 Earthquake Near San Agustín, Colombia

On September 30, 2012, at 16:31 local time, a magnitude 7.3 earthquake struck 11 km west-northwest of San Agustín in Colombia’s Huila Department. The event occurred at a focal depth of 170 km, classifying it as an intermediate-depth earthquake within the subducting slab.

Colombia lies along the convergent margin where the Nazca Plate subducts eastward beneath the South American Plate at rates of approximately 6–7 cm per year. This subduction generates the Andean orogen and produces both shallow crustal seismicity and deeper intraslab events. The 2012 earthquake originated within the Wadati-Benioff zone, where the descending oceanic lithosphere experiences phase changes and dehydration embrittlement that can trigger large ruptures at depths between 100 and 300 km.

Since 1 January 2000, this M7.3 event remains the strongest earthquake recorded within 0 km of its epicenter in the national catalog. No other events of comparable magnitude have occurred in the immediate vicinity during that period, underscoring the relative infrequency of great intraslab earthquakes directly beneath southern Huila.

Intermediate-depth earthquakes in this segment of the subduction zone typically produce moderate to strong ground shaking over wide areas but rarely generate surface rupture. Their energy is attenuated before reaching the surface, resulting in lower peak ground accelerations compared with shallow crustal events of similar magnitude. Historical records document similar deep events in the Colombian Andes, including several in the 20th century that caused regional intensity VI–VII shaking without widespread structural collapse.

The San Agustín region sits within the Eastern Cordillera transition zone, where active fault systems such as the Algeciras and Garzón faults accommodate part of the oblique convergence. Although these faults can produce shallow earthquakes, the 2012 rupture occurred well below the seismogenic crust, illustrating the dual seismic regimes present in the northern Andes.

Seismic monitoring by the Colombian Geological Survey and regional networks has improved since 2012, allowing better characterization of slab geometry and stress transfer. Updated slab models confirm that the 170 km depth corresponds to the interior of the subducted Nazca Plate, consistent with global observations of large intraslab events elsewhere along the Andean margin.

References

USGS Earthquake Catalog (event page for 2012-09-30 Colombia M7.3)
Colombian Geological Survey – National Seismological Network annual reports (2012–2023)
Global CMT Catalog – Centroid-moment-tensor solution for 2012 Colombia event