Abstract

We present new geologic, tectonic geomorphic, and geochronologic data on the slip rate, timing, and size of past surface ruptures for the right-lateral Limón and Pedro Miguel faults in central Panamá. These faults are part of a system of conjugate faults that accommodate the internal deformation of Panamá resulting from the ongoing collision of Central and South America. There have been at least three surface ruptures on the Limón fault in the past 950–1400 years, with the most recent during the past 365 years. Displacement in this young event is at least 1.2 m (based on trenching) and may be 1.6–2 m (based on small channel offsets). A well-preserved 4.2 m offset suggests that the penultimate event also sustained significant displacement. The Holocene slip rate has averaged about 6 mm/yr, based on a 30-m offset terrace riser incised into a 5-ka abandoned channel.

The Pedro Miguel fault has sustained three surface ruptures in the past 1600 years, the most recent being the 2 May 1621 earthquake that partially destroyed Panamá Viejo. At least 2.1 m of slip occurred in this event near the Canal, with geomorphic offsets suggesting 2.5–3 m. The historic Camino de Cruces is offset 2.8 m, indicating multimeter displacement over at least 20 km of fault length. Channel offsets of 100–400 m, together with a climate-induced incision model, suggest a Late Quaternary slip rate of about 5 mm/yr, which is consistent with the paleoseismic results. Comparison of the timing of surface ruptures between the Limón and Pedro Miguel faults suggests that large earthquakes may rupture both faults with 2–3 m of displacement for over 40 km, such as is likely in earthquakes in the M 7 range. Altogether, our observations indicate that the Limón and Pedro Miguel faults represent a significant seismic hazard to central Panamá and, specifically, to the Canal and Panamá City.

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