Coseismic Fault Displacements on the Pedro Miguel Fault for Design of Borinquén Dam, Panamá Canal Expansion

Abstract eng:
As part of the seismic hazard assessment for the Panamá Canal Expansion Project’s design studies, we completed detailed paleoseismic investigations of the Pedro Miguel fault. Using tectonic geomorphic mapping, the previously assumed inactive fault was shown to right-laterally deform all fault-crossing streams. Paleoseismic trenching then demonstrated that the fault has experienced recurrent Holocene displacements. As such, the fault not only poses a shaking hazard to the Panamá Canal structures, it also crosses the proposed footprint of Borinquén Dam, a critical part of the Canal’s expansion program. Borinquén Dam is composed of four discrete segments, is nearly 5 km in length and 10-12 meters high, and forms the approach channel from Gatún Lake to the new Pacific locks. Our studies of the fault are based in great part on the excavation and logging of about 90 trenches, including at four locations where we excavated the fault in 3-D to determine earthquake recurrence, displacement magnitudes and fault slip kinematics. At multiple sites we were able to determine and measure the displacement of three surface-rupturing earthquakes in the past 1600 years. The minimum displacement from all three events was 8.1 m, with the last event a 2.8 to 3.0-m rupture across the ca. AD 1533 Camino de Cruces occurring on May 2, 1621, indicating that all three earthquakes were of similar rupture magnitude. This paper reports on the last 3-D paleoseismic investigation conducted directly within the Borinquén Dam footprint that measured a 3.1 +0.3/-0.1 m rightlateral offset of a gravel-filled channel thalweg during the most recent earthquake, the AD 1621 Panamá Viejo event. At the surface, the fault exploits weak, low-angle, west-dipping bedding planes of the La Boca Formation to rupture as a series of north-stepping, en-echelon, west-dipping fault petals that roll over near the surface to near horizontal. The challenge for the dam’s design was to correctly understand the location and geology of the fault crossing at the base of the dam’s foundation, and to model the fault rupture kinematics through the foundation’s geology and the dam’s embankments.

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