Archeoseismological study of the AD 1621 “Panamá La Vieja” Earthquake: insight on the seismic source (Panamá, Central America)

As part of the revision of the seismic hazard investigations for the Panamá Canal Expansion Project in the Pacific new set of locks, we completed a detailed archeoseismological investigation on the existing ruins of the ancient Panamá La Vieja, which was affected by an earthquake of intensity ≥VIII in AD 1621 (6.9 Mw). Additionally, geomorphic, paleoseismic research together with the analysis of the historical and instrumental seismicity in central Panama allowed to develop different macroseismic scenarios (ShakeMaps) to check the suitability of the different proposed seismic sources in this zone of the isthmus where convergence rates are low (c. 0.7 – 0.8 mm/yr).

The archaeological site of Panamá La Vieja is the only place in which that event is truly documented by the historical report of the vicar Requejo Salcedo (earthquake witness), but also for the different earthquake archaeological effects (EAEs) preserved in the buildings of the present ruins. There were only eight stone buildings and about seventeen masonry buildings (convents, city jail, hospital, etc.) in the year 1621. The old cathedral was under construction then and the rest of the houses were wooden structures. At present, the convents of San Francisco, Sto. Domingo, La Compañia de Jesús, La Concepción and the old Hospital, display severe earthquake damage, the last three buildings practically collapsed. The measured EAEs are (a) penetrative and conjugate fractures in masonry walls; (b) tilted walls; (c) rotated and displaced masonry blocks; and (d) a large amount of dipping broken corners in stone blocks. The structural measures of the EAEs indicate a N10-20E regular orientation for ground movement, consistent with the offshore current seismic activity in the Pacific south of the city. There the NNW-SSE left-lateral Las Perlas Fault (LPF), responsible for two c. 5.0 Mw instrumental events (years 1971 and 2017), that struck the Panama City (c. 15 -20 km far away) with intensity VI MM. This scenario it is not consistent with other proposed seismic sources, such as the right-lateral Pedro Miguel Fault (PMF), cutting across the new set of locks of the Panamá Canal onshore. ShakeMaps (USGS methodology) elaborated to check the PMF and LPF seismic sources strongly suggests that the PMF 6.9 Mw earthquake solution do not explain the oriented damage recorded in the archaeological site. On the contrary, the offshore LPF solution only will need of a lower 6.0 – 6.5 Mw event to explain the destruction at the archaeological site with PGA values c. 0.4g (VIII MM). In addition, the LPF solution can account for the small tsunami flooding the littoral sector of the old city soon after the event described in the historical chronicle of Requejo Salcedo during the evening of 2 May1621. Recent research denies the Holocene and historical and activity of the PMF and our analyses strongly suggest that offshore faults (i.e. PLF) in the Gulf of Panama can be more suitable and realistic candidates than the PMF as the source of the 1621 earthquake.

Contribution supported by the Spanish Research Project I+D+i PID2021-123510OB-I00 (QTECIBERIA-USAL) funded by the MICINAEI/10.13039/501100011033/