Paleoseismic Investigation of the Western End of the 1976 Surface Rupture of the Motagua Fault, Guatemala

Angelika A. Jones, Tina Neimi, Christoph Grützner, Emma Pollice, Omar Flores Beltetón, Luis A. Romero, & Rupert Hauschild

Submitted August 30, 2026, SCEC Contribution #15251, 2026 SCEC Annual Meeting Poster #TBD

The Motagua Fault is a left-lateral, strike-slip fault along the North American-Caribbean plate boundary in Guatemala. It last slipped in the devastating 1976 Mw 7.5 earthquake with a 230 km-long surface rupture. Field surveys documented 142 cm of left-lateral displacement at the western end of the rupture at Finca La Concepción, near San Martín Jilotepeque. To investigate the earthquake history of this fault section, a paleoseismic trench was excavated across the 1976 rupture. Nine stratigraphic units were identified within the trench, separated by a major erosional unconformity between a lower sequence of floodplain and channel sediments (Units 6–9) and an upper sequence of channel, overbank, and soil deposits (Units 1–5). Unit ages were constrained by Bayesian modeling of 14C charcoal ages in OxCal, supplemented by Maya pottery sherds and modern cultural debris. Fault-related deformation is expressed by distributed, clay-lined fractures that deform the lower sequence, with several fractures extending upward into the upper deposits. Three paleoearthquake horizons were recognized based on upward-terminating fractures, displaced strata, and fissure fills. The oldest earthquake (Event 3) is broadly constrained to date to ~3 ka. The penultimate earthquake (Event 2) produced fractures and faults at the base of Unit 4 and dates to between 1678 and 1904 CE. Historical earthquakes that fall within this interval include those of 1717, 1751, and 1773. Historical macroseismic data suggest that the one of the major (~Mw 7.5) earthquakes that destroyed Antigua in 1773 is the most likely candidate for Event 2. The youngest earthquake (Event 1) occurred between 1682 and 2018 CE. Because fractures extend to the base of the modern soil, this event is interpreted as the 1976 earthquake. A nearly 4,000-year hiatus between the lower and upper sedimentary sequences indicates that a substantial portion of the earthquake record is absent at this site. The lack of significant lateral displacement preserved on fractures suggests that strain is distributed across multiple faults or a broader shear zone within the basin. These results indicate that the location of past surface ruptures at this site remains uncertain and raise new questions regarding westward rupture propagation and the long-term behavior of the Motagua Fault.

Key Words
Paleosiesmology

Citation
Jones, A. A., Neimi, T., Grützner, C., Pollice, E., Flores Beltetón, O., Romero, L. A., & Hauschild, R. (2026, 08). Paleoseismic Investigation of the Western End of the 1976 Surface Rupture of the Motagua Fault, Guatemala. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology