The anatomy of a strike-slip plate boundary fault in a pull-apart basin – The Motagua Fault in Guatemala

Off-fault deformation in surface-rupturing earthquakes can be detected using geodetical methods, but field evidence is rare. Here we present data from the North American-Caribbean Plate boundary, documenting off-fault deformation in the geological record in great detail.

The Motagua Fault in Guatemala is part of the plate boundary between the North American and Caribbean plates. It ruptured in a M7.5 earthquake in 1976, producing a 230 km-long surface rupture with an average slip of about 1 m. At the Estanzuela site, the fault-parallel, elongated topographic depression “Laguneta Los Yajes” is about 2 m lower than its surroundings as revealed by new airborne LiDAR data. It is interpreted as a pull-apart basin, either caused by a fault stepover or by a fault bend. Since it was seasonally filled with water, the surface rupture of the 1976 Earthquake could not be mapped precisely here. We trenched the northern topographic scarp of the depression to investigate the boundary fault but did not encounter a distinct major shear zone. Instead, we found distributed deformation manifested as fractures. Two additional trenches in the center of the depression found the main fault zone and additional structures that accommodate distributed shear. We interpret the fault geometry to be a fault bend rather than a stepover, and we document the evidence for off-fault deformation over 80 m around the main strand at this site. These data shed light on the anatomy of the plate boundary and its associated off-fault deformation.