The Polochic and Motagua faults define the active plate boundary between
the North American and Caribbean plates in central Guatemala. A splay
of the Polochic Fault traverses the rapidly growing city of San Miguel
Uspantan that is periodically affected by destructive earthquakes.
This fault splay was located using a 2D electrical resistivity tomography
(ERT) survey that also characterized the fault damage zone and evaluated
the thickness and nature of recent deposits upon which most of the
city is built. ERT images show the fault as a similar to 50 m wide,
near-vertical low-resistivity anomaly, bounded within a few meters
by high resistivity anomalies. Forward modeling reproduces the key
aspects of the observed electrical resistivity data with remarkable
fidelity thus defining the overall location, geometry, and internal
structure of the fault zone as well as the affected lithologies.
Our results indicate that the city is constructed on a similar to
20 m thick surficial layer consisting of poorly consolidated, highly
porous, water-logged pumice. This soft layer is likely to amplify
seismic waves and to liquefy upon moderate to strong ground shaking.
The electrical conductivity as well as the major element chemistry
of the groundwater provides evidence to suggest that the local aquifer
might, at least in part, be fed by water rising along the fault.
Therefore, the potential threat posed by this fault splay may not
be limited to its seismic activity per se, but could be compounded
its potential propensity to enhance seismic site effects by injecting
water into the soft surficial sediments. The results of this study
provide the basis for a rigorous analysis of seismic hazard and sustainable
development of San Miguel Uspantan and illustrate the potential of
ERT surveying for paleoseismic studies.