Geoelectrical Imaging of a Major Active Fault and Implications for Seismic Hazard Assessment in the City of San Miguel Uspantan, Quiche, Guatemala (original) (raw)

A geophysical survey was carried out in the city of San Miguel Uspantan, Quiche, Guatemala, located along the seismically active Polochic fault which forms, with the Motagua Fault, the plate boundary between the N- American and Caribbean plates. During its history, the city of San Miguel Uspantan and its close region had experienced severely damaging earthquakes induced by the Polochic fault. The earthquakes generated by the fault during the last two centuries, although of moderate intensity, produced important damages. The importance of the destructions and the fact that even earthquakes of small magnitude can cause such important destructions indicate a possible site effect. Within the city, very little is known about the affected ground and the location and character of the investigated active fault as its surface expression was removed by human activity. For this reason, 2D Electrical Resistivity Tomographies (ERT) have been carried out in order to contribute to the geological knowledge of the recent tectonic structures by identifying and locating the active fault that traverses the city and by characterizing the thickness, nature, and physical proprieties of the ground on which the city was constructed. Firstly, geomorphologic observations and ERT profiles showed that the city was constructed on a Quaternary deposit of 20 m thickness with thixotropic properties that can be responsible for an amplification of seismic waves during an earthquake. Secondly, ERT profiles confirmed the presence of the fault within the city limits. The fault is expressed as a pronounced, near-vertical anomaly characterized by a sharp lateral resistivity contrast that not only allows interpreting the location of the fault zone to within a few meters, but also confirms that the fault was active until recent time. Finally, this study has significant implications for seismic hazard analysis in this tectonically active and populated region. The results of this work can help plan urban development and diminish the population exposure to natural hazards.