Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

148 PATA Days 2024 of the Highway CA10 road offset near Zacapa, where a sag pond called Laguneta Los Yajes, lies on the property of a melon packing plant. The goal of this study is to define the subsurface structure of the Motagua fault using Electrical Resistivity Tomography (ERT) and to help identify paleoseismic trench locations. M E T H O D S ERT data were collected along the western and central sections of the 1976 earthquake rupture of the Motagua fault. We used a Ground Tester Model 6472 configured in a Wenner array. In 2023, electrode positions were placed at 5 m, 10 m, and 20 m at La Laguna and Gualán. In 2024, we used an electrode spacing of 2 m, 4 m, 8 m, and 16 m at Estanzuela, and intervals of 5 m, 10 m, 20 m, and 40 m at La Laguna with the intention of obtaining data to deeper depths compared to the previous field season. Topographic data in 2023 were collected for electrode positions with an RTK GNSS system. In 2024, topography of the ERT lines is derived from LiDAR data. The data were processed using RES2DINV software to develop an inverse model with topographic corrections. R E S U LT S At La Laguna, a 230 m ERT line crosses a limestone gravel alluvial fan on the south, fluvial terraces through the middle of the valley, and ends on volcanic tuff to the north (Fig. 2). The line crossed a dirt road from 145 m to 150 m at this site. Two vertical discontinuities separate a 30-m-wide high resistivity area from low resistivity, which defines a fault zone (Fig. 3). The high resistivity zone may represent fault-bound bedrock sliver or a buried shutter ridge. Within the high resistivity along the upper portion is a low resistivity wedge that is about 4-m- wide that is likely filled with alluvium deposits. The 1976 earthquake rupture bounds the north side of this alluvium. Fig. 2: Drone photograph of the Motagua fault valley at La Laguna showing the location of the ERT line. View toward the west. Photo by F. Gomez.