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

356 PATA Days 2024 The epicenter of the earthquake is located near the ancient Mayan site of Quiriguá, toward the eastern side of the Motagua Valley (Fig. 1), but the maximum surface displacement of 340 cm was measured at the western end of the fault rupture at La Laguna (Plafker, 1976, 1977, 1978). The average displacement was 108 cm, but significant afterslip slip was measured up to a year after the earthquake (Bucknam et al., 1978). In this study, we present preliminary data from ongoing research to investigate the tectonic geomorphology of the Motagua fault and potential aseismic slip using LiDAR data and to help define the subsurface structure of the fault zone and models of earthquake recurrence from geophysical and paleoseismic data. M E T H O D S This project began with the exceptional collaboration of the U.S.G.S. Menlo Park office in sharing the archival materials of George Plafker, who led a team to investigate the ground rupture and geologic effects of the 1976 Guatemalan earthquake starting just four days after the earthquake and again in April and October 1976 and October 1977. We digitized and georeferenced annotated topographic maps, 1:10.000 scale black-and-white aerial photographs, 35 mm slides, helicopter and ground photographs, and various notes and papers from the U.S.G.S. archive of Plafker (McEnaney and Niemi, 2021; Niemi et al., 2022). These data were used to exactly locate the 1976 earthquake rupture in the field in 2021 and subsequent years. Knowing the location Fig. 2: Left, Quadcopter; Right, Fixed-wing UAS. Both were equipped with LiDAR instrumentation. of the 1976 earthquake ground rupture helped us to define sites for further study. We collected high resolution LiDAR during two field campaigns. In January 2023, with a quadcopter drone at La Laguna, where the maximum coseismic slip was recorded, at Rio Tambor across a flight of offset terraces (Schwartz et al., 1979; Mendez et al., 2023), and at Gualán, where an offset concrete canal is still visible (Niemi et al., 2022, 2024). In January 2024, additional LiDAR data at higher altitude and over a larger area were collected using a fixed-wing drone at the previous sites and at Estanzuela, Quriguá, and Los Cerritos. Paleoseismic trenching has been conducted at Gualán at the Finca Los Limones site (McEnaney et al., 2022) and in Estanzuela at the melon plant site. Although we have preliminary data for six sites (Fig. 1), here we will describe data from just the Gualán site. The January 2023 LiDAR data for Gualán covered an area of ~400,000 m2 with five flights using an DJI Matrice 200 quadcopter flying at an altitude of ~50 m above the ground surface (Fig. 2). A tightly- coupled kinematic solution integrating PPK GNSS and a 9-DOF IMU was calculated using the TerraPos software processer, and this was used to align the laser returns into the point cloud with an average density of 170 points/m2. Noise and foliage were removed using the PDAL software to create a bare-ground point cloud. ArcGIS was used to convert the point cloud into a DEM with high-resolution contour lines. Furthermore, a microslope map was created to highlight the trace of the concrete canal that is built above the ground surface.