MEASURING OFFSETS OF THE MOTAGUA FAULT USING LIDAR DRONE SURVEYING NEAR RIO EL TAMBOR, GUATEMALA

The left-lateral Motagua fault, part of the boundary between the North American and Caribbean plates, produced a large (M 7.5) earthquake in 1976. River terraces along Rio El Tambor, a tributary of the Motagua River in central Guatemala, show cumulative, late Quaternary offsets along the Motagua Fault, which can help constrain the fault slip rate. Although these features have been previously studied, new LiDAR drone surveying allows unprecedented mapping and measurement of terrace offsets. In total, 4 LiDAR flights over an area of ~0.7 square kilometers produced a point cloud with a density of ~ 200 points/square meter (point spacing of approximately 7 cm). The LiDAR point cloud was filtered, and ground points were classified using PDAL (Point Data Abstraction Library). The resulting bare-earth point cloud was gridded to a 10 cm pixel digital elevation model (DEM). Shaded relief images and high-resolution contour lines facilitated mapping of terrace surfaces and the fault zone. The fault zone was characterized by a left stepping, left lateral pull apart basin. Five terrace surfaces were identified, which correspond with those identified in previous studies. Four offset terrace risers were measured in ArcGIS. These results, from oldest (uppermost) to youngest (lowermost): The offset between terraces 7 and 5 has a range of 55-65 m. The offset between terrace 5 and 4 has a range of 51-54 m. The third offset measured between terraces 4 and 3 is 30-31 m. The offset between terrace 3 and 2 has a range between 24-25 m. Two estimates of vertical slip measurements were made using the elevation profiles across the fault scarp. Overall, the ratio of the vertical is 2-4 % of the strike-slip at the fault. Future work efforts to date the river terraces will allow calculating better constrained slip rates using these offset measurements.