EARTHQUAKE-INDUCED SEDIMENTATION EVENTS IN LACUSTRINE ENVIRONMENTS ALONG THE NORTH AMERICAN-CARIBBEAN PLATE BOUNDARY IN GUATEMALA

The Polochic-Motagua Fault System (PMFS) is a left-lateral transform fault system in central Guatemala and represents an onshore extension of the North American-Caribbean Plate Boundary. Despite its significance, the earthquake cycle along the PMFS remains poorly understood and documented. This study aims to shed light on the timing of recent earthquakes by investigating earthquake-induced sedimentation events (SEs) in lakes near the fault system. Thirty-four sediment gravity cores were collected during the Spring of 2023 from four lakes— Atitlán, Ayarza, Lachua, and Chichoj - all located near faults of the PMFS. We performed detailed physical and sedimentological analyses, including magnetic susceptibility and gamma density scanning, core descriptions, facies analysis and lithostratigraphic correlation, and particle size analysis. In all four lakes, four distinct facies types are present: 1) Laminated mud, consisting of alternating dark and light laminae, interpreted as varves; 2) Turbidites, with a sharp erosive base and a fining upward sequence, interpreted as event deposits; 3) Homogeneous, massive beds of silty mud and sand, interpreted as homogenites; and 4) Bedded mud, interpreted as background sedimentation. Several facies are persistent within lakes, based on preliminary lithostratigraphic correlation between cored sites. The observed variability in thickness and facies presence likely represent spatial variations in sediment source and bathymetry. Analyses of the event deposits and their stratigraphic correlation suggest that they occur basin-wide, but unit thickness decreases with distance from inferred sediment sources. Preliminary results relying on varve counting from the mud-water interface in lakes Ayarza and Lachua indicate that the first SE occurs at an age of 73-45 years before the present, potentially representing an event deposited during the 1976 Motagua Earthquake. Constraining the timing of these events with ²¹⁰Pb, ¹³⁷Cs, and ¹⁴C will help establish a potential relationship between known historical and instrumental earthquakes and the event deposits and contribute to our understanding of lake sensitivity to earthquakes, furthering our knowledge of the earthquake cycle along the PMFS.