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Abstract On 4 February 1976, a Mw 7.5 earthquake along the Motagua fault, Guatemala, ruptured ~230 km of the North American and Caribbean plate boundary. Today, the plate boundary remains poorly monitored, and the 1976 earthquake is still not fully understood. Here, we present seismic reflection profiles and radiometrically dated sediment core data from six lakes around the Motagua fault, together with reports of destruction and a quasi-dynamic rupture model, which show that the 1976 earthquake experienced strong directivity that impacted the distribution of shaking. The earthquake left behind a detailed record of event deposits (EDs) in five of the six study lakes. Thicker EDs are present in Lake Atitlán, near the terminus of the earthquake rupture, whereas thinner EDs were found in lakes off-axis of the rupture direction. We argue that EDs can be utilized to constrain asymmetrical distribution of shaking during earthquakes and that paleoseismic studies should consider directivity as a factor controlling the thickness of EDs. 

Coastal freshwater ecosystems are economically and ecologically important and provide multiple environmental services worldwide. They sequester carbon at rates ten times faster, and store five times more carbon per unit area than mature tropical forests. Vulnerability of these carbon sinks to marine inundation, however, is expected to increase in response to global sea-level rise (GSLR). To better understand the implications of future GSLR, we investigated the geochemical and biological consequences of episodic Holocene marine incursions into Lake Izabal, a large coastal freshwater ecosystem on the Caribbean coast of Central America. About 8,300 cal yr BP, marine incursion transformed Lake Izabal into a sulfur-rich anoxic waterbody, altered its biogeochemical cycles, eliminated several aquatic species, and reduced sediment organic carbon (OC) concentration by as much as to 90%. After that Early Holocene seawater incursion, it took almost 5,000 years for the lacustrine ecosystem to return to low-salinity status. And even when it did, the system did not fully recover to pre-inundation conditions. Some freshwater taxa failed to return, and sediment carbon content remained lower than pre-inundation values. A subsequent, but less intense marine incursion ca. 1,900 cal yr BP led to the formation of a sulfur-rich, hypoxic, brackish-water ecosystem that triggered a similar biodiversity loss and further sediment OC decline. These findings suggest that future marine incursions into coastal freshwater ecosystems, driven by ongoing GSLR, could have dramatic consequences, leading to losses of environmental services, including the ability of these systems to maintain high rates of blue carbon storage.