The recording of floods and earthquakes in Lake Chichój, Guatemala during the twentieth century

Abstract

Laguna Chichój (Lake Chichój) is the only deep permanent lake in the central highlands of Guatemala. The lake is located in the boundary zone between the North American and Caribbean plates. The lake has been struck by devastating earthquakes and tropical cyclones in historical times. We investigated the imprint of twentieth century extreme events on the sedimentary record of this tropical lake using a bathymetric survey of the lake, coring the lake floor, and providing a chronology of sediment accumulation. The lake occupies a series of circular depressions likely formed by the rapid dissolution of a buried body of gypsum. 210 Pb and 137 Cs inventories and varve counting indicate high rates of sedimentation (1-2 cm year -1 ). The annually layered sediment is interrupted by turbidites of two types: a darker-colored turbidite, enriched in lake-derived biogenic constituents, and interpreted as a seismite, and a lightercolored type, enriched in catchment-derived constituents, interpreted as a flood layer. Comparison of our 137 Cs-determined layer ages with a catalog of twentieth century earthquakes shows that an earthquake on the Motagua fault in 1976 generated a conspicuous darker-colored turbidite and slumped deposits in separate parts of the lake. The entire earthquake inventory further reveals that mass movements in the lake are triggered at Modified Mercalli Intensities higher than V. Tropical cyclonic depressions known to have affected the lake area had limited effect on the lake, including Hurricane Mitch in 1998. One storm however produced a significantly thicker flood layer in the 1940s. This storm is reportedly the only event to Electronic supplementary material The online version of this article (have generated widespread slope failures in the lake catchment. It is thus inferred that abundant landsliding provided large amounts of concentrated sediment to the lake, through hyperpycnal flows.

Guatemala paleoseismicity: from Late Classic Maya collapse to recent fault creep

“…1). They are encountered at shallow depth, and crop out most extensively near the site of trenching22, where they may therefore promote aseismic creep4041.…”

“…Cores were opened and photographed at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) in Duebendorf, Switzerland. Lake sediments are dominantly composed of silty clays, with dark- and light-colored laminae produced by the annual alternation of dry and wet seasons (true varves), interlayered with turbidites and mass-movement deposits22.…”

“…Flood-induced turbidites are light-colored and rich in pumice, magnetite, and quartz, whereas earthquake-induced turbidites (seismites) remobilize dominantly shallow lacustrine sediment which contains organic debris and biochemically precipitated carbonate22. Comparison of 20 th century seismites and of isoseismal maps of damaging 20 th century earthquakes shows that seismite generation in Lake Chichój occurs at ground-shaking Mercalli Modified Intensities (MMI) ≥ VI22.…”

“…The age model of the composite section is based on three independent datasets. (1) a 137 Cs- 210 Pb inventory performed on the topmost 70 cm of the section constrains the age of sediments deposited during the 20 th century22. (2) Varve counting down-core starting at turbidite A (1976 CE earthquake) provides precise age estimates down to ~1820 CE.…”

“…It has been stressed that the collective inventory of earthquakes along the Polochic and Motagua faults over the last millennium seems insufficient to accommodate the total left-lateral motion accumulated across the plate boundary over the period, suggesting a deficit of major seismic events20. Over the 20 th century however, the instrumental record of M < 5.5 earthquakes does not display a severe deficit212223. Direct geological observations of displacements across the plate boundary are thus far restricted to observations in 10–17 ky-old alluvium along the Polochic fault24.…”