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Abstract Paleoenvironmental records from a southern California coastal saltmarsh reveal evidence for repeated late Holocene coseismic subsidence events. Field analysis of sediment gouge cores established discrete lithostratigraphic units extend across the wetland. Detailed sediment analyses reveal abrupt changes in lithology, percent total organic matter, grain size, and magnetic susceptibility. Microfossil analyses indicate that predominantly freshwater deposits bury relic intertidal deposits at three distinct depths. Radiocarbon dating indicates that the three burial events occurred in the last 2000 calendar years. Two of the three events are contemporaneous with large-magnitude paleoearthquakes along the Newport-Inglewood/Rose Canyon fault system. From these data, we infer that during large magnitude earthquakes a step-over along the fault zone results in the vertical displacement of an approximately 5-km²area that is consistent with the footprint of an estuary identified in pre-development maps. These findings provide insight on the evolution of the saltmarsh, coseismic deformation and earthquake recurrence in a wide area of southern California, and sensitive habitat already threatened by eustatic sea level rise. 

Klong Thap Lamu, a large mangrove-fringed tidal channel along the northern Andaman Coast of Thailand, provides an ideal location to test the hypothesis that a paleotsunami record can be preserved in the sediments of a mangrove forest. The 2004 Indian Ocean tsunami destroyed local swaths of mangrove forest with highly variable widths — up to 300 m. Left in the wake of the tsunami is a thin mantling of laterally discontinuous sand, macerated shells, and localized coral rubble that is being mixed rapidly into the underlying mangrove peat. Transects across the channel's tsunami-modified shore show that the sand layer thins abruptly at the border of the undisturbed mangroves, suggesting that the energy of the wave dissipated quickly as it entered the forest. The distribution and sedimentology of the 2004 tsunami deposit (Unit tI) suggest that any paleotsunami deposit within this mangrove environment should be spatially restricted and thoroughly bioturbated. Sediment cores collected from within the 2004 tsunami zone penetrate a buried coral-shell peat unit (Unit tIII) that tapers inland. Unit tIII is strikingly similar to Unit tI, except for Unit tIII's diffuse sedimentology, which we attribute to extensive bioturbation. Unit tIII also cross-cuts an identified facies boundary that is traceable across the width of the 2004 tsunami zone. Rather than a facies boundary associated with the regional early-to-late Holocene sea level regression, stratigraphic correlations suggest that Unit tIII represents an event horizon (i.e. tsunami). AMS 14C dates on material from within Unit tIII combined with an upper bracketing age suggest that the tsunami event occurred sometime between 2720 and 4290 cy BP. If correct, this tsunami predates the 3–4 tsunami events recognized to the north at Koh Phra Thong. Unit tIII is, however, a potential far-field equivalent of a recently recognized paleotsunami deposit on the southwestern Indian coast ca. 3,710 years before present (Nair et al., 2010).