Coastal and nearshore sediments preserve a range of signals that geologists use to identify past tsunamis. Field investigators look for abrupt sand sheets that overlie organic soils or marsh peat, marine microfossils within otherwise terrestrial deposits, distinct sedimentary structures, and correlated evidence offshore. These features, when combined and dated, form a robust case for tsunami inundation.
Physical and sedimentary signatures
Sand layers that extend inland beyond the modern shoreline and contain marine microfossils such as foraminifera and diatoms are a primary indicator. Minoru Minoura University of Tokyo emphasized microfossil transport as a diagnostic marker for tsunami flow because such organisms come from offshore environments. Sedimentary structures within those sands — basal erosion surfaces, rip-up clasts, graded bedding, and poorly sorted coarse material — differ from the well-sorted deposits typical of storm waves. However, storms and river floods can produce overlapping signatures, so context is essential.
Corroborating offshore and geomorphic evidence
Submarine records provide independent corroboration. Christopher Goldfinger Oregon State University has shown that turbidites on the continental slope, generated by earthquake-triggered slope failure, often line up in time with coastal tsunami deposits. Onshore, sudden subsidence or buried tree stumps beneath sand sheets record permanent landscape change. Brian Atwater U.S. Geological Survey used buried marsh peat overlain by sand along the Pacific Northwest to link a widespread deposit to a large Cascadia tsunami, integrating the stratigraphy with Native oral histories to strengthen the interpretation.
Dating, geochemistry, and multidisciplinary confirmation
Absolute dating is critical. Radiocarbon dating of displaced peat, charcoal, or shells establishes the age of inundation layers, while optically stimulated luminescence dating can directly date buried sands. Geochemical markers such as elevated marine salts or trace elements and heavy minerals that match offshore sources add further weight. When multiple lines of evidence—sedimentology, microfossils, geochemistry, offshore correlation, and precise dating—converge, the interpretation of a tsunami deposit becomes strong.
Recognizing ancient tsunamis has direct human and environmental relevance: it refines seismic hazard estimates, informs community planning on coasts with cultural and archaeological resources, and reveals how past societies and ecosystems responded to catastrophic inundation. Reliable identification depends on rigorous fieldwork and interdisciplinary synthesis rather than any single diagnostic feature.