Paleoseismology is the study of the timing, location, and magnitude of prehistoric earthquakes
preserved in the geologic record. Knowledge of the pattern of earthquakes in a region and over long periods of time helps to understand
the long-term behavior of faults and seismic zones and is used to forecast the future likelihood of damaging earthquakes.
Paleoseismology is proving especially useful in regions where of the time between large earthquakes
is usually longer than the historical record. In western North America, paleoseismology usually focuses on faulting of near-surface
sediment whose age can be determined. In eastern North America, where near-surface faulting is uncommon or difficult to identify,
paleoseismology often employs liquefaction features to learn about prehistoric earthquakes. Earthquake-induced liquefaction features
are distinctive and form as the result of strong ground shaking. Liquefaction features include sand blows, dikes, and sills. Sand blows
are deposits that form on the ground surface as the result of venting of water and sand. Sand dikes are sediment-filled cracks through
which water and sand flowed. Sand sills usually take the form of lenses intruded below clay layers and are connected to sand
dikes. Most large earthquakes around the world have induced liquefaction.
Photograph of sand blow
deposit and related feeder dike exposed in excavation. Sand blow buries soil that was at ground surface at time of event. Sand dike
fills fissure that formed in soil. For scale, shovel blade is 20 cm wide. Photograph by Martitia Tuttle.
Photograph of large, 80-cm-wide sand dike exposed in cutbank of Hatchie River in western Tennessee.
Note clasts of mottled silt in sand dike. Weathering is especially pronounced in upper 15 cm of sand dike and extends downward for
another 70 cm, suggesting it is probably prehistoric in age. Hoe is 1 m long with each color band representing 25 cm. Photograph by
Photograph of sand dike and sill exposed in drainage ditch in southeastern Missouri. Sand
dike intruded weathered sand; sill emplaced below weathered clay. Layering within the dike and sill indicate that they
formed during two or more events. For scale, knife is 8 cm long. Photograph by Martitia Tuttle.