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Paleoseismology is the study of ancient Earthquake using geologic evidence, such as geologic Sediment and rocks. It is used to supplement Seismology to calculate seismic hazard. Paleoseismology is usually restricted to geologic regimes that have undergone continuous sediment creation for the last few thousand years, such as Swamp, Lake, river beds and shorelines.
Paleoseismic investigations are commonly performed through trenching studies. A trench is dug, and geologists record the attributes of the rock layers. On-fault evidence includes warping and disconformity, angular unconformity, fracturing, Fissure, and colluvial wedges. Off-fault evidence includes liquefaction of sand, tsunami deposits, turbidite, and marine terrace uplift.
Another example involves the megathrust earthquakes of the Pacific Northwest. It was thought that seismic hazard in the region was low because relatively few modern earthquakes have been recorded. However, paleoseismology studies showed evidence of extremely large earthquakes (the most recent being in 1700), along with historical tsunami records. In effect, paleoseismology found that the subduction zone under British Columbia, Washington, Oregon, and far northern California, is hazardous in the long term and can generating coastal tsunamis of several hundred feet in height at the coast. Periodically, a slip will occur, which causes the coastal portion to reduce in elevation and thrust toward the west, leading to tsunamis in the central and eastern north Pacific Ocean (with several hours of warning) and a reflux of water toward the coastal shore.
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