Tsunamiites and seismites

Article By:

Rodolfo, Kelvin S. Department of Earth and Environmental Sciences, University of Illinois, Chicago, Illinois.

Last reviewed:2003

DOI:https://doi.org/10.1036/1097-8542.YB030915

Records of some of nature's most catastrophically powerful, short-lived phenomena are preserved in sediments and ancient sedimentary rocks called tsunamiites and seismites, which are important not only for reconstructing ancient events but also for evaluating future hazards. Tsunamiites (also spelled tsunamites) are composed of predominantly marine or lake sediments that were transported by huge water waves called tsunami, which are set into motion by earthquakes, massive landslides, volcanic explosions, and asteroid impacts. Tsunamiites also include continental sediments that were transported when tsunamis suddenly dislodged offshore sediment and mixed it with water to form turbidity currents—dense, turbid masses that flow down the slopes of large lakes and oceans and then commonly travel for hundreds of kilometers over the deep sea floor before being deposited. A seismite is a sediment or sedimentary rock that contains textures and structures produced by earthquake shaking; unlike a tsunamiite, the sediments in seismites were already accumulating when the disturbance occurred. By comparing their pre- and postearthquake features, seismites can be used to calculate the timing, frequency, and local intensities of historic or prehistoric earthquakes, the orientation of their stresses, and even the basin slopes on which the sediments were accumulating. Various seismite features may even provide estimates of the magnitude of the earthquake that produced them, if the distance to the epicenter and the depth of focus are reasonably well known.

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