Paleoseismology - Atlas Introduction
Atlas of Reconnaissance Data from USGS Paleoseismic Studies of the Puget Sound Region, Washington
Introduction | Contacts | References | Site Map | Site Table- The Search for Prehistoric Earthquakes -
Knowledge of when, where, how big, and how often large earthquakes occur is crucial for evaluating the seismic hazards of a region. Paleoseismology(the study of the geologic record of past earthquakes) seeks to decipher the history of large earthquakes along individual faults and throughout a region over hundreds to tens of thousands of years. For earthquakes of the past several hundred to several thousand years, paleoseismic studies complement historical and instrumental records. For example, no large (m>7) earthquakes have been recorded historically in the Puget Sound region. However, paleoseismic studies show that a large earthquake about 1100 years ago suddenly uplifted the land more than 20 feet along a fault passing through Seattle.
Geologists are studying coastal marshes and other landforms along Puget Sound in their search for evidence of prehistoric earthquakes. Large earthquakes may produce changes in elevation of several meters near faults that slip during an earthquake. The relatively constant level of the sea in coastal areas provides a marker for recognizing prehistoric changes in relative sea level, such as shown by:
- sudden changes in fossil assemblages of coastal marshes - Coastal plants and microorganisms that live near the level of high tide are very sensitive to changes in salinity or the length of time they are submerged by tides. If exposed to a sudden change in elevation, these organisms will be replaced by others that can better tolerate the conditions at the new elevation. Fossils in the peat and other tidal sediments beneath coastal salt marshes record these changes. We are trying to uncover and interpret the history of these changes by systematically studying the fossil record of coastal marshes throughout Puget Sound.
- uplifted shorelines provide dramatic evidence of uplift produced by earthquakes
Other earthquake related features that record earthquakes and that are found in the Puget Sound region, are
- sandy sediment liquefied by strong shaking, and
- sediment deposited by surges of water(tsunami) generated by sudden movements of the ocean floor during an earthquake.
