Preliminary estimates of the strength of prehistoric shaking in the Columbia River valley and the southern half of coastal Washington, with emphasis for a Cascadia subduction zone earthquake about 300 years ago
Year: 1995
Type: report
Abstract: Liquefaction-induced features recently discovered in islands of the Columbia river show that seismic shaking accompanied coastal downdropping about 300 years ago.These findings agree with previous studies that attributed the downdropping to tectonism along the plate-boundary of the Cascadia subduction zone.A regional inventory of liquefaction-induced features has been used as a means to bracket the strength of prehistoric subduction earthquakes.The searched areas focused on islands in the Columbia River, and the banks of the Chehalis and Humptulips Rivers in western Washington.A preliminary geotechnical engineering analysis, made to assess the strength of shaking required to form the dikes in the islands, yields results consistent with theoretically and statistically derived accelerations for a M~8 earthquake whose energy source is located about 35 kilometers offshore.The liquefaction effects are evaluated by using the procedure of Seed et al., in terms of 0.5 probability estimated in a logistic regression.The method is not dependent on locating the most distal effects of liquefaction in the field, but rather requires testing at sites of marginal liquefaction.In the Chehalis River valley scattered dikes of earthquake-induced liquefaction origin were also discovered, but only at distances further than 60 km from the coast.Source beds commonly were sandy gravel, indicating strong shaking was required for their formation.Ages of the dikes are not well constrained, but most formed within the past two to three thousand years.Some of these dikes greatly predate the subduction event of about 300 years ago.Most or all of these dikes in the Chehalis River valley likely had a local seismic source not directly related to rupture of the plate boundary.Nearer the coast, in the Humptulips River valley, there is a virtual absence of dikes in sandy gravel that probably should liquefy during very strong shaking.The paucity of dikes very near the coast indicates that onshore seismic shaking could not have been very strong within the past few thousand years.Therefore, for any very large plate-boundary subduction earthquake of the past few thousand years, an offshore energy source of shaking is probable.If the magnitudes of subduction earthquakes within the past two thousand years much exceeded M 8, the accelerations seem to have been extraordinarily low or alternatively the locked zone was much further than 35 km offshore.The dominant vibration frequency may also have been very low.
DOI: 10.3133/ofr94589
Author: Stephen F. Obermeier
Language: English
Subfield: Geophysics
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W197250800