Washington's largest historic crustal earthquake, in 1872, was probably near Entiat
A Dec. 2002 article suggests that Washington's largest historic crustal earthquake occurred near Entiat, and had a magnitude of around 6.8.
PDF Version of the Article - "The December 1872 Washington state earthquake", by William H. Bakun, Ralph A. Haugerud, Margaret G. Hopper, and Ruth S. Ludwin, Bulletin of the Seismological Society of America, Vol. 92, No. 8, pp. 3239-3258 (2002).
On the evening of December 14, 1872, severe earthquake shaking
was widely felt in Washington, Oregon, British Columbia,
Idaho, Montana, and Alberta. Dozens of communities reported severe shaking, but
no surface faulting was found. The 1872 earthquake was thought to have originated in the North Cascades, a rugged area inaccessible for most of the year. Reports of shaking came from more populous areas some distance away, making the location and size of the earthquake very hard to determine. Magnitude estimates have ranged as high as 7.4, and location estimates have spanned a wide area of the North Cascades.
The new magnitude and location were determined from reports of shaking intensities from communities throughout the northwest and an improved model of how
shaking intensities fall off with distance. Hundreds of trial magnitude/location combinations were modeled and compared to the observed shaking intensities.
The best fit was a magnitude 6.8 crustal earthquake near the southern end of
Lake Chelan.
Several other observations offer some support:
Following the 1872 earthquake, additional felt events - likely aftershocks - were reported in the "Winesap" area, near Entiat. Felt events continue to occur in the vicinity, including magnitude 5.0 quakes on Dec. 30, 1926 and August 6, 1959. Since 1975, more than 20 earthquakes larger than M 3.0 have been felt in the Entiat area. These are part of a persistant cluster of seismicity that has been continuously active in the Entiat area as long as the PNSN has recorded data there.
Placing the 1872 earthquake in the Entiat area helps to focus the search for candidate faults. The area lies on the margin between the Cascades and the Columbia River Plateau, and geologically young structures within the Plateau may include
thrust faults large enough to have produced the M 6.8
1872 North Cascades quake.
The 2002 study was authored by Bill Bakun, Ralph Haugerud, and Margaret Hopper of the US Geological Survey and Ruth Ludwin of the
University of Washington.
More Links about The 1872 Earthquake
This is a lengthy compilation of newspaper clippings related to the 1872 earthquake and its aftershocks. It is part of the "gray literature" of technical reports, which are published in limited quantities, and are often difficult or impossible to find out about or access.