|This event has, in the past, been referred to as the 1872 "North Cascades" earthquake. It
is the largest crustal earthquake known to have occurred
in Washington or Oregon in historic times.
Magnitude and location given here are from Bakun et al. [2002),
which concluded that it probably originated near Entiat (near 119.9 W longitude, 47.75 N latitude), and had a magnitude of around 6.8.
Dozens of communities reported severe shaking, but no surface faulting was found.
The 1872 earthquake was originally 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 preferred 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.
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).
The maximum intensity
is Milne's projection of the likely intensity in the epicentral area. Intensity values of VIII and IX (MM) were reported in the
Chelan/Entiat area by some later investigators. No primary faulting has been definitively connected with this earthquake. Prior
Estimates of the location of this earthquake vary from southern British Columbia, to the Entiat, Washington area. Much of the extensive
information available on this earthquake is in "grey literature", technical reports that are not included in most library catalogs.
The best information regarding the timing of the mainshock and its felt aftershocks at Olympia was provided by Capt. James S. Lawson of Olympia, whose observations were published in the Oregonian of 12/20/1872, and also elsewhere. The Oregonian says:
"... It was timed with a chronometer watch, absolutely correct, ... A shock occurred precisely at 9:40½ PM. It commenced with a light movement, gradually increasing for eighteen or twenty seconds. Then came the heavy shock, lasting four or five seconds; then it gradually decreased. In six minutes after the first shock there was another, followed by two others one minute apart. At 10:12 2/3 there was another shock, lasting four or five seconds; then it gradually decreased. In six minutes after the first shock there was another, followed by two others. During the night other shocks were reported, (I did not feel them) at 3 and 5 o'clock. On Sunday evening, at 6:37½, a light shock. December 16th at 9:17½ AM another light shock."
Some catalogs and newspapers give the date as 12/15 or 12/17, but it is my opinion (RSL) that these dates are misstatements, as it seems clear that the main event was on Saturday, December 14 at 9:40½ PM.
An extensive collection of articles and information on the North Cascades earthquake was made by Weston Geophysical Research, Inc. (1976; 'The 1872 earthquake; significant data and conclusions, prepared for United Engineers and Constructors, Inc.', unpaginated - /HIST_CAT/1872/Weston), and much or all of the same material is also included (in a rather different arrangement) in the "Report of the review panel on the December 14, 1872 earthquake" (1976) by Howard Coombs, W.G. Milne, Otto W. Nuttli,and David B Slemmons. This report has extensive appendices titled: a) landslide discussion, b) reports related to the 1872 earthquake, c) earthquake catalogs and isoseismals, and d) selected supporting information. Volume 2A of the Preliminary Safety Analysis Report for
WPPSS Nuclear Project No. 1, includes the Coombs et at. report (with a listing of sources for appendices b and d, rather than the entire articles), and a 1977 addendum to it. The volume also contains a "Review of North Cascade Earthquake of 14 December 1872" by Woodward-Clyde Consultants, with an iso/meizo-seismal map indicating several suggested locations. Other grey-literature resources are :
Bechtel, Inc. (1975) Investigation of the December 14, 1872 earthquake in the Pacific Northwest, Puget Sound Power and Light Company Skagit Nuclear Power Project, Preliminary Safety Analysis Report, Appendix 2J, 128 p.
Scott, N.H. (1976) Evaluation of the epicenter and Intensity of the Pacific Northwest earthquake of December 1872, Prepared for Bechtel, Inc., September, 1976, 24 p.
See /CATDAT/northcascades.html for extensive information on this event.