Data concerning the British Columbia earthquake of December 14, 1872 were reviewed by Weston Geophysical Research, Inc. with regard to epicentral intensity and epicentral location. These data consist of period and later descriptive accounts researched and transcribed by WPPSS, as well as data researched by Weston Geophysical Research, Inc. included in the PSAR for Units 1 & 4 (1973). Intensity values based on primary descriptive accounts and quotations or sum- maries are shown in Plates I through III of this report. A compilation of localities, documentary sources, and evaluated intensities are provided in Tables 1 and 2. Newspapers from 1872 to 1874 from which period accounts were compiled are shown on Plate IV. We have attempted at this time to present significant data without duplication or repetition of reports stemming from the same source. We have assembled data which control the epicentral location and intensity. These data are presented in concise form on the plates included with this report. In addition, the published epicentral region after Milne (1956) and earthquake epicenters are shown on the physiographic map, Figure 1. Earthquake epicenters are also shown on ERTS imagery, Figure 2.
We believe that these additional data disclosed by the WPPSS research provide significant information with respect to the 1872 earthquake, but because of the sparsity of popu- lation, especially literate population, in the region in 1872, they fall short of the amount and kind of data needed to draw an accurate isoseismal map for this earthquake. Although an isoseismal map was produced internally by Weston Geophysical as a working hypothesis during the PSAR preparation, such a map was produced to evaluate the reasonableness of the pub- lished epicenters for this earthquake (USGS, 1973; Milne, 1956). We believe, that even in the absence of an accurate epicentral map several lines of evidence can lead to conclusions with respect to epicentral intensity and location of this earthquake.
EPICENTRAL INTENSITY
Exclusive of landslides or soils effects, the largest
intensity, Plate I, reported is intensity VII (MM). How-
ever, there are broad areas into which higher epicentral
intensities could be assigned where there are no existing
reports. This leaves us with the problem of "upper bounding"
the intensity on the basis of other data. The limits of
perceptibility for this earthquake appear to be Eugene,
Oregon to the south, and an area north of Quesnel, British
Columbia; that is, an area approximately 620 miles long.
If we use this distance as a diameter, we compute a per-
ceptible area of 300,000 square miles. Allowing that the
population was sparse and that the area could be larger, we
have added 20% to the diameter and computed an area of about
435,000 square miles. Using Figure 3 (PSAR, Unit 1, Figure
2.5-42), this would correspond to an epicentral intensity of
VIII(MM).
Table 1 which accompanies this report, is a tabular
listing of localities for which descriptions of earthquake
effects have been made available to us; intensity values
which could be assigned and/or remarks are include in this
table. Documentation upon which the intensity evaluation is
based is presented in Appendix A.
Discussion of specific localities are presented in
Appendix B.
EPICENTRAL LOCATION
Isoseismal and Soils Failure and Data
Fixing the epicentral location is difficult because of
the sparsity of population and lack of reports to which a
definite intensity can be assigned. The reports of inten-
sity values not including soils failures are shown in
Plate I. Soils failures are shown separate by our Plate II,
since they require special attention when they are used to
assign intensities. The reason for this is that soils
failures can result from sustained ground motions of low
amplitude at substantial distances from the epicenter where
local conditions of instability exist; such conditions with
associated failures are not indicative of high intensity
values. This perhaps is better stated by Hawkins and
Crosson (1975):
"The presence of localized destruction in West Seattle
on compact Pleistocene materials and on sons of
Group 2 along the Duwamish and in Issaquah suggests that
factors other than surface soil effects are operative
in the region. Most areas of heavy damage along filled
river basins and adjacent to lakes are known to be
underlain by clay layers. Seed and Idriss (11) have
shown that in the Union Bay area of Seattle, motions in
the underlying glacial till are amplified three to ten
times in such clay layers. The lack of damage in
Issaquah in the 1949 quake and the significant damage
in the 1965 quake suggest that while the transmission
path and perhaps the source radiation pattern may be
important factors controlling the basic excitation of
the region local soil conditions are the dominant
factor (12)."