INTRODUCTION

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)."

Aftershock Data

In addition to the intensity values (Plate I), and the soils failures (Plate II), the aftershock information from this earthquake have been assembled and are presented on Plate III and discussed in the following paragraphs.

There is an asymmetrical distribution both in time and frequency in the reporting localities. Aftershocks from reporting localities within the Puget Sound region are limited generally to within a week of the primary event. There are individual small earthquakes in the area during 1873 but they are not identified as aftershocks of the December 14 earthquake.

The distribution of reports of aftershocks which oc- curred in or subsequent to December 1872 is characterized by apparent longer duration and more events from Chelan, east- ward to Colville, Plate III. South and west of this region there are sufficient control points (reporting localities) to suggest that if the epicentral region had been more southerly than that selected by Milne (1956), numerous aftershocks would have been noted.

Yale, British Columbia is a controlling reporting locality for aftershocks west of the epicentral region; dispatches noting repeated shocks define a time extending through the latter part of December and possibly through February. The frequency is inferred to be greater than the Puget Sound region; it is less than the Chelan-Colville area. The distribution of aftershocks indicates a southerly constraint on the published epicentral area but a more easterly translation cannot be discounted.

Conclusion

We believe that the weight of the report's intensity, soils failures, and aftershocks, indicate that the most probable location of the epicenter for the 1872 earthquake is the original location shown by Milne (1953). However, on the basis of the existing data we also believe that the epicenter could be moved as far east as the western border of the Okanogan Highlands. This is based on an equal inten- sity attenuation east and west. Should the intensity atten- uation be more rapid to the west, then the epicenter would be in the location given by Milne.

If we assume that the intensity attenuation to the south is the same as to the north, the data (Plates I, II, and III) would indicate a location in British Columbia at or slightly north of the latitude given by Milne.

References