from Mineral Information Service (California Geology), July 1965, Vol. 18, No. 7.

The Seattle Earthquake of April 29, 1965

From May 5th through 8th, Deputy Chief Gordon B. Oakeshott visited Seattle, Washington to study the damage from the earthquake of April 29. His report to Resources Agency Administrator Hugo Fisher follows:

On April 29, 1965, at about 0830 Pacific Daylight Time, the Seattle-Tacoma- Olympia area suffered a moderately strong earthquake. The University of California has determined the Richter magnitude as about 6 1/2 to 6 3/4 and the possible epicenter as north and west of Gig Harbor, Washington.

Depth of focus was placed at 30-40 km., considerably deeper than most of California's very shallow-focus earthquakes (about 10-20 km.). The U.S. Coast and Geodetic Survey recorded a maximum acceleration of 0.204 gravity at Olympia and calculated the focal depth at greater than 30 km.

The University of Washington located the epicenter in the vicinity of Renton, although their sensitive seismograph did not obtain a complete record.

No aftershocks of the earthquake were felt.

Minor damage was widespread through Seattle to Everett on the north and to Olympia on the south and extended from a few miles east of Renton to almost as far west as the Hood Canal. Three persons were killed by falling debris, one in downtown Seattle and two on Harbor Island.

Most of the damage consisted of broken chimneys, cracked mortar between concrete blocks, fallen fluorescent light fixtures, and loosened brick facing.

Damage in Olympia-Tacoma was spotty. No damage was noted on the mud flats at Olympia, but the State Capitol dome was cracked and shear walls and columns were cracked in the Capitol building. Total damage is estimated at $800,000. The largest building damage was undoubtedly incurred by the Boeing aircraft plants; one at Renton and the other in southwestern Seattle. In these two plants, both located on natural mud flats and artificial fill, floors settled away from the foundation piling, much interior concrete block was cracked, fluorescent light fixtures were down, acoustical ceiling tile fell, and concrete tiles fell away from structural steel members.

Geology

General geology of the area is shown on the Washington State Division of Mines and Geology map, scale 1 inch equals 8 miles. Geology of the Seattle area is shown on U.S. Geological Survey Map 1-354, on a scale of l inch equals 1/2 mile, published in 1962. The accompanying map shows broad units of the geology, modified from the state map.

There are three broad geologic units in the area. The oldest, which crops out in very limited areas, consists of Early Tertiary folded sedimentary and volcanic rocks, locally called "bedrock", labeled "T" on the accompanying map. The second, which covers the greatest part of the entire region, consists of silt, clay, sand, and gravel deposited by the extensive continental glaciers of Quaternary age. The ice was in part of this area as recently as 13,000 years ago. Glacial till and outwash is as much as 2,000 feet thick. Major features of the land surface, both depositional and erosional, are the result of the glaciation. The glacial deposits are marked "Qg". The third and latest unit consists of artificial fill, alluvium of stream valleys, lake sediments and peat, and muds of the margins of Puget Sound, labeled "Qa".

The larger scale map by the U.S. Geological Survey gives a generalized description of the engineering properties of these units, including comments on such items as ground water, ease of excavation, foundation stability, slope stability, seismic stability and reported or possible use. It is interesting to note that seismic stability of the artificial fill and the alluvium is listed as "very poor. Maximum destruction during quake of April 1949 occurred on fill and alluvium." Seismic stability of all the other post-glacial materials ranges from "very poor" to "fair". On this map, seismic stability of all glacial units and the Early Tertiary rocks is shown as "good".

In general, the damage was directly related to geology. There were, however, certain puzzling exceptions to a simple damage pattern correlating with the geology: 1) in several areas of glacial sediments and "bedrock" there was chimney damage and other superficial building damage comparable to that in areas of fill and alluvium - in West Seattle, for example; 2) although no surface faulting was observed, such minor ground breaking as occurred was seen at Harbor Island, Port Orchard, on the Gig Harbor-Purdy road, and on the county highway between Allyn and Belfair; 3) all damage was light and was quite widely dispersed.

Geophysical work (a gravity survey) by the physics department of the University of Puget Sound disclosed two probable faults, buried beneath glacial sediments: one striking in a west-northwest direction through Renton and Bremerton, and the other parallel to it and passing through Gig Harbor and Allyn. Movement on one of these faults could explain the eastwest trend of minor ground breakage. The considerable depth of focus is probably a factor in the widespread, minor-damage pattern.

Conclusion

Rather unusual features of the Seattle earthquake, compared to most California earthquakes of similar magnitude, were: 1) widespread and spotty minor damage but no surface faulting; 2) rather deep focus; and 3) no aftershocks.

Every earthquake has unique or unusual features. Only by well-coordinated, immediate interdisciplinary study of every earthquake along the Pacific margin can we hope to gain that knowledge that will be most useful in minimizing damage and loss of life in future earthquakes.