Abstract

The Puget Sound earthquake of 29 April 1965 occurred in an area that is part of the Circum-Pacific Seismic Belt, a region of current crustal instability and adjustment of which earthquakes are one expression. In Figure 1, the location (47.4 degrees N, 122.3 degrees W) of this quake's epicenter, as determined by Algermisson and Harding (1965), is shown in relation to all the other known epicenters in the area. It is described as having a Richter scale magnitude of 6.5 and a focal depth of 60 km. The strikes and dips of the two possible fault plane solutions are shown with conventional symbols. Unlike the San Andreas Fault of California, the thick layer of glacial deposits in the PugetFIG. 1. Earthquakes in the Puget Sound, Washington, area. FIG. 2. Bouguer Gravity Map of the United States in the epicentral area of the Puget Sound earthquake of 29 April 1965. Sound area tends to hide any surface evidence of fault zones with which the epicenters could be correlated. Nevertheless, it is of interest to note that the epicenters in Figure 1 show linear patterns which may mark active fault zones. One in particular, shown with a dashed line, is close to the 1965 epicenter and has a direction (N 35 degrees W) which lies half way between the strike directions of the two fault plane solutions. The direction of this line is also more or less parallel to the structural trend of the region.Figure 2 shows the location of these epicenters on a portion of the Bouguer Gravity Map of the United States (1964) for the Puget Sound area. The 1965 epicenter lies on the nose of a high trend from the west which drops off steeply to the northeast, into a deep trough-like negative feature centered approximately in Seattle. Although it has been known for decades that large gravity anomalies exist in the Puget Sound area, their relationship to the geology and tectonic features in the area has not been clear. In relatively recentFIG. 3. Variation of the vertical component of the earth's magnetic field in the epicentral area of the Puget Sound earthquake of 29 April 1965. work by the USGS, Stuart (1961) has interpreted the gravity variations in the area as being due to the combined effects of a mass deficiency in a sedimentary basin centered in Seattle and a mass excess in some thick slabs and blocks of basaltic rock intruded into the earth's section to the southwest of the basin. In a first approximation, he considers the basin to be a hemisphere, 10 to 20 km in radius, filled with relatively unconsolidated glacial and marine sediments. More recently, Danes (1965) has interpreted the Puget Sound structure, based mainly on surface geology and his own recently completed gravity survey. It should be noted that Danes' gravity map differs in some details from the USGS map reproduced in Figure 2. Danes attributes the high gravity trend, on whose nose the 1965 epicenter is located, as being due to an igneous horst. If the cause of the gravity variations in the epicentral area is the density contrast between partially consolidated basin sediments and basic igneous intrusive rock, there should be corresponding magnetic variations since the magnetic susceptibility of basaltic rock is several magnitudes greater than most sediments.To attempt a verification of this theory, magnetic surveys for the area were sought but without success. A magnetic survey was therefore carried out in the summer of 1965 with a McPhar Vertical Field Variometer, Model M500, which measures the difference in intensity of the vertical component of the earth's magnetic field between stations. Reading accuracy of this instrument is + or -10 gammas. Checks for magnetic field variation during the time of the survey were made by periodic base and subbase measurements. Local effects at stations were eliminated by taking several readings in the station area at points separated one or two hundred ft. In Figure 3 the results of this survey are shown. It can be seen that the 1965 epicenter, as in the gravity picture, occupies a position on or near the nose of a high magnetic trend from the west which drops off to the northeast into a trough passing through Seattle.To further verify the correlation between magnetic and gravity data in the epicentral area, a total intensity magnetic survey was carried out with an El Sec proton precession magnetometer along the line A-B shown in Figure 3. In Figure 4 the gravity and total intensity magnetic profiles along A-B are compared and the correlation between them in the epicentral area is clearly confirmed.The data indicates that a contact zone between large masses of basaltic rock of relatively high density and magnetic susceptibility and sedimentary rock of relatively low density and magneticFIG. 4. Gravity and total magnetic intensity profiles along line A - B in Figure 3. susceptibility underlies the 1965 epicenter, and that fault movements are closely associated with this zone. The large size and areal extent of the gravity and magnetic anomalies can reasonably be explained only by having this contact zone extend deep into the crust. This interpretation is in keeping with the vertical distribution of the earthquake foci whose depths range from less than 17 to 60 km.Extended and more detailed magnetic work in this area would provide a better background than exists at present, against which plausible interpretations of both magnetic and gravity data could be made. However, it is believed that, if geophysical work is to continue in this highly interesting area, the most conclusive results in terms of the structure and composition of the rocks at depth and their past tectonic history could be obtained from explosion seismology. It would be useful to establish first the depth to, and the attitude of, the Mohorovicic discontinuity in the area. From this datum other geological and geophysical interpretive work could proceed with some confidence. ACKNOWLEDGMENTSMy thanks are due to Georgia Institute of Technology, Atlanta, Georgia, and to Shannon and Wilson, Inc., Seattle, Washington, for the loan of magnetic survey equipment and to Mr. E. Mannery for his very able assistance in the field.

First Page Preview

First page PDF preview
You do not currently have access to this article.