The Puget Sound is lined with a string of deep sediment-filled basins that will influence groundshaking during a large earthquake. Understanding the shapes of the basins is important to assessing the earthquake hazard potential of the region.

The map on the left (Click on any image to enlarge) shows variations in gravitational attraction in central Puget Sound. The big, bright blue "anomalies" are caused by low-density sediments that fill deep basins. The white-dotted lines are important crustal faults.

We used these anomalies to estimate the three-dimensional shapes of the basins. This technique works by separating observed gravity into two parts: that caused by the basins and that caused by volcanic rocks beneath the basins. The method "inverts" the first of these quantities to find a shape for the basin, one that also honors seismic data and deep well information.

The map on the left (Sediment Thickness) shows the estimated thickness of sedimentary rocks on top of volcanic basement. It is based on gravity measurements, seismic reflection data, and a few deep wells. Note the huge basins at Everett and Seattle. They are deep, isolated from each other, and have relatively steep margins. The basins are obviously related to important crustal faults (white-dotted lines).

The images at left and below show 3D views of the Seattle basin, the largest of the Puget Sound basins. The City of Seattle lies on its southern lip. It is approximately 60 km wide in the east-west direction, 30 km across in the north-south direction, and more than 9 km deep at its deepest point. The basin was formed by motions on various faults in the area, especially the Seattle fault that lies along its entire southern margin.

The maps above compare the shape of the Seattle basin with the seismic velocity of the sediments that fill it. We see, for example, a particularly deep hole in the middle of Seattle basin that we think might indicate the shape of the basin when it first began to form. One possibility is illustrated on the right-hand pair of maps: Perhaps two strike-slip faults existed during the early stages of the formation of Seattle basin, and a rectangular-shaped sub-basin developed where the two faults overlap.