GRAVITY MODELING OF SUMAS VALLEY IN WASHINGTON STATE AND BRITISH COLUMBIA: BOUNDED BY ACTIVE FAULTS?

In northwestern Washington there are two lineaments, heading approximately northeast from Bellingham, known locally as the Vedder Mountain and Sumas Faults. The Vedder Mt. Fault is known to have been an active fault in the geologic past; there is no published evidence on the past activity of the Sumas Fault. There is a growing body of evidence suggesting that the faults have moved in the last 10,000 years, including Quaternary sediment in the basin and a change in course of the Nooksack River. In addition, there have been recent earthquakes in the vicinity of the potential faults. Although the surface traces of these faults are well expressed, the subsurface geometry is unknown. In order to try to determine whether the faults are active, more information about the faults in the subsurface is required.

Because gravitational acceleration is a product of the mass of two objects, various geologic units will produce slightly different gravitational accelerations. Measuring the gravity near the lineaments will help determine the subsurface structure of the valley. This survey consists of approximately 70 locations, along three transects perpendicular to the lineaments. Measurements were collected using a LaCoste & Romberg (G-127) gravimeter. Each measurement is corrected for free-air, drift, latitude, tide, and Bouguer corrections.

Three alternative hypotheses have been proposed regarding the formation of the valley: that it is bound by a pair of normal faults, that it is bound by thrust faults, or that it was formed glacially. Because the various hypotheses require different subsurface structures, they should produce slightly different gravity variations. Each hypothesis is modeled and compared to the observed gravity pattern. The results help determine whether the valley is consistent with other known active structures in the region.