Attenuation of Earthquake Ground Motions
Investigators since the late 1960s recognized that the Snake River Plain subsurface conditions seem to dampen seismic waves. As part of the INL seismic hazards assessment, subsurface characterization, measurement of rock and sediment properties, modeling, and earthquake monitoring studies were conducted to understand the manner in which seismic waves are affected by the Snake River Plain.
Deep drill holes at INL reveal alternating layers of basalt and sediments. The basalt layers are lava flows from repeated volcanic eruptions occurring from 3.2 millions years ago to as recently as 2,100 years ago. During times in between volcanic eruptions, soils form and cover the basalt flows. The soils accumulate from wind blown dust, erosion of rocks due to precipitation in local drainages, and deposition of sediments from rivers (e.g., the Big Lost River) and occasional lakes (e.g., Lake Terreton).
The passage of seismic waves through alternating layers of hard basalt (higher seismic velocities) and loosely consolidated sediments (lower seismic velocities) scatter and dampen seismic energy. The net effect is to reduce earthquake ground motions by 15 to 25% of the motions expected for uniform rock (all basalt and no sediment layers). An independent review panel convened by the State of Idaho and U.S. Department of Energy in 1997 concluded that alternating layers of basalt and sediments was highly effective in damping ground motion. They also concluded that the geometries of the sedimentary layers did not cause focusing or amplification of ground motions.