Analyzing the Relationship between Earthquakes and Geology: A Key to Understanding the Seismic Gap in Central Idaho. SADIE SPRAGUE (University of Idaho Moscow, ID 83843) STEVE REIDEL (Pacific Northwest National Laboratory, Richland, WA, 99352)

Since 1873 there have been 2,507 earthquakes in Idaho. Central Idaho is the site of most earthquakes. There is a seismic gap located between Borah Peak and the Idaho/Montana border. To understand the gap, one solution is to relate earthquakes to geology. Strong earthquakes may occur in igneous and metamorphic rocks. Earthquakes in these rocks may take place at great depth. Small magnitude earthquakes may occur in continental and marine rocks. Earthquakes in the continental rocks should occur near the surface. If there is a relationship between earthquakes and geology the seismic gap may be better understood. It may also explain why few earthquakes occur on the Snake River Plain and the Idaho Batholith. Earthquake data were found at the Digital Atlas of Idaho website before beginning the project. Knowing crust layer thickness, rock types in Idaho, and earthquake data, rock layer thicknesses were then found. The rock layer thicknesses were found online and then the layers were associated with rock units using the Idaho Geologic Map. The rock units were then assigned to the rock types on the geologic map. Knowing the rock type thickness allowed geology data to be analyzed with earthquake depth and magnitude. The result of the project is that there is a relationship between earthquakes and geology. This relationship allowed the seismic gap to be better understood. The rocks found in this area are marine rocks and they are located near the surface. With marine rocks near the surface there isn't enough pressure to cause them to fracture creating earthquakes.

Location and Trends of Ice Age Floods Erratics in the ALE Unit, Hanford Reach National Monument. RICK EDWARDS (Montana State University Bozeman, MT 59715) BRUCE BJORNSTAD (Pacific Northwest National Laboratory, Richland, WA, 99352)

The Ice Age Floods in Montana, Idaho, Washington and Oregon dramatically changed the landscape along their paths. Along with massive erosion, large quantities of ice rafted debris, or erratics, were deposited in slackwater areas along the paths of the floods. The study area is located on the northeast slope of Rattlesnake Mountain in south central Washington, an area relatively undisturbed by humans. The goal of the study is to map the erratics and determine from the data collected as much as possible with regard to water height and possible extrapolation of iceberg sizes. The ice rafted debris was mapped using a Global Positioning System unit. At the same time lithology, size, roundness, shape and degree of weathering were also determined and recorded. This data was entered into a mapping program and an Excel spreadsheet. With the data in a manageable format, trends were noted and hypotheses were generated as to water depth and why certain features were found in particular places. There were three types of erratic groupings found: singular erratics, erratic clusters with many rocks and negligible topographic relief and bergmounds with many rocks and up to several feet or more of topographic relief. A majority of the erratics were found on the up current side of arroyos on Rattlesnake Mountain. Erratics were found from 528 feet to 1177 feet above sea level (asl) with an average of 846 feet asl with a standard deviation of 135 feet. Erratic clusters found from 549 feet to 1063 feet asl with an average elevation of 937 feet and a standard deviation of 119 feet. The bergmounds were found between the elevations of 554 and 970 feet with an average elevation of 793 feet and a standard deviation of 116 feet asl. The lack of any ice rafted debris above 1177 feet asl indicates this elevation as the likely maximum elevation of the floodwaters. The size and extent of the feature found, erratic, erratic cluster or bergmound were controlled by the size, thus draft of the iceberg that deposited the debris. Single erratics were carried by all sizes of icebergs and thus were deposited more widely whereas erratic clusters and bergmounds were deposited from much larger icebergs, thus needed much deeper water, therefore are distributed over a lower range of elevations.