Title

Geophysical and Geological Investigation of Sediment Boils at Chilly Buttes, Idaho

Publication Date

4-2000

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

John R. Pelton

Abstract

The 28 October 1983 magnitude 7.3 (Ms) Borah Peak earthquake ruptured the Thousand Springs segment of the Lost River normal fault in east-central Idaho. This event is the largest historic earthquake in Idaho and caused significant hydrogeologic changes over an 18,000 km2 area, including spectacular groundwater eruptions at several locations. Adjacent to Chilly Buttes, in the epicentral area, at least 47 eruptive features, ranging in size from 0.6 m to 23.0 m in diameter, formed during the earthquake. The violent upward migration of ground water observed at Chilly Buttes most likely resulted from tectonic disturbance of the underlying limestone aquifer, rather than from compaction of the over- lying alluvium as in liquefaction events.

Joint data for the limestone bedrock were collected and eruptive features mapped as part of this thesis in order to better determine the correlation between jointing and coseismic groundwater eruptions. From these data, a 200-m wide NNW trending zone was delineated which could be associated with one of the two primary joint sets identified in the study area. Seismic reflection and refraction experiments were conducted across this zone to measure the depth to the upper surface of the limestone bedrock. These data delineate a northeastward sloping contact between the limestone and the overlying saturated sediments; the estimated maximum overburden thickness from the reflection data is less than 90 m beneath the zone of sediment boils. The fact that no boils developed in areas with greater depth to the limestone aquifer suggests that overburden thickness is one factor in localizing the formation of the sediment boils.

A ground penetrating radar (GPR) experiment was conducted to obtain an image of the subsurface structure of one sediment boil which had erupted during the 1983 earthquake. Following the GPR experiment, the sediment boil was trenched to visually examine its structure and establish any correlations with the radar images. The radar images and the trench logs give supporting evidence for the repetitive nature of sediment boil eruptions and the reuse of groundwater flow pathways, including indications of two pre-historic earthquakes. Furthermore, the data indicate that multiple downward curving reflections and disrupted and truncated bedding may be diagnostic of buried sediment boils in radargrams.

As a test, the radar profiles acquired in the primary study area were examined and a site was identified with characteristics indicative of a buried sediment boil. Trenches of the site revealed evidence for a paleo-sediment boil. Additionally, the excavation of the site of an anomalous point diffraction revealed a void. This void is evidence that one origin of eruptive sediments is their elutriation from subsurface deposits. Sediment removal is, in turn, a likely cause of areas of collapsed ground which are prevalent in the primary study area.

Since a specific combination of tectonic, sedimentary and hydraulic conditions were required for the formation of sediment boils at Chilly Buttes, the identification of other areas which have similar characteristics is possible. As small closed basins, sediment boils can fill in a relatively short period of time. Such rapid degradation may help preserve material which can be used to date major earthquakes. GPR is a useful tool in the search for ancient boils which no longer have surface expression, and thereby may have application in a program to determine earthquake recurrence intervals.

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