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INL seismic stations record about 2000 earthquakes a year in southeast Idaho with the exception of years when earthquake swarms or large magnitude earthquakes with aftershocks occur. For a typical year, earthquakes are magnitude 3.0 or less and are not felt. Earthquakes occur in this region in response to extensional stresses that are pulling apart the crust to form mountains and valleys or the Basin and Range. This process has occurred over millions of years., so repeat times for M 7-7.5 earthquakes are on the order of thousands of years. For example, the earthquake that occurred prior to the 1983 M 7.3 Borah Peak, Idaho earthquake occurred 5,000-7,000 years earlier.

Historical Earthquake Data

INL earthquake data have been combined with earthquake data from nearby seismic networks to produce a historical earthquake record. The historical earthquake record (1884-2001) shows that the eastern Snake River Plain is seismically quiet (aseismic) relative to the surrounding active Basin and Range province, with the exception of the 1905 M 5.7 Shoshone earthquake. Since the installation of INL’s seismic network in 1971, only 29 small magnitude microearthquakes (M < 1.5) have been detected within the eastern Snake River Plain. In contrast, thousands of earthquakes have occurred in the Basin and Range Province surrounding the eastern Snake River Plain. Two large historic earthquakes, the 1983 M 7.3 Borah Peak, Idaho earthquake and the 1959 magnitude M 7.5 Hebgen Lake, Montana earthquake were felt at INL but caused no damage.

The 11 November 1905 Shoshone earthquake occurred before there was instrumental monitoring in Idaho and, since its location was based on felt reports, it may have an error of 100 km or more. An evaluation of the intensity contours (or damage reports) for the 1905 earthquake and similarity with intensity contours for earthquakes occurring near the Idaho-Utah border in 1934 (M 6.6), 1962 (M 5.7), and 1975 (M 6.0) suggest the 1905 Shoshone earthquake originated outside the Snake River Plain. However, INL seismic design criteria include ground motion estimates from an earthquake similar in size to Shoshone occurring at the INL.

Locations of magnitude 5.0 and greater earthquakes occurring from 1884 to 2001.

INL earthquake catalog from 1972 to 2001 within a 100-mile radius of INL.

Seismic and Volcanic Hazards

Levels of Earthquake Ground Shaking

In the past 12 years, INL has supported efforts to estimate the levels of ground shaking that can be expected at INL facilities from all earthquake sources in the region. The estimates are in the form of levels of ground shaking that will not be exceeded in specified time periods (such as 500, 1000, 2500, 10,000 years). A probabilistic ground motion study for all facility areas was finalized in 2000. The method incorporated the possible ranges of seismologic and tectonic interpretations:

1. Earthquake source characteristics such as type of faulting, earthquake magnitude, location and depth.
2. Attenuation models or the manner in which seismic waves dissipate as they travel through the earth.
3. Subsurface geologic conditions beneath facilities, the manner in which seismic waves are affected by the near-surface sediment and basalt layers.

The INL ground motion evaluation incorporated results of all geologic, seismologic, and geophysical investigations conducted by many investigators since the 1960’s. Faults closest to INL facilities, the Lost River and Lemhi faults, were studied in detail to estimate their maximum earthquake magnitudes, distances to INL facilities, when the last earthquakes occurred, and how often they have occurred in the past. The results of these investigations indicate that these faults are capable of generating earthquakes of magnitude 7 to 7.2 and that the most recent earthquakes occurred more than 15,000 years ago.

Excavated trench across the Arco segment of the Lost River fault to determine the sizes and times of past earthquakes (courtesy of Susan Olig URS Corporation, Oakland, CA).

Investigators since the late 1960’s recognized that ESRP subsurface conditions seem to dampen seismic waves. Subsurface characterization, modeling, and earthquake monitoring studies were conducted to understand the manner in which seismic waves are affected by the ESRP basalt layers. The alternating sequence of basalt and sediment interbeds that comprise the ESRP subsurface dissipate (or attenuate) seismic energy to a greater extent than uniform basalt rock. The passage of seismic waves through alternating layers of hard basalt and loosely consolidated (soft) sediments scatter and dampen seismic energy, resulting in earthquake ground motions which are 15-25% lower than they would be for uniform basalt rock. In 1997, an independent scientific review panel convened by the State of Idaho concluded:

1. Sedimentary interbeds within the basalt are highly effective at damping earthquake ground motions.
2. The geometry of sedimentary interbeds do not cause focusing or amplification of ground motions.

The expected levels of earthquake ground motions determined by the INL probabilistic seismic hazards assessment are now part of the seismic design criteria for new and existing INL facilities. These criteria indicate that most facilities were built conservatively to provide safety to workers, the public, and the environment. Older facilities that do not meet current seismic design criteria are being decommissioned or structurally upgraded.

Volcanism

Dike-induced fissures in the Great Rift volcanic rift zone looking northwest towards Craters of the Moon National Monument.

Because the Yellowstone Hotspot is no longer present beneath the INL area, there is no threat of catastrophic rhyolitic volcanism such as at Yellowstone. An independent panel of volcanologists convened to address volcanic hazards at INL concluded that the main volcanic hazard at INL was inundation by basaltic lava flows. Investigations therefore focus on recurrence intervals (or repeat times) for lava flow eruptions, the possible locations of future volcanic vents with respect to INL facilities, and the distance and directions that lava flows travel on the eastern Snake River Plain near INL. This information is used to assess volcanic hazards for INL facilities.

Several INL seismic stations are located near or within identified volcanic rift zones to provide early warning in the event of potential volcanic eruption. The seismic stations enable detection of characteristic low-magnitude microearthquake swarms that accompany the upward movement of magma through the earth’s crust and advanced warning of potential renewed volcanism.

INL in cooperation with the Crater’s of the Moon National Monument have two seismic stations located in the Great Rift where basalt volcanism has occurred as recently as 2,000 years ago. The public can view current earthquake activity in southeastern Idaho and from around the world at the Crater’s of the Moon National Monument visitor’s center.

Aligned volcanic vents in the Lava Ridge-Hell’s Half Acre volcanic rift zone looking to the northwest.

Crustal Deformation

Continuous recording GPS (Global Positioning System) receivers are collocated at two INL seismic stations located in the eastern Snake River Plain. The crustal movement data are collected primarily in support of the Yellowstone Geodynamics project being conducted by the University of Utah and other collaborators. These data also provide quantitative data to assess seismic and volcanic hazards at INL. In the early 1990’s, INL provided to support to the National Geodetic Survey to establish geodetic base stations near and within the INL. These stations have also been reoccupied in support of the Yellowstone Geodynamics project.