The 1978 Yellowstone Eastern Snake River Plain Seismic Profiling Experiment

In 1978 a major seismic profiling experiment was conducted in the Yellowstone-eastern Snake River Plain region of Idaho and Wyoming. Fifteen shots were recorded that provided coverage to distances of 300 km. In this paper, travel time and synthetic seismogram modeling was used to evaluate an average P wave velocity and apparent Q structure of the crust from two seismic profiles (reversed) across the Yellowstone National Park region. This area includes the well-known hydrothermal features of Yellowstone National Park (geysers, fumeroles, etc.), a large collapse caldera, and extensive silicic volcanism of Quaternary age?features attributed to shallow crustal sources of magma. The averaged crustal structure for this region as interpreted from the seismic data consists of (1) a highly variable, near-surface layer approximately 2 km thick with variable velocities of 3.0 to 4.8 km/s and a low apparent Q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill, (2) an upper crustal layer 3 to 4 km thick with variable velocities of 4.9 to 5.5 km/s and apparent Q of 50 to 200 that is thought to represent the accumulation of the Pleistocene-Quaternary rhyolite flows, ash flow tuffs, and possible Paleozoic and Precambrian metamorphic equivalents, (3) the crystalline, upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4.0 to 6.1 km/s, averaging 5 km thick with a Q of 300. This layer appears to be a cooling but still hot body of granitic composition beneath the Yellowstone caldera. It is thought to be a remnant of the magma chambers that produced the Quaternary silicic volcanic rocks of the Yellowstone Plateau and may still be a major contributor to the high heat flow, (4) a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6.5 km/s and apparent Q of 100 to 300, (5) a homogeneous 25-km-thick lower crust with a velocity of 6.7 to 6.8 km/s and an apparent Q of 300, and (6) a total crustal thickness of ?43 km. The upper crustal layer, 5.5 to 6.0 km/s, is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed Archean crust. An interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of Yellowstone. The high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation, partial melting, and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the Yellowstone Plateau. This seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern Snake River Plain to their present location beneath the Yellowstone hydrothermal system. While these findings do not bear directly on the origin and source of the heat, i.e., mantle lumes, lithospheric fractures, mantle radiogenetic heat, basal lithospheric shearing, etc., they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries.

Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 305. This field trip was conceived as a way to introduce one of the major volcano-tectonic features of the North American continent to visiting scientists from abroad. Its objectives are to allow the participants to observe first hand the geologic relationships relevant to the formation of the Snake River Plain (SRP) and to discuss various interpretations of SRP genesis. The approach to these objectives is to travel the length of the plain from northeast to southwest and to examine in a logical manner, from younger to older volcanic rocks, the relationships important to an understanding of its origin and evolution (Fig. 1). Even though basaltic volcanism is commonly thought of in association with the SRP, this field trip will emphasize the importance of silicic volcanism because of its much greater volume and because of its profound effect on the upper crustal structure of the SRP.

The subject of this NATO Advanced Study Institute was seismic monitoring under a nuclear test ban - an application of scienti fic knowledge and modern technology for a political purpose. The international political objective of a comprehensive nuclear test ban provided in turn the motivation for our technical and scientific discussions. In order to obtain a historical perspec tive on the progress of the work towards a comprehensive test-ban treaty (CTB), it is necessary to go back to 1958, when a confer ence of scientific experts in Geneva made the first steps toward an international seismic monitoring system. However, agreement on actual capabilities of a monitoring system for verifying compliance with such a treaty was not achieved, and thus the conference did not lead to immediate political results. After the Partial Test Ban Treaty of 1963, which banned nuclear explosions in the atmosphere, outer space and under the seas, renewed interest in the seismological verification of a CTB took place. A number of countries initiated large-scale research efforts toward detecting and identifying underground nuclear explosions, and it was in this context that the large aperture seismic arrays NORSAR and LASA were established. This type of development resulted in excellent seismic data in digital form and was thus of great irnprotance to the seismological com munity.

A review and evaluation of our knowledge of the structure of the crust and upper mantle of the continental United States, exclusive of Alaska, as determined from geophysical observations. Covers geophysical methods of studying the crust and upper mantle; a region-by-region review of crustal and upper-mantle structure; continental overviews based on the different geophysical methods; and geologic and petrologic syntheses based largely on the geophysical results.

For many centuries people living on volcanoes have known that the outset of seismic activity is often a forerunner of a volcanic eruption. This understand ing allowed people living close to the sites of the Mt. Nuovo 1538 eruption at Campi Flegrei, Italy, and of the Mt. Usu 1663 eruption, in Hokkaido, Japan (to quote only two examples) to flee before the eruptions started. During the second half of the 19th century seismographs were installed on some volcanoes, and the link between seismic and eruptive activity started to be assessed on a firmer scientific basis. The first systematic observations of the correlations existing between seismic activity and volcanic eruptions were probably those carried out at Mt. Vesuvius by Luigi Palmieri in 1856. Palmieri was the Director of Osservatorio Vesuviano and built an electromagnetic seismograph with the aim of "making visible the smallest ground motions by recording them on paper and indicating direction, intensity and duration". He was able to show the relationship between earthquakes and the different phases of volcanic activity. He identified the harmonic tremor which he indicated was a precursor of volcanic activity: "the characteristic feature of the ground mo tions preceding eruption is its continuity . . . (before the eruption of 1861) the electromagnetic seismograph began to show a continuous tremor". The Palmieri seismograph was also utilized in Japan until 1883, when it was replaced by the new Gray-Milne seismographs, and, later, by the Omori in struments.