Large teleseismic delays, exceeding 1 second, are found near Mount Hannah in the Clear Lake volcanic field and in the steam-production area at The Geysers. The delays are superimposed on a general delay field of about 0.5 second extending over the volcanic rocks and the steam reservoir. It is postulated that a magma chamber under the surface volcanic rocks with a core of severely molten rock beneath Mount Hannah and a highly fractured steam reservoir probably underlain by partially molten rock at The Geysers are responsible for the observed delays. Both zones extend to depths of 20 kilometers or more.

In September and October, 1972 the U. S. Geological Survey made an investigation of seismic noise associated with the known geothermal phenomena in Yellowstone National Park. Eighty‐four stations, each recording for at least 48 hours, were operated. All major geyser basins were covered by the experiment. L-shaped three‐element arrays, three‐component stations, and single vertical component stations were operated. Four eight‐element mobile arrays were operated to study propagation characteristics of the noise. Preliminary analysis of data shows that high noise levels are associated with all the major thermal areas in the park. An elongated band of high noise envelops Lower and Upper Geyser Basins; noise levels are high around Norris Basin, Mammoth Hot Springs, Sulphur Mountain, and Mud Volcano; and a strong noise field exists around Lower and Upper Falls of the Yellowstone River. The seismic waves generated by the waterfalls have very different spectral characteristics from the waves associated with geothermal activity. The geothermal noise is predominantly in the spectral band of 2–8 hz, whereas the waterfall noise is predominantly around 2 hz. A mobile array operated near Norris Basin showed coherent wave trains radiating from seismic sources in the basin. Seismic noise measured around 50 m from Old Faithful Geyser showed amplitude fluctuations that followed the eruption cycles of the geyser. A few minutes after each eruption, the noise level starts rising slowly in ramplike fashion. Twenty to thirty minutes before the next eruption, sharp bursts of noise activity occur with increasing rapidity and continue for a few minutes after the eruption. The predominant energy of seismic noise generated by Old Faithful is at frequencies well above 8 hz. We postulate that only such high frequency noise is generated by the surface activity of geysers and hot springs and that the lower frequency noise found in and around the geyser basins is generated by a deeper convection system associated with the geothermal activity.

The available studies on upper-mantle structure in North America can be broadly divided into two categories: delineation of one-dimensional models, that is, the determination of P-and S-velocities as a function of depth; and computation of two-and three-dimensional models to take into account lateral heterogeneities in structure. About 50 one-dimensional models based on traveltimes, synthetic seismograms, and surfacewave velocities are currently available for continental North America. The gross features of these models are sharp velocity increases at depths near 400 and 650 km in the upper mantle beneath the whole continent and the presence of a low-velocity layer in the uppermost part of the upper mantle in the western half of the continent. A few other velocity discontinuities and velocity-gradient changes have also been documented. The most important finding from the available studies is a quantitative confirmation of what was suspected even in the early 1950s, namely, that the upper-mantle structure, particularly the structure related to the low-velocity layer, is drastically different in the tectonically active Cordillera from the stable central and eastern shield of North America. In western North America, in general, the upper-mantle velocities are low, and the lowvelocity zone is well developed and occurs at shallow depths. On the other hand, in the central and eastern parts of the continent the upper-mantle velocities are higher than in the west, and a low-velocity layer-if present at all-tends to be deep and to have a smaller velocity contrast than in the west. Available data and modeling techniques are inadequate to unambiguously resolve spatial variation in the depths to the 400-km and 650-km discontinuities and the boundaries of the low-velocity layer in North America. Apart from the broad division of the one-dimensional models into tectonically active and shield structures, any further finer scale quantitative division of the models within each tectonic unit is not warranted by the available data. Qualitatively, however, it is clear that such finer scale differences do exist, particularly in the upper 250 to 300 km of the upper mantle.The laterally heterogeneous structure of the upper mantle in North America has been studied by three-dimensional modeling using teleseismic P-and S-wave residuals. Three-dimensional inversion of P-and S-wave residual data collected over a substantial part of the North American continent show the existence of long-wavelength heterogeneous structure extending throughout the upper mantle. In addition, shortwavelength lateral heterogeneities are revealed by regional investigations. These include heterogeneous velocity structures associated with: (

In June 1973, seismic noise measurements were made in Long Valley, California, as part of the U.S. Geological Survey's geothermal investigations. Spatial variation of the average noise power shows high levels of noise extending over most of the eastern half of the Long Valley caldera. Since the noise high is almost similar in extent to the soft sedimentary Owens River basin, it is possible that ground amplification of seismic waves is at least partially responsible for the noise anomaly. Two lines of evidence indicate that geothermal noise may be present in Long Valley. (1) Relative amplification of teleseismic waves over soft ground, with respect to a reference station on hard rock, is about 12 dB. The noise anomaly, however, is at least 12 dB higher than this value. It is therefore difficult to explain the anomaly by postulating ground amplification of regional noise, thus indicating that a noise source may be present in the area of the anomaly. At wave frequencies below 2 Hz, river and cattle noise do not contribute much to the anomaly. (2) Group velocities of seismic noise, measured by using arrays, are in general quite low except at a few stations along the southern edge of the noise anomaly. The wave azimuths in the low‐velocity areas show random propagation, whereas azimuths associated with the high‐velocity waves point to the area where surface geothermal phenomena are found. The high‐velocity waves also have frequencies below 2 Hz. If a noise source is present under the southern edge of the sedimentary basin, it could excite the basin much more than it does the hard ground directly above it and thus produce the observed noise anomaly.