Seismic noise survey in Long Valley, California

Abstract

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.

Microseisms in geothermal exploration—studies in Grass Valley, Nevada

“…the surface waves, the shallow structure can provide a waveguide at particular frequencies corresponding to the maxima and minima in the group velocity dispersion curve. Similar effects have been reported for seismic noise, where amplitude and dominant frequency are characterized by the geology of the recording site (Kanai and Tanaka, 1961;Borcherdt, 1970;Iyer and Hitchcock, 1976).…”

“…Measurements near roads in the Waiotapu region show that Iyer and Hitchcock (1976) reported that At the power drops (small waterfalls) c along the canals, however, a distinctive noise component is generated i n a narrow frequency band around 2.5 Hz (Iyer, 1974). Iyer and Hitchcock -7-(1974) found t h a t i n Yellowstone National Park the waterfalls generate a narrow band seismic noise at 2 Hz whilch i s clearly different i n appearance from the noise generated by the hydrothermal process.…”

The hydrothermal system of Long Valley Caldera, California

“…A seismic-noise survey indicated the possible presence of geothermal noise beneath Long Valley (Iyer and Hitchcock, 1976). Microearthquakes were recorded for only a brief period in 1973 Pitt, 1973, 1976).…”

Repeat Measurements of Seismic Noise at the Waiotapu Geothermal Area, North Island, NZ

“…Hz (Liaw & McEvilly 1979, Iyer & Hitchcock 1976, Whiteford 1070,1975. The Occurrence of high levels of seismic noise in geothermal regions was first noticed during seismic surveys reflection and refraction surveys being undertaken to explore the geothermal regions.…”