The microtremors generated by pressure variations and microcracking of the hydrothermal system have been considered to be direct indicators of geothermal reservoirs. Despite the number of studies on this subject, it is still controversial. It is clear that a prime hypothesis has yet to be tested: that geothermal reservoirs generate a detectable seismic signal. Two passive seismic studies have been conducted in the Basin and Range province of the western United States, using state-of-the-art seismic data acquisition and processing techniques. The purpose of these studies was to investigate the correlation between microtremor activities and existing geothermal reservoirs.

A 256-element modified eight-arm geophone array was deployed to monitor microtremors at six sites in the vicinity of Roosevelt Hot Springs, Utah. Some 2 Hz coherent microtremors were detected, intermittently, in four recording sites; however, the directions of propagation did not indicate that the existing reservoir system was the common source region.

The microtremor data in Beowawe geothermal area, Nevada, were acquired by a 60-element cross array near the geyser. Low frequency (2 to 3 Hz) microtremors clearly appear on every record. Those microtremor activities are associated with the geothermal system at depth, as indicated by seismic body wave components propagating with extremely high apparent velocities.

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