Abnormal P-Wave Delays in The Geysers—Clear Lake Geothermal Area, California

Abstract

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.

The Yellowstone “hot spot” track results from migrating basin-range extension

“…Christiansen et al (2002) reprocessed the data, finding strengths of typically -4-5% throughout the main low-V P body ( Figure 2). They confirmed the results of Iyer et al (1981b), that the anomaly terminates at ~ 200 km depth, ruling out deeper bodies with V P anomalies stronger than about -1% and dimensions comparable to the shallower anomaly. At greater depth, immediately beneath Yellowstone, high-V P was imaged at 300-400 km.…”

“…Teleseismic tomography to study the mantle beneath Yellowstone was pioneered in the 1970s by Iyer et al (1981b) who deployed a 57-station network of vertical short-period seismometers over an area of 430 x 250 km. That experiment detected a low-V P anomaly in the upper crust, decreasing in strength in the lower crust and upper mantle.…”

Research in the Geysers-Clear Lake geothermal area, Northern California

“…Such a velocity contrast appears reasonable, based on observations at Long Valley (Steeples and [yer, 1976a, b) and Yellowstone (Iyer and Stewart, 1977;Iyer, 1979) , for a crustal magma chamber. However, in order to explain the large delays in excess of l s at GBO (over the production zone) and at GGL and sur-.…”

“…Kubota and Berg (1967) (Iyer, 1979). Steeples andIyer (1976a, 1976b) observations of large delays in the vicinity of the Geysers geothermal area.…”

Comparative assessment of five potential sites for hydrothermal-magma systems: energy transport

“…Bufe et al (in press)] suggest that the magma body consists of a molten chamber 14 km in diameter. The magma body is thought to be as shallow as 5 km [Crow (1979), Iyer et al (1979»). The two-phase hydrothermal zone beneath the Geysers has temperatures as high as 240°C and the zone extends to a depth of at least 3 kmas evidenced by numerous.…”