Chapter 3 - The geology and geophysics of The Geysers geothermal area Figure 3.1 Diagram showing the northward progression of Tertiary and Quaternary volcanism with time, the San Andreas fault and the extrapolated positions of the Mendocino Triple Junction between 3 and 5 Ma (from McLaughlin, 1981). 37

Chapter 3 - The geology and geophysics of The Geysers geothermal area Figure 3.2 Map of California showing major fault zones. Faults are shown as black lines (dotted where concealed). Arrows and numbers indicate direction and amount of motion of the Pacific and Gorda plates with respect to the North American plate (from Hill et al., 1990). Key: B = Banning; BP = Big Pine; BS = Bartlett Springs; BZ = Brawley seismic zone; C = Calaveras; CN = Concord; CU = Cucamonga; DV = Death Valley; E = Elsinore; FC = Furnace Creek; G = Garlock; GV = Green Valley; H = Hayward; HG = Hosgri; HRC = Healdsburg-Rodgers Creek; IM = Imperial; LVC = Long Valley Caldera; M = Maacama; M = Mission Creek; NI = Newport-Ingelwood; OT = Ortigalita; PM = Pinto Mountain; PV = Palos Verdes; PVA Panamint Valley; R = Rinconada; RC = Rose Canyon; SA = San Andreas; SC = San Clemente Island; SG = San Gregorio; SJ = San Janinto; SN = Sierra Nevada; SNA = Sur-Nacimiento; W = Whittier; WM = White Mountains; WW = White Wolf. 38

Figure 3.3 (A) Seismicity adjacent to the Mendocino Triple Junction. (B) Cross- section of seismicity from (A). The hypocentral pattern shows the subduction of the Gorda plate beneath the North American plate (from Walter, 1986). Chapter 3 - The geology and geophysics of The Geysers geothermal area 40

Figure 3.4 Locations of earthquakes with M  in California and western Nevada , and mapped Holocene faults (from Hill et al., 1990). Chapter 3 - The geology and geophysics of The Geysers geothermal area 41

Figure 3.5 (a) North-west to south-east cross-section showing earthquakes with M  1.5 adjacent to The Geysers (b) As (a) but for a south-west to north-east cross-section (from Hill et al., 1990). Key: M = Maacama, BS = Bartlett Springs, SA = San Andreas Chapter 3 - The geology and geophysics of The Geysers geothermal area 42

Figure 3.7 Structural model of The Geysers geothermal system (from McLaughlin, 1981). Chapter 3 - The geology and geophysics of The Geysers geothermal area 45

Figure 3.8 Major crustal features of northern California and their relation to the emplacement of magma beneath The Geysers (from McLaughlin, 1981). Chapter 3 - The geology and geophysics of The Geysers geothermal area 46

Figure 3.9 Schematic conceptual model of The Geysers geothermal system. HTR = high temperature reservoir. Open arrows represent the flow of steam. Closed arrows represent the flow of water (condensate and water) (from Trusdale et al., 1993). Chapter 3 - The geology and geophysics of The Geysers geothermal area 50

Chapter 3 - The geology and geophysics of The Geysers geothermal area Figure 3.10 (a) Map of The Geysers, showing residual gravity based on reduction densities of 2.67 g/cm 3. Contour interval = 2 mGal, (from Isherwood, 1975). (b) Map of The Geysers showing magnetic field continued upward to 3 km. Contour interval, 20 gamma (from Isherwood, 1975). The south-west to north-east line refers to the cross-section used for gravity and magnetic field modelling (Figure 3.11). 53

Figure 3.11 Gravity and magnetic field modelling for the cross-section shown on Figure (a) Calculated and observed gravity fields, along with the gravity field calculated for a spherical body centred at 13.5 km depth. (b) Calculated and observed magnetic fields along the same profile. (c) Crustal model from gravity and magnetic modelling (from Blakely & Stanley, 1993; Isherwood, 1975). Figure 3.12 Geoelectrical modelling for the cross-section shown on Figure The model was compiled from direct current soundings, bipole-dipole mapping data, time domain electromagnetic soundings and a two-dimensional magnetotelluric (MT) model. Numbers indicate resistivities (in ohm-m) of the units in the model. (A) and (B) represent alternative 2 ohm-m magma bodies added to test the sensitivity of the data (from Blakely & Stanley, 1993). Chapter 3 - The geology and geophysics of The Geysers geothermal area 54

Figure 3.15 Map of The Geysers production area showing the area operated by each developer in 1992 (from Barker 1992). The location of power generating units is indicated by the symbol with associated number. Information on the generating units is given in Table 3.3. Chapter 3 - The geology and geophysics of The Geysers geothermal area 65

Figure 3.17 As 3.14, but with the locations of power generating units operating during the year represented superimposed on the seismicity (from Ross, 1996). Chapter 3 - The geology and geophysics of The Geysers geothermal area 71

Figure 3.19 (a) Epicentres (dots) located by the UNT network (Section 4.1.1) between 11/1988 and 8/1989 with M d > 0.7. Open circles represent UNOCAL injection wells, with circle area proportional to the volume injected over the aforementioned period. Numbered squares are selected PG&E power plants. (b) Cross-section showing earthquakes located in the vicinity of injection well DX-61 September to December 1986 (from Stark, 1992). Chapter 3 - The geology and geophysics of The Geysers geothermal area 74

Figure 3.20 Plot of monthly average delivery rates for the south-east Geysers effluent pipe project ( Chapter 3 - The geology and geophysics of The Geysers geothermal area 75