The Geysers reservoir is situated in a 60--80 km-wide right-lateral shear zone associated with the North American-Pacific plate boundary. A small fraction of the shear may be occurring as permanent aseismic creep, which is being converted to stick-slip movement in the Geysers reservoir due to steam production. Two mechanisms could be responsible for the induced seismicity: a large (>100 bars) increase in effective rock pressure; and an increase in the coefficient of friction. The first mechanism requires that the initial state of the reservoir be predominantly liquid-dominated. The second mechanism is a consequence of dehydration of the reservoir causing both the 'hardening' of clay and fault gouge, and the precipitation of silica.