A seismic network comprising 18 stations was established to study the seismicity associated with the Olkaria geothermal field. During the period of network recording, 4765 local earthquakes originating within the study area (ts-tp6 km, Vp=6.4 km/s, and Vs=3.74 km/s. The magnitude threshold is Mc=1.4 and the observed b value is 1.05. Vp/Vs ratios were determined for the six individual fields composing the study area and were found to be low (1.58) in the east production field (EPF) and high (1.82) in the Olkaria central field (OCF). Hypocenter analysis show that the main geothermal heat source for the area is at a depth of at least 6 km. Poisson's ratio (&sgr;) is low (0.16) in the EPF and Olkaria west field (OWF) but high (0.26) in the central (OCF and Olkaria south field (OSF)) N-S zone parallel to the Ololbutot fault zone. The low &sgr; corresponds to a decrease in P wave velocity in areas with low reservoir pressure, high heat flow, and steam/gas saturation. Evaluation of &sgr; and Vp/Vs with regard to directly measured reservoir parameters such as pressure, temperature, and fluid chemistry give consistent and comparable results. This shows that &sgr; and Vp/Vs values are indicators of reservoir characteristics and tools for monitoring natural or induced changes. ¿ 2000 American Geophysical Union