Seismic monitoring of the Olkaria Geothermal area, Kenya Rift valley

Seismic monitoring of the Olkaria Geothermal area in the southern Rift Vall ey region of Kenya has been carried out since 1985. The initial purpose of this effort was to determine the background level of seismicity before full exploitation of the geothermal resource was started. This monitoring began with one seismic station. However, since May 1996, a seismic network compr ising six stations was operated and focused mainly on the East Production F ield. During the 5 months of network recording up to mid-September 1996, mo re than 360 local events originating within the Olkaria Geothermal area (T- s - T-p < 5 s) were recorded, out of which 123 were well-located. Also, 62 events were recorded at regional distances (5 s < T-s - T-p < 40 s), and 44 events at teleseismic distance (T-s - T-p > 40 s). During this period, the local microseismicity was found to be continuous with swarms occurring eve ry 4-5 days. Duration magnitudes based on the coda length did not exceed 3. 0. Preliminary spectral analysis shows three kinds of seismic signals, with only the first type displaying well-defined P- and S-phases. The seismicit y is mainly concentrated in the central area of the recording network, and the linear alignments in the epicenters are striking. A prominent alignment occurs along the Ololbutot fault zone extending from the northern end of t he greater Olkaria volcanic complex to the south near the southern terminus of Hell's gorge. Two other prominent alignments occur along NW-SE trends t hat coincide with fault zones which have been detected by geological and gr avity studies. Consequently, they are interpreted to be associated with flu id movement in the geothermal field. These preliminary results suggest that seismic monitoring will be useful to both monitor the field during product ion and to help site additional wells. (C) 2000 Elsevier Science B.V. All r ights reserved.