Locating Seismic Events Under The Menengai Volcanic Field of The Kenyan Rift Based On Data From The Georthermal Development Company

Abstract

Seismic stations comprising of 3 accelerometers, 6 broadband and 5 short period seismometers were installed in the Menengai volcanic field to monitor the level of microseismicity over a one year period. This was aimed at identifying the seismic events mapping them and interpreting their cause. A total of 7241 seismic events depths up to 8.5 km below sea level were located and using Hypocentral algorithm then relocated using waveform cross correlation and double difference algorithm. The results of the location show significant epicentral clustering and hypocentral cluster on vertical structures aligned towards the Molo- Tectonovolcanic axis, Solai-Tectonovolcanic axis and in the caldera. The hypocentral distribution shows depths up to 8.5km depth in the Menengai caldera and shallower depths of about 2km in the northeastern part of the Menengai Volcano. Earthquake swarm activity with rates of more than 150 events per month was noted. These activities were high in the wet season and less in the dry season an indication of triggering by pore pressure diffusion within a heterogenous fault zone. The computed one-dimensional (1-D) velocity model used in relocating most of the earthquakes occurring at the center of the Menengai Volcano allowing one to constrain hypocentral locations effectively. Analysis of focal mechanism, showed a region predominantly of normal faulting with a north east trending strike consistent with the northeast-southwest low resistivity anomaly obtained in magneto telluric studies carried out for the Menengai Volcanic field. An indication of the existence of a geothermal system. Local stress conditions favored failure along fault zones of different orientation. A complex network of faults and fractures suggests that the tensional strain oblique to the primary rift axis is still occurring.