Ambient Noise Imaging of Menengai Caldera in the Central Kenya Dome

The Geothermal Development Company (GDC) and the University of Texas at El Paso (UTEP) have deployed fourteen seismic stations around the Menengai geothermal field along the Kenya rift system to monitor the seismicity around the Menengai Caldera. The goal of the project is to identify active faults and fracture systems that may contain hydrothermal fluids and favorable drilling targets, plus image the magma chamber. The deployment has a variety of seismic sensors with different frequency responses, and the instruments where deployed in two stages, with seven stations recording continuously since Mar. 2011, and another seven stations being deployed in Aug. 2011. We use vertical component waveform data from Mar. 2011 to Mar. 2012 to image the caldera using ambient noise tomography. We first cut waveform data every 5 hours recorded at single seismic station, applying a 1-bit normalization to eliminate earthquake signals and any instrumentation irregularities. We also apply spectral whitening in order to flatten the spectrum to minimize the source contamination. We cross-correlate waveforms to retrieve the Green's functions for all combinations of stations within our seismic network, accounting for different frequency responses, given the different sensors frequency responses (Guralp 3T's and 40Ts). We introduce a period cut-off in which it is acceptable for closely spaced station-pairs to have inter-station distance greater than ~3 wavelengths. This avoids deterioration of the cross correlation result. We compute signal-to-noise-ratios (SNR) of the cross-correlation to determine the quality of the data, where the SNR is defined as the peak amplitude divided by the root-mean-square noise in the window. The computed cross-correlations were stacked in 3-month bins to account for seasonal variability in order to estimate the group velocity uncertainties at each period. We used Frequency Time Analysis (FTAN) and a match filter analysis (MTA) in order to approximate the group velocities. Our next step will be to use the dispersion curves to perform tomography, which will map subsurface shear-wave velocities. This information, along with the micro-events, should help the GDC target future drill sites by identifying the location of the magma chamber.