Investigating Short-Period Microseisms near Lake Malawi using a Broadband Array of Onshore and Lake-Bottom Seismometers

Newly-available broadband seismic data surrounding Lake Malawi provide an opportunity to investigate interactions between lake processes and the ambient seismic noise field. Recent studies near lakes identify pervasive noise signals between periods of 0.5 and 5 s which are observable as far as 300 km away from the interpreted lacustrine source, but the precise physical mechanism producing these signals remains unclear. The SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania) experiment included the deployment of fifty-seven onshore broadband seismometers and six broadband lake-bottom seismometers (LBS) in water depths between 270 and 680 m in Lake Malawi. Lake Malawi covers nearly 30,000 km² of the southern East African Rift and by volume is the fifth largest lake in the world. This project provides one of the only broadband seismic datasets collected in a sub lacustrine environment.

Power spectral analyses at stations surrounding and within Lake Malawi reveal a transient noise signal that manifests daily as swells in power that persist for several hours and peak near 1-3 s — a signature distinct from those of ocean microseisms. Onshore, the signals are strongest close to the lake and decay with distance from the shoreline. Offshore, the signals extend to slightly longer period (1-5 s). In both cases, signal amplitude is highest during the local overnight hours. Preliminary application of frequency-dependent polarization analysis suggests that the onshore signals are Rayleigh-waves consistent with a source near the northern lake shoreline, while the lake-bottom signals are largely unpolarized. Further spectral characterization and comparison between onshore and lake-bottom stations may illuminate the mechanism generating these microseisms.