Investigating Short-Period Lake-Generated Microseisms Using a Broadband Array of Onshore and Lake-Bottom Seismometers

Recent analyses of ambient seismic noise proximal to large lakes identify excitations between periods of 0.5 and 5 s as lake-generated microseisms, though the mechanisms producing them remain unclear in the absence of distinct spectral peaks. We present a unique data set combining onshore and offshore seismometers to investigate the dominant parameters controlling water-to-solid-earth seismic coupling in a lacustrine environment. Using a set of broadband observations that include six lake-bottom seismometers, we compare signals collected both within and around Lake Malawi (Nyasa) in southeastern Africa, the fifth largest freshwater lake in the world. We document clear evidence for two peaks within the lake microseism band, with pervasive diurnal excitations between periods of similar to 0.6-1.3 and similar to 1.3-3 s. Variations in spectral behavior as a function of recording depth and proximity to both steep lake-floor slopes and the shoreline suggest that the spectral bands may correspond to single- and double-frequency generation processes, akin to primary and secondary ocean microseisms generated by the shoaling of wind-driven gravity waves along shorelines and wave interactions within the open body of water, respectively. However, both the absolute amplitudes and temporal evolution we observe in these two bands are complex and often inconsistent with traditional microseism theory. Therefore, these spectral bands may alternatively result from complex wave-wave interaction at the lake surface within the enclosed northern end of the lake. The diurnal behavior of lake microseisms, as well as their polarizations, suggest that the wave energy is driven by a combination of seasonally varying nighttime winds funneled by lakeshore topography and waveguide effects. This work also provides a glimpse into seasonal variation in lacustrine ambient seismic noise and thus also into lake processes.