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


Northern Malawi Rift (Accardo et al., 2018)

Motivation

The Malawi Rift sits at the southern end of the larger East African Rift system, a region over which the African plate is currently splitting into two tectonic plates. Continental plates are typically thick, cold, and strong, so a weakening mechanism is needed to break them up. Over much of the East African Rift system, intruding magma heated up from a mantle plume below (and erupting onto the surface as lava in many places) weakens the African plate and allows it to rift apart. However, very little surface evidence for volcanism exists along the Malawi rift, leading us to wonder what is responsible for its weakness: hidden magmatism below or some other mechanism?

Map of Broadband Seismometers for SEGMeNT From 2013-2015, the SEGMeNT experiment blanketed the northern and central Malawi Rift with a network of 63 seismic stations as part of a multidisciplinary effort to better understand how a relatively cold, strong continental plate rifts apart in its early stages in the apparent absence of a weakening magmatism. However, sometimes we discover exciting new observations we weren’t even looking for. This network included 6 lake-bottom seismometers (LBS) deployed during 2015 within Lake Malawi (Nyasa). From the bottom of the fifth largest lake in the world, amongst the many earthquake recordings, we made an exciting and unexpected discovery: the clearest observations of the Loch Malawi Monster! lake-generated microseisms to date. That’s right, these LBS instruments detected pervasive daily pulses of energy in a usually quiet part of Earth’s background seismic noise!

Wait… Microseisms? Seismic noise? Isn’t noise bad for clear earthquake recordings? Technically, that depends on your scientific question and what you consider signal.

Ambient Seismic Noise

Overprinting and surrounding a typical earthquake arrival, seismometers around the world record continuous quasi-random signal from any kinetic energy coupling with the solid earth.

To understand the various sources of ambient noise, we look in the frequency domain. The plot below shows power spectral density (PSD) – a measure related to signal amplitude as a function of frequency, or here, its reciprocal, period. Most signals recorded on earth fall within those two gray dashed lines, including these PSDS from our lake-bottom seismometers. These PSD plots act as seismic fingerprints, with peaks and troughs corresponding to noise sources and gaps.

Power spectral density plot for the LBS stations.

A Seismic Fingerprint

Oceans cover about 70% of Earth’s surface, so it is no surprise that interactions between ocean storms and the solid Earth generate the strongest ambient noise signals, within a period band of 2 to 20 s. Two mechanisms explain the bimodal ocean microseism peaks.

The long-period peak, known as the primary microseism (OPM), is caused by shoaling processes as waves beat the shoreline, while the stronger secondary microseism at shorter period is caused by the nonlinear interaction of waves traveling in opposite directions. The secondary microseism (OSM) is often called the double-frequency microseism as the wave interactions generate microseismic energy at twice the frequency of the initial ocean waves.

Infragravity waves contribute to the longest period ambient noise signals as they deform the seafloor, while cultural noise such as traffic, machinery, or electrical wires, dominate the high frequency end of the spectrum.

As seen in the noise envelope, normally the ambient noise spectrum has a quiet zone around 1 s, but from lake bottom we are seeing strong excitations impacting the deployment-averaged seismic fingerprints. These are lake microseisms. If we scale things down, lakes generate microseisms similar to those in the oceans, but which mechanism or mechanisms are responsible? Up until very recently, lake microseism studies focused on their broad impacts to the wavefield from beyond the lake itself. Fortunately, these LBS deployed on the bottom of Lake Malawi provide us with a unique perspective from which to observe lake microseisms.
Spectrograms of Malawi LBS

Lake-Bottom Spectrograms

If we look at a shorter time window, the intricacies and pervasiveness of lake microseisms are revealed. Two distinct subpeaks within the lake microseism band are observed, particularly at stations at shallower depth. Our paper goes on to explore these variations in spectral behavior as a function of recording depth and proximity to both steep bathymetric slopes and shorelines and suggest that the two lake microseism bands may correspond to single- and double-frequency generation processes, akin to primary and secondary ocean microseisms. The diurnal behavior of lake microseisms, as well as their polarizations, is consistent with wave energy driven by a combination of seasonally-varying nighttime winds funneled by lakeshore topography and waveguide effects.

Check out our paper to learn more!

Carchedi, CJW., JB. Gaherty, SC. Webb, and DJ. Shillington (2022). Investigating short-period lake-generated microseisms using a broadband array of onshore and lake-bottom seismometers.SRL,93 (3), 1585–1600. https://doi.org/10.1785/0220210155