Analysis of Very Long-Period Noise at Flexible-Array Stations in the
North-American Midcontinent
Abstract
The Superior Province Rifting Earthscope Experiment (SPREE) deployed
seismic stations in 2011-2013 throughout Wisconsin, Minnesota, and
Ontario. To protect equipment from groundwater damage, SPREE stations
were buried at unusually shallow depths, increasing the power of long
period noise and facilitating an investigation into the regional effects
of atmospheric tides and soil properties (Wolin et al., 2015). Here we
utilize the SPREE array to study the effects of solid-earth tides and
meteorological conditions, on very long-period seismic noise in the U.S.
midcontinent. Continuous seismic data was collected from SPREE and
Transportable Array (TA) stations located in Wisconsin and Minnesota
(WIMN) between July 2011 and September 2013. This data was “cleaned”,
filtered, and averaged to produce a monthly representation of the
very-long period signals recorded by the SPREE stations. The signals
showed diurnal (24 hr) and semidiurnal (12 hr) periodicities, whose
magnitudes and dominance vary seasonally. Using cross correlations, we
compare our very-long period observations with theoretical solid-earth
tides (Milbert, 2018) as well as meteorological components in the WIMN
region. Meteorological data, specifically temperature and pressure, was
obtained from the National Oceanic and Atmospheric Administration’s
(NOAA) National Center for Environmental Information (NCEI). Solid-earth
tides result from the gravitational pull of the moon and sun, and have
previously been documented in seismic data (e.g. Pillet et al.,1994;
Lambotte et al., 2005). We observe a distinct correlation between
theoretical solid-earth tides and very-long period signals in seismic
data from SPREE and TA stations in the WIMN region, where one frequency
component is correlated while the other appears delayed. In addition, we
observe a remarkable seasonal change in SPREE recordings of these
signals, but not in TA recordings. We will report our findings from
testing the hypothesis that the observed very-long period signals in
SPREE data are a combination of both tidal and thermal effects and that
these cumulative effects are the result of the unusual burial depth of
SPREE stations.