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.

We are engaging citizen scientists in an experiment to test if many human ears can replace the process of a professional seismologist in identifying dynamically triggered seismic events. Ordinarily, this process involves interactive data processing and visualization efforts on a volume of earthquake recordings (seismograms) that exploded during the recent big-data revolution, for example through EarthScope. In this citizen seismology project, we ask citizens to listen to relevant sections of seismograms that are accelerated to audible frequencies. This approach has five advantages: 1) The human ear implicitly performs a time-frequency analysis and is capable of discerning a wide range of different signals, 2) Many human ears listening to the same data provides statistics that rank seismograms in order of their likelihood to contain a recording of a triggered event, which is helpful to researchers’ analysis of this data as well as to 3) the ability of a deep-learning algorithm to model the boolean identifications or bulk statistics of the analyses, 4) the project has the potential to enhance informal learning because of the online platform that hosts the project, Zooniverse, is available to people of all identities and hosts many other citizen science projects, and 5) it helps prepare our team for diverse post-graduation careers as part of IDEAS, an NRT program at Northwestern University. The events we are asking citizens to help identify via listening are small seismic events such as local earthquakes and tectonic tremor, that occur in response to transient stresses from passing seismic surface waves from a large, distant earthquake. While much research progress has been made in understanding how these events are triggered, there is no reliable deterministic recipe for their occurrence. The aim of our project is to enlist the help of citizens to increase the data set of known triggered seismic events and known absences of triggered events in order to help researchers unravel key aspects of that recipe. A better understanding of triggered seismic events is expected to provide important clues towards a fundamental understanding of all seismic activity, including damaging earthquakes.