Cluster Analysis of Thermal Icequakes Using the Seismometer to
Investigate Ice and Ocean Structure (SIIOS): Implications for Ocean
World Seismology
Abstract
Introduction: Ocean Worlds are of high interest to the planetary
community [1, 2] due to the potential habitability of their
subsurface oceans [3–5]. Over the next few decades several missions
will be sent to ocean worlds including the Europa Clipper [6],
Dragonfly [7], and possibly a Europa lander [8]. The Dragonfly
and Europa lander missions will carry seismic payloads tasked with
detecting and locating seismic sources. The Seismometer to Investigate
Ice and Ocean Structure (SIIOS) is a NASA PSTAR funded project that
investigates ocean world seismology using terrestrial analogs. One goal
of the SIIOS experiment is characterizing the local seismic environment
of our field sites. Here we present an analysis of detected local events
at our field sites at Gulkana Glacier in Alaska and in Northwest
Greenland approximately 80 km North of Qaanaaq, Greenland (Fig. 1a).
Both field sites passively recorded data for about two weeks. We
deployed our experiment on Gulkana Glacier in September 2017 (Fig. 1b)
and in Greenland in June 2018 (Fig. 1c). At Gulkana there was a nearby
USGS weather station [9] which recorded wind data. Temperature data
was collected using the MERRA satellite [10]. In Greenland we
deployed our own weather station to collect temperature and wind data.
Gulkana represents a noisier and more active environment: Temperatures
fluctuated around 0C, allowing for surface runoff to occur during the
day. The glacier had several moulins, and during deployment we heard
several rockfalls from nearby mountains. In addition to the local
environment, Gulkana is located close to an active plate boundary
(relative to Greenland). This meant that there were more regional events
recorded over two weeks, than in Greenland. Greenland’s local
environment was also quieter, and less active: Temperatures remained
below freezing. The Greenland ice was much thicker than Gulkana
(~850 m [11] versus ~100 m [12,
13]) and our stations were above a subglacial lake. Both conditions
can reduce event detections from basal motion. Lastly, we encased our
Greenland array in an aluminum vault and buried it beneath the surface
unlike our array in Gulkana where the instruments were at the surface
and covered with plastic bins. The vault further insulated the array
from thermal and atmospheric events. Event Detection and Clustering: To
detect local events we filtered the data between 5-20 Hz. Using the
Obspy module in python [14], we performed a short-term
average/long-term average (STA/LTA) approach to determine where
amplitudes spiked. For short term we used 1.5 seconds and 40 seconds and
a ratio of 20 to detect events [15]. Through this approach we
detect-ed 104 events at our Greenland site and 2252 events at our
Gulkana site. The Gulkana site showed a strong correlation with both
temperature and changes in temperature, while Greenland did not show
this relationship [16]. Once we had a catalog of events, we
performed a hierarchal cluster analysis to cluster events.