Ocean-Bottom Seismology of Glacial Earthquakes: The Concept, Lessons Learned, and Mind the Sediments

Abstract

Cite this article as Podolskiy, E. A., Y. Murai, N. Kanna, and S. Sugiyama (2021). Ocean-Bottom Seismology of Glacial Earthquakes: The Concept, Lessons Learned, and Mind the Sediments, Seismol. Res. Lett. XX, 1–16, doi: 10.1785/ 0220200465. About 70% of Earth’s surface is covered by ocean, for which seismic observations are challenging. Seafloor seismology overcame this fundamental difficulty and radically transformed the earth sciences, as it expanded the coverage of seismic networks and revealed otherwise inaccessible features. At the same time, there has been a recent increase in the number of studies on cryoseismology. These have yielded multiple discoveries but are limited primarily to land and ice-surface receivers. Near ice calving fronts, such surface stations are noisy, primarily due to crevassing and wind, are hazardous to maintain, and can be lost due to iceberg calving. To circumvent these issues, we have applied ocean-bottom seismology to the calving front of a tidewater glacier in northwest Greenland. We present details of this experiment, and describe the technical challenges, noise analysis, and examples of recorded data. This includes tide-modulated seismicity with thousands of icequakes per day and the first near-source (∼200–640 m) underwater record of a major kilometer-scale calving event in Greenland, which generated a glacial earthquake that was detectable ∼ 420 km away. We also identified a decrease in bottom-water temperature, presumably due to modified water stratification driven by extreme Greenland glacial melting, at the end of July 2019. Importantly, we identify glacial sediments as the key reason for the anomalously long (∼9:7 hr) delay in the sensor release from the fjord seafloor. Our study demonstrates a methodology to undertake innovative, interdisciplinary, near-source studies on glacier basal sliding, calving, and marine-mammal vocalizations. Introduction One of the fundamental questions in glaciology is what controls glacier basal sliding. In the Antarctic and Greenland, the rapid slip of marine-terminating glaciers and ice streams drains interior ice to the ocean (Zoet and Iverson, 2020). This is important for predicting sea-level rise, which may displace up to 180 million people in the twenty-first century (Bamber et al., 2019). One of the fundamental questions in seismology—what controls tectonic fault slip—is conceptually similar, as it relates to shear zone conditions. During the past two decades, seismology has been revolutionized by the discovery of slow earthquakes and the recognition that their continuous seismic tremor can be used to monitor otherwise inaccessible faults (Obara, 2002; Rouet-Leduc et al., 2019). In this respect, dense seismic monitoring networks have enabled key discoveries in the earth sciences (Beroza and Ide, 2011). In some regions, seafloor seismic observations were instrumental in detecting nonvolcanic tremors (Todd et al., 2018). Polar regions have fast-flowing glaciers, which can be considered analogous to a slow earthquake (Podolskiy and Walter, 2016; Lipovsky and Dunham, 2017). However, testing this analogy is challenging, because seismic stations in polar regions are scarce, dangerous to maintain, moved by ice flow by tens of meters per month, and influenced by noise due to near-surface, tide-modulated icequakes, supraglacial and englacial hydrology, and wind (Podolskiy et al., 2016, 2017; Frankinet et al., 2020; Podolskiy, 2020). Furthermore, for long-term monitoring, the extremely cold temperatures and long polar nights require a large number of batteries, which complicates logistical operations significantly. In this study, we investigated glacier microseismicity and other terminus processes by seafloor seismology (Fig. 1a). 1. Arctic Research Center/GiCORE, Hokkaido University, Sapporo, Hokkaido, Japan, https://orcid.org/0000-0002-3050-4897 (EAP); 2. Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, Sapporo, Hokkaido, Japan, https://orcid.org/0000-0002-5207-893X (YM); 3. Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba, Japan, https://orcid.org/0000-00031874-3868 (NK); 4. Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido, Japan, https://orcid.org/0000-0001-5323-9558 (SS) *Corresponding author: [email protected] © Seismological Society of America Volume XX • Number XX • – 2021 • www.srl-online.org Seismological Research Letters 1 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220200465/5296214/srl-2020465.1.pdf by Hokkaido Univ Rigakubu user on 06 May 2021 Specifically, we deployed an ocean-bottom seismometer (OBS) near a calving front of grounded Greenlandic glacier. This approach can: (1) protect the seismometer from destruction; (2) provide direct coupling between the sliding base and seismometer; (3) greatly decrease the high-frequency (>5–10 Hz) seismic noise (Webb, 1998); and (4) provide a potentially powerful method to observe the frictional state of the glacier base (e.g., Hudson et al., 2020; Zoet et al., 2020) and monitor other seismic sources in the fjord, including iceberg calving and anthropogenic noise. Moreover, our approach is not only of interest to glaciologists and seismologists, but also to marine biologists. It has been recognized that it is possible to detect and classify the seasonal occurrence of different species and to track whales with seafloor seismic networks using their vocalizations (Dreo et al., 2019). In Arctic glacier fjords, which contain biologically rich assemblages (Lydersen et al., 2014), animal monitoring is extremely limited and challenging (Podolskiy and Sugiyama, 2020). However, seismometers can detect acoustic vocalizations by some cetaceans (whales). High-frequency sounds of fish, pinnipeds (seals), and delphinids can be recorded with hydrophones. Figure 1b shows examples of seismoacoustic sources of biological and geophysical origin in Greenland. Integrating hydrophones into the OBS system is relatively easy and less demanding than preparing a standard oceanographic mooring. Moreover, since the 1960s 10 10 10 10 10 10 10 10 10 Frequency (Hz) HTI-99 HF AMAR-G4 HF HTI-04PCA/ULF HF NAMMU OBS, 120s Aquarius HF Aquarius OBS, 120s SoundTrap ST300 HU-ISV OBS, 4.5Hz Calving-tsunami Cryo-seismoacoustic Narwhal Beluga Long-fin. pilot whale Bearded seal Sperm whale Bowhead whale Arctic cod Fin whale