First Ocean Bottom Seismometer network underneath the ice-covered Arctic Ocean: Operational challenges and chances for monitoring the state of the sea ice cover

Abstract:

Seismic monitoring of the cryosphere is mostly done with land seismometers on the surface of ice masses. Seismic monitoring beneath sea ice at the bottom of ice covered oceans has hardly been attempted, because ocean bottom seismometers (OBS) are difficult to recover in perennial sea ice. As a result, for example the tectonic activity of the Arctic mid-ocean ridge system is poorly known. Recently, ambient seismic noise in long-term seismic records proved a useful tool to monitor the state of the sea ice cover.

From September 2018 - September 2019, we deployed a trial network of 4 broadband OBS in Arctic sea ice. OBS were positioned at distances of 10 km at a water depth of 4 km on eastern Gakkel Ridge. We used a modified Lobster-type OBS of the German Instrument Pool for Amphibian Seismology (DEPAS) that includes a detachable Posidonia transponder to yield precise OBS positions also underneath ice floes. We then carefully broke the ice with RV Polarstern until the OBSs appeared in open water and could be recovered.

The network was designed to record local earthquakes along Gakkel Ridge, but it also yields valuable year-round data on the microseismic noise signal at the bottom of the Arctic Ocean in a marginal ice zone. Noise levels under the Arctic ice cover are considerably lower than for OBS in the Greenland Sea and comparable to those of Arctic land stations. Above 5 Hz, noise levels increase when sea ice cover is present. The secondary microseismic noise peak has two clearly separable components with opposite seasonal evolution. Microseisms at 3-10s periods relate to swell events outside the Arctic Ocean with a higher incidence of such events during winter time. In contrast, secondary microseisms originating in the Arctic Ocean peak in September during the annual sea ice minimum. Their periods increase from 0.5 s to 5 s as the fetch area for wave evolution increases from June to September.

Peculiar shaking events affected one of four OBSs and produced both long period (>10 s) and high frequency (>5 Hz) disturbances lasting about 1.5-2.5 days mainly during the summer season. Narrow banded, potentially hydrodynamically generated tremor with tidally modulated frequencies between 10 and 30 Hz occurs exclusively at this station and is disrupted by the shaking events.