20We observe remarkably periodic patterns of seismicity rates and magnitudes at the Fimbul 21 Ice Shelf, East Antarctica, correlating with the cycles of the ocean tide. Our analysis covers 22 19 years of continuous seismic recordings from Antarctic broadband stations. Seismicity 23 commences abruptly during austral summer 2011 at a location near the ocean front in a 24 shallow water region. Dozens of highly repetitive events occur in semi-diurnal cycles, with 25 magnitudes and rates fluctuating steadily with the tide. In contrast to the common 26 unpredictability of earthquake magnitudes, the event magnitudes show deterministic trends 27 within single cycles and strong correlations with spring tides and tide height. The events 28 occur quasi-periodically and the highly constrained event sources migrate landwards during 29 rising tide. We show that a simple, mechanical model can explain most of the observations. 30Our model assumes stick-slip motion on a patch of grounded ice shelf, which is forced by 31 the variations of the ocean-tide height and ice flow. The well fitted observations give new 32 insights into the general process of frictional triggering of earthquakes, while providing 33 independent evidence of variations in ice shelf thickness and grounding. 34 35
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