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Recently, slow earthquakes (slow EQ) have received much attention relative tounderstanding the mechanisms underlying large earthquakes and to detectingtheir precursors. Low-frequency earthquakes (LFE) are a specific type of slowEQ. In the present paper, we reveal the relevance of LFEs to the 2011 GreatTohoku Earthquake (Tohoku-oki EQ) by means of cluster analysis. We classifiedLFEs in northern Japan in a data-driven manner, based on inter-time, the timeinterval between neighboring LFEs occurring within 10 km. We found that thereare four classes of LFE that are characterized by median inter-times of 24seconds, 27 minutes, 2.0 days, and 35 days, respectively. Remarkably, inexamining the relevance of these classes to the Tohoku-oki EQ, we found thatactivity in the shortest inter-time class (median 23 seconds) diminishedsignificantly at least three months before the Tohoku-oki EQ, and becamecompletely quiescent 30 days before the event (p-value = 0.00014). Furtherstatistical analysis implies that this class, together with a similar class ofvolcanic tremor, may have served as a precursor of the Tohoku-oki EQ. Wediscuss a generative model for these classes of LFE, in which the shortestinter-time class is characterized by a generalized gamma distribution with theproduct of shape parameters 1.54 in the domain of inter-time close to zero. Wegive a possible geodetic interpretation for the relevance of LFE to theTohoku-oki EQ.

Classes of low-frequency earthquakes based on inter-time distribution reveal a precursor event for the 2011 Great Tohoku Earthquake