Geodetic Matched Filter Slow Slip Event Detection Along the Northern Japan Subduction Zones.

We apply a template matching method on GNSS data for stations located in Honshu, Japan, to detect slow slip events associated with the subducting Philippine Sea and Pacific plates during the period from 1997 to 2020. A measure of the minimum detectable moment magnitude is proposed, from which we infer that the method could potentially detect SSEs as small as Mw 5.2 on the westernmost part of the Philippine Sea plate and Mw 6 on the Pacific plate below Honshu eastern coastline. We find 12 slow slip events on the Philippine Sea plate, among which eight are located on the known Boso slow slip event asperity and the four others are located offshore north‐east relative to the Boso SSEs, at the transition with the Pacific plate. We find 9 SSEs on the Pacific plate, mainly on the northern section, offshore Iwate prefecture. A clear gap with no SSEs coincides with the main asperity that broke during the 2011 Tohoku earthquake. Most event locations correlate with low locking areas. We do not find any clear temporal pattern apart from the regular occurrence of the largest Boso SSEs. Plain Language Summary: Slow earthquakes are known to occur in northern Japan, especially off the Boso Peninsula. They are however harder to detect on the Pacific plate, likely due to their small sizes and/or their remoteness. We here apply a method that search for such slow ruptures, that process all the GNSS stations at once so to improve the signal‐to‐noise ratio. We find 21 such events, 13 of them being so far unrecognized. A clear lack of slow slip events is observed for the asperity that broke during the 2011 Tohoku‐Oki earthquake. The absence of any regular pattern, outside the well‐known Boso slow ruptures, is noticed, implying that the role of these events in the overall seismic cycle is still unclear for the Pacific plate subduction zone. Key Points: We search for slow slip events along the northern Japan subduction zones using a template matching approach on GNSS time series (1997–2020)We find 12 slow slip events on the Philippine Sea plate and 9 on the Pacific plate, coinciding with low locking areasA large gap devoid of any SSE correlates with the locked asperity of the 2011 M9 Tohoku‐Oki earthquake [ABSTRACT FROM AUTHOR]