Comprehensive Detection of Very Low Frequency Earthquakes Off the Hokkaido and Tohoku Pacific Coasts, Northeastern Japan

Very low frequency earthquakes (VLFEs), classified as one type of slow earthquake, occur near seismogenic zones and can be a proxy for interplate slow slip. We investigated VLFE activity off the Hokkaido and Tohoku Pacific coasts, where the Pacific plate subducts under the North American plate. Comprehensive detection of VLFEs is performed for the period from January 2003 to July 2018 by a matched-filter technique utilizing synthetic waveforms calculated based on a three-dimensional velocity structure model. The spatiotemporal consistency between the afterslip of the 2003 Tokachi-Oki earthquake (Mw 8.0) and VLFE distribution off the Hokkaido coast suggests that VLFE activity can reflect interplate slow slip. Inside a large coseismic slip area of the 2011 Tohoku earthquake (Mw 9.0), the VLFE activity was low thereafter, whereas outside the area, VLFE activity increased after the 2011 Tohoku earthquake. These characteristics are consistent with aseismic slip distributions estimated from repeating earthquakes. VLFEs were sometimes activated by afterslips of interplate earthquakes with Mw of 6-7, such as the 2008 Fukushima-Oki earthquake and aftershocks of the 2011 Tohoku earthquake. Even in the coseismic slip areas of very large earthquakes, we found the occurrence of episodic bursts every 3 months to a year during an interseismic period between very large earthquakes. Such VLFE bursts may suggest small slips during the interseismic period. Plain Language Summary The activity of very low frequency earthquakes (VLFEs), classified as one type of slow earthquake, can reflect the spatiotemporal distribution of interplate slip. We investigated VLFE activity off the Hokkaido and Tohoku Pacific coasts from January 2003 to July 2018. VLFEs were activated by afterslips of large earthquakes, that is, the 2003 Tokachi-Oki earthquake (Mw 8.0) and the 2011 Tohoku earthquake (Mw 9.0). Inside a large coseismic slip area of the 2011 Tohoku earthquake, the VLFE activity was low thereafter. Episodic bursts that occurred every 3 months to a year and were observed in large coseismic slip areas of very large earthquakes may indicate small slips during an interseismic period between very large earthquakes.