Tracking very low frequency earthquakes into long continuous records: application to the Southern Ryukyu subduction zone

Very low frequency earthquakes (VLFEs) are generally absent from the standard seismicity catalogs because of their depleted seismic radiation at frequencies around and above 1 Hz. With the aim of improving their detection, we have developed an approach where the continuous three-component records of a station pair are first template-matched with the corresponding surface-wave time windows of previously known earthquakes. As a time delay is allowed for one of the stations of the pair, detected events may be not collocated with their templates, and their epicenters can be determined as soon as a second pair is considered. In a second stage, based on their high-frequency radiation, we determine whether the detected events are standard earthquakes absent from the template catalog or VLFEs. This two-stage method, referred as VLFE\_DRL (VLFE Detection and Relative Location), is applied to the southern Ryukyu subduction zone where VLFEs were already known to occur. When compared with existing VLFE catalogs of the area, VLFE\_DRL is shown to provide robust estimates of the VLFE source parameters. Between 2004 and 2024, VLFE\_DRL detects and locates there more than 160 VLFEs with moment magnitude greater than 4, occurring in areas distinct from the standard interplate seismicity.