The Spatial Relationship Between Contemporaneous Tremor Detections in Relatively Low- and High-Frequency Bands

Although tremor is believed to consist of myriad low-frequency earthquakes (LFEs), it also contains longer-period signals of unknown origin. We investigate the source of some of the longer-period signals by locating tremor windows independently in relatively high-frequency (“HF”, 1.25–6.5 Hz, containing typical LFEs) and low-frequency (“LF”, 0.5–1.25 Hz) bands. We hypothesize that if tremor consists entirely of LFEs, such that the lower-frequency signals come from the non-uniform timing of higher-frequency (∼2 Hz) LFEs, then contemporaneous LF and HF signals should be nearly co-located. Here we search for a systematic offset between the locations of contemporaneous LF and HF detections during rapid tremor migrations (RTMs). This first requires correcting for apparent offsets in location that arise simply from filtering in different passbands. To guard against possible errors in our empirical filtering effect corrections, we focus on a region of the subduction interface beneath southern Vancouver Island that hosts migrations propagating in nearly opposing directions. We find that the LF energy appears to occur roughly 500 m farther behind the propagating fronts of RTMs than the HF energy, whether those fronts propagate to the ENE or to the WSW. This separation is small compared to the location error of individual LF detections, but the result seems robust owing to the large number of detections. If this result stands, it suggests that tremor consists of more than just a collection of LFEs, with longer-period energy being generated farther behind the migrating fronts of RTMs, where slip speeds are presumably lower.