The Spatial Relationship Between Contemporaneous Tremor Detections in
Relatively Low- and High-Frequency Bands
Abstract
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 ($\sim$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.