On the Apparent Duration of Low-Frequency Earthquakes

The broadband stacks (templates) of velocity seismograms of nearly co-located low-frequency earthquakes (LFEs) detected using a 1-8 Hz passband beneath southern Vancouver Island tend to exhibit a simple dipolar shape with a characteristic duration of ~0.3-0.5 s, which is also found to be nearly independent of the seismic moment. An important question left unanswered is whether the duration is due to the nature of the source, is set by attenuation near the source region, or is just a bias introduced by the narrow passband used to detect LFEs. In tremor catalogs detected using a relatively low-frequency passband, 0.5-1.25 Hz, we have found some tremor windows that contain relatively isolated dipole arrivals similar to LFEs. A few of these have a duration apparently longer than that of the LFE templates. Notably, the same location on the fault also seems capable of generating signals with a shorter duration at other times. Figure 1 shows seismograms, at 3 stations, of one such example in the vicinity of LFE family 001 of Bostock et al. (2012), in which the main arrival has a duration of ~1 s, whereas another signal 3 s earlier with a duration of only ~0.4 s comes from roughly the same location (same move-out between the stations). This significant variability in duration at approximately the same location suggests that the long-duration events owe their duration to source processes and not attenuation, provided that attenuation does not vary on extremely short time and space scales during the episodic tremor and slip episode. The relative isolation in time also makes the longer duration less likely to result from the temporal clustering of multiple typical LFEs. We will undertake a more systematic search of our longer- and shorter-period tremor catalogs to assess this possibility. Addressing this question will shed more light on the factors that control the apparent duration of LFEs. Figure 1 The top panel shows the long-duration tremor signal in a relatively lower-frequency band, 0.5-1.25 Hz, whereas the second panel from the top is the same 32-s segment in a higher-frequency band, 1.25-6.5 Hz. The third panel shows the trace in a broader passband, 0.5-6.5 Hz. The bottom panel shows the stacked LFE templates of the same family filtered through 0.5-6.5 Hz.