Currently there are two end-member views of low-frequency earthquakes (LFEs). One is that they result from stick-slip behavior of Mxed brittle patches that are mechanically distinct from the surrounding, creeping, fault. The other is that they represent the high-frequency limit of stochastic accelerations and decelerations of slip on the fault, with boundaries that may vary with time. Among the many unknowns concerning LFEs are their physical dimensions, knowledge of which might place constraints on their underlying nature. 

Estimates of LFE source sizes range from 100 m (if they have a stress drop similar to regular earthquakes) to more than 1 km (if they rupture at local shear wave speeds, given a ~0.5-s duration). Our 4-s tremor catalogs have an estimated location uncertainty smaller than 1 km, and many consecutive and nearly consecutive detections are spaced closer than this. This suggests that if LFEs are close to the upper size limit, successive events are strongly overlapping, which seems more consistent with the stochastic acceleration and deceleration model of LFE generation. Moreover, many 4-s windows themselves contain multiple nearly co- located LFE-like arrivals. This encourages us to use LFE templates to deconvolve tremor seismograms, to obtain an LFE catalog with a temporal resolution conceivably as high as one event per LFE duration. Our preliminary catalog shows that many LFEs as close in time as 0.5 s are separated by less than 1 km. Figure 1 shows LFE detections from a 250-s window with an overall 2-km migration to the southeast. The median event has 75 other detections within 1 km. In the propagation direction, also the direction of low error, the median separation between consecutive events is 360 m. If LFEs are km-scale, possible explanations for the signiPcant overlap between events sometimes as close as 0.5 s in time (see the zoom) include reQected waves from boundaries of a low-velocity shear zone, or inertial vibrations at low normal stress (Im and Avouac, 2021). If, instead, LFEs are brittle asperities closer to 100 m in size, successive events need not overlap, but one must explain both their long duration and why, with so many sources in close proximity, nearly none are observed to grow larger in both duration and magnitude than is characteristic of LFEs.