The zones of spatially coherent recurrence intervals (Figure 2.9) are further divided into segments where individual events recur over roughly the same location. While the average recurrence intervals of ETS are similar within a given zone, the relative timing between ETS events shows variation with location, a phenomenon that is particularly clear when comparing northern and southern Vancouver Island (Dragert et al., 2004). The extent of these segments is now emerging from the increased number of ETS observations. Figure 2.10B illustrates the phase shift in time between different segments by displaying the timing of ETS observations all along Cascadia, with horizontal lines estimating the along-strike extent of a given episode. Since these are station locations instead of source locations, we would expect the grey lines to extend on the order of 50 km beyond the actual source locations. Dashed vertical lines are approximate boundaries defined by events on either side that are separated in time by over a month for greater than 50 of the episodes. We find 7 large segments with along-strike widths of 100-200 km (Figure 2.10B).
The largest segments of ETS occur immediately landward from the proposed locations of asperities on the Cascadia megathrust (Wells et al., 2003). The asperity locations are based on large, low gravity, sedimentary basins in the forearc that have been interpreted to indicate potential seismogenic segmentation at depth. Figure 2.10B shows the along-strike pattern of prominent fore-arc basins for comparison with the spatial extent of ETS segments. The apparent correlation between segmentation of the seismogenic zone and segmentation of the ETS zone suggests that effects of locking (or lack thereof) on the megathrust are transmitted to greater depths where slow slip is believed to occur (Dragert et al., 2001). This spatially links megathrust structure and anticipated seismogenic behavior with ETS characteristics.
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