ETS Recurrence Intervals

While ETS is observed throughout the Cascadian subduction zone, the characteristics vary coherently along-strike revealing clear segmentation in the recurrence interval and relative timing of ETS events. First, there are 3 broad geographic zones with different recurrence intervals of ETS (Figure 2.9). The average interval across the Siletzia Zone (19$\pm$4) is longer than those observed on Vancouver Island to the north (14$\pm$2) and is nearly twice as long as that from California to the south (10$\pm$2). The broader geographic extent of our ETS measurements relative to previous studies allows us to identify that a coherent Wrangellia Zone extends from northern Vancouver Island down to $\sim$47.5$^{\circ}$N, and that a Klamath Zone extends up from the southern end of the subduction zone to $\sim$42.8$^{\circ}$N (Figure 2.9).

This pattern of recurrence intervals is not tied to the overall rate of subduction which drives the earthquake cycle as a whole. Overall convergence velocities decrease slowly from the north to the south (Figure 2.9), while the longest recurrence interval occurs in the middle of the subduction zone. We also find the 3 zones of relatively uniform recurrence intervals cannot be explained by age of the subducting plate, implying along strike variations in ETS are not due to temperature changes.

We suggest that the recurrence interval of ETS is related to properties of the overriding continental plate instead of the subducting oceanic plate. The age and temperature of the subducting plate likely has some impact on generating ETS, because initial work has shown SSE and/or NVT are prominent in other young, warm subduction zones like southwest Japan and Mexico (Hirose and Obara, 2006; Larson et al., 2004; Lowry et al., 2001; Obara, 2002). Yet the oceanic plates subducting beneath Cascadia are relatively uniform compared to the heterogeneity of the continental plate they dive beneath. In fact, the central Siletzia Zone with an $\sim$18 month recurrence interval corresponds to the relatively low lying and young Coastal Range Block of central and northern Oregon and southern Washington (mostly thick Siletzia terrane). The shorter-recurrence interval zones to the north (Wrangellia) and south (Klamath) correspond to older Pre-Tertiary blocks with higher topography consisting of a melange of old oceanic material with later silicic intrusion in a continental environment. Figure 2.10A shows how ETS recurrence intervals are inversely proportional to onshore fore-arc topography. Correlation of these continental blocks with along-strike patterns of ETS is also consistent with the observation that NVT appears to occur throughout the continental crust at depths above the interface with the subducting oceanic crust.

Figure 2.10: Plot of along strike patterns of ETS and upper plate features. (A) Top panel shows distinct variations in ETS recurrence with symbols as in Figure 2.9 and vertical bars show boundaries of observed intervals. Bottom panel shows topography above the 40 km depth contour of the subduction interface, in the middle of ETS observations. Topography is inversely correlated with ETS recurrence, roughly matching the primary continental terranes of different age and composition (Wrangellia, Siletzia, Klamath). (B) Top panel shows "phase" of ETS for 7 different segments along the subduction zone from ETS timing at individual stations. Horizontal grey lines connect stations that record ETS within a month of one another. Bottom panel shows color shaded gravity anomalies and locations of offshore fore-arc sedimentary basins (white lines), features which have been correlated with megathrust asperities on the subduction interface in recent global studies (Wells et al., 2003). Vertical dashed lines show apparent edges of ETS segmentation from currently available data that seem to correlate with megathrust segmentation from the 5 largest sedimentary basins. To deal with trench curvature, station latitudes are those when projected on to 40-km contour (black curve).
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