Buoyant Asthenosphere Beneath Cascadia Influences Megathrust Segmentation

Abstract

Great megathrust earthquakes (magnitude 8+) do not typically rupture an entire convergent margin but rather are limited to one or more along‐strike segments. A fundamental question of subduction zone dynamics and hazard assessment is what physical properties or dynamic processes govern megathrust segmentation. Here we use onshore‐offshore teleseismic delay time data to tomographically image the upper mantle seismic structure of the Cascadia subduction zone. Our results reveal along‐strike segmentation in the oceanic asthenosphere beneath the subducting plate with pronounced low‐velocity anomalies below regions of increased plate locking and greater occurrence of episodic tremor and slip. We attribute the anomalous asthenospheric velocities to independent mantle upwellings associated with hot spot‐derived material in northern Cascadia and fragmentation processes at a diffuse plate boundary in southern Cascadia. Based on these relations, we hypothesize that subslab buoyancy modulates the plate coupling force at the thrust interface, thereby contributing to the localization of subduction zone segmentation.

Cascadia Convergent Zone: An Example of Primary Convergent Seismogenic Structure

“…Slightly higher-densities are shown in northern Washington and southern Oregon/northern California, relative to central Oregon. The higher-density ETS contours that are presented in this article are not short ellipses, as suggested by Bodmer, Toomey [66], but rather are substantially elongated along-margin. Bodmer, Toomey [66] propose that the concentrated ETS clusters are associated with relatively greater inter-plate coupling, but modern convergence strain rates in the upper-plate ( Figure 5) do not support that hypothesis.…”

“…The higher-density ETS contours that are presented in this article are not short ellipses, as suggested by Bodmer, Toomey [66], but rather are substantially elongated along-margin. Bodmer, Toomey [66] propose that the concentrated ETS clusters are associated with relatively greater inter-plate coupling, but modern convergence strain rates in the upper-plate ( Figure 5) do not support that hypothesis. The greatest density of ETS events occurs in the southern Cascadia margin, the Gorda Plate segment, where upper-plate convergence stain rates are moderate by comparison to the central Cascadia margin [24].…”

“…The greatest density of ETS events occurs in the southern Cascadia margin, the Gorda Plate segment, where upper-plate convergence stain rates are moderate by comparison to the central Cascadia margin [24]. Bodmer, Toomey [66] also suggest that the northern high-density clusters of ETS events could represent along-margin segmentation of the inter-plate coupled zone, thus limiting mega-thrust rupture lengths. The rupture boundary proposed by Bodmer, Toomey [66] is between the southern end of the Olympic Range and the Northern Coast Range in southwest Washington and Oregon (Figure 1).…”

“…Bodmer, Toomey [66] also suggest that the northern high-density clusters of ETS events could represent along-margin segmentation of the inter-plate coupled zone, thus limiting mega-thrust rupture lengths. The rupture boundary proposed by Bodmer, Toomey [66] is between the southern end of the Olympic Range and the Northern Coast Range in southwest Washington and Oregon (Figure 1). Ironically, the best paleo-seismic records of along-margin ruptures occur across that proposed boundary, with at least 5 out of the last 6 major mega-thrust ruptures, during the last ~2.6 ka, crossing from the central Washington coast to the northern Oregon coast [23] [61] [63], and 3 out of the last 4 major mega-thrust ruptures, during the last 1.3 ka, crossing from the northern Washington coast to the central Oregon coast [67].…”

The Fragmented Death of the Farallon Plate

“…The active expression of the HLP trend lies directly above the southern edge of the slab hole we image in CASC19-S. Slab holes have been inferred in other subduction zones (Obayashi et al, 2009;Portner et al, 2017), sometimes accompanied by anomalous volcanism (Berk Biryol et al, 2011;Rosenbaum et al, 2008). Dry basalts in the HLP have been explained by upwelling mantle in the backarc due to rollback of the coherent JdF slab (Ford et al, 2013;Long et al, 2012;Till et al, 2013), but as the tomography models do not show a coherent slab, we explore the possibility that the volcanism is generated by asthenospheric flow through the hole and subsequent decompression melting.…”