Margin-wide variations in slab dehydration in Cascadia and their relationship to slow slip

Abstract

Slow slip events are observed in subduction zones globally, with a wide range of variations in behaviour, including the periodicity and magnitude of events. These variations are observed both within and across different subduction zones. Due to their role in controlling strain accumulation and potential nucleation of regular earthquakes, understanding the physical environment of slow slip events and the source of their observed variability is critical to understanding seismogenic processes. Here we calculate thermal models for 10 trench-normal profiles along the entirety of the Cascadia subduction zone. The models incorporate lateral changes in incoming plate age and sediment input, as well as slab geometry and overriding plate structure. Temperatures at the location of peak slow slip and tremor occurrence (depths of 30–40 km) vary from 525 °C in southern Cascadia, to 475 °C in central Cascadia, to 550 °C in northern Cascadia. The variability in thermal structure and slab geometry leads to changes in the rate of calculated fluid flux in the vicinity of the mantle wedge corner where slow slip events are observed. Fluid flux rates calculated within 25 km and 50 km downdip of the mantle wedge corner show an inverse relationship with the recurrence time of slow slip with highest value observed in southern Cascadia, followed by northern Cascadia, and then central Cascadia. Other along-strike variability in tremor behaviour, such as tremor strength, correlates spatially with changes in calculated fluid flux pattern but the relationship between these parameters is not readily defined. This study provides support for models where variations in the rate of fluid production in the vicinity of the mantle wedge corner are a potential mechanism controlling the rate of pore fluid pressure buildup and fluid release during slow slip. • Changes in subduction parameters affect fluid release along Cascadia. • Major sites of slab dewatering are observed beneath the mantle wedge corner. • Fluid flux rates show an inverse relationship with the recurrence time of slow slip. • Variability in tremor strength and density correlate spatially with flux pattern.