Fluid Controls on the Heterogeneous Seismic Characteristics of the Cascadia Margin

Abstract

The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate-depth seismicity. In warm subduction zones, shallow dehydration can also lead to the buildup of pore-fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate-depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate-depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate-depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.Plain Language Summary Fluids are released from subducting oceanic lithosphere as temperature and pressure within the Earth increases. The release of these fluids is manifest by seismicity within the subducting lithosphere and nonvolcanic tremor near the subduction interface. Along the northern and southern Cascadia margin, the spatial distribution of seismicity within the slab and nonvolcanic tremor correlates with very slow shear wave velocities in the lower crust of the overriding plate. These low-velocity zones likely represent underthrust sediments containing slab-derived fluids. In central Cascadia, however, seismicity and tremor are relatively low, and a low-velocity zone in the lower crust of the forearc is not observed. Our results suggest that a combination of variations in the distribution of underthrust sediments and fluid flux along the margin may be the ultimate control on the tremor and seismicity distribution along the Cascadia margin.

Constraining Crustal Properties Using Receiver Functions and the Autocorrelation of Earthquake‐Generated Body Waves

“…One can see that there is no observable Pmp phase associated with the H ‐ κ stacking solution of H = 25.6 km and V P / V S = 1.66 assuming a V P of 6.2 km/s in the autocorrelations (Figure , black dashed line). The resulting estimate of average crustal V S based on the H ‐ κ ‐ V P stacking solution ( V S = 3.58 km/s) is also more consistent with studies that utilize the sensitivities of surface wave dispersion data in this area to constrain crustal shear velocity (~3.3 km/s, Schmandt et al, ; ~3.5 km/s, Delph et al, ) than the H ‐ κ solution ( V S = 3.73 km/s).…”

Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

“…However, the spatial offset between the correlation of phase velocities and seismic behavior suggests that a compositional or thermal transition from slow (weak) to fast (stronger) overriding crust may delineate the updip extent of tremor. The relationship with intraslab seismicity additionally supports that both observations may have a common cause such as variations in fluid release/retention along the megathrust, perhaps also related to variations in underthrusting sediment (Delph et al 2018).…”

Seismic imaging of a mid-crustal low-velocity layer beneath the northern coast of the South China Sea and its tectonic implications

“…Such distribution may be determined by inherited conditions: 1) the lateral variations in the degree of hydration of the incoming plate, 2) variations in permeability structure of the subduction oceanic plate, and 3) variations in permeability of the upper plate. Heterogeneity of the incoming plate hydration are controlled by variations in density and the maximum fault throw (vertical component of fault displacement) of bending-related faults (Boneh et al, 2019), and the permeability structure of the downgoing slab ultimately determines the release of fluids (Delph et al, 2018). The difference in rock porosity or permeability of the upper plate controlled by fluid and rock composition can also lead to the observed lateral variation of the LVLs (Hyndman, 1988;Araya Vargas et al, 2019), even though we cannot give direct evidence.…”