[1] The mantle wedge above subducting slabs is associated with many important processes, including the transport of melt and volatiles. Our understanding of mantle wedge dynamics is incomplete, as the mantle flow field above subducting slabs remains poorly understood. Because seismic anisotropy is a consequence of deformation, measurements of shear wave splitting can constrain the geometry of mantle flow. In order to identify processes that make first-order contributions to the pattern of wedge flow, we have compiled a data set of local S splitting measurements from mantle wedges worldwide. There is a large amount of variability in splitting parameters, with average delay times ranging from~0.1 to 0.3 s up to~1.0-1.5 s and large variations in fast directions. We tested for relationships between splitting parameters and a variety of parameters related to subduction processes. We also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge. We find that no simple model can explain all of the trends observed in the global data set. Mantle wedge flow is likely controlled by a combination of downdip motion of the slab, trench migration, ambient mantle flow, small-scale convection, proximity to slab edges, and slab morphology, with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology. There is also a likely contribution from B-type olivine and/or serpentinite fabric in many subduction zones, governed by the local thermal structure and volatile distribution.
The observation of widespread seismic discontinuities within Archean and Proterozoic lithosphere is intriguing, as their presence may shed light on the formation and early evolution of cratons. A clear explanation for the discontinuities, which generally manifest as a sharp decrease in seismic velocity with depth, remains elusive. Recent work has suggested that midlithospheric discontinuities (MLDs) may correspond to a sharp gradient in seismic anisotropy, produced via deformation associated with craton formation. Here we test this hypothesis beneath the Archean Superior and Wyoming Provinces using anisotropic Ps receiver function (RF) analysis to characterize the relationship between MLDs and seismic anisotropy. We computed radial and transverse component RFs for 13 long‐running seismic stations. Of these, six stations with particularly clear signals were analyzed using a harmonic regression technique. In agreement with previous studies, we find evidence for multiple MLDs within the cratonic lithosphere of the Wyoming and Superior Provinces. Our harmonic regression results reveal that (1) MLDs can be primarily explained by an isotropic negative velocity gradient, (2) multiple anisotropic boundaries exist within the lithospheric mantle, (3) the isotropic MLD and the anisotropic boundaries do not necessarily occur at the same depths, and (4) the depth and geometry of the anisotropic boundaries vary among stations. We infer that the MLD does not directly correspond to a change in anisotropy within the mantle lithosphere. Furthermore, our results reveal a surprising level of complexity within the cratonic lithospheric mantle, suggesting that the processes responsible for shaping surface geology produce similar structural complexity at depth.
Seismic hazard associated with Cascadia megathrust earthquakes is strongly dependent on the landward rupture extent and heterogeneous fault properties. We use 3D numerical simulations and a seismic velocity model for Cascadia to estimate coseismic deformation due to M 9–9.2 earthquake scenarios. Our earthquake source model is based on observations of the 2010 M 8.8 Maule and 2011 M 9.0 Tohoku earthquakes, which exhibited distinct strong motion‐generating subevents in the deep portion of the fault. We compare our estimates for land‐level change to paleoseismic estimates for coseismic coastal subsidence during the A.D. 1700 Cascadia earthquake. Results show that megathrust rupture extending to the 1 cm/yr locking contour provides a good match to geologic data. In addition, along‐strike variations in coastal subsidence can be matched by including low slip, strong motion‐generating subevents in the down‐dip region of the megathrust. This work demonstrates the potential to improve seismic hazard estimates for Cascadia earthquakes by comparing physics‐based earthquake simulations with geologic observations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.