3:30 PM - 4:30 PM
[J06-P-16] Anisotropy in the subducted oceanic crust and the overlying continental crust coincides with slow slip phenomena in the flat portion of the Mexican subduction zone
A novel approach has been developed to isolate the anisotropy observed by receiver functions (RF) into the separate layers through which the seismic waves pass (Castillo et al., 2017). This approach has proven particularly effective in the Mexican flat slab, which remains in near contact with the continental crust to 300 km from the trench. There are 3 regions that can be observed with this approach: (1) The lower subducted oceanic crust; (2) the remnant mantle wedge between the oceanic and continental crust; and (3) the continental crust. Slow earthquakes are also found in the flat slab region, and they strongly correlate with changes in the anisotropy in the different layers. The flat slab region is a double slow earthquake zone: from updip to down-dip (1st slow earthquake zone) Mw ~7.5, 4-year recurrence-time slow slip events (SSE) with little tectonic tremor (TT) on the down-dip edge; and (2nd slow earthquake zone) Mw <6.5, 2 – 3 month recurrence-time SSEs with a large amount of TT on down-dip edge. The highest anisotropy percentage in the subducted crust aligns with the TT in the two slow earthquake regions. In the remnant mantle wedge, high anisotropy only aligns with the TT in the second slow earthquake region. The overlying crust is the reverse with low anisotropy directly above the TT in each of the slow earthquake regions. This pattern may be explained by a seal at the base of the continental crust (fluids trapped beneath the crust where the TTs are located, leading to high anisotropy beneath the seal and low anisotropy above) broken by SSEs (high anisotropy above the SSEs and low anisotropy beneath as fluids are transported into the upper crust). The anisotropy direction changes with distance from the trench and correlates with LFE activity, but it is difficult to explain at this time.