Localized seismic anisotropy associated with long-term slow-slip events beneath southern Mexico

• Published in: Geophysical research letters 2012-05, Vol.39 (9), p.n/a
• Main Authors: Song, Teh-Ru Alex, Kim, YoungHee
• Format: Article
• Language: English
• Subjects: Anisotropy; Crusts; Deformation; Earth sciences; Earth, ocean, space; Earthquakes; Exact sciences and technology; foliation; Geology; Marine; Mexico; Oceanic crust; Plate tectonics; Rheology; Seismic activity; Seismic engineering; Seismic phenomena; Seismology; Shear deformation; Shear zone; Slip; slow slip; Transition zone
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2012GL051324

Slow slip events (SSEs) are recently discovered in the transition zone downdip of the locked zone where megathrust earthquakes repeatedly occur at subduction zone interface. Structural changes from the locked zone to the transition zone and the nature of in situ deformation in the transition zone, however, remain unclear. We model teleseismic converted P‐to‐S waves to investigate seismic structural transition and anisotropy associated with shear deformation near the deep locked zone and the transition zone beneath southern Mexico. The deep locked zone appears to be consistent with normal oceanic crust velocity. In the transition zone, we find that seismic anisotropy larger than 5% is associated with the ultra‐slow velocity layer (USL) near the top of the subducted oceanic crust. The foliation plane is plunging 20 ± 10 degrees steeper than the plate interface, and it is consistent with crystallographic preferred orientation developed in S‐C mylonites. We suggest that long‐term SSEs likely coincide with a near‐lithostatic semiductile talc‐rich shear zone. Key Points Use seismic anisotropy to probe structural control on slow slip events Llong‐term slow slip associated with weak clay minerals and high fluid pressure Semi‐ductile shear zone may display composite slip behavior