[S-CG46] スラブ内地震とその発生メカニズム

[SCG46-01] Preliminary results from an amphibious MT study of the Mw 8.2 intermediate depth intraslab Tehuantepec, Mexico earthquake

★Invited Papers

*Allen L Husker¹、Diego Ruiz Aguilar²、Steven Constable³、Carlos Mortera⁴、Claudia Arango Galvan⁴ (1.California Institute of Technology、2.Center for Scientific Research and Higher Education of Baja California, Mexico、3.Scripps Institution of Oceanography、4.National Autonomous University of Mexico)

キーワード：Intermediate depth earthquake, Magnetotelluric, Fluids

This study aims to understand the 2017 Mw 8.2 Tehuantepec, Mexico intraslab earthquake within the subduction zone. It was an unusual earthquake that did not follow many standard seismological hypotheses. It was a normal faulting earthquake parallel the trench and just below the slab interface of the megathrust seismogenic zone. Worldwide there have been 27 Mw 8+ earthquakes since 2000. 6 of those have been normal. 3 of those had hypocenters > 100 km depth and 2 < 20 km depth. The Tehuantepec earthquake was the only one at intermediate depth at 47 km. The slab interface there has never recorded a Mw 7+ earthquake and is considered a long-standing seismic gap and possibly aseismic. It is also considered highly coupled since the trench is deformed there by the subducting Tehuantepec Ridge into the seismic gap, although highly coupled typically implies that there is a record of many large earthquakes. The Tehuantepec rupture initiated at the base of the slab where it was too hot to be brittle. The theories for how this earthquake occurred include (1) that slab pull combined with the highly coupled upper plate caused it to break, (2) that slab bending led to a weakened zone, and (3) that the earthquake occurred along a reactivated outer rise fault that possibly had fluid infiltration. This study uses magnetotelluric soundings to look for evidence of possible fluids in the fault region as well as the role of the Tehuantepec ridge. We also explore the explanations for the dichotomy of the region being aseismically highly coupled. Slow slip events (SSE) at the slab interface could explain the lack of large magnitude earthquakes while also being highly coupled. SSEs account for the expected slip build-up in other highly coupled parts of Mexico. We successfully used magnetotelluric surveys in those regions of Mexico by identifying associated zones high conductivity, which are a proxy for high pore fluid pressure. Worldwide high pore fluid pressure in subduction zones is identified with SSEs. We present the preliminary findings after our recent field season.