Rheological separation of the megathrust seismogenic zone and episodic tremor and slip

• Published in: Nature (London) 2017-03, Vol.543 (7645), p.416-419
• Main Authors: Gao, Xiang, Wang, Kelin
• Format: Article
• Language: English
• Subjects: 704/2151/210; 704/2151/508; 704/2151/562; 704/4111; Earthquakes; Geological research; High temperature; Humanities and Social Sciences; letter; multidisciplinary; Natural history; Plate tectonics; Rheology; Science; Seismology; Slabs; Strike-slip faults; Subduction zones
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature21389

A model of the conditions required for episodic tremor and accompanying slow slip to occur, near the megathrust seismogenic zone, reconciles seemingly contradictory observations. Zone separation in subducting slabs Xiang Gao and Kelin Wang propose a unifying model for the spatial separation between the seismogenic zone and the zone of episodic tremor and accompanying slow slip (ETS zone) that is observed in some subduction zones of young and warm subducting slabs. Using numerical thermal models, they infer that high temperatures in the warm-slab environment cause the megathrust seismogenic zone to terminate at a depth much shallower than that of the mantle wedge corner. They conclude that high pore-fluid pressures around the mantle wedge corner give rise to an isolated friction zone that is responsible for episodic tremor and slow slip, and that this friction zone is separated from the seismogenic zone by a segment of semi-frictional or viscous behaviour. Episodic tremor and accompanying slow slip, together called ETS, is most often observed in subduction zones of young and warm subducting slabs 1 , 2 , 3 . ETS should help us to understand the mechanics of subduction megathrusts 3 , 4 , but its mechanism is still unclear. It is commonly assumed that ETS represents a transition from seismic to aseismic behaviour of the megathrust with increasing depth, but this assumption is in contradiction with an observed spatial separation between the seismogenic zone and the ETS zone 5 , 6 , 7 , 8 . Here we propose a unifying model for the necessary geological condition of ETS that explains the relationship between the two zones. By developing numerical thermal models, we examine the governing role of thermo-petrologically controlled fault zone rheology (frictional versus viscous shear). High temperatures in the warm-slab environment 9 cause the megathrust seismogenic zone to terminate before reaching the depth of the intersection of the continental Mohorovičić discontinuity (Moho) and the subduction interface, called the mantle wedge corner. High pore-fluid pressures around the mantle wedge corner 10 give rise to an isolated friction zone responsible for ETS. Separating the two zones is a segment of semi-frictional or viscous behaviour. The new model reconciles a wide range of seemingly disparate observations and defines a conceptual framework for the study of slip behaviour and the seismogenesis of major faults.