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

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Abstract

Episodic tremor and accompanying slow slip, together called ETS, is most often observed in subduction zones of young and warm subducting slabs1,2,3. ETS should help us to understand the mechanics of subduction megathrusts3,4, 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 zone5,6,7,8. 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 environment9 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 corner10 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.

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Figure 1: Observed relationship between the seismogenic zone and the ETS zone in Nankai.
Figure 2: Schematic illustration of fault stress and slip phenomena for subduction zones that produce great earthquakes and ETS.
Figure 3: Models of megathrust rheology for warm-slab subduction zones Nankai, Northern Cascadia and Mexico, and cold-slab subduction zones Japan Trench and Hikurangi.
Figure 4: Rheologically controlled slip phenomena along the San Andreas Fault.

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Acknowledgements

We thank J. He for developing the computer code PGCtherm2D employed in this work. X.G. was supported by the Chinese Academy of Sciences’ Strategic Priority Research Program (grant XDA11030102) and the National Natural Science Foundation of China (grant 41406063). K.W. was supported by Geological Survey of Canada core funding and Discovery Grant from Natural Sciences and Engineering Research Council of Canada through the University of Victoria. This is Geological Survey of Canada contribution 20160265.

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X.G. conducted the thermal and rheological modelling. X.G. and K.W. jointly designed the research and contributed equally to the writing.

Correspondence to Kelin Wang.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks P. Audet and D. Shelly for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 The function (see equations (4) and (5)) used to scale fault shear stresses to simulate the effect of high Pf.

The shown updip skewed (q = 0.3) and broad (b = 0.3) distribution is used for all models in this work. The distance is measured from the updip end of the high-Pf zone and normalized by its width W.

Extended Data Figure 2 Map view of the distribution of megathrust seismogenic zone, slow slip events, and tremor at the northern Cascadia, Mexico, Japan Trench, and Hikurangi subduction zones.

Data are based on references given in Extended Data Table 2. Red patches show tremor distribution (or ETS zone). Blue patches indicate slip areas of long-term SSEs (labelled with year of occurrence). Yellow shading shows megathrust earthquakes (labelled with year of occurrence) or inferred seismogenic zone. Thick blue line indicates thermal model profile. References for the depth contours (in kilometres) of the plate interface (dashed lines) are given in Extended Data Table 1. a, Northern Cascadia. The megathrust coseismic slip scenario is from ref. 8. b, Mexico. c, The Japan Trench. Green patches are rupture areas of Mw > 7.0, moderate to large interplate earthquakes51,52. Bold green line marks the downdip limit of reported interplate earthquakes of all sizes25. d, Hikurangi. The megathrust of the northern two-thirds of the margin is almost fully creeping, as inferred from GPS observations26,53.

Extended Data Table 1 Summary of subduction zones studied here
Extended Data Table 2 Fault slip phenomena in subduction zones studied here and in the SAF

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Gao, X., Wang, K. Rheological separation of the megathrust seismogenic zone and episodic tremor and slip. Nature 543, 416–419 (2017) doi:10.1038/nature21389

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