The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes


The recognition of a previously unknown spectrum of slow earthquake phenomena has ignited one of the most dynamic fields in modern seismology and fault mechanics. These slow events can last for a few seconds to years and occur in a wide range of settings. They are most extensively studied on the deep portion of the plate interface in subduction zones, where they are typically detected by land-based instrument networks. Recent investigations reveal that similar events are also common along the shallow, more accessible reaches of subduction zone megathrust faults, near the trench. These shallow slow earthquakes may be linked to tsunamigenesis and the triggering of large interplate earthquakes. Geophysical surveys, drilling, numerical modelling and laboratory studies focused on this near-trench region collectively show that shallow slow earthquake phenomena occur in regions of highly overpressured fluid and low effective stress, and within fault rocks characterized by transitional frictional behaviour. However, slow earthquakes are not restricted to specific temperature regimes or depths. They are also linked with the subduction of rough seafloor, where the plate interface is likely to be compositionally and geometrically heterogeneous. The physical conditions conducive to slow earthquakes are thought to be met on many shallow megathrust faults. We therefore expect that shallow slow slip occurs at many, if not most, subduction zones.

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Figure 1: Selected subduction margins with well-characterized shallow slow slip events and slow earthquakes.
Figure 2: In situ thermal, metamorphic and hydrologic conditions of slow slip events and slow earthquake source regions.
Figure 3: Fault rocks and rock properties.
Figure 4: Summary of the shallow slow earthquake and slow slip event environment.


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We thank T. Dixon, Y. Jiang, S. Ellis, R. Harris, H. Kitajima, K. Ujiie, and M. Ikari for providing data and model results shown in Figs 1,2,3, and K. Ujiie and A. Fagereng for providing photos shown in Figs 3 and 4. This work also benefitted from detailed discussions with M. Ikari and R. Harris.

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Both authors contributed equally to data analysis, planning, and writing the manuscript.

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Correspondence to Demian M. Saffer.

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Saffer, D., Wallace, L. The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes. Nature Geosci 8, 594–600 (2015).

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