Abstract
Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab, spanning ~50 major deep slow slip with tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of in-slab seismicity are temporally associated with the episodes. Furthermore, although stress orientations in the slab below these slow slips may rotate slightly, in-slab orientations 20–50 km updip from there rotate farther, suggesting that previously-unrecognized transient slow slip occurs on the plate interface updip. We infer that fluid pressure propagates from slab to interface, promoting episodes of slow slip, which break mineral seals, allowing the pressure to propagate tens of km further updip along the interface where it promotes transient slow slips. The proposed methodology, based primarily on in-slab seismicity, may help monitor plate boundary conditions and slow slip phenomena, which can signal the beginning stages of megathrust earthquakes.
Highlights
Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts
We refer to these quasi-characteristic slow earthquakes as “slow slip with tremor” (SST) to emphasize that tremor is driven by slow slip and is not a separate process
We determined times of approximately 50 large SST events at depths of ~30 km beneath the Kii Peninsula from January 2002 to March 2019 using hypocenter information for tremors detected by NIED7,9,10 (Fig. 1b)
Summary
Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. B-values, and stress orientations of in-slab seismicity are temporally associated with the episodes. Studies have examined the interaction between slow slip and in-slab earthquakes, showing that even slow deformation on the plate interface has the potential to change the stress field[2] and seismicity[3] within the subducting slab. An analysis[2] of the Hikurangi subduction zone, which is generally in downdip tension, found a change in orientation of the least compressive stress σ3 of ~15°, using 14 months of in-slab focal mechanisms associated with four slow slip events of up to M6.8. We find temporal changes of the in-slab seismicity rate, b-values, and stress orientations associated with the slow slip episodes, and infer a key role for fluid pressure propagation in promoting slow slip
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