Interactions between deep seismicity and shallow deformation in the Japan trench and Chile subduction zones

Recent great subduction earthquakes have been preceded by an accelerated rate of both interface seismicity along the megathrust and intermediate depth seismicity within the slab at ~100km depth (e.g. Bouchon et al., 2016, 2018), sometimes along with seismicity lineaments along dip over some hours (Bouchon et al., 2022, 2023). These may also be associated with large-scale gravity and mass changes in the subduction zone (Panet el al. 2018). Such interactions between deep and interface seismicity can last several years and can be associated with deformation within the upper plate (Durand et al., 2014; Jara et al. 2018, Rousset et al. 2023, Mitsui et al. 2021).

However such interactions between deep seismicity and shallow deformation have been observed only on rare occasions. In addition, assessing better how they relate to fluid transfer and slab force balance is key to improved understanding of the driving mechanisms of the plate interface destabilization.

Here we present some intriguing examples of interactions between intraslab seismicity and shallow deformation, and assess their statistical significance. We show that the occurrence of the Tohoku earthquake significantly changed the deep (>150km) seismicity rate, suggesting that this major megathrust event modified the slab equilibrium down to the lower mantle.

We also revisit the interactions between intermediate-depth and shallow seismicity in the Japan trench and the northern Chile subduction zone, during the decade preceding the Tohoku-oki (Mw 9.0, 2011) and Iquique (Mw 8.2, 2014) megathrust events. Cross correlations highlight different periods with significant interactions between intermediate-depth and shallow earthquakes, including the 8 months before the Tohoku-Oki megathrust in Japan, over which multiple bursts of ~7 days are synchronized. In Chile, the 4 months preceding the Iquique megathrust also show strong interactions, with successive bursts of ~4 days. Unlike some other periods, no stress transfer implied by Mw>6 earthquakes can explain the correlations observed before both megathrust events. Clustering of the seismicity allowed to identify along-dip lineament patterns. Their occurrence rate shows a significant increase when approaching the date of the megathrusts. If only a few are observed in Chile, the numerous lineaments downdip Tohoku highlight some structures along which lineaments concentrate. These elongated seismicity features seem to connect intermediate-depth and shallow seismicity and could be explained by fluid migrations.