Intermediate-depth earthquakes have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Recent experiments on serpentinized peridotites at upper mantle conditions allow to decipher the mechanism of these earthquakes (Ferrand et al., 2017). At 1.1 GPa, dehydration of deforming samples with 5 vol% antigorite triggers seismic ruptures; at 3.5 GPa, acoustic emissions are recorded for samples with up to 50 vol% of antigorite. Experimental faults are sealed by fluid-bearing micro-pseudotachylytes. Antigorite dehydration triggers dynamic shear failure, demonstrating that little dehydration is required to trigger embrittlement. These experiments lead to the “dehydration-driven stress transfer” (DDST) model, which does not require fluid overpressure, contrary to the “dehydration-embrittlement” model.
Field observations in the Balmuccia massif, NW Italy (Ferrand et al., 2018), show the exact equivalent of the latter experimental earthquakes. The natural pseudotachylyte is the frozen record of an intermediate-depth (>40 km) earthquake. Its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite‐like veins and deciphered by electron backscattered diffraction. When in the lab the largest acoustic emission corresponds to a Mw -6 earthquake, the natural pseudotachylyte is demonstrated to correspond to a Mw 6 to 6.5 earthquake, i.e. about one million times larger than the experimental ones. Combined together, experimental and natural pseudotachylytes show scaling of the fault thickness with relative displacement (Ferrand et al., submitted).
Seismological data (Kita & Ferrand, 2018) show that lower-plane events reveal significantly larger b-values beneath Tohoku (0.96) than Hokkaido (0.86), implying that the brittle deformation beneath Hokkaido is more localized and leads to a higher ratio of relatively large lower-plane events than beneath Tohoku. According to observations, lower-plane peridotite is more hydrated beneath the Tohoku region, which is compatible with oceanic-plate velocity structures near the trench identified in OBS studies offshore Tohoku and Hokkaido. These results suggest that lower-plane events occur in fresh peridotite in between serpentinized faults.
Hypocentre P-T conditions do not fit the dehydration curve of nominal serpentine minerals (Abers et al., 2013), yet recent experimental results confirm that serpentine minerals dehydrate very fast (Liu et al., 2019) and cannot be metastable in subduction conditions (Ferrand, 2019b). Serpentinized faults mostly consist of variable amounts of serpentine minerals, along with a mixture of numerous H2O- and/or CO2-bearing phases, which have their own stability limits and may explain the earthquakes P-T conditions that do not fit serpentine dehydration (Ferrand, 2019a). In other words, a myriad of minor metamorphic reactions could participate in a “transformation-driven stress transfer” (TDST), even though the stability limits of serpentine minerals correlate with most of the observed seismicity (Ferrand, 2019a).