What controls seismicity at intermediate depths in subducting slabs : a study of the M7.1 2003 Miyagi-oki intraslab earthquake sequence

In subduction zones, earthquakes at depths between 60 and 100 km occur within the subducting slab rather than at the slab interface. The presence of fluids resulting from dehydration reactions in the medium is often called to explain the occurrence of these earthquakes, as fluids would favour the rupture (by fluid embrittlement). But the relation between the two processes is yet not fully constrained.

We study the aftershock sequence following the M7.1 2003/05/26 intraslab earthquake which was located off the Miyagi prefecture coast, in Japan, at 70 km-depth. This sequence displays characteristics that are promising for studying the intraslab and the relations between seismicity and fluid pressure (high aftershock rate, rupture of both the crust and the mantle of the slab, expanded instrumentation...).

The analysis of the catalogue of seismicity and focal mechanisms provides information on the principal characteristics of the aftershock sequence (Omori-Utsu law, Gutenberg-Richter law). In particular, the aftershock sequence follows a nearly perfect Omori’s law with a p-exponent depending on depth. This extremely good agreement between the data and the model appears to be due to the absence of large aftershocks, as confirmed by a significant deviation of the frequency-magnitude relationship from the Gutenberg-Richter law at large magnitudes. An application of the ETAS model to the sequence suggests that most of the sequence would be triggered by the M7.1 itself, i.e the aftershocks play no role in triggering more aftershocks. Moreover, the temporal distribution (although it has to be confirmed after Template Matching reevaluation of the catalogue) and inversion of stress field in the small aftershock zone show that, unlike the slab interface, the area inside the slab does not seem to be disturbed by the nearby occurrence of M9 Tohoku-oki earthquake (2011/03/11).

We conclude from our analyses that this intraslab sequence is characteristic of a very critically stressed crustal and upper mantle volume implying strong faults that are not sensitive to large stress perturbations. Moreover, if fluids are involved, then they are likely to be drained off from the top of the activated volume as suggested by the depth dependence of the Omori-Utsu’s p-value, possibly playing a role in the subsequent occurrence of the 2011 megathrust Tohoku-Oki earthquake which hypocenter is updip this sequence.