Program > Browse abstracts by speaker > Gasc Julien

The rheology and the seismicity of subducting lithosphere is extremely complex. In some descending slabs, the brittle-ductile transition is marked by the occurrence of Low-Frequency Earthquakes (LFEs) causing non-volcanic tremors. And in rapidly sinking slabs, Deep-Focus Earthquakes (DFEs) are observed in the transition zone, i.e., at depths of ~ 400-700 km. Both types of earthquakes are characterized by unusual relationships between their main properties, (e.g., magnitude, duration, frequency content). In other terms, the scaling laws of deep seismicity −because of elevated PT− do not follow those of frictional rupture. Both LFEs and DFEs have been explained by invoking other mechanisms. DFEs define a cluster of seismicity, with its upper and lower bounds matching precisely the expected appearance of wadsleyite and the breakdown of ringwoodite, respectively, thus strongly suggesting a tie to olivine transformations. LFEs, on the other hand, occur in a fluid rich region owing to dehydration reactions and have often been attributed to high fluid pressure.

In recent years, several studies have successfully recorded microseismicity in high pressure experiments in the form of Acoustic Emissions (AEs), which can be treated as analogues to actual earthquakes. DFEs were reproduced during deformation experiments by transformational faulting, where the growth of the high-pressure phase in olivine aggregates causes strain localization and AEs. Similarly, low-frequency tremor-like AEs were collected at the PT conditions of LFEs. In both cases, when assessed with a seismological approach, the obtained AEs reveal scaling laws that are similar to those of their natural counterparts and that may therefore explain some of their characteristics. For instance, the magnitude vs occurrence rate of AEs obtained during transformational faulting, i.e., their b-value, is different than that of brittle failure and strongly depends on stress level, which clarifies the large span of b-values observed for DFEs across subduction zones.