3:00 PM - 3:15 PM
[SIT20-06] Learning About Transition Zone Rheology from the Scaling Laws of Deep-Focus Labquakes
★Invited Papers
Keywords:mantle transition zone, deep-focus earthquakes, acoustic emissions, high pressure experiments, rheology, deformation experiments
The rheology and the seismology of subducting lithosphere in the transition zone of the mantle is extremely complex. Descending slabs generate large thermal anomalies in the mantle; and in those that sink most rapidly, deep-focus earthquakes are observed. In terms of temperature, these earthquakes may be compatible with what is known of the brittle-ductile transition. However, frictional processes at those depths predict unrealistic stress drops. Deep-focus earthquakes tend to define a cluster of seismicity, with its upper and lower bounds matching precisely the expected appearance of wadsleyite and the extinction of ringwoodite, respectively, thus strongly suggesting a tie of deep focus earthquakes to these transformations.
In recent years, several studies have successfully produced high-pressure experiments where transformational faulting proved to be a satisfactory mechanism to reconcile these observations. The growth of the high-pressure phase in strong olivine aggregates considerably weakens the samples and consistently leads to strain localization. In certain cases, this strain localization causes brittleness and associated acoustic emissions (aka labquakes) are collected. These experiments are extremely valuable because they allow a quantification of how sluggish kinetics must be, relative to the imposed strain, for transformational faulting to occur. In addition, when processed with a seismological approach, the obtained acoustic emissions reveal scaling laws that are similar to those of their natural counterparts and that may therefore explain some of their characteristics. Their magnitude vs occurrence-rate distribution, ie., their b value, is different than that of regular brittle failure and strongly depends on stress level, which can be related to the large span of b-values observed for deep-focus earthquakes across subduction zones. However, their frequency-moment-duration scalings seem to agree with those of shallow earthquakes.
In recent years, several studies have successfully produced high-pressure experiments where transformational faulting proved to be a satisfactory mechanism to reconcile these observations. The growth of the high-pressure phase in strong olivine aggregates considerably weakens the samples and consistently leads to strain localization. In certain cases, this strain localization causes brittleness and associated acoustic emissions (aka labquakes) are collected. These experiments are extremely valuable because they allow a quantification of how sluggish kinetics must be, relative to the imposed strain, for transformational faulting to occur. In addition, when processed with a seismological approach, the obtained acoustic emissions reveal scaling laws that are similar to those of their natural counterparts and that may therefore explain some of their characteristics. Their magnitude vs occurrence-rate distribution, ie., their b value, is different than that of regular brittle failure and strongly depends on stress level, which can be related to the large span of b-values observed for deep-focus earthquakes across subduction zones. However, their frequency-moment-duration scalings seem to agree with those of shallow earthquakes.