Earthquake nests as natural laboratories for the study of intermediate-depth earthquake mechanics
The physical mechanism of intermediate-depth earthquakes is still under debate. In contrast to conditions in the crust and shallow lithosphere, at temperatures and pressures corresponding to depths > 50 km, rocks ought to yield by creep or flow rather than brittle failure. Some physical process has to enable brittle or brittle-like failure for intermediate-depth earthquakes. The two leading candidates for that are dehydration embrittlement and thermal shear runaway. Given their great depth, intermediate-depth earthquake processes can't be observed directly. Instead we must rely on a combination of seismology and the study of laboratory analogs to understand them. Earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity. In this paper we focus on three intermediate-depth earthquake nests — Vrancea, Hindu Kush and Bucaramanga, and what they reveal about the mechanics of intermediate-depth earthquakes. We review published studies of tectonic setting, focal mechanisms, precise earthquake locations and earthquake source physics at these locations, with an emphasis on the Bucaramanga nest. All three nests are associated with subducting lithosphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity. In contrast, the Bucaramanga nest has a larger b-value, larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two. We also report for the first time finding a significant number of repeating earthquakes, some with reverse polarity. Given the nature and characteristics of earthquake nests, they can be thought as natural laboratories. Future seismological studies of intermediate-depth earthquakes in nests will likely enlighten our understanding of their physical mechanisms.
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