Geology of the Tremor Source: What are the Active Processes?

Abstract:

Tectonic tremor is a low frequency seismic signal lacking impulsive body wave arrivals, but including low and very-low frequency earthquakes. The seismic signal is interpreted to be associated with shear failure at the tremor source, and has been detected on well-instrumented subduction and transform plate boundary interfaces. Tectonic tremor has now been detected at a wide range of depths, but appears to predominantly occur in regions of transitional frictional behavior. To create the low and very-low frequency earthquakes within the tremor signal, frictional failure with a low stress drop is required at the tremor source. A common inference is therefore that tremor involves shear failure at low effective normal stress.

Incrementally formed slickenfibre veins are commonly observed in exhumed fault zones. Based on the angle between shear surfaces and tensile veins, slickenfibres can in some locations be shown to have formed incrementally at low effective stress, comparable to tectonic tremor. The veins show slip increments of 10 - 100 micrometers, and along-strike lengths of tens to hundreds of meters, in agreement with values predicted from low frequency earthquake signals. 

Although slickenfibre veins are a candidate for the geological fingerprint of tectonic tremor, the tremor source occurs over a depth and temperature range that involves several grain scale deformation mechanisms, and both pro- and retrograde metamorphic conditions. The mechanism may therefore be more fundamental, and relate to shear failure in a mixed rheology rock assemblage where ambient stress levels are low. This includes shear failure on weak, frictional discontinuities within bulk viscous rheologies, as can be expected within the frictional transition zones where tremor is observed, independently of metamorphic grade. Slickenfibre veins represent one type of preserved shear discontinuity, on which it is easy to determine past slip increments. We show a number of other examples of frictional-viscous shear zones where bulk viscous deformation is accompanied by localized shear, and suggest that tremor may be a common byproduct of finite strain accumulation across mixed-rheology shear zones.