Creep events at the brittle ductile transition

Abstract

[1] We present an analytic formulation to model creep events at the transition between brittle behavior in the crust and viscous behavior in ductile shear zones. We assume that creep events at the brittle ductile transition (BDT) are triggered by slip on optimally oriented fractures or network of fractures filled with weak ductile material. These events are expressed as transient flow in ductile shear zones likely aided by the release of crustal fluids. We show that the creep in the shear zone can be modeled as the motion of a forced damped oscillator composed of a brittle viscoelastic crust, a ductile shear zone and a creeping zone of fractures at the BDT. The time scale of the events varies between seconds to thousands of years depending on the viscous, elastic and brittle-plastic properties of the fractured BDT, the shear zone and the crust. The nature of the events depends on the aspect ratio, of the shear zone thickness, H w to the length of the fractured zone, w. We find that thick shear zones with small fractures at the BDT are stiff and generate creep oscillations. Thin shear zones with well-connected fractures over a large width have very small stiffness and are well lubricated. They generate slow creep events or steady creep event. The former are similar to transient slip events and the latter to creep at the far field tectonic rates. The viscosity of the shear zone, w enhances lubrication if it is small and stiffness if it is large.

The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes

“…High fluid pressures and concomitant low effective stress inferred from seismological studies have also been widely invoked as a key factor in controlling deep SSE, tremor and LFE occurrence 9,40,41 . As is the case for shallow slow events, these mechanisms are consistent with numerical investigations showing that both heterogeneous frictional or rheological properties [48][49][50] and low effective stress linked to high pore pressure 26 can lead to emergent quasi-periodic slow slip transients.…”

“…Such models are able to reproduce transients over a broader range of conditions than models that assume uniform rate-state friction parameters 26 . Other models have achieved the same result by incorporating rheological heterogeneities through a mix of brittle, viscous and elastic materials along the fault 48,50 .…”

“…Most of these examples are characterized by abundant microseismicity (M w of 2.0-4.0 and larger) within and surrounding the slow slip regions 6,8,12,13 . It is likely that basement relief on the incoming plate leads to a heterogeneous and geometrically complex subduction interface, composed of a mosaic of lithologies, including carbonate, basalt and altered basalt, clays and quartz, that would be prone to a mix of stable and unstable behaviours 32,33,[48][49][50] . This heterogeneity is most clearly imaged at the northern Hikurangi margin, where seismic reflection data reveal diverse plate interface properties, including zones of high-amplitude reflectivity (interpreted as fluid-rich, underplated sediments) with intervening lower amplitude zones, and non-reflective areas interpreted as subducting seamounts 29 (Fig.…”

“…Schematic cross section showing in situ conditions and processes common to shallow slow slip event source regions, including: (1) geometric complexity and heterogeneous lithologies along the plate interface at multiple scales29,32,33 ; (2) elevated pore fluid pressure and concomitant low effective normal stress27,28 ; and (3) foliated quartz-and clay-rich fault zones33,44 characterized by patterns of rate-weakening behaviour conducive to slow slip transients30,31,33,48 . The seismic reflection profile shows an example from the Hikurangi margin slow slip event source.…”

Stationary points in activation energy for heat dissipated with a power law temperature‐dependent viscoelastoplastic rheology

“…Since the main focus of our study is on the effects of activation energy, which determines the degree of temperature dependence of rheology, we have paid attention to the thermally activated positive feedback (i.e., shear heating). The shear zone formation with shear heating in various materials has been extensively investigated to understand the transition from ductile to brittle regimes [e.g., Lavier et al, 2013] and the shear instability with bimaterial elastic heterogeneities [e.g., So et al, 2012]. However, the influence of activation energy on plastic flow for given environmental conditions, such as initial temperature and amount of deformation, has not been studied up to now.…”