The depth of seismic faulting and the upper transition from stable to unstable slip regimes

Abstract

Abstract. A number of observations indicate that an upper stability transition occurs along well-developed faults, such as the San Andreas, as a result of unconsolidated gouge within shallow regions of these faults. These observations include the depth distribution of seismicity along faults with and without well-developed gouge zones, correlations between seismicity and shallow crustal structure, and modeling of coseismic and post-seismic slip. In addition, recent experimental friction studies indicate that thick layers of simulated gouge exhibit a positive slip-rate dependence of frictional resistance (velocity strengthening) and thus inherently stable slip, whereas bare rock surfaces and thin gouge layers exhibit potentially unstable velocity weakening behavior. Subduction zones with large accretionary wedges also exhibit an upper stability transition in that slip is aseismic within the accretionary wedge. A stability transition due to the presence of unconsolidated material can also be invoked in this case.

Expedition 344 summary

“…Studies of interplate seismicity have shown that large earthquakes generally initiate within specific depth limits, generally between ~10 and 40 km (Byrne et al, 1988;Pacheco et al, 1993;Tichelaar and Ruff, 1993;Scholz, 2002), but the precise limits vary with temperature (Hyndman et al, 1997). This observation led to a conceptual model in which the subduction thrust is divided into three zones (e.g., Scholz, 1988). In the shallowest zone, plate convergence is accommodated by stable (aseismic) slip.…”

“…Pore pressures in excess of hydrostatic pressures decrease the effective normal stress, thereby limiting the shear stress, and, coupled with the low consolidation and lithification state of sediment in the updip aseismic portion of the fault, may explain the lack of seismicity. A downdip decrease in pore fluid pressure increases the normal stress, leading to greater instability in plate sliding (Scholz, 1988). …”

“…This hypothesis posits that the updip limit of the seismogenic zone occurs near a threshold of consolidation and/or lithification in which fault zone behavior changes from distributed shear (stable sliding), where shear is accommodated by granular processes, to localized shear (unstable sliding) along discrete surfaces. Among other factors, the processes of lithification and consolidation are influenced by fault slip rate, mineralogy, and heat flow.Pore pressure within the fault zone is a principal factor controlling effective stress and strength along the plate interface (Sibson, 1981;Scholz, 1988). Pore pressures in excess of hydrostatic pressures decrease the effective normal stress, thereby limiting the shear stress, and, coupled with the low consolidation and lithification state of sediment in the updip aseismic portion of the fault, may explain the lack of seismicity.…”

“…Sediment properties including porosity, permeability, consolidation state, and alteration history also exert a strong influence on fault zone behavior. At erosive margins, where the plate boundary cuts into the overriding plate, the composition and strength of the upper plate is also important (McCaffrey, 1993).Field observations and laboratory experiments suggest that a prerequisite for unstable sliding is that the incoming sediment is consolidated and lithified (Davis et al, 1983;Byrne et al, 1988;Marone and Scholz, 1988;Marone and Saffer, 2007;Fagereng and Sibson, 2010). This hypothesis posits that the updip limit of the seismogenic zone occurs near a threshold of consolidation and/or lithification in which fault zone behavior changes from distributed shear (stable sliding), where shear is accommodated by granular processes, to localized shear (unstable sliding) along discrete surfaces.…”

Seismic variability of subduction thrust faults: Insights from laboratory models

“…Its relation with stress drop has been experimentally simulated with granular flow models [e.g., Higashi and Sumita, 2009]. Similar factors can play a role in subduction thrusts, favored by dehydration reactions in the subducting lithosphere [e.g., Pytte and Reynolds, 1988], changes in lithification state [Marone and Scholz, 1988;Marone and Saffer, 2007] and diagenetic processes [Moore and Saffer, 2001]. Thus, the initially rough profile of the lithosphere at the trench will be gradually smoothed as subduction evolves, minimizing roughness fluctuations with depth [Wang, 2010a].…”