The frictional properties of faults at shallow depths: implications for rupture propagation.

Abstract:

Following the surge of great earthquakes from 2004-2014, revised synoptic models have been proposed, where 2D spatial heterogeneity in frictional properties of continental and subduction fault zones is invoked to explain earthquake behaviours. However, none of the proposed models accounts for the changes in frictional properties that past ruptures may have caused on the rupture zones of recent events.

We performed velocity step friction experiments at sub-seismic slip rates (10-100 mm/s) on dry, water- and brine-saturated granite and calcite rocks (continental crust) showing that velocity strengthening behaviour evolves to velocity-neutral/-weakening behaviour after critical displacements of a few tens to hundreds of mm. Dry, water- and brine-saturated gabbros (oceanic crust) show velocity-weakening behaviour and slip localization for any applied displacement and normal stress. Dry, water- and brine-saturated phyllosilicate-rich gouges (typical of subduction zones and continental sedimentary deposits) show velocity-strengthening behaviour for any applied displacements and normal loads. Cyclic slide-hold-slide experiments show that, after sliding at sub-seismic slip rates, static friction increases with time according to a logarithmic relationship (fault healing) for almost all tested materials under dry, water- and brine-saturated conditions. The only exceptions are organic-rich black shales, which show a decrease in static fault friction with time (negative fault healing). The positive and negative healing rates tend to increase under water- and brine-saturated conditions, respectively. After sliding at seismic velocities (1 m/s), granite, calcite and phyllosilicate-rich gouges show an evolution toward less velocity strengthening to velocity-weakening behaviour, as opposed to grabbros gouges showing an evolution towards velocity strengthening behaviour.

Our experimental results suggest that patches of small and large seismic slip of past events are critical in controlling the frictional healing and frictional stability of both crustal and subduction fault zones. Hence, the distribution and amount of slip recorded by past events should be included in synoptic models describing the spatial heterogeneity of frictional properties along fault zones.