A new talk
This is a brief outline of the first project we conducted with the osmotically driven flux meter systems. The were deployed within Ocean Botton Seismometers (LeRoy Dorman’s) on the forearc of the Costa Rica Subduction System.
The flux meters inject a tracers into the water flowing through a chamber embedded in the sea bed. the dilution of the tracers and pump rate gives the rate of flow thought the chambers exit. Very low flow rates (0.01 to1500 cm/d) through the seabed are measurable.
The tectonic activity related to slow slip cases pulsing of flow through the seabed in response to poro-elastic loading acroost the propagating tip.
Different types of pulsing flow events are seen at the toe of the wedge and on the incoming plate.
The pulsing flow correlated with seismic tremor (RMS noise) forming from the seismogenic fault
The downward directed flow pulsing on the incoming plate proceed a large normal earthquake.
A numerical poro-elastic model verified a slowly propagating rupture on the subduction thrust could cause the flow events.
This project seeks to evaluate the frictional weakening mechanisms associated with the nucleation of unstable slip and eventual propagation of earthquake ruptures. Slip rates on faults during nucleation are low <0.1 mm/s but greatly accelerate to between 1-10 m/s during rupture. The rupture front propagate at km/s and generates destructive shaking.
The heated shear zones develop and well developed fabric. Later studied were conducted on thick heated gouge samples.
Unstable slip can occur because dynamic friction varies with slip speed and can lead to transient fault weakening.
Comparison of new and previous a-b data. Negative a-b values denote unstable slip.
To slip unstably the a-b values must become negative.We observed together with previous studies that both temperature and the presence of water control the stability of quartz dominated lithologies.
The dip of the slip nucleation region varies with fault conditions.
HIGH SPEED FAULT WEAKENING DURING RUPTURE
Early Prototype Rotary Shear System
Frictional heating of stressed faults and elevated slip rates can Lead to melt weakening and Pseudotachylite formation. Pseudotachylites are glassy previously melted rock that can form veins around earthquake faults.
The onset of weakening at time Tc is quite sharp. At low stresses weakening is followed by strengthening as a thin viscous melt layer forms.
We attribute early weakening to initial melt wedging of the fault.
The wreakening show very systematic relationships to the rate of fricti0nal heat