Cyclic Pore Fluid Pressure Fluctuations Throughout The Laboratory Earthquake Cycle

Fluids are ubiquitous in fault zones and play a key role in the mechanics that generate catastrophic earthquakes. Previous studies have shown that pore fluid pressure transients can exert a dominant control on fault strength and stability, however, it is unclear how these transients evolve over multiple slip events and how they lead to a spectrum of slip behaviors, including aseismic creep, slow slip, and seismogenic rupture. We use a heavily instrumented 1-meter scale experimental fault that allows for direct fluid injection into a sealed fault with on-fault pore-pressure measurements that are time synchronized with measurements of shear stress, normal stress, fault slip, and fault dilation and compaction. Consistent with previous studies, we show that the stress drop and slip rate of laboratory earthquakes scale inversely with the ambient pore-pressure. Our data also show systematic fluctuations in pore-pressure throughout the seismic cycle. Specifically, pore-pressure increases during the inter-seismic period as the fault compacts, reaches a maximum, then starts to decrease during the nucleation phase, before dropping rapidly during the co-seismic phase as the fault dilates. The magnitude of these transient changes scale inversely with the magnitude of the ambient pore-pressure, so that they are most prevalent at the highest effective normal stress. These results provide insight for the role of pore fluids in fault mechanics, which has important implications for seismic hazards.

For more information:  cegraham@eckerd.edu