Semi-constant source radius and variable stress drop of repeating earthquakes: a fluid-invasion hypothesis for earthquake generation

SUMMARY Earthquakes with very similar waveforms have been recognized as repeating earthquakes, or repeaters, which are interpreted as repeated ruptures of an isolated, frictionally locked asperity patch that are caused by stress loading due to aseismic slip in the surrounding stable regime of the fault. Ideally, the repeated ruptures of the same asperity result in the same source dimension and stress drop. However, significant variations in the stress drop have been observed during fluid-injection experiments even for a single repeater sequence. This study focuses repeater sequences in three different tectonic regimes beneath the Japanese Islands and investigates variations in the source dimension and stress drop in the same repeater sequences. The obtained results show that the variation in the source radius is mostly within 20–30 per cent in individual repeater sequences but the stress drop is largely variable by one order of magnitude or more. We propose a fluid-invasion hypothesis to explain our observations. When pore-fluid pressures gradually invade into an asperity, the edge of the asperity tends to behave as aseismic because of the reduction of the effective normal stress to values below the critical value of effective normal stress, which makes the source radius somewhat smaller. Simultaneously, the amount of the fluid invasion into the asperity fluctuates the shear strength of the asperity, with the stress drop being smaller for higher pore-fluid pressures. The reduced source radius and stress drop expected from the fluid-invasion hypothesis can explain positive correlations between the source radius and seismic moment. We propose a conceptual model for earthquake generation by involving the triggering of aseismic slip by the enhancement of pore-fluid pressures and subsequent rupture of asperity by the stress loading due to the aseismic slip, which can be potentially applied to the genesis of repeaters and non-repeaters.