Understanding Fault Slip Modes Through AE Scaling and Seismic Partitioning

Recent observations reveal that faults can host both slow and fast slip events, raising fundamental questions about their governing physics. Initially considered rare phenomena, slow slip events are now recognized as widespread features of fault zones, capable of releasing seismic moments comparable to large earthquakes, however, over longer time-scales.

We use laboratory experiments in a double-direct shear configuration with quartz gouge to explore fault slip behavior under varying normal stresses (8–22 MPa) and loading stiffness. The experiments were conducted on the BRAVA2 biaxial deformation apparatus, which features servo-controlled loading and high-frequency acoustic emission (AE) monitoring. By varying the stiffness ratio (k/k_c), defined as the ratio of system stiffness (k) to fault stiffness (k_c), we reproduced a spectrum of slip behaviors.

Our results reveal that the transition between slip modes is governed by the ratio (k/k_c) of the loading system stiffness (k) to the fault critical stiffness (k_c). We observe stable sliding when k/k_c > 1.4, slow slip events clustering around k/k_c ≈ 1, and fast events occurring when k/k_c < 0.8, with peak slip velocities ranging from hundreds of μm/s to over 25 mm/s.

These slip modes exhibit distinct seismic signatures: slow slip events produce swarms of low-amplitude AEs (M₀<10⁻² Nm) corresponding to fault acceleration, while fast events generate concentrated high-amplitude bursts (M₀>10⁻² Nm). Despite differences in seismic signatures, our findings reveal continuous scaling across slip modes, with breakdown work (W_b​) scaling with slip (W_b∝δ^1.47) and seismic moment (W_b∝M₀^0.25). The key difference lies in deformation partitioning: during slow slip, AEs occur in ~0.02% of the total slip duration, defined by the time during the stress drop, while fast slip events exhibit AEs over >10% of their duration. This indicates that slow slip is dominated by aseismic processes, whereas fast slip involves more seismic energy release.

Based on these observations, we suggest that slow and fast earthquakes represent end-members of a continuum. During slow slip, multiple small seismic patches develop during short inter-seismic periods, leading to distributed deformation. In contrast, fast slip events develop under longer inter-seismic periods, enabling the formation of larger, more coherent patches that fail simultaneously. This finding has important implications for understanding the spatiotemporal variations in fault slip behavior and suggests that changes in fault zone properties could trigger transitions between slip modes.