Seismic–Aseismic Slip Partitioning on a Frictionally Heterogeneous Fault: An Experimental Approach

Faults in the shallow brittle crust are rarely frictionally homogeneous. Structural and mineralogical heterogeneities such as phyllosilicate-rich shear zones mixed with competent lenses generate, respectively, velocity-strengthening (VS) and velocity-weakening (VW) domains that strongly influence earthquake nucleation and rupture dynamics. Geological observations and laboratory experiments show that VW patches typically nucleate unstable stick-slip, whereas VS regions promote stable creep and can transfer stress on neighbouring VW patches. Although this heterogeneous patch framework supports models of shallow seismicity, induced seismicity, and subduction zones, direct experimental investigations with rock samples and realistic patch geometries remain limited.Here we present a new experimental framework for testing heterogeneous fault slip in cm-scale rock samples. Using the newly developed “BeeAx” servo-controlled biaxial apparatus, we sheared 15 × 17 cm Pennant Sandstone blocks at slow displacement rates (~1 µm/s) and 2, 5 and 8 MPa of normal stress. We compare three frictional sliding configurations: (1) homogeneous sandstone–sandstone (VW-dominated), (2) homogeneous graphite-coated sandstone (VS-dominated), and (3) heterogeneous samples with four circular uncoated sandstone patches embedded within a graphite background, comprising 50% of the sliding surface. High-resolution and calibrated acoustic emission (AE) monitoring (16 sensors) allows hypocentre location and source parameter retrieval, enabling direct comparison of microseismicity and frictional stability across configurations.The homogeneous graphite experiment produced stable sliding with very low friction (µ≈0.15), while the homogeneous sandstone samples exhibited unstable stick-slip and higher friction (µ≈0.5). The heterogeneous samples displayed hybrid behaviour: a low overall friction (µ≈0.20) comparable to graphite, yet persistent dynamic stick-slip events. AE hypocentres concentrated on the sandstone patches perimeters, revealing that aseismic creep in the weak graphite transfers shear stress onto the stronger patches, which subsequently fail seismically. Compared to homogeneous sandstone, heterogeneous samples showed larger stress drops, stronger localization of microseismicity, and reduced Gutenberg-Richter b-values. Temporal b-value evolution differed between configurations: constant and low for graphite, cyclic for sandstone (decreasing during interseismic loading and increasing post-mainshock), and intermediate but systematically lower in the heterogeneous case, consistent with enhanced stress transfer and patch interaction.These results demonstrate that discrete weak VS regions can modulate and even enhance the seismicity of stronger VW patches by acting as creeping load reservoirs. This provides laboratory support for models invoking patchy asperities on shallow faults and in induced seismicity settings, where strong and weak rock patches coexist within a fault. More broadly, the experimental platform enables controlled studies of rupture nucleation, asperity geometry, interaction, and seismicity evolution in frictionally heterogeneous fault systems.