Size‐Dependent Melting Behavior of Ultrafine Fault Rocks

Abstract Dynamic weakening during earthquake ruptures is essential for accommodating fault slip and controlling seismic energy release. This weakening is localized within narrow principal slip zones (PSZs) that commonly contain nanoparticles in both natural and experimental faults. Although weakening is dominated by thermally activated mechanisms, the thermal and mechanical effects of extreme grain‐size reduction in PSZs remain poorly constrained. We examine the melting behavior of dry granitoid fault rocks ball‐milled to mean grain sizes from 150 μm to 300 nm. Differential scanning calorimetry reveals a decrease in melting temperature, from 1,270°C to 1,090°C, with decreasing grain size. Our results demonstrate that extreme grain‐size reduction lowers the temperatures required for dynamic weakening via melting. Comparisons with previous studies suggest PSZs are enriched in ultrafine, mechanically weak mineral phases that can influence frictional melt composition. In natural faults, this effect is likely amplified by complex mineralogy, high pressure–temperature conditions, and fluids.