Concordant Dilatancy in Stick-Slip Events and Fast/Slow Earthquakes: Insights from High-Resolution Studies

Fast and slow earthquakes represent distinct modes of energy release during tectonic fault rupture. While laboratory stick-slip experiments have observed both fast and slow slips, limitations in sampling rates have obscured detailed insights into fault thickness variation. Specifically, the underlying reasons for a single fault exhibiting different slip modes have remained elusive. In this study, we conducted ring shear experiments employing an ultrahigh sampling rate (10 MHz) to shed light on the contrasting physical processes between fast and slow slip events. Our findings reveal that slip durations varied from dozens to hundreds of milliseconds. Fast slip events exhibit continuous large-amplitude Acoustic Emission (AE) signals alongside complex variations in sample thickness: a brief compaction pulse during rapid stress release, succeeded by sample dilation and thickness vibrations. As the slip concludes, the sample thickness initially undergoes slow compaction, followed by dilation preceding the nucleation of subsequent slip events. Conversely, during slow slip events, shear stress reduction coincides with intermittent bursts of low-amplitude AE and sample dilation. Fast and slow slips have similar AE spectra. Detailed observations of thickness variations during slips indicate that dilation occurs in both fast and slow slips, aligning with natural observations of coseismic dilatation. This study offers insights into the mechanisms governing fault slips during fast and slow earthquakes, potentially explaining their impact on stress redistribution and structural reorganization within faults.