Seasonal modulation of seismicity in an intraplate setting, the case of southeastern Australia

While most earthquakes occur at plate boundaries, significant seismic events also occur within stable continental regions (SCRs), despite their low strain rates. These intraplate earthquakes, including rare but damaging events, raise fundamental questions about how elastic strain cumulates, is stored, and released in slowly deforming crust.

We develop a high-resolution seismicity catalog for southeastern Australia, a tectonically stable intraplate region, spanning 2005-2025. The catalog was constructed using the BPMF workflow which integrates backprojection-based detection, deep learning phase picking, nonlinear probabilistic relocation, and matched filtering. Relative to the Geoscience Australia catalog, our approach increases the number of detected events by approximately a factor of six and achieves a magnitude of completeness of Mc = 2.1, enabling robust statistical analyses over two decades. This enhanced resolution enables the exploration of seismicity statistics, clustering behavior, and temporal variability in a low-seismicity environment. 

Using this catalog, we identify a statistically significant seasonal modulation of seismicity, with earthquake rates peaking during winter–spring and reaching a minimum during summer–autumn. The seasonal signal persists after declustering and is observed across a range of magnitude thresholds above completeness, indicating modulation of background seismicity rather than dominance by individual earthquake sequences. 

Further analysis of GNSS displacement, GRACE-derived hydrological loading, and seismicity using multichannel singular spectrum decomposition identifies coherent temporal modes shared across all datasets. This correspondence suggests that hydrological loading drives elastic stress perturbations that are temporally linked to variations in earthquake occurrence. Together, these results imply that even modest seasonal and environmental stresses can modulate seismicity in stable continental regions, providing new insights into fault stability in intraplate settings.