Using foreshocks to constrain earthquake nucleation: low foreshock to aftershock ratios in the Hikurangi subduction zone

Rebecca Louise Colquhoun , Jessica Cleary Hawthorne

The complexity and duration of earthquake nucleation is an open question. Our understanding of this process is limited by a lack of high-quality observations of foreshocks and aftershocks. We therefore apply a coherence-based template matching approach to search for more foreshocks and aftershocks. We examine the half-hour before thousands of M3+ mainshocks on the Hikurangi subduction zone in New Zealand, and make new detections of foreshocks and aftershocks which are close in space and time to the mainshocks. For M4+ events, we find 68% fewer foreshocks than expected if earthquake nucleation is explained by the most intuitive type of earthquake-earthquake triggering: single-mode triggering. The number of foreshocks suggests that nucleation must be more extended and complex, perhaps driven by external processes like pore-pressure changes.