Dr. Takamasa Kanaya - How rock deformation experiments can help understand when and where large earthquakes happen – A tale of ‘Swimmy’ cracks and a ‘Big Fish’ fault

How do earthquakes begin? Under increasing tectonic stresses, the crust deforms initially by microfracturing throughout broad regions, but, as the stresses approach the rock strengths, microfractures grow, interact, and eventually coalesce to nucleate large fault rupture. However, a limited number of laboratory studies show inconsistent amounts of crack interactions at the time of fault nucleation. We quantified crack interactions for microfractures formed in experimentally deformed sandstones through determining the ratios of microfracture length to spacing over a range of length scales. We find that faults nucleate when the largest mesoscale shear fractures critically interact, with the length to spacing ratios showing the critical values expected from theoretical models. Precursory slow slip patches observed in the hypocentral regions of many tectonic earthquakes show similar length to spacing ratios to those found in our experimentally deformed samples just before the mainshocks, indicating that the interaction of slow slip patches is an important step of earthquake nucleation. Understanding of the nucleation processes of laboratory faults helps geophysical interpretations of earthquake precursory signals that are essential for predicting when and where large earthquakes may occur.