Laboratory Hydrofractures as Analogs to Tectonic Tremors

The fracture of Earth materials occurs over a wide range of time and length scales. Physical conditions, particularly the stress field and Earth material properties, may condition rupture in a specific fracture regime. In nature, fast and slow fractures occur concurrently: tectonic tremor events are fast enough to emit seismic waves and frequently accompany slow earthquakes, which are too slow to emit seismic waves and are referred to as aseismic slip events. In this study, we generate simultaneous seismic and aseismic processes in a laboratory setting by driving a penny-shaped crack in a transparent sample with pressurized fluid. We leverage synchronized high-speed imaging and high-frequency acoustic emission (AE) sensing to visualize and listen to the various sequences of propagation (breaks) and arrest (sticks) of a fracture undergoing stick-break instabilities. Slow radial crack propagation is facilitated by fast tangential fractures. Fluid viscosity and pressure regulate the fracture dynamics of slow and fast events, and control the inter-event time and the energy released during individual fast events. These AE signals share behaviors with observations of episodic tremors in Cascadia, United States; these include: (a) bursty or intermittent slow propagation, and (b) nearly linear scaling of radiated energy with area. Our laboratory experiments provide a plausible model of tectonic tremor as an indicative of hydraulic fracturing facilitating shear slip during slow earthquakes. When the ground breaks or fractures, it can happen in different ways and at different speeds. Some fractures happen very quickly and send out waves that we can feel as earthquakes. Other fractures happen more slowly and don't cause noticeable shaking; we call these slow earthquakes. Both types of fracture often happen together. To better understand this, we make a small-scale model of a fracture in our lab using a transparent material and fluid under pressure. We use a high-speed camera and acoustic sensors to watch and listen to how the fracture behaves, noticing patterns of stopping and starting over time. Interestingly, we find that slow fractures involve quick, smaller fractures and are influenced by the viscosity and pressure of the fluid. These lab findings are similar to real-world observations of tremors in the Cascadia region of the United States. This study helps us understand these tremors as components of slow earthquakes happening deep underground. A new hydrofracturing experiment simulates tectonic tremors and aseismic slips by visualizing and listening to stick-break instabilities Both laboratory and tectonic tremors behave with bursty or intermittent slow propagation and radiate energy that scales with rupture area Our experiments suggest tectonic tremors are manifestations of hydraulic fracturing that contributes to induced slow earthquakes