Taxonomy of shallow aseismic creep

Shallow aseismic fault creep has been observed for more than half a century (Steinbrugge et al., 1960). Aseismic creep can be manifest as continuous creep at a steady rate, can occur episodically in events with durations of hours or weeks, or can take the form of afterslip decaying in rate following an earthquake.

Upon closer inspection of the different types of aseismic slip recorded on creepmeters, this simple view may not be complete. Here, we present a new taxonomy for shallow aseismic slip based on 40 years of creepmeter data recorded at seventy-nine locations on creeping faults in California, Utah, Türkiye, and Pakistan. We identify at least six major forms of creep: steady creep, episodic creep events, afterslip, triggered slip, months-long creep surges, and creeplets (slip of ≤100 μm). The last two of these modes of aseismic slip have hitherto not been recognized, since they are often difficult to distinguish from environmental perturbations and/or instrumental artifacts. The creep event and creeplet classes of aseismic slip can form sequences of events that occur in close temporal spacing to one another. These six modes of creep may sometimes occur simultaneously (notably afterslip and creeplets/smaller creep events), combining to accommodate the overall fault motions.

Using these formalised definitions of aseismic slip, we have catalogued more than 5000 aseismic slip transients. In this global catalogue of mostly sub-centimetre amplitude shallow aseismic slip transients we observe over 4,400 creep events, over 800 creeplets and over 150 months-long creep surges. Using this catalogue we are able to form the following conclusions: the median duration of episodic creep events is 2.7 days with a median slip amplitude of 0.6 mm, creeplets have a median duration of 11 hrs with a median slip amplitude of 50 μm, and creep surges have a median duration of 1.5 months, and a median slip amplitude of 1.7mm. The catalogue will shortly be made available for public access.