“…In a few cases, upper bounds on L c have been made from borehole strain measurements (e.g., Johnston et al, 2006), but this disqualifies only nucleation zones larger than 10 km and only at specific fault locations. Instead, seismic measurements such as foreshocks, tremors, and initial P wave signatures primarily shape our understanding of earthquake initiation (Bouchon et al, 2011(Bouchon et al, , 2013Chen & Shearer, 2013;Dodge et al, 1995;Ellsworth & Bulut, 2018;Kato et al, 2012;Ruiz et al, 2017;Tape et al, 2018). A coalescence of seismicity is sometimes observed (Savage et al, 2017) in a source volume near the eventual hypocenter of a larger earthquake.…”