Debris flows occur infrequently, with average recurrence intervals (ARIs) ranging from decades to millennia. Consequently, they pose an underappreciated hazard. We describe how to make a preliminary identification of debris-flow-susceptible catchments, estimate threshold ARIs for debris flows that pose an unacceptable risk to life, and identify the “window of non-recognition” where debris flows are infrequent enough that their hazard is unrecognised yet frequent enough to pose a risk to life.

Griffith (1921) assumed that energy used to create new surface area by breaking intact rock immediately becomes surface energy which is not available for further breakage. Our lab data disprove this assumption; we created much more new surface area, 90 % on submicron fragments, than the energy involved should allow. As technology allows ever smaller fragments to be measured, continued use of the Griffith assumption will lead to incorrect energy budgets for earthquakes and rock avalanches.

Rockfall boulders can travel long distances downslope, and it is important to predict how far fatalities can be prevented. A comparison of earthquake data from New Zealand during summer and full-scale rockfall experiments in the same soil during winter shows that during dry seasons boulders travel further downslope because the soil is harder. When using predictive tools, engineers and geologists should take soil conditions (and seasonal variations thereof) into account.