Scaling law and repeat times for Ruapehu volcanic events

Technical Abstract

Ruapehu volcano produces a wide variety of seismic signals ranging from individual volcanic earthquakes to long duration weak tremor. An analysis of the size and numbers of volcanic earthquakes at Ruapehu volcano reveals that the process of earthquake generation follows a predictable relationship, with very few large earthquakes and more numerous small earthquakes. This predictable relationship allows us to estimate how often (but not when) larger more potentially hazardous volcanic earthquakes can occur at Ruapehu.

The analysis also shows that the earthquake generation process is quite random in time when looking over longer time periods (weeks to years). For shorter time periods however (less than 10 days), the occurrence of a volcanic earthquake increases the likelihood of additional activity. This observation is completely analogous to the increased chance of an aftershock after a large main-shock earthquake has occurred.

The data also reveal that the range of sizes for volcanic earthquakes (individual volcanic events) is fundamentally different from the range of sizes for volcanic tremor (long duration volcanic signals). While the volcanic earthquakes do not reveal an “upper” size limit, the tremor observations suggest that its size is constrained.

Because larger volcanic earthquakes are strongly associated with volcanic eruptions, the analysis of volcanic earthquake numbers and their sizes can provide information about the chance of volcanic eruptions. Our analysis can thus be used to inform the likelihood of volcanic eruptions during periods of no activity and also when there is increased activity at Ruapehu. The results also provide a better understanding of internal processes of the volcanic system.