Establishing a spatiotemporal benchmark for ongoing crustal stress monitoring in the Southern Taupo Volcanic Zone

Technical Abstract

Changes in the direction of stresses within the crust have been observed at some active volcanoes and if these can be identified in near real-time they have the potential to highlight episodes of magma intrusion before an eruption occurs. The active volcanism and high level of earthquake activity in the southern Taupo Volcanic Zone (the area from Ruapehu to Taupo) make this the best area in New Zealand for researching such changes. We have therefore established the background crustal stress pattern throughout a large part of New Zealand and have developed a method to look for changes in stress with time.

Using data from the GeoNet earthquake catalogue we have calculated more than 600 focal mechanisms that indicate the directions of fault movement during earthquakes. From the focal mechanisms we have estimated the stress directions in about 30 areas. These data establish the background crustal stress pattern for New Zealand as they are internally consistent and in agreement with previously published data.

For five areas (three in the southern Taupo Volcanic Zone and two control areas) we have tested a method for looking for changes in the stress directions by dividing a data period into two intervals and comparing “before” and “after”. The key to this step is a statistical test that determines the significance of any changes. We believe that we can detect rotations of stress directions greater than about 30 degrees if we use 20 or more focal mechanisms.