CASCADING EARTHQUAKE SWARMS IN THE NORTHERN TAUPO VOLCANIC ZONE, NEW ZEALAND

Over-pressurized magmatic systems in extensional arc settings are susceptible to changes in crustal stress, which ultimately can lead to volcanic unrest. We report >10 swarms of elevated seismic activity that occurred along ~200 km of the northern and central Taupō Volcanic Zone (TVZ) between March-August 2019; all swarms lack clear mainshock-aftershock distributions, indicating involvement of magmatic or high-pressure fluids. The most energetic swarm occurred beneath the southwestern flank of the highly active Whakaari/White Island volcano and was accompanied by elevated SO2 emissions and southwest displacement of the volcanic edifice. Focal mechanisms of the Whakaari/White Island swarms suggest horizontal dilation along normal faults in the direction of tectonic extension, as well as strike-slip motion along north-south striking faults. Given the small GNSS displacements, we favor the interpretation that magmatic volatiles were released by changes in crustal stress. On the basis of the punctuated, cascading nature of swarms along the northern TVZ, we hypothesize that a rapid reduction in the minimum horizontal stress occurred along the entire northern TVZ over of ~6 months during a slow-slip episode along the Hikurangi subduction zone, leading to widespread unrest.