Cascading Earthquake Swarms in the Northern Taupō Volcanic Zone, New Zealand

The spatiotemporal characteristics of infrequent back-arc rifting events and their relationships to volcanic unrest as well as other transient processes within the subduction system are not well known. We report 10 spatio-temporal swarms of earthquakes that occurred along ∼175 km of the northern and central Taupō Volcanic Zone (TVZ) March–September 2019. The swarms lack clear mainshock-aftershock distributions, suggesting involvement of pressurized fluids. The most energetic swarms occurred beneath the southwestern flank of Whakaari/White Island volcano (WI) and were accompanied by elevated SO₂ emissions and ∼8 mm southwest displacement of the edifice. Focal mechanisms of the WI swarms suggest horizontal dilation in the direction of tectonic extension achieved by slip along networks of closely spaced, NE-striking normal faults linked by oblique-slip to strike-slip faults. Given the small GNSS displacements and upward-migrating swarms, we favor the interpretation that magmatic volatiles were released along faults in response to changes in crustal stress. Based on the punctuated, cascading nature of swarms along the northern TVZ, we hypothesize that faults and magmatic systems are fluid-rich, experiencing aseismic creep, and critically stressed, raising the possibility that small changes in crustal stress from slow slip along the Hikurangi subduction zone caused a reduction in the minimum horizontal stress. This change occurred along the entire northern TVZ over ∼5 months, potentially contributing to widespread volcanic unrest. The probable release of magmatic fluids from shallow magma bodies in the vicinity of WI between May and June argues against a causal relation to the 9 December 2019 WI eruption.