A study and interpretation of the distribution and kinematics of the 2001 Taupō Fault Belt seismic swarm using relative relocation techniques
The Taupō Volcanic Zone (TVZ) is the southern extent of the Tonga-Kermadec volcanic arc, forming a rifted arc due to its interaction with the nearby Hikurangi Subduction Zone. The TVZ aligns closely with the Taupō Rift, with the two structures accommodating extension in the region via magmatic and tectonic processes. Rates of seismicity are high in this area, especially around Taupō Volcano in the central TVZ. A swarm of earthquakes occurring between January and June 2001 contrasts with other periods of increased seismicity, occurring slightly north of the volcano within the Taupō Fault Belt. We have analysed this swarm of earthquakes to investigate potential interaction between the tectonic and magmatic systems at Taupō.
We obtained data from two temporary seismic arrays deployed at the time of the swarm and processed the data using non-linear location, matched-filter detection and differential relocation to yield over 2000 more earthquakes than initially detected by GeoNet. The relocated events indicated that earthquakes occurring within the fault belt formed vertical clusters near small, unidentified faults. Prior to seismicity within the Taupō Fault Belt, a large earthquake cluster was located beneath Lake Taupō’s Western Bay, previously thought to be aseismic. Focal mechanisms calculated for six of the eight largest magnitude earthquakes within the swarm are consistent with a rotation of the maximum compressive stress axes to horizontal during the swarm.
We interpret the earthquakes detected beneath the Western Bay as signalling unrest from an intruding, possibly mafic, magma body, such as have been identified at numerous other calderas. Fluids and increased horizontal stress from this intrusion triggered the resulting seismicity in the Taupō Fault Belt, which display vertical clustering and small-scale propagation to support this. Increased horizontal pressure from the intrusion rotated the axis of maximum compressive stress, indicated by strike-slip focal mechanisms and mirroring the same process in other caldera settings. This is a further example of volcano-tectonic interactions in the central TVZ, involving a previously unexplored area beneath Lake Taupō’s Western Bay.