Crustal seismicity in Taranaki, New Zealand using accurate hypocentres from a dense network

Abstract

A large, dense network of three-component, broad-band seismographs was used to determine
\naccurate hypocentres for earthquakes in Taranaki, New Zealand. They allow us to characterize
\nseismicity around Mt Taranaki, a large, dormant, andesite, cone volcano, and to map precisely
\ntwo major lineations of crustal seismicity. A minimum 1-D velocity model was used to locate
\n389 local earthquakes using the probabilistic, non-linear earthquake location program
\nNONLINLOC. There are few earthquakes beneath Mt Taranaki itself, and all are relatively small
\nand shallow (≤10 km deep). The shallow seismogenic zone can be explained by the crust
\nbeing unusually hot, thus causing the base of the brittle–ductile transition to be shallower
\nthan normal beneath Mt Taranaki. This is supported by a high heat flow anomaly in this area.
\nThe absence of any volcanic earthquakes beneath Mt Taranaki suggests that active volcanic
\nprocesses are currently unlikely, and the shallow brittle–ductile transition depth means that
\nprecursory volcano–tectonic seismicity from any future magmatic intrusion is unlikely to occur
\nbelow 10-km depth. The permanent seismic network can locate earthquakes in Taranaki
\nreasonably accurately and can reproduce most of the details seen by the temporary seismograph
\ndeployment provided that only the best hypocentres are considered. However, beneath
\nMt Taranaki, which is the most important area for volcano monitoring, hypocentres determined
\nby the permanent network are too deep by 4–12 km. The active Cape Egmont fault
\nzone (CEFZ), west of Mt Taranaki, is the most seismically active area, with earthquakes in the
\nupper crust to about 22-km depth. Spatial and temporal clustering, earthquakes with similar
\nwaveforms, and an absence of obvious main shocks imply that earthquake swarms make up
\na significant proportion of the seismicity in this area. Earthquakes in eastern Taranaki occur
\nprimarily along the Taranaki–Ruapehu Line (TRL), thought to be a major boundary across
\nwhich the crustal thickness changes by about 10 km. These earthquakes are less clustered,
\nhave a b value typical of tectonic earthquakes, and occur in the lower crust to a depth of 35 km,
\nwith the upper crust almost aseismic. The abrupt cessation of seismicity at 35-km depth is
\nconsistent with this boundary marking the Moho, with no earthquakes in the mantle. The
\nconcentration of earthquakes in the lower crust requires it to be drier and more mafic than
\nthe wet, quartzo-feldspathic composition often used to model crustal rheology. There is no
\nchange in maximum earthquake depth across the currently accepted location of the TRL, but
\nthere is a 10-km decrease in maximum earthquake depth some 25 km to the north of the currently
\naccepted location. This suggests that the true position of the TRL is 25 km north of the
\nhitherto accepted position.