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

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