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 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.