Results from the new seismic monitoring network at Egmont Volcano, New Zealand: Tectonic and hazard implications

Abstract

Data from the new Taranaki Volcano-Seismic Network provide the first detailed information on microseismicity in the vicinity of Egmont Volcano (Mt Taranaki) and central Taranaki. During the period January 1994 to March 1995, 133 earthquakes were located within a 40 - 40 km area centred on Egmont Volcano. Earthquake magnitudes varied from 1.5 to 3.4, with five events being of magnitude 3 or greater. Most earthquakes were located in the northwest of the study area, with a distinct cluster of earthquakes occurring c. 12 km WNW of the summit of Egmont Volcano. Levels of seismicity were low in the remainder of the area and occurred throughout the crust, with depths ranging to just over 30 km. The location and character of the observed seismicity, and especially that of the cluster, suggest sources related to tectonic processes rather than volcanic activity. A likely cause of this earthquake cluster is movement on the Oaonui Fault, which is known to be recently active. A composite fault-plane solution for the cluster events gives a nodal plane of similar orientation and dip to this fault and indicates left-lateral slip with a minor normal component. No other correlation between observed seismicity and active faults is apparent; the only other known active faults in the region, the Inglewood and Norfolk Faults, were aseismic during the period of observation. Frequency-magnitude analysis of earthquakes in the cluster shows that the dataset is complete for magnitudes of 2.5 and greater and yields a b-value of 2.1 - 0.4. This value is anomalously high for both tectonic and A-type volcanic earthquakes elsewhere in New Zealand, though it may be a consequence of the limited dataset. Although a tectonic source is deduced for the cluster, there is an important implication for volcanic hazard. Because the volcanic history of Egmont Volcano is characterised by debris avalanches, possibly triggered by tectonic events, then the proximity of the cluster to the potentially unstable edifice is significant. These results not only identify present-day seismicity very close to Egmont Volcano, but also link it to a fault known to have generated large magnitude earthquakes in the recent past, and therefore constitute an important factor in any assessment of volcanic hazard at Egmont.