RESULTS FROM THE NEW SEISMIC MONITORING NETWORK AT EGMONT VOLCANO, NEW-ZEALAND - TECTONIC AND HAZARD IMPLICATIONS

Data from the new Taranaki Volcano-Seismic Network provide the first d etailed information on microseismicity in the vicinity of Egmont Volca no (Mt Taranaki) and central Taranaki. During the period January 1994 to March 1995, 133 earthquakes were located within a 40 x 40 km area c entred on Egmont Volcano. Earthquake magnitudes varied from 1.5 to 3.4 , with five events being of magnitude 3 or greater. Most earthquakes w ere located in the northwest of the study area, with a distinct cluste r of earthquakes occurring c. 12 km WNW of the summit of Egmont Volcan o. Levels of seismicity were low in the remainder of the area and occu rred throughout the crust, with depths ranging to just over 30 km. The location and character of the observed seismicity, and especially tha t 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 pl ane of similar orientation and dip to this fault and indicates left-la teral slip with a minor normal component. No other correlation between observed seismicity and active faults is apparent; the only other kno wn active faults in the region, the Inglewood and Norfolk Faults, were aseismic during the period of observation. Frequency-magnitude analys is of earthquakes in the cluster shows that the dataset is complete fo r magnitudes of 2.5 and greater and yields a b-value of 2.1 +/- 0.4. T his value is anomalously high for both tectonic and A-type volcanic ea rthquakes elsewhere in New Zealand, though it may be a consequence of the limited dataset. Although a tectonic source is deduced for the clu ster, there is an important implication for volcanic hazard. Because t he volcanic history of Egmont Volcano is characterised by debris avala nches, possibly triggered by tectonic events, then the proximity of th e cluster to the potentially unstable edifice is significant. These re sults not only identify present-day seismicity very close to Egmont Vo lcano, but also link it to a fault known to have generated large magni tude earthquakes in the recent past, and therefore constitute an impor tant factor in any assessment of volcanic hazard at Egmont.