article: Jul 2 2005 - Te Aroha Earthquake Swarm

Several small earthquakes have been occurring near Te Aroha in the eastern Waikato since Friday afternoon (1 July).

The distribution of shallow (less than 20km deep) earthquakes recorded over the last 25 years; Te Aroha is situated at the centre.

The swarm started at 2:48 pm on Friday with a magnitude 3.0 event at 15 km depth (it was not reported as felt, but may well have been by local people), and was followed at 6:41 pm by a 3.1 event, and then a 3.3 event at 1:22 am on Saturday morning. During the next 10 hours at least 14 events were recorded, the larger being two 3.1 events at 6:32 am and 8:10 am. Only four events were recorded during the next four hours, before the largest earthquake so far, a magnitude 3.7 occurring at 4:46 pm. This event was felt in Te Aroha, Katikati and Waihi, and has been followed by several mainly small aftershocks, with the largest being one of 3.5 at 6:29 pm. These latter events have been at 4 to 6 km depth.

Swarms of earthquakes are common in the volcanic region. In 1972 a magnitude 5.2 event occurred in this area damaging some homes. Most such swarms last only hours or a few days. From historical trends we would expect the swarm to gradually settle down and not trigger a larger event, but we cannot rule out that possibility.

Earthquake activity is lower in this area than the Rotorua - Taupo region. The area of the Hauraki Plains between Matamata, Te Aroha and Ngatea is bounded on both the western and eastern sides by geological faults. The Kerepahi fault also trends across the plains.