Earthquake swarm develops in outer Bay of Plenty

Published: Wed Nov 1 2017 3:00 PM

A moderate earthquake swarm is now under way off shore Bay of Plenty

On Sunday 29 Oct, what appeared as a small earthquake sequence, consisting of some small foreshocks and a moderate mainshock started. On Sunday evening the largest event was a M4.1 event at 11.24 pm and 9 km depth. This was followed by what appeared as normal aftershocks on Monday. This has now developed into a typical Bay of Plenty earthquake swarm. Swarms come in a variety of forms and it is not possible to easily judge the future of them. Some have the stronger phase near the start, others in the middle, while some swarms have the stronger activity at the end. We anticipate this one will continue in the short term.

On Tuesday evening at 8.36 pm a M4.7 event occurred at 6 km depth. This was followed by many smaller aftershocks. Then this morning at 11.58 am there was a M4.8 event at 7 km depth. This is the largest event so far in the swarm. These events are being felt by coastal Bay of Plenty residents. The swarm is located about 12-15 km east of White Island. White Island is the nearest geographic location so our automated location software naturally reports them as being nearby. White Island is a large volcano with a significant footprint under the sea, being about 6 x 10 km at 300 m depth. However, these earthquakes are well beyond the footprint of the volcano, hence we consider them to be caused by regional faulting, rather than related to the volcano.

So far, we have located 137 earthquakes in the swarm that range in magnitude from M1.7 to 4.8. The depths range from 2-13 km. Many active faults are mapped in the offshore Bay of Plenty area, hence it is seismically active and we record numerous earthquakes every year in the area. In the last 12 months, we have located 114 earthquakes in the same area as the current earthquake swarm. These have ranged in magnitude from M1.6 to 4.1, while the depths have ranged from around 2-10 km. In the last 5 years there have been 3 earthquakes over M4.0 in the same area. There have 3 over M4.0 so far in this swarm.

Earthquakes located so far in the Bay of Plenty swarm

Earthquakes located so far in the Bay of Plenty swarm