Deep Earthquake Strikes Bay of Plenty

At 12:57pm NZDT on the 5th January 2016 a local magnitude 4.7 earthquake struck 15 kilometres southeast of Opotiki in the north of the North Island, just south of the Bay of Plenty. The earthquake was given a focal depth of 78 kilometres, consistent with most quakes in this area (although shallow seismicity does occur as well). This certainly places it near the plate interface as the Pacific Plate dives very deep below New Zealand and the Indo-Australian Plate at this point. The earthquake was felt relatively well with more than 300 felt reports received by GeoNet, from Te Kaha in the north to Wellington in the south. The highest intensities were received from more epicentral population centres, Whakatane, Opotiki and Opape at MM5 (moderate).

This is the third earthquake so far this year in New Zealand which has had a magnitude greater than 4.0. The 4th January had two such events: a moment magnitude 4.6 (local magnitude 5.1) near Akitio which was the result of Pacific in-slab normal faulting and a moment magnitude 4.2 (local magnitude 4.0) near Akitio which was (with very low confidence from GeoNet*) was a result of Pacific in-slab reverse faulting at 36 kilometres depth. This latest event near Opotiki was likely oblique normal or normal faulting, although I could not say in which plate regardless of any focal depth given by a moment tensor solution.

*The Akitio Earthquakes on the 4th January 2016 occurred an hour after the moment magnitude 6.7 earthquake in Manipur Province, India - this earthquake killed at least 23 people. The low frequency body waves from the India Earthquake masked the local signal from the Akitio Earthquake, making any moment tensor solution questionable. I have been assured that the moment tensor for the local magnitude 5.1 earthquake is quite reliable, but owing to its smaller magnitude the later local magnitude 4.0 event has a far poorer confidence in the solution given (personal email, John Ristau, GeoNet).

Written by J H Gurney, 10:44pm NZDT, 5th January 2016.
From Hampshire, United Kingdom.