Matata Quake Count Near 700

Nearly 700 earthquakes have been identified in the earthquake swarm which has been occurring near Matata in the Bay of Plenty since December 2006. GNS Science has released a small article comparing the current earthquake swarm with the 2005 event during which more than 1000 quakes were identified.

The article published on the Geonet website on Friday 11th, reports that the earthquake swarm currently occurring near Matata is similar to the 2005 swarm in terms of the duration of the swarm, the number of events and the total energy release.

The quakes are occurring close to large fault structures on the western boundary of the Rangitaiki Plains, an area of about 27,000 ha. of former wetlands which extends from near Kawerau to the coast at Matata and Whakatane. The towns of Te Teko, Edgecumbe and Thornton are located within the plain, with Awakeri on its south-eastern border. The Rangitaiki Plains cover the lower flood plains of the Rangitaiki, Tarawera and Whakatane Rivers, according to Environment Bay of Plenty.

The plain lies at the northern end of the Taupo Volcanic Zone, a tectonic structure that runs from Mt Ruapehu to the Bay of Plenty. It developed about 2 million years ago, and is characterised by having only 10 km of brittle crust which is getting thinner owing to the tectonic forces which are stretching the area by about 10 mm per year, causing the Coromandel Peninsula and East Cape to move further apart.

The east-west extension of the area is causing the earthquakes close to the large scale fault structures near Matata, according to the GNS Science article. The 2005 swarm of more than 1000 earthquakes occurred in a slightly different location from the present swarm which has one distinct cluster of activity about 1 km south of Matata at a depth of 5 km and another about 10 km offshore at a depth of about 2 km.

The magnitude 6.1 Edgecumbe earthquake of March 1987 accounted for about a thousand year’s worth of extension activity when it caused more than a metre of horizontal extension and caused the western side of the Rangitaiki Plains to drop by up to 2 metres, making gravity drainage systems less effective.

The quake had been preceded by swarms of earthquakes felt near Matata and Thornton, so the possibility of a larger event in the present swarm cannot be ruled out. However, the article suggests that the current activity’s similarity to the 2005 swarm and to other swarms in the Bay of Plenty leads them to expect the earthquakes to gradually die away over several months without any significantly larger events.

The article on the Geonet website provides plots of the number of earthquakes and their magnitude over time, with a map showing the location of the 2005 and 2007 swarm events. Of the nearly 700 earthquakes in the current swarm, more than 50 events have been over magnitude 3 and 5 over magnitude 4.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST.]

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