article: Jun 15 2005 - Earthquakes Near Matata

Since early February, many small earthquakes have occurred near Matata on the Bay of Plenty coast, west of Whakatane. Hundreds of events have been recorded, with the two largest being a magnitude 4.0 on April 14 and a magnitude 4.2 on June 13. These events are very shallow, between 5 and 10 kilometres deep, so even quite small events have been felt locally.

Swarms of earthquakes are common in this region. Last July near Lake Rotoehu, a swarm included events up to magnitude 5.4. Most such swarms last only hours or days, so this one is slightly unusual in continuing for such a long period. 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.

Because of the increase in frequency of the earthquakes in the Matata area, and the sparse distribution of permanent seismographs in the Bay of Plenty region, the GeoNet team has installed 3 temporary portable stations to collect extra information about what's going on. One is in Matata itself, and the other two are within 10 km to the south and west of the town.

The distribution of epicentres recorded over the last two months is shown on the accompanying map. It will be updated regularly within this article whilst the swarm continues. The magnitude of events is shown by their comparative size and colour, and the mapped faults are shown in grey. The symbol 'h' refers to the depth of the earthquake, and 'thetis' is the name of one of our earthquake analysis computers. Permanent seismograph sites are shown by white triangles together with their international station codes. The white circles shown on the diagrams highlights the most recent earthquake displayed. The bottom two graphs show the distribution of the earthquakes by magnitude and depth over the past two months.

The Rangitaiki Plains (the area between Matata and Whakatane) are bounded on both the western and eastern sides by large scale fault structures. The current Matata earthquake swarm is occurring along the northern end of the western boundary faults and out to sea on the extension of these faults. The faults mapped by geologists tend southward towards Kawerau and Lakes Rotoma and Rotoehu, defining the edge of the high ground along the boundary of the Plains.