Mt. Ruapehu and the Matata Earthquake Swarm

Another volcanic earthquake at Mt. Ruapehu overnight followed a few hours later by an earthquake near Matata in the Bay of Plenty again raises the question of a link between the two events.

Tuesday night’s eruption at Mt. Ruapehu was followed 3½ hours later by a burst of earthquake activity near the coastal town of Matata in the Bay of Plenty. During the 8:23 p.m. eruption at Mt. Ruapehu on Tuesday 25th September 2007, a 7 minute-long magnitude 2.9 volcanic earthquake was recorded by geologists.

Last night, a smaller 3 minute-long volcanic earthquake was recorded at Mt Ruapehu at 11:05 p.m. Two hours later a magnitude 4.5 earthquake was recorded at Matata. The 1:13 a.m. quake on Sunday September 30th was felt from Waihi to Opotiki, and as far south as Rotorua. The very shallow quake, which was 2 km deep, has attracted 316 reports from members of the public who felt the event, and slight damage has been reported in 3 reports from Edgecumbe and Matata.

At the time of writing, the Geonet website reports two quakes at Matata one minute apart, with magnitudes of 4.7 and 4.5, and the seismograph traces from this morning appear to show two events 30-40 seconds apart, but the traces overlap. However, both currently have the same reference number, and only the magnitude 4.5 event appears in Geonet’s earthquake database at this time. In-depth analysis of the seismic data tomorrow will probably resolve the issue when seismologists analyse traces from more instruments to more clearly identify the quakes.

No other eruptions at Mt. Ruapehu have occurred during the current Matata earthquake swarm which began in December last year. The lahar at the volcano in March this year was due to the collapse of eroded crater wall and, though tremor was recorded, it was due to material spilling from the crater lake as the tephra dam was washed away and not volcanic in origin.

The previous eruption at Mt. Ruapehu was on October the 4th last year before the Matata swarm activity recommenced. Since this morning’s activity, both Matata and Mt. Ruapehu have again been quiet, so it will be interesting to see whether the unusual relationship between the two events continues.

Other earthquake activity of note in New Zealand today has been a shallow magnitude 4.1 quake off-shore to the north-west of East Cape at 12 minutes after midday, and a magnitude 5.1 quake 125 km north-east of Raoul Island in the Kermadecs at a depth of 35 km at 5:10 this morning.

In the Pacific, another burst of activity in New Caledonia’s Loyalty Islands occurred between twenty-seven minutes past midnight and thirty-eight minutes past midday today New Zealand time. The six quakes with magnitudes between 4.7 and 5.7 indicate a reduction in the strength of earthquakes when compared with similar bursts of activity in recent days.

A magnitude 6.8 quake in the Mariana Islands just after three o’clock this afternoon continues the activity that commenced on September 28th, which includes the magnitude 7.4 quake of September 29th. Background on this earthquake activity in the Pacific has appeared in recent items on this website.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST; and the US Geological Survey and its contributing agencies.]

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