Matata Earthquake Swarm Continues

The swarm of earthquakes occurring within 10 km of the Bay of Plenty township of Matata has continued throughout March.

To date, Geonet’s website has detailed 24 earthquakes within 10 km of Matata during March 2007. The quakes range between magnitude 1.9 and 4.0 and are occurring at depths between 1 and 8 km.

These reports focus on the quakes that were likely to, or have actually been felt by nearby residents. On March 16th, Hazardwatch reported that the swarm was continuing with daily rates between 5 and 20 earthquakes, many of which would have been too light to have been felt.

The current swarm began in December last year when four earthquakes ranging in magnitude between 2.8 and 3.4 were reported near Matata at depths between 3 and 5 km. Six quakes were reported during January 2007, and nine were reported during February.

The 43 earthquakes reported during this latest swarm are occurring at several distinct locations – within 5 km of Matata, or 10 km north, north-east, east and north-west of the town. At times activity seems to concentrate at one location for a short period, but there seems to be no clear longer-term trend as to the centre of activity at this stage.

During 2005, three distinct swarms of earthquakes struck Matata between the 12th of February and 25th of August. Some of the 55 earthquakes plagued the town at the same time as a flash flood which caused considerable damage to the settlement.

The magnitude 6.1 Edgecumbe earthquake of the 2nd of March 1987 was preceded by a month of small earthquakes that unnerved the residents of Kawerau and Edgecumbe. Distinct foreshocks were felt at Matata and Thornton prior to the main earthquake, and long-term residents of the area are naturally wary as the present earthquake swarm runs its course.

Seismologists are, of course, monitoring the Matata earthquake swarm but they are unable to say whether it can be classed as a premonitory swarm – a swarm of quakes that precedes a larger earthquake.

Studies into premonitory earthquake swarms have been carried out in New Zealand and elsewhere, with scientists studying past earthquake swarms that have preceded a larger nearby quake in the hope of determining whether the premonitory swarm has any special characteristics. However, it is a complex subject as earthquake swarms in compression zones (like Wellington), spreading zones (like the Bay of Plenty) and subduction zones (like the area off East Cape) behave differently.

Some progress seems to have been made in the study of premonitory swarms in subduction zones, but the ability to identify an actual premonitory swarm still lies in the future.

The Bay of Plenty experiences earthquake swarms from time to time and, as the 2005 Matata swarms showed, not all swarms precede large earthquakes. In the meantime, earthquake swarms of all kinds serve a useful purpose to remind people to be ready to manage their affairs for several days following a larger earthquake if they live in one of New Zealand’s high earthquake-risk areas. Local councils and the Ministry of Civil Defence publish tips and guidelines on their websites to help residents survive natural disasters.

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

Leave a Reply