Data published on the Geonet website for quakes associated with the Matata earthquake swarm indicate that the earthquakes have become shallower during the first week of April. However, this is not the case, according to seismologist Bryan Field.
The Bay of Plenty town of Matata has been experiencing shallow earthquakes since December last year, with 60 events being detailed on the Geonet website to date. The quakes have ranged up to magnitude 4.1, with 4 quakes reported during December 2006, 6 during January 2007, 10 during February, 30 during March and 10 so far during April.
The 60 quakes reported on the website represent those most likely to have been felt by residents nearby and have been analysed to determine location and depth.
Geonet operates several systems which monitor seismic instruments deployed throughout New Zealand. A selection of the seismograph drums appears in near real-time on the Geonet website to provide public information on larger earthquake events.
There are 41 seismograph recording sites in the national network, feeding information into the data centres at Wellington and Wairakei for analysis. Data is monitored and, if information indicates that shaking intensity has reached a certain threshold, a duty seismologist is paged to begin analyzing an earthquake’s size and location.
Hundreds of earthquakes are recorded within New Zealand every week, with most too small to be felt. However, in the case of the Matata earthquake swarm, even the smaller 2nd magnitude earthquakes may be felt owing to their shallow depth.
Which brings us back to the question ”“ are the Matata quakes getting shallower with time? It doesn’t seem so, according to Bryan Field. The sheer volume of earthquakes being recorded in any given day means that not all are analysed to a high degree within a few hours ”“ there simply isn’t time or resource available. Only those quakes likely to be felt receive immediate attention.
For most of the Matata quakes, this means that data from several stations is analysed to give an approximate position (such as “within 5 km of Matata”) and a depth to within a few kilometres to allow publication on the website within 10 to 30 minutes of the earthquake happening. Hence the proliferation of quakes appearing at a standardised depth of 5 km.
Subsequent analysis can refine this information, and this can sometimes be seen in the weekly summaries provided Hazardwatch and, of course, by searching the publicly available databases.
As the Matata swarm has continued, it has attracted more interest from the public and seismologists alike. When time allows, some of the Matata quakes are being analysed to a higher degree using the Rotorua volcanic network instruments, allowing more accurate depths to be determined. Hence the appearance of more reports on the Geonet website showing 2 km deep events.
The apparent trend toward shallower quakes is simply a result of increased focus as the swarm persists, and the opportunity to analyse each event more deeply.
This is commonly seen in earthquake analysing networks around the globe. Initial estimates of earthquake magnitude and location are steadily refined over hours, days and weeks, as information from more seismograph stations comes to hand and is analysed.
[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST.]
I was thinking about the Matata swarm today – this is now 4 months of almost daily seismic activity in the same locale, that’s fairly impressive.
I’ve heard of swarms last days, even weeks (up to about 2 months) but 4 months is…out there. From what I know anyway….
Don’t forget the 4 or 5 month swarm at Matata in 2005. There were about 1000 quakes in that one too.