Dec 13 2007 - Update on Matata
Since 2005 GeoNet has being recording many earthquakes in the Matata region. We have also installed a new permanent site just to the west of Matata so we can get better information. We have now located over 2,300 earthquakes in the sequence.
Locations of earthquakes for each month during 2007. Earlier in the year there were 2 or 3 active locations; now there is just one offshore.
The reason that Matata is experiencing so many earthquakes is that it lies close to large fault structures on the western boundary of the Rangitaiki Plains. The area between Matata and Whakatane is what geologists call a graben. Basically the two masses of land are very gradually pulling apart, albeit at an extremely slow rate, about 1 cm per year.
These faults are moving as a result of east-west extension in the region. It is this movement that is causing the earthquakes.
We now appear to be starting a third phase of activity (see the accompanying plots). The good news is that activity is moving offshore and the latest events are mostly further away. This will also mean less are felt.
In the first phase (January - December 2005) we located about 1,000 earthquakes, and only one was over magnitude 4. In the second phase there were about 1,200 earthquakes, and six of magnitude 4 or greater occurred in March - May 2007. Activity then started to decline, like it did in 2005, but then the largest events to date occurred in October 2007 (five events over magnitude 4). A lot of energy was released then. Activity almost stopped in early November but since then about 100 events have occurred offshore.
We will continue to post information about the ones you feel as they happen.
Plots of cumulative energy, cumulative number of events and magnitude with time show how the sequence has developed.
