Waking up this morning to read about a new 5.3 earthquake Christchurch New Zealand. This is the latest but not last of quakes to hit the city in recent days. And the news is not good because this is the end of a line of faults that lead south from Tonga. And that series of faults is lit up right now with quakes from top to bottom. (There were 14,000 quakes on this zone this last year.)
The fault system is a series of trenches and not so deep subduction zones where the Pacific plate meets the Indo/Australian plate and along most of it the Pacific plate is subducted. Starting at the top is the Tonga Trench. It starts as a curve from the west bending around to the south as it curves around the Tonga islands. This trench has quakes from the trench bottom all the way down to where the subducted material finally melts and becomes part of the mantle. this trench continues south until a spot just north of the Kermadec islands.
At this point a string of under water sea mounts, extending from a transform area of the pacific mid ocean ridge. This line of mounts is hitting the subduction zone and create an area where the trench is not a trench and is the delineation between the Tonga trench and the Kermadec trench. This area with mountains being subducted is not the sticking point it would seem to be but seems to be lubricated by the mounts or possibly because it is formed from a transverse portion of the mid ocean ridge is weaker and bends to allow the subduction with less preasure.
The Kermadec trench extends from this point, past the Kermadec islands and to a point about 300 km north of New Zealand. This trench is about as deep as the Tonga trench, 32,000 feet deep, about the same depth below the surface as air lines fly above. One difference between the Kermadec and Tonga trenches is the Tonga trench keeps showing quakes on the subducted crust to a depth of 600+ km where quakes at that depth are not seen as often on the Kermadec trench. We don’t know why yet.
While the fault that makes up the subduction zone continues south past the North island the trench stops when it gets to a point about 300 km north of the north island. This trench system is has a lot of quakes, and they seem to come in packs of quakes that start with large 7 and 8 magnitude quakes at the north end of the trench and cause cascades to flow south along the system.
These waves of quakes can go all along the trench but usually stop at the south end of the Kermadec trench. But not always and they also seem to stack. What we are seeing in Christchurch today and for the last week is the results of this with damaging quakes and parts of the city in ruins. But they are not the only quakes on the fault zone. There are quakes today in the Kermadec islands and Tonga, and 2 days ago there was a 6 in Tonga region. Is that the start of a new earthquake storm?
But why do these quakes hit Christchurch? It is not on the subduction zone directly, on the south island and off of the fault zone. But it is on the edge of the zone where the fault goes from the east side of the country to the west side. This area is under a lot of stress twisting and folding. Satellite data shows parts of the city are rising while others are dropping. And the faults are surface faults, not the part of the earth being subducted. This is all stress due to other quakes on the subduction zones north and south of the country that are allowing the pacific and Indo/Australian plates slide by each other but here they are head-butting and instead of sliding past each other are just causing a train wreck effect.
And with more quakes happening on the subduction zones this just builds the pressure in this intermediate zone and so the quakes at Christ church will never stop, in fact they can only get worse. While today’s quake seems to have not hurt or killed anyone this is more because no-one is allowed into the downtown core than from any other reason, once all of the quake damaged and damageable buildings are empty no one gets hurt.
- Subsea mountains’ ‘march to ruin’ (bbc.co.uk)
- Back to the Kermadec Trench (echlinm.wordpress.com)
- New Zealand Earthquake Dec 2011 (echlinm.wordpress.com)