Central NZ Earthquakes in Cook Strait: A Complex Tectonic Region

[davenn]

hi Gang :)

interesting set of earthquakes in the central NZ region in Cook Strait, between the 2 main islands

the last M6+ event that I recall in this area ( and almost in the same spot) was a 6.2 in the 1980's. This a complex tectonic region where there is a change from oblique slip across the South Island changing to a subduction zone under the northeastern most corner of the South Island and under the North Island.
These events have occurred in the centre of that zone and appear to delineate a fault striking approx. NW-SE. The USGS moment tensor supports this conclusion.

I was in Christchurch, some 400km to the SW at the time of the M5.7 ( the first event) but didn't feel it. It would appear that only a few people in Christchurch felt the M6.5.
The first event (M5.7) was preceded by several foreshocks of ~M1.5 - M2.5 in the 2 day before the event. The 5.7, 5.8 and 6.5 had their own good set of aftershocks so far totally in excess of 300 events.
Unfortunately my digital seismograph computer decided to have a hard drive failure at the time of the M6.5 so am unable to show you a seismogram of the event.

Regards
Dave

 

[256bits]

the last M6+ event that I recall in this area ( and almost in the same spot) was a 6.2 in the 1980's. This a complex tectonic region where there is a change from oblique slip across the South Island changing to a subduction zone under the northeastern most corner of the South Island and under the North Island

So does that mean that NZ is getting smaller, and the two isalnds will evenually meet up. Is the Cook Straight a result of the subduction. I am just wondering if the two islands have relative motion to one another or are experiencing uplift or whatnot.

Unfortunately my digital seismograph computer decided ...

At home I assume. That is interesting. I always had the idea to set something up but never ever got around to it. I am now having moments of regret.

 

[StevieTNZ]

There is more discussion over the recent earthquakes here - https://www.physicsforums.com/showthread.php?t=702290

I'm based in a suburb of Lower Hutt; ~8-10 minutes drive from Wellington.

 

[davenn]

Hi 256

So does that mean that NZ is getting smaller, and the two isalnds will evenually meet up. Is the Cook Straight a result of the subduction. I am just wondering if the two islands have relative motion to one another or are experiencing uplift or whatnot.

Both the North and South Islands are under compression but by different methods. As a result they are both shortening in they East-West dimension

in the above image, Nth Is. cross-section the tectonics motion is primarily subduction with a bit of a strike slip component. Most of that subduction shortening is taken up by the uplift of the coastal ranges ( east coast) through reverse faulting. A lot of that east coast material (land) is Pacific seafloor that has been accreted to the Nth Is mainland.

In this cross-section of the Sth Is., the tectonics is oblique slipping its roughly a 3:1 ratio. That is for every 3 metres of horizontal motion there's ~ 1 metre of vertical motion.
The Southern Alps mountain chain is one of the youngest in the world at ~ 5 - 8 million years.
Prior to then there was extensional tectonics across the Sth Is., resulting in a lot of volcanism

At home I assume. That is interesting. I always had the idea to set something up but never ever got around to it. I am now having moments of regret.

Yes at home. have a look here for info my system

cheers
Dave

 

[davenn]

There is more discussion over the recent earthquakes here - https://www.physicsforums.com/showthread.php?t=702290

I'm based in a suburb of Lower Hutt; ~8-10 minutes drive from Wellington.

yeah I happened to see that the other day :)
that discussion would have probably been better off in the Earth forum ;)
I always wondered when in NZ you were

Dave

 

[davenn]

Dangit, only just got my online seismo running for several days and my www site host decides to do some changes to their servers and now the images are not uploading.
Am still fighting with them trying to sort it out.

Dave

 

[StevieTNZ]

Didn't feel this morning's 5.4.

 

[davenn]

deep sleeper huh ;)

I see the USGS only gave it a M4.9, but they are often quite low in their magnitudes for NZ events

and a M4.7 in upper Canterbury this afternoon

cheers
Dave

 

[StevieTNZ]

Have made a recent posting regarding the two 4.9 earthquakes that have struck within 24 hours of each other: https://www.physicsforums.com/showthread.php?t=702290

Didn't feel today's one, just after 1am.

 

[billiards]

Hi Dave,

I was struck by the difference in tectonic regime between the N and S islands in your diagrams. After looking into it seems as though New Zealand is the stage of an epic battle between the Pacific and the Australian plates! They are colliding and are fighting to remain afloat. To the North the Australian plate is winning, but to the south the Pacific remains afloat.

 

[davenn]

Hi Billiards

yes, NZ is a geologists "field of dreams" or is that nirvana ? LOL
I loved doing my geology studies there, there's just so much variety packed into such a small landmass

and yes something I didn't point out was in the lowest corner South Island and further down south through the Puysegur - Macquarie trench, the subduction returns but in the opposite direction compared to the North Island, as can be see by the B-B' cross-section.
The subduction zone there dips very steeply just a few degrees (~15) off the vertical. I personally don't know of another such steeply dipping zone.

cheers
Dave

 

[billiards]

Wow that really is steep. I guess that means its a mature subduction zone. So is the theory that it will eventually detach and subduction of the Pacific plate will takeover?

 

[StevieTNZ]

Just had two severe ones.

First one 6.2, 8km deep followed by a 5.7 8km deep. Data being revised. First it was 6.9 at 12km deep.

 

[davenn]

yup its coming in on my seismo here in Sydney as we speak

http://www.sydneystormcity.com/sydln.gifand unlike the previous ones in this region over the last month ... these ones are on land only a few km's from the Seddon township :(
Dave

 

[StevieTNZ]

http://www.stuff.co.nz/national/9051982/Strong-quake-rocks-central-New-Zealand

 

[StevieTNZ]

Am noticing a lot of small aftershocks. Let's hope there is no more big one.

 

[Evo]

Hold on Stevie!

 

[davenn]

a quick little graphic I whipped up

for non-New Zealanders, this is the NE tip of the South Island of NZ

EDIT: updated graphic and another update

Dave

 

[StevieTNZ]

A few severe aftershocks have occurred since the last post.

 

[davenn]

OK now that I am home from work and have had time to process the data
here's the seismograms for the 2 largest events

Mw 6.5 (MB 6.2) at 02:31UT (14:31 NZST)

Mw 5.9 (MB 6.0) at 05:31UT (17:31 NZST)

exactly 3 hours apart, give or take a few seconds ... freaky!

Dave

 

[StevieTNZ]

Yes, I was on the phone to someone when the ones hit at 5.30/5.31pm. And the house just started shaking. It was as scary as the 2.31pm earthquake.

 

[StevieTNZ]

http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10913861

Didn't realize that a 5.5 aftershock hit last night.

GNS scientists warned of a 20 per cent risk of another 6+ Richter scale quake in the next seven days.

God I hope not.

 

[davenn]

ahhh you didnt feel that one huh

yeah, it was the getting away down the SW end of the activity zone.
There was a M5.0 a bit further SW ~ 2 hours earlier

Dave

 

[davenn]

now that most of the locations have been finalised,here's an updated graphic ( as of midnite 17 Aug 2013)
I have included all M5+ events and also the events of several weeks ago that started all this activity.

NOTE: eg. 16/1709NZ = 16 August @ 1709 NZST ( New Zealand Standard Time)
other than the ones noted in the list in the lower right corner which are July events

the focal mechanism for these events are predominatly strike-slip, trending NE-SW
The M6.5 of July ( out in Cook Strait) had a small amount of obique slip component to it
A line through the 3 x M6 events also nicely delineates the direction of the slip


cheers
Dave

 

[davenn]

GNS scientists warned of a 20 per cent risk of another 6+ Richter scale quake in the next seven days.

God I hope not.

really ... it wouldn't surprise me... this region has been really quiet for a very long time
the last major even in this region was the estimated M7.5 event of 1848 on the Awatere Fault
a few km's to the west of this activity

Dave

 

[Greg Bernhardt]

Thanks for the info davenn! I'm headed to NZ this January. I hope to feel a little one, but that is it :)

 

[davenn]

where specifically in NZ are you heading to?
or is it a wide ranging tour ?

Dave

 

[Greg Bernhardt]

where specifically in NZ are you heading to?
or is it a wide ranging tour ?

Dave

yeah I'll be there a month driving around, so everywhere :)

 

[StevieTNZ]

Another aftershock (5.1) just after midnight:

http://nz.news.yahoo.com/a/-/top-stories/18787570/high-winds-quakes-in-welly-overnight/

At least we didn't sustain the damage that Christchurch did:

 

[lisab]

Another aftershock (5.1) just after midnight:

http://nz.news.yahoo.com/a/-/top-stories/18787570/high-winds-quakes-in-welly-overnight/

At least we didn't sustain the damage that Christchurch did:

Hang on! (Do you have padding around your bed?)