Wellington Earthquake warning ! - New Zealand

[The Matrix Traveller]

I think that the area with no activity has only low depth earthquakes, as the chart shows only earthquakes at depths bigger than 40km.

From my understanding of it, shallow earthquakes are probably the result of other movements, like aftershocks, as if things keep on rearranging themselves for some time after a large earthquake.

I will agree with you that Shallow Earthquakes often are after shocks, but not all shallow Earthquakes
are associated with after shocks.

I can tell you that Seismologists are intently interested in the Southern Alps, which has been well documented.

They have said they have concerns regarding the possibility of a Major earthquake occurring
in the Alpine Ridge (Main Divide) in the future which has also been well documented.

Laser stations have been set up along the Alps recording movement within the South Island Ridge.

Note the Strongest Earthquake that hit Wellington was a shallow Earthquake.. 

Public Id: 2013p543824
NZST: Sunday, July 21 2013 at 5:09:30 pm
New Zealand region intensity ?: severe
Maximum intensity ?: severe
Depth: 13 km
Magnitude: 6.5
Location:  25 km east of Seddon

Rather Shallow...

Not regarded as an after shock.


So this is why I suspect these shallow earthquakes may also indicate Subterranean subsidence in some cases.

From Public Id: 2013p543824 Seismologists surveyed the Ocean floor in Cook Strait soon after
this Earthquake, looking for subsidence (NOT in this case Subterranean subsidence)
on the Wellington side of Cook Strait.



http://www.eqc.govt.nz/research/research-papers/slip-rate-holocene-subsidence

Technical Abstract

The Vernon fault lies within the northern part of Marlborough fault system; it splays from the Awatere fault
~12 km from the east coast and continues into Cook Strait for a further ~25 km northeast-wards.

Although a relatively minor component of the plate boundary zone, the Vernon fault is significant
because it lies adjacent to Big Lagoon, a coastal wetland that potentially holds a geologic record
of tectonic subsidence related to subduction earthquakes.

The southern Hikurangi subduction interface dips westward beneath the Cook Strait region,
and has the potential to rupture in large to great earthquakes. A subduction earthquake
on the southern Hikurangi margin is potentially one of the biggest hazards in central New Zealand.

http://www.stuff.co.nz/national/8950508/Swarm-could-trigger-Wellington-fault

Seismologists say the weekend's swarm of earthquakes could be on a new fault
they have not mapped before, and could possibly trigger movement on the feared Wellington fault.

The news comes as Wellington wakes from another rocky night, which included a 4.9-magnitude quake
in the early hours. Through the night, a swarm of quakes occurred which were centred off Seddon
in the upper South Island -
the biggest being was a 4.9 quake at 1.31am at a depth of 25km. All other quakes since midnight
were 3.8 or lower.

GNS scientists and teams from the National Institute of Water and Atmospheric Research have been
working to establish which fault line was the catalyst for the series of shakes, including the magnitude 6.5 quake
on Sunday evening.

Seismologist John Ristau said the earthquakes were lying in an area of Cook Strait between the Vernon fault
and Needles fault.

"There are a number of active faults in the area. It's an issue because they are offshore so it's difficult
to get the most accurate location we can."

A third fault to the left of Needles, called the London Hill fault, was also a possible location if it was found
to extend offshore.

"The other possibility is it is either on an unmapped fault or something brand new."

Quantifying the contribution of Vernon fault displacement to the tectonic subsidence of Big Lagoon
is the main objective of this study.

http://www.niwa.co.nz/news/scientists-set-to-delve-into-the-secrets-of-the-cook-strait-mega-canyon

This week, scientists led by NIWA Marine Geologist Dr Joshu Mountjoy will use a new dynamic positioning
(DP) system onboard NIWA's research vessel Tangaroa to collect sediment cores, rock samples
and DTIS (deep-towed imaging system) footage of the seafloor.

The DP system uses electrically powered thrusters and computerised controls to fix the vessel
to a specific area, guided by satellite positioning or transponders on the seabed. It will allow the scientists
to take very accurate samples from the canyon floor, landslide scars and fault scarps, in 500-1,000 metre depths.

Data collected will provide information about active sediment transport through the canyon,
how often landslides and earthquakes occur, and give scientists a first glimpse of the biology in the canyon.

"This voyage is a step towards understanding one of New Zealand's most remarkable seafloor features.
Internationally, mega-scale canyon systems like Cook Strait are regarded as the focus of deep ocean
sedimentary activity. We know that dramatic sediment movement occurs through much of Cook Strait
in response to tide related currents but we have no information on the canyon system. It is really exciting
to be on the verge of collecting the first images and samples from this extraordinary landscape
including the first footage of the biology living in these canyons," says Joshu.

"Data collected will also help us to quantify hazards that could be created by landslides and earthquakes
in the canyon system, ensuring New Zealand is better prepared to respond to natural disasters in the future.
Landslides in the canyon are huge, up to four times the size of Wellington's Mount Victoria, and have
the potential to generate hazardous tsunami.

"We need to know two things - one is to understand how often they occur and the other is to model
the tsunami generation potential. This voyage will provide information on how often the landslides happen,
while NIWA and GNS Science are working together under the Natural Hazards Research Platform
to quantify the tsunami generation potential."

While there are several canyon systems in New Zealand, Cook Strait Canyon is one of the largest
and the closest to any major city. NIWA has mapped more than 150 individual landslides throughout
the Cook Strait canyon system, ranging in volume from 2.5 million cubic metres to approximately
ten cubic kilometres. Understanding the causes and recurrence of these landslides is part
of a long-term goal of quantifying the landslide tsunami risk to New Zealand's coastal communities.

The research is funded by the Ministry of Science and Innovation under the "Consequences of Earth
Ocean Change and Impacts of Resource Use on Vulnerable Deep Sea Communities" programmes,
and through the Natural Hazards Research Platform.

[The Matrix Traveller]

http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Major-Faults-in-New-Zealand/Alpine-Fault

Alpine Fault

The Alpine Fault, which runs for about 600km up the spine of the South Island,
is one of the world's major geological features. It's the "on-land" boundary of the Pacific
and Australian Plates.

This fault has ruptured four times in the past 900 years, each time producing an earthquake
of about magnitude 8. Approximate rupture dates are 1717AD, 1620 AD, 1450 AD, and 1100 AD.

Recent research (published in 2012) by GNS Science has extended our knowledge of the Alpine fault earthquake
record back through the past 8000 years. Click here for more details of these findings.

Horizontal movement of the Alpine Fault is about 30m per 1000 years — very fast by global standards.
Each time it has ruptured, it has also moved vertically, lifting the Southern Alps in the process.
In the last 12 million years the Southern Alps have been uplifted by an amazing 20 kilometres,
and it is only the fast pace of erosion that has kept their highest point below 4000 metres.
The glaciers and rivers have removed the rest of the material and spread it out across the lowland plains
or onto the sea floor. The rapid uplift also means that faulted rock from deep down has been brought
to the surface, and can be studied by scientists.

The Alpine Fault has a high probability (estimated at 30%) of rupturing in the next 50 years.
The rupture will produce one of the biggest earthquakes since European settlement of New Zealand,
and it will have a major impact on the lives of many people. In between earthquakes, the Alpine Fault
is locked.
All these things mean that the Alpine Fault is a globally significant geological structure.

[The Matrix Traveller]

Public Id: 2013p695265
NZST: Sunday, September 15 2013 at 5:57:23 pm
Maximum intensity ?: moderate
Depth: 18 km
Magnitude: 4.1
Location:  15 km south-east of Seddon

[The Matrix Traveller]

Public Id: 2013p698529
NZST: Monday, September 16 2013 at 10:56:40 pm
Maximum intensity ?: moderate
Depth: 9 km
Magnitude: 3.6
Location:  15 km south of Seddon

[The Matrix Traveller]

3 more Moderate Earthquakes near Seddon last Night (Near Cook Strait)


Public Id: 2013p707091
NZST: Friday, September 20 2013 at 2:58:17 am
Maximum intensity ?: moderate
Depth: 16 km
Magnitude: 4.4
Location:  25 km east of Seddon



Public Id: 2013p706672
NZST: Thursday, September 19 2013 at 11:15:14 pm
Maximum intensity ?: moderate
Depth: 20 km
Magnitude: 4.3
Location:  20 km south-east of Seddon



Public Id: 2013p706591
NZST: Thursday, September 19 2013 at 10:32:08 pm
New Zealand region intensity ?: light
Maximum intensity ?: moderate
Depth: 26 km
Magnitude: 4.1
Location:  20 km east of Seddon

[Shasta56]

Matrix, I am going to be seriously unhappy if the ground opens up and swallows you.

Shasta

[robomont]

i agree.

[Amaterasu]

Me too!!!!!

[The Matrix Traveller]

Thank you ALL for your concern. At present though things appear to be relatively quiet, except for all these
small shallow earthquakes around the rest of NZ.

I am certainly no expert in this field,     but I am wondering why so many shallow earthquakes are occurring
right across NZ ?

I always thought our normal activity took place at a much deeper depth, than what we have been getting lately.

NZ experiences hundreds of small earthquakes every year, which is Normal for NZ.

But recently there have been hundreds of these smaller earthquakes, at depths between 5 km and 30 km.
(NOT "after shocks") springing up all over the country, many of which are not felt.
(around the Magnitude 1.5 to 3.0)

In the past it would get our attention, when sometimes shallow earthquakes occurred.

Most of of the shallow earthquakes appear around Seddon, (Near Cook Strait) but between these
we are getting many other small shallow Earthquakes, from the bottom of the South Island right up to White Island
on the north eastern coast.

When I get a moment I hope to obtain records over the last 20 years or so referring to only shallow earthquakes
of any magnitude, to see IF this has been going on over the past years or not.

Swarms of Earthquakes are fairly common in NZ but often deeper than these, I thought ?

[Shasta56]

It will be interesting to find out what the records say.  Our perceptions can definitely mislead us.  I get the feeling though, that your perceptions are pretty accurate on this.

Shasta

[The Matrix Traveller]

A 4.1 Magnitude Earthquake (Moderate) 15 Km west of Christchurch.

Public Id: 2013p708602
NZST: Friday, September 20 2013 at 4:23:27 pm
Maximum intensity ?: moderate
Depth: 9 km
Magnitude: 4.1
Location:  15 km west of Christchurch

[Flux]

My phone alerts me of near by quakes but when It did I was surprised I never felt that 4.1 at such a shallow depth.

[The Matrix Traveller]

My phone alerts me of near by quakes but when It did I was surprised I never felt that 4.1 at such a shallow depth.

Interesting you should say that, because I noticed some other Quakes have also been listed as being weak
or not felt also ?

For example;

Public Id: 2013p707306
NZST: Friday, September 20 2013 at 4:53:10 am
Maximum intensity ?: light
Depth: 16 km
Magnitude: 3.0
Location:  60 km west of Milford Sound.

My guess is that the ground acceleration is different.

In the case of the 4.1 Magnitude Earthquake (Moderate) 15 Km west of Christchurch today
the ground acceleration must have been low.

I know there is argument over identifying the strength of Earthquakes i.e. should we use "magnitude"
or "acceleration" scales (using an "Accelerometer") to record them.

I understand we can get high magnitude earthquakes ,with either low Acceleration or high Acceleration rates.

And we can get low magnitude Earthquakes which can have high acceleration rates.

[1Worldwatcher]

As I pondered all of the reports you have been putting for us here Matrix, I was taken back to a conversation I had with an on line friend that worked for the USGS and he discussed the "Possibilities" of cause and effect's of different tectonic depth scenario's. One of these conversations led to a discussion of how 'Earthquake Liquefaction' is introduced to a known area of rather rocky or solidified terrestrial land masses.

For those of you that don't know "Earthquake Liquefaction" :

Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.
In soil mechanics the term "liquefied" was first used by Hazen[1] in reference to the 1918 failure of the Calaveras Dam in California. He described the mechanism of flow liquefaction of the embankment dam as follows:
If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand... the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquefied.
The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied; dense sands by contrast tend to expand in volume or 'dilate'. If the soil is saturated by water, a condition that often exists when the soil is below the ground water table or sea level, then water fills the gaps between soil grains ('pore spaces'). In response to the soil compressing, this water increases in pressure and attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loading is rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that it does not flow out in time before the next cycle of load is applied, the water pressures may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contacts between grains are the means by which the weight from buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose all of its strength (the ability to transfer shear stress) and it may be observed to flow like a liquid (hence 'liquefaction').
Although the effects of liquefaction have been long understood, it was more thoroughly brought to the attention of engineers after the 1964 Niigata earthquake and 1964 Alaska earthquake. It was also a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake, and in Port of Kobe during the 1995 Great Hanshin earthquake. More recently liquefaction was largely responsible for extensive damage to residential properties in the eastern suburbs and satellite townships of Christchurch, New Zealand during the 2010 Canterbury earthquake[2] and more extensively again following the Christchurch earthquakes that followed in early and mid-2011.[3]

Soil liquefaction

My USGS friend thought that he may have the reasons why these event's  occur when there has been no known report's of these event's in previous history or record. And if he is correct with how Island types of continental land masses eventually all acquire these events due to their Geological location's, "Could we be witnessing the 'Possibility' of that part of the world becoming one of these Soil Liquefaction sub tectonic boundaries?"

After all, it is prime real-estate for this type of scenario, and is reported around the world of have happening with these Earthquake liquefaction zones apparently out of the blue, there by leaving them virtually undetectable until it happens.

Has there ever been report's of this happening around your area in the past?

As always Matrix, sending you Protective Vibes my friend, and as the rest here have already extrapolated on, I don't want to see you get swallowed up by the ground my Friend. "Be safe and well."

1WW

[The Matrix Traveller]

Hi 1WW,

I suspect you friend might be correct..   

YES we had a lot of "Liquefaction" occur during the Christchurch earthquake a couple of years ago.
Made a hell of a mess which took some time to clear.

I understand we had seismologists from around the globe come to Christchurch to view and study "Liquefaction" !

Another Earthquake in or near Cook Strait this morning NZ Time.

Public Id: 2013p710152
NZST: Saturday, September 21 2013 at 6:09:19 am
Maximum intensity ?: strong
Depth: 6 km
Magnitude: 4.1
Location:  20 km east of Seddon