Sunday, June 26, 2011

Banks Peninsula Rising II

This video clip animation (which is silent by the way) made by GeoNet NZ is a helpful illustration of the way the New Zealand land mass has been formed between the interaction or collision between the Pacific Plate and the Indian Australian Plate. It's all interesting but what is especially interesting for the South Island is that the Southern Alps were formed by the two plates rubbing together and forcing the edges to buckle and bend and form the huge upthrusts that are now the Southern Alps.

This graphic shows the situation today. What is of particular interest is the fact that the Pacific Plate is "subducting" under the Indian Australian Plate to the north of New Zealand. ie it is diving under it. The arrow heads show where that is happening. But along the Southern Alps the edge of the Pacific Plate is apparently sliding along the edge of the Indian Australian Plate.

This difference in interaction is further illustrated in this graphic. The transverse movement/fault known as a "strike slip region" it appears. Even though the Plates are sliding along each other in the South Island, along the Southern Alpine fault, it's not as if this movement is well oiled. Any movement is in huge and violent jerks. Enormous pressures will still be exerted in an East-West direction (because the plates still press together), the edges will bind, causing a shearing stress on the Pacific Plate that lies under the Canterbury Plains. As a result a number of smaller faults have developed from the top of the South Island, parallel to the Southern Alps fault, that run down into the Canterbury Plains.

Some of these smaller fault lines are shown in this recent graphic prepared by NIWA which is now mapping new faultlines. You can just make out other red fault lines under the earthquake dots on the land around in Christchurch. It is movement in these smaller faults that give rise to smaller earthquakes (compared to those released in and by the Southern Alp fault.). But these faultlines are close to the surface and close to a city. So they can be much more destructive than a big distant earthquake.

This is an East/West cross section of Banks Peninsula (looking North). It shows the volcanic rock that formed Banks Peninsula after volcanic activity a million or more years ago. These volcanoes are extinct. The underlying rock is known to be faulted. (You can see the faultlines in the graphic.) This base rock was once flat but has been pushed up out of shape, and it has been squeezed by East-West tectonic plate pressure, and must have been cracked allowing volcanic magma to push through. One scenario is that this underlying rock is folding up slowly now. It may be a weak point in the plate. New faults may be forming. But this is speculation.

Intensive monitoring is the duty of authorities now.

Labels: , , , , ,

1 comment:

Joel Cayford said...

I have had some helpful technical explanation by email which I share here:

"...When Banks Peninsula was an active volcano about 9 million years ago, it was a a pretty typical oceanic basalt volcano, about 350 km offshore from the current coastline. Since then its been carried along at about 40 mm/yr on top of its bit of oceanic crust (also basalt) travelling towards the bit of continental crust that makes up NZ. At some point that big chunk of basalt on top of the regular oceanic crust has to bump into the crumpled up continental crust and hard older sedimentary rocks that make up NZ, underlying the softer Tertiary sediments and Pleistocene gravels of the Canterbury Plains. When that happens (maybe now) there'll be hell to pay.

Large plate-margin earthquakes happen when the sinking and over-riding plates which have become "stuck together" by friction, suddenly become unstuck because the accumulated strain overcomes the friction forces. The 1931 Hawkes Bay and the recent Japan earthquakes are good examples. In HB the westward-moving, sinking Pacific plate had dragged down the eastern margin of North Island in Hawke Bay, and when the accumulated strain was released that dragged down margin "flicked" up by 2m. In the Japan quake, the over-riding Asian plate, carrying Japan on top of it shot 25 m or so eastwards, relative to the sinking, westward-moving Pacific plate, when the accumulated strain was released.

Other large earthquakes occur when plate-margin transcurrent (side-slip) faults move suddenly, again because accumulated strain overcomes the friction stopping their movement, eg various earthquakes in California including the 1906 San Francisco earthquake and the 1855 Wellington earthquake.

In the central part of the South Island on the west side we have the Alpine Fault which, like the San Andreas Fault, will one day release another huge earthquake as it moves sideways (over about 5 million years it has moved about 500 kilometres - an average of 10 cm/year). It seems to have large earthquakes about every 800 years, so it might move on average about 80 metres in a "typical" large earthquake! The fault takes up the huge torsion applied to the NZ chunk of continental crust by the fact that the Pacific Plate sinks under NZ from the east at about 70mm/year, and the Australian plate sinks under the NZ bit of continental crust as a slightly slower rate, from the west, south of about Milford Sound. As well as the side-ways shift on the Alpine Fault, on its east side there has been about 20 km of upllift (of which about 4 km remain) in addition to a lot of crustal shortening by folding and many other small faults with uplifted blocks. That uplifting and "scrunching" has absorbed some of the combined 40mm/yr + c 50mm/year at which the two plates approach each other in the mid-section of South Island.

But at some point the big, hard lump of the Banks Peninsula volcano will start to get caught up in that crustal shortening, uplift etc. The bit of the volcano we can see is the "tip of an iceberg" with its westernmost edge going some distance (who knows how far) under the Canterbury Plains. That big hard lump will be hard to bend or break and other rocks will have to deform around it. Maybe that's starting to happen now, or maybe this is a renewed phase of that process which has been dormant for many decades or centuries.

In your later piece you are right that the liquefaction hazards in parts of ChCh were well documented in the 80's and 90's. Much of the damaged areas were built on before this was done, and liquefaction was only really recognised as a major earthquake hazard in the early 1960's Alaskan earthquake. But some parts like part of Bexley were built on in the 90's and later in spite of their flood hazard and the liquefaction hazard...."

Thought you might be interested.

July 7, 2011 at 9:22 AM

Post a Comment

Subscribe to: Post Comments (Atom)

Sunday, June 26, 2011

Banks Peninsula Rising II

This video clip animation (which is silent by the way) made by GeoNet NZ is a helpful illustration of the way the New Zealand land mass has been formed between the interaction or collision between the Pacific Plate and the Indian Australian Plate. It's all interesting but what is especially interesting for the South Island is that the Southern Alps were formed by the two plates rubbing together and forcing the edges to buckle and bend and form the huge upthrusts that are now the Southern Alps.

This graphic shows the situation today. What is of particular interest is the fact that the Pacific Plate is "subducting" under the Indian Australian Plate to the north of New Zealand. ie it is diving under it. The arrow heads show where that is happening. But along the Southern Alps the edge of the Pacific Plate is apparently sliding along the edge of the Indian Australian Plate.

This difference in interaction is further illustrated in this graphic. The transverse movement/fault known as a "strike slip region" it appears. Even though the Plates are sliding along each other in the South Island, along the Southern Alpine fault, it's not as if this movement is well oiled. Any movement is in huge and violent jerks. Enormous pressures will still be exerted in an East-West direction (because the plates still press together), the edges will bind, causing a shearing stress on the Pacific Plate that lies under the Canterbury Plains. As a result a number of smaller faults have developed from the top of the South Island, parallel to the Southern Alps fault, that run down into the Canterbury Plains.

Some of these smaller fault lines are shown in this recent graphic prepared by NIWA which is now mapping new faultlines. You can just make out other red fault lines under the earthquake dots on the land around in Christchurch. It is movement in these smaller faults that give rise to smaller earthquakes (compared to those released in and by the Southern Alp fault.). But these faultlines are close to the surface and close to a city. So they can be much more destructive than a big distant earthquake.

This is an East/West cross section of Banks Peninsula (looking North). It shows the volcanic rock that formed Banks Peninsula after volcanic activity a million or more years ago. These volcanoes are extinct. The underlying rock is known to be faulted. (You can see the faultlines in the graphic.) This base rock was once flat but has been pushed up out of shape, and it has been squeezed by East-West tectonic plate pressure, and must have been cracked allowing volcanic magma to push through. One scenario is that this underlying rock is folding up slowly now. It may be a weak point in the plate. New faults may be forming. But this is speculation.

Intensive monitoring is the duty of authorities now.

Posted by at
Labels: , , , , ,

1 comment:

Joel Cayford said...

I have had some helpful technical explanation by email which I share here:

"...When Banks Peninsula was an active volcano about 9 million years ago, it was a a pretty typical oceanic basalt volcano, about 350 km offshore from the current coastline. Since then its been carried along at about 40 mm/yr on top of its bit of oceanic crust (also basalt) travelling towards the bit of continental crust that makes up NZ. At some point that big chunk of basalt on top of the regular oceanic crust has to bump into the crumpled up continental crust and hard older sedimentary rocks that make up NZ, underlying the softer Tertiary sediments and Pleistocene gravels of the Canterbury Plains. When that happens (maybe now) there'll be hell to pay.

Large plate-margin earthquakes happen when the sinking and over-riding plates which have become "stuck together" by friction, suddenly become unstuck because the accumulated strain overcomes the friction forces. The 1931 Hawkes Bay and the recent Japan earthquakes are good examples. In HB the westward-moving, sinking Pacific plate had dragged down the eastern margin of North Island in Hawke Bay, and when the accumulated strain was released that dragged down margin "flicked" up by 2m. In the Japan quake, the over-riding Asian plate, carrying Japan on top of it shot 25 m or so eastwards, relative to the sinking, westward-moving Pacific plate, when the accumulated strain was released.

Other large earthquakes occur when plate-margin transcurrent (side-slip) faults move suddenly, again because accumulated strain overcomes the friction stopping their movement, eg various earthquakes in California including the 1906 San Francisco earthquake and the 1855 Wellington earthquake.

In the central part of the South Island on the west side we have the Alpine Fault which, like the San Andreas Fault, will one day release another huge earthquake as it moves sideways (over about 5 million years it has moved about 500 kilometres - an average of 10 cm/year). It seems to have large earthquakes about every 800 years, so it might move on average about 80 metres in a "typical" large earthquake! The fault takes up the huge torsion applied to the NZ chunk of continental crust by the fact that the Pacific Plate sinks under NZ from the east at about 70mm/year, and the Australian plate sinks under the NZ bit of continental crust as a slightly slower rate, from the west, south of about Milford Sound. As well as the side-ways shift on the Alpine Fault, on its east side there has been about 20 km of upllift (of which about 4 km remain) in addition to a lot of crustal shortening by folding and many other small faults with uplifted blocks. That uplifting and "scrunching" has absorbed some of the combined 40mm/yr + c 50mm/year at which the two plates approach each other in the mid-section of South Island.

But at some point the big, hard lump of the Banks Peninsula volcano will start to get caught up in that crustal shortening, uplift etc. The bit of the volcano we can see is the "tip of an iceberg" with its westernmost edge going some distance (who knows how far) under the Canterbury Plains. That big hard lump will be hard to bend or break and other rocks will have to deform around it. Maybe that's starting to happen now, or maybe this is a renewed phase of that process which has been dormant for many decades or centuries.

In your later piece you are right that the liquefaction hazards in parts of ChCh were well documented in the 80's and 90's. Much of the damaged areas were built on before this was done, and liquefaction was only really recognised as a major earthquake hazard in the early 1960's Alaskan earthquake. But some parts like part of Bexley were built on in the 90's and later in spite of their flood hazard and the liquefaction hazard...."

Thought you might be interested.

July 7, 2011 at 9:22 AM

Post a Comment

Subscribe to: Post Comments (Atom)