Sunday, June 26, 2011
Banks Peninsula Rising II
1 comment:
- Joel Cayford said...
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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
1 comment:
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.
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