Volcanoes greatest risk to this region

Volcanoes greatest risk to this region

There are 3 comments on the Bay of Plenty Times story from Aug 26, 2012, titled Volcanoes greatest risk to this region. In it, Bay of Plenty Times reports that:

SIGNIFICANT THREAT: The Bay's volcanoes have earned the unenviable title of being major civil defence risks, led by the Taupo Volcanic Zone which is the most active caldera system in the world.

Join the discussion below, or Read more at Bay of Plenty Times.

“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Aug 29, 2012
Nice article.

Watch out for Tsunamis.

Tonga-North island backarc basin is divided into sectors. North island sector last underwent Rebound/long duration Mercalli X event (r/lx) during Hao Whenua event (~1450AD). That sector is still in early-mid stage of squeezing in magma filled fractures into the volcanoes (hence volcanoes being biggest threat).

Area between Kermadec sector & Brothers volcano (N. part of North island sector)... is a very large volcanic (all dormant) and seismic (low rate of deformation due to tight solid crust)gap. This sector is in the very late stage of recompression following r/lx and will eventually go "pop". The r/lx vector is variable ranging from NE. ENE.(most typical), E. ESE. or SE. resulting in wide variation in the strike of fractures formed by sudden decompression and as a result variability in size & location of volcanism during the squeeze in stage.

Recompression rate is ~7.5cm/yr (tonga), down to ~4.5cm/yr (North island) with recompression vector at NE-SW or closer to NNE-SSW near Tonga. This vector tends to squeeze magma filled fractures generated by crustal relaxation of initially tightly compressed crust lying over magma discs at the base of the crust... into the arc at a strike of NNW-SSE or NW-SE where they can go no further.

Result is a quake (locally 8.3 or so richter) and a runout of 2004 size or bigger (they are usually bigger in this backarc basin) with tsunami impact on NZ of 10+ metres (up to 20 metres if event has runout of expected size of 60m). Runout to the backfault may reach 500m @ tonga about 2/3 of which is achieved during a slow crawl lasting a few weeks!

As the runout is large, the recompression interval time is quite long, therefore (as with S. part of Atacama Altiplano) the latest stage is only recognisable by overall volcanic dormancy and generally tight overriding plate and decreasing seismicity from the descending plate.

Keep watching that space.

Also note: Monowai sector is in the late stage but with Monowai stratovolcano still squeezing in a long fracture and occasionally going "phut". r/lx there is still a long way off.

Tonga arc is in the early stage as a result of large event resulting in palaeotsunami in Wallis & Futuna. Also Tonga region had a historic arc size r/lx in the "curl" with tsunami in Samoa, and a recent (2009) plate bend rift event & tsunami in the same area due to free rapid motion of the descending plate following 1st event.

Have a nice day: Ag

“Educating Utus”

Since: Aug 10

Papakura, New Zealand

#2 Aug 29, 2012
Considering part Maoris reckon they own the mountains, seabed, air [what have I missed?] Can the rest of us claim compensation from them when a mountain blows up?

“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#3 Sep 3, 2012
Torqueing Heads wrote:
Considering part Maoris reckon they own the mountains, seabed, air [what have I missed?] Can the rest of us claim compensation from them when a mountain blows up?
Sadly no.

Probably other way round.

Have a nice day: Ag

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