How did they measure Earthquakes back in 1100 AD ?

[JennyLeez]

This is probably a good question for Suezy.
In my roaming around I have found documented earthquakes from the years 1100 AD,  1450 AD, 1610, 1717 and 1826.
These were magnitude 8, 7, 8, 8 and 8 respectively.
These are listed under The Alphine Fault and Wellington Region.

How 'on earth' did they measure these ways back in 1100 please?

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[Suezy]

In reply to finding the info for the quakes
The Alpine Fault line is the boundary between the Australian and Pacific Plates which runs for approx 650 km from the Wairau River valley through the central South Island down the western side of the Southern Alps out to sea near the Milford Sound entrance.This is also very visible from outer space. The plates are moving past and pushing against each other forcing the Southern Alps higher.
In about 2008 scientists from NewZealand, Switzerland and the United States  dug a 26m long and 2m deep trench across the fault line at Inchbonnie.  So that what they found could be carbon dated. 
Where the fault crosses the trench, layers of gravel and sand laid down by the Taramakau River have been twisted and squeezed almost vertically.  Any wood found in the cross section can then be carbon dated to calculate when the last major earthquake caused by the Alpine fault occurred.
Inchbonnie was at the junction of several faults, including the Hope fault and movement on the Alpine Fault there could effectively shift earthquake shaking along those other faults.
At Inchbonnie the avg slip rate of the fault  was about 10mm a year but that increased to more that 25mm further south west on the fault.
The last major rupture happened in 1717, which generated an earthquake of mag 8 on the Richter scale also evidence of a similar sized quake about 1620 and other fault movement about 1450 and 1100.
Evidentially the Alpine fault occurs every 250 to 280 years and that New Zealand is overdue for a big one.
It has also been mentioned in other articles where the Alpine fault may not necessarily erupt in one long fault but in sections.
They have also done research on previous trenches too on this fault line.
So the Wellington ones you mention Jenny would be the result of our South Island one and from what I gather their are no other fault lines that is joining the Wellington ones from the scanning they did on the Cook strait area.  Never the less if our ones goes the whole of New Zealand would feel it rather badly as well.
Hope this answers your question as to how they can find the years etc.
When the early settlers came to NZ they had some really big ones too and how they judged the magnitude of a lot of those are going by the diaries that they used to keep and as to how severe they were.

PS Please excuse any typo errors as lousy typist.

[gabba]

Good question and thanks suezy fpt a great answer. Interesting that there is no connection between the Alpine fault and the Wellington region although the geographic features of Marlborough and Wellington suggest otherwise. Thanks again

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[Mark]

The last major rupture happened in 1717, which generated an earthquake of mag 8 on the Richter scale
Suezy do you known what part of the fault ruptured in this Quake?.

[Suezy]

I cant recall from memory Mark unfortunately - the poor old brain gets a bit overloaded at times with the different research I do - but think it may have been the southern part but will check it out for you and let you know.
 really don't want to think about the difference o our one of 7.1 and hope we never see it but the calculations are=
8.1 mag is 10 times bigger BUT  31.622 stronger in energy release.

[Suezy]

Found it Mark finally - you can disregard the other links if you like and just go to the last one.


Mark - I have been looking through my papers here and using google - but to no avail - unable to find which portion ruptured etc.  I will have to start and read the University papers that I have downloaded to see where I have read it.
Found these sites on my wander through the google and will put them below.

http://all-geo.org/highlyallochthonous/2010/09/all-quiet-on-the-alpine-fault/

Video from GNS



They have cut trenches across the alpine fault for researching and last year I think it was they have also bored down into the fault itself  for their recording devices to go down and record what is down there.

http://www.teara.govt.nz/en/active-faults/3

http://www.orc.govt.nz/Information-and-Services/Natural-Hazards/Great-Alpine-Fault-Earthquake/

1717 - Found it the lower part from Milford sound  to Haupiri

1620 – from Paringa north to near the Matakitaki River

called “Danger line of the alpine fault”  from stuff – 1/01/09

http://www.stuff.co.nz/national/31699/Danger-line-of-Alpine-Fault

[Mark]

Observation of Long Supershear Rupture During the Magnitude 8.1 Kunlunshan Earthquake

Michel Bouchon* and
Martin Vallée

+ Author Affiliations

Université Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géophysique Interne et Tectonophysique, Boîte postale 53, 38041 Grenoble, France.

* To whom correspondence should be addressed. E-mail: Michel.Bouchon@ujf-grenoble.fr


Abstract

The 2001 Kunlunshan earthquake was an extraordinary event that produced a 400-km-long surface rupture. Regional broadband recordings of this event provide an opportunity to accurately observe the speed at which a fault ruptures during an earthquake, which has important implications for seismic risk and for understanding earthquake physics. We determined that rupture propagated on the 400-km-long fault at an average speed of 3.7 to 3.9 km/s, which exceeds the shear velocity of the brittle part of the crust. Rupture started at sub–Rayleigh wave velocity and became supershear, probably approaching 5 km/s, after about 100 km of propagation.

[Suezy]

The length of the fault line and the longer it is can usually produce the magnitude size  - for eg the long one off the Pegasus Bay can produce up to a 7 as well.