Evison's Wall, Lewis Pass, New Zealand

Evison's WallDuring March 2011 I accompanied the 2nd AEES Earthquake Reconnaissance Mission to Christchurch, and during that time in New Zealand I took the opportunity to visit a place I recalled from my youth.  As a young hiker I'd chanced across a curious unplacarded concrete wall located in a relatively odd place, and pondered its purpose.  Only recently I learned that this structure has some seismological purpose, and was built in 1964 at the instigation of legendary New Zealand geophysicist Frank Evison.  Fault Creep is slow steady movement of an earthquake fault without an earthquake (aseismic) as opposed to sudden movement resulting in an earthquake (seismic), and to monitor this motion Evison had a concrete wall built orthogonally across a section of the Alpine Fault, at a river flat in the Lewis Pass (between Maruia Springs and Springs Junction).  These days this route through the Lewis Pass is well touristed, and an informative sign has now been placed at the wall to explain its purpose.

Evison's WallDuring my brief visit to the wall I measured its precise whereabouts using a hand held GPS receiver.  One of the handy features of this particular GPS model, is the ability to obtain a more precise position measurement by integrating many measurements over time.  That is, position estimates are summed and averaged.  If one looks closely at the ends of Evison's Wall, one will notice stainless steel reference markers set into the concrete.  These are used in conjunction with a theodolite to determine the precise geographic orientation of the wall, which would alter if the wall was rotated by earth creep (I estimate the orientation of the wall is about 150/330º, or N150ºE).  By placing my GPS receiver's antenna precisely over the stainless steel markers, and integrating for 1000 1-second measurements, the error circle estimate indicated by the GPS receiver was about 1-metre.  The two position measurements I obtained for the stainless steel wall markers were:

42º 20' 56.7" South,  172º 13' 29.1" East  =  -42.34908º, 172.22476º (i.e. northern end of the wall)

42º 20' 57.3" South,  172º 13' 29.6" East   =  -42.34926º, 172.22489º (i.e. southern end of the wall)

Evison's WallThese reference locations may be viewed as the map pin markers in Google Earth.  The average of these two positions (i.e. centre point of the wall) is -42.34917, 172.22483, and the geodetic datum used is WGS84, which is the normal default standard used with GPS receivers.  The accuracy of the above GPS measurements is sufficiently high that should a major earthquake occur in this region, and Evison's Wall gets translated/rotated, it may be possible to measure the changes using a similar GPS receiver.

In summary, Evison's wall is:
EPSO, 2011-09-24