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MECHANICAL STRAIN G A U G E M E A S U R E M E N T S AT TE M A R A U
D. J. Darby* and N. D. Perrin*
A B S T R A C T
In 1981 an array of five steel rods linking four m o n u m e n t w a s i n s t a l l e d across the trace of the W e l l i n g t o n Fault, at the site of the Te M a r a u w a t e r s t o r a g e p r o j e c t . T h i s array w a s i n s t r u m e n t e d w i t h e l e c t r o n i c d i s p l a c e m e n t t r a n s d u c e r s and data l o g g e r s to serve as a h o r i z o n t a l s t r a i n m e t e r . C o r r e s p o n d i n g m e a s u r e m e n t s h a v e been m a d e from time to time using a W h i t t e m o r e m e c h a n i c a l strain g a u g e . Though t h e s e m e a s u r e m e n t s are c o n s i s t e n t w i t h a m o d e l of right lateral shear d e f o r m a t i o n at an average rate of (5 ± 2) x 1 0 "6/ y r (engineering u n i t s ) s i n c e 1 9 8 2 , some or all of t h i s e s t i m a t e m a y b e due to f l e x u r e of the rods; it m a y t h e r e f o r e be c o n s i d e r e d as an u p p e r limit to d e f o r m a t i o n at the s i t e . V e r i f i c a t i o n by p r e c i s e s u r v e y i n g m e t h o d s may be p o s s i b l e if t h e r e is a s u f f i c i e n t l y b r o a d z o n e of d e f o r m a t i o n .
INTRODUCTION
T h e W e l l i n g t o n fault h a s b e e n d e s c r i b e d by L e n s e n [1958]; its strike near Te M a r u a is about 060° and, citing B e r r y m a n (in p r e p . ) , B r o w n and Wood [1983]
g i v e an a v e r a g e r i g h t lateral offset r a t e of 7.4 m m / y r o v e r t h e last 1 4 0 , 0 0 0 y e a r s . T h e s t r a i n m e t e r at the site of the Te M a r u a w a t e r s t o r a g e p r o j e c t w a s installed in 1981 to m e a s u r e d e f o r m a t i o n about the fault, and is d e s c r i b e d by B r o w n and W o o d [ 1 9 8 3 ] . It w a s assembled in a 10 m d e e p trench, now b a c k f i l l e d , and c o n s i s t s of an a p p r o x i m a t e l y h o r i z o n t a l array of f i v e s t a i n l e s s steel r o d s ; two of t h e s e c r o s s the t r a c e of the W e l l i n g t o n fault d i a g o n a l l y , two are p a r a l l e l to it, and the fifth is nearly p e r p e n d i c u l a r to it (Figure 1 ) . The rods h a v e l e n g t h s from 5 m to 10 m. One end of each rod is a n c h o r e d to rock and the d i s p l a c e m e n t of the other end can be m e a s u r e d r e l a t i v e to an e n g r a v e d m o u m e n t . A c c e s s is by two m a n h o l e s referred to as n o r t h m a n h o l e (NM) and south m a n h o l e (SM) ; the a n c h o r e d ends are r e f e r r e d to as north anchor (NA) and south anchor ( S A ) . T h e s e a b b r e v i a t i o n s are u s e d for naming the r o d s . E l e c t r o n i c d i s p l a c e m e n t and temperature t r a n s d u c e r s , and d a t a l o g g e r s , w e r e
i n s t a l l e d as the p r i n c i p a l d a t a a c q u i s i t i o n system, but p u n c h e d h o l e s in the rods and m o n u m e n t s w e r e also p r o v i d e d to permit s u b s i d i a r y m e a s u r e m e n t s w i t h a m e c h a n i c a l s t r a i n g a u g e . W h i l e s o m e of the
e l e c t r o n i c a l l y logged d i s p l a c e m e n t s have b e e n r e p o r t e d [Brown and W o o d 198 3 ] , there h a v e b e e n p r o b l e m s w i t h c o n t i n u i t y and c a l i b r a t i o n of t h e o u t p u t s . T h e following d i s c u s s i o n is l i m i t e d to the m e c h a n i c a l s t r a i n g a u g e r e s u l t s .
STRAIN GAUGE M E A S U R E M E N T S
T h e gap l e n g t h b e t w e e n the punched h o l e at one end of each rod and a c o r r e s p o n d i n g h o l e a l i g n e d with the rod on the m o n u m e n t h a s b e e n m e a s u r e d 13 times NZ G e o l o g i c a l Survey, L o w e r H u t t .
b e t w e e n M a r c h 198 2 and September 1 9 8 5 , w i t h a W h i t t e m o r e six-inch m e c h a n i c a l s t r a i n g a u g e . T h e set of five m e a s u r e m e n t s t a k e n on each o c c a s i o n w i l l be called a survey.
A g a u g e m e a s u r e m e n t consists of the
d i f f e r e n c e b e t w e e n a m e a n dial reading w i t h the g a u g e set in d r i l l holes in an invar s t a n d a r d b a r . This p r o c e d u r e e l i m i n a t e s any c h a n g e in the zero error of the g a u g e . A l t h o u g h the dial is g r a d u a t e d to 0.0001 inch i n t e r v a l s (estimation 0.00002 inch) e x p e r i e n c e shows that m e a s u r e m e n t s are r e p e a t a b l e only to about 0.001 inch or 25 urn. Some expertise is r e q u i r e d to a c c o m p l i s h this r e p e a t a b i l i t y . T h e m e a n d i a l r e a d i n g s are m e a n s of i n d i v i d u a l r e a d i n g s taken with the g a u g e held in each of -the two p o s s i b l e o r i e n t a t i o n s . O n s e v e r a l o c c a s i o n s the d i f f e r e n c e b e t w e e n t h e s e i n d i v i d u a l readings w a s q u i t e large
(over 0.00200 inch or 50 urn). W h e n r e p e t i t i o n s w e r e not m a d e in t h e s e c i r c u m s t a n c e s , the m e a s u r e m e n t s are of d o u b t f u l r e l i a b i l i t y , and 8 such m e a s u r e - m e n t s h a v e been rejected from the d a t a set.
T h e p r o b l e m is attributed to the age and c o n d i t i o n of the gauge and the p o o r q u a l i t y of the p u n c h e d holes used in this e x p e r i m e n t . N e v e r t h e l e s s , a m e a s u r e m e n t of linear strain over a 5 m d i s t a n c e should be a c c u r a t e to 5 x 1 0 - 6 . M e a s u r e m e n t s in the five d i f f e r e n t o r i e n t a t i o n s over the three year time p e r i o d m a y permit e s t i m a t i o n of areal s t r a i n rate components to an a c c u r a c y
a p p r o a c h i n g 1 x 10~*°/yr, if the s t r a i n is s p a t i a l l y u n i f o r m and at a c o n s t a n t rate t h r o u g h o u t the time period.
F i g u r e 2 shows the g a u g e m e a s u r e m e n t s for each rod. The ordinate o r i g i n s depend in each case on the o r i g i n a l gap lengths and are of no significance, but the similar trend of each trace i n d i c a t e s that the m e a s u r e m e n t s on each o c c a s i o n show c o n s i s t -
ency amongst t h e m s e l v e s , and that t h e r e is a common, p o s s i b l y systematic, b e h a v i o u r of each of the five m e a s u r e d g a p s . T h i s could be attributed to areal d i l a t a t i o n s at the site, but other m o r e o b v i o u s e f f e c t s
BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol. 19, No. 2, June 1986
north manbote
W i l l 8.4 m
R.L 109.S
concrete * pillar ^
manhoia
stainless steel rod enclosed In protective conduit
F I G U R E 1 P L A N A N D S E C T I O N O F S T R A I N M E T E R I N S T A L L A T I O N A T TE M A R U A (AFTER B R O W N A N D W O O D (1983)) A N D ITS R E L A T I O N S H I P TO THE T R A C E OF THE W E L L I N G T O N F A U L T .
106
•such as t e m p e r a t u r e v a r i a t i o n s , changes in the g a u g e or invar standard, and flexure of the rods m u s t be c o n s i d e r e d .
DATA A N A L Y S I S
T h e e l e c t r o n i c a l l y logged t e m p e r a t u r e s of the rods v a r i e d b e t w e e n 9°C and 13°C.
T h i s d i f f e r e n c e w o u l d induce a thermal change in l e n g t h of about 600 ym for the longest rod, i.e. some 25 times the r e s o l u t i o n of the g a u g e . It is t h e r e f o r e crucial t h a t account be taken of t e m p e r a t u r e e f f e c t s ; it is r e c o g n i s e d that access to the rod e n d s , v i a the m a n h o l e s , m a y itself affect the t e m p e r a t u r e at the rod end at the time of a m e c h a n i c a l m e a s u r e m e n t . T h e o r i g i n a l d e s i g n of e l e c t r o n i c a l l y logged t r a n s d u c e r o u t p u t s c i r c u m v e n t s this p r o b l e m .
T h e number and p o s i t i o n s of the m o n u m e n t s w i t h r e s p e c t to the fault trace do not a l l o w any d i f f e r e n t i a t i o n b e t w e e n t r a n s c u r r e n t slip of the fault itself and shear d e f o r m a t i o n p a r a l l e l to its s t r i k e . In the small area t h i s array samples, w e feel such a d i s t i n c t i o n is not r e l e v a n t from a r e g i o n a l p o i n t of view, and strain c o m p o n e n t s w i l l b e e s t i m a t e d , rather than relative d i s p l a c e m e n t s , to take a d v a n t a g e of m e a s u r e m e n t r e d u n d a n c y . T e m p e r a t u r e m e a s u r e m e n t s are not a v a i l a b l e to correct all the m e a s u r e m e n t s for thermal changes of rod l e n g t h s . W e t h e r e f o r e e s t i m a t e
p a r a m e t e r s of the m o d e l :
g = g ° + d L + y , t ,L cos 20 + y0t ,L
yr s ^r s r '1 si r r '2 si r
s i n 2 8r + er s (1)
w h e r e r = 1, , 5; s = 1, --, — , 13 except as n o t e d .
g is the g a u g e m e a s u r e m e n t for rod r in
rs s u r v e y s;
g° is the ideal i n i t i a l m e a s u r e m e n t for
r rod r, in t h e a b s e n c e of r a n d o m or m o d e l l i n g e r r o r s , and m u s t be estimated;
d is the a p p a r e n t linear d i l a t a t i o n
s c o m p a r e d to the first survey for each s u b s e q u e n t s u r v e y s (s > 1 ) ; these are to b e e s t i m a t e d and are d i s c u s s e d in d e t a i l b e l o w ;
y• (i = 1,2) are the two {tensor) strain
1 rate c o m p o n e n t s , assumed c o n s t a n t in time, w h i c h d e s c r i b e u n i f o r m shear strain; t h e y are to be estimated;
t , is the t i m e of survey s after the
S1 first (s > 1 ) ;
Lr is the l e n g t h of rod r;
6 is the azimuth of rod r;
r
e is the r a n d o m e r r o r for rod r in survey s; e r r o r s are a s s u m e d to h a v e a normal d i s t r i b u t i o n w i t h m e a n zero, and a standard d e v i a t i o n w h i c h w i l l be e s t i m a t e d from the r e s i d u a l s .
F o r 13 s u r v e y s of 5 rods there are thus 65 o b s e r v a t i o n s to d e t e r m i n e 19 p a r a m e t e r s ; t h e r e w o u l d t h e r e f o r e be 46 d e g r e e s of freedom, but the 8 d u b i o u s m e a s u r e m e n t s h a v e b e e n rejected. In
g e n e r a l , for S s u r v e y s there are p o t e n t i a l l y 4S-6 d e g r e e s of freedom.
T h e a p p a r e n t d i l a t a t i o n d for each survey after the first m a y h a v eSa
c o n t r i b u t i o n from a real areal d i l a t a t i o n A , as w e l l as from thermal e x p a n s i o n or c o n t r a c t i o n of the steel r o d s , w h i c h w e a s s u m e to h a v e all the same t e m p e r a t u r e . T h e r e f o r e
d =s 2 s s 1 ~ A - a(T - T, ) (s > 1) (2) w h e r e a is the coefficient of thermal
e x p a n s i o n for the rods (15.9 x 10~6/C°) and T is the rod t e m p e r a t u r e at the time of
survey s.
A f t e r estimating the apparent
d i l a t a t i o n s , it would be of some i n t e r e s t to compare them w i t h the r e c o r d e d
t e m p e r a t u r e w h e r e this is a v a i l a b l e , in o r d e r to s u b s t a n t i a t e this m e t h o d of using the array itself as a t h e r m o m e t e r . A t p r e s e n t there are i n s u f f i c i e n t t e m p e r a t u r e d a t a to do t h i s .
S i n c e the invar standard is o c c a s i o n - ally subjected to inertial d a m a g e (by being d r o p p e d , etc.) in the field, and h a s never b e e n c a l i b r a t e d against one m a i n t a i n e d u n d e r l a b o r a t o r y c o n d i t i o n s , it m a y be of i n t e r e s t to extend the model to allow for any d i s c r e t e change the standard m a y s u f f e r . T h i s m a y be p o s s i b l e by adding a term
f (s £ 2) to the r.h.s. of e q u a t i o n ( 1 ) , and e s t i m a t i n g these as fiducial c o r r e c t i o n s i n d e p e n d e n t of rod length. T h i s h a s yet to be d o n e .
T h e s y s t e m a t i c and important e f f e c t s of rod flexure are discussed after the r e s u l t s of a p p l i c a t i o n of m o d e l ( 1 ) . It w i l l be seen that these e f f e c t s depend i n v e r s e l y u p o n rod length. T h o u g h p e r h a p s of a c a d e m i c concern at p r e s e n t , an array of e q u a l l y long rods (as in an a s t e r i s k
f o r m a t i o n , coupled at their crossing) w o u l d p e r m i t the fiducial c o r r e c t i o n s and flexure c o n t r i b u t i o n s to be incorporated in the a p p a r e n t d i l a t i o n s w i t h o u t reducing the number of d e g r e e s of freedom in fitting the m o d e l .
STRAIN E S T I M A T E S
T h e e s t i m a t e d apparent d i l a t i o n s are shown in F i g u r e 3. The r a n g e , if d u e to t e m p e r a t u r e e f f e c t s alone, c o r r e s p o n d s to a t e m p e r a t u r e v a r i a t i o n range of some 2°C, w e l l w i t h i n that indicated by the
e l e c t r o n i c a l l y logged t e m p e r a t u r e t r a n s d u c e r s .
T h e r e s u l t s for and y^ c o r r e s p o n d to a m a x i m u m engineering shear rate of
(6.0 ± 1.8) x 1 0 "6/ y r right l a t e r a l l y at an a z i m u t h of 076 ± 0 1 0 ° . T h i s can be resolved into a r i g h t lateral shear rate of (5.0 ± 1.8) x 1 0_ 6/ y r p a r a l l e l to the s t r i k e , 060°, of the p r e d o m i n a n t l y t r a n s c u r r e n t W e l l i n g t o n fault at the locality of the strain m e t e r
[Lensen 1 9 5 8 ] , together with o r t h o g o n a l e x t e n s i o n - c o n t r a c t i o n components w h o s e m a g n i t u d e and i n t e r p r e t a t i o n cannot be p r o p e r l y d i s c u s s e d w i t h o u t k n o w l e d g e of the real d i l a t a t i o n s A . (If the 8 d u b i o u s m e a s u r e m e n t s are nSt rejected, the r e s u l t s are s i m i l a r , but the shear r a t e is s o m e w h a t g r e a t e r ) .
Gauge Measurements
2 2 0 0 2 0 0 0 I- 1 8 0 0
2 0 0 0 1 8 0 0 1 6 0 0
1 8 0 0 1 6 0 0 1 4 0 0 6 0 0 4 0 0 2 0 0
SM-NM
V
SM-SA
NM-SA
NM-NA
\
0 - 2 0 0 - 4 0 0
1982 1984
Time ( y r s )
1 9 8 6
FIGURE 2 : M E C H A N I C A L STRAIN GAUGE M E A S U R E M E N T S OF T H E GAP B E T W E E N T H E E N D O F E A C H R O D A N D THE C O R R E S P O N D I N G M O N U M E N T .
Estimated Apparent Dilatations
1982 1983 1984
Time (yrs)
1985 1986
F I G U R E 3 : T H E ESTIMATED APPARENT DILATATIONS, AS D E F I N E D IN T H E T E X T .
108
A r e p r e s e n t a t i o n of the m e a s u r e m e n t s in terms of the m o d e l is shown in F i g u r e 4; for each rod the c o n t r i b u t i o n s to the g a u g e m e a s u r e m e n t of the e s t i m a t e d a p p a r e n t d i l a t a t i o n , d , and ideal initial m e a s u r e - m e n t , g ° , h a v e b e e n subtracted, leaving shear strain and r a n d o m error c o n t r i b u t i o n s . T h e p l o t t e d s t r a i g h t lines show the trend e x p e c t e d for the e s t i m a t e d shear s t r a i n a l o n e . T h e s e lines are d e t e r m i n e d by the two c o m p o n e n t s of shear strain and the rod o r i e n t a t i o n ; they are not i n d e p e n d e n t fits to the p l o t t e d p o i n t s . A p a r t from showing the m e a s u r e m e n t s w i t h a p p a r e n t d i l a t a t i o n s removed, t h e s e p l o t s show the scatter of m e a s u r e m e n t s about the m o d e l e x p e c t a t i o n . T h e e r r o r bars show the e s t i m a t e of the s t a n d a r d d e v i a t i o n of the q u a n t i t i e s e , 35 urn, w h i c h , as e x p e c t e d , is g r e a t e r tfian, b u t c o m p a r a b l e to, the e x p e r i e n c e d r e p e a t - a b a i l i t y of m e a s u r e m e n t s . It does appear that m o n t h l y m e a s u r e m e n t s are d e s i r a b l e . A p o s s i b l e i n t e r p r e t a t i o n of d e t a i l s of t h e s e p l o t s in t e r m s of rod flexure w i l l b e treated in the f o l l o w i n g s e c t i o n . R O D F L E X U R E
T h e e f f e c t s of rod f l e x u r e are i m p o r t a n t and d e s e r v e some d e t a i l e d d i s c u s s i o n . O t h e r s y s t e m a t i c e f f e c t s , as w e h a v e seen, can be i n c o r p o r a t e d in the m o d e l m u c h m o r e e a s i l y .
T h o u g h each rod lies inside a g a l v a n i s e d iron p i p e , of d i a m e t e r 65 mm, they can and h a v e b e e n observed to sag.
W i t h i n a few d a y s of i n s t a l l a t i o n of the s t r a i n m e t e r , in O c t o b e r 1 9 8 1 , it w a s noted that the sag of two p i p e s , NM-NA and SM-NA, w a s so g r e a t that sight through them w a s no longer p o s s i b l e , and that some lesser sagging of the o t h e r p i p e s had occurred.
T h i s w a s p r e s u m a b l y d u e to i n s u f f i c i e n t c o m p a c t i o n of some u n d e r l y i n g b a c k f i l l m a t e r i a l , a c i r c u m s t a n c e w h i c h arose from u n a v o i d a b l e c o n s t r u c t i o n site a c t i v i t i e s . A n y sag of a rod w i l l result in an increase of the m e a s u r e d g a p at its end. If a rod of l e n g t h L sags into a c i r c u l a r a r c , w i t h its m i d - p o i n t d e f l e c t e d a d i s t a n c e a d o w n - w a r d s from its p o s i t i o n w h e n s t r a i g h t , then the gap w i l l l e n g t h e n by an a m o u n t 6g g i v e n a p p r o x i m a t e l y b y
6g s 8 a2/ ( 3 L ) (3)
The n u m e r i c a l f a c t o r d e p e n d s on the shape of the sag, but to a first and sufficient a p p r o x i m a t i o n , the d i m e n s i o n a l factor m u s t be a /L for any s h a p e . The i n v e r s e d e p e n d e n c e on rod length and d i r e c t d e p e n d e n c e on the s q u a r e of the sag d i s t a n c e should be noted. Some gap m e a s u r e m e n t s m a d e in o n l y one m a n h o l e over the first few d a y s after i n s t a l l a t i o n and not i n c l u d e d in the s t r a i n a n a l y s i s , w e r e q u i t e c o n s i s t e n t w i t h the extent of sag o b s e r v e d at that t i m e . T h e s e initial m e a s u r e m e n t s also showed a rapid d e c r e a s e in the r a t e of c h a n g e of gap l e n g t h s , and w i t h o u t f u r t h e r d i s t u r b a n c e it m a y be supposed that at p r e s e n t any remaining settlement should b e n e g l i g i b l e . U n f o r t u n a t e l y , c o n t i n u e d c o n s t r u c t i o n o p e r a t i o n s at the T e M a r u a site and v a r i a t i o n s of g r o u n d w a t e r level may c o n t i n u e to c a u s e v e r t i c a l flexure of the
rods, and it is not c e r t a i n that this w o u l d always be downward. T h e r e is also no r e a s o n to suppose that all rods will suffer the same f l e x u r e , though the sense m a y be the same. If gap m e a s u r e m e n t s are used to e s t i m a t e rod flexure a l o n e , as they can b e , then it is not p o s s i b l e to e s t i m a t e any h o r i z o n t a l strain c o m p o n e n t s nor fault creep, and the p u r p o s e of the e x p e r i m e n t w o u l d b e frustrated.
T h e o n l y r e a s o n a b l e assumption is that the two d i a g o n a l l y - c r o s s i n g rods suffer similar flexures since they cross near their m i d - p o i n t s and the pipes w h i c h c a r r y them almost c e r t a i n l y r e m a i n in contact at their crossing as installed. T h e crucial p o i n t is that the gaps a s s o c i a t e d w i t h these d i a g o n a l rods are most sensitive to the e x p e c t e d shear d e f o r m a t i o n , yet, since they h a v e d i f f e r e n t l e n g t h s , their gaps will c h a n g e d i f f e r e n t l y from a given amount of m i d p o i n t d e f l e c t i o n , and their
o r i e n t a t i o n s are favourably disposed for this d i f f e r e n c e to be m i s t a k e n for a c o n s e q u e n c e of t r a n s c u r r e n t creep on the fault.
In d e t a i l , if the two d i a g o n a l rods w e r e each at 45° to the fault t r a c e , then the e s t i m a t e d e n g i n e e r i n g shear strain, y, right l a t e r a l l y p a r a l l e l to the t r a c e , w o u l d be the d i f f e r e n c e of the ratios of gap change to rod l e n g t h .
s 0 . 0 1 7 a2 x 1 0 "6 (a in mm) (4) T h i s s p u r i o u s shear increases rapidly w i t h sag a, and the total estimated shear, as d e r i v e d in the p r e c e d i n g section, w o u l d result from about 3 0 mm sag of i n i t i a l l y s t r a i g h t r o d s , or any 7 mm a d d i t i o n a l sag of rods a l r e a d y sagging 65 mm.
M o r e o v e r , any r e d u c t i o n in the sag, d u e for example to g r o u n d surface u n l o a d i n g or g r o u n d w a t e r c h a n g e s , w o u l d appear as l e f t - l a t e r a l shear or so-called "fault r e v e r s a l " if i n c o r r e c t l y i n t e r p r e t e d . E v e n if rod flexure d o e s not cause the o v e r a l l t r e n d s in F i g u r e 4, it m a y c a u s e the d e t a i l e d v a r i a t i o n s .
T h e p i p e s w e r e again inspected for v i s i b l e sag in S e p t e m b e r 1985. Two m o r e , N M - S A and SM-SA, are now sagging s u f f i c i e n t - ly that sight through them is no longer p o s s i b l e , leaving only o n e , NM-SM, w i t h a sag less than its d i a m e t e r of 65 mm. T h e a r r a y has t h e r e f o r e b e c o m e h i g h l y
s e n s i t i v e to rod f l e x u r e .
It is not k n o w n w h e n and how rapidly this sag occurred; any of the three d i l a t a t i o n a l m a x i m a shown in F i g u r e 3 may c o r r e s p o n d to some sagging e p i s o d e of s u f f i c i e n t m a g n i t u d e . O n l y the first, h o w e v e r , h a s e f f e c t s on the d i a g o n a l l y crossing rods c o n s i s t e n t with their sagging e q u a l l y , some 20 mm. T h e second and third m a x i m a are q u i t e u n l i k e l y to be e n t i r e l y d u e to sag since the longer of the two d i a g o n a l r o d s , NM-SA, is a f f e c t e d to a m u c h g r e a t e r extent than the s h o r t e r ,
Shear Contributions : N o n - d i a g o n a l Rods
& -100
1964
Time (yrs)
Shear Contributions : Diagonal Rods
&-100
Time (yrs)
F I G U R E 4 : THE C O N T R I B U T I O N S TO M E A S U R E D G A P S M O D E L L E D A S B E I N G D U E TO SHEAR (POINTS WITH E R R O R B A R S ) , A N D THE M O D E L E X P E C T A T I O N ( L I N E S ) . T H E S E A R E N O T R E G R E S S I O N L I N E S OF THE P L O T T E D P O I N T S (SEE T E X T ) .
110
SM-NA, (the former lies under the latter at their c r o s s i n g ) .
D e s p i t e these c o n s i d e r a t i o n s , there is no secular u p w a r d trend in the plots of gauge m e a s u r e m e n t s in F i g u r e 2, indicating that the initial sagging has ceased.
E p i s o d i c sagging and r e c o v e r y m a y still occur from time to t i m e . The strong d e p e n d e n c e of gap length on f l e x u r e d o e s , h o w e v e r , m e a n that some part of the
e s t i m a t e d shear w o u l d then be s p u r i o u s . It is of the u t m o s t i m p o r t a n c e that some m e a n s of m e a s u r i n g the sag w i t h , for e x a m p l e , suitably a d a p t e d i n c l i n o m e t e r s , be
i m p l e m e n t e d if at a l l p o s s i b l e . It w o u l d be of g r e a t a d v a n t a g e to p r e v e n t sagging in any future s i m i l a r i n s t a l l a t i o n . C O N C L U S I O N S
R E F E R E N C E S
[1] B e r r y m a n , K.R. in p r e p : L a t e Q u a t e r n a r y T e c t o n i c Map of U p p e r Hutt, 1:25 0 0 0 . DSIR, W e l l i n g t o n , N e w Zealand.
[2] B r o w n , I.R. and Wood, P.R. 1 9 8 3 : S t r a i n M e a s u r e m e n t s A c r o s s the W e l l i n g t o n F a u l t at T e M a r u a . P r o c . T h i r d South P a c i f i c R e g i o n a l C o n f . on E a r t h q u a k e E n g i n e e r i n g , W e l l i n g t o n , N e w Zealand. T h e N . Z . N a t i o n a l Society for E a r t h q u a k e E n g i n e e r i n g , V o l . 3, p p . 5 0 9 - 5 1 8 . [3] L e n s e n , G.J. 1 9 5 8 : The W e l l i n g t o n
F a u l t from Cook Strain to M a n a w a t u G o r g e . N . Z . Journal of G e o l o g y and G e o p h y s i c s , V o l . 1, p p . 1 7 8 - 1 9 6 .
A shear m o d e l w h i c h ignores rod flexure i n d i c a t e s t h e m e c h a n i c a l s t r a i n g a u g e m e a s u r e m e n t s are c o n s i s t e n t w i t h right l a t e r a l s h e a r d e f o r m a t i o n on the W e l l i n g t o n fault at an a v e r a g e rate of
(5 ± 2) x 1 0 ~ 6 / y r (engineering units) since 1 9 8 2 . T h i s w o u l d c o r r e s p o n d to (25 ± 10) um/yr d i s p l a c e m e n t a c r o s s the array.
C o n s i d e r a t i o n of t h e e f f e c t s of rod sag shows some s i g n i f i c a n t part of the e s t i m a t e d d e f o r m a t i o n may b e s p u r i o u s . It must
t h e r e f o r e be c o n s i d e r e d as an upper limit to e i t h e r fault c r e e p o r d i s t r i b u t e d d e f o r m a t i o n . T h i s limit is two o r d e r s of m a g n i t u d e less than the g e o l o g i c a l l y d e t e r m i n e d a v e r a g e slip r a t e . If shear d e f o r m a t i o n is o c c u r r i n g , and is
d i s t r i b u t e d b r o a d l y a b o u t the fault trace, it should b e m e a s u r a b l e by p r e c i s e
s u r v e y i n g m e t h o d s . N Z G S does already h a v e a n e t w o r k of s u r v e y i n g m o n u m e n t s installed about the fault t r a c e at T o t a r a Park, a d i s t a n c e of some 5 km from the Te M a r u a site. T h i s type of i n d e p e n d e n t v e r i f i c a t - ion is n e c e s s a r y , e v e n in the a b s e n c e of the sagging p r o b l e m , since the s t r a i n m e t e r e x p e r i m e n t d e p e n d s u p o n the s t a b i l i t y of the four m o n u m e n t s . Some improvement in site s t a b i l i t y is to be e x p e c t e d after c o n s t r u c t i o n a c t i v i t i e s c e a s e .
A C K N O W L E D G E M E N T
S o m e of t h e ideas c o n t a i n e d h e r e h a v e a r i s e n out of s t i m u l a t i n g d i s c u s s i o n s w i t h P e t e r Wood, o n e of the o r i g i n a t o r s of the e x p e r i m e n t .
T h e array i n s t a l l a t i o n and m e a s u r e m e n t s h a v e b e e n p o s s i b l e only d u e to continued c o o p e r a t i o n and f i n a n c i a l support of the W e l l i n g t o n R e g i o n a l C o u n c i l .
