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SEISMIC MICROZONING IN NEW ZEALAND
W. R. Stephenson*
1. Introduction
" S e i s m i c Microzoning" m e a n s m a n y d i f f e r e n t t h i n g s to d i f f e r e n t p e o p l e . T h e r e is a l w a y s i n c l u d e d t h e e l e m e n t of d i f f e r e n t d a m a g e in n e a r b y a r e a s , b u t h o w t h e d i f f e r e n c e s arise, h o w we s h o u l d s t u d y them, a n d h o w we s h o u l d a p p l y the r e s u l t s of o u r studies, a r e s t i l l u n c e r t a i n .
T o s o m e p e o p l e , m i c r o z o n i n g r e f e r s t o s t r u c t u r a l d a m a g e d u e to g r o u n d f a i l u r e ;
faulting, slumping and 1iquefact ion a l1 belong in t h i s category. To others, micro zoning is the effects of the focussing of seismic wave s by boundaries, r e s u l t i n g in modified ground
damage and b u i l d i n g d a m a g e .A third very popular view of microzoning holds that it concerns multiple reflection of s e i s m i c wave s in layers, with interference of the wave trains giving rise to maxima, where
g r o u n d and s t r u c t u r a l damage w i l l beaccentuated.
Microzoning can be defined as the division of land areas into small regions of differing
l o c a l g e o l o g y for which d i f f e r e n c e s in earth-quake attack on structure s are specified.
Thi s paper i s an a t t e m p t to set down aspe c t s of microzoning in a 1ogical manner, and t o relate therm It also discusses activities here and overseas, and con sider s where microzoning and microzoning research in New Zeal and should he ad.
2. Ground Damage, Relative Displacement, and Inertia
Seismic microzoning is the division of
1and areas into small regions, down todimensions of a few hundred feet, for the purpo se of de scribing differences in earth- quake attack on structures. Three main mechanisms be come immediately obvious in this cont e xt
cThe first me chan i sm of attack is ground failure
cThi s take s in land siide s, liquefaction, fault movement and slumping.
Secondly there is the relative di splacement attack, whereby the ground at different parts of an extended structure can move differently, cau sing s tructural failure, Thi s relative displacement can be elastic, so that after the earthquake the ground is back as it was before. Pipe 1ine s crossing geological boundarie s are target s for thi s type of attack*
* Engineering Sei smology Section, Phy si c s and Engineering Laboratory, Department of Scientific and Indu s trial Re search.
The third mechanism giving rise to micro- zoning effects is variat ion of inertia at tack
0The inertia attack is caused by forces which are gene rated by ground accelerati on acting on structural mass
0- Variations in ground
acceleration give ri se to variations in the inertia attack.
3. Microzoning and Microzoning Research
Microzone s are being assigned in New Zealand on the basis of exist ing informat ion.
Re srach at present under way should lead to a more accurate def init ion of these microzone s«
The specification of micro zone s on the basis of the three common mechani sms of attack is at present crude. Only a slow advance can be expected in microzoning for ground failure attack. However, rapid progre s s can be
expected in microzoning for inertia attack.
The variat ion of the inertia at t ack is po ss ibly the least inve st igated mechani sm, the one 1 ikely to show the mo st dramat ic damage effects, and the one most amenable to an approach along the lines of conventional physical and engineering theory.
Structural attack due to ground damage should only occur in severe earthquakes, but inertia attack amplifi cat ion will occur at all ampli tude s, including those of moderate earth- quake s. Thus inertia attack amplification will be a factor in many earthquakes•
The New Zealand D.S.I-R. Report "Micro- zoning for earthquake effects in the Pauata- hanui area" repre sent s the be st that can be done on the basi s of existing knowledge. It is likely to be of limited value to engineers
04. Importance of Instrumental Studies
To be of any value, studies of the micro- zone inertia attack must relate to building damage, taking building character into account
0Intensive studies of building damage after an earthquake yield limited information on inertia attack and the re suit s obtained are usually capable of various interpretations.
The se objections arise from the complex dynamic characteri sties of buildings. No damage to a building of 1 second period does not mean that there wil 1 be no damage to a -J- se cond one, and even if they have the same natural period, a shear wall structure and a frame structure can perform quite differently under earthquake loads. By the time one admits asymmetric buildings, or allows structural changes above a certain height, the situation becomes com- plex.
Any inertia attack study must start with
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At chis p o i n t it s h o u l d be n o t e d t h a t i n e r t i a g e n e r a t e d g r o u n d f o r c e s c o n t r o l t h e
r e l a t i v e d i s p l a c e m e n t a n d g r o u n d f a i l u r e e f f e c t s , a n d t h a t t h e s e w i l l c o n s e q u e n t l y show an
i n e r t i a m i c r o z o n e d e p e n d e n c ec T h u s the i n e r t i a a t t a c k m e c h a n i s m p l a y s a c e n t r a l r o l e in
s e i s m i c m i c r o z o n i n g *
I n e r t i a a t t a c k m e c h a n i s m s c a n be s p l i t I n t o w e l l d e f i n e d g r o u p s . T h e m a j o r d i v i s i o n i s i n t o w a v e l e n g t h d e p e n d e n t a n d w a v e l e n g t h I n d e p e n d e n t p r o c e s s e s „ T h e f o r m e r a r e c o m - p r i s e d of i n t e r f e r e n c e , m u l t i p l e r e f l e c t i o n a n d r e s o n a n c e p h e n o m e n a , w h i l e the l a t t e r a r e c o m p r i s e d of e n e r g y c o n s e r v a t i o n and f o c u s s i n g effectSo U n d e r s u i t a b l e c i r c u m s t a n c e s a n y c o m b i n a t i o n of t h e s e p r o c e s s e s m a y be i m p o r t a n t ,
5 • 1 I n t e r f e r e n c e
H i g h i n t e n s i t y v i b r a t i o n s m a y b e c a u s e d by s e v e r a l w a v et r a i n s f o r t u i t o u s l y a d d i n g at s o m e p o i n t in s p a c e to g i v e a l a r g e r e s u l t a n t . T h e w a v e t r a i n s a l l come f r o m the p r i m e e n e r g y s o u r c e , b u t b y d i f f e r e n t r o u t e s0 A c h a n g e in t h e w a v e f o r m s e v e n t h o u g h t h e y s t i l l p r o p a - gate b y the same p a t h s , c a n c h a n g e t h e i n t e r - f e r e n c e p a t t e r n * T h i s I n t e r f e r e n c e c a n h a p p e n f o r the i n c o m i n g w a v e s w h i l e they are s t i l l t r a v e l l i n g In r o c k , a n d by I t s n a t u r e c a n p o t e n t i a l l y s w a m p t h e l o c a l g e o l o g y e f f e c t s in a r a n d o m f a s h i o n * T h e p r o c e s s Is the o n l y a r g u m e n t a g a i n s t the " r o c k r e f e r e n c e " c o n c e p t ( s e e s e c t i o n 6,k), a n d i s d e s e r v i n g of i n v e s t ™ i g a t i o n . T h e H u t t V a l l e y s t r o n g m o t i o n n e t - w o r k i n c l u d e s t h r e e r o c k b a s e d r e c o r d e r s a s a f i r s t s : e : c r i s a i _ e c t i o n ,
5« 2 r u ~i ^-a _r._e '_iac_,:yi
T m s is £ 3D S :I. _ c i n t e r f e r e n c e p r o c e s s , w h e r e "he saps. _ e p a t h s a r i s e f r o m m u l t i p l e r e f l e c t i o n s f r o m t h e i n t e r f a c e s b e t w e e n h o r i z o n t a l s l a b s o f l o c a l g e o l o g y . It w a s t h e f i r s t i n e r t i a a t t a c k p r o c e s s p o s t u l a t e d . 5 o3 R e s o n a n c e
In a sense r e s o n a n c e a n d m u l t i p l e r e f l e c t i o n g o h a n d in h a n d , b e c a u s e a n I n f i n i t e h o r i z o n t a l
s l a b b e h a v e s the s a m e , w h e t h e r t r e a t e d a s a m e c h a n i c a l r e s o n a t o r o r m u l t i p l y r e f l e c t i n g
s y s t e m . It i s e v i d e n t t h a t a g u l l y f i l l e d w i t h a l l u v i u m w i l l r e s o n a t e , a n d that a n e z c x t a t i o n at a m o d a l f r e q u e n c y w i l l g i v e a r e s o n a n t r i s e o f a m p l i t u d e * H o w e v e r , a m u l t i p l e r e f l e c t i o n t r e a t m e n t o f t h e s a m e p r o b l e m w o u l d n o t
n e c e s s a r i l y g i v e t h e s a m e a n s w e r s . T h i s is b e c a u s e b o u n d a r y v a r i a t i o n s of the o r d e r of a w a v e l e n g t h in s c a l e c a n d i f f r a c t w a v e s a s w e l l a s r e f l e c t a n d r e f r a c t t h e m. M u l t i p l e
r e f l e c t i o n t e c h n i q u e s a r e o n l y a p p r o x i m a t e s o l u t i o n s of t h e w a v e e q u a t i o n , w h i l e r e s o n a n t b e h a v i o u r c a n be p r e d i c t e d f r o m e x a c t s o l u t i o n s of t h e w a v e e q u a t i o n ,
5 * U Energy__Con^6r^a_tijO_n
If a s e i s m i c w a v e p r o g r e s s e s from a m e d i u m
of g r e a t e r a c o u s t i c i m p e d a n c e to a m e d i u m of l e s s e r a c o u s t i c i m p e d a n c e in a r e f l e c t i o n l e s s m a n n er , c o n s e r v a t i o n of e n e r g y d e m a n d s a g r e a t er a m p l i t u d e on the s o f t e r roed 1 u m , by a f a c t o r of t h e r a t i o of a c o u s t i c i m p e d a n c e s 0 in the case of a n a b r u p t t r a n s i t i o n a s p e c i f i c a m o u n t o f r e f l e c t i o n o c c u r s a n d t h e r a t i o is of the s q u a r e r o o t s of the a c o u s t i c i m p e d a n c e s „ R e a l i t y u s u a l l y l i e s s o m e w h e r e b e t w e e n t h e s e two c a s e s , b u t in any e v e n t it is a r e a s o n f o r e x p e c t i n g g r e a t e r s t r u c t u r a l d a m a g e on s o f t e r g r o u n d .
Due to the l a w s of r e f r a c t i o n ^ on t r a v e r s i n g a b o u n d a r y f r o m a h i g h v e l o c i t y t o a l o w
v e l o c i t y m e d i u m , a s e i s m i c r a y w i l l b e b e n t t o w a r d s t h e n o r m a l of the i n t e r f a c e . T h u s a c u r v e d I n t e r f a c e c a n f o c u s or d e f o c u s s e i s m i c w a v e s, F o r i n s t a n c e , a s e m i c y l i n d r i c a l v a l l e y f u l l of s o i l , w i t h i t s r e t a n g u l a r f a c e in the g r o u n d s u r f a c e p l a n e c a n p o t e n t i a l l y c a u s e a n a b n o r m a l l y h i g h r e s p o n s e a l o n g i t s a x i s , i n d e p e n d e n t of w a v e l e n g t h . F o c u s s i n g is t h e s a m e s o r t of m e c h a n i s m a s t h a t g i v i n g r i s e to a c a u s t i c c u r v e in a t e a c u p ( i n d e - p e n d e n t of c o l o u r ) a s it s t a n d s in s u n l i g h t ,
6, 1 La 7. U n i q u e s of Studying Modified Inertia Attack
Of trie t e c h n i q u e s e v o l v e d to p r e d i c t v a r i a t i o n s in the I n e r t i a a t t a c k , s o m e a r e s p e c i f i c w i t h r e s p e c t to m e c h a n i s m s , w h i l e o t h e r s a r e g e n e r a l in n a t u r e • S p e c i f i c m e t h o d s h a v e b e e n e v o l v e d for m u l t i p l e r e f l e c t i o n , w h e r e a p o s t u l a t e d i n p u t Is p r o g r e s s i v e l y d e l a y e d , a t t e n u a t e d , a n d a d d e d to i t s e l f ; a n d f o r f o c u s s i n g , w h e r e r a y t r a c i n g t e c h n i q u e s a r e performed,* F o r m u l t i p l e r e f l e c t i o n , the w a v e f o r m a n d d i r e c t i o n of p r o p a g a t i o n a r e v a r i a b l e s , a n d f o r f o c u s s i n g the w a v e f r o n t s a r e v a r i a b l e ,
6« 1 Gejiejral M e t h o d s
G e n e r a l m e t h o d s s t a r t e d w i t h t h e J a p a n e s e , w h o m e r e l y s p e c i f i e d that f l e x i b l e s t r u c t u r e s s h o u l d be b u i l t o n s t i f f g r o u n d , a n d s t i f f s t r u c t u r e s on f l e x i b l e g r o u n d • T h i s a p p r o a c h c e r t a i n l y a v o i d s d o u b l e r e s o n a n c e s , b u t d o e s n o t a l l o w m a n y s u b d i v i s i o n s o f g r o u n d t y p e s•
6 . 2 Mjncrjpj^em or t e c h n i q u e s
S t u d i e s b a s e d o n m i c r o t r e m o r o b s e r v a t i o n s h a v e b e e n a c t i v e l y c a r r i e d o u t b y t h e J a p a n - e s e . T h e s e m e t h o d s r e l y on m e a s u r i n g t h e f r e q u e n c y of the d o m i n a n t g r o u n d v i b r a t i o n * T h i s is a s s u m e d to b e the f r e q u e n c y w h i c h w i l l be a m p l i f i e d d u r i n g a s e v e r e e a r t h q u a k e , a n d b u i l d i n g d e s i g n i s b a s e d a c c o r d i n g l y • T h e o n l y w o r k o f a c o r r o b o r a t i v e n a t u r e w h i c h h a s b e e n d o n e , c o n s i s t s of c r u d e g r o u n d p r o p e r t y m e a s u r e m e n t s a p p l i e d to a m u l t i p l e r e f l e c t i o n m o d e l * V a r y i n g d e g r e e s of s u c c e s s h a v e b e e n o b t a i n e d ,
T h e m e t h o d s h o u l d b e v i e w e d w i t h some s c e p t i c i s m b o t h b e c a u s e t h e m e c h a n i s m b y w h i c h m i c r o t r e m o r s a c q u i r e t h e i r d o m i n a n t f r e q u e n c y i s n o t k n o w n , a n d on a c c o u n t o f t h e l a c k o f c o r r o b o r a t i v e e v i d e n c e of a f i r m n a t u r e , M i c r o t r e m o r s are a t l e a s t p a r t i a l l y i n d u c e d b y t r a f f i c a n d w e a t h e r , a n d w h o c a n say t h a t s e i s m i c a l l y u n i m p o r t a n t c h a r a c t e r i s t i c s w i l l n o t d o m i n a t e t h e m i c r o t r e m o r p r o c e s s ? g r o u n d m o t i o n s , a n d d e d u c e s t r u c t u r a l r e s p o n s e ,
t h e n c e d a m a g e . It s h o u l d c o n c e n t r a t e on f i n d i n g w a y s o f c h a r a c t e r i s i n g t h e r e s p o n s e of a n y
p a r t i c u l a r s i t e .
5. The Mechanisms Involved In Modified Inertia Attack
6 . 3 Brute Force Analysis
Some schools of thought favour a finite element approach for analysis of inertia attack microzone effect s. Of course this is an approach which cannot be logically faulted.
One merely obtains modulus and density data on a suitable three dimensional grid for the locale in question (tne moduli being functions of strain), and using a large computer, follows the surface consequences of some bedrock
mot ionc The practical and financial problems encountered in doing this may be large.
6 .k Smal1 Earthquake s
There appears to be much merit in observing smal1 earthquakes, in order to predic t feature s of future larger ones » (If the modificat ion was dependent on the direct ion of p r o p a g a t i o n , a representative selection of small earthquakes from different directions would have to be u s e d ) .
Smal1 earthquakes w i1 1 encounter the same geological configurations in the same fashion as large ones, and will be modified by almo st the same proce s s. The only diff erence s will be due t o material propertie s be ing functions of amplitude. Thu s depo sit s w i1 1 tend to ha v e lower re sonant frequencie s in large earthquake s due to a decrease in the shear modulus of soil at large amplitudes 0
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An uncertainty w h i c h must be cleared up be fore smal1 earthquake s can be used conf id- en 1 1 y , and indeed be fore sense is made of other m e t h o d s , i s the p r e c i s e nature of "rock r e f e r e n c e1 8® T h i s t e r m is used to stress the f a c t that it i s t h e m o t i o n of the bedrock surface which d r i v e s the soil, Before soil s u r f a c e motion can b e deduced from any model, we must know the variat ion of the "drive"
across the bedrock0 The drive i s certainly u n i f o r m o v e r a small e n o u g h d i s t a n c e , but no f i e l d work o n t h i s aspect is reported in the literature. Before mechanisms can be deduced from any scaling method such as the use of small earthquakes, t h e c h a r a c t e r of "rock reference" must b e known•
In this u n t o u c h e d f i e l d of small earth- quake microzoning there is the question of a n a l y s i s . One c o u l d contemplate measuring the r a t i o of peak a c c e l e r a t i o n s o f records, or better, the ratio of power spectra, the latter presumably showing up re sonant phenomena. Our group w i s h e s to obtain a model which can predict both horizontal com- ponent s of ground motion in detail, given both horizontal component s of bedrock motion.
we f e e l that a person able to do this under- stands the processes, and is then in a position to generalise and to simplify. A m p l i t u d e and spectrum methods applied to one component d o not l e a d to detailed predictions, apparently because one bedrock component can affect the other component on the surface.
7. Intensity Effects
The magnitude of the ground strain is an important parameter w h i c h enters microzone inertia attack studie s . Hysteretic character- istic s in the stress-strain relations of the ground can influence the amount of energy which 1s used in ground deformation and i s consequently not available for building damage.
Inert ia attack then, will dominate micro- zoning thought s for the smaller ampli tude
damaging earthquakes. Because smal1 amplitudes are more common, modification of the inertia attack is a very important part of microzoningc
8. Scale of Microzoning
Nothing has been said so far about where microzoning ends and detailed site investiga- tion begin s , Thi s is a controversial topic, but in view of the physical fact s, guideline s can be set easily•
Where the problem is particular to a structure, it is one of detailed site invest- igation. ' On the other hand anything concerned with the ground alone is microzoninga
In the case of large buildings, two approache s are evident. One may regard the total re sponse of the system from bed rock u ps
or one may predict the surface mot ion independ- ently and apply it to the very local building - plus - foundation system•
The first me thod is analytically correct but may involve very unwieldy computations and be rendered very inaccurate by difficulties in assessing ground p r o p e r t i e s ,
The second method is an approximation, but a readily applied one, and in ail likeli- hood wilI give truer re suit s, The surface layers have a great enough mas s that the build- ing w i1 1 be driven by the surface ground motion. Foundations are easily analysed now, and the experimental smal1-earthquake me thod should give accurate surface motion predicts ions, given an assumed mot ion of the bedrocks
If the second me thod is adopted, de tai1ed site inve stigation will extend t o g e t t i n g enough data to solve foundation p r o b l e m s , a n d Non-linear characteristics alter the soil modulus and thence the natural frequency of a
soil resonator. Both hysteresis and non-line- arity become important at large ampli tude s 0
There are two aspects of this intensity question. One concerns changes in soil parame ter s as the intensity is increased, and the other concerns the lower and upper limits of intensity which must be considered.
If smal1 earthquakes are used as a tool in s tudie s of the inertia at tack, they may be expected to reflect in a qualitative way, all important characteristies. That is, if a piece of ground behaves 1 ike a re sonator of
1 second natural period and 10% of critical damping, in a strong earthquake, the smal1 earthquake w i1 1 correctly indicate a re sonator.
The frequency and damping indicated could be in error, and the appropriate correction would have to be applied as a matter of experience.
On the other hand, the range of strain in large ear thquakes is likely to be re stricted.
Below the structural damage thre shold we are not int ere sted. At high levels, where the ground absorbs energy in failure and the inertia a t tack zonal contrast falls, such effect s a s subsidence and 1iquef action may dominate, and once again we lose interest in the inertia attack0 This is the second aspect of intensity referred to before.
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mi c r o z o n i n g w i l l c o n c e r n the r e l a t i o n b e t w e e n b a s e m e n t m o t i o n and s u r f a c e m o t i on 0
9, The Importance of Shear
In m o s t r e c o r d s of s t r o n g e a r t h q u a k e a c c e l e r a t i o n5 the maximum a c c e l e r a t i o n is a t t a i n e d d u r i n g t h e s p h a s e . F u r t h e r m o r e, in m o s t r e c o r d s , t h e s p h a s e i s a s s o c i a t e d w i t h p r e d o m i n a n t l y h o r i z o n t a l g r o u n d m o t i o n * A g a i n , t h e inertia, a t t a c k on b u i l d i n g s is m o s t
i m p o r t a n t in the h o r i z o n t a l d i r e c t i o n } b e c a u s e v e r t i c a l a c c e l e r a t i o n a m o u n t s to an i n c r e a s e In t h e g r a v i t a t i o n a l s t a t i c l o a d on the b u i l d i n g { o f m a y b e 30%) w h i l e the h o r i z o n t a l I n e r t i a l o a d i n g d u r i n g a n e a r t h q u a k e is far g r e a t e r t h a n the u s u a l h o r i z o n t a l l o a d s d u e to w i n d .
T h u s b e c a u s e t h e y c a r r y the m o s t e n e r g y , a n d u s e It the m o s t d e s t r u c t i v e l y , s - w a v e s a r e of p r i m a r y I m p o r t a n c e In the m i c r o z o n i n g p r o b l e m . M i c r o z o n i n g s t u d i e s s h o u l d c o n c e n t r a t e m a i n l y on e f f e c t s d u e to s h e a r w a v e s .
In t h e s o f t e s t s o i l s , w h e r e m ic r o z o n e e f f e c t s m a y be m o s t p r o n o u n c e d , s-wave
v e l o c i t i e s m a y b e a r l i t t l e r e l a t i o n to p - w a v e v e l o c i t i e s . T h i s r u l e s o u t the u s e o f p - w a v e v e l o c i t y d a t a to c a l c u l a t e s-wave v e l o c i t i e s a n d r e s o n a n t f r e q u e n c i e s. L i k e w i s e It c a s t s a d o u b t on the v a l u e o f m i c r o t r e m o r s p e c t r a l p e a k s , b e c a u s e t h e s e c o u l d a r i s e f r o m p - w a v e r e s o n a n c e s .
10. Earthquake Character
G r o u n d m o t i o n in a p a r t i c u l a r e a r t h q u a k e d e p e n d s on the c h a r a c t e r of the e a r t h q u a k e , a s w e 11 a s that of t h e g r o u n d e Thu s the ear th- q u a k e s p e c t r u m on r o c k , a n d the I m p u l s i v e or n o i s e - l i k e c h a r a c t e r of the e a r t h q u a k e m u s t b e c o n s i d e r e d .
10 . 1 R o c k S p e c trum
T h e a t t e n u a t i o n of s h e a r w a v e s in r o c k i n c r e a s e s w i t h t h e i r f r e q u e n c y . H e n c e the p e a k of the s p e c t r u m on r o c k w i l l shift to l o w e r f r e q u e nci e s as a p a r t i c u l a r e a r t h q u a k e Is- re c o r d e d p r o g r e s s i v e l y f urt her f r o m the ep i c e n t r e . In a d d i t i o n , b e c a u s e l a r g e r e a r t h - q u a k e s i n v o l v e l a r g e r s o u r c e d i m e n s i o n s s it is p l a u s i b l e that t h e y h a v e t h e i r s p e c t r a l p e a k s at l o w e r f r e q u e n c i e s *
N e i t h e r of t h e s e ef f ect s h a s b e e n s t r i c t - ly q u a n t i f i e d o w i n g to l a c k of s t r o n g m o t ion d a t a , A n I m p r e s s i o n of the e x i s t e n c e of the s e c o n d e f f e c t c o u l d be d u e to the fact that s t r o n g e a r t h q u a k e a c c e 1 e r a t i o n s t e n d to be re c o r d e d at di s t a n c e , a n d s m a l l o n e s a r e o n l y re c o r d e d c l o s e to ttie s o u r c e .
1 0 . 2 I m p u l s i v a n e s s
It i s p o s s i b l e f o r two e a r t h q u a k e s to h a v e i d e n t i c a l p o w e r s p e c t r a e v e n t h o u g h o n e is i m p u l s i v e a n d the o t h e r m o r e n o i s e - l i k e in c h a r a c t e r o T h e y c o u l d h a v e d i f f e r e n t r e s p o n s e s p e c t r a a n d c a u s e d i f f e r e n t d a m a g e .
One c o n s e q u e n c e of thi s is that in e x a m i n i n g the e f f e c t s of 1 o c a l ge ology on the i n e r t i a a t t a c k , c h o o s i n g t y p i cal r o c k m o t i o n s f r o m a r a n g e of m a g n i t u d e s a n d e p i c e n t r a l d i s t a n c e s is i n s u f f i c i e n t . One m u s t a l s o s e l e c t some " d i f f e r e n t l o o k i n g1 1 ear t h q u a k e s f o r e a c h m a g n i tude a n d d i s t a n c e b e f o r e a
r e p r e s e n t a t i v e p i c t u r e is o b t a i n e d o
One fact Is a b u n d a n t l y c l e a r - that m o r e s t r o n g m o t i o n e a r t h q u a k e r e c o r d s a r e n e e d e d b e f o r e a d e f i n i t i v e p a t t e r n o f r o c k v i b r a t i o n erne r g es 0
1 \. Dhection of Propagation
At f i r s t sight it a p p e a rs t h a t the t r a n s - fer c h a r a c t e r i s t i c s of soil 1 a y e rs wi 11 v a r y g r e a t l y f r o m ear t h q u a k e to e a r t h q u a k e be cau s e e a c h e a r t h q u a k e a p p r o a c h e s by a u n i q u e set of p a t h s , H o w e v e r , c o n s i d e r a b l e s i m p l i f i c a t i o n s m a y be po s s i b l e , s i n c e it m a y be p e r m i s s i b l e to ass u m e that the b a s e m e n t is t r a nsl a t i n g b o d i l y, in w h i c h case the d i r e c t i o n of w a v e p r o p a g a t i o n in the r o c k is I r r e l e v a n t .
C o n s i d e r s u c h a sy s terr- a s chat in F i g . 2 . The bed r o c k e x c i t e s the soil at p o i n t A, a n d we m u s t a s k what b e d r o c k m o t i o n I s impor tant in the r e s p o n s e ^ P o i n t s s u c h as D w i 1 1 h a v e l i t t l e c o n t r i b u t i o n s b e c a u s e the e n e r g y at D w i l l be r e f r a c t e d at the r o c k - s o i l i n t e r f a c e to p r o p a g a t e v e r t i c a l l y0 T h e m a i n c o n t r i b u t i o n w i 1 1 be f r o m the r e g i o n B - Z} of a b o u t a q u a r t e r w a v e l e n g t h in the soil « y?/4> ) • O v e r thi s r e g i o n there w i l l be l i t t l e c h a n g e in the b e d r o c k m o t i o n ( e v e n : o r he ri z o n t a l p r o p a g a t i o n } b e c a u s e , t y p i c a l l y , the w a v e - 1 e n g t h in the r o c k ( Ar ) w i l l be of an o r d e r g r e a t e r t h a n X& . H e n c e w h e n g r o u n d resonance- o c c u r s it Is r e a s o n a b l e to a s s u m e f o r the p u r p o s e of c a l c u l a t i o n that a l l p o i n t s on the r o c k m o v e in p h a s e .
12. M i c r c z s^ n g Overseas
In s e i s m i c c o u n t r i e s w i t h a d v a n c e d e n g i n - e e r i n g i/crks, the i d e a that l o c a l c o n d i t i o n s can a f f e c t e a r t h q u a k e a t t a c k to a d r a m a t i c e x t e n t is a c c e p t e d . T h e e x t e n s i o n of the se i d e a s i n t o c ode s of p r a c t i c e h a s p r o c e e d e d m o r e s l o w l y , b u t e n l i g h t e n e d e n g i n e e r s c o n s i d e r site c o n d i t i o n s and m a y m o d i f y d e s i g n l o a d i n g s a c c o r d i n g l y .
In J a p a n , the c o n c e p t of " d o m i n a n t g r o u n d p e r i o d " Is a c c e p t e d . In a d d i t i o n , f o r m i c r o - z o n i n g p u r p o se s, they t e n d to a s s i g n s o i l s to one of 3 c a t e g o r i e s of g o o d n e s s ; r o c k , d i l u v i u m a n d a l l u v i u m .
T h i n k i n g in the U S S R s u g g e s t s m o d i f i c a t i o n of de sign 1 o a d s a c c o r d i n g to soil s t i f f n e s s a n d d e p t h , a n d to b u i l d i n g p e r i o d , T h i s Is a way of r e c o g n i s i n g re s o n a n c e •
T h e t e n d e n c y in the U . S . A . is to r e j e c t m i c r o z o n i n g as s u c h , but to c a r e f u l l y i n v e s t - i g a t e s u b - s i t e c o n d i t i o n s f o r i n d i v i d u a l b u i l d i n g s . S o p h i s t i c a t e d a n a l y s e s of w h o l e b u i l d i n g - f o u n d a t i o n s y s t e m s h a v e b e e n u n d e r - t a k e n ,
13. Microzoning in New Zealand
T h e p r e s e n t s i t u a t i o n in N e w Z e a l a n d Is that e n g i n e e r s a p p r e c i a t e the d r a m a tic c h a n g e s in e a r t h q u a k e a t t a c k that l o c a l g e o1o g y c a n c a u s e • H o w e v e r , the re Is a m a r k e d r e l u c t a n c e to be b o u n d by c o d e s to take l o c a l g e o l o g y i n t o ac c o u n t . S u c h c ode s w o u l d act a s a c o n - s t r a i n t w h i c h e n g i n e e r s w o u l d l i k e to a v o i d .
At p r e s e n t de sign s a r e c o n t r o l l e d on the b a s i s of i n f o r m a t i o n g a i n e d by the u s u a l site
investigation dril1ings. As thi s paper shows, this information would be complemented by infor- mation from an adequate microzoning m a p
cAt the moment, microzoning in New Zeal and amoun t s to the production of microzone maps of city areas by the D„ S.I. R. The se maps are the be st that can be produced by the close co-oper- ation of experts in the relevant fields.
The maps delineate zones which are progre ss- ively "worse", and as such are adequate for town planningo It is difficult to apply this information to designs but it does provide an overall geological picture of the site
0The maps as they are will give no criteria for modifying design loadings
0The D. S.I
0R.
divi s ions contributing to data for the map s are Geophysics Division, Soil Bureau, and Geological Survey. The Physics and Engineering Laboratory are involved at the stage at which data are co-ordinated are interpreted.
Engineers cannot be blamed for seeing a depre s sing pic ture in the informat ion now available, but may be heartened to know that research proceeds at Auckland university, Canterbury University, and the Physics and Engineering Laboratory
eAll three institutions have attempted to make sense of micro-tremor observations, with little progre s s as yet. It could well be that microtremor spectra may give building periods adequately, but not ground natural periods.
In situ shear wave properties are being studied by Physics and Engineering Laboratory and Canterbury University, the Physics and Engineering Laboratory current work aiming at shear velocity profiles down to bed rock.
Physics and Engineering Laboratory, and in a smaller way Canterbury, are trying to carry out strong motion measurement s on soil, because thi s must be the final arbiter in theories of inertia attack modificationo To date only one such measurement has been made• The soil at Physics and Engineering Laboratory, in a 0.lg earth- quake , has been shown to have a 0.55 second natural periodo Insufficient is known about the local ground condi tions to calculate thi s re sonant period.
Auckland workers have concentrated on laboratory measurement s of dynamic soil characteristics, and this w i
1 1 be a vitalfact or in tho se microzone investigations of interest to engineers.
Theoretical inve s t igat ions of ideal geome trie s at Auckland and Canterbury have given insight into approx imate mechani sm s, in the same way that shaking a plaster mould
containing jelly has at Physics and Engineering Laboratory.
Mention must be made of the Physics and Engineering Laboratory projections of the
appropriate pattern for inertia attack research, It Is hoped that work will proceed in three stages; ground mode 1s valid for smal1 earthquakes, ground models valid for severe earth-quake s, and modificat ion of the code » One critical advance expected is the determin- ation of ground character by direct measurement during frequently occurring small earthquakes•
Thi s give s a low ampli tude model of the ground.
A further critical advance will lie in learning how to modify the 1 ow amplitude m o d e1s by taking into account the hysteresis and non-linearity of soils at the high strains which occur in
destructive earthquake s
0Modifying the code in accordance with the strong motion model should then follow easilyo
There is e nough diverse re search effort be ing made that advances in microzoning seem certain, but it must always be borne in mind that the inertia attack picture is incomplete until strong motion accelerograms are obtained^
and that bo th strong earthquake s and strong motion accelerographs are sufficiently rare that some time must elap se before en ough data is forthcomingo For the Wellington area effective
strong motion data Is expected at intervals of about eight years. However, the smal1 earth- quake techniques which are under development wi 1 1 have u sef ul events about on ce a week,
14. Microzoning for Engineers
The fact that damage in earthquakes can be a dramatic function of local ground conditions is now establi shed beyond doubt, Caracas and Manila being prime examples. It se ems reason- able that knowledge of the cau se of the se phen- omena must be of benefit to humanity, but how do we apply such knowledge ? Town planners could undoubtedly shuffie things so that areas prone to 1iquefaction become recreational, and so that centres of commerce and admini stration, which attract tall buildings, are on the be st available ground. However, town planners may have to weigh aesthetic values against earth- quake probabilities, and who can blame them for belitt
1ing an event of low probability?We can operate through the design engineer in a completely automatic way by modifying his de sign code
QThi s would penali se the owner who insists on a building inappropriate to the site » by requiring higher de sign loads, and hence a more expensive structure. Microzoners, and codes, would take into account the possible surface mot ion, leaving the engine er to deal with the response of the building-foundat ion system to that motion•
One can justify such an approach by compar- ing the mas s of soil with that of the building
aFrom this point of view it is seen that during an earthquake, surface motion will be effect- ively independent of the structure
0This approach Is urged as an alternative to the engineer attempting to evaluate the total sy stem from the bed rock up. The latter could be more expensive and involved, and also less ac curate.
The day may come when the microzoning map
in its present form has transparent overlays
supplied. The se would carry contours, perhaps
of ground re sonant frequency and/or ground
amplificat i on, or perhaps derived parameters *
The code would refer to the se value s, and ask
that de sign loads be modified according to the
values, Then, who knows, we may see a city area
without buildings between three and ten stories,
surviving an earthquake without the selective
damage that has occurred during recent earth-
quake S o
48
T h e i n t e n s i t y of the m i c r o z o n e i n e r t i a a t t a c k I n c r e a s e s w i t h the s o f t n e s s of the s o i l , T h e d e p t h of soft l a y e r s m u s t be s u f f i c i e n t t o b r i n g the f i r s t r e s o n a n t p e r i o d u p to a v a l u e a t w h i c h c o n s i d e r a b l e v i b r a t i o n is o c c u r r i n g in the u n d e r l y i n g r o c k s . It i s t h o u g h t t h a t t h e s p e c t r u m of r o c k v i b r a t i o n h a s a b r o a d m a x i m u m c e n t r e d on a p e r i o d w h i c h i n c r e a s e s w i t h i n c r e a s i n g e p i c e n t r a l d i s t a n c e a n d
p r o b a b l y w i t h e a r t h q u a k e m a g n i t u d e . H e n c e t h e m i n i m u m d e p t h of soft g r o u n d f o r s e v e r e m i c r o - z o n e e f f e c t s s h o u l d I n c r e a s e w i t h I n c r e a s e d d i s t a n c e a n d p r o b a b l y a l s o w i t h m a g n i t u d e .
F o r e a r t h q u a k e s w i t h I n c r e a s i n g I n t e n s i t y o f r o c k v i b r a t i o n m i c r o z o n e e f f e c t s s h o u l d r e m a i n v e r y p r o n o u n c e d u n t i l soil, s t r a i n s r e a c h a l e v e l at w h i c h t h e r e i s a l a r g e l o s s of
e n e r g y d u e to p l a s t i c d e f o r m a t i o n or u n t i l t h e r e i s d a m a g e by r u p t u r e o r 1i q u e f a c t i o n0 H e n c e f o r m o d e r a t e l y s e v e r e e a r t h q u a k e s we e x p e c t l a r g e l o c a l i n c r e a s e s in the I n e r t i a a t t a c k on s u f f i c i e n t l y d e e p l a y e r s of soft g r o u n d . D u r i n g v e r y s e v e r e e a r t h q u a k e s we e x p e c t that l o c a l i n c r e a s e s i n t h e i n e r t i a a t t a c k w i l l be m u c h r e d u c e d b y the e n e r g y a b s o r b e d d u r i n g p l a s t i c d e f o r m a t i o n . A s the e a r t h q u a k e i n t e n s i t y is i n c r e a s e d t h e r e m a y b e a s m a l l or a l a r g e r a n g e o f i n t e n s i t i e s at w h i c h r e s o n a n c e is l i m i t e d b y p l a s t i c i t y b e f o r e trie i n t e n s i t y f o r d a m a g e by r u p t u r e or
l i q u e f a c t i o n is r e a c h e d * 16. Conclusion
T h i s p a p e r h a s s h o w n h o w i n e r t i a a t t a c k v a r i a t i o n s ari s e , h o w t h e y c a n be s t u d i e d , a n d h o w t h e y a r e b e i n g s t u d i e d . H o p e f u l l y , it h a s o u t l i n e d t h e p r e s e n t c r i t i c a l p o i n t s in
I n e r t i a a t t a c k s t u d i e s , a n d e n c o u r a g e s a b i g p u s h at t h e s e p o i n t s .
T h e b e n e f i t s to t h e c o m m u n i t y of t a k i n g l o c a l e f f e c t s i n t o a c c o u n t w i l l b e g r e a t , a n d t h i s p a p e r is I n t e n d e d to g i v e e n g i n e e r s an o v e r a l l p i c t u r e o f t h e s e e f f e c t s . D e s i g n i n g f o r l o c a l g r o u n d e f f e c t s is a c o n s t r a i n t o n trie e n g i n e e rs b u t o n e w h i c h i s l i k e l y to b e i n c o r p o r a t e d i n t o e x i s t i n g c o n s t r a i n t s , a n d o n e w h i c h s h o u l d h a v e e c o n o m i c a n d h u m a n i t a r i a n b e n e f i t s f o r t h e c o m m u n i t y .
15. Summary — Inertia Microzone Attack on Structures
GROUND DAMAGE INERTIA .A— RELATIVE DISPLACEMENT
GROUND DAMAGE INERTIA RELATIVE DISPLACEMENT
WAVELENGTH INDEPENDENT WAVELENGTH DEPENDENT
0 NTERFERENCE MULTIPLE REFLECTION RESONANCE FOCUSSING ENERGY CONSERVATION Fig. 1. Block diagram—microzone processes.
A
ROCK
< — — • ~ ' ' >
Fig. 2. Soil motion at A is primarily due to rock motion in the region B-t.
