changes in vpvs with depth-implications for appropriate velocity models improved eq...

8/13/2019 Changes in VpVs With Depth-implications for Appropriate Velocity Models Improved EQ Location_nicholson_1985

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Bulletinof the SeismologicalSociety of America, Vol. 75, No. 4, pp. 1105-1123,August 1985

C H A N G E S I N V J V W I T H D E P T H : I M P L I C A T I O N S F O RA P P R O P R I A T E V E L O C IT Y M O D E L S, I M P R O V E D E A R T H Q U A K EL O C A TI O N S , A N D M A T E R I A L P R O P E R T I E S O F T H E U P P E R C R U S TBY CRAIG NICHOLSON AND DAVID W. SIMPSON

ABSTRACTP r o p e r t i e s a s s o c i a t e d w i t h t h e a r r iv a l t i m e s o f s h a l l o w m i c r o e a r t h q u a k e s a r ed e t e r m i n e d f o r t h r e e t e c t o n i c a l l y d if f e r e n t s i t e s : S o v i e t C e n t r a l A s i a ; t h e C e n t r a lU n i t e d S t a t e s ; a n d s o u t h e r n C a l i f o r n i a . S i m p l e g r a p h i c a l p r e s e n t a t i o n s o f t h ee a r t h q u a k e d a t a i n c l u d i n g W a d a t i a n d R i z n i c h e n k o d i a g r a m s a r e u s e d t o r e s o l v et h e t r a d e - o f f i n o r i g in t im e a n d f o c a l d e p t h . E s t i m a t e s o f t h e a v e r a g e h a l f - s p a c ev e l o c i t i e s fo r P a n d S w a v e s a n d o f t h e t ra v e l - t i m e r a t io t . / t p ) a r e a l s o o b t a i n e dt h a t a r e r e l a t i v e l y f r e e o f a n y m o d e l c o n s t r a i n t s u s e d t o i n i ti a l ly lo c a t e t h ee a r t h q u a k e . I n a l l c a s e s P a n d S a r r iv a l s e x h i b i t a s y s t e m a t i c d e c r e a s e i n t , / t pw i t h d e p t h i n t h e u p p e r f e w k i lo m e t e r s . T h i s d e c r e a s e i s s i g n i fi c a n t a n d m a t c h e ss i m i l a r v a r i a t i o n s o b s e r v e d i n s i t u f o r t h e v e l o c i t y r a t io V p / V . ) . B a s e d o n l a b o r a t o r ya n d t h e o r e t ic a l s tu d i e s s u c h o b s e r v a t i o n s a re c o n s i s t e n t w i th t h e c l o s i n g : o fs a t u r a t e d m i c r o c r a c k s w i t h in c r e a s i n g c o n f i n i n g p r e s s u r e . I n d e p e n d e n t d e t e r m i -n a t i o n s o f v e l o c i t y s t r u c t u r e a n d r o c k p r o p e r t i e s fr o m b o t h r e f r a c t i o n s t u d i e s a n db o r e h o le e x p e r i m e n t s s h o w a n a l o g o u s r e s u l ts . B y s p e c i f y in g s e p a r a te v e l o c i t ym o d e l s f o r b o t h P a n d S w a v e s t h a t r e f le c t t h i s v a r i a t io n r a t h e r t h a n a s s u m i n g ac o n s t a n t v a l u e o f V p / V . , w e f i n d i m p r o v e d s t a b i l it y i n h y p o c e n t e r d e t e r m i n a t i o na n d i n c r e a s e d r e s o l u t i o n o f s h a l lo w s e i s m i c i t y p a t t e r n s .

INTRODUCTIONT h e u s e o f m i c r o e a r t h q u a k e s to d e t e r m i n e t h e o r i e n t a t i o n o f a ct iv e f a u lt s, f o c alm e c h a n i s m s o l u t i o n s , a n d d e t a il s o f lo c a l c r u s t a l s t r u c t u r e s t r o n g l y d e p e n d s o n t h ev e l o c it y m o d e l u s e d i n t h e e a r t h q u a k e l o c a ti o n p r o c e ss a n d , a s a c o n s e q u e n c e , iss u b j e c t t o s y s t e m a t i c e r r o r s i n t r o d u c e d b y i n a p p r o p r i a t e m o d e l p a r a m e t e r s . W i t ht h e a d v e n t o f d i g it a l d a t a a n d t h e i n c r e a s in g u s e o f t h r e e - c o m p o n e n t s t a ti o n s , l a rg en u m b e r s o f g o o d q u a l it y S - w a v e a r ri v a ls a r e b e i n g r e c o rd e d b y m i c r o e a r t h q u a k en e t w o r k s . T h e s e S - w a v e a r r iv a l s r e q u i r e p r o p e r m o d e l i n g i f f u ll a d v a n t a g e i s t o b et a k e n o f t h e i n c r e a s e d r e so l u t i o n s u c h se c o n d a r y ar r iv a l s c a n p r o v id e B u l a n d ,

1976) . F o r t he s e r e a s ons , w e be ga n t o i n ve s t i ga t e t he c h a r a c t e r i s t ic s o f m i c r oe a r t h -qua k e t r a ve l t i m e s a s a m e a n s o f i de n t i fy i ng : 1 ) m ode l - i nduc e d e r r o r s; 2 ) a ve r a gev a r i a t i o n s i n l o c al v e lo c i ty st r u c t u r e ; a n d 3 ) g e n e r al m a t e r i a l p r o p e r t i e s o f t h euppe r c r u s t .TECHNIQUES

T h e p r i n c i p a l t e c h n i q u e s i n v o lv e t h e u s e o f W a d a t i 1 93 3) a n d R i z n i c h e n k o1 95 8) d i a g r a m s a s s h o w n i n F i g u r e 1 . B o t h m e t h o d s h a v e h a d a l o n g h i s to r y o f u s ea s p r i m a r y t o o l s i n t h e e a r t h q u a k e l o c a t i o n p r o c e s s . W a d a t i d i a g r a m s h e l p e d t ov e r if y t h e e x i s t e n c e o f d e e p f o cu s e a r t h q u a k e s u n d e r t h e i s l a n d s o f J a p a n W a d a t i,1 93 3) a n d w e r e s u b s e q u e n t l y a d o p t e d a s a c o m m o n p r o c e d u r e f o r d e t e r m i n i n g o r i g i nt i m e s o f l oc a l a n d r e g i ona l e a r t h qu a ke s e .g ., Bu ne e t a l . 1960) . J ames e t aL 1969)c o m m e n t o n t h e i m p r o v e m e n t i n e a r t h q u a k e l o c a ti o n s i f t h e s t a n d a r d f o u r p a r a m -e t e r h y p o c e n t r a l s o l u t io n is r e d u c e d to t h r e e s p a t ia l p a r a m e t e r s b y u s i n g t h e W a d a t i

Also at the Department of Geological Sciences of Columbia University.1105


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1 1 0 6 C R A IG N I C H O L S O N A N D D A V I D W . S I M P S O Nm e t h o d . T h e s i m p l e g e o m e t r i e s i n v o l v e d i n t h e R i z n i c h e n k o d i a g r a m w e r e f i r s td i s c u s s e d b y G r e e n ( 1 9 3 8 ), a l t h o u g h t h e s e m i - lo g f o r m o f p r e s e n t a t i o n f o r u s e i nd e t e r m i n i n g v e l o c i t i e s a n d e a r t h q u a k e f o c a l d e p t h s w a s d e v e l o p e d b y R i z n i c h e n k o( 1 9 5 8 ) . M o r e r e c e n t l y , H a l e s e t a l . ( 1 9 8 1 ) a n d T o t h a n d K i s s li n g e r ( 1 9 8 4 ) h a v es h o w n h o w b o t h t e c h n i q u e s m a y b e u s e d f o r d e t e r m i n i n g o r i g in t im e s , f o c a l d e p t h s ,a n d v e l o c i t ie s i n s u b d u c t i o n z o n e s , p r o v i d e d te r m s t o a c c o u n t f o r t h e s p h e r i c it y o ft h e e a r t h a r e u s e d a n d h i g h e r o r d e r c o e f f i c i e n t s a r e i n c lu d e d i n t h e W a d a t i d i a g r a m .

T h e W a d a t d i a gr a m ( F ig u r e 1 A ) is b a s e d o n a l in e a r r e la t i o n s h i p b e t w e e n t h ea r r iv a l t i m e o f P ( T p ) a n d t h e t i m e d i f fe r e n c e b e t w e e n P a n d S ( T , - T p)

T - T ~ = T p - T o ) t , / t , , - 1 . - } -T h i s p r o d u c e s a n i n t e r c e p t t h a t i s a n e s t i m a t e o f t h e o r i g in t i m e ( To ) a n d a sl o p et h a t i s a f u n c t i o n o f th e a v e r a ge t r a v e - t i m e r a t io t ~ / t p ) .

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F I G . 1 . E q u a t i o n s a n d fig ur e,~ fo r A ) W a d a t i d i a g r a m s a n d B ) R i z n i c h e n k o d i a g r a m s . T o th e l f ta r e i n p u t a n d o u t p u t p a r a m e t e r s , i n t h e mi le a r e t h e g r a p h i c a l f o rm s , a n d t o t h e r i gh t a r e t h e e q u a t i o n s .

T h e R i z n i c h e n k o d i a gr a m ( F ig u r e 1 B ) i s b a s e d o n a l in e a r r e l a ti o n s h i p b e t w e e nt h e s q u a r e o f t r a v e l t i m e ( t 2) a n d t h e s q u a r e o f e p i c e n t r a l d i s t a n c e ( A 2 )

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a p p r o p r i a te f o r c l o s e d i s t a n c e s a n d s h a l l o w f o c a l d e p t h s . T h i s p r o d u c e s a s lo p e t h a ti s a f u n c t i o n o f t h e a v e r ag e h a l f - s p a c e v e l o c i t y ( V a v ) a n d a n i n t e r c e p t t h a t i s af u n c t i o n o f t h e V e r t ic a l t r a v e l t i m e t z = H / V a v , w h e r e H i s t h e e a r t h q u a k e f o c a l

A r r i v a l t i m e s a r e u p p e r c a s e e . g . , T p ); t r a v e l t i m e s a r e l o w e r c a s e e . g . , T p - T o = t p ) .


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CHANGES IN VjV WITH DEPTH 11 7d e p t h ) . F o l l o w i n g t h e S o v i e t p r a c t i c e B u n e et a l . 1 9 6 0 ), w e p l o t t h e l o g o f t h et r a v e l t i m e a g a i n s t d i s t a n c e t o a c c e n t u a t e t r a v e l t i m e s t o th e n e a r e r , m o r e i m p o r t a n ts t a t i o n s .

T h e s e m e t h o d s h a v e t h e a d v a n t a g e o f b e i n g s im p l e a n d r o b u s t. T h e W a d a t id i a g r a m d e p e n d s o n l y o n t h e o b s e r v e d ar r iv a l t i m e s o f P a n d S w a v e s a n d i sc o m p l e t e ly i n d e p e n d e n t o f a n y p a r a m e t e r s u s e d i n th e h y p o c e n t r a l l o ca t io n p r o c e -d u r e . T h e R i z n i c h e n k o d i a g r a m n o r m a l l y r e q u i re s e s t i m a t e s o f t h e o r ig i n t i m e T o)a n d e p i c e n t r a l d i s t a n c e A ). I n p r a c ti c e , w e u s e t h e o r i g in t i m e a s d e t e r m i n e d f r o mt h e W a d a t i d i a g r a m a n d d i s t a n c e s a s c a lc u l a t e d f ro m t h e t r i a l c o m p u t e r lo c a t io n .I f s t a t i o n s a r e w e l l d i s t r i b u t e d i n a z i m u t h a r o u n d t h e e p i c e n t e r, t h e s e d i s t a n c e s a r eg e n e r a l ly w e l l c o n t r o ll e d . H a l f - s p a c e v e l o c i t ie s f o r b o t h P a n d S w a v e s c a n t h e n b ee a s i ly d e t e r m i n e d , a s w e l l a s s e v e r a l i n d e p e n d e n t e s t i m a t e s o f f o c a l d e p t h . F o re x a m p l e , i f v a l u e s f o r T~ - T p a r e u s e d i n th e R i z n i c h e n k o d i a g r a m , a v e r a g e e f f e c t i v ev e l o c i ti e s f o r t h e s e p a r a t i o n t i m e b e t w e e n S a n d P a r e c a lc u l a te d , a n d f o c a l d e p t h si n d e p e n d e n t o f o r i g in t i m e a r e p r o d u c e d . T h e d i f f e r e n t e s t i m a t e s o f f o c a l d e p t h c a nt h e n b e u s e d t o i d e n t if y c a s es w h e r e c o m p e n s a t i n g e r ro r s i n d e p t h a n d o r ig i n t im eh a v e c o n s t r a i n e d t h e f o ca l d e p t h t o r e m a i n n e a r t h e s t a rt i n g d e p t h u s e d i n th ei t e r a t i v e c o m p u t e r l o c a t i o n p r o g r a m .

T h e v e l o c i t y i n f o r m a t i o n t h e s e m e t h o d s p r o v i d e is a l s o u s e f u l i n a n u m b e r o fo t h e r w a y s . W a d a t i d i a g r a m s f o r a s u i te o f e a r t h q u a k e s a t v a r io u s f o c a l d e p t h s g i v e st J t p a s a f u n c t i o n o f d e p t h . T h i s i n t u r n c a n b e u s e d to r e l a te t h e S - w a v e v e l o c i t ys t r u c t u r e t o t h a t o f t h e P w a v e . T h e a v e r a g e h a l f- s p a c e v e l o c it ie s d e t e r m i n e d u s i n gt h e R i z n i c h e n k o m e t h o d r e p r e s e n t a v e r a g e s o v e r a l l r a y s t o a l l s t a t i o n s . T h e y a r en e i t h e r i n t e r v a l v e l o c i t i e s , n o r s t r i c t l y s i m p l e a v e r a g e v e r t i c a l v e l o c i t i e s f r o m t h ef o c u s t o t h e s u r f ac e . T h e y c a n , h o w e v e r , p r o v i d e s u f f i c ie n t i n f o r m a t i o n t o r e s o l v e an u m b e r o f f i r s t- o r d e r v e l o c i t y v a r i a t i o n s i n d e p e n d e n t o f t h e o r ig i n a l v e lo c i t y m o d e lu s e d f o r lo c a t io n . C o n v e r s i o n o f t h e a v e r a g e v e l o c i t y e s t i m a t e s i n t o n o m i n a l i n t e r v a ll a y e r v e l o c i t i e s i s a c c o m p l i s h e d b y e i t h e r d i f f e r e n t i a t i n g t h e v e r t i c a l t r a v e l - t i m ev e r s u s d i s t a n c e c u r v e B u n e et a l . 1 9 60 ), o r b y b a c k s t r i p p i n g f r o m t h e s u r f a c e t h ea v e r a g e v e l o c i t y - d e p t h p r o f i l e . L a y e r b o u n d a r i e s a r e i d e n t i f i e d b y s i g n i f i c a n tc h a n g e s i n t h e t r a v e l - t i m e o r a v e r a g e v e l o c i ty p l o t s .

V a r i a t i o n s o f v e l o c i t y w i t h i n t h e n e t w o r k c a n b e i d e n t i f i e d b y a s li g h tl y d i f f e r e n tt e c h n i q u e . I n t h i s m e t h o d , a v e r a g e v e l o c i t i e s a r e c a l c u l a t e d u s i n g t h e t r a v e l t i m ea n d t h e h y p o c e n t r a l d i s t a n c e t o i n d i v i d u a l s t a t i o n s . T h e s e e s t i m a t e s a r e m o r ed e p e n d e n t o n l o c a l g e o m e t r y a n d s o a r e m o r e u s e f u l i n i d e n t i f y i n g g r o s s l a t e r a lc h a n g e s w i t h i n a s m a l l re g i o n . A s t h e s e d a t a a r e i l o t, h o w e v e r , r e a li s ti c e s t i m a t e s o fr e g i o n a l a v e r a g e s , t h e y a r e n o t s u b s e q u e n t l y c o n v e r t e d t o i n t e r v a l v a l u e s .

I n t h i s p a p e r , w e s h o w t h a t f o r sh a l l o w e a r t h q u a k e s r e c o r d e d a t c l o s e d i s ta n c e s ,t J t p i n i ti a ll y d e c r e a s e s w i t h d e p t h . T h i s d e c r e a s e is s ig n i f i c a n t a n d m u s t b e c o n s id -e r e d i f a c c u r a t e l o c a t i o n s f o r s h a l lo w e a r t h q u a k e s a r e to b e a c h ie v e d . F u r t h e r m o r e ,i f ts/t~ i n i ti a ll y d e c r e a s e s , s o to o m u s t V p / V ~ t h e r a t i o o f t h e c o m p r e s s i o n a l t o s h e a rw a v e v e l o c i ti e s . S i n c e V p / V s e v e n t u a l l y b e g in s t o in c r e a s e w i t h d e p t h i n t o t h e u p p e rm a n t l e H a l e s et a l . 1981) , V p / V s a t t a in s a b r o a d m i n i m u m t h a t a p p r o x i m a t e l yc o in c i de s w i th t h e d e p t h r a n g e o f m o s t c o m m o n c r u s ta l e a r t h q u a k e s a n d r e f l ec t sc e r t a i n d e p t h - d e p e n d e n t f r a c t u r e p r o p e r t i e s o f t h e u p p e r c r u s t.

RESULTS FROM SOVIET CENTRAL ASIAO u r o r ig i n a l i n t e n t i n a p p l y i n g t h e s e m e t h o d s w a s to p r o v i d e a c o n v e n i e n t m e a n s

o f c h e c k i n g c o m p u t e r l o c a t e d e a r t h q u a k e h y p o c e n t e r s f o r so u r c e s o f e r ro r a n dp r o p e r c o n v e r g e n c e . F i g u r e 2 s h o w s a n e p i c e n t r a l m a p a n d v e r t i c a l c r o s s s e c t i o n


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C H N G E S I N V J V W I T H D E P T H 1109near Toktogul Reservoir located along the Naryn River in the Soviet republic ofKirgizia. The network was installed to monitor induced earthquake activity asso-ciated with the filling of the reservoir and consists of 10 high-gain stations, eachconnected by radio telemetry to a central recording site Simpson et aL, 1981).Three of the stations are three-component , with station spacing on the order of 10km or less. In addition, network data are supplemented by data supplied by severalregional Soviet stations at greater distances. The location program used for deter-mining the hypocenters shown in Figure 2 was HYPOINVERSE Klein, 1978). Thevelocity model used by the program assumed homogeneous layers and a constantvalue for V J V s of 1.78.The cluster of events directly below the dam is interpreted as the seismicityinduced by the reservoir. Other well-located events that extend to depths of 40 kmform part of the natural background seismicity. As seen from the figure, a largenumber of earthquake hypocenters remain constrained at the starting depth of 3km, a depth chosen as appropriate for the shallow induced events. For many ofthese earthquakes, sufficient data were available to determine that their actual focaldepths were much greater. Using the methods of Wadati and Riznichenko, we wereable to determine that one of the primary causes for this result was the incorrectassumption used in the computer location program that V p / V ~ was constant anddid not vary with focal depth.Changes in t~ /tp w i th depth . Figure 3 shows Wadati and Riznichenko diagramsfor six events near the center of the Toktogul array. Focal depths range from 1 to38 kin. The Riznichenko diagrams clearly show the change in shape of the travel-time versus distance curve with increasing focal depth, indicative of both a geomet-rical effect and an increasing P-wave velocity. The Wadati diagrams show asystematic decreases in ts/tp, suggesting that the velocity for shear waves increasesfaster than that for the compressional waves in the upper kilometers of the crust.An interesting feature, however, is the linearity of the Wadati diagrams. If t~/tpand therefore V J V ~ indeed changes with depth, the linear relation assumed in theWadati diagram is no longer valid Kisslinger and Engdahl, 1973). Yet it is apparentthat, at close distances and shallow focal depths, the expected curvature of theWadati diagram is not sufficiently resolvable with the available data.Average ve loc it y s truc tures for P an d S. A summary of average half-space velocitiesfor P and S at Toktogul calculated using the Riznichenko technique and values fort J t p from the Wadati diagram are shown in Figure 4. Values shown are those datareported during a 1-month interval and are plotted as a function of focal depth foreach individual earthquake. The internal consistency of the data suggests that thesevalues are useful as estimates of the first-order variation in velocity structure andcan be used to improve the velocity model used for location. Consequently, the datawere smoothed by a polynomial fit dashed lines), and then backstripped to yieldinterval velocities solid lines). Backstripping simply starts with the surface valueand then determines the incremental velocity that, when averaged with all thep~eceding values, maintains the shape of the best-fit ting curve. Depth intervals overwhich the backstripped velocities remained relatively uniform were chosen for layerboundaries. Interval values for V J V s were calculated using the ratio of the intervalP and S velocities. At depths greater than 20 km, where the earthquakes were fewand velocities less well resolved, the velocity for the S wave was restr icted to a valuecorresponding to a V J V ~ of 1.68.Earthquake re locat ions us ing P and S veloci ty models . Because of the systematicvariation in t J t p with depth, the location program was modified to accept separate


6/19

1 1 1 0 C R A I G N I C H O L S O N A N D D A V I D W . S I M P S O Nve loc i ty models for P and S waves The va lues used were those show n in F igure 4as interval velocities Figure 5 show s a comp arison of this m odif icat ion on the dep thdis tr ibut ion o f earthquake hypocenters a t Toktogul Th e data chosen for comparisoninc lude on ly those earthquakes for which depth contro l and a good az imutha l

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FIG. 3 . V a r i a t i o n in W ad a t i top) an d Rizn ich en k o bottom) d iag r am s wi th ea r th q u ak es o f d i f f e ren tfoca l dep ths a t Toktogul . Inset sh o ws a m ap o f t h e ea r th q u ak e lo ca t io n s n u m b er ed ) r e la t i v e t o t h ear ray , tJtp f r o m th e W ad a t i d i ag r am s sho w a sy s t em a t i c d ec r ea se wi th f o ca l d ep th a s d e t e r m in ed b y th eRizn ich en k o d iag r am s , s td d ev i s t h e s t an d a r d d ev ia t io n in secon d s) b e tween th e d a t a an d th e l i n ea rl ea s t - sq u a r e s f i t o n th e W ad a t i p lo t .c o v e r a g e e x i s t . I n t h e f i r s t c r o s s s e c t i o n F i g u r e 5 A ) , a c o n s t a n t v a l u e f o r V p / V s w a su s e d . T h e s h a l l o w i n d u c e d e v e n t s d e f i n e a t i g h t c l u s t e r b e n e a t h t h e d a m , h o w e v e r ,a s n o t e d e a r l ie r , f e w of~ t h e e a r t h q u a k e l o c a t i o n s i t e r a t e d t o d e e p e r d e p t h s a n d al a r g e n u m b e r w e r e a r t i f i c i a l l y h e l d a t t h e s t a r t i n g d e p t h o f 3 k m . C h a n g i n g t h e


7/19

C H A N G E S I N V V s W I T H D E P T H 1 1 1 1s t a r t i n g d e p t h t o 1 0 k m F i g u r e 5 B ) a ll o w e d m o r e h y p o c e n t e r s to r e lo c a t e d e e p e r,b u t m a n y w e r e s ti ll c o n s t r a i n e d a t t h e S t a r ti n g d e p t h . M o r e o v e r , t h e i n d u c e d e v e n t se x h i b i t a p a t t e r n o f m i g r a t i o n t o w a r d s t h e s u r f ac e , i m p l y i n g s e v e r a l d i d n o t p r o p e r l yc o n v e r g e . I f s e p a r a t e v e l o c i t y m o d e l s fo r P a n d S a r e u s e d , - i n c o r p o r a t i n g t h eo b s e r v e d v a r i a t i o n i n t J t ~ w i t h d e p t h , t h e n b o t h t h e s h a l l o w i n d u c e d e v e n t s a n dt h e d e e p e r e a r t h q u a k e s a r e w e l l l o c a t e d a n d t h e e f f e c t o f t h e s t a r t i n g d e p t h i s n o ta s p r o m i n e n t F ig u r e 5 C ). S t a r t i n g d e p t h f o r t h e e a r th q u a k e h y p o c e n t e r s s h o w n i nF i g u r e 5 C w a s 1 0 k m .

R E S U L T S F R O M S O U T H E R N C A L I F O R N I A

A n o t h e r v a r i a t io n o f t h e s e s i m p l e t e c h n i q u e s i s t o d e t e r m i n e , f o r a g iv e n s t a ti o n ,t h e a v e r a g e v e lo c i ty b e t w e e n t h a t s t a ti o n a n d e a c h e a r t h q u a k e h y p o c e n t e r . T h i s

V p ( k r n / s e c ) V s ( k m / s e c ) t s / f p ( W o d a fi )5 0 5 5 6 0 8 5 7 0 5 5 3 0 3 5 4 0 4 5 1 5 1 7 1 9

I o 1 ~ o 0I - i~ e,4.,.--I cl,~0 ] 43D

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/ Io i :3 0 3 o ~ o 3 0 oFIG. 4. Averagehalf-space value s of Vp, V,, and t J tp for each earthquake as a function of focal depthat Toktogul. Foca l depths are those de rived from the Riznichenko diagram. Dotted lines are polynomialfits to the data; solid lines are the backstripped interval velocitiesused to relocate the earthquakes shownin F igure 5C.

2:

2 1

v e l o c i t y i s c a l c u l a t e d u s i n g t h e t r a v e l - t i m e b a s e d o n t h e W a d a t i o r ig i n t im e a n d t h el in e a r h y p o c e n t r a l d i s t a n c e f r o m t h e R i z n i c h e n k o d i a g r a m . A n e x a m p l e is t a k e nf r o m t h e s o u t h e r n C a l if o r n ia n e t w o r k , S C A R L E T , o p e r a t e d b y t h e U . S . G e o lo g ic a lS u r v e y a n d t h e C a l if o r n ia I n s t i tu t e o f T e c h n o l o g y . N e t w o r k g e o m e t r y is sh o w n i nF i g u r e 6 , a l o n g w i t h t h e l o c a t i o n s o f e a r t h q u a k e s u s e d i n t h e s t u d y .

M o d e l i n d u c e d e r ro r s. F i g u r e 7 c o m p a r e s t h e v e l o c i t i e s f r o m t h i s m e t h o d w i t hs i m i l a r r e s u l t s u s i n g t h e h y p o c e n t r a l p a r a m e t e r s o f o r ig i n ti m e a n d f o c a l d e p t h f r o mt h e H Y P O I N V E R S E c o m p u t e r l o c a ti o n p r o g r am . A v e ra g e V p, V s, a n d t s / t p t os t a ti o n M L L a r e p l o t t e d f o r ea c h e a r t h q u a k e a s a fu n c t i o n o f f o ca l d e p t h . T h e


8/19

1 1 1 2 C R A I G N I C H O L S O N A N D D A V ID W S I M P S O N

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9/19

C H A N G E S I N Vp/V W I T H D E P T H 1113o v e r al l r a n g e i n h y p o c e n t r a l d e p t h s i s s im i l a r i n t h e t w o s e ts o f d i ag r a m s , b u t t h r e ed i ff e re n c es s t a n d o u t . T h e d e p t h s o f t h e H Y P O I N V E R S E h y p o c e n t e r s F ig u re 7,t op ) a r e d i s t i nc t l y c l u s t e r e d a bove d i s c on t i nu i t i e s i n t he ve l oc i t y m ode l u s e d f o rl o c a t io n l o n g - d a s h e d l in e s ), w h e r e a s t h e m o d e l - i n d e p e n d e n t h y p o c e n t e r s F i g ur e7 , b o t t o m ) a r e m o r e u n i f o r m l y s c a t t e re d w i t h d e p t h . T h i s s u g g e st s t h a t t h e c l u s te r i n gi s m ode l de pe n de n t , as i s t he t e nd e n c y f o r t he ve l oc it i es t o a l i gn a l ong t he a ve r a geo f t he i np u t ve l oc i t y m o de l s ho r t - d a s he d li ne s) . S e c ond , t he v e l oc i ty p r o f i le s ba s e d

1 1 6 . 5 0

33 65 ~io.uwmi 1 7 5 0

O ~ 1 KMF IG . 6 . E a r t h q u a k e e p i c e n t e r s d e t e c t e d b y t h e s o u t h e r n C a l i f o r n i a n e t w o r k S C A R L E T f r o m 1 97 7 t o1 98 0 an d u s ed i n t h e v e l o c i t y s t u d y . Hea v y l in e s a r e m a j o r f au l t s ; s h ad ed a r ea s r ep re s en t e l ev a t i o nco n t o u r s ab o v e 6 0 00 f t . C i r c l ed s t a t i o n s ex h i b i t v e l o c i t y m i n i m a ; s t a t i o n s w i t h s q u a re s d o n o t . He x ag o n a ls t a t i o n s e x h i b i t v e lo c i t y m i n i m a b u t o n l y f o r a z im u t h s t h a t g e n e r a l ly c o r r e s p o n d t o p r o p a g a t i o na l o n g o r a c r o s s th e S a n J a c i n t o f a u lt . D o u b l e - b o r d e r e d s t a t i o n s a r e d i s c u s se d i n t h e t e x t a n d s h o w n i nFigure 8 .

o n t h e W a d a t i o r i g i n t i m e s a n d R i z n i c h e n k o f o c a l d e p t h s s h o w a w e l l d e f i n e dm i n i m um i n V p, Vs, a n d t s t p a t a bo u t 8 t o 10 km , s ug ge s t i ng a l ow - ve l oc i ty l a ye ra b o v e th i s d e p t h . N o e v i d e n c e fo r s u c h a v e l o ci ty m i n i m u m i s a v a il a b le f r o m t h ev e l o c it y p r o f il e s b a s e d o n t h e c o m p u t e r - l o c a t e d h y p o c e n t e r s . T h i r d , t h e r e is a nove r a l l de c r e as e i n t s t p w i t h d e p t h t h a t is i n sh a r p c o n t r a s t t o t h e a s s u m p t i o n o f ac o n s t a n t v a l u e f o r V J V ~ u s e d in t h e c o m p u t e r lo c a t io n p r o g r a m F i g u re 7, t o p r i g h td i a g r a m ) .


10/19

1114 C R A I G N I C H O L S O N A N D D A V I D W S I M P S O NT h e r e l o c a te d h y p o c e n t e r s , b a s e d o n t h e m e t h o d s o f W a d a t i a n d R i z n i c h e n k o ,

w e r e t h e n u s e d t o d e f i n e t h e o r i e n t a ti o n o f a c ti v e f a u l t s a n d t h e p a t t e r n o fd e f o r m a t i o n a s d e f i n e d b y th e r e su l t in g f o ca l m e c h a n i s m s o l u t i o n s N i c h o l s o n e ta l . 1 9 8 4 a ) . T h e r e s u l t s w e r e s u f f ic i e n t t o r e so l v e a n u m b e r o f s m a l l - s c a l e s e c o n d a r ys t r u c t u r e s t h a t h a d n o t b e e n p r e v i o u s l y i d e n t i f i e d .Re g i o n a l v a r i a t i o n s i n v el o c i ty . I n d e a l i n g w i t h a c o m p l i c a t e d g e o l o g i c e n v i r o n -m e n t , s u c h a s th e S a n A n d r e a s f a u l t t h r o u g h t h e E a s t e r n T r a n s v e r s e R a n g e s , l a te r al

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F IG . 7 . A v e r a g e h a l f s p a c e v e lo c i ti e s c a l c u l a t e d t o s t a t i o n M L L . T o p t h r e e f i gu r e s sh o w v a l u e s b a s e do n o r ig i n ti m e s a n d f o c a l d e p t h s d e t e r m i n e d b y t h e c o m p u t e r l o c a t io n p r o g ra m ; t h o s e a t t h e b o t t o m a r eb a s e d o n W a d a t i o r i g i n t im e s a n d R i z n i c h e n k o f o c al d e p t h s . L o n g d a s h e d l in e s i n to p d i a g r a m s a r ei n t e r v a l v e l o c it i e s u s e d i n t h e l o c a t i o n m o d e l ; s h o r t d a s h e d l i n e s a r e a v e r a g e v e r t i c a l v el o c it ie s . D a s h e dl i n e s in b o t t o m d i a g r a m s a r e p o l y n o m i a l le a s t s q u a r e s a p p r o x i m a t i o n s . Top da ta s h o w p r o n o u n c e dc l u s t e r i n g a b o v e d i s c o n t i n u i t i e s i n t h e l o c a t i o n v e l o c i t y m o d e l . B o t t o m d a t a s h o w l e s s c l u s t e r i n g a n ds u g g e s t a l o w v e l o c i t y l a y e r a t o r a b o v e a d e p t h o f 8 t o 9 k m . V a l u e s o f t / tp a l s o s h o w a s i g n i f i c a n td e c r e a s e i n t h e f i r s t f e w k i l o m e t e r s .


11/19

CHANGES IN V ~ / V WI T H D E P T H 1 1 1 5v a r i a t i o n s i n v e l o c i t y a r e t o b e e x p e c t e d . A s i m p l e p r o c e d u r e t o d e t e c t t h e s ev a r i a t i o n s is to c o m p a r e t h e a v e r a g e v e l o c i t y p r o f i l es t o v a r io u s s t a t i o n s w i t h i n t h en e t w o r k . S i g n i f i c a n t d i f f e r e n c e s w i ll t h e n i d e n t i f y l ar g e - sc a l e h e t e r o g e n e i t i e s . F i g u r e8 s h o w s a v e r a g e v e l o c i t y p r o f i l e s fo r si x s ta t i o n s i n s o u t h e r n C a l i f o r n ia . T h r e e o f

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FIG. 8. M odel-independent half-space velocities calculated for six stations o f the California network.A t top are three stations located nor th o f the n orthern branch o f the San And reas fault zone, in thevicinity of the San Bernardino Mountains; those at ottom are located south of this fault, in the SanJacinto M ountains see map , Figure 6).t h e s t a t i o n s i n c lu d i n g M L L ) l o c a t e d n o r t h o f t h e n o r t h e r n b r a n c h o f S a n A n d r e a sf a u l t e x h i b i t v e l o c i t y m i n i m a , w h i l e t h e t h r e e s t a t i o n s s o u t h o f t h i s f a u l t d o n o t .T h i s s u g g e s ts a m i d - c r u s t a l l o w - v e l o c i t y l a y e r is p r e s e n t u n d e r t h e S a n B e r n a r d i n oM o u n t a i n s w h e r e t h e t h r e e s t a t io n s t h a t s h o w t h e v e lo c i t y m i n i m a a r e l oc a te d )b u t n o t u n d e r t h e S a n J a c i n t o M o u n t a i n s F i gu r e 6 ). O t h e r s t a t io n s e x h i b i t e d


12/19

6 CRAIG NICHOLSON AND DAVID W. SIMPSONvelocity minima, but only for earthquakes recorded at certain azimuths. Most ofthese stations are located adjacent to the San Jacinto fault, and the azimuths forwhich velocity minima were most prominent corresponded to propagation along oracross the fault zone. Evidence for such localized low velocities associated witheither fault zones or regional mid-crustal layers have been previously identified onthe basis of seismic refraction surveys Mooney and Colburn, 1985), seismic reflec-tion profiles Feng and McEvilly, 1983), sonic well-log studies Moos e t a l . 1983),microearthquake travel-time delays Healy and Peake, 1975), and by the presenceof regional phase arrivals Py or P Gutenberg, 1951).If the data presented here are taken at face value, then the possibility exists tha tthe overthrust San Bernardino Mountains are rootless, and that the dividing linebetween this st ructure and that of the San Jacinto Mountains to the south is indeed

89 . 6 6 89 .S 1 L V /

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o o.PUMP / .. e , . _ a \ ] J / n f ] ~ , fGRT _~.~~ P ~ / ~ . j / ~ V ~ e S G T ~ ( : ' .~ .~

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8 9 3 63 6 . 3 8

3 6 . 2 9

3 6 . 2 036FIG. 9. Earthquakes located during the operation of the portable array 10 May to 11 June 1978) inthe central Mississippi valley 56 events). The town of Ridgely is in the cen ter ; Mississippi river is to

the left Horizontal errors in location are less than 0.5 km.the northern branch of the San Andreas fault through San Gorgonio Pass. Regionalgravity data and the distribution of Pg velocities also support this interpretationHearn and Clayton, 1984) Earthquake phase data are, therefore, useful not onlyto improve local velocity models, but to identify significant lateral inhomogeneitiesand possible allochthonous terrains.

RESULTS FROM THE NEW MADRID AREA IN THE CENTRAL UNITED STATESThe methods of Wadati and Riznichenko can also be used as a basis for more

sophisticated techniques. Figure 9 shows a map of a dense network of portableseismographs installed to monitor a portion of the earthquake activity associatedwith the New Madrid seismic zone in the Central United States. The collected datawere analyzed for earthquake arrival times and subsequently inverted for velocitystructure and hypocentral locations Nicholson e t a l . 1984b). The main purpose of


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CHANGES IN VJV, WITH DEPTH 1117the study was to investigate the influence of hypocenter relocations using a variableVp/V in resolving details of local seismicity patterns and to compare the velocitystructure obtained from the earthquake data with models determined from refrac-tion studies borehole data and a formal velocity inversion.

Determinat ion o f the f i r s t -order ve loc i t y s t ruc ture . Figure 10 reproduces theprincipal results of this analysis. In each case the average half-space velocity fromthe Riznichenko diagram and 4 / t p from the Wadati diagram are plotted against the

:z lOi . u

20

v C k m / s e c3.0 5.0 7.0I I I I

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1 . 0 1 . 8 2 . 2 2 . 6 3 . 0I I I I I I I I I

10

Vs k m / s e )2.0 3.0 4.0 1.4i L _ _ _ . . I . . . I f

9 oo =

~ ,,o oo,,

~ o 1 0

20 20 o

T s / T p W a d a f l )I

- i o

E~ I 0o

20

Vp AVG k m / s e c )3.0 5.0 7.0o Ib 01 I l

eJ -

Vs AV+ (km/sec)1 . 0 2.0 3.0

I J o ~ t _ l I I I~ o ~ ~ _ _ ~0 ~ . 4 b - -

1 0 - -

2 0 - -

Ts/Tp AVG (Wodofi)4.0 1.4 1.8 2.2 2.6I | I 1 0 I < > L . I I I

lO

2O

3.0I

FI6. 10. Average half-space velocities for P and S waves as a function of focal depth for eachindividual earthquake shown in Figure 9. Travel-time ratios t+/ tp) are from Wadati diagrams; focaldepths are from Riznichenko diagrams. T o p are original data; b o t t o m are the same data smoothe d by amoving-window average startin g from known surface values. Arrows identi ty significant inflection points,repres enting discontinuities in velocity structure. The heavy dashed line in Vp AVG plot is from refractionstudy (Mooney e t a l . , 1982); dot ted lines are result s of inversion study for both P and S waves. Bothhave been sm oothed by a filter to account for averages over up-going rays.


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1 1 1 8 CRAIG NICHOLSON AND DAVID W. SIMPSONe a r t h q u a k e f o ca l d e p t h d e t e r m i n e d f ro m t h e R i z n i c h e n k o d i a g ra m . A t t h e t o p a r et h e o r i g i n a l d a t a , a n d a t t h e b o t t o m a r e t h e s a m e d a t a i n t e r p o l a t e d a t c o n s t a n ti n te r v a ls a n d s m o o t h e d b y a m o v i n g - w i n d o w a v er a ge . L i k e t h e r e s u l ts f r o m T o k t o g u la n d s o u t h e r n C a l if o r n ia , t s / t~ i s n o t c o n s t a n t , b u t c h a n g e s s y s t e m a t i c a l l y w i t h d e p t h .B o t h V ~ a n d V s i n c r e a s e r a p i d l y i n th e u p p e r 2 k m f r o m t h e i r k n o w n s u r f a c e v a l u e sa n d e x h i b i t a m i d - c r u s t a l l o w v e l o c i t y z o n e b e l o w a d e p t h o f a b o u t 2 . 5 t o 3 k m .A n o t h e r s i g n i f ic a n t i n f l e c ti o n o c c u r s a t a b o u t 5 t o 6 k m d e p t h , a n d m a r k s t h e r e t u r nt o i n c r e a s in g v e l o c i t ie s t h a t a s y m p t o t i c a l l y a p p r o a c h c o n s t a n t v a l u e s b e l o w a d e p t ho f 1 0 k m . A v e l o c it y m o d e l c o m p o s e d o f u n i fo r m l a y er s w o u l d r e p r e s e n t s u c h as t r u c t u r e w i t h v e l o c i ty d i s c o n t i n u i t i e s a t 2 .5 t o 3 a n d 5 t o 6 k m d e p t h . M o o n e y e tal. 1 9 8 2) i d e n t i fy j u s t s u c h b o u n d a r i e s a t 2 . 0 a n d 5 .0 k m f o r t h i s a r e a b a s e d o n al o n g - b a s e l in e re f r a c t i o n e x p e r i m e n t T a b l e 1 ). T h e y a l so f o u n d u n u s u a l l y h ig hv e l o c it ie s a t a d e p t h o f 1 t o 2 k m , a n d a l o w - v e l o c i ty zo n e . A c o m p a r i s o n o f t h e i rm o d e l w i t h t h e e a r t h q u a k e v e l o c i t y d a t a i s s h o w n i n F i g u r e 1 0 a s t h e h e a v y d a s h e dl in e . T h e d a t a i m p l y t h a t a n y a t t e m p t t o a c c u r a t e l y m o d e l t h e l o c a l v e l o c i t y s t r u c t u r er e q u i r e s s e p a r a t e s t u d i e s f o r b o t h S a n d P .

TABLE 1C R U S T A L V EL O C IT I ES A N D B U L K P R O P E R T I E S I N T H E N E W M A D R I D A R E A

Case 1 Case 2 Case 3 Mooney Bulk She arDepth* Mooney F ina l F ina l I n t e rva l et al Poisso n s Modulus Modulus(kin) et a l Model Model Vp/V8 Dens i ty R a t io K

Vp Vp V~ 2/3 -1/3 p ~ (101 Pa) (101 Pa)(km/sec) (km/sec) (km/sec) (grn /cm )0.0 1.80 1.68 1.68t 4.20-4.50~: 2.2 0.47-0.47 0.57-0.67 0.0040.5 5.95 6.00 3.04 1.97-1.95 2.65 0.33-0.32 6.28-6.09 2.452.0 4.90 4.62 2.76 1.67-1.77 2.55 0.22-0.27 2.85-3.53 1.945.0 6.20 6.14 3.65 1.69-1.70 2.75 0.23-0.24 5.48-5.69 3.66

18.0 6.60 6.50 3.79 1.72-1.74 2.80 0.25-0.25 6.47-6.83 4.0230.0 7.30 7.35 4.35 1.69-1.68 3.00 0.23-0.23 8.64-8.42 5.6840.0 8.00 8.10 4.70 1.73-1.72 3.25 0.25-0.24 11.80-11.20 7.18Depth to top of layer.

t Held fixed after first iteration;:~ Assumes a value of 0.4 km/sec

S conver ts to P at the base of this layer.for S.

V e l o c i t y in v e r s i o n s f o r P a n d S . W e l l r e c o rd e d e a r t h q u a k e s f r o m t h e N e w M a d r i da r e a w e r e t h e n s e l e c t e d f o r a v e lo c i t y i n v e r s i o n s t u d y . T h e t e c h n i q u e u s e d w a s t h a td e v e l o p e d b y C r o s s o n 1 9 7 6) fo r s i m u l t a n e o u s i n v e r s i o n o f b o t h v e l o c it i e s a n de a r t h q u a k e h y p o c e n t e r s . U n l i k e p r e v i o u s s t u d i e s o f t h i s ty p e , v e l o c i ty s t r u c t u r e s f o rb o t h P a n d S w e re d e t e r m i n e d w i t h n o a s s u m p t i o n s m a d e r e g a rd i n g t h e s i m i la r it yo f t h e S v e l o c i ty to t h a t o f t h e P v e l o c i t y e x c e p t f o r p o s i t i o n o f la y e r b o u n d a r i e s .L a y e r b o u n d a r i e s w e r e t h o s e c h o s e n b y M o o n e y e t a l . 1 9 8 2 ) f o r v e l o c i t y d i s c o n t i n -u i t ie s i n t h e a r e a o f t h e p o r t a b l e a r r a y . D e t a i l s o f t h e i n v e r s i o n p r o c e d u r e , a s w e lla s i ts t e c t o n i c i m p l i c a ti o n s a r e d i s c u s s e d b y N i c h o l s o n et a l . 1 9 8 4 b ) . T h e r e s u l t sa r e s u m m a r i z e d i n T a b l e 1 . A l s o s h o w n a r e t h e i n t e r v a l v a l u e s o f V p / V s d e r i v e df r o m t h e t w o i n v e rs i o n s t u d i e s a n d t h e P - w a v e v e l o c it y s t ru c t u r e d e t e r m i n e d f r o mt h e se is m i c r e f ra c t io n e x p e r i m e n t M o o n e y e t a l . 1982) .B o t h i n v e r s i o n s , w h e t h e r f o r P o r S , p r o v e d t o b e c r i t i c a l l y d e p e n d e n t o n t h ei n i ti a l v e l o c i ty m o d e l u s e d t o s t a r t t h e i n v e r s i o n p r o c e d u r e . S e v e r a l t ri a l s w e r e m a d ea n d , i n e a c h c a s e , t h e a t t e m p t t h a t c o n v e r g e d r a p i d l y a n d h a d t h e g r e a t e s t s t a b i l i t yd u r i n g t h e i n v e r s i o n p r o c e s s w a s t h e o n e t h a t u s e d a s t a r t i n g m o d e l t h a t c l o s e l ym a t c h e d t h e f i r s t- o r d e r v e l o c i t y s t r u c t u r e i d e n t i fi e d b y t h e h a l f - s p a c e v e l o c i ty


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CHANGES IN Vp/V~WITH DEPTH 1119t e c h n i q u e ( F i g u re 1 0). I n t h e c a s e o f t h e P - w a v e i n v e r s io n , i n c l u s i o n o f a lo w -v e l o c i ty l a y e r re s u l t e d i n a n 8 3 p e r c e n t r e d u c t i o n i n v a r i a n c e a n d p r o d u c e d s t a t i o nc o r r e ct i o n s t h a t w e r e le ss e x t r e m e a n d s h o w e d a m u c h m o r e c o h e r e n t s p at ia l p a t t e r nt h a n a n y o f t h e o t h e r a t te m p t s .

D u r i n g t h e S - w a v e i n v e rs i o n , t h e v e l o ci ty in t h e u p p e r m o s t l a y e r t e n d e d t o w a r dt h a t o f t h e P w a v e . T h i s w a s n o t u n r e a s o n a b l e s in c e th i s l a y e r i s p r i m a r i l y c o m p o s e do f s a t u r a t e d u n c o n s o l i d a t e d s e d i m e n t s , t h e s u r f a c e v e lo c i t y f o r t h e S w a v e is k n o w nt o b e l ow , a n d t h e c o n v e r s i o n c o e f f ic i e n t f r o m S t o P i s s u b s e q u e n t l y h i g h . M o r ei m p o r t a n t l y , o b s e r v a ti o n s f r o m h o r i z o n ta l s e i s m o m e t e r s o p e r a t e d w i t h i n t h e e m -b a y m e n t s h ow t h a t SH a r r iv e s l a t e r t h a n w h a t w o u l d n o r m a l l y b e i d e n t i f ie d a s S o nt h e v e r t i c al c o m p o n e n t ( A n d r e w s a n d M o o n e y , 1 98 3), s u g g es ti n g t h a t t h e la t t e rp h a s e i s S v c o n v e r t i n g t o P . C o n s e q u e n t l y , t h e v e l o c i ty i n t h e u p p e r m o s t l a y e r w a sh e l d f i x e d a t t h e v e l o c i ty p r e v i o u s l y d e t e r m i n e d f r o m t h e P - w a v e i n v e r s io n . T h et o t a l r e d u c t i o n i n v a r i a n c e b e t w e e n t h e f i n a l S - w a v e m o d e l a n d t h e o n e u s e d t oi n i t i a l l y l o c a t e t h e e a r t h q u a k e s w a s 9 8 p e r c e n t , i n d i c a t i n g t h a t t h e h i g h r e s i d u a l sc o m m o n l y o b s e r v e d f o r S a r r iv a l s a r e m o r e l i k e ly a r e s u l t i f i m p r o p e r m o d e l i n gr a t h e r t h a n i n h e r e n t s c a t t e r in t h e S - w a v e d a t a . T h e f a c t t h a t t h e i n v e rs i o n p r o c e ssp r o v e d s t a b le , c o n v e r g e d r ap i d l y , a n d a c h i e v e d a c o n s i d e r a b le r e d u c t i o n i n v a r i a n c ei m p l i e d t h a t t h e S w a v e w a s i n d e e d c o n v e r t i n g t o P a t t h e n e a r - s u r f a c e , a n d t h a tt h e b o u n d a r y f o r c o n v e rs i o n w a s c o n s i s t en t l y t h e b a s e o f th e u n c o n s o l i d a t e ds e d i m e n t s .W e i g h t e d m o v i n g - w i n d o w a v e r a g e s o f t h e f i n a l m o d e l s f o r P a n d S a c c o u n t i n gf o r a v e ra g e s o v e r u p g o i n g r a y s a r e s h o w n i n F i g u r e 1 0 a s t h e f i n e d o t t e d l i n es . T h ec o r r e l a t i o n w i t h e a r l i e r h a l f -s p a c e v a l u e s i s r e a d il y a p p a r e n t . F u r t h e r m o r e , r e l o c a -t i o n o f t h e m i c r o e a r t h q u a k e s u s in g a v a r ia b l e V ; / V s m o d e l r e s u l t e d i n a c l e a r e rh y p o c e n t r a l d i s t r i b u t i o n t h a t s h o w e d g r e a t e r s p a ti a l c o h e r e n c e a n d w a s in b e t t e ra g r e e m e n t w i t h r e g i o n a l g eo l o gy a n d f a u l t p l a n e s o l u t i o n s.Es t ima tes o f general m ater ia l proper ti es . B e s i d e s i m p r o v i n g e a r t h q u a k e l o c a t io n s ,a n a l y s i s o f t h e d a t a f o r b o t h P a n d S v e l o c i t y s t r u c t u r e s a l lo w s f o r t h e c a l c u l a t i o no f r e l a t e d b u l k p r o p e r t i e s a n d e l a s ti c c o n s t a n t s . T a b l e 1 s h o w s c r u s t a l p a r a m e t e r sb a s e d o n t h e v e lo c it ie s d e t e r m i n e d f r o m b o t h t h e N e w M a d r i d i n v e r si o n s t u d y a n dt h e r e f r a c t i o n e x p e r i m e n t , a s w e l l a s d e n s i t y v a l u e s d e r i v e d f r o m a r e g i o n a l g r a v i t yp r o f i l e ( M o o n e y e t a l . 1 98 2). A l t h o u g h n o e s t i m a t e s o f t h e s u r f a c e s h e a r v e l o c i tyw e r e a v ai la b le f r o m t h e e a r t h q u a k e d a t a, a v a l u e o f a b o u t 4 0 0 m / s e c w a s d e t e r m i n e dd u r i n g t h e s e i s m i c r e f r a c t i o n e x p e r i m e n t ( W . M o o n e y , p e r s o n a l c o m m u n i c a t i o n ,1 98 1). T h i s v a l u e , c o m b i n e d w i t h t h e m e a s u r e d P v e l o c it y , i m p l i es t h e u p p e r m o s tl a y e r e x h i b i ts l o w P a n d S v e l o c i ti e s, h i g h P o i s s o n s r a ti o , a n d l o w s h e a r s t r e n g t hc o n s i s t e n t w i t h i ts b u l k c o m p o s i t i o n a s u n c o n s o l i d a t e d s a t u r a t e d s a n d a n d c l a y. O nt h e o t h e r h a n d , l a y e r 2 e x h i b i t s b o t h a h i g h V p/ V s a n d r e l a ti v e l y h i g h P a n d Sv e l o c i t i e s . A h i g h l y f r a c t u r e d c a r b o n a t e s e q u e n c e w o u l d t e n d t o p r o d u c e s u c hc h a r a c t e r i s t ic s . A t e s t w e l l d r i ll e d n e a r t h e p o r t a b l e a r r a y c o n f i r m e d t h a t t h i s l a y e rw a s i n d e e d c o m p o s e d o f c a r b o n a t e s a n d w a s b o t h h i g h l y f r a c tu r e d a n d h i g h l ys a t u r a t e d , w i t h u n u s u a l l y h i g h p o r e p r e s s u r e ( C r o n e a n d R u s s , 1 9 79 ). T h e h i g h p o r ep r e s s u r e s w o u l d re d u c e t h e e f f e ct i v e s t r es s l e v e ls a n d i n s u r e t h a t c r a c k s r e m a i no p e n , a s r e q u i r e d t o p r o d u c e t h e o b s e r v e d h i g h - v e l o c i t y r a ti o .Vp/V~ c a n , t h e r e f o r e , h e l p t o d i s c r i m i n a t e l i t h o l o g i e s ( T a t h a m , 1 9 8 2). F o r i n -s t a n c e , t h e l ow v e l o c i ty f o r t h e P w a v e f o u n d f o r l a y e r 3 c o u ld b e p r o d u c e d b y ah i g h - v e l o c i t y s a t u r a t e d m a t e r i a l w i t h h i g h c r a c k d e n s i t y . H o w e v e r , t h e l ow v e l o c it yc o m b i n e d w i t h t h e o b s e r v e d l o w - v e l o c it y r a t io i m p l i es t h a t t h i s m a t e r i a l i s l i k e ly toh a v e l o w c r a c k d e n s i t i e s , lo w i n t r i n s i c v e l o c i ti e s , a n d p o s s i b l y h i g h e r e f f e c t i v e s t r e s s


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1120 CRAIG NICHOLSON AND DAVID W. SIMPSONl ev e ls . B o t h t h e P - w a v e v e l o c i ty a n d t h e v e l o c i ty r a t io a r e t h u s c o n s i s t e n t w i t h t h i sl a y e r b e in g c o m p o s e d o f e a r l y P a le o z o i c s y n - r i f t s e d i m e n t s ( m a r i n e s h a le s , s i l ts t o n e s ,a n d s a n d s to n e s ) , a s p ro p o s e d b y M o o n e y e t al. (1982).

I t is i n t e r e s t i n g to n o t e t h a t t h e m i c r o e a r t h q u a k e a c t i v i ty is g e n e r a l l y a b s e n t i nt h e u p p e r f e w k i l o m e t e r s . T h e u p p e r c r u s t i s p r e s u m a b l y t o o w e a k ( e . g . , l a y e r 1 )a n d / o r a t i n s u f f i c i e n t s t r e s s l e v e ls ( e.g ., l a y e r 2 ) t o f a i l b y b r i t t l e f r a c t u r e . T h ee a r t h q u a k e s o n l y o c c u r w e l l i n t o t h e l a y e r s e x h i b i t i n g l o w e r V v / V s i n d i c a t i v e o fh i g h e r e f f e c t iv e s tr e s s l e v e ls a n d h i g h e r s t r e n g t h . I n f a c t, t h e e l a st ic m o d u l i o f l a y e r4 , i n w h i c h m o s t o f t h e e a r t h q u a k e s o c c u r , a r e f a ir l y t y p i c a l o f t h o s e a s s u m e d f o rt h e p r o p e r t i e s o f r o c k s a t d e p t h s i n v o l v e d i n t h e b r i t t l e d e f o r m a t i o n a s s o c i a t e d w i t he a r t h q u a k e f a u l t i n g .

DISCUSSIONI n a l l o f t h e c a s e s s t u d i e d , t s / tv i n i t i a l l y d e c r e a s e s w i t h d e p t h . T h i s i m p l i e s a

s i m i l a r v a r i a t io n i n V ; / V ~ a n d r e q u i r e s s e p a r a t e v e l o c i t y s t r u c t u r e s f o r P a n d S i fa c c u r a t e h y p o c e n t e r s a re t o b e a c h i e v e d . S i m i l a r e ff e c t s h a v e b e e n w e l l - d o c u m e n t e de l s e w h e r e , b o t h i n t h e a n a l y s i s o f n e a r - f ie l d s t r o n g m o t i o n r e c o r d s ( A r c h u l e t a , 1 9 82 ;C r a m e r a n d S h a k a l , 1 9 83 ) a n d i n d i r e c t b o r e h o l e m e a s u r e m e n t s o f S a n d P v e l o c it ie s(e .g ., Z o b a c k a n d H i c k m a n , 1 98 2; M o o s a n d Z o b a c k , 1 98 4). I n i t i a l d e c r e a s e s in V JVs w i t h d e p t h h a v e a l s o b e e n o b s e r v e d i n s e i s m i c v e lo c i t y e x p e r i m e n t s c o n d u c t e dt h r o u g h y o u n g o c e a n c r u s t ( B r a t t a n d S o l o m o n , 1 9 84 ). F i g u r e 1 1 is a c o m p i l a t i o nf r o m s e v e r a l s o u r c e s o f i n s it u V J V m e a s u r e d i n d e e p b o r e h o l e s , i n f e r r e d f r o mb o r e h o l e d a t a , o r d e r i v e d f r o m l o n g - b a s e l i n e r e f r a c t i o n e x p e r i m e n t s . I n a l l c a s e s ,V ; / V ~ d e c r e a s e s ra p i d l y i n t h e f i r s t fe w k i l o m e t e r s , m a t c h i n g t h e o b s e r v e d d e c r e a s ein t / tp f r o m m i c r o e a r t h q u a k e t r a v e l t i m e s . T h e h i g h d e g r e e o f c o r r e l a t io n s t r o n g l ys u g g e s ts t h a t t h e s e t e c h n i q u e s a r e d e t e c t i n g t h e s a m e f i r s t - o r d e r sp a t i a l v a r i a t i o ni n v e l o c i ty s tr u c t u r e ; a v a r i a t i o n t h a t r e f l e c ts c e r t a i n c o m m o n m a t e r i a l p r o p e r t i e so f t h e u p p e r c r u s t .

B o t h l a b o r a t o r y a n d t h e o r e t i c a l s t u d i e s p r e d i c t a d e c r e a s e i n V v / V w i t h t h ec lo s in g o f s a t u r a t e d m i c r o c r a c k s u n d e r i n c re a s i n g c o n f i n in g p r e s su r e ( N u r a n dS i m m o n s , 1 96 9; O C o n n e l l a n d B u d i a n s k y , 1 97 4). I n s i t u m e a s u r e m e n t s o f P a n d Sv e l o c i t i e s s h o w t h a t a t l o w e f f e c t iv e s t r e s s l e v e ls , h i g h V v / V r a t io s c o r r e l a t e w i t hh i g h c r a c k d e n s i t i e s ( M o o s a n d Z o b a c k , 19 83 ). T h e m i c r o e a r t h q u a k e d a t a a n d t h ed a t a f r o m b o r e h o l e s tu d i e s t h u s i m p l y t h a t h i g h Vv/V~ h i g h c r a c k d e n s i t i e s , t h ep r e s e n c e o f p o r e f l u id s , a n d l o w e f fe c t i v e s tr e s s l e v e l s a r e a f a i r l y u b i q u i t o u s f e a t u r eo f t h e n e a r - s u r f a c e r e g im e . A m o r e i m p o r t a n t r e s u l t, h o w e v e r , i s t h a t m a n y o f t h e s ep r o p e r t i e s c a n a n d d o e x t e n d t o d e p t h s o f se v e r a l k i l o m e t e r s .

I n v e r s io n o f t h e d a t a f r o m N e w M a d r i d f o r b o t h P - a n d S - w a v e v e lo c it ie sc o n f i r m e d t h e p r e s e n c e o f a l o w - v e l o c i ty zo n e ; a h i g h - v e l o c i ty li d w i t h h i g h V v / V sa n d t h u s h i g h c r a c k d e n s i t y , a n d a b o u n d a r y a t t h e n e a r - s u r f a c e f o r S t o Pc o n v e r s i o n . T h e r e s u l ts f r o m s o u t h e r n C a l i f o r n i a a l so e x h i b i t a l o w - v e lo c i t y z o n e a tm i d - c r u s t a l d e p t h s , o n l y i n t h i s c a s e, t h e l a y e r w a s c o n f i n e d t o a r e g i o n u n d e r t h eS a n B e r n a r d i n o M o u n t a i n s , o r l o c a li z e d t o t h e S a n J a c i n t o f a u l t . C o n v e r s e l y , t h ed a t a f r o m T o k t o g u l s h o w e d n o s t r o n g e v i d e n c e f o r a n y l o w - v e l o c i ty m i d - c r u s t a ll a y e r . T h e s e s i m p l e m e t h o d s t h u s s e r v e a s u s e f u l e x p l o r a t i o n t o o l s i n a r e a s w h e r em i d - c r u s t a l i n f o r m a t i o n i s la c k i n g o r s u r fa c e e x t r a p o l a t i o n s a r e s u s p e c t.I t i s t e m p t i n g t o i n f e r t h a t v a l u e s o f t s / tv f r o m a W a d a t i d i a g r a m re p r e s e n t v a lu e so f Vv/V~. U n f o r t u n a t e l y , t h i s c o r r e s p o n d e n c e is n o t e x a c t a s S a n d P t a k e d i f f e r e n tr a y p a t h s . O t h e r c o m p l i c a t i o n s ar is e w h e n t h e s e c o n d a r y a r r iv a l i s n o t a t ru e Sw a v e, a s is o f t e n t h e c a s e w i t h v e r t ic a l c o m p o n e n t s e is m o g r a p h s m o s t c o m m o n l y


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CHANGES IN Vp/V WITH DEPTH 1121u sed in m ic r o ea r th q u ak e s t u d ie s ( K an asew ich et al. 1973) . S can conver t to P,g iv ing r i se to a f as te r ap pare n t S-wave ve loc i ty and a lower ts/tp. Car e m u s t t h e r e f o reb e t ak en i n t h e i n t e r p r e t a t i o n o f tdt v values. L inear i ty o f the W adat i d iagram doesno t p rec lude var ia t ions in Vp/Vs nor does an unusual ly low value o f tJtp ind ica tethe p resen ce o f a mater ia l a t dep th wi th unusua l p roper t ies . U se o f r eg ionald e t e rm i n a t io n s o f Vv/Vs ( e .g . , Smi th , 1983) may no t then be the mos t appropr ia temea ns o f model ing S waves , s ince they ma y on ly r e f lec t the average Vp/Vs asd e t e r m in ed b y t h e m ean f oca l d ep th o f t h e m o s t ab u n d an t ea r t h q u ak es . Bu t i f

1.4 1.8I I I I P I J I i ~ I

F A / I1 E-- :5 :A :1 / 1 1

/ rI l/Veloc i ty Rat io Vp/ Vs)2 .2 2 .6 3 .0 3 .4 3 .8 4 .2

FIG. 11. Vp/V~ measured in situ from boreholes, inferred from borehole data or derived from refractionstudies: A) Michigan basin Stewart et al. 1981); B) Imp erial Valley, Califor nia Archuleta , 1982); C)Tadjikistan , U.S.S.R. after Kulagin and Nikitina, 1968); D) and E) Tokyo, Japa n area Ohta et al.1980); F) Gulf Cost, Tex as Lash, 1980); G) San Andreas, cent ral Califo rnia Malin et al. 1981).

second ary ar riva l s a re p roper ly mo deled ( as in the case o f the S to P conve r tedwaves) , increased r eso lu t ion and improved ear thquake loca t ions can be ach ieved .

C O N C L U S IO N ST r av e l t im es f r o m m ic r o ea r th q u ak es ex h ib i t ts/t; r a t ios tha t decrease wi th foca ldep th . Th is decrease i s mo s t r ap id in the upp er f ew k i lometer s and m atches s imi lar

var ia t ions observed in situ fo r the ve loc i ty r a tio . S epara te ve loc i ty s t ruc tu res fo r Pand S are therefore nec essary i f accura te and r ea l i s t ic ear th models a re to bedetermined . I f separa te models fo r P and S are used , r e loca ted ear thqua kes a re


18/19

1 1 2 2 CRAIG NICHOLSON AND DAVID W. SIMPSONg e n e r a l ly d e e p e r a r e le s s l ik e l y t o e x h i b i t m o d e l - d e p e n d e n t e r ro r s a n d a l l o w g r e a t e rr e s o l u t i o n o f s h a ll o w s e i s m i c i ty p a t t e r n s .

T h e t e c h n i q u e s o f W a d a t i a n d R i z n i c h e n k o s e rv e a n u m b e r o f u s e s in t h e r o u t i n ep r o c e ss i n g o f m i c r o e a r t h q u a k e s l o ca ti o ns : h y p o c e n t e r s c a n b e c h e c k e d f o r p r o p e rc o n v e r g e n c e ; s i m p l e e a r t h m o d e l s c a n b e d e v e l o p e d ; s p a t i a l v a r i a t i o n s i n t s t p c a nb e m a p p e d ; f o c a l m e c h a n i s m s o l u t i o n s i m p r o v e d ; a n d r e g i o n a l v a r i a t i o n s i n v e lo c i t yc a n b e id e n t if i e d . A l th o u g h t h e i r m o s t c o m m o n a p p l i c a t io n h a s b e e n t o r e s o lv e t h et r a d e - o f f i n o r i g in t i m e a n d f o c a l d e p t h w e fi n d t h e s e m e t h o d s h a v e p r o v e n u s e f u li n a m o r e f u n d a m e n t a l s e n s e o f i d e n t i f y i n g si g n i fi c a n t e a r t h s t r u c t u r e a n d i m p r o v i n go u r g e n e r a l u n d e r s t a n d i n g o f m a t e r i a l p r o p e r t i e s o f t h e u p p e r c r u s t .

ACKNOWLEDGMENTSThis paper originated from conversations with I. L. Nersesov. We thank Lynn Sykes, Chris Scholz,Keith Evans, and Dan Moos for their critical and substantial reviews of the manuscript. Support for theSoviet network was conducted under the auspices of the United States-USSR exchange program on

earthquake predict ion and Contracts USGS 14-08-0001-21278 and 14-08-0001-21869. Data from theCentral Uni ted States was acquired in cooperation with St. Louis Universi ty and the TennesseeEarthquake Information Center under Contract USGS 14-08-0001-16794. We gratefully acknowledgethe generosity of the California Institute of Technology and the U.S. Geological Survey for the use ofthe data from SCARLET.REFERENCES

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Manuscript received 26 September 1984

changes in vpvs with depth-implications for appropriate velocity models improved eq...

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