27, 1980 1981 the 0

UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY

R E V I E W OF EARTHQUAKE ACTIVITY AND CURRENT STATUS OF SEISMIC MONITORING

I N THE REGION OF THE BRADLEY LAKE HYDROELECTRIC PROJECT, SOUTHERN K E N A I PENINSULA, ALASKA:

NOVEMBER 27, 1980 - NOVEMBER 30, 1981

by Chr is topher D. Stephens, John C. Lahr, and John A. Rogers

Submit ted t o U.S. Department of t h e A m y

Alaska D i s t r i c t , Corps o f Engineers Post O f f i c e Box 7002

Anchorage, A1 aska 9951 0

Open-Fi 1 e Report 82-41 7

Th i s r e p o r t i s p r e l i m i n a r y and has not been reviewed f o r c o n f o r m i t y w i t h U. S. Geo log ica l Survey e d i t o r i a l standards and s t r a t i g r a p h i c nomenclature. Any use o f t r a d e names i s f o r d e s c r i p t i v e purposes o n l y and does no t i m p l y endorsement by t h e U. S. Geol og i c a l Survey.

Menlo Park, C a l i f o r n i a 1982

HIGHLIGHTS OF RECENT RESULTS

Over 900 earthquakes were l o c a t e d near t h e southern Kenai Peninsu la d u r i n g

t h e p e r i o d November 27, 1980 - November 30, 1981. O f these earthquakes, 127

were 1 ocated a t depths o f 20 km o r sha l lower and c o n f i r m t h e presence of

a c t i v e f a u l t s w i t h i n t he c r u s t . Al though many o f t he earthquakes occur c l ose

t o t h e p r i n c i p a l mapped f a u l t s and may i n d i c a t e a c t i v i t y on p o r t i o n s o f these

f a u l t s , o t h e r as y e t unmapped and p o s s i b l y b u r i e d f a u l t s must be a c t i v e t o

account f o r t h e observed d i s t r i b u t i o n o f sha l low s e i s m i c i t y . The 1 a rges t

earthquake t h a t occur red i n t h e s t u d y area d u r i n g t h i s t i m e had a

coda-durat ion magnitude 4.8 ( 5.1mb), and was l oca ted a t 106 km depth beneath

western Cook I n l e t . For ty - two events had coda-dura t ion magnitudes o f 3 o r

l a r g e r , b u t a l l of these earthquakes were l oca ted a t depths o f 40 km o r

g rea te r , i n t he B e n i o f f zone beneath t h e southern Kenai, Pen insu la and Cook

I n l e t . The l a r g e s t sha l low earthquake w i t h i n 25 km o f Brad ley Lake had a

coda-durat ion magnitude o f 2.1.

There i s no s t r o n g c o r r e l a t i o n of t h e sha l low s e i s m i c i t y w i t h mapped f a u l t

t races . Focal mechanisms determined f o r s e l e c t e d shal low earthquakes near

Brad ley Lake a re compatable w i t h p redominan t l y normal f a u l t i n g c o n t r o l l e d by

east-west t o southeast -nor thwest o r i e n t e d t ens ion .

A s t rong-mot ion acce lerograph was i n s t a l l e d near t h e main Brad ley Lake

h i gh -ga in se ismic s t a t i o n , b u t t o da te has n o t been t r i g g e r e d by an

earthquake.

INTRODUCTION

The A1 aska D i s t r i c t , Corps o f Engineers p lans t o c o n s t r u c t a h y d r o e l e c t r i c

f a c i l i t y on t h e sou thern Kenai Peninsu la , Alaska. The p r o j e c t i n v o l v e s

damming Brad ley Lake, which i s l o c a t e d i n t h e Kenai Mountains a t an e l e v a t i o n

o f 1,090 f e e t , and f e e d i n g t h e water th rough a tunne l t o a power p l a n t a t sea

l e v e l . I n t h i s r e g i o n o f t e c t o n i c i n t e r a c t i o n between t h e P a c i f i c and N o r t h

h e r i c a n p l a t e s ( F i g u r e I ) , t h e p o t e n t i a l f o r s t r o n g earthquakes needs t o be

addressed so t h a t t h e hazards t h e y c o u l d pose can be minimized, The most

s e r i o u s e f f e c t o f earthquakes on man-made s t r u c t u r e s i s s t r u c t u r a l damage due

t o s t r o n g shaking. Other p o t e n t i a l l y damaging aspects o f earthquakes i n c l u d e

sur face f a u l t i n g as we1 1 as shak ing - i nduced e f f e c t s such as l i q u e f a c t i o n ,

1 andsl i d e s , d i f f e r e n t i a l s e t t l i n g , and se iches.

The Corps o f Engineers has asked t h e U.S. Geo log ica l Survey, O f f i c e of

Earthquake S tud ies (USGS-OES) t o i n v e s t i g a t e t h e problem o f se ismic hazards i n

t h e B r a d l e y Lake r e g i o n . T h i s e n t a i l s c o l l e c t i n g and a n a l y z i n g ear thquake

d a t a i n t h e r e g i o n of t h e proposed Brad ley Lake H y d r o e l e c t r i c P r o j e c t i n o r d e r

t o develop a more d e t a i l e d model f o r t h e t e c t o n i c framework. P a r t i c u l a r

emphasis i s be ing p laced on t h e d i s t r i b u t i o n o f sha l low c r u s t a l earthquakes

and t h e i r re1 a t i onsh i p t o mapped o r i n f e r r e d f a u l t s .

The purpose o f t h i s r e p o r t i s t o summarize t h e work completed t o date,

i n c l u d i n g t h e o p e r a t i o n o f t h e se ismic s t a t i o n s and a r e v i e w o f t h e se ismic

da ta c o l l e c t e d between November 27, 1980 (when t h e Brad ley Lake network was

i n s t a l l e d ) and November 30, 1981. P a r t o f t h e d a t a presented i n t h i s r e p o r t

was d iscussed i n an e a r l i e r r e p o r t ( L a h r and Stephens, 1981 ) a long w i t h o t h e r

a v a i l a b l e d a t a f r o m t h e USGS-OES network t h a t has been o p e r a t i n g i n sou thern

A laska s i n c e 1971. The d a t a p resen ted i n t h i s r e p o r t g e n e r a l l y conform t o t h e

F i g u r e 1. Upper- Curren N o r t h American p l a t e , and 61°w. R o t a t i o n o f p l a t e about t h i s p o l e i E p i c e n t e r s o f t h e 1958, Lower- Enlargement o f - s e t t i n g o f B r a d l e y Lake vo lcanoes are i n d i c a t e d

~t m o t i o n of P a c i f i c p l a t e w i t h r e s p e c t t o P r o j e c t i o n i s o b l i q u e M e r c a t o r u s i n g a p o l e a t t h e P a c i f i c p l a t e w i t h respect t o t h e Not t h Am€ s e q u i v a l e n t t o v e r t i c a l t r a n s l a t i o n i n t h i s f i

1964, and 1979 ear thquakes a r e shown. t h e area o u t l i n e d i n upper f i g u r e , showing t h e . The l o c a t i o n s o f Spurr , Redoubt, and I l i a m n a . M o d i f i e d f r om Woodward-Clyde C o n s u l t a n t s ( 19

54'~ ! r i c a n gu re .

e s s e n t i a1 f e a t u r e s of t h e d i s t r i b u t i o n of ea r thquake hypocen te rs and

magni tudes d e s c r i b e d i n t h e e a r l i e r r e p o r t . The g r e a t e r q u a n t i t y o f

h igh-qua1 i t y d a t a now a v a i l a b l e a1 lows us t o make more r e 1 i a b l e e s t i m a t e s o f

such parameters as t h e depth t o t h e Benio f f zone beneath B r a d l e y Lake and t h e

l o c a t i o n o f areas t h a t a r e c u r r e n t l y e x p e r i e n c i n g h i g h r a t e s o f s h a l l o w

s e i s m i c i t y . I n a d d i t i o n , new d a t a a re p resen ted about t h e s t y l e of f a u l t i n g

and c u r r e n t s t a t e of s t r e s s i n t h e c r u s t as i n f e r r e d f r o m f o c a l mechanisms

determined from P-wave f i r s t - m o t i o n s .

SEISMOTECTONIC FRAMEWORK : CURRENT STATE OF KNOWLEDGE

The B r a d l e y Lake r e g i o n i s l o c a t e d i n t h e t e c t o n i c zone o f i n t e r a c t i o n

between t h e N o r t h American p l a t e and t h e r e 1 a t i v e l y northwestward-moving

P a c i f i c p l a t e ( F i g u r e 1) . The average r a t e o f convergence n e a r t h e s o u t h e r n

Kenai P e n i n s u l a ove r t h e p a s t 3 m.y. i s 6.5 cm ly r ( M i n s t e r and Jordan, 1978).

D i r e c t ev idence f o r c o n t i n u e d convergent mo t ion comes f r o m s t u d i e s o f r e c e n t

1 arge earthquakes a long p o r t i o n s o f t h e P a c i f i c - N o r t h American p l a t e boundary

a d j a c e n t t o t h e Gul f of Alaska. For example, t h e 1964 A laska ear thquake

r e s u l t e d from d i p s l i p m o t i o n of about 12 m ( H a s t i e and Savage, 1970) on t h e

p o r t i o n of t h e A l e u t i a n megathrust e x t e n d i n g f r o m P r i n c e W i l l i a m Sound t o

s o u t h e r n Kodiak I s l a n d and d i p p i n g no r thwes tward beneath t h e c o n t i n e n t .

The s e i s m i c i t y a s s o c i a t e d w i t h t h e processes of convergent p l a t e mot ion i n

A laska may g e n e r a l l y be d i v i d e d i n t o f i v e s p a t i a l l y d i s t i n c t groups:

1. Earthquakes which occur on t h e g e n t l y d i p p i n g A l e u t i a n mega th rus t

( t h e i n t e r f a c e zone between t h e P a c i f i c and N o r t h American P l a t e s ) ;

2. Earthquakes which occur i n t h e wedge of c r u s t above t h e a c t i v e

megathrus t zone;

3. Earthquakes which occur w i t h i n t h a t p o r t i o n o f t h e P a c i f i c p l a t e

which has been t h r u s t beneath Alaska ( B e n i o f f zone events) ;

4. Earthquakes w i t h i n t h e P a c i f i c p l a t e seaward o f t h e A l e u t i a n

megathrust ;

5. Shal low earthquakes near t h e a c t i v e volcanoes.

The Brad ley Lake r e g i o n i s most d i r e c t l y a f f e c t e d by t he f i r s t t h r e e types of

events .

INSTRUMENTATION

Dur ing most o f t h e pas t year t h e h i gh -ga in se ismic s t a t i o n s a t t he f i v e

B rad ley Lake s i t e s ( F i g u r e 2 ) operated s a t i s f a c t o r i l y . Several o f t h e

s t a t i o n s were n o t o p e r a t i n g f o r severa l weeks between l a t e A p r i l and e a r l y

J u l y 1981, as a r e s u l t o f unexpected env i ronmenta l problems. An unusual i c e

problem and f l o o d i n g almost dest royed t h e main Brad ley Lake s i t e (BRLK). The

i c e snapped t h e c o a x i a l cab les i n s i d e o f t h e antenna masts and t h e h i g h water

d ischarged t h e b a t t e r i e s . The i n s t r u m e n t a t i o n was moved t o t he t o p of a sma l l

k n o l l t o p rov i de f o r b e t t e r drainage. To inc rease r e l i a b i l i t y , t h e r a d i o

r epea te r f o r BRNW, BRNE, and BRSE was separated f rom t h e l o c a l three-component

seismograph. Th i s i nvo l ved i n s t a l l i n g another antennalmast as w e l l as b a t t e r y

c u l v e r t . I n a d d i t i o n , a l l mast guys were anchored t o t h e rocks us i ng a rock

d r i l l and concrete anchors.

BRSE and BRSW were i n good c o n d i t i o n . A t BRNW t h e b a t t e r i e s drowned; new

b a t t e r i e s were i n s t a l l e d on a r a i s e d base. A t BRNE t h e l i d had blown o f f t h e

c u l v e r t and was l o s t i n 2 f e e t o f snow. The e l e c t r o n i c s were undamaged and a

new l i d was p u t on t h e c u l v e r t .

On a l a t e r v i s i t an SMAl acce lerograph was i n s t a l l e d a t t h e main Brad ley

Figure 2 Map of t h e study area in t h e Bradley Lake region. Symbols represent seismograph stations funded by t h e U.S. Geological Survey (diamonds), the Corps of Engineers (circles), and the University of Alaska (triangles). Heavy sol id 1 ines indicate principle faults, after Beikrnan (1980), Plafker (1969), Tysdal and Case (1979)~ and Plafker and others (1977). Locat ion of Bradley Lake i s indicated by small oval next to station BRLK near center of map.

Lake s i t e . Our s tandard f r e e - f i e l d i n s t a l l a t i o n was used except t h a t a new

charge r e g u l a t o r c i r c u i t was added t o t r i c k l e charge t h e SMAl b a t t e r i e s f r om

t h e h i gh -capac i t y a i r c e l l b a t t e r i e s used t o power t h e h i gh -ga in seismograph.

Thus t h e need t o change t h e S M A l b a t t e r i e s has been reduced f r om every year t o

every 5 years .

A t Diamond Ridge t h e RF-2 r e c e i v e r was re tuned and a pre-amp1 i f i e r

i n s t a l l e d . An A l V C O was a l s o i n s t a l l e d a t t h e l o c a l Homer s t a t i o n , HOM

( F i g u r e Z ) , operated by t h e U n i v e r s i t y of Alaska, so t h a t t h e da ta cou ld be

recorded on the USGS Develocorder f i l m s i n Palmer.

DATA PROCESSING TECHNIQUES

The da ta recorded f rom USGS-OES se ismic s t a t i o n s i n the Brad ley Lake

reg ion a re ma i led weekly f r om Palmer, Alaska, t o Menlo Park , C a l i f o r n i a , where

t h e y are processed us i ng t h e f o l l o w i n g m u l t i - s t e p r o u t i n e :

1. P r e l i m i n a r y scanning: The paper records o f BRLK and BRSW a r e scanned

t o i d e n t i f y and no te t imes o f se ismic events w i t h i n t he Brad ley Lake

network and t h e sur round ing area.

2. F i n a l scanning: The events no ted i n p r e l i m i n a r y scanning are found on

t h e Develocorder f i l m r e c o r d and any event w i t h a P- t o S-phase t ime

i n t e r v a l o f l e ss than o r equal t o 10 seconds a t one of t he Brad ley

Lake s t a t i o n s i s noted f o r subsequent t im ing .

3. T iming: For each of t h e i d e n t i f i e d events t h a t has been recorded on

t h e 16-mm f i l m s a t four o r more s t a t i o n s i n t h e Brad ley Lake r e g i o n

t h e f o l l o w i n g da ta a re read f r om each s t a t i o n : P- and 5-phase a r r i v a l

t imes; d i r e c t i o n of f i r s t mot ion o f t h e P wave; d u r a t i o n o f s i g n a l i n

excess of 1 cen t ime te r t h r e s h o l d ampl i tude; and p e r i o d and amp l i tude

o f maximum recorded s i g n a l .

4. I n i t i a l computer process ing: The da ta read f r om t h e f i l m s a re ba t ch

processed by computer us i ng t h e program HYPOELLIPSE (Lahr , 1980) t o

o b t a i n t h e o r i g i n t ime, hypocenter, magnitude, and f i r s t - m o t i o n p l o t

f o r each earthquake.

5. Review of i n i t i a l computer r e s u l t s : Each hypocent ra l s o l u t i o n i s

checked f o r l a r g e t r a v e l t i m e r e s i d u a l s and f o r a poor s p a t i a l

d i s t r i b u t i o n o f s t a t i o n s . A r r i v a l t imes t h a t produce l a r g e r e s i d u a l s

a re re- read, For shocks w i t h a poor azimuthal d i s t r i b u t i o n of

s t a t i o n s , read ings f rom a d d i t i o n a l s t a t i o n s are sought.

6. F i n a l computer p rocess ing : The da ta f o r those events w i t h poor

hypocent ra l s o l u t i o n s a re r e r u n w i t h c o r r e c t i o n s and t h e new s o l u t i o n s

are checked once again f o r l a r g e r e s i d u a l s t h a t m igh t be due t o

rema in ing e r r o r s .

ANALYS I S OF HYPOCENTER QUALITY

Two types o f e r r o r s en te r i n t o t he de te rm ina t i on o f hypocenters :

sys temat i c e r r o r s l i m i t i n g t h e accuracy and random e r r o r s l i m i t i n g t h e

p r e c i s i o n . Systemat ic e r r o r s a r i s e p r i n c i p a l l y f rom i n c o r r e c t model ing of t h e

se ismic v e l o c i t y w i t h i n t h e ea r t h . Random e r r o r s r e s u l t f rom e f f e c t s such as

random t i m i n g e r r o r s , and t h e i r e f f e c t on each earthquake i s es t imated through

t h e use o f s tandard s t a t i s t i ca1 techniques.

The magnitude of t h e sys temat i c e r r o r s can be g r e a t l y reduced by c l ose

spac ing o f seismographic s t a t i o n s w i t h i n t h e area of i n t e r e s t , as t h e

hypocen t ra l s o l u t i o n i n t h i s s i t u a t i o n i s much l e s s s e n s i t i v e t o t h e v e l o c i t y

model assumed f o r t h e ea r t h . For t h i s reason, t h e earthquakes loca ted i n t h e

Bradley Lake r e g i o n s i nce t h e i n s t a l l a t i o n o f t h e a d d i t i o n a l f i v e s t a t i o n s i n

l a t e 1980 a re expected t o have sma l l e r systemat ic o f f s e t s than those l oca ted

e a r l i e r w i t h t h e l e s s dense r e g i o n a l network.

For each earthquake t h e l eng ths and o r i e n t a t i o n s o f t h e p r i n c i p a l axes o f

t h e j o i n t con f idence e l l i p s o i d are ca l cu l a ted . The one-s tandard -dev ia t ion

con f idence e l l i p s o i d descr ibes t h e r e g i o n o f space w i t h i n which one i s

68 percent c o n f i d e n t t h a t t h e hypocenter l i e s , assuming t h a t t h e o n l y source

o f e r r o r i s t h e es t imated random read ing e r r o r . The s i z e and o r i e n t a t i o n o f

t h e e l l i p s o i d i s a f u n c t i o n of t h e s t a t i o n geometry. For earthquakes w i t h i n

t h e network, t h e l eng ths of t h e e l l i p s o i d axes a re g e n e r a l l y on t h e o rder o f 2

t o 3 km.

To f u l l y eva lua te t h e q u a l i t y o f a hypocenter, bo th t h e s i z e and

o r i e n t a t i o n of t h e conf idence e l l i p s o i d , t h e r o o t mean square (RMS) r e s i d u a l

f o r t h e s o l u t i o n , and t h e s t a t i o n geometry must be considered.

RESULTS

The ep i cen te r s o f 907 earthquakes t h a t occur red between November 27, 1980,

and November 30, 1981, and were l o c a t e d near t h e southern Kenai Pen insu la are

shown i n F i gu re 3. Most of these events occurred w i t h i n t h e northwestward-

d i pp ing B e n i o f f zone t h a t u n d e r l i e s t h e southern Kenai Pen insu la and Cook

I n l e t (F i gu re 4 ) . The l a r g e s t ear thquake t h a t occurred d u r i n g t h i s t ime

p e r i o d was an event of coda-durat ion magnitude 4.8 (5 .1mb) which was l oca ted

a t a depth of 106 km near t h e nor thwes t p a r t of t h e s t udy area nea r I l i a m n a

volcano. For ty - two earthquakes had coda-durat ion magnitudes o f 3 or l a r g e r

(Tab le 1 and F i g u r e 5 ) , b u t a l l o f these events had computed depths o f 40 km

o r more which would p lace them i n t h e Beniof f zone. E i g h t o f t h e l a r g e s t

earthquakes were r e p o r t e d i n t h e P r e l i m i n a r y De te rmina t ion of Ep icen te rs as

e- .- C m V , . N L a, U E a - K A a, +

0 0 L C .-a no 2 a, ro E m b m u C 7- w Al .- W m L L + J hi La, rU

W O L C c c u m a - - I .- 3 m a , r a, W T >

r u a m a ~ 2 2 ," .-L -0 0 I% E .- *- In - Y W -

0 c o I . m m w !n m - E 0 - 0 003 > I . - - c m m o -

7 m m m n n w E a, - c - u m u o m a, m m m - L > U - m u 9 0 L U C - a, a, m - 0

u . - 7 a, I n E m L 3 - a , a , u c r m m A - G U m _ o a .- .- =I& C E T - 0 0- . - A @ C

IW a .- +J c m o a L m f u T * \ m V, m v E -- fll .. -

F i g u r e 4 Hypocenters of F i g u r e 3 p r o j e c t e d o n t o v e r t i c a l p l a n e a long A - A ' . P l o t t i n g symbol i s p r o p o r t i o n a l t o magni tude as i n d i c a t e d i n F i g u r e 3. E x t e n t o f B r a d l e y Lake ne twork i s shown a t t o p o f f i g u r e . Nor thwestward d i p p i n g zone o f s e i s m i c i t y be low 3 5 krn depth i s t h e B e n i o f f zone, w h i c h i s i n f e r r e d t o l i e w i t h i n subducted P a c i f i c p l a t e . S o l i d l i n e a t 3 5 km dep th be low B r a d l e y Lake n e t w o r k i n d i c a t e s t o p o f S e n i o f f zone.

TABLE 1

F A L T t - C J A K F r , :IT C:i l3A-OUFAT IOCJ M A G N I T U D F 3 AND L A R G C K N E A P SOOTHEUN K F N A I P E N I V S U L A NGV'M;3EH 7 7 . 1 9 8 0 - NOVEMBI I k 30. 1 9 6 1

C L I u I N T l 4 ' L A T h L O N G b DEPTH MAG M 3 NU N S G 4 P D 7 R M 5 E R H 1 4 3 3 Hk UP. SiC at(, F I N D E G M I N K M DtG K M S E C ICY

N!,V 2 7 lE 51 54.6 b O 6.5 1 5 2 5 1 . 6 l l R . 7 3.3 1 8 5 8 5 R 3 O . 5 Y 3.1 2u 1 7 44 4 . 2 6 0 1 7 . 1 I S 2 1 3 . 3 €44.5 3.6 4.6 28 5 55 5 b 0.4b 1.1

3 C C I 1 2 7 1 1 3.7 6 0 2.8 1 5 2 3V.9 9 1 . 1 3.8 24 3 7 4 76 0.38 1.5 F E L T i 1 1 4 T K F N A I hND S U L D C T N A

2t 1 7 2 44.8 6 0 4 . 7 1 5 2 55.6 1 1 6 . 6 3.3 2 4 9 7 3 f R 0 . 6 9 1.3 1 9 s l

J A N 7 0 1 9 47.6 6 0 9.5 1 5 2 5 7 . 4 2 3 . 3 R 2 209 126 0 . 2 0 3 . 4 2 3 1: 39 42.2 519 5 7 . 7 152 53.6 1 1 2 . 0 3 . 6 2 4 5 5 9 74 0 .50 1.4 2t 1 2 4 6 0 0.5 1 5 3 75.0 4 8 . 1 3.2 4.0 35 8 99 5 1 0 .54 1.0

F F t l 2' I 3 1 H.R b 9 1 4 . 5 1 5 0 2 7 . 6 40 .2 3.8 4.H 32 1 Q 5 55 O.4B 1 .4 F E L T I b AT K E N A I v HOMFU* MOCSF P A S S * AND SEWAQD

W A F - t L 4 A 33.5 h O 5 . 2 15.2 22.3 7 9 . 1 3 .1 4.6 ? 5 7 5 5 7 2 0 . 3 1 1.3 A D C 2 2 3 1 a e . 3 b o 21.0 152 r7.9 1 1 8 . 1 3.7 25 3 8 1 9 3 0 . 2 6 1.8

24 7 52 55.5 5 9 2C.3 1 5 1 51.9 4 9 . 4 3.4 4 . 0 2 3 6 134 95 0.38 1.8 Ft.LT 4 7 HiJHER

7 1 2 4 45.7 6 0 28.3 152 49.G 1 2 7 - 4 3.2 4 . 0 2 2 R 6 7 89 0.39 1.6 Y A Y 1 5 1 4 7 2b.l b O 0 . 1 1 5 3 32.6 3 O . R 3 . 0 2 3 6 1 1 3 6 1 0 . 5 5 1 .6

1 7 P 5 1 9. 7 5 0 2 J . l 1 5 2 2 7 . 7 9 7 . 8 3.0 23 5 7 9 1 0 8 O . & b 1 .B 2 1 20 2q 3 0 . 3 59 4 6 . 1 1 5 2 56.5 1 0 ? . 9 4.1 4 6 7 0 1 1 4 2 95 0 .30 2.9

F E L T O F K E N A I P E Y I h S U L A A N D A T ANCHORAGE 2 7 F 1 9 2 e 1 s 9 3 2 1 2 4 4

JUL 1 7 7 7

2 1 3 1 7 AUG 1 1 42

5 1 C 2 1 7 1 1 3 1

1 1 7 1 2 f 2 4 1 3

5 t F 7 c 57 G I H 3

2 7 1: I > O C T 3 1 3

1 1 4 21 I3 2' 1' 1 4 1 1 3 lr; - 1 1 - L C 1" 21 1 t' I T 3t,

ERZ 0 KM

2.8 h 2 -8 H 3.5 8

1.8 A

2.6 d 3.2 9 2.6 H 2-8 9

2.0 A 3 .8 6 3.3 a

3.3 n 4.7 4 3.4 9 4.3 kl

53 5b.3 152 54.4 9 8 . 8 3.1 23 11 8 3 76 0 . 3 4 1 .2 1.b A 59 2 4 . 7 1 5 1 5P.b 5 3 . 1 .4* 1 2 5 5 1 1 6 66 0.38 1 . 7 2-0 A

F E L T 1 1 1 4 T HOMER PND S E L D O V I A 59 17 .0 152 18.1 74.2 3.5 4.5 28 7 1 2 1 96 0 . 2 4 1.5 3.1 H h 3 3.5 152 57.4 106.3 4.e 5 .1 2 8 2 1 1 3 t a 0.39 1 . 0 4 . 1 o

F F L T I V I N AYCHO"AGF-HOMFP AOEA

. . . - - F E L T I r A T noucu

b O 15 .0 1 5 1 32.7 65.2 4.3 4 - 7 3 5 2 54 5 7 0 . 4 8 1. 1 2.7 d FELT I V AT K E Y A ! . HGUER. N I N I L C U ~ K I S L O O T N A . C L A 4 GULCH. CaOpFR L A N D I N G * TYO"lErC. S T E R L I N G . R A S I L O F * A N D GIR3WOL)D; ALSO FELT I 1 I A f ANCYO2AGE.

C F f G I N T I M E : U Y I V E R S f i L T I M E ( U f ) . T O CONVERT TO A L A S K A S T A N D A R D T I M E S U B T R A C T 10 HR. M A C . C3*>4-O3R h f 1Clt.I M A C N I T U D F YE: t C D Y l A V t Y A G Y I T U C E L I S T E D I N THE P R E L l M f N 4 R 7 D E T E R M I N A T I O N O F E P l C E N T E U S NP: hUMt3ch OF P - W A V k A k R f V A L S U S E C I N L O C A T I N G T H F E A U T H O U A K E NS: ~ L M Y E R rn s-WAVE LRRIVALS USEC IY LOCATING THE EARTHOUAKF G A P : L A s L E S T A Z I U Y T t I A L S t P A q A T I U h I r D f GHEFS H E T W E F N S T A T I O N S 03: E P f C E N T Y A L D I S T A Y C F . I N K I L C M E T E H S T U T H E T H I M D C L O S E S T S T A T I O N b W S : F 0 3 T - M E A Y - S 9 U A R E I N SECONDS C F T R A V E L T I Y E R E S I D U A L S E R H : L A R G F S T H O i i l Z O N T Y D E V I A T I O N F R O M T H E H V P O C E N T E R r ITHIN THE O N E - S T A N O A R > - D E V I A T I O r j

COUF IDErJCE E L L I P C 0 1 7 . T H I S Q U A N T I T Y IS A MEASURE OF THE E P I C E N T R 4 L ORECIS I O N * € H Z : L A R , ~ S ~ V E P T I C A L D E V l A T t O N F C G U T H F W P O C E N T E k * I T l i f N THE O N E - S f A N D A R D - D E V I A T I O N

C L W I D E ' 4 C E F L L I P S O l l m T H I S O U 4 N T I T Y I S A MEASURE OF T H E D E P T H PRECI S t O V . 9: a u b L l r Y OF T H = HYPCCEUTFR. THIS LNOEX IS A MFIASWE OF THE P R E C I S I O N U F THF

I - Y g O C F k T E h 4N3 I S C A L C U L A T E D FROM ERb AND E U X AS FOLLOWS:

E R H

F t C R OF T E P H I Y A T I O N S O F T H E O A R A M T E R E S M A Y R E S U T FROM RANDOM ERRORS PRESFlvT f N T H E PhASE D A T A . 09 F R O Y S Y S T E M A T I C ERQORS I h f R O O U C E O E I T H E R B Y THE V F L O C I T Y M O D E L S U S E D 3 K A Y THF L E L P T I V F P O S I T I O N S O F THE TRUE H Y F O C E N T E R A N D T H E S T A T I O N S USED I N THE S O L U T I O N . ONE SHUbLO BE P A M T I C U L A R L Y C A U T I O U S U S I N G S O L U T I D N S T H A T H A V E G A P S 1 8 0 DEGREES. NP 5 . h S = 01 03 * 75 KM. R V S b 0 . 7 5 s ERH ' 5 K Y , OH ERZ 1 0 KM. S O L U T I O N S W I T H A A N D B Q U A L I T Y A R E G E N E R A L L Y MOQE R E L I A B L E * B U T T H I S DOES NUT G U A R A N T E E T H A T THE ACCURACY C F THE S O L U T I O N S IS m I T H I N T H E L f W I T S I M P L I E D BY ERH A N 0 E R Z -

FELT R E P O P T S A R E T A K E h FROM THE P R E L I M I N A F V D E T E R M I N A T I O h OF E P I C E N T E R S . A COPY OF I h T E h S l T Y S C A L E C W R E h T L Y I N USE B Y N E l S I S L I S T E D I N T H E A P P E N O I X .

F i g u r e 5 Map o f e p i c e n t e r s f o r 42 ear thquakes from F i g u r e 3 w i t h c o d a - d u r a t i o n magni tude 3 or l a r g e r . Hypocenter parameters o f t h e s e even ts a r e l i s t e d i n Tab le 1 . Note t h a t s h a l l o w e s t even t o c c u r r e d a t 40 km dep th . P l o t t i n g symbols a r e d e s c r i b e d i n F i g u r e 3 . Heavy s o l i d l i n e s co r respond t o mapped f a u l t t r a c e s i d e n t i f i e d i n F i g u r e 2 . Locat i o n o f B r a d l e y Lake i s i n d i c a t e d by s o l i d o v a l a t c e n t e r o f map.

be ing f e l t w i t h maximum i n t e n s i t i e s r ang ing f r om about I 1 t o I V (Tab le 1 and

Appendix) i n t he Kenai-Anchorage area.

Other f ea tu res o f t h e s e i s m i c i t y t h a t were noted i n an e a r l i e r r e p o r t

(Lahr and Stephens, 1981 ) and which can be observed i n t h e da ta presented here

a r e t h a t t h e B e n i o f f zone s e i s m i c i t y d i es ou t near Brad ley Lake (near 115 km

a long t h e s e c t i o n i n F i gu re 4), t h a t t h e sha l low c r u s t a l a c t i v i t y i s

concent ra ted eas t o f Cook I n l e t , and t h a t few events were l oca ted a long t h e

A l e u t i a n megathrust .

B e n i o f f zone earthquakes a re g e n e r a l l y assumed t o occur w i t h i n a subducted

p l a t e and near i t s upper sur face. A t sha l low depths t h e upper l i m i t of t he

B e n i o f f zone a c t i v i t y w i l l be near and p o s s i b l y co i nc i de w i t h t h e zone o f

t h r u s t con tac t between t h e p l a t e s . Thus t h e maximum depth t o t h e t h r u s t zone

beneath Brad ley Lake i s 35 - + 5 km based on t h e abrupt inc rease i n t h e r a t e of

a c t i v i t y a t t h i s depth ( F i g u r e 4) .

Dur ing t h e t ime p e r i o d o f t h i s study, f i v e earthquakes t h a t occur red

beneath t he Brad ley Lake s t a t i o n a r r a y were l o c a t e d a t depths between 20 and

30 km. The n a t u r e o f t h i s in te rmed ia te -dep th se ismic a c t i v i t y i s unce r t a i n ;

i t may i n d i c a t e t h a t some f a u l t s i n t h e upper c r u s t ex tend down i n t o t h e lower

c r u s t , o r a l t e r n a t i v e l y i t may be assoc ia ted w i t h t h e zone o f t h r u s t con tac t

between t h e two converg ing p l a t e s . F u r t h e r work, i n c l u d i n g a d d i t i o n a l data,

i s needed t o h e l p r e s o l v e t h i s ques t ion .

PATTERN OF SHALLOW EARTHQUAKES

Shal low (dep th l ess than 20 km) earthquakes (F i gu re 6) occur w i t h i n t h e

c r u s t and c o n f i r m t h e presence o f a c t i v e f a u l t s w i t h i n t h e c r u s t . O f t h e

l oca ted sha l low earthquakes, t h e s m a l l e s t had coda-durat ion magnitudes of 0.3,

and t h e two l a r g e s t had coda-dura t ion magnitudes o f 2.8. The l a r g e s t sha l low

F i g u r e 6 Map o f e p i c e n t e r s o f 127 s h a l l o w ( d e p t h <20km) ear thquakes t h a t o c c u r r e d near t h e s o u t h e r n Kenai P e n i n s u l a between November 2 7 , 1980, and November 30, 1981. Smal l s o l i d c i r c l es i n d i c a t e s e l e c t e d even ts f o r w h i c h foca l mechanisms from F i g u r e s 7 A and 7B a r e shown. Event numbers co r respond t o t h o s e i n F i g u r e 7 and T a b l e 2. Foca l mechanisms a r e lower hemisphere p r o j e c t i o n s w i t h compress iona l quadran ts shaded. Heavy s o l i d l i n e s co r respond t o mapped f a u l t t r a c e s i d e n t i f i e d i n F i g u r e 2.

TABLE 2

SELECTED SHALLOW EARTHQUAKES FOR WHICH FOCAL MECHANISMS HAVE BEEN DETERMINED

ORIGIN T I M E , UT NUMBER W

1 81 02 0 8 11 43 35.9

2 81 02 17 00 26 32.7

3 81 06 1 0 13 40 15.5

4 81 10 22 04 15 14.7

5 81 02 02 04 38 37.4

6 81 05 29 04 10 48.6

7 81 09 04 16 2 0 02.6

8 81 11 24 14 40 04.5

LAT, , N E r n

LONG., W E r n

151 0.83

151 0.77

151 4.56

150 38.75

149 9.4

149 18.58

DEPTH -KM

CODA MAG

earthquake t h a t occur red w i t h i n 25 km e p i c e n t r a l d i s t ance from Brad ley Lake

was an event o f coda-durat ion magnitude 2.1.

Most of t h e c r u s t a l a c t i v i t y occur red beneath t h e Kenai Pen insu la and

southeast o f t h e Border Ranges f a u l t . I t should be noted, however, t h a t t h e

magnitude d e t e c t i o n t h r e s h o l d f o r l o c a t i n g events i s no t un i form across t h e

s tudy area, which p a r t i a l l y accounts f o r t h e apparent concen t ra t i on o f

s e i s m i c i t y near t h e Brad ley Lake s t a t i o n s . I n general t h e r e i s no t a s t r o n g

c o r r e l a t i o n o f t h e earthquakes w i t h t h e t r aces of t he mapped f a u l t s , b u t t he

e p i c e n t e r s do t end t o c l u s t e r s p a t i a l l y . Some of t h e more prominent c l u s t e r s

are: one t h a t s t r a d d l e s t h e p o r t i o n o f an unnamed f a u l t t h a t runs a long t h e

south o f Kachemak Bay; two t h a t occur a few k i l o m e t e r s west o f t h e Eagle R i v e r

f a u l t , about 15 km n o r t h and 20 km sou th of B rad ley Lake; a more d i f f u s e

c l u s t e r t h a t occurs eas t o f t h e Eagle R i v e r f a u l t about, 30 km no r t heas t o f

Brad ley Lake; and one near t h e southern end of t h e P lace r R i v e r f a u l t and

Kenai l ineament . It i s i n t e r e s t i n g t o no te t h a t numerous earthquakes a re

l o c a t e d o n l y a few k i l o m e t e r s west of t he Eagle R i ve r f a u l t f rom about 15 km

no r t heas t o f Brad ley Lake t o t h e p o i n t where t h e f a u l t i s t r unca ted by t h e

lower map boundary. Th i s s e i s m i c i t y p a t t e r n f o l l ows a major d e v i a t i o n of t h e

f a u l t a long i t s leng th . One p o s s i b l e exp lana t i on f o r t h i s p a t t e r n i s t h a t t h e

earthquakes a re o c c u r r i n g a long t h e down-dip ex tens ion o f t he Eagle R i v e r

f a u l t . Whether o r no t t h i s c o r r e l a t i o n i s s i g n i f i c a n t w i l l be i n v e s t i g a t e d

f u r t h e r as a d d i t i o n a l da ta become avai 1 able.

As d iscussed p r e v i o u s l y b y Lahr and Stephens (1981), t h e c u r r e n t p a t t e r n

of s e i s m i c i t y may be s t r o n g l y i n f l uenced by t h e s t r e s s r e d i s t r i b u t i o n

f o l l o w i n g t h e 1964 earthquake, and t h i s p a t t e r n may change s l o w l y over tens of

years as s t r esses b u i l d up p r i o r t o another l a r g e earthquake.

FOCAL ME CHAN ISMS

P r e l i m i n a r y f o c a l mechanisms were determined f r om P-wave f i r s t mot ions o f

se lec ted , we1 1 - recorded s h a l l ow earthquakes throughout t h e southern Kenai

Peninsu la (F i gu res 6, 7A, and 7B) . Cons ider ing t h e g e n e r a l l y poor coverage of

t h e f o c a l sphere f o r many of t h e events, t h e r e i s a reasonable cons is tency i n

t h e t y p e o f f a u l t i n g i n d i c a t e d by t h e mechanisms. The f i v e events near t h e

cen te r o f F i g u r e 6 (even ts 1-4 and 8) a1 1 i n d i c a t e o r a re c o n s i s t e n t w i t h

p redominan t l y normal f a u l t i n g . For t h r e e of these events (2 , 4, and 8) t h e

s t r i k e o f a t l e a s t one o f t h e nodal p lanes i s reasonably w e l l cons t ra ined t o

be o r i e n t e d between NNE-SSW and NE-SW, s u b p a r a l l e l t o t he t r e n d of nearby

mapped f a u l t t r aces . The t ens ion axes o f these t h r e e mechanisms a re o r i e n t e d

e i t h e r east-west o r southeast -nor thwest . T h i s o r i e n t a t i o n o f s t resses i s

c o n t r a r y t o what m igh t be expected i n a zone o f no r t hwes t -d i r ec ted p l a t e

convergence and indeed i s suggested by t h e r e g i o n a l geology. However, some

t h e o r e t i c a l s t ud ies ( f o r example, Melosh and F l e i t o u t , 1982; Bischke, 1974)

suggest t h a t p o r t i o n s o f t h e o v e r r i d i n g p l a t e ad jacen t t o a subduc t ion zone

may be i n t ens i on d u r i n g t h e e a r l y p a r t o f t h e se ismic c y c l e f o l l o w i n g a l a r g e

earthquake ( i n t h i s case t h e 1964 earthquake), and t h a t these zones change t o

compression p r i o r t o t h e nex t l a r g e earthquake.

The t h r e e mechanisms ( 5 , 6, 7) i n t h e no r t heas t p a r t of F i g u r e 6 suggest

t h a t t h e l o c a l s t r e s s p a t t e r n i n t h i s area may be ve ry compl icated. One event

( 6 ) i n d i c a t e s normal f a u l t i n g , w h i l e t h e o t h e r two are t h r u s t mechanisms. The

d i f f e rence between mechanisms 5 and 7 i s supported by bo th events hav ing

seve ra l s t a t i o n s r e p o r t i n g i n common, b u t w i t h oppos i t e p o l a r i t i e s . F u r t h e r

work i s needed t o r e s o l v e these comp lex i t i e s .

F igu re 7A P-wave f i r s t - m o t i o n p l o t s f o r se l ec ted sha l low (dep th 420krn) earthquakes t h a t occur red near t h e southern Kenai Peninsu la . Compressions and d i l a t a t i o n s a r e i n d i c a t e d by s o l i d and open symbols, r e s p e c t i v e l y , and l a r g e r symbol s i z e corresponds t o more r e l i a b l e readings. Nodal compressions and d i l a t a t i o n s a r e i n d i c a t e d by ' + I and ' - ' s igns . An ' x ' i n d i c a t e s two o r more readings o f oppos i t e p o l a r i t y a t t h e same p o i n t . The axes o f maximum and l e a s t compressive s t r e s s a r e represented by the symbols P and T . Symbols a r e p l o t t e d on equal -area p r o j e c t ions of t h e lower hemi sphere.

810904 16:20 81 1124 14:40

F i g u r e 7B F i g u r e d e s c r i p t i o n same as f o r F i g u r e 7 A .

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

The ope ra t i on o f t h e Brad ley Lake network has proved h i g h l y success fu l t o

date. Hi gh-qua1 i t y hypocenter and P-wave f i r s t - m o t i o n data, p a r t i c u l a r l y f o r

earthquakes i n t h e c r u s t and sha l low p a r t o f t h e B e n i o f f zone, a r e h e l p i n g t o

r e f i n e our unders tanding o f t h e t e c t o n i c processes a c t i v e i n t h e Brad ley Lake

reg ion . Earthquake a c t i v i t y be1 ow Brad ley L a k e i n d i c a t e s t h a t t he maximum

depth t o t h e megathrust zone between t h e P a c i f i c and o v e r r i d i n g No r th American

p l a t e s i s 35 - + 5 km. If t h e megathrust i s a t 35 km depth, then t h e events

l o c a t e d a t depths between 20 and 30 km may i n d i c a t e t h a t some f a u l t s i n t h e

upper c r u s t ex tend down i n t o t h e lower c r u s t . A l t e r n a t i v e l y , t h e zone of

megathrust i n t e r a c t i o n may be sha l lower than 35 km and i n v o l v e a complex zone

o f s p l a y f a u l t i n g . The w e l l - l o c a t e d sha l low earthquakes c o n f i r m t h e presence

o f a c t i v e f a u l t s i n t h e upper 10-15 km o f t h e c rus t . The p a t t e r n o f c r u s t a l

a c t i v i t y revea led b y c u r r e n t l y a v a i l a b l e da ta i s i n s u f f i c i e n t t o determine

whether o r n o t t he p r e v i o u s l y mapped f a u l t s a re a c t i v e , a l though t h e r e i s a

suggest ion o f s e i s m i c i t y c l o s e l y p a r a l l e l i n g 100 km of t h e Eagle R i v e r f a u l t ,

Focal mechanisms determined f o r s e l e c t e d sha l low c r u s t a l events w i t h 35 km of

B rad ley Lake a re compatable w i t h normal f a u l t i n g i n response t o east-west t o

southeast -nor thwest tens ion . A d d i t i o n a l da ta and r e l o c a t i o n of c u r r e n t da ta

w i t h r e l a t i v e l o c a t i o n techniques may h e l p c l a r i f y t h e n a t u r e o f t h e sha l low

earthquakes.

ACKNOWLEDGEMENTS

We thank Robert Page f o r h e l p f u l comments t o improve t h e manuscr ip t , and

J. P. Eaton and W. H. K. Lee f o r t h e i r t e c h n i c a l rev iews o f t h e manuscr ip t .

Kent Fogleman, Jan Meln ick , Roy Tam, Jane F re i be rg , and Paula Brown

a s s i s t e d i n much o f t h e da ta r e d u c t i o n and ana l ys i s .

We a l s o thank t h e s t a f f of t h e NOAA Tsunami Warning Center f o r t h e i r

ass is tance i n m a i n t a i n i n g our r e c o r d i n g equipment i n Palmer, Alaska.

L a r r y Gedney k i n d l y p rov i ded read ings f o r an event recorded by U n i v e r s i t y

o f A l a s k a se ismic s t a t i o n s .

REFERENCES

Beikman, H. M., comp i le r , 1980, Geologic map o f Alaska: U.S. Geo log ica l

Survey, s c a l e 1:2,500,000.

Bischke, R. E., 1974, A model o f convergent p l a t e marg ins based on t h e r e c e n t

t e c t o n i c s o f Shikoku, Japan, Journa l o f Geophysical Research, v. 79,

p. 4845-4857.

Hast ie , L. M., and Savage, J. C., 1970, A d i s l o c a t i o n model f o r t h e 1964 Alaska

earthquake, B u l l e t i n o f t h e Se ismo log ica l S o c i e t y of America, v. 60, no. 4,

p. 1389-1392.

Lahr, J. C., 1980, HY POELLI PSE/MULTICS, A computer program f o r d e t e r m i n i n g

l o c a l ear thquake hypocen t ra l parameters, magnitude, and f i r s t - m o t i o n

p a t t e r n , U.S. Geo log ica l Survey Open-Fi le Repor t 80-59, 59 p.

Lahr, J. C., and Stephens, C. D., 1980, Review o f ear thquake a c t i v i t y and

c u r r e n t s t a t u s o f se ismic m o n i t o r i n g i n t h e r e g i o n o f t h e B r a d l e y Lake

H y d r o e l e c t r i c P r o j e c t , Kenai Peninsula, A1 aska, U.S. Geo log ica l Survey

Open-F i le Repor t 81-736, 33 p.

Melosh, H. J., and F l e i t o u t , L., 1982, The ear thquake c y c l e i n subduct ion

zones, Geophysical Research L e t t e r s , v. 9, p. 21-24.

P l a f k e r , George, 1969, Tec ton ics of t h e March 27, 1964, Alaska earthquake,

U.S. Geo log ica l Survey Pro fess iona l Paper 543-1, 74 p.

P l a f k e r , George, Jones, D. L., and Pessegno, E. A., Jr., 1977, A Cretaceous

a c c r e t i o n a r y f l y s c h and melange t e r r a n e a long t h e Gul f o f A laska margin,

i n The U.S. Geo log ica l Survey i n A laska: Accomplishments d u r i n g 1976: - U.S. Geo log ica l Survey C i r c u l a r 751-B, p. B41-042.

Tysda l , R. G., and Case, J. E., 1979, Geo log ic map o f t h e Seward and B l y i ng

Sound quadrangles, A laska : U.S. G e o l o g i c a l Survey Map 1-1 150,

s c a l e 1 : 250,000.

Woodward-Clyde Consu l tan ts , 1979, Reconnaissance geo logy B r a d l e y Lake

H y d r o e l e c t r i c P r o j e c t : s u b m i t t e d t o Department o f t h e Army, A laska

D i s t r i c t , Corps o f Eng ineers , Anchorage, A1 aska, 1979.

A P P E N D I X : INTENSITY SCALE USED BY NEIS

MODIFIED MERCALLI INTENSITY SCALE OF 1931

Adap ted f rom S i e b e r g ' s M e r c a l l i - C a n c a n i s c a l e , m o d i f i e d a n d c o n d e n s e d .

I . N o t f e l t - o r , e x c e p t r a r e l y u n d e r e s p e c i a l l y f a v o r a b l e c i r - c u m s t a n c e s . Under c e r t a i n c o n d i - t i o n s , a t a n d o u t s i d e t h e boun- d a r y o f t h e a r e a i n wh ich a g r e a t s h o c k is f e l t : s o m e t i m e s b i r d s , a n i m a l s , r e p o r t e d u n e a s y o r d i s - t u r b e d : s o m e t i m e s d i z z i n e s s or n a u s e a e x p e r i e n c e d ; s o m e t i m e s t r ees , s t r u c t u r e s , l i q u i d s , bodies of w a t e r , may sway--doors may s w i n g , v e r y s l o w l y .

11. F e l t i n d o o r s by few, e s p e c i a l l y o n u p p e r f l o o r s , o r by s e n s i t i v e , o r n e r v o u s p e r s o n s . A l s o , a s i n g r a d e I , b u t o f t e n more n o t i c e - a b l y : s o m e t i m e s h a n g i n g o b j e c t s may s w i n g , e s p e c i a l l y when d e l i - c a t e l y s u s p e n d e d : some t imes t r e e s , s t r u c t u r e s , l i q u i d s , b o d i e s of w a t e r , may sway , d o o r s may s w i n g , v e r y s l o w l y ; s o m e t i m e s b i r d s , a n i m a l s , r e p o r t e d u n e a s y o r d i s t u r b e d : s o m e t i m e s d i z z i n e s s o r n a u s e a e x p e r i e n c e d .

111. F e l t i n d o o r s b y s e v e r a l , m o t i o n u s u a l l y r a p i d v i b r a t i o n . Some- times n o t r e c o g n i z e d t o b e a n e a r t h q u a k e at f i r s t . D u r a t i o n e s t i m a t e d i n some c a s e s . V i b r a - t i o n l i k e t h a t d u e t o p a s s i n g o f l i g h t , or l i g h t l y l o a d e d t r u c k s , o r h e a v y t r u c k s some d i s t a n c e away. Hang ing o b j e c t s may s w i n g s l i g h t l y . Movements may b e a p p r e c i a b l e o n u p p e r l e v e l s of t a l l s t r u c t u r e s . Rocked s t a n d i n g m o t o r c a r s s l i g h t l y .

I V . F e l t i n d o o r s by many, o u t d o o r s by f ew. Awakened f e w , e s p e c i a l l y l i g h t s l e e p e r s . F r i g h t e n e d n o o n e , u n l e s s a p p r e h e n s i v e f r o m p r e v i o u s e x p e r i e n c e . V i b r a t i o n l i k e t h a t d u e t o p a s s i n g o f h e a v y or h e a v i l y l o a d e d t r u c k s . S e n s a - t i o n like h e a v y body s t r i k i n g b u i l d i n g or f a l l i n g o f h e a v y o b j e c t s i n s i d e . R a t t l i n g o f d i s h e s , windows, doors; g l a s s w a r e and c r o c k e r y c l i n k and clash. C r e a k i n g o f v a l l s , f r a m e , e s p e - c i a l l y i n t h e u p p e r r a n g e of t h i s g r a d e . Hang ing o b j e c t s s u u n g , i n numerous i n s t a n c e s . D i s t u r b e d l i q u i d s i n o p e n v e s s e l s s l i g h t l y . Rocked s t a n d i n g m o t o r c a r s n o t i c e a b l y .

V. F e l t i n d o o r s by p r a c t i c a l l y a l ! o u t d o o r s by many or m o s t : o u t - d o o r s d i r e c t i o n e s t i m a t e d . Awak- e n e d many, o r m o s t . F r i g h t e n e ( ; f e w - - s l i g h t e x c i t e m e n t , a f ew r a t , o u t d o o r s . B u i l d i n g s t r e m b l e ? t h r o u g h o u t . B r o k e dlshe. g l a s s w a r e , t o some e x t e n t . C r a c k e d windows-- in some c a s e s , b u t n o t g e n e r a l l y . O v e r t u r n e d v a s e s , s m a l l or u n s t a b l e o b l e c t s , i n many i n s t a n c e s , w i t h o c c a - s i o n a l f a l l . Hang ing o h l e c t s , d o o r s , s w i n g g e n e r a l l y o r c o n s i d - e r a b l y . Knocked p i c t u r e s a g a i n s t w a l l s , or swung them o u t o f p l a c e . Opened , o r c l o s e d , d o o r s , s h u t t e r s , a b r u p t l y . Pendulum c l o c k s s t o p p e d , s t a r t e d or r an f a s t , o r s l o w . Moved s m a l l o b j e c t s , f u r n i s h i n g s , the l a t t e r t o s l i g h t e x t e n t , S p i l l e d l i q u i d s i n s m a l l a m o u n t s f r o m w e l l - f i l l e d o p e n c o n t a i n e r s - T r e e s , b u s h e s , s h a k e n s l i g h t l y .

VI . F e l t by a l l , i n d o o r s a n d o u t - d o o r s . F r i g h t e n e d many, e x c i t e - ment g e n e r a l , some a l a r m , many r a n o u t d o o r s . Awakened a l l . P e r s o n s made t o move u n s t e a d i l y . T r e e s , b u s h e s , s h a k e n s l i g h t l y t o m o d e r a t e l y . L i q u i d s e t i n s t r o n g m o t i o n . S m a l l b e l l s r ang- - c h u r c h , c h a p e l , s c h o o l , e t c . Damage s l i g h t i n p o o r l y b u i l t b u i l d i n g s . F a l l o f p l a s t e r i n s m a l l amoun t . C r a c k e d p l a s t e r somewha t , e s p e c i a l l y f i n e c r a c k s c h i m n e y s i n some i n s t a n c e s . Broke d i s h e s , g l a s s w a r e , i n con- s i d e r a b l e q u a n t i t y , a l s o s o m c windows . F a l l o f k n i c k - k n a c k s , b o o k s , p i c t u r e s . Overturne? f u r - n i t u t e i n many i n s t a n c e s . V ~ u v e d f u r n i s h i n g s o f m o d e r a t e l y h+.av;j k i n d .

: Y I 1 F r i g h t e n e d a l l - - g e n e r a l a l a r m , *- a l l r a n o u t d o o r s . Some, or many,

f o u n d i t d i f f i c u l t t o r t a n d . F ' N o t i c e d by p e r s o n s d r i v i n g m o t o r i c a r s . T r e e s a n d b u s h e s s h a k e n

m o d e r a t e l y t o s t r o n g l y . Waves o n p o n d s , l a k e s , and r u n n i n g w a t e r . W a t e r t u r b i d f r o m mud s t i r r e d u p . I n c a v i n g t o some e x t e n t o f s a n d or g r a v e l s t r e a m b a n k s . Rang l a r g e c h u r c h b e l l s , e t c . S u s p e n d e d o b j e c t s made to q u i v e r . Damage n e g l i g i b l e i n b u i l d i n g s o f good d e s i g n a n d c o n s t r u c t i o n , s l i g h t t o m o d e r a t e i n w e l l - b u i l t o r d i n a r y b u i l d i n g s , c o n s i d e r a b l e i n p o o r l y b u i l t o r b a d l y d e s i g n e d b u i l d i n g s , a d o b e h o u s e s , o l d v a l l s ( e s p e c i a l l y w h e r e l a i d u p w i t h o u t m o r t a r ) , s p i r e s , etc. C r a c k e d c h i m n e y s t o c o n s i d e r a b l e e x t e n t , r a l l s t o some e x t e n t . F a l l o f p l a s t e r i n c o n s i d e r a b l e t o l a r g e a m o u n t , a l s o some s t u c c o . Broke numerous windows , f u r n i t u r e t o some e x t e n t . s h o o k down l o o s e n e d b r i c k w o r k and t i l e s . B r o k e weak c h i m n e y s a t t h e r o o f - l i n e ( s o m e t i m e s damaq i n g r o o f s ) . F a l l o f c o r n i c e s f rom t o w e r s and h i g h b u i l d i n g s . D i s l o d g e d b r i c k s and s t o n e s . O v e r t u r n e d h e a v y f u r n i t u r e , w i t h damage f r o m b r e a k i n 9 . Damage c o n s i d e r a b l e t o c o n c r e t e i r r i g a - t i o n d i t c h e s .

V ' T I I . F r i g h t g e n e r a l - - a l a r m a p p r o a c h e s p a n i c . D i s t u r b e d p e r s o n s d r i v i n g m o t o r c a r s . T r e e s s h a k e n s t r o n g l y - - b r a n c h e s , t r u n k s , b r o - k e n o f f , e s p e c i a l l y pa lm t r e e s . E j e c t e d s a n d and mud i n s m a l i a m o u n t s . C h a n g e s : t e m p o r a r y , p e r m a n e n t ; i n f l o w o f s p r i n g s a n d w e l l s ; d r y wells renewed f l o w : i n t e m p e r a t u r e o f s p r i n g a n d w e l l w a t e r s . Damage s l i g h t i n s t r u c - t u r e s ( b r i c k ) b u i l t e s p e c i a l l y t o w i t h s t a n d e a r t h q u a k e s . C o n s i d e r - a b l e i n o r d i n a r y s u b s t a n t i a l b u i l d i n g s , p a r t i a l c o l l a p s e : r a c k e d , t u m b l e d down, wooden h o u s e s i n some c a s e s : t h r e w o u t p a n e l w a l l s i n f r a m e s t r u c t u r e s , b r o k e o f f d e c a y e d p i l i n g . F a l l o f w a l l s . C r a c k e d , b r o k e , s o l i d s t o n e w a l l s s e r i o u s l y . W e t g r o u n d t o some e x t e n t , a l s o g r o u n d on s t e e p s l o p e s . T w i s t - i n g , f a l l , o f c h i m n e y s , c o l u m n s , monuments , a l s o f a c t o r y s t a c k s . t o w e r s . noved c o n s p i c u o u s l y , o v e r t u r n e d , v e r y h e a v y f u r n i t u r e .

Z X . P a n i c g e n e r a l . C r a c k e d g r o u n d c o n s p i c u o u s l y . Damage c o n s i d e r - a b l e i n ( m a s o n r y ) s t r u c t u r e s b u i l t e s p e c i a l l y t o w i t h s t a n d e a r t h q u a k e s : T h r e v o u t o f p lumb some wood-f rame h o u s e s b u i 1 t e s p e c i a l l y t o w i t h s t a n d e a r t h - q u a k e s ; g r e a t i n s u b s t a n t i a l ( m a s o n r y ) b u i l d i n g s , some c o l - l a p s e i n l a r q e p a r t ; or w h o l l y s h i f t e d f r a m e b u i l d i n g s o f f f o u n - d a t i o n s , racked f r a m e s : s e r i o u s t o r e s e r v o i r s ; u n d e r q r o u n d p i p e s s o m e t i m e s b r o k e n .

X . Cracked g r o u n d , e m p e c i a l l y when l o o s e a n d w e t , up t o w i d t h s o f s e v e r a l i n c h e s : fisaures u p t o a y a r d i n w i d t h r a n p a r a l l e l t o c a n a l a n d s t r e a m b a n k s . L a n d s l i d e s c o n s i d e r a b l e f r o m r i v e r b a n k s a n d s teep c o a s t s . S h i f t e d sand a n d mud h o r i z o n t a l l y o n b e a c h e a a n d f l a t l a n d . Changed l e v e l o f w a t e r i n w e l l s . Threw w a t e r on b a n k s o f c a n a l s , l a k e s , r i v e r s , e t c . Damaqe s e r i - o u s t o dams, d i k e s , embankmen t s , S e v e r e t o w e l l - b u i l t wooden s t r u c t u r e s a n d b r i d g e s , some d e s - t r o y e d . Deve loped d a n g e r o u s c r a c k s i n e x c e l l e n t b r i c k w a l l s . D e s t r o y e d most masonry a n d f r a m e s t r u c t u r e s , a l s o t h e i r f o u n d a - t ions. B e n t r a i l r o a d r a i l s s l i g h t l y . T o r e a p a r t , o r c r u s h e d e n d w i s e , p i p l i n e s b u r i e d i n e a r t h . Open c r a c k s and broad wavy f o l d s i n c e m e n t p a v e m e n t s a n d a s p h a l t r o a d s u r f a c e s .

X I . D i s t u r b a n c e s i n g r o u n d many a n d w i d e s p r e a d , v a r y i n q r i t h g r o u n d m a t e r i a l . Broad f i s s u r e s , e a r t h s l u m p s , a n d l a n d s l i p s i n s o f t , w e t g r o u n d . E j e c t e d w a t e r i n l a r g e a m o u n t s c h a r g e d w i t h s a n d a n d mud. Caused sea -waves ( " t i d a l * w a v e s ) o f s i g n i f i c a n t m a g n i t u d e . Damage s e v e r e t o wood-f rame s t r u c t u r e s , e s p e c i a l l y n e a r shock c e n t e r s . G r e a t t o dams, d i k e s , embankments o f t e n f o r l o n g d i s t a n c e s . Few, i f a n y ( m a s o n r y ) s t r u c t u r e s r e m a i n e d s t a n d i n g . D e s t r o y e d l a r q e well- b u i l t b r i d g e s by t h e w r e c k i n g o f s u p p o r t i n g p i e r s , or p i l l a r s . A f f e c t e d y i e l d i n g wooden b r i d g e s l e s s . B e n t r a i l r o a d r a i 1 s g r e a t l y , and t h r u s t them e n d w i s e . P u t p i p e l i n e s b u r i e d i n e a r t h c o m p l e t e l y o u t o f s e r v i c e .

XII. Damage t o t a l - - p r a c t i c a l l y a l l w o r k s o f c o n s t r u c t i o n damaged g r e a t l y o r d e s t r o y e d . D i s t u r - b a n c e s i n g r o u n d g r e a t and v a r i e d , numerous s h e a r i n g c r a c k s . L a n d s l i d e s , f a l l s o f rock of s i g - n i f i c a n t c h a r a c t e r , s l u m p i n g o f r i v e r b a n k s , e t c . , numerous and e x t e n s i v e . Wrenched l o o s e , t o r e o f f , l a r g e r o c k m a s s e s . F a u l t s l i p s i n f i r m r o c k , w i t h n o t a b l e h o r i z o n t a l and v e r t i c a l o f f s e t d i s p l a c e m e n t s . W a t e r c h a n n e l s , s u r f a c e and u n d e r g r o u n d , d is- t u r b e d a n d m o d i f i e d g r e a t l y . Dammed l a k e s , p r o d u c e d w a t e r - f a l l s , d e f l e c t e d r i v e r s , e t c . Waves s e e n o n g r o u n d s u r f a c e s ( a c t u a l l y s e e n , p r o b a b l y , i n some c a s e s ) . D i s t o r t e d l i n e s of s i g h t a n d l e v e l . Threw o b j e c t s upward i n t o t h e a i r .