The Alaska Earthquake March 27, 1964

Regional Effects

Ground Breakage in the Cook Inlet Area

GEOLOGICAL SURVEY PROFESSIONAL PAPER 543-F

THE ALASKA EARTHQUAKE, MARCH 27, 1964: REGIONAL EFFECTS Ground Breakage and Associated Effects in the Cook Inlet Area, Alaska, Resulting from the March 27, 1964, Earthquake

By HELEN L. FOSTER and THOR N. V. KARLSTROM

A description of the ground cracks ,and deposits from ground-water eruptions and crustal changes, particularly in the Kenai Lowland

GEOLOGICAL SURVEY PROFESSIONAL PAPER 543-F UNITED STATES DEPARTMENT OF THE INTERIOR

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY

William T. Pecora, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1967

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 THE ALASKA EARTHQUAKE SERIES

The U.S. Geological Survey is publishing there­ sults of investigations of the earthquake in a series of six Professional Papers. Professional Paper 543 describes the regional effects of the earthquake. Other Professional Papers describe the history of the field investigations and reconstruction effort; the effects of the earthquake on communities; the effects on hydrology; and the effects on transportation, communications, and utilities.

CONTENTS

Page Page Page Abstract______F 1 Ground breakage-Con. Causes of ground breakage_ _ _ _ _ F24 Introduction______1 Kenai Lowland______F3 Crustal changes in the Cook In- Scope of report and source of Northeast-trending zone____ 3 let region______25 data______1 Coasts______12 Observed coastal changes_____ 25 Acknowledgments______2 Large lakes______14 Tilting of lake basins______26 Regional geologic setting_____ 2 Southern interior______17 Pattern of subsidence______27 Geologic history and local geo- Northern Cook Inlet area_ _ _ _ 19 Conclusions______28 logic setting______2 Northwest side of Cook Inlet_ 23 References______28 Ground breakage______3 Southeast side of Kachemak Types of breakage______3 Bay______24

ILLUSTRATIONS

PLATES

[Plates are in pocket] 1. Map showing ground breakage reSJUlting from the 1964 Alaska earthquake in the Kenai Lowland. 2. Map of Cook Inlet area. 3. Map of dis:ruption on outwash delta of Tustumena Lake.

FIGURES

Page Page Page 1. Index map ______VI 5-17. Photographs-Continued 21-23. Photographs of Eklutna 2. Azimuth-frequency diagram 12. Small vent which valley: showing dominant trend of extruded watery 21. Sites of avalanche ground cracks in the Kenai silt ______Fll activity ___ - ___ _ F19 Lowland ______F4 13. Detrital coal ex­ 22. Dust cloud ______19 3, 4. Sketch maps of ground truded with sand_ 13 23. A crack in gravel cracks in Kenai Low- 14. Ground cracks at outwash ______20 land ______5,6 Kasilof ______13 24. Photograph of failure of the 5-17. Photographs: 15. Collapse pits at delta front at Troublesome 5. Tree split by ground Kasilof ______14 Creek, Lake George ____ -- 21 crack ______7 16. Outwash delta at 25. Sketch map of Lake George 6. Typical large ground head of Tustu­ area ______21 crack in Kenai mena Lake a few 26-27. Photographs of Lake Lowland______8 days after the 7. Vertical displace- earthquake ______15 George: ment ______9 17. Crack between delta 26. Wall of silt punch­ 8. Sand ridges depos­ front and bedrock ed up on the ited along ground at Tustumena emerged bottom_ 22 Lake ______crack ______9 15 27. Silt squeezed up 18. Diagramshowingtypeof ground 9. Large fissure ______10 along cracks in disruption on the north shore 10. Large ground crack the emerged bot­ of Tustumena Lake ______16 tom sediments __ 22 with vertical dis- 19. Diagram showing a type of placement ______28. Photograph of postearth­ 10 slumping on south shore 11. Microrelief features of Tustumena Lake ______17 quake high-tide mark along formed in ex- 20. Sketch map of Eklutna val- Turnagain Arm sea bluffs ley ______near Hope ______25 truded silt ______11 18 v 6Qo

~~ -11\' Active volcano

Fault -=-}...--+-_.:..--~Dashed where approximate

---/5000---­ Submarine contour, in feet

164° 160° 156° 152° 148° 144° 140°

1.-Index map showing location of Cook Inlet area. THE ALASKA EARTHQUAKE, MARCH 27, 1964: REGIONAL EFFECTS

GROUND BREAKAGE AND ASSOCIAT:ED EFFECTS IN THE COOK INLET AREA, ALASKA, RESULTING FR.OM THE MARCH 27, 1964, EARTHQUAKE

By Helen L. Foster and Thor N. V. Karl strom

ABSTRACT

The great 1964 Alaska earthquake Cracks were as much as 30 feet across scattered localities. Some tidal flats caused considerable ground breakage and 25 feet deep. Sand, gravel, and were cracked. However, in view of the in the Cook Inlet area of south-central pieces of coal and lignite were extruded many thick sections of unconsolidated Alaska. The breakage occurred largely along many fissures. It is suggested that sediments and the abundance of steep in thick deposits of unconsolidated sedi­ the disruption in this zone may be due slopes, the cracking was perhaps less ments. The most important types of to movement along a fault in the under­ than might have been expected. ground breakage were ( 1) fracturing lying Tertiary rocks. Observations along the coasts indi­ or cracking and the extrusion of sand The outwash deltas of Tustumena cated changes in sea level which, al­ and gravel with ground water along and Skilak Lakes in the Kenai Low­ though caused partly by compaction of fractures in various types of landforms, land, of Eklutna Lake and Lake George unconsolidated sediments, may largely and (2) slumping and lateral extension in the Chugach Mountains, of Bradley be attributed to crus1tal deformation ac­ of unconfined faces, particularly along Lake in the Kenai Mountains, and at companying the earthquake. Most of delta fronts. the outlet of upper Beluga Lake at the the Cook Inlet area was downwarped, The principal concentration of base of the Alaska Range showed much although the northwest side of Cook ground breakage within the area cov­ slumping, as did the delta of the Inlet may have been slightly unwarped. ered by this report was in a northeast­ Susitna River. Parts of the flood plains Maximum change in the Cook Inlet area trending zone about 60 miles long and of the Skilak River, Fox River, and was probably less than 6 feet. Little or 6 miles wide in the northern part of the Eagle River were extensively cracked. no regional tilting was detected in the Kenai Lowland. The zone cut across A few avalanches and slumps oc­ lake basins of Tustumena and Skilak diverse topography and stratigraphy. curred along the coast of Cook Inlet in Lakes.

INTROD1UCTION

The great 1964 Alaska earth­ part of the Chugach Mountains ticularly the more populated and quake oaused considerable ground ~and the Kenai Mountains. The accessible Kenai Lowland. De­ breakage in the Cook Inlet area of greatest ground breakage outside scriptions of the damage in the south-central Alaska. Cook Inlet of the Anohorage and Portage Anchorage and Portage areas are is the major marine reentrant of areas was in the Kenai Lowland, available in previously published the Gulf of Alaska; it extends in­ about 115 air miles west of the reports (Grantz and others, 1964; land between the Kenai and Chu­ epicenter. Hansen, 1965). Some observations gach Mountains and the Alaska of ground breakage in the north­ Range (fig. 1). It is bordered by SCOPE OF REPORT AND western part of the Chugach extensive lowlands on the east, SOURCE OF DATA Mountains are included here. northwest, and north. The epicen­ This report is concerned pri­ Most of the information on ter of the earthquake (fig. 1) was marily with the ground breakage earthquake effects presented in to the east and northeast of Cook and related phenomena in the low­ this report is based on field obser­ Inlet and separated from it by land areas of Cook Inlet-par- vations made by K a r 1 s t r o m

F1 F2 ALASKA EARTHQUAKE, MARCH 2·7, 1964

between May 10 and July 15, 1964, ian, all of the U.S. Geological Sur­ Tertiary. Near the end of the and by Foster from May 10 to the vey, made their unpublished data Cretaceous, there was downwarp­ end of May. Ground observations available and gave assistance dur­ ing and subsidence of the Cook were made in the parts of the ing the preparation of the manu­ Inlet trough and, in the Tertiary, I\.:enai Lowland accessible by road script. rocks that are now locally more and along beaches. I\.:ar lstrom than 15,000 feet thick were de­ made boat traverses along the REGIONAL GEOLOGIC posited. By the end of Tertiary shores of Tustumena Lake, Skilak SETTING time, the major topographic ele­ Lake, and Lake George. The ments of the area were established. The Cook Inlet area is near the ground observations were supple­ The subsequent geologic history juncture of the western Pacific mented by reconnaissance from has consisted largely of erosion island arc system and the orogenic fixed-wing planes and by the study and modification of the mountain­ belts of the western part of North of aerial photographs made after ous areas during repeated glacial­ America. Just northeast of Cook the earthquake by the U.S. Army, interglacial cycles ·and of pa~tia;} Inlet the trends of the mountain the U.S. Coast and Geodetic Sur­ filling of lowland area.s and valleys ranges abruptly change from vey, and the U.S. Geological Sur­ with glacial drift and associated northwest to southwest and extend vey. Localities near the head of deposits (l{arlstrom, 1964). Off­ in broad subparallel arcs (fig. 1). Cook Inlet were checked primarily sets of surficial deposits, along pre­ Near the apices of these arcs the by road traverses. On the west side existing faults in bedrock, indicate granite-cored Talkeetna Moun­ of Cook Inlet only one ground continuing sporadic tectonism in tains separate the Chugach Moun­ traverse was made near Tyonek. the region through the Quater­ tains from the Alaska Range and Other observations there were nary and to the present. In the form the divide between the head from fixed-wing planes. Cook Inlet area, it was primarily of Cook Inlet and the intermon­ Ground breakage is difficult to the thick m1consolidated deposits tane Copper River basin to the detect from the air, especially in of Quaternary age that locally east. Several arcuate fault zones wooded areas. Consequently, some failed by fissuring, slumping, and have been 1napped in central and localities with ground breakage subsidence during the 1964 Alaska southern Alaska; these in part fol­ undoubtedly have been overlooked, earthquake. low and in part transect the moun­ particularly west of Cook Inlet. The Kenai Lowland part of the tain-·range structures. One of the Cook Inlet area had the most faults, the Lake Clark-Castle ACKNOWLEDGMENTS ground breakage and therefore Mountains fault, cuts the Alaska was examined in more detail; it is Special acknowledg1nent is due Range just north of Cook Inlet a broad low shelf 20 to 50 miles Mr. David M. Spencer, Mr. Robert (Dutro, 1957), there having a wide and 106 miles long. Most of Ward, Mr. Avery Thayer, and Mr. northeasterly trend. A belt of high the lowland is less than 400 feet Will Troyer of the U.S. Fish and seismicity, coincident with the above sea level, surfaces are flat to Wildlife Service in Kena.i, Alaska, a pical zone of the arcuate moun­ undulating, and local relief varies who provided essential logistic tain structures, extends from support and supplied personal ob­ from a few feet to more than 200 Prince William Sound beyond feet. The Caribou Hills, a broad servations on changes in areas not Fairbanks north of the Alaska visited by U.S. Geological Survey glaciated upland north of Homer, Range and includes the epicenter personnel. Thanks are also due Mr. rise abruptly 1,000 to 2,000 feet of the main shock of the 1964 Joe Magargl and Mr. George above the general lowland surface. earthquake. Calvin of Kasilof for their assist­ Remnants of this same upland sur­ ance during the boat traverse face occur as piedmont slopes GEOLOGIC HISTORY AND adjacent to the Kenai Mountains around the shores of Tustumena LOCAL GEOLOGIC SETTING Lake, and to numerous members between Skilak and Tustumena of the U.S. Geological Survey sta.­ The geologic structure of the Lakes (I{arlstrom, 1964, p. 12). tioned in Anchorge and Palmer, Cook Inlet area is highly complex. Drainage in the Kenai Lowland who supplied critical information The rocks of the region record a is poorly integrated, and numer­ based on personal observations of history of repeated geosynclinal ous lakes, marshes, and muskeg the earthquake effects. George sedimentation, deformation, and areas make up 1nore than a third Plafker, David McCulloch, Roger intrusion beginning in Paleozoic of the total surface. Two major Waller, and Reuben I\.:achadoor- time and extending through the lakes, Tustumena and Skilak, GROUND BREAKAGE:, COOK INiLIET AREA, AL.A:S'KA F3 occupy glacially scoured and mo­ Kachemak Bay and along the ments underlie terraced and chan­ raine-dammed troughs and are Cook Inlet shoreline south of neled surfaces between major mor­ drained respectively by the Kasilof Kasilof. ainal belts on coastal lowlands and and Kenai Rivers, which empty A complex of Quaternary mo­ in mountain valleys. The lacus­ into Cook Inlet. Along the shore­ raines and associated drift and out­ trine sediments range in thickness from a few inches to at least 100 line of l{enai Lowland are wave­ wash deposits of several ages cov­ feet. Many of the streams have cut cliffs that range in height from ers the greater part of the Kenai built large deltas; other smaller 800 feet in the Kachemak Bay area Lowland (pl. 1). Locally, the de­ streams have deposited sediments to less than 50 feet near the mouths posits are more than 400 feet thick. to form alluvial fans and alluvial of 1najor drainage lines (Karl­ The moraines have been modified fan deltas. Thick and extensive strom, 1964, p. 12) . Tertiary bed­ to different degrees by erosion, by elevated tidal-flat and beach de­ rock is exposed locally beneath gla­ an irregular cover of loess, and by posits occur in places along the cial deposits in the cliffs, especially mantling with proglaciallake sedi­ coasts (pl. 1), particularly near along the northwest shore of ments. Other proglacial lake sedi- l{enai and l{asilof.

GROUND BREAKAGE

TYPES OF BREAKAGE tion and downslope movements of part of the zone (pl. 1). The mo­ underlying water-saturated fine­ raines are mantled by a variable Ground failures in the Cook In­ grained deposits. thickness of proglacial lake silt, let area include most of the types There was also extensive ground sand, and gravel of Naptowne that occurred elsewhere during the breakage and extrusion of sand (classical Wisconsin) age. The 1964 earthquake and that are de­ and gravel along fractures which proglacial lake deposits are thin­ scribed in detail from adjoining cut across the topography and so ner and coarser on the flanks of regions in other chapters of the seem, at least in part, unrelated the moraine and become thicker U.S. Geological Survey 1964 to it. and finer grained in the intermo­ Alaska earthquake series. Types of raine areas of the lowland. Inter­ KENAI LOWLAND failure include ( 1) rock falls and bedded glacial, glaJciolacustrine, avalanches resulting from dis­ NORTHEAST-TRENDING ZONE and glaciofluvial deposits, gener­ lodgement of bedrock, colluvium, ally hundreds of feet thick, under­ Most localities of major ground and snow on steep rocky slopes; lie the surface drift units and over­ breakage in the Kenai Lowland ( 2) landslides on moderate to lie bedrock of Tertiary age. are roughly alined northeast­ gentle slopes with downslope mi­ The intensity of ground crack­ southwest in a zone 60 miles long gration of surface materials giving ing and ground-water eruption and 6 miles wide which extends rise to tensional cracking and fold­ varies within the zone. The areas from Kasilof on the east shore of ing (pressure ridges) ; (3) slump­ of broken ground are separated by Cook Inlet to Chickaloon Bay on ing and lateral extension of sur­ large areas in which surface cracks the south sho.re of Turnagain Arm ficial deposHs toward steep uncon­ are inconspicuous or absent. No fined faces (river banks, lake (pis. 1, 2). consistent relationship is apparent The zone of ground breakage shores, delta fronts, and sea bluffs) between ground cracking and crosses an area of diverse land­ that cause tensional and rotational either the local topography or the forms underlain by Quaternary underlying stratigraphy. Within fracture patterns; ( 4) transverse surficial deposits of differing the zone the ground-cracked lo­ and longitudinal tensional crack­ origin and age (Karlstrom, 1964). ing of surface gravel in va.Iley flood Included are elevated tidal flats of calities ooour (1) on terraced mo­ plains and valley trains ; and ( 5) Recent age, end moraines of l{nik rainal slopes and crests underlain concentric and transverse cracking ( preclassical Wisconsin) age near by thick sections of compact till of frozen surface layers of muskeg Kasilof and Chickaloon Bay, and 1nantled by thin sand and gravel and elevated tidal flats resulting an interlobate moraine of Eklutna deposits, ( 2) on channeled plains largely from differential compac- (Illinoian?) age in the central bordering moraines and underlain 262-870 0-67-2 F4 ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4 N

Number of measurements 2.-Azimuth-frequency diagram showing dominant trend of ground cracks in the Kenai Lowland re­ sulting from the 1964 Alaska earthquake. Includes measurements of 139 cracks in the central part of the northeast-trending zone of ground breakage in the Kenai Lowland. by thick sections of predominantly and N. 75°-80° W. (fig. 2). Ex­ F e a t u r e s associated with fine- to medium-grained sand, (3) amples of broken ground patterns ground-water eruptions were com­ in intermoraine depressions un­ within the zone are shown in fig­ mon throughout the zone. These derlain by sand and silt, and ( 4) ures 3 and 4. In forested areas, features included ridges and on elevated tidal flats underlain by cracking of the ground split nu­ mounds of sand alined along estuarine silt and sand. The cracks merous trees (fig. 5). cracks (fig. 8) ; sand sheets as cut undeflected across small hills Most of the cracks within the much as 2 feet thick, locally cov­ and depressions In morainal zone ranged in width from knife ering acres of ground associated topography. edge to a foot (fig. 6). They were with the cracks (fig. 9) ; vents from The broken ground is character­ generally very sharp, vertical, and which erupted sand either spread ized by a mosaic of ground cracks straight with abrupt angular in all directions or was extruded that trend in many directions. Lo­ changes in direction and sharp primarily in one direction-and cally the cracks show topographic angular intersections with other made a sand apron extending from control by slumping on slopes, but cracks (fig. 7). Major cracks had the vent; collapse pits, as much as in many places no such control is many smaller branch cracks that 30 feet in diameter; and fissures as evident. In all localities observed intersected the main cracks at much as 20 feet wide and 25 feet on the ground, the northeast­ sharp acute angles or sometimes at or more deep (figs. 9, 10), result­ trending cracks were the most per­ right angles. In places, major ing from the removal of material sistent and in many places could cracks abruptly changed direction by copious outpouring of ground at crack intersections. Vertical and water. In a few places great out­ be traced diagonaily across slopes horizontal displacements occurred pourings of sand were followed by and ridges without apparent de­ along some cracks; most were less small extrusions of watery silt flections in trend. Measurements than a foot but a few were as much which formed low ( 2 to 3 inches of the most conspicuous crack di­ as 2 feet (fig. 7). Most displace­ high) silt ridges and mounds and rections in the southern half of ments appeared to be primarily the irregular microrelief features on the zone suggest preferred frac­ result of adjustments in the sur­ the surface of the sand (figs. 11, ture trends of N. 50°-55° E. (par­ ficial materials by slumping and 12). These silt extrusions must allel to the zone itself), N. 20°- differential compaction, or of re­ have occurred in the very last 250 E., N. 0°-5° W., N. 10°-15° moval of underlying materials by stages of the ground-water erup­ W., N. 30°-35° W., N. 50°-55° W., ground-water eruption. tions and after the main shaking GROUND BRE'AKAGE., COOK IN1IJET AREA, ALASKA F5

150"52'40"

I I I I I I I

I I I

60°35'55"

'\ ' I I ''{ I 'l( ~-- I ' I I '

APPROXIMATE SCALE 0 100 200 FEET

EXPLANATION D Terraced and channeled sand plain

Muskeg ~ ~ Ground-water eruptive deposits of sand and silt with pebbles of clastic coal and lignite

Trends of main ground crack sets

'"'"'"'"''""'''""" Margin of well-drained ground

Craterlet which erupted ground water and sand 8 Letter indicates location of photograph

3.-Ground cracks (loc. 1, pl. 1), Kenai Lowland. F6 ALASKA EARTHQUAKE, MARCH 27, 1964

150°49'20"

I I y /I (I \I I

0 50 100 FEET I I I I

Open ground cracks 1-16 inches wide, 2-6 feet deep; locally displacements of 1-16 inches form graben (G) and horsts (H)

-----=-----

Thin ground cracks cutting forest turf and peat EXPLANATION deposits D Forested, terraced, and channeled moraines under­ Pressure ridges of forested turf and peat formed by lain by variable thickness of stratified silt, sand, landsliding and gravel over till

- - ~ Collapse pits and fissures associated with erupted Muskeg underlain by 6-20 feet of peat and organic sand deposits silt

~ ~ Open ground cracks with associated eruption of sand and ground water Ground-water eruptive deposits of sand and silt with pebbles of clastic coal and lignite ---250'--- D Topographic contour

u B Fault Letter indicates location of photograph U, upthrown side; D, downthrown side

4.-Ground cracks (loc. 2, pl. 1), Kenai Lowland. GROUND BRE1AKAGE, COOK J:NIDET AREA, AL~SKA F7

5.-Tree split by ground cracking (loc. 2, pl. 1), Kenai Lowland. The grow1d crack which trends into the split in the tree (loc. A, fig. 4) cuts ·across the top of a small knoll in moraine. F8 ALASKA EARTHQUAKE, MARCH 27, 1964

G. - 1'ypical large ground crack ( loc. 1, pl. 1), Kenai Lowland. Crack was 4 inches to 1¥2 feet wide; it cut a sand plain. Sand was not extruded along this crack (loc. A, fig. 3). GROUND BREAKAGE., COOK INLIET ARE'A, ALASXA F9

7.-Vertical displacement (loc. 2, pl. 1). A scarp more than 200 feet long and 2 to 3 feet high wa'S produced in a thick section of frozen peat and organic silt (loc. B, fig. 4). Crack at the base of the scarp was 6 inches to 1 foot wide. An­ other crack intersects scarp at right angles; this displacement probably re­ sulted from differential compaction and downslope extension of silty deposits in a muskeg bordering a small lake.

8.-Sand ridges deposited along a ground crack (loc. 2. pl. 1) by eruption of ground water during the earthquake. A sand ridge on the right side of the picture intersects the main ridge at nearly right angles. Person has right leg in deep crack (loc. C, fig. 4). 9.- Large fissure at locality 2 (pl. 1). This fissure was 25 or moro feet deep and as wide as 20 feet in places. A thickness of 2 or more feet of sand which was extruded from the fissure can be seen around the tree. Photo­ graph was taken at locality D (fig. 4).

10.- Large ground crack with vertical displacement (loc. 2, pl. 1). Thick de­ posits of sand border the left side of the crack. Photograph was taken near E (fig. 4). GROUND BREAKAGE, COOK INil.IEri' AREA, ALASKA Fll

H.-Microrelief features formed in ex­ truded silt (loc. 2, pl. 1) . Area pictured is about 48 inches wide) 3-inch-long jackknife is slightly above center of picture). The delicate silt mounds and ridges were formed in the last stages of ground-water eruption on the sur­ face of previous sand extrusions.

12.-Small vent which extruded watery silt (loc. 2, pl. 1). Jackknife is 3 inches long. Near locality D (fig. 4), watery silt from small vents such as this was extruded in the last stJages of eruption and spread out over the top of previ­ ously erupted sand.

26~-870 0-67--3 F12 ALASKA EARTHQUAKE, MARCH 2 7, 1964 had ceased, otherwise the small In several, but not all, places large landslides and very little delicate microrelief features would where the northeast-trending zone slumping. not have been so well preserved. crossed roads, cracks were found. At Kasilof, which is approxi- Although the greatest outpourings The zone crossed the paved Ster- mately in line with the northeast- of sand were in relatively flat areas ling Highway east of Soldatna, trending zone of ground breakage or depressions, some sand was ex- and several cracks in the pavement previously described (p. P3), truded from cracks extending up mere noted together with minor cracks as much as 1foot wide were the sides of small morainal hills. slumping in fill. Large cracks formed. The cracks were traceable The deposits laid down by and large outpourings of sand for several hundred feet and cut ground-water eruption within the occurred where the zone crossed sand dunes and beach deposits at zone are predominantly fine- to the oil-well access road and several the coast (fig. 14). Crack walls medium-grained sand. In most lo- of the side roads connecting with were generally sharp-in fact, ex- calities the sand contains scattered it (pl. 1). ceptionally sharp for occurrence pebbles and cobbles of detrital coal Although this zone of ground in loose sand. Some had small and lignite as much as 6 inches breakage passed along the south- vertical displacements as much as long (fig. 13). The largest amounts ern margin of the Swanson River 10 inches high. They had the usual of sand and much of the coal were oil field, no ground breakage was pattern of many branches and commonly concentrated at the in- noted or reported from the field cracks intersecting from several tersection of two or more major itself. Apparently the oil wells directions. Collapse occurred along cracks or at the intersection of a were not damaged or significantly some cracks where large amounts wide and a narrow one. Many, if disturbed by the earthquake. of sand had been extruded (fig. not all, of the coal fragments car- In the spring-fed Finger Lakes 15). Pressure ridges a few inches ried to the surface during the (about one-half mile southwest of high and 2 to 3 inches across were earthquate by ground water could loc. 2, pl. I), there was a gradual noted in silt on tidal flats; some have been deriwed from near-sur- lowering of water during the sum- radiated from a central point in face deposits that contain detrital mer following the earthquake, an several directions for distances of coal originally derived from un- occurrence which had not been ob- 25 feet or more. derlying Tertiary coal beds. How- serv.edin previous summers. It was At Chickaloon Bay a complex ever, derivation of some of the coal suggested that the lowering might network of cracks extended sea- from the Tertiary coal beds at be due to fissures in the lake bot- ward from the shore and perpen- depths of several hundred feet, toms or to a restriction of the flow dicular to it onto the tidal flats. though unlikely, is not excluded of spring water as a result of the Major crack systems also paral- as a possibility by present evidence. compaction of sediments (Alaska leled the shore, but these were ob- The quantity of sand deposited Dept. Fish and Game, 1965, p. 29). served only from the air. Although locally indicates copious ground- The lakes are in glacial moraine the tidal flats at Kasilof and and the earthquake caused slump- water discharge during the earth- Chickaloon Bay were considerably ing on the sides of the moraines fractured, other tidal flats in quake and suggests the tapping of along the margins of the lakes. similar geologic and topographic large reservoirs. Sand and silt de- At several places within the situations had no ground breakage. posited on tree branches indicate zone, ancient collapse pits and For instance, no ground cracking that water was ejected out of some linear troughs covered by mature was observed on the tidal flats at cracks to a height of at least 20 forest vegetation record previous Kenai, Moose Point, Point Pos- feet. Ground-water supplies in the episodes of ground cracking and session, Stariski, or in the Nin- general area of the central part of ground-water eruption. ilchick area. the zone are obtained from gravel On the mainland near Homer, aquifers under hydrostatic head. COASTS ground fissures occurred in the The aquifers are 100 to 200 feet Much of the western coast of bluff on which the U.S. Bureau of below the surface and beneath the the Kenai Peninsula mas exam- Land Management station is lo- surface till units. It is probable ined, and few changes attributable cated. However, as elsewhere on that ground cracking penetrated to the earthquake mere found. Al- the coast, ground breakage due to through the surface till units at though steep high bluffs of uncon- the earthquake was slight and less many places and tapped these deep solidated glacial deposits line than might have been expected ground-water reservoirs. much of the coast, there were no where steep slopes in materials GROUND BREAKAGE, COOK INLET AREA, ALASKA F13

13.-Detrital coal extruded with sand (loc. 1, pl. 1). Copious amounts of sand were extruded from a fissure cutting a gravel road ( loc. B. fig. 3). Pieces of coal and lignite as long as 6 inches (some indicated by arrows) were ex­ truded with the sand.

14.- Ground cracks at Kasilof in ele­ vated beach deposits. Cracks were as much as 1 foot wide. Vertical displace­ ment is evident along crack near man. Considerable clastic coal was brought up from beneath the surface and de­ posited with the sand. F14 ALASKA EARTHQUAKE, MARCH 27, 1964 The fracture pattern in the dis­ rupted part of the delta consisted of two sets of diagonal fractures formed on northwest and north­ east trends and a less well de­ veloped set formed about per­ pendicular to the main fissured zone. It is believed that this frac­ ture pattern formed during the earthquake when the lakeward un­ confined face of the delta extended forward and pulled away from the main mass of the delta. McCulloch has described and explained sim­ ilar crack patterns in Rocky Creek and Lakeview deltas at Kenai Lake {1966, p. A35). The ground cracks in the out­ wash delta ranged in width from a fraction of an inch to 4 feet (fig. 17). Most cracks showed no verti­ cal displacement, but a few indi­ cated subsidence of several inches 15.-Collapse pits at Kasilof formed after eruptions of ground water and sand. to 11/z feet on the lake side. Little Pit in foreground is about 3 feet in diameter. Erupted sand covers the ground. A large crack extends from the pits in the middle background. overall evidence of subsidence of the delta front was observed, and susceptible to sliding abound Skilak Lakes were mostly in un­ it is estimated to have been less (Waller, 1966, p. D6-D7). Damage stable slopes of alluvial fan deltas than 10 feet. to Homer Spit, due largely to sub­ or at the tips of peninsulas or The southern edge of the out­ marine sliding and flooding, has spits projecting into deep water. wash delta is bordered by a low been described by Waller ( 1966). TUSTUMENA OUTWASH DELTA forested terrace (pl. 3, fig. 16). LARGE LAKES The outwash delta at the head Cracks 3 to 4 feet wide and 2 to 3 feet deep spaced at intervals of 10 Tustumena Lake and Skilak of Tustumena Lake is 411z miles to 50 feet cut through the forested Lake are the two largest lakes in wide; it partly buries two groups area in a north-south direction the Kenai Lowland (pl. 1). Tustu­ of ice-rounded bedrock hills that along the lake edge. Many trees mena Lake is 25 miles long and 3 divide the outwash plain into two were split and offset along cracks, to 6 miles wide; Skilak Lake is 14 branches with separate delta the offset indicating a right-lateral miles long and has a ma.ximum fronts (pl. 3). During the earth­ ground displacement of 1 to 3 feet. width of about 4 miles. Both lakes quake, both delta fronts failed by The walls of the deeper cracks ex­ occupy glacially scoured troughs fracturing and slumping into the posed the following terrace stra­ that hwve been dammed by lake. tigraphy from top to bottom :vege­ moraines. Photographs obtained after the tation mat and organic silt about Ground cracking around these earthquake (fig. 16) indicate that 1 foot thick over stratified pebble­ lakes was extensive in the large a major zone of fissures oriented to-boulder gravel containing lenses outJwash deltas at the heads of the N. 5°-10° '\¥.formed subparallel to and beds of sand and silt. lakes. Damage elsewhere along the the southern delta front about half Sand, silt, and some gravel were shores of the lakes was minor, al­ a mile from the lake shore. This brought to the surface by forceful though the lakes are surrounded break defined the inland margin of primarily by thick unconsolidated the pa·rt of the delta most dis­ ejection of ground water from the deposits of Quaternary sediments. rupted by the earthquake. Much cracks. The extruded material The several small occurrences of sand was extruded along this frac­ spread out to form large sand ground cracking which were ob­ ture zone and flowed out upon the sheets covering a few acres. Else­ served along Tustumena and snow-covered surface (fig. 16). where sand ridges were deposited GROUND BREIAKAGE , COOK IN[)ET AREA, ALA:S:KA F15

16.-0utwash delta at head of Tustu­ mena Lake a few days after the earth­ quake. View is south (pl. 3). The ground was snow covered, and water and sand erupted along major cracks (dark-gray areas) partly removing the ·Snow and covering the ground sur­ face with sand. The cracked ice-covered lake surface is to the right. A major fissure system paralleling the delta front is seen to the left of center. Photograph by Avery Thayer, Fish and Wildlife Service, Kenai, Alaska.

17.-Crack between delta front and bedrock at Tustumena Lake (loc. A. pl. 3). A wedge of outwash deposits pulled away from the stable bedrock face during the earthquake S·haking. ALASKA EARTHQUAKE, MARCH 27, 1964

[%Old beach

cracks

Lake level

APPROXIMATE SCALE 0- 50 100 150 FEE1 18.-Diagram showing type of ground dis~uptionon the north shore of Tustumena Lake (loc. 3, pl. 1). along the margins of the cracks. The northern delta front was end of Caribou Island the narrow Locally, boulders as much as 8 fractured in a pattern similar to tip of beach gravel cracked but did inches in long dimension were that of the southern front, but less not slump. Ground cracking oc- brought to the surface from the severely. The diagonal and per- curred over fairly large areas on underlying gravel beds by the pendicular crack sets and the sub- shoreline terraces on opposite sides ejecting ground water. parallel fissure zone separating the of the narrowest part of Tustu- Such forceful ejection at this extended block from the inland mena Lake. single locality may have been part of the delta were present, but Near Tanya Lake (loc. 3, pl. caused by conditions which dif- the patterns mere not as strongly I), a northeast-trending set of fered slightly from those in the developed as in the southern front. ground cracks spaced 10 to 30 feet delta to the north. Water moving A considerable amount of sand was apart cut the beach deposits and a through the outwash terrace under extruded along many of the frac- low forested gravel terrace. A higher hydrostatic head (probably tures. The fracturing was appar- graben 10 to 30 feet wide, about because the surface gradient of the ently caused by lateral spreading 100 feet long, and downdropped terrace is steeper than that of the of the deltaic sediments toward the 4 to 6 feet formed and filled with bordering outwash) may have been lake as a result of the earthquake water (fig. 18). Gravelly terrace confined between more firmly shaking. deposits underlain by lake silt ap- frozen sediment layers. The firmer parently broke into blocks along freezing could obtain because of OTHER TUSTUMENA LAKE GROUND northeast fractures and slid lake- the thicker organic silt layer, con- CRACKING ward upon a slightly inclined slip tinuous vegetation mat, and exten- Local slumping at the fronts of plane in the silt (fig. 18). The for- sive forest cover. The difference in alluvial-fan deltas occurred at the ward translation and slumping of the pattern of cracking between mouths of Indian Creek, Moose the lakeward blocks permitted the the terrace and the outwash delta Creek, and Bear Creek (pl. 1). interior block to drop down. This may reflect differences in the un- Slumping took place largely by mechanism is similar to that de- derlying bedrock topography. Be- v e r t i c a 1 displacement along scribed for some of the landslides cause of the general configuration ground cracks parallel to the lake in the Anchorage area (Hansen, of this valley, the bedrock slope margin. At Windy Point a block 1965). The submerged block with under the terrace may be north- of beach and terrace gravel 50 by mature spruce trees indicated ver- westward and that under the out- 100 feet slumped into the lake tical slump of at least 20 feet at the wash delta may be more to the largely submerging spruce and shore. west. Slippage of the lake front birch trees 40 to 60 feet tall. Sub- On the opposite shore of the lake sediments over a northwest-slop- parallel fractures 1 to 6 inches (loc. 4, pl. I), a section of a low ing bedrock floor would explain wide developed landward of the forested terrace 500 feet long and the observed displacement. slump-block scarp. At the western 100 feet wide slumped toward the GBOUND BRE~~E.,COOK LN(IIET AREA, ALASKA F17 lake. The disrupted block is de- level changes were observed any- stresses set up by slumping con- fined by an arcuate set of large where along the shores in the up- fined to the upper layers of the fractures that intersect the lake per part of the lake suggests that unconsolidated deposits or whether margin at both ends; however, no the amount of material that the deformation resulted from ac- displacement of bhe terrace is visi- slumped from the delta front at commodation of the unconsoli- ble where the fractures meet the the time of the earthquake was not dated section to displacements at lake shore. Tilted trees occur on large enough to generate large depth. The possibility of forma- low ridges that appear to have waves. Fracturing of the outwash tion by movement of the lake ice formed by slight rotation along plain drained a beaver-dammed during the earthquake must also subordinate cracks in the block lake along the south side of the be considered. (fig. 19). Visible vertical displace- valley about 2 miles from the head The north- to northeast-trending ments were slight, and modern of the Skilak Lake (Avery beaches at the lower end of the lake beach deposits fronting the block Thayer, written commun., 1964). were fractured in two places. At were probably lowered only a few The beach fronting the Pipe locality 7 (pl. 1) the cracks are 3 inohes relative to the unaffected Creek alluvial-fan delta (Iw. 5, pl. inches to 1 foot wide and are con- beaches to the north and south of 1) ms cut by a system of inter- fined to the modern beach gravel the slumped area. secting fractures one-half inch to deposits. The largest crack was at 3 inches wide. The most con- the base of the scarp defining the SKILAK LAKE spicuous cracks trended north or inner edge of the beach. It was The entire outwash plain be- nortrheast, that is, transverse to intersected by a few transverse tween the front of Skilak Glacier rather than parallel to the shore. cracks that extended diagonally and Skilak Lake was extensively At the lower campground (loc. across the beach toward the lake. fractured, and the delta front at 6, pl. 1) near the outlet of Skilak At locality 8 (pl. 1) a north- to the head of Skilak Lake slumped Lake, ground cracks, mole tracks, northeast-trending crack 5 to 15 into the lake. The amount of and pressure ridges formed on a inches wide cut diagonally across slumping of bhe delta front below low forested terrace. The pressure the beach and was traceable into lake level is not known, but exami- ridges, 6 inches to 2 feet high, higher ground a few hundred feet nation of aerial photographs and border the muskeg back of the from the lake before being lost in ground observations suggest that campground and trend northwest- dense forest. a segment of the delta front about ward. The mole tracks developed half a mile wide may have sub- by overthrusting (or underthrust- SOUTHERN INTERIOR sided sufficiently to be partially ing) of the forest litter layer; they Ground breakage in the interior submerged during the low mean are slightly offset along transect- of the Kenai Lowland outside of lake-level phase. Aspen and brush ing cracks which trend northeast. the northeast zone was limited to 10 to 20 feet high were standing in This pattern suggests formation a few scattered localities (pl. 1) water 3 to 10 feet deep as muoh as under compression, but the mech- where the following types of dis- 1,000 feet offshore from the new anism is not clear. In the ab- ruption mere observed : shore line. However, the fact that sence of subsurface data, it can- 1. Landslides in unconsolidated no freshly ice-scoured trees or not be determined whether the other evidence of major water- surface deformation resulted from deposits on moderate to steep slopes. Northeast of Homer. 2. Large- avalanches. Anchor River. 3. Slumps in unconsolidated de- posits on or toward uncon- Lake level fined faces such as river banks, lake shores, and delta fronts. Chakok River, Box River, Tustwnena Lake, Ski- APPROXIMATE SCALE lak Lake, and Stariski Creek. 0 25 50 75 FEET IIIIII I I 4. Ground cracks in lowlands cov-

19.-Diagram showing a type of slumping on the south shore of Tustumena Lake ered by silty lake deposits, (lot. 4, pl. 1). alluvium, or fluvioglacial de- F18 ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4

EXPLANATION APPROXIM ATE SCALE 1 M ILE N L_L_~~~--~

Valley train

Alluvial terrace and glacial moraine. Dark lobes are Recent mo­ raines

Rock glacier or moraine

Area of slope deposits

Ground cracks produced by earthquake; asso­ ciated locally with ground-water eruptive deposits ® Collapse pit produced by the earthquake ••• Sites of unusually per­ sistent avalanching

20.-Sketch map of IDklutna valley showing sites of perSii.stent avalanching and location of ground breakage. posits. Upland in headwaters The cracking in the Fox River longitudinal and t r an s v e r s e of Bear Creek. flood plain was particularly spec­ cracks. The alluvial flats on the in­ 5. Ground ·cracks and pressure tacular on the alluvial fan formed side of meanders were cracked nnd ridges in muskeg areas. Half 'at the confluence between Sheep channel banks sl urn ped at n u­ a mile south of Clam Creek, Creek and the Fox River. Copious merous places. The cracking ap­ Slikok Creek. amounts of sand erupted along an peared to be concentrated along 6. Cracks in flood plains and val­ extensive mosaic of cracks. Along the axis of the Fox River valley. ley trains underlain by sand the channels of the Fox River, No flood-plain failures were ap­ and gravel. Skilak River, Fox numerous alluvial deposits on parent in tributaries heading m River. slip-off slopes were broken along the Kenai MountJains. GROUND BRE1AKAGE, COOK INIJEfi AREA, ALA:SKA Fl9 NORTHERN COOK INLET AREA The major ground breakage in the northern Cook Inlet ,area was in and around Anchorage and Portage. Ground cracks and land­ slides were abundant and destruc­ tive and have been described in de1Jail by Hansen ( 1965), Mc­ Culloch ( 1966) , Kachadoorian (1965) and others. Ground break­ age in this area also occurred in the Eklutna valley, around Lake George, in valleys in the western Chugach Mountains, and west of Anchorage including the delta of the Susitna River. At the head of Eklutna valley, unusual avalan0he activity fol­ lowed the earthquake. It began after the earthquake and con­ tinued throughout the summer of 1964. Periodic observations made in the valley by U.S. Geological 21.-Sites of avalanche activity in Eklutna valley. These talus cones were present before the earthquake but have been enlarged by avalanche activity since the Survey personnel stationed in earthquake. The dust clouds are created by the aV'alanching. Anchorage indicate that it com­ menced again after the spring thaw in 1965 and continued throughout that summer. As late as August 2,1966, minor avalanche activity was reported (Ruth Schmidt, written commun., 1966). However, the total activity in tJhe summer of 1966 was apparently considerably less than that in 1964, hut more than before the earthquake. In the early part of the summer of 1964, rock debris ca.scaded nearly continually from three principal places on the steep high cliffs above Eklutna valley (fig. 20). Talus cones at the base of the cliffs were much enlarged (fig. 21). Dense dust clouds emanated from the avalanches and resulted in erroneous reports of volcanic eruptions when spotted by air­ plane pilots (figs. 21, 22). Later in the summer, several of the ava­ lanches became inactive, and by 22.- Dust cloud in Eklutna valley caused by avalanche activity. The avalanching August 1965 activity was largely has been persistent during the summers since the earthquake. 23.-Cr

EXPLANATION

Outwash

Moraine of Recent age

Emerged lake bottom sediments; line pattern indicates areas of severely cracked sedi­ ments and eruptions of silt, sand, and water from cracks • Front of delta that failed during earthquake

Earthquake cracks paralleling front of moraine 61° 15'

l!!llllllll!!ll!!l!!l ! ! Margin of bedrock areas

A Letter indicates location of photograph

25.- Lake George area showing location of ground breakage. ALASKA EARTHQUAKE, MARCH 27, 1964

26.-Wall of silt punched up on the emerged bottom of Lake George. The wall was 8 to 24 inches high, 30 feet long, and 6 inches wide (loc. B, fig. 25).

27.-Ridges of silt squeezed up along mclrs in the emerged bottom sediments of Lake George (lw. A. fig. 25). GROUND BREAKAGE:, COOK INILIET ARE'A, AL.AISKA F23

Lake silt was squeezed out of the valley from these cracked zones. 2. Inland from Trading Bay, 15 underlying water-saturated sedi­ Ground observations were not miles to the northeast. Ex­ ments and erupted through cracks made in the other valleys of the tensive fracturing occurred in the seasonally frozen surface mountain range. in a zone extending north­ layer. At one locality a wall of West of Anchorage, tidal flats east more or less parallel to frozen silt, 6 inches wide and about were cracked and slumping oc­ the shore of Trading Bay 30 feet long, was punched up 8 to curred at the mouth of Knik Arm and along the approximate 24 inches by pressures in the un­ on the west side. The delta of the postulated trace of the Lake derlying unfrozen 1naterials dur­ Susitna R i v e r was severely Clark-Castle Mountain ing the earthquake (fig. 26). The cracked; parts of its front subsid­ fault. The zone is about 12 undisturbed structure of the fine ed into Cook Inlet. In some small miles long. The dominant laminated silts exposed in the wall lakes in the Susitna River low­ trend of the fractures is also indicated that the silt must have land, the sediments in the bottoms northeast. Much sand was been rigid and frozen at the time of the lakes were cracked. At the extruded along many of the of displacement. Disruption and base of Mount Susitna along the fractures. minor dislocation of the wall along projected trace of the Lake Clark­ 3. Northeast shore of Chakat­ northeast-trending cracks indicate Castle Mountain fault, sand erup­ chama Lake. Cracks oc­ subsequent development of and tions occurred along the margins curred in beach deposits minor displacements along cracks of lake terraces cut in till. To the along the shore of the lake that cut transversely the northwest northeast about 18 miles, also (Gordon Giles, written com­ set of parallel fractures that de­ along the trace of the Lake Clark­ mun., 1964). fined the wall structure itself. At Castle Mountain fault, there was 4. Tyonek. Lake ice and muskeg several locations, mounds of silt disruption and slumping along a were cracked. About 5 miles 1 to 2 feet high and 2 to 3 feet in bluff face. north of Tyonek, old slumps diameter and showing internal along the coast were reacti­ NORTHWEST SIDE OF flow structure formed at the inter­ vated, but displacement was COOK INLET section of the cracks. Unfrozen silt less than 1 foot. was squeezed up as ridges along Limited observations on the 5. Trading Bay. On the coast Ph some of the cracks (fig. 27). On northwest side of Cook Inlet indi­ miles east of Kustatan, small the north side of the moraine along cate that ground breakage was slumps occurred in bluffs of the south side of lower Lake relatively minor and confined glacial till (George Plafker, George, some sand eruption oc­ mostly to deltas, tidal flats, and oral commun., 1966). curred at the contact of the mo­ alluvial flats, and occurred in 6. Redoubt Bay. Cracks were raine with the lake sediments widely scattered localities. Ground common along the Drift through cracks transecting the breakage was noted in the follow­ River from the mouth in­ moraine. ing places (pl. 2) : land 2 miles or more. All Reconnaissance observations 1. Beluga Lake. Beluga Lake is breakage was in alluvium. within the Chugach Mountains be­ divided into upper and lower North from the mouth of tween Knik Valley and Turnagain parts by deltaic outwash Drift River, cracks associ­ Arm revealed only two other local­ sediments d ep o sited by ated with extruded sand oc­ ities of ground breakage: cracks drainage f~om Capps Gla­ curred along the coast for a cutting forest turf on thin allu­ cier. This outwash delta was distance of about 2 miles vium over bedrock near the mouth considerably fractured. At (George Plafker, oral com­ of south fork valley of Campbell the head of upper Beluga mun., 1966). Creek and cracks in thick muskeg Lake, terraces along outwash 7. Near the mouth of Tuxedni and alluvial deposits in the middle channels cut below the main Channel. On the south side, part of Eagle River valley (pl. 2). surface level of the large cracks formed on steep The major crack set in Eagle River outwash delta were locally slopes. Two months later, valley was roughly parallel to the fractured. At the head of following spring thaw, land­ axis of the valley, and the most lower Beluga Lake a large slides occurred in the cracked conspicuous transverse set of block slumped into the lake terrain (George Plafker, cracks trended at right angles to submerging trees. M i n o r oral commun., 1966) . the valley axis. No ground damage fracturing occurred at the 8. North shore of Tuxedni Bay. was observed upvalley or down- outlet of lower Beluga Lake. Along the east side of F24 ALASKA EARTHQUAKE, MARCH 27, 1964 Squarehead Cove, the bluff were close to the shore, a reconnaissance flight in 1964 :face sloughed off along the others were higher on the (George Plafker, oral commun., shore :for about 1 mile slope and subparallel to it. 1966; pl. 2). Extensive :fracturing (George Pla:fker, oral com­ Slumps and small slides oc­ resembling that o:f Skilak Lake mun., 1966). curred in places along the occurred in the outwash delta at 9. l{algin Island. Slumps devel­ cracks, and the surface was the head o:f Bradley Lake. Fine oped in unconsolidated de­ "stepped" downward toward closely spaced cracks were noted posits on the south end. the coast along some cracks in the tidal flats along the south 10. Augustine Island. Cracks more (Bruce Reed and George shore o:f China Poot Bay. A rock­ or less parallel to the coast Pla:fker, oral commun., fall took place on a steep cliff were noted :from the air on 1966). nearby. The outwash flats along the north end and on the the glacial drainage which enters SOUTHEAST SIDE OF northeast side. All appeared China Poot Bay from the south KACHEMAK BAY to be in recent unconsoli­ were considerably fissured. Ground dated deposits o:f volcanic Ground breakage at two places breakage extended :from the bay sand, gravel, and bouldery along the southeast side o:f Kache­ margin inland along the drain­ gravel. Some o:f the cracks mak Bay was noted by Pla.:fker in ~age :for about 2 miles.

CAUSES OF GROUND BREAKAGE

The northeast-trending zone o:f define clearly the bedrock struc­ cides in position with the zone of ground breakage in the Kenai tures beneath the Kenai Lowland, broken ground developed during Lowland may not be caused en­ but some o:f the data suggest a the 1964 earthquake. At one place tirely by simple lateral movements buried :fault; none preclude the along the trace o:f this disconti­ or compaction in unconsolidated possibility o:f such a :fault. Field nuity the seismic data can be inter­ sediments. Several :features indi­ mapping in the l{enai Lowlands preted as recording a basement cate that additional :factors were (I{arlstrom, 1964) shows that the :fault o:f northeast strike, a steep involved. For example, in many top o:f the Tertiary bedrock along southeast dip, and a downdropped places there is no consistent re­ the coast generally is above sea south block (C. E. Kirschner, oral lationship between the cracks and level south of, and below sea level commun., 1964). either the local topography or north of, Kasilof. Thus there could The unusual avalanche activity stratigraphy; the cracks are un­ be either an erosional break or a at the head o:f Eklutna valley :fol­ usually deep and are very per­ structural dislocation o:f the bed­ lowing the earthquake and con­ sistent and have a marked linear rock floor where the inferred :fault tinuing into the summer o:f 1966 trend. zone crosses the coast. might also be related to move­ The :following explanations :for In the Swanson River oil field, ment along faults. The rocks in these :features are suggested : ( 1) oil and gas have been produced this area are highly sheared and The disrupted zone may have re­ :frmn the Tertiary sedimentary crushed. I:f these rocks were made sulted :from movement triggered rocks northwest of the zone of dis­ unstable by small movements by the earthquake along a buried rupted ground. To date, all wells along :faults, the avalanche ac­ :fault. (2) There may have been drilled south o:f the zone have been tivity could have been set in mo­ differential compaction o:f Qua­ dry. This change in subsurface tion. Furthermore, the continued avalanche activity might be due to ternary sediments along a buried conditions may be due to a :fault, minor readjustments in the dis­ ridge o:f Tertiary rock. (3) Grav­ or it may indicate a :facies change. turbed blocks :following the main ity sliding may have taken place By report, seismic profiles ob­ displacement. Although the bed­ along the interface between the tained :from the northern part o:f rock o:f this looality has not been unconsolidated Quaternary de­ the l{enai Lowland are difficult to mapped in detail, the :fractured posits and the Tertiary bedrock. interpret because o:f poor signal rock suggests that :faults are pres­ ( 4) A combination o:f these proc­ returns. They do suggest, however, ent. I:f simple displacement o:f esses could have been involved. an important subsurface disconti­ sheared blocks took place as a re­ The subsur£ace data available nuity, either lithologic or struc­ sult o:f the shaking o:f the earth­ to the authors are insufficient to tural, that approximately coin- quake without :faulting, it seems GROUND BRE'AKAGE, COOK SNILIE,T AREA, ALASKA F25

unlikely that the avalanching of the Lake Clark-Castle Moun- can be explained by the presence would have continued so long. The tain fault. To the northeast of this of thick sections of silt and water- disruption in the vialley floor at locality along the trend of the logged sediments which are par- the head of Eklutna Lake and the fault, two other localities of dis- ticularly susceptible to disruption by seismic activity when in topo- damage to the powerplant facili- ruption were noted within the graphically unfavorable situa- ties (Logan, 1967) also suggest Cook Inlet area. This ground tions. Despite many such unfavor- breakage may be indicative of that more than just the usual seis- able situations in the Kenai LOW- mic effects from a distant earth- movement, along the Lake Clark- land and the mountains bordering quake were in operation here. Castle Mountain fault, but other it, the overall ground breakage in On the northwest side of Cook evidence of fault movement, par- this area was relatively minor. Inlet northwest of Trading Bay, ticnlarly in bedrock, has not been This stability may be explained, at a concentration of cracks and sand detected. least in part, by the fact that the extrusions in flood-plain deposits The remainder of the ground ground was still firmly frozen at was noted near the projected trace breakage in the Cook Inlet area the time of the earthquake.

CRUSTAL CHANGES IN THE COOK INLET AREA

I I Crustal deformation that accom- of Hope were flooded. Inun- sulted from crustal subsid- panied the 1964 earthquake af- dation during high spring ence and not from compac- fected an area of 65,600 to 77,200 ticies extended to just be lo^^ tion or sl~lmpingof the sur- 1 square iniles in south-central the entrance of the General ficial deposits. Alaska. The deformation consisted Store. High-water marks on 2. Along the south shore of Turn- I of two major northeast-trending forested slopes of a thick sec- again Arm in Chickaloon I zones of uplift and subsidence bc- tion of coarse alluvial fan east Bay, there was 3 to 4 feet of tween the Aleutian Trench and the of Hope indicate inundations subsidence. Vegetated ele- i Aleutian volcanic arc (Plafker, approximately 5 feet above vated tidal flats previously I I 1965, p. 1686). Most of the Cook the preearthquake high-tide above the reach of high tide Inlet area is in the depressed area, mark (fig. 28). The coarse were inundated following the I but the region also includes areas gravel substratum in the area earthquake. The estimate of ranging from those with no detect- and the absence of cracking 3 to 4 feet of subsidence is able change to some on the north- or slumping along the coast based on observed changes in west side of Cook Inlet that suggest that most, if not all, tidal levels in relation to the I are slightly uplvarped. However, of the recorclecl subsidence re- floor of a hunter's cabin in maximum change in the Cook In- I let area was small, probably not I more than 6 feet, as compared with i 50 feet to the southeast. OBSERVED COASTAL CHANGES I Coastal observations in the Cook I Inlet area indicate the following approximate changes in land-sea I levels : 1. Along the south shore of Turn- I again Arm near Hope, there was 4 to 6 feet of subsidence. Vegetated areas previously above storm beach levels were inundated during high tides 28.-Postearthquake high-tide rnnrlc along Turnagain Arm sea bluffs near Hope. following the earthquake. Arrows indicate the pre- and postearthquake high-tide positions. The vertical dis- tance of about 8 feet between these two pasiitions approximates the amount of Houses in the low-lying parts crustal subsidence that accompanied the 1% earthquake. Fa6 ALASKA EARTHQUAKE, MARCH 2 7, 19 64 the area that was for the first survey of the highway from Service (Theron Smith, oral time partly inundated during Kenai southwest to Ninilchik commnn., 1964). Probably the high tides (Avery Thayer, indicated no measurable most of this subsidence can oral commun., 1965). How- change that could be attrib- be attributed to slumping and ever, a thick section of fine- uted to regional warping dur- compaction of the delta front grained deposits underlies the ing the earthquake (Alaska and adjoining elevated tidal tidal flats, so there is a possi- Highway Dept. personnel, flats between the Susitna bility that part of the ob- oral commun., 1964). Short- River and McKenzie Point. served subsidence may have term postearthquake tidal- This conclusion seems reason- resulted from compaction. gage records s u g g e s t a able because (1) the coastal 3. Along the east coast of Cook subsidence of 0.9 foot near m a r g i n was extensively Inlet from Point Possession Nikishka (Small, 1966, p. 20). cracked during the earth- to Anchor Point, observed 4. Along the south coast of the quake and (2) changes in subsidence ranged from a lit- Kenai Lowland between An- bathymetry of the bordering tle less than a foot to none chor Point and Homer Spit, seaway (determined by U.S. it all. Undercutting of the regional tilting ranged from Coast and Geodetic Surveys stabilized and vegetated col- none to less than 1 foot of in the summers of 1963 and luvial slopes along the sea subsidence at Anchor Point, 1964) record a major slump bluffs was approximately the 2 feet at Homer, and 4 to 6 of material along the coast, same as had been observed by feet at the tip of Homer Spit. presumably resulting from Karlstrom during s p r i n g These figures are based on a the earthquake. No changes traverses in previous years. rapid resurvey of the high- were observed along the coast Vertical rellations between way after the earthquake by near Tyonek except in one storm beaches, elevated tidal- the Alaska Highway Depart- area where subsidence of flat surfaces, and postearth- ment. Of the 4 to 6 feet of storm beaches suggested a quake high-tide levels ap- subsidence at the end of downdrop of about 1 foot. peared comparable to those Homer Spit-a low sand and This area of obvious subsid- observed in previous years gravel bar that extends 5 ence occurs at the foot of a near Point Possession, Moose miles southeastward into Ka- section of coastal bluff where Point, Kenai, Kasilof, Ninil- chemak Bay from Homer-a large preearthquake slump chik, and Anchor Point. Lo- minimum of 2 feet has been blocks moved locally during cal fishermen estimated a sub- attributed to crustal subsid- the quake. The anomalously sidence of about half a foot ence and the remainder to low tide levels following the near Ninilchik on the basis of differential compaction of earthquake that were re- apparent changes in low tides. the underlying unconsoli- ported by residents Iliamna Careful observations on a pier d ate d materials (Grantz, and Tuxedni Bays suggest an gage at the cannery on the Plafker, and Kachadoorian, uplift along the Kamishak south side of the mouth of the 1964, p. 9,24). Bay coast of 1to 2 feet. Kenai River indicated that 5. Along the west shore of Cook The estimates of land-sea water levels during the spring Inlet from Point McKenzie changes along the south shore of high tides *following the to Kamishak Bay, there was Turnagain Arm are of the same earthquake did not quite 'at- 1to 2 feet of subsidence along order of magnitude as the changes tain the levels reached during the slumped front of the determined by releveling bench the high tides of the preced- Susitna Delta area ; no change marks in bedrock along the north ing fall (Clarence Platt, oral to slightly down near Ty- shore of the arm. This agreement comrnun., 1964). B 0 c a u s e onek; 1 to 2 feet of uplift suggests that the estimates else- high tides in this area are along the coast of Kamishak where in the area based on shore- normally higher in fall than Bay. The estimate of subsid- line relations may also be approxi- in spring, this observation is ence in the Susitna Delta area mately correct. consistent with the geologic is based on observations of evidence of very little or no postearthquake tidal changes TILTING OF LAKE BASINS change in datum along the at a hunter's cabin by person- The large lake basins in the coast. A postearthquake re- nel of the Fish and Wildlife Cook Inlet area serve as natural GROUND HRE 1AKAGE:, COOK J.NlL/EiT ARE'A, ALASKA F27

tiltometers because their long di­ merged vegetation be related to by Avery Thayer (written com­ rections are n1ore or less parallel regional tilting. Hand-leveling by mun., 1964) revealed unusually to the known direction of regional Brunton compass at numerous lo­ high lake levels hut no changes tilt. Regional tilting should be de­ calities about the margin of the that could be attributed to dif­ tectable in changes of water-plane lake revealed no systematic differ­ ferential tilting. However, Kenai levels. The positions of preearth­ ences in the vertical interval be­ Lake, about 25 miles east of Skilak quake water levels were checked in tween lake level and high-water Lake in the Kenai Mountains, was a few of the lake basins by refer­ beaches and watermarks. Thus, if in a zone of greater deformation ring to established bench marks. the lake basin had been tilted, the and showed about 3 feet of tilt A preliminary resurvey of bench amount of tilting was apparently (McCulloch, 1966, p. A29). marks in relation to lake levels less than the accuracy of measure­ PATTERN OF SUBSIDENCE after the earthquake indicated ment by Brunton compass, which southeastward tHting of Eklutna is estimated to be + 1 foot. The combined geologic observa­ Lake; the head of the lake was pos­ Observations made at the head tions and instrumental releveling sibly down 1 to 2 feet relative to of Tustumena Lake during the data in the Cook Inlet area, to­ its mouth (Russell Wayland, writ­ earthquake (Joe Secora, oral com­ gether with data from adjoining ten commun., 1964). There was mun., 1964) indicate that the areas, indicates differential sub­ little or no tilting of Lake Chakat­ frozen surface of the lake cracked sidence, maximum crustal depres­ chama (Gordon Giles, written into a mosaic fracture pattern as sion occurring along a linear zone commun., 1964) ; and little or no the lake level rapidly rose and fell. between Portage and Aialik Bay tilting of Bradley Lake near Lake level receded about 2 feet or approximately coincident with Homer (Marvin Slaughter, writ­ and then rose ; sloshing of the lake the axis of the Kenai Mountains ten commun., 1964) . waters continued for about 2 (Plafker, 1965). U.S. Coast and Air reconnaissance observations hours. General lake level was Gl~odetic Survey releveling indi­ made of Tustumena Lake immedi­ judged to have been somewhat cah~s that maximum subsidence ately after the earthquake sug­ lower during the following day or (nearly 6 feet) oocurred near gested southeastward tilting of the two and then to have risen to about Portage and that the amount of I a k e basin. Outlet drainage half a foot higher than the pre­ subsidence diminishes progres­ through the Kasilof River was re­ earthquake level. Whether this sively to about 2 feet northwest­ ported to have decreased to such rise resulted from belated tilting ward towards Anchorage (U.S. an extent that the day following of the lake basin or from the nor­ Coast and Geodetic Survey, 1966, the earthquake a galosh-shod biol­ mal inflow during increased p. 122) . These data indicate re­ ogist was able to walk up its chan­ spring thaw is not clear. Mr. gional tilting to the southeast, ap­ nel (Alaska Dept. Fish and Game, Secora's account, however, is con­ proximately at right angles to the 1965, p. 29). Forested areas at the sistent with the later crude Ineas­ axis of the J{enai Mountains. The lake head were reported to have urements on shoreline features gradient of regional tilt is about been submerged. At the time of a which suggest that tilting was 0.1 foot per mile between Anchor 2-day boat traverse around the probably less than 2 feet and con­ Point and the Homer area, and in­ ceivably less than 1 foot over a dis­ margins of the lake in J nne 1964, creases to 0.2 foot per mile between tance of 25 miles between the lake level was below the normal Homer and Aialik Bay where the mouth and head of the lake. seasonal level (Joe Magargl, oral greatest subsidence ( feet) Similar geologic observations 71h commun., 1964). It was about 5 and Brunton compass measure­ measured in the region occurred feet below the average late autumn ments made in June 1964 along the (George Plafker, written com­ high -lake level, as marked by the margins of Skilak Lake suggest mun., 19·66). The releveling and inner edge of generally barren the same general amount of reg­ geologic data ~are insufficient to ex­ gravel beaches and by conspicuous ional tilting during the earth­ clude the possibility of some local high-water marks on rocky cliffs quake as on Tustumena Lake. Ex­ vertical fault displacements, but, along the margin of the lake. Sub­ tensive flooding of forests >around in general, subsidence appears to Inerged trees were found to be as­ the margins of Skilak Lake was have resulted largely from down­ sociated only with locally dis­ observed by local pilots in Sep­ warping or folding of the crustal turbed and slumped shoreline tember 1964. A subsequent boat rocks. deposits; in no area could the sub- traverse around the lake margin The line of zero change separ·at- F28 ALASKA EARTHQUAKE, MARCH 2 7, 1 9 6 4

ing the depressed area of Cook In­ data. Apparently the line of zero Cook Inlet along its western side let from the more stable area to change passes southwestward (Thor Karlstrom, unpub. data, the north can be only approxi­ across the Susitna Lowlands, a 1964; George Plafker, unpub. mately located by the available few miles west of Tyonek 1and into data, 1965).

CONCLUSIONS

Ground breakage in the Cook and localities with seemingly simi­ ments along previous faults also Inlet area occurred largely in un­ lar conditions were not necessarily may have contributed to the ava­ consolidated deposits. It consisted similarly affected. Many areas, lanche activity in the Eklutna area of slumps toward unconfined faces such as steep bluffs of unconsoli­ and to the distribution of ground such as river banks, delta fronts, dated materials along sea coast, cracks along the west shore of and lake shores; ground cracks in were notable for their lack of visi­ Cook Inlet in line with the Lake low land and valley bottom sur­ ble earthquake effects. Regional Clark-Castle Mountain fault. faces ; transverse and marginal structural elements may have had The distribution of ground cracks in modern and elevated some control over the location and breakage in the 1964 earthquake tidal flats; and cracks in muskeg type of damage incurred in the indicates that in the lowlands of areas. Most of these breaks can be Cook Inlet area, particularly in the the Cook Inlet area the most dam­ attributed to various processes and northeast-trending zone of ground combinations of them such as dif­ breakage in the Kenai Lowland. age from future seismic activity ferential com paction and down­ Here, disruption in a thick se­ can be expected on outwash deltas, slope movements of underlying quence of unconsolidated deposits lowland areas underlain by thick water-saturated fine-grained sedi­ may have been due to movement marine or lake silt, and in alluvium ments. However, distribution of along a fault in the underlying and outwash along the larger this type of damage was irregular, Tertiary rocks. Faulting or adjust- streams.

REFERENCES CITED

Alaska Department of Fish and Game, Prof. Paper 542-B, p. B1-B21. Plafker, George, 1965, Tectonic defor­ 1965, Post-earthquake fisheries eval­ Karlstrom, T. N. V., 1964, Quaternary mation associated with the 1964 uation ; an interim report on the geology of the Kenai Lowland and Alaska earthquake: Science, v. 148, March 1964 earthquake effects on glacial history of the Cook Inlet re­ no.3678,p.1675-1687. Alaska's fishing resources: Juneau, gion, Alaska: U.S. Geol. Survey Prof. Small, J. B., 1966, Alaskan surveys to Alaska, 72 p. Paper 443, 69 p. determine crustal movement ; Pt. 1, Dutro, J. T., Jr., and Payne, T. G., 1957, Logan, Malcolm, 1967, Effects of the vertical bench mark displacement : Geologic map of Alaska: U.S. Geol. March 27, 1964, Alaska earthquake U.S. Coast and Geodetic Survey, 24 p. Survey, scale 1 : 2,500,000. on the Eklutna hydroelectric project, U.S. Coast and Geodetic Survey, Wood, Grantz, Arthur, Plafker, George, and Anchorage, with a section .on Televi­ F. G., ed.-in-chief, 1966, The Prince Kachadoorian, Reuben, 1964, Alaska's sion examination of earthquake dam­ William Sound, Alaska, earthquake Good Friday earthquake, March 27, age to underground communication of 1964 and aftershocks, VI: U.S. 1964, a preliminary geologic evalua­ and electrical systems in Anchorage, Coast and Geodetic Survey Publica­ tion: U.S. Geol. Survey Circ. 491, 35 p. by Lynne R. Barton: U.S. Geol. Sur­ tion 10-3, 263 p. Hansen, W. R., 1965, Effects of the vey Prof. Paper 545-A, p. A1--A30. Waller, R. M., 1966, Effects of the earth­ earthquake of March 27, 1964, at McCulloch, David S., 1966, Slide-in­ quake of March 27, 1964, in the Anchorage, Alaska: U.S. Geol. Sur­ duced waves, seiching and ground vey Prof. Paper 542-A, p. A1-A68. fracturing caused by the earthquake Homer area, Alaska, with a section on Kachadoorian, Reuben, 1965, Effects of of March 27, 1964, at Kenai Lake, Beach changes on Homer Spit by Kirk the earthquake of March 27, 1964, at Alaska: U.S. Geol. Survey Prof. Paper M. Stanley: U.S. Geol. Survey Prof. 'Vhittier, Alaska: U.S. Geol. Survey 543-A, p. A1-A41. Paper 542-D, p. D1-D28.

-U.S. GOVERNMENT PRINTING OFFICE: 1967 Q-262-870