Displacement of the South Flank of 

Kilauea Volcano: The Result of 

Forceful Intrusion of Magma 

Into the Rift Zones 

By DONALD A. SWANSON, WENDELL A. DUFFIELD, and RICHARDS. FISKE 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 963 

Interpretation of geodetic and geologic 

information leads to a new model for the 

structure of Kilauea Volcano 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Swanson, Donald A. 

Displacement of the south flank of Kilauea Volcano. 

(Geological Survey Professional Paper 963) 

Bibliography: p. 37-39. 

1. Kilauea. 2. Magmatism- Hawaii- Kilauea. 3. Earth movements- Hawaii- Kilauea. I. Duffield, Wendell 1 .,joint author. 

II. Fiske, Richard S.,joint author. III. Title. IV. Series: United States Geological Survey Professional aper 963. 

QE523.K5S9 551.2'1'099691 76-17918 

For sale by the Superintendent of Documents, U.S. Government Printing Ofliice 

Washington, D.C. 20402 

Stock Number 024-001-02837-9 
DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO: 

THE RESULT OF FORCEFUL INTRUSION OF MAGMA 

INTO THE RIFT ZONES 

By DoNALD A. SwANSON, WENDELL A. DuFFIELD, and RICHARD S. FISKE 

ABSTRACT 

Seismic evidence has long indicated that the south flank of 

Kilauea Volcano is mobile. Examination of triangulation, trilateration, and leveling data obtained throughout the 20th century shows 

that the south flank of Kilauea has been displaced upward and away 

from the rift zones by as much as several metres. The amount of 

horizontal displacement approximates the probable amount of dilation that accompanies the intrusion of magma as dikes in the rift 

zones and is greatest for p~riods of most intense intrusive activity, as 

evidenced by the frequency of eruptions. Displacement and seismic 

events on the south flank take place soon after intrusive activity, 

indicating that the displacement is the result of forceful intrusion in 

the rift zones, not the cause of relatively passive intrusion. The dikes 

are thought to be nearly vertical or to dip steeply southward, on the 

basis of both interpretation of seismic and displacement data for 

Kilauea itself and comparison with dikes exposed in older, eroded 

Hawaiian shield volcanoes. 

In contrast to the south flank, seismic and geodetic data indicate 

that the north flank is virtually immobile. This contrast is believed 

to reflect the fact that Kilauea was built on the south slope of Mauna 

Loa and was consequently influenced by the gravitational stress system of Mauna Loa, which favors displacement away from the volcanic edifice. The north flank of Kilauea is effectively buttressed by 

Mauna Loa, whereas thr south flank is unbuttressed and free to 

move away from the edifice when prompted by forceful intrusion of 

magma. 

The active part of the east rift zone of Kilauea has apparently 

migrated several kilometres southward with time. This is shown by 

the location of recent vents and by the location of the axis of a positive gravity anomaly along the north edge of the active part of the 

rift zone. The southward migration helps explain several features of 

the geometry of the east rift zone, particularly its prominent bend 

near Kilauea Caldera. The southwest rift zone and the caldera also 

show some evidence of southward migration. 

The Hilina fault system is considered to be a gravity-controlled 

system not directly related to the rift zones. Gravitational instability 

resulting from uplift and seaward displacement is eventually relieved by normal faulting along the seaward part of the south flank. 

The Hilina faults are thought to bottom at shallow depth without 

interseeting magma reservoirs, except possibly along part of the 

lower east rift zone, where the fault system impinges upon the rift 

zone. Strains have been accumulating within the Hilina system 

throughout this century, and a high level of instability may have 

been reached. We anticipate a subsidence event in the not too distant 

future, possibly similar to the damaging events of 1823 and 1868. 

(While this paper was in press, such an event occurred on November 

29, 1975.) 

INTRODUCTION 

Kilauea Volcano is the southeasternmost shield volcano in the Hawaiian-Emperor chain of islands and 

seamounts that extends some 6,000 km across the 

central Pacific Ocean. Kilauea and its giant neighbor 

Mauna Loa (fig. 1) are among the most active volcanoes on earth, presently erupting tholeiitic basalt at 

an average rate of 0.1 km3/yr (Shaw, 1973; Swanson, 

1972). Kilauea is built on the southeast flank of Mauna 

Loa, and flows from the two volcanoes probably interfinger extensively at depth. 

Much has been learned about the structure and 

magma-conduit system of Kilauea during the past 20 

years. A wealth of geodetic, geologic, geophysical, and 

seismic data indicates that magma rises from the mantle through a nearly vertical conduit to a holding reservoir about 2 to 5 km beneath the summit caldera (fig. 

2). This reservoir, almost certainly an intricate plexus 

of dikes, sills, and irregularly shaped chambers, swells 

as it fills, causing easily measurable tumescence at the 

surface. With continued magma supply, the strength of 

the reservoir rocks is eventually exceeded, and magma 

is leaked from the system, resulting in a shallow intrusion and usually, but not always, in an eruption. 

Virtually all eruptions and shallow intrusions occur 

either in the summit area or along the east or southwest rift zones, which extend outward from the caldera 

(table 1). Summit eruptions, most of which take place 

in Halemaumau Crater, are accompanied by numerous 

earthquakes as magma works its way toward the surface. Local uplift is generally observed very near the 

site of summit eruption or jntrusion, but the overall 

summit area may unde_rgo net exp~nsion (table 1, May 

1970, August 1971, July and September 1974), contraction (table 1, 1~24, 1938, 1959, 1967, December 

1974), or virtually no change (table 1, 1907-24, 1927, 

1929, 1930, 1931, 1934, 1952, 1961), depending upon 

complex interrelations among rate of eruption, rate of 

magma recharge from the mantle, and degree of shallow intrusion versus extrusion. 

During rift eruptions, the summit area subsides (table 1, 1955, 1960, 1961, 1962, August and October 

1963, March and December 1965, August and October 

1 
2 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

155° October 1968, and all rift eruptlo sin 1969--74), which 

forms a ridge commonly contaihi g an axial graben. 

The forceful intrusion of magTI\a as dikes in the rift 

zones produces considerable dil tion, which is manifested at the surface by gapin cracks and fissures 

whose ((projections on one wall COfnmonly fit into reentrants in the other wall* * *" (fv!acdonald, 1956, p. 

278). The net long-term effect of such dike-induced dilation must be substantial, for h ndreds or thousands 

of dikes are necessary to account for the gravity anomaly (Kinoshita and others, 1963)l along the rift zones 

and the core of high-velocity material at depth (Hill, 

1969). Not all of these forcible ntrusive events are 

accompanied by eruptions. Dur ng this century, at 

least five inferred intrusive eve ts with no eruption 

caused dilation of the rift zone, as evidenced by observed ground cracking (table 1, 938, 1950, May and 

July 1963, May 1970). 

We have studied the effects ofi rift dilation, principally through geodetic means, and find that the south 

flank of Kilauea has been uplifted and displaced 

southward as a result of forcible i+trusion of magma as 

dikes. During this century, maxlmum net horizontal 

and vertical displacements of the mobile south flank 40 MIL Es have each been several metres, in icating that the volol-----. 10___._2,_0 --r30_,__4..,0-l..5,---0 -K-ILO-'METREs cano grows by ground deformatio as well as by addi10 20 30 

EXPLANATION 

tion of lava flows, though to a 1 sser degree. Our results suggest that the south flan of the volcano has 

--------- been displaced southward, away from the rest of the 

Caldera Apt:t:,~~:~~~~~~~:ry island, throughout the history of Kilauea, whereas the 

Axis of Bouguer gravity 

anomaly along rift zones 

of Kilauea 

Zone of young volcanic 

activity along rift zones 

Generalized scarps in 

Hilina fault system 

Bar and ball on 

down thrown side 

FIGURE 1.-Map of Kilauea Volcano and adjacent areas, showing 

generalized subaerial and submarine topography (from Moore and 

Fiske, 1969), zones of young volcanic activity along rift zones, axis 

of Bouguer gravity anomalies along rift zones (see also fig. 18), and 

generalized scarps in Hilina fault system. Cross sections A-A', 

B-B', and C-C' shown in figure 15. Land and bathymetric contour 

interval 900 m. 

1968, and all rift eruptions in 1970--7 4), apparently 

because magma migrates outward from the central 

reservoir into a rift zone. During east rift eruptions, 

few earthquakes occur between the reservoir and a 

point near the eruption site, suggesting a virtually unobstructed interconnecting conliuit. Many earthquakes, however, take place in the immediate vicinity 

of the eruption, as magma forcibly wedges its way toward the surface from the supply conduit or secondary 

holding reservoir. The site of rift eruption is further 

characterized by local uplift (table 1, 1919, 1955, 1960, 

1961, 1962, March and December 1965, August and 

north flank has remained com~aratively stable because of the gravitational and b ttressing effects imposed by neighboring volcanoes, e pecially Mauna Loa. 

The results of this study help con m the importance of 

the ((edifice effect" (Fiske and Jac son, 1972) in determining the orientation of the str ss field in clustered 

oceanic volcanoes. I 

ACKNOWLEDG ENTS 

Most staff members of the Haw iian Volcano Observatory took part in the 1970, 197 , and 1973 surveys; 

we thank them all, especially R. T. Okamura, M. K. 

Sako, J. B. Judd, and J. C. Fo:rb s. Wilbur Griswold, 

helicopter pilot, skillfully ferrie~ us into and out of 

very tight places during the 1~7 trilateration. R. Y. 

Koyanagi contributed greatly th ough his continuing 

seismic investigation of KilaueJ. . K. Meade supplied 

data from the 1949 U.S. Coas~ nd Geodetic Survey 

triangulation, and J. P. Church coured the files for 

past U.S. Geological Survey levJli g and triangulation 

data and gave much advice regar ing their interpretation. T. Harano and A. C. Loo o the Hawaii Department of Transportation provided t iangulation data for 

the 1955 eruption. R. W. Declte allowed the use of 
STRUCTURAL SUBDIVISIONS OF KILAUEA 3 

EXPLANATION 

Pit crater 

Cone 

Fault 

Bar and ball on downthrown side 

Approximate boundary between 

Kilauea and Mauna Loa 

FIGURE 2.-Map of Kilauea and adjacent part of Mauna Loa, showing main structural features an~ locations of important faults, cones, 

and pit craters. Modified from Stearns and Macdonald (1946, pl. 1). Aloi Crater, mentioned in text, is buried beneath the southwest 

flank of Mauna Ulu; Alae Crater, beneath the southeast flank of Mauna Ulu. 

unpublished leveling data, and W. T. Kinoshita did 

likewise with gravity data. R. W. Decker, J. P. Eaton, 

D. B. Jackson, W. T. Kinoshita, J. G. Moore, Kazuaki 

Nakamura, D. W. Peterson, T. L. Wright, and many 

others contributed stimulating ideas and willingly 

served as sounding boards and critics. R. W. Decker, E. 

T. Endo, R. C. Greene, E. D. Jackson, J.P. Lockwood,~· 

G. Moore, D. W. Peterson, and R. I. Tilling reviewep 

the manuscript at various stages, substantially improving it. I 

STRUCTURAL SUBDIVISIONS OF KILAUEA 

For this paper, we can divide the subaerial part 9f 

Kilauea Volcano into six structural subdivisions (fig. 

3) on the basis of the principal structures shown in 

figure 2: 

1. The summit area is dominated by Kilauea Caldera 

and Halemaumau and Kilauea Iki Craters. The summit is the site of numerous eruptions and contains the 

center of surface deformation over the central magma 

reservoir. 

2. The east rift zorte is an elongate region as much as 

5 km wide along the axis of the constructional ridge 

that forms the eastern part of the volcano. This zone is 

characterized by open cracks, pit craters, and small 

cones and shields and is the site of numerous eruptions. With increasing distance from the summit area, 

the subaerial part of the east rift zone may be loosely 
TABLE !.-Eruptions and inferred intrusive events at Kilauea Volcano, 1900-1974 

[For volume and duration of eruptions, see Stearns and Macdonald (1946) and Wright (1971)) 

Date' 

1900--1907 

1907-1924 

December 1919 

May 1922 

August 1923 

April-May 

1924 

July 1927 

February 1929 

Location of 

eruption 

Halemaumau, sporadically 

active. 

Halemaumau, virtually continuously active. 

Southwest rift zone, creating 

Mauna Iki. 

Makaopuhi and Napau 

Craters. 

Upper east rift zone, between 

Alae and Makaopuhi 

Craters. 

Phreatic explosions in Halemaumau. 

July 1924 } 

July 1929 Halemaumau 

November 1930 

December 1931 

September 1934 

May, June, No eruption 

August 1938 

December 1944 No eruption 

December 1950 No eruption 

Deformation in 

summit area2 

None described 

Small reversible tilts of unknown significance. 

None described, but magma 

column in Halemaumau 

withdrew about 75 m. 

None described, but magma 

column in Halemaumau 

withdrew at least 300 m. 

None described, but magma 

column in Halemaumau 

withdrew about 170 m. 

Subsidence of at least 4 m; 

inward-directed horizontal displacement possibly as much as 1.4 m. 

None described 

Subsidence indicated by 

measurements of ground 

tilt; incomplete leveling 

data for September 1934 

to January 1939 indicate 

subsidence of 42 em, probably due largely to 1938 

events. 

Abrupt summit subsidence, 

3.6 seconds of arc on pendulum seismometer. 

Subsidence of at least 30 em calculated-fram ground tilt 

Deformation in areas outside summit area 

Location 

None described 

____ __ do ____ _____________ _ 

From Halemaumau to 

Mauna Iki. 

Southwestward from Makaopuhi Crater for more than 1 km into Koae fault 

system. 

Southwestward from the 

eruptive fissures for an unknown distance into 

Koae fault system. 

(1) East rift zone near Cape 

Kumukahi, 40 km from 

Halemaumau. 

(2) Koae fault system, about 

2 km southwest of Aloi 

Crater. (3) East rift zone, near Alae 

and Makaopuhi Craters. 

None described 

Upper east rift zone and 

eastern Koae fault system. 

None described 

Upper east rift zone and 

eastern Koae fault system. 

Nature 

None described 

_____ _ do _________________ _ 

Ground cracks and uplift of 

at least 48 em. 

Ground cracks 

______ do _________________ _ 

(1) Ground cracks, normal 

faults, and graben; subsidence up to 3.6 m. (2) and (3) Ground cracks. 

None described 

Ground cracks. New steaming area formed just 

northwest of Kokoolau 

Crater. 

None described 

Ground cracks 

References 

Macdonald and Abbott (1970). 

Jaggar (1947). 

Jaggar (1947); Wilson (1935). 

Bull. Hawaiian Vole. Obs., v. 10, p. 54, 1922. 

Bull. Hawaiian Vole. Obs., 

v. 11, p. 72, 1923. 

Bull Hawaiian Vole. Obs., v. 12, p. 18-19, 66, and 68, 

1924; Wilson (1935). 

Stearns and Macdonald 

(1946). 

Volcano Letter, no. 459, p. 2; no. 460, p. 3; no. 462, p. 

1-4, 1938; no. 463, p. 4, 

1939. 

Macdonald (1954). 

Volcano Letter, no. 510, p. 

-3, 1950. 

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--~~--~--------==~-=~--------------~--~~at~------------------------------------------------------~~--------------------~~====== June 1952 Halemaumau ____________ Probably less than 5 sec of None described __________ None described __________ Macdonald (1955). 

May 1954 

February-April 

1955 

Halemaumau and floor of 

caldera within 1 km northeast of Halemaumau. 

East rift zone, from near 

Heiheiahulu to Kapoho 

Cone. 

inward tilt. 

Ground cracks; probably ______ do__________________ _ _____ do__________________ Macdonald and Eaton (1957). 

horizontal displacements 

of 1 m or more accompanied 

by opening of new fissure. 

Subsidence of at least 42 em Same area as eruption ___ _ 

calculated from ground 

tilt data. 

Ground cracks, uplift, normal faults, graben; horizontal movements up to 

1.6 m. 

Macdonald and Eaton (1964). 
November 1959 

JanuaryFebruary 1960 

Kilauea Iki Crater ___ ____ _ 

East rift zone, near Kapoho 

Cone. 

February, March, Halemaumau 

July 1961 

September 1961 East rift zone, Napau Crater 

to near Heiheiahulu Cone. 

December 1962 

May 1963 

July 1963 

August 1963 

October 1963 

March 1965 

December 1965 

November 19673 

August 1968 

October 1968 

February 1969 

May 19694 

December 1969 

April1970 

May 1970 

July 1970 

January 1971 

East rift zone, Aloi Crater 

to Kane Nui o Ramo. 

No eruption 

East rift zone, in and near Alae Crater. East rift zone, between Napa u Crater and Kalal ua Cone. 

East rift zone, Makaopuhi 

Crater to near Kalalua 

Cone. 

East rift zone, Aloi Crater to 

Kane N ui o Ramo. 

Halemaumau 

East rift zone, Hiiaka Crater 

to near Kalalua Cone. 

East rift zone, Kane Nui o 

Ramo to Puu Kamoamoa. 

East rift zone, Aloi Crater to 

Puu 'Kamoamoa. 

East rift zone, from 1.6 km 

west of Aloi Crater to Alae 

Crater. East rift zone, Mauna Ulu, 

between Aloi and Alae 

Craters. 

East rift zone, in and west of 

Aloi Crater. 

No eruption 

East rift zone, north and 

northeast of Alae Crater. East rift zone, south of Puu 

Huluhulu. 

Inward tilt of 15 p.rad, followed soon thereafter by 

outward tilt of 40 p.rad; 

ground cracks. Subsidence of about 1.5 m, 

calculated from ground 

tilt data. 

None detected ___________ _ 

Subsidence, indicated by 

ground tilt data. 

___ ___ do 

Subsidence of 25 to 30 em 

Subsidence, indicated by 

ground tilt data. 

Subsidence of 4.5 em _____ _ 

Subsidence of 19 em; in warddirected horizontal displa~ement of 14 em. 

Subsidence of more than 15 

em; large horizontal con- traction. 

Subsidence of12 em; inwarddirected horizontal displacement of 11 em. 

Small subsidence and con- traction. 

Uplift of 18 em and outwarddirected horizontal displacement of 29 em in 

south part of caldera. 

Small subsidence and contraction. 

Small subsidence ______ ___ _ 

None described 

(1) Same area as eruption. 

(2) Koae fault system, 7 km 

south of Halemaumau. 

None described 

Same area as eruption 

(1) Same area as eruption. 

(2) Koae fault system 1 km 

west of Aloi Crater. Central and western part of 

Koae fault system, 3 to 8 

km south and southwest 

of Halemaumau. 

Central and eastern part of 

Koae fault system, 4 to 5 

km south and southeast of 

Halemaumau. 

Same area as eruption ___ _ 

From Kane N ui o Ramo across Koae fault system 

for 20 km to Kamakaia 

Hills. 

None described 

Same area as eruption ___ _ 

North and northeast of Alae 

Crater. 

Same area as eruption ___ _ 

East rift zone adjacent to 

south part of caldera. 

Same area as eruption ___ _ 

None described 

(1) Ground cracks, graben. 

(2) Ground cracks, normal 

faults. 

None described 

Ground cracks and normal 

faults, vertical displacements up to 6 m. Ground cracks, normal 

faults, uplift. 

Ground cracks and normal 

faults 

Ground cracks, normal 

faulting; local buckling. 

Ground cracks ____ _____ _ 

Ground cracks, graben ___ _ 

Ground cracks, normal 

faults, uplift, graben. 

None described 

Ground cracks, uplift, 

graben. 

Ground cracks and uplift. __ 

_ _____ do _________________ _ 

______ do _________________ _ 

______ do _________________ _ 

Eaton (1962); Richter and 

others (1970). 

Richter and others (1970; 

Eaton (1962); Hawaiian 

Volcano Observatory (unpub. data). Richter and others (1964). 

Do. 

Moore and Krivoy (1964). 

Kinoshita (1967). 

Hawaiian Volcano Observatory, Summary 31, 1963. 

Peck, Wright, and Moore 

(1966). 

Moore and Koyanagi (1969). 

Wright, Kinoshita, and Peck 

(1968). 

Fiske and Koyanagi (1968). 

Kinoshita and others (1969). 

Jackson and others (1975). 

Do. 

Swanson and others (1976). 

Hawaiian Volcano Observatory (unpub. data). 

Do. 

Do. 

Duffield, Jackson, and Swan- son (1976). 

Hawaiian Volcano Observatory (unpub. data). 

Do. 

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TABLE I.-Eruptions and inferred intrusive events at Kilauea Volcano, 1900-1974-Continued 

Deformation in areas outside summit area 

Date' 

August 1971 

September 1971 

February 19725 

May 1973 

November 1973 

July 1974 

September 197 4 

December 197 4 

Location of eruption 

Southeast part of Kilauea 

Caldera, between Keanakakoi Crater and Hale- maumau. Southwest part of Kilauea 

Caldera and southwest 

rift zone to 3 km southwest of Mauna Iki. 

East rift zone, Mauna Ulu __ 

East rift zone and adjacent 

Koae fault system, Hiiaka 

Crater to 1 km west of 

crater, and Pauahi Crater. 

East rift zone, Pauahi Crater 

to 1 km downrift of Puu 

Huluhulu. 

Southeast part of Kilauea 

Caldera, in and near 

Keanakakoi Crater. 

Halemaumau and southwest 

part of Kilauea Caldera. 

Southeast edge of upper 

southwest rift zone, northeast of Puu Koae. 

Deformation in summit area2 

Uplift of 22 em and outward 

directed horizontal displacement of 34 em. 

Uplift of 48 em; outward 

directed horizontal displacement of more than 

1m. 

Very slight subsidence ___ _ 

Inward tilting of more than 

15 JLrad. 

Subsidence of more than 4 

em; small horizontal con- traction. 

Uplift of more than 15 em; 

large horizontal expansion. 

Outward tilting of more than 15 JLrad just before 

eruption, inward tilting of 

10 JLrad during eruption; 

horizontal contraction of 

16 em. Inward tilting of more than 

100 JLrad; horizontal con- traction of as much as 80 em. 

Location 

None described 

Same area as eruption __ _ _ 

None detected 

Same area as eruption, but 

extended several kilometres farther west into 

Koae fault system. 

Same area as eruption 

None described 

______ do _________________ _ 

Same area as eruption ___ _ 

1Date for commencement of eruption or intrusive event. Where several dates are listed, each indicates a period of er'!Ption or deformation. Range of dates indicates t jme span_during which_mosLdeformation-occuned .... .Undel'-- -- ---nnea date indicates that no inferred forcible intrusions accompanied the eruption; most of these eruptions took __ ....,""""',..-.plae :: alemaum~.;;.;.;,;,~.;.;;,;;.;;.;;,.;.;;,;;.;.;;,;;,;...:....:::,;;;..;,;;.;,;,.;;;;..;;.;..~;.;,.;~::....:.::..;..;;.;;.;,;,:;.;.;.;.;;.;,;.;;;,;,;.;.;..;.;...;,;,;.;,;,;~..;:.;o-..._.-..-. 

Nature 

None described 

Ground cracks and uplift__ 

None detected _____ ______ _ 

Ground cracks and uplift __ 

______ do ___________ ___ ___ _ 

None described 

______ do _________________ _ 

Ground cracks and uplift__ 

References 

Hawaiian Volcano Observatory (unpub. data). 

Do. 

Do. 

Duffield (1975); Koyanagi, Unger, and Endo (1973). 

Hawaiian Volcano Observatory (unpub. data). 

Do. 

Do. 

Do. 

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EARTHQUAKE DISTRIBUTION 7 

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EXPLANATION 

Structural subdivisions of Kilauea 

D . 

Summit area Koa

-

e fault system 

~ [ill 

H Unfaulted p art 

m w "' illiiiiJ Inactive part Hilina fault system 

~ ELj Southwest rift one North flank 

Boundary between volcanoes 

• Triangulatio n or trilatera tio n station 

:<'IGURE 3.--Structural subdivisions of Kilauea Volcano and locations 

of triangulation st ations referred to in this paper and established 

before 1970. Boundaries of subdivisions are gradational in most 

places. Compare with principal structures shown in figure 2. 

3ubdivided into upper, middle, and lower parts. The 

upper part extends from the summit area to about 

Makaopuhi Crater, the middle part from Makaopuhi to 

Heiheiahulu, and the lower part from Heiheiahulu to 

che seacoast at Cape Kumukahi. The rift zone can also 

be divided into active and inactive parts on the basis .gf_ 

data presented in this paper. The east rift zone forms a 

prominent submarine ridge that extends far beyond 

che coastline (fig. 1). 

3. The southwest rift zone, about 4 km wide, is 

~haracterized by structures similar to those of the east 

rift zone. It is the site of many eruptions, though fewer 

chan the east rift zone. The southwest rift zone extends 

only a short distance beyond the shoreline (fig. 1; 

Moore and Fiske, 1969). 

4. The Koae fault system is an east-northeast trending, 3-km-wide zone of open cracks and normal faults 

connecting the east and southwest rift zones. 

5. The south flank is that part of the volcano seaward 

of and bound~d on the north and west by the east and 

southwest rift zones and the Koae fault system. The 

northern part of the subaerial section of the south 

flank is characterized by unbroken, seaward-dipping 

lava flows. The southern part is disrupted by the 

Hilina fault system, a set of predominantly southdipping normal faults. 

6. The north flank is bounded by the summit area 

and the east rift zone. This flank is characterized by 

east- to northeast-dipping lava flows that abut and interfinger with flows from Mauna Loa. 

The rift zones are believed to have a structure similar to that outlined by Fiske and Jackson (1972). 

Chiefly on the basis of seismic evidence, the dikes are 

considered to be shallow, blade like bodies, largely if 

not entirely confined within the volcanic edifice and fed 

either directly from the summit reservoir system or 

from secondary high-level reservoirs in the rift zone 

itself (Swanson and others, 1976). The dips of the dikes 

are believed to be steep to vertical on the basis of (1) 

observations in older, deeply eroded Hawaiian shields 

such as Niihau (Stearns, 1947) and Koolau (Stearns 

and V aksvik, 1935), (2) seismic evidence, which shows 

that earthquakes and tremor directly associated with 

the intrusion of magma occur in a narrow zone directly 

below the point where lava reaches the surface, and (3) 

interpretations of ground deformation data related to 

the intrusion of magma on the upper east rift zone in 

August 1968 (Jackson and others, 1975). Contrary interpretations regarding the dip are examined later. 

The flanks of the volcano are assumed to consist of 

lava flows above sea level and pillow lavas below sea 

level, with a layer of hyaloclastic debris between; data 

for this interpretation are given by Moore and Fiske 

(1969). 

EARTHQUAKE DISTRIBUTION 

The distribution and timing of crustal earthquakes 

provide probably the most unequivocal evidence for instability of the south flank related to magmatic events 

in the rift zones. Koyanagi, Swanson, and Endo (1972) 

-summarized this seismic evidence, some of which is 

shown in figure 4. Numerous earthquakes take place 

beneath the summit area and are presumably related 

to stress release in and near the main reservoir system. 

Almost all other earthquakes occur at shallow or intermediate depths beneath the south flank or its boundary zones, the east and southwest rift zones and Koae 

fault system. Thousands of earthquakes ranging up to 
8 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

_y;~ 

EAST RIFT ZONE ot::~c$ -:::;-"/:00 o o. 

0 10 MILES 

0 10 KILOMETRES 

EXPLANATION 

Earthquake epicenters 

• Earthquake during initial 

period-of activity 

0 

Earthquake during middle 

or late period of activity 

FIGURE 4.-Location of earthquake epicenters from brief seismic 

swarms related to eruptions and ground-cracking events between 

1963 and 1969. Earthquakes plotted are of M~2 and occurred at 

depths less than 15 km. Note that earthquake epicenters migrate 

south-southeastward across south flank following initial seismicity along east rift zone and Koae fault system and that earthquakes are virtually absent from north flank, a distribution also 

typical of periods between eruptions (Koyanagi and others, 1972, 

fig. 1). Slightly modified from Koyanagi, Swanson, and Endo 

(1972, fig. 6). 

magnitude (M) 5.0 take place each year in these areas 

away from the summit, furnishing clear evidence of 

instability within the volcanic pile. Study of the focal 

mechanisms of south-flank earthquakes shows that 

most maximum stress axes are oriented southeast and 

are horizontal or plunge gently seaward (Koyanagi 

and others, 1972; Endo, 1971; E. T. Endo, oral commun., 1974). 

In contrast, the north flank of Kilauea has few 

earthquakes, and most of those that do occur emanate 

from upper mantle depths well beneath the base of 

Kilauea. Thus, an immediate conclusion is that the 

north flank is relatively stable seismically compared 

with the south flank. 

The high seismicity of the rift zones is not surprising, for these are areas where intrusion and eruption 

take place. The reason for the high seismicity of the 

south flank, on the other hand, is not so readily apparent, but the timing of earthquake activity relative to 

rift eruptions and intrusive events appears pertinent 

to an explanation. Figure 4 shows that earthquakes 

related to magmatic events occur on the south flank 

only during the middle and late stage of seismic activity, after the intrusive event has largely or completely 

ended, whereas earlier earthqu kes cluster along the 

east rift zone and adjacent part of the Koae fault system, near where intrusion is aking place. This sequential relation, discusse~ in more detail by 

Koyanagi, Swanson, and Endo (1972), indicates that 

intrusion is a precursor to major south-flank seismicity, which in turn suggests a cause-effect relation between them. The inference is t at stress is built up in 

the south flank by intrusion al ng its northern edge, 

then relieved over a period of few days during the 

late-stage earthquake activity. 

One effect of forceful intrusion of magma is lateral 

displacement of wallrock on e · her side · of the dike. 

This may provide a mechanis to account for the 

south-flank earthquakes. If th, 1 observed south-flank 

seismicity is caused by such di, placement away from 

the rift zone, it should be po s ble to detect this displacement by geodetic mea s. For this reason we 

undertook a geodetic study ofKi auea, results of which 

demonstrate that large ground displacements on the 

south flank and elsewhere have ndeed taken place in a 

remarkably systematic fashion. 

The following presentation of he horizontal and vertical · deformation data is long nd detailed as we attempt to document our case. Ma y readers may wish to 

examine figures 5-14 and then roceed directly to the 

three summary sections on lp g-term horizontal deformation, south-flank horizontal deformation, and 

vertical deformation on the anks. The principal 

interpretive part of the paper b gins with the section, 

~~A Structural Model for the Sou h and North Flanks of 

Kilauea." 

LONG-TERM HORIZONTA DISPLACEMENTS 

DURING THE 20th CENTURY 

EXISTING DATA AND T EIR QUALITY 

Horizontal ground displdc ments over several 

periods a decade or more long ave been derived from 

six regional horizontal control surveys. Four of the 

surveys are triangulations, irl 896 (1914)1 and 1949 

by personnel of the U.S. Coa t and Geodetic Survey, 

and in 1958 and 1961 by pers9n el of the U.S. Geological Survey. The other two sur eys are trilaterations 

conducted in 1970 and 1971 witf _a model 8 laser-beam 

geodimeter by personnel of the r awaiian Volcano Observatory; the general procedu es and measured distances for the 1970 survey, the rilateration most used I 

in this paper, are given by ~ anson and Okamura 

(1975). The regional triangulat on surveys by Wilson 

(1935) and Wingate (1933) arF ot used in this paper 

because neither was tied to a , a eline that we consider 

1 Most of the 1896 (1914) survey took place in 1896 but stations on the south flank were 

not triangulated until 1914. See text discussion of fi re 5D for details. 
LONG-TERM HORIZONTAL DISPLACEMENTS DURING THE 20TH CENTURY 9 

to be stable. Figure 3 shows the location of all triangulation stations occupied at least once prior to reoccupation in 1970. 

Each of these surveys took place over a period of 

weeks to months, and the 1896 (1914) survey was split 

into two intervals separated by 18 years. As Kilauea 

sometimes deforms greatly over much shorter periods 

of time, the effect of possible contemporaneous ground 

deformation on the quality of each survey must be considered. This is done in the section ~~supplemental Information," with the conclusion that the problem of 

contemporaneous ground deformation is minimal, with 

the possible exception of the summit area in 1961. 

The general quality of the four triangulation surveys 

is given in table 2. The 1949 and 1958 surveys are 

adequate to define reliable long-term displacements of 

the magnitudes shown by Kilauea. The 1896 (1914) 

and 1961 data are poor, owing to large closure errors 

and, in 1961, very poor network geometry on the south 

flank. The precision of the 1970 trilateration is on the 

order of ~8 mm/km, far superior to precisions of all the 

triangulations. 

The problems of imprecision and the generally poor 

geometry of the survey networks make the triangulation data unrealiable for strictly quantitative studies. 

Nonetheless, most long-term ground displacements 

have been large enough to overshadow these problems, 

so that the triangulation data can be compared with 

the trilateration data to derive general patterns and 

semiquantitative amounts of displacements. The selection of stable baselihes and the procedure used in determining ground displacements are outlined in the 

section ~~Supplemental Information." 

RESULTS 

Horizontal displacements derived from comparison 

of the four triangulation surveys with the 1970 trilateration survey are shown in figure 5. The net displacements for most stations, especially those on the 

south flank, are large and systematic for the survey 

periods, which range from 9 years to 7 4 years in duration. In this section we correlate these displacements 

with magmatic (eruptive or intrusive) events that occurred during the survey intervals and find that the 

TABLE 2.-Precision of the triangulation surveys 

Triangulation 

1896 (1914) 

1949 

1958 rift summit 

1961 

Closure of triangles used(sec) 

Average 

3.9 

1.19 

i:g} 3.6 

4.8 

Maximum 

12 

3.09 

13.6 

5.6 

9.6 

Order' Probable error in length 

less than' 

3 1 in 5,000 (20 cm/km) 

2 1 in 20,000 (5 cm/km) n3 (good) 1 in 10,000 (10 cm/km) 

3 1 in 5,000 (20 cm/km) 

1Taken from Gossett (1959, p. xiv-xv). Results of the present study suggest that the 1896 

(1914) and 1961 surveys have prJbable errors of 1 in 10,000 (10 cm/km) or less. 

displacements on the south flank are systematically 

related to the opening of new ground cracks in the east 

and southwest rift zones and in the Koae fault system. 

However, the exact timing between specific magmatic 

and displacement events is unknown because the survey intervals are long and may include several events; 

later we examine shorter survey intervals in which 

specific magmatic and displacement events can be correlated. In this section, the results are presented in the 

order of decreasing reliability instead of chronologically. 

1949--70 

Displacement vectors on the lower east rift zone generally point perpendicularly away from nearby fissures 

that opened in 1955, 1960, and 1961 and are of magnitudes close to the probable aggregate amount of 

opening of these fissures, judging by observations of 

the fissures soon after they formed (fig. 5A). Kaliu and 

Honuaula are both located within the rift zone, less 

than 400 m northwest of some of the 1955 fissures 

(Macdonald and Eaton, 1964). Kapoho is southeast of 

the easternmost 1955 vent and the main 1960 vents 

(Richter and others, 1970). Heiheiahulu is southeast of 

the nearest 1961 fissure (Richter and others, 1964). 

The radial pattern of displacement defined by the 

Volcano House Flag, Keanakakoi, Uwekahuna, and 

Ohaikea vectors indicates tumescence of the summit 

region centered northwest of Keanakakoi. Such 

tumescence is reasonable in view of the similar location of swelling centers documented for many later 

periods of inflation (for example, Jackson and others, 

1975). This radial pattern is defined by comparatively 

small displacements, 30 em or less, and would probably 

have been distorted or even completely obscured if the 

baselines had moved. For this reason we have confidence in the stability of the baselines between 1949 

and 1970. The displacement of Keanakakoi is larger 

than at the other summit stations, perhaps partly the 

result of the opening in 1954 of a fissure oriented 

N.75°E. about 1.5 km north of the station (Macdonald 

and Eaton, 1957). 

Puu Huluhulu shows a large northwest displacement of about 1. 7 m that is not perpendicular to the 

trend of nearby eruptive fissures. This displacement is 

possibly the net result of major north-northwest 

movement directly-away from the rift zone during the 

nearby eruptions of 1962, 1963, 1965, and 1968--70 

combined with lesser westward movement away from 

an area of uplift centered over a magma reservoir near 

Makaopuhi Crater (Swanson and others, 1976). 

Ohale, in the Koae fault system, was displaced about 

2.6 m south-southeastward, perpendicular to cracks 

that formed in the area during major faulting events in 
10 

N 

t 

A 1949-70 

c 1949-58 

E 1896(1914)-1958 

DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

P A 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

t 

\ c 

c 1 f 

0 

EXPLANATION 

Displacement vector 

Eruptive fissure 

xxxxxxxx 

xxxxxxxx 

Area of ground cracking 

Trilateration station 

0 40 80 1201NCHES 

0 100 200 300 CENTIMETRES 

VECTOR SCALE 

D 1896(19141-1970 

10 15MILES 

I I I 

10 15 20 KILOMETRES 

MAP SCALE 
LONG-TERM HORIZONTAL DISPLACEMENTS DURING THE 20TH CENTURY 11 

1950 (Finch, 1950), 1963 (Kinoshita, 1967), and December 1965 (Fiske and Koyanagi, 1968), and several 

minor events as recently as May 1970 (Duffield and 

others, 1976). 

The pattern of displacement directly away from the 

southwest rift zone suggests rift dilation, for Koae and 

Kamakaia are southeast of the axis of the rift zone and 

Puu Ulaula and Kapapala northwest. The several 

episodes of faulting in the nearby Koae fault system 

could also have contributed to the displacement of 

Koae and Kamakaia. 

1958-70 

All stations within the lower east rift zone were displaced southeastward, directly away from the rift zone, 

in a pattern consistent with rift dilation during the 

1960 and 1961 eruptions (fig. 5B). The 1960 vents were 

near Kapoho, and the eastern 1961 vents near Heiheiahulu. The four stations between Kapoho and Heiheiahulu are not located near vents active since 1958, but 

their displacements may record the effects of intrusion 

accompanying the two eruptions. 

Stations on the south flank (Pohoiki, Malama Ki, 

YY66, and all stations southwest of YY66) were displaced by large amounts directly away from the east 

rift zone. These displacements are largest (2.3 m at 

Panau) at stations opposite the most active part of the 

rift zone between 1958 and 1970, even though the 

nearest vent areas are more than 5 km away. The displacements generally decrease away from the rift zone; 

for example, compare Panau with Laeapuki and 

Pulama with Kupapau. 

FIGURE 5.-Horizontal ground displacements (solid vectors) at 

Kilauea Volcano derived from comparison of 1896 (1914), 

1949, 1958, and 1961 triangulation surveys with 1970 trilateration survey. Procedures used outlined in the section "Supplemental Information." Base stations considered stable are · 

underlined. Southwest rift zone and active part of east rift 

zone (fig. 3) are shown by shading. Locations of rift eruptions 

and major episodes of ground cracking that occurred during a 

given survey interval are indicated. Vectors in C were obtained by subtraction of vectors inB from A; vectors inE were 

obtained by subtraction of vectors in B from D. Dashed vectors in C and E were computed by Lloyd (1964) using standard least-squares analysis with no geologic constraints. Dotted vector for Honuaula in A indicates station was intersected 

but Ii'bt occupied during 1949 survey. Alii in F shows no displacement greater than survey error. See text for discussion 

of the vectors at Panau and Laeapuki in F. Parts of network 

common to both surveys are indicated for each survey interval; further information can be obtained from the U.S. Geological Survey, the Nation~l Geodetic Survey, or the senior author. 

W aikahekahe and Halona, located a comparable distance from the axis of the rift zone as Laeapuki and 

Kupapau but on the north flank, registered little if any 

displacement. 

The radial pattern of displacements in the summit 

area during this period indicates expansion around a 

center 1-2 km southwest of Keanakakoi. This center, 

south of that for the 1949-70 interval (fig. 5A) corresponds to an area of uplift defined by leveling between 

1958 and 1971 (Okamura and Swanson, 1975). The fact 

that these small horizontal displacements define the 

center of deformation quite precisely is evidence that 

the assumption of baseline stability is justified. 

Puu Huluhulu shows about the same displacement 

for the 1958--70 interval as the 1949-70 interval; this 

indicates that most displacement took place after 1958, 

consistent with the absence of upper east rift eruptions 

between 1949 and 1958. The displacement at Ohale 

during the 1958--70 interval has about the same 

azimuth as between 1949 and 1970 but is somewhat 

smaller, probably because it does not reflect the 1950 

ground-cracking event in the surrounding Koae fault 

system. Koae moved away from the southwest rift 

zone, and its magnitude of displacement suggests that 

about half of its 1949-70 displacement took place after 

1958. 

194!}.-58 

Vector subtraction of the 1958--70 displacements 

from the 1949-70 displacements yields the 1949-58 

values. Kaliu, Honuaula, and Kapoho, all within the 

rift zone, clearly responded to dilation accompanying 

the 1955 eruption (fig. 5C). Heiheiahulu, Puu 

Huluhulu, and Uwekahuna did not move measurably, 

Ohale reflects the 1950 ground-cracking episode near 

it, and Keanakakoi was displaced away from the 1954 

vent fissure nearby in the caldera. 

The dashed vectors in figure 5C are displacements 

calculated by Lloyd (1964, fig. 3A) using data from the 

1949 and 1958 triangulation surveys adjusted by 

least-squares methods with no geologic constraints. 

The pattern of these displacements is rather similar to 

the one we derived, except for Puu Huluhulu and 

Heiheiahulu, where Lloyd's data show large displacements along azimuths nearly parallel to the east rift 

zone, and ours show none. This discrepancy clearly relates to the methods used in the derivation of the displacements. We believe it important that there is no 

geologic evidence for deformation that would give rise 

to displacements at Puu Huluhulu and Heiheiahulu of 

the azimuths and magnitudes indicated by the leastsquares adjustment. 
12 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

1896(1914)-1970 

In 1896, stations Kulani, Olaa, Kaloli, West 

Heiheiahulu and Heiheiahulu (100 m apart), Kaliu, 

Honuaula, Kapoho, Moana Hauae, and Hakuma 

(1891) were triangulated, and our derivation of their 

displacements up to 1970 is shown in figure 5D. In 

1914, Heiheiahulu-Kaliu was used as the base for 

triangulating Hakuma (150 m south of Hakuma 

(1891)) and all stations farther west. Assuming that 

this base was stable between 1896 and 1914, we derived the shortest vector shown in figure 5D at 

Hakuma for the period 1914--70. The Hakuma and 

Hakuma (1891) vectors are so similar in magnitude 

and azimuth, with the vector for the longer period 

somewhat larger, that the assumption of stability of 

the Heiheiahulu-Kaliu baseline for the period 189&-

1914 seems justified. Furthermore, this period was one 

of almost continuous weak eruption in the famous 

Halemaumau lava lake at the summit of Kilauea (table 1) and on the basis of recent work at Kilauea it is 

unlikely that significant deformation took place ~long the rift zone during such summit behavior, especially 

as far as 3~50 km downrift. We therefore assumed 

baseline stability and derived the 1914--70 displacements for Kupapau, Pulama, Laeapuki, and Panau. 

The general relations between displacements and 

eruptive fissures were the same for the 1896(1914)-

1970 period as for the previously discussed examples. 

Heiheiahulu, Kaliu, Honuaula, and Kapoho were all 

displaced at right angles away from nearby vent fissures and ground cracks. Moana Hauae and other 

south flank stations farther west were also displaced 

away from the nearest active parts of the east rift zone, 

and the seaward stations (Laeapuki and Kupapau) 

were displaced less than landward stations (Panau and 

Pulama) (compare fig. 5D with 5B). The largest displacement, nearly 4.4 m at Panau, was opposite the 

most active part of the rift zone. 

1896 (191 4)-1958 

Displacements for this period (fig. 5E) were derived 

by subtracting vectors of figure 513 from corresponding 

vectors of figure 5D. Lloyd (1964, fig. 3A) also computed vectors (dashed in fig. 5E) for this period, again 

by a least-squares analysis. The displacements derived 

by Lloyd and by us are rather similar, although ours 

are smaller and more systematic, lacking the clockwise 

swing suggestive of adjustment or survey error. Both 

sets of displacements show that the south-flank stations moved seaward by large amounts, most likely 

along azimuths nearly perpendicular to the trend of 

the east rift zone. Comparison of figures 5E and 5D 

shows that only about half of the displacement of the 

south flank from 1914 to 1970 took place in the 44 

years before 1958. Thus the a e of displacement has 

been almost four times greater n the last 12 years, a 

time during which eruptionJ long the middle and 

upper east rift zone have been r more frequent than 

during the earlier period (table 1). 

Displacements ofHeiheiahul , Kaliu, and Honuaula I 

between 1896, 1958, and 1970 re reasonable in light 

of the eruptions in these areas. owever, the displacement of Kapoho between 18 6 and 1958 is unexpectedly . small, considering t~e f ~or episode of ground 

cracking and graben subs1de:mc that took place just 

north of Kapoho in April 19214 (Finch, 1925; Jaggar, 

1924). In fact, the overall displ ement at Kapoho between 1896 and 1970 can be la gely accounted for by 

the 1960 eruption (compare figs 5B and 5D). We cannot eliminate the possibility th t significant displacement during the 1924 event has one undetected owing 

to unrecognized survey errors. , owever, the mode of 

ground deformation in 1924 ( aximum subsidence 

along the north edge of the grab n and northward tilting of the graben floor: Finch, 1925; Jaggar, 1924) 

suggests the development of a asymmetric graben 

similar to that which Cloos (19 8, fig. 18) modeled in 

clay. In this model, the north si e of the graben would 

have moved northward, and lit le if any ground displacement south of the graben ould be expected. 

1961-701 

Three major episodes offaul i g (May and July 1963 

and December 1965) in the Ko fault system (fig. 2) 

and thirteen upper and middl ~ e st rift eruptions (December 1962, August and Ocro er 1963, March and 

December 1965, August and ([) ober 1968, February 

1969, and five events associat d with the Mauna Ulu 

eruption in 1969 and 1970) wer the dominant structural events of this interval. 0 ale moved southeastward more than 2m, and Puu "fi luhulu moved northwestward more than 1. 5 m, ~erpendicular to new 

ground cracks and vent fissu es (fig. 5F). Dilation of 

the southwest rift zone is sugge ted by displacements 

at Koae, Kamakaia, and possib Puu Ulaula. South 

flank stations within, and inlf, tl of, the Hilina fault 

system (fig. 2) moved seaward. The displacement at 

Kaena Point is s~milar to that Ia nearby Laeapuki for 

the 1958-70 perwd (fig. 5B), qo surprising since the 

two stations are equidistant fr m the east rift zone 

only 4 km apart, and are not sJp rated by intervenin~ ground cracks of faults. I 

Laeapuki itself was not tria gulated in 1961, as 

shown in figure 5F, but Panau as triangulated from 

Kaena Point and Laeapuki. In rder to estimate the 

displacement of Panau betwee'n. 1961 and 1970 it is ' 

necessary to assume a di p acement vector at 
LONG-TERM HORIZONTAL DISPLACEMENTS DURING THE 20TH CENTURY 

Laea puki. If we assume that the displacement at 

13 

Laeapuki for 1961-70 is the same as that measured at 

Kaena Point for the same period, the displacement derived for Panau compares closely with that for the 

1958-70 period (compare figs. 5F and B). This close 

agreement seems reasonable and constitutes evidence 

that the 1961-70 displacements are indeed reliable, 

despite the poor quality of the triangulation (table 2) 

and the weak network geometry. 

Just as for earlier periods, south flank stations opposite the most active parts of the east rift zone and Koae 

fault system were displaced more than other stations 

(for example, compare displacements west of Goat with 

those east). The north flank apparently remained stable, as the displacement at Alii is well within probable 

survey error. 

AUGUST-SEPTEMBER 1970 TO OCTOBER 1971 

This period is bracketed by two trilateration surveys 

conducted on the western half of Kilauea. A more 

closely spaced survey network was used than during 

the previous periods discussed. Major eruptions and 

related ground cracking occurred in August 1971 in 

Kilauea Caldera and September 1971 in the caldera 

and along the southwest rift zone. In addition, a small 

eruption took place in late January and early February 

1971 from new cracks south ofPuu Huluhulu (fig. 6) on 

the east rift zone; this episode was related to the contemporaneous eruption at nearby Mauna Ulu (Swanson and others,.1971). 

The effects of the August and September eruptions 

dominate the displacement map. HVO 34, about 1.5 

km west-southwest of HVO 10, shows the largest displacement, more than 1.25 m, which is best interpreted 

as the net result of deformation attending the two 

eruptions combined with lesser inflation of the summit 

area. Most stations in the summit area show some effect of this inflation, which took place largely before 

the August eruption. Displacements decrease abruptly 

down the southwest rift zone beyond the eruption site. 

The south flank was markedly displaced during this 

period, although far less so than areas closer to the 

eruption sites. Displacements are directed away from 

the general trend of the east rift zone and Koae fault 

system and are oblique to the southwest rift zone. The 

magnitude of displacement is greatest in the sector directly south-southeast of the area in which the three 

eruptions took place and decreases both to the east and 

west. In contrast to previous periods, the eastern part 

of the Hilina fault system dilated slightly as indicated 

by direct measurement of extension between Laeapuki 

and Panau, and Kupapau and Pulama; this extension 

is not shown clearly in figure 6 because of the scale. 

The western part of the Hilina system was displaced 

Kulani 0 

MAP SCALE 

10 

10 15 20 25 INCHES 

20 30 40 50 60 CENTIMETRES 

VECTOR SCALE 

15 MILES 

N 

• Trilateration station. 

Open dot if no displacement detected 

------7 

Displacement vector 

Eruptive fissures and 

associated ground 

cracks 

10 15 KILOMETRES 

FIGURE G.-Horizontal ground displacements betweeJJ. August 24-

September 23, 1970 and October 3-18, 1971, derived from trilateration surveys. Holei was first occupied on November 18, 1970. 

Stations held fixed (Kulani, Strip, and Keakapulu) are underlined. 

Eruptive fissures and associated ground cracks that opened during 

survey interval are shown. Southwest rift zone and active part of 

east rift zone (fig. 3) are indicated by pattern. Stations west of 

Karnakaia and Nali show no displacement greater than survey 

error. Diagram of network showing lines and stations occupied is 

given by Swanson and Okamura (1975, fig. 2). 

seaward as a unit, with little or no buildup of horizontal strain. 

SUMMARY OF LONG-TERM HORIZONTAL DEFORMATION 

Long-term horizontal displacements, as measured 

over periods of years to decades, are of considerable 

magnitude at Kilauea. The summit and southwest rift 

zone showed small net expansion for the periods 

1949-70 and 1958-70 and considerable dilation related 

to the 1971 eruptions. Throughout this century, stations within the east rift zone and Koae fault system 

were displaced along azimuths nearly perpendicular to 

the rift zone and fault system; stations southeast of 

active fissures were displaced south-southeastward, 

those northwest of active fissures, northnorthwestward. The entire south flank moved southeastward in a direction almost exactly perpendicular to 

the east rift zone and Koae fault system; this direction 

is more nearly perpendicular to the trend of the rift 

zone as a whole than to individual fissures within the 

rift zone. The direction of displacement is similar to 

that of maximum stress axes found from the study of 
14 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

the focal mechanism of south-flank earthquakes. The 

displacements on the south flank are large, as much as 

4.4 mat Panau between 1914 and 1970 and 2.3 mat 

Panau between 1958 and 1970. Displacement of the 

south flank is greatest south-southeast of the most active part of the rift zone for a given survey interval, 

and observations show that the magnitude of this displacement approximately accounts for the probable 

total width of new fissures and ground cracks in the rift 

zone. For at least the 1960 eruption, north-flank stations about 10 km from the vents did not definitely 

move, whereas south-flank stations as far as 10 km 

from active vents moved tens of centimetres southward. This is consistent with the seismic evidence indicating stability of the north flank relative to the south 

flank. In contrast to north-flank stations, those close to, 

but west of, the southwest rift zone were displaced toward Mauna Loa during the 1970--71 period. 

Most of this evidence implies a direct causal relation 

between magmatic events and ground displacement. 

However, the periods of measurement between these 

regional surveys were so long that they contain more 

than one magmatic event; consequently the measured 

displacements are the net effect of several episodes of 

deformation. These displacements alone are therefore 

not adequate to correlate a specific eruptive or intrusive event with a specific episode of ground deformation, and we must turn to shorter survey periods to 

demonstrate such a correlation. 

HORIZONTAL DEFORMATION DURING 

SPECIFIC INTRUSIVE OR EXTRUSIVE EVENTS 

In this section, we present examples of short-term 

deformation events keyed to specific eruptions or episodes of ground cracking associated with intrusions. It 

is well known that filling and emptying of the magma 

reservoir system beneath the summit of Kilauea is 

generally accompanied by largely reversible ground 

deformation (Wilson, 1935; Eaton, 1962; Decker and 

others, 1966; Fiske and Kinoshita, 1969). We focus attention here on other types of events that apparently 

reflect the forceful intrusion of magma as dikes into 

the rift zones. 

Most of our examples are controlled by geodimeter 

data of excellent quality, indicated by both repeated 

measurements and small closure errors. The triangulation data for the 1955 eruption are poor (third order), 

but the pattern of displacement is compelling. 

FEBRUARY-MAY 1955 EAST RIFT ERUPTION 

The early part of the February 28--May 26, 1955 

eruption took place in an area where a new highway 

was under construction. Fortuitously, triangulation 

surveys were carried out just and just after the 

eruption; all stations in this l network are located 

along the east rift zone. The -..a, •. ,, .... ,,a,, indicated that 

substantial ground m place during the 

eruption. 

The ground displacements in figure 7 were 

derived using two different as!;;;ul~ptiOinS. The preferred 

assumption is that all of Honuaula, 

Kapoho, and Kaliu between and 1958 (fig. 5C) 

took place during the 1955 The other assumption, used by Macdonald Eaton (1964) in 

computing displacement, is that apoho did not move 

during the eruption. Both · rpretations indicate 

northwest displacement of stat s north of the fissure 

zone, and displacement on the preferred 

assumption indicate of stations 

south of fissures. Macdonald Eaton (1964, p. 107) 

noted that the pattern of ts is similar to 

that produced by ~~a simple open of the fissure 

zone," with no evidence of stri slip movement. Stations opposite the central part of · s zone, where eruptive activity was concentrated, edmore than those 

at either end. 

The displacement at Honuaul • 

1 

30' 

0 20 40 60 80 INCHES 

0 50 100 150 200 CENTIMETRES 

VECTOR SCALE 

• 

Heiheiahulu 

\ 

Displacement vector 

is less than that at 

5 MILES 

• 

Triangulation station 

FIGURE 7.-Horizontal ground during February 

28-May 26, 1955 east rift eruption. vectors were derived on 

preferred assumption that all ui:;pu:t'l;~l~ ~~nt of Honuaula, Kapoho 

and Kaliu between 1949 and 1958 took place during 1955 

eruption. Dashed vectors were Macdonald and Eaton 

(1964) on assumption that Kapoho move during eruption. 

Data for triangulation· stations of Honuaula-Kapoho 

baseline were obtained in late 19 eruption) and MayJune 1955 (after eruption) by the of Hawaii· Highway 

Department. 
HORIZONTAL DEFORMATION DURING SPECIFIC INTRUSIVE OR EXTRUSIVE EVENTS 15 

stations farther northwest. This may result from the 

combined effect of horizontal dilation and uplift along 

the fissure zone (Macdonald and Eaton, 1964, p. 105--

107), for such uplift should in theory cause the 

maximum horizontal displacement to be located some 

distance away from the crest of the uplift (Mogi, 1958; 

Dieterich and Decker, 1975). Alternatively, the intrusion of northwest-dipping dikes could have caused such 

a displacement pattern, although first-motion studies 

of earthquakes during the eruption suggest normal 

faults dipping 70° to 90° southeast (Macdonald and 

Eaton, 1964, p. 122). 

FEBRUARY-MAY 1969 EAST RIFT ERUPTIONS 

Eruptions on February 22-28 and May 24--29, 1969 

on the upper east rift zone took place within the newly 

established geodimeter network (fig. 8A). Surveys over 

the February eruption were conducted on February 

11-13 and 24--27; most ground deformation occurred 

between February 22 and 24 (Swanson and others, 

1976). Surveys over the May eruption were conducted 

on April 21-23 and June 2-4. The two bench marks 

closest to the eruption sites, Puu Huluhulu (within the 

rift zone) and HVO 117 (along the northern margin of 

the south flank), moved away from the vent fissures 

and associated ground cracks. The displacement vectors are nearly perpendicular to the new fissures and 

reflect the divergence in trend of the fissures for each 

eruption. 

SEPTEMBER 1971 ERUPTION 

An eruption took place on September 24--29, 1971 

from a line of fissures between Kilauea Caldera and a 

point along the southwest rift zone nearly 12 km away 

(fig. 8B). The eruption site lies within a trilateration 

network that was occupied August 16--19 and October 

4--7. Most of the stations are either within the southwest rift zone or near the caldera. Horizontal displacements were directed away from the new fissures 

and ground cracks and were of roughly equal magnitude on both sides of the eruptive zone. The average 

extension across this zone, indicated by the displacement vectors, was ~bout 1.4 m, virtually the same as 

the total amount of opening on new cracks as determined by direct measurement. Stations HVO 111 and 

HVO 109, on the north edge of the south flank of 

Kilauea about 6 km from the eruption zone, were displaced an order of magnitude less than stations nearer 

the zone. This dramatic decrease may largely reflect 

the partial closing of preexisting open cracks in the 

intervening Koae fault system. Such closing was 

documented by detaliled measurements of deformation 

related to the forceful intrusion of magma into the 

southern part of the caldera in May 1970 (Duffield and 

others, 1976). 

3 KILOMETRES 

MAP SCALE 

5 10 15 20 INCHES 

I I I I I I I I I 

0 10 20 30 40 50 CENTIMETRES 

VECTOR SCALE 

A 

EXPLANATION 

/ 

Displacement vector 

3 MILES 

3 KILOMETRES 

MAP SCALE 

0 10 20 30 CENTIMETRES 

~ 

B 

N 3 MILES 

r 

~auahi '- Puu Huluhulu / ~:;/"' 

vcrater / / .-- ~ V/\Aiae Crater 

~- /v~V HVO 117 ~e'O~ 

EXPLANATION 

/ /-:// Eruptive fissure 

1\ February 1969 

\displacement vector 

f\ May 1969 

\ displacement vector 

\ 

N 

1 

FIGURE B.-Horizontal ground displacements related to specific 

recent eruptions at Kilauea. A, February and May 1969 

upper east rift eruptions; base stations are HVO 135 and 

HVO 136 (fig. 6). February 1969 data from Swanson and 

others (1976). B, September 1971 summit and southwest rift 

eruption; base stations are Kulani, Strip, and Keakapulu (fig. 

3). Kalanaokuaiki Pali is southernmost fault scarp in Koae 

fault system. 

SPECIFIC DISPLACEMENT EVENTS ON THE SOUTH FLANK 

The foregoing e~amples demonstrate that horizontal 

deformation accompanying single eruptions is similar 

in style to that suggested by the long-term geodetic 

surveys. We feel confident that the long-term displacements are the summation of several smaller 

episodes of deformation. All of the examples of specific 

displacement events, however, dealt with the rift zones 

themselves. The behavior of the south flank of Kilauea 
16 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

must be related in some way to the rift zones, but what 

is this relation? 

The 1970--71 survey period (fig. 6) is the only reasonably short period for which measured ground displacements are available for the south flank. Even this 

period, however, includes three eruptions, a prolonged 

episode of summit inflation, and more tqan a year during which other processes could have affected the south 

flank. The measured displacements for this period are 

consistent with the interpretation that they directly 

reflect the effects of the eruptions, but we now present 

more specific examples of deformation events on the 

south flank. 

Strains between March 6 and August 27, 1970.-

Displacement vectors on the south flank are nearly 

parallel to one another, so that there is little, if any, 

extension or contraction along lines perpendicular to 

them. However, there has been measurable extension 

or contraction across the Hilina fault system, parallel 

to the direction of displacement, for all survey periods, 

so that coastal stations are displaced by different 

amounts, generally smaller, than inland stations. 

Thus we can use the observation of strain along a 

south-southeast direction, with little or no strain perpendicular to that direction, to infer displacement of 

the type indicated by the more complete survey. This is 

not the only interpretation, but we believe it reasonable in the context of Kilauea deformation. 

As an example, figure 9 shows the results of 

geodimeter measurements for three triangles on the 

south flank between March 6 and August 27, 1970. 

Most lines that cross the Hilina fault system at high 

angles contracted, whereas lines approximately parallel to the trend of the system did not change length 

significantly. Contractile strain characterized the 

area. The principal axis of contraction (E2) of the calculated strain ellipse is oriented approximately perpendicular to the east rift zone and Koae fault system and 

parallel to the azimuth of overall south-flank displacement. The minor axis of the ellipse is very small, 

probably within the limits of survey error. These relations suggest that the south flank underwent seaward 

displacement during this short survey period. 

During this period, two ground-cracking events took 

place in the upper east rift zone and adjacent part of 

Kilauea Caldera (fig. 9). On April 9, wide ground 

cracks opened in and west of Aloi Crater, and the crater was filled by new lava. On May 15, a major earthquake swarm (Endo, 1971) accompanied the inferred 

forceful intrusion of magma into the sout~east part of 

the caldera; this intrusion resulted in severe uplift, 

dilation, and some ground cracking (Duffield and 

others, 1976). A reasonable interpretation is that the 

measured contraction and inferred displacement of the 

south flank were caused by one o both of these deformation events. 

Linear contraction of a geodimet r line between 1965 

and 1970.-We can extend the ob ervation that strain 

accumulates across the Hilina fa t system during displacement events to the interpretation of long-term 

measurements of a geodimeter ine oriented nearly 

parallel to the azimuth of displ ement of the south 

flank. Figure 10 shows the locatio of this 5.4-km-long 

geodimeter line, which was e sured 30 times between August 1965 and Dece er 1970; measurements were effectively halted in December 1970, when 

the line began to skim the top of newly erupted lava 

flow, and soon thereafter. the inl nd station was covered by lava. In the 5¥:3-year peri d of measurements, 

the distance contracted a net arh nt of 32 em (an extensional strain of-5.9 x 10-5). T e rate of contraction, 

at least during 1969--70 when r easurements were 

most frequent, was related in systematic way to 

major magmatic and structural vents on the upper 

east rift zone and adjacent area in the caldera and 

Koae fault systems. Each new ruption or groundcracking event in 1969--70 was llowed 2 to 4 weeks 

later by an episode of rapid contra tion. Measurements 

were infrequent before 1969 but are consistent with 

the later pattern. The period ofl aximum contraction 

includes three upper east rift er ptions, all of which 

were accompanied by substantiJl ound cracking and 

large measured horizontal displf c . ments (Jackson and 

others, 1975; Swanson and other , 1976). 

Beginning in early 1970, the g odimeter line began 

to show periods of extension alte~ ating with periods of 

contraction. Most of these exteh !ions could be correlated in time with earthquakes of M ~ 3.5 having 

epicenters within a few kilometr s of the geodimeter 

line and focal depths between ' ut 5 and 9 km. All 

such earthquakes between Fe r ary and November 

1970 are noted with respect to cha ' ges of line length in 

figure 10. After the earthquakes in July and September, we predicted extensions later confirmed by 

measurements. There was no laf e earthquake on the 

south flank before the November extension, but a 30-

km-deep earthquake with M of · I took place beneath 

the southern part of the caldera ja the end of October. 

Summary and interpretation of fo izontal deformation 

on the south flank-The data prF nted in this section 

show that the south flank resp nds to specific rift 

eruptions and related structura' ents by undergoing 

displacement away from the ri£t ones. The resulting 

deformation is similar in style j t that shown by the 

long-term survey periods. Almqs certainly, the longterm displacements are the cumulative effect of several magmatic and structural e ents along the rift 

zones and within the adjacent su mit caldera. 
VERTICAL DISPLACEMENT OF SOUTH AND NORTH FLANKS OF KILAUEA 17 

KOAE FAULT SYSTEM 

N 

0 

0 

Aloi 

Crater 

2 

RIFT 

Alae 

Crater 

C) 

2 

3 4 

zoNE 

Makaopuhi 

Crater 

a 

3 4 

Napau 

Crater 

5 KILOMETRES 

5 MILES 

EXPLANATION 

Pit crater or caldera 

Area of ground 

deformation on 

indicated date 

L::.= -7.6 

Dilatation, in units of 10-6 

)>------<( ~ 

Maximum Maximum 

contraction extension 

Axes of strain ellipse 

• Trilateration station 

FIGURE 9.-Changes in line length, extensional and dilational strains, and orientation and magnitude of principal axes of strain ellipse 

for three triangles on south flank between March" 6 and August 27, 1970. Changes in length are in millimetres; numbers in parentheses are extensional strains in units of 10-5• Dilation and center of strain ellipse are plotted near center of gravity of each survey 

triangle, following usual convention. Kalanaokuaiki Pali forms south side of Koae fault system. 

The response of the south flank to rift events is not 

immediate; instead, there is a lag time of 2 to 4 weeks 

before the wave of deformation reaches the area 5 to 10 

km from the rift zones, an average propagation velocity on the order of 100m/day. The lag time shows that 

the displacement is a result, not a precursor, of the 

magmatic event in the rift zones, a conclusion consistent with the timing of seismicity on the south flank 

relative to the rift er ents. 

Throughout this qentury, deformation of the south 

flank has resulted in net contraction across the Hilina 

fault system (fig. 5D), and this contraction was closely 

monitored between 1965 and 1970. Such long-term accumulation of contractile strain may have reached a 

critical level by early 1970, so that comparatively 

small but nearby earthquakes could trigger brief 

episodes of extension. We return to this contention in a 

later section on the Hilina fault system. 

VERTICAL DISPLACEMENT OF 

SOUTH AND NORTH FLANKS OF KILAUEA 

Repeated leveling surveys on Kilauea during this 

century document substantial vertical displacement of 
18 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

5404.20 

.18 

.16 

.14 

.12 

.10 

.08 

.06 

.04 

.02 

CJ) 

w 

a: 

1-

w 5404.00 ~ 

z 

:r:' 

1- 5403.98 (.!J 

z 

w 

....J 

.96 

.94 

.92 

.90 .90 

.88 CJ) 

w .88 a: 

1-

w 

~ 

.86 z .86 

£ 

1-

(.!J 

.84 z 

w .84 A ....J 

.82 

5403.80 

1969 

FIGURE 10.-Contraction of 5.4-km-long geodimeter line (upper right inset) on south flank of Kilauea u""•·•n='""u 1965 and early December 1970. Major magmatic and structural events are: 1, December 1965 eruption and ground cntc~ n!g; , November 1967-July 

1968 summit eruption; 3, August 1968 eruption; 4, October 1968 eruption; 5, February 1969 eruption; 6, e! nmrtg of May 1969-0ctober 1971 Mauna Ulu eruption; 7, new fissure north of Alae Crater, December 1969; 8, ·new fissure in and west of Aloi 

Crater, April 1970; 9, intrusion and cracking in southern part of Kilauea Caldera, May 1970; 10, new east of Mauna Ulu, 

July 1970. Inset in lower left shows changes in line length relative to times of all south-flank of magnitude (M) ;;. 3.5 

between February and early December 1970 and one caldera (Cal) earthquake at the end of October: A, M 4.1; B, M=3.8; C, M=3.9; 

D, M=4.1; E, M=3.5; F, M=4.3; Cal, M=4.0. 
VERTICAL DISPLACEMENT OF SOUTH AND NORTH FLANKS OF KILAUEA 19 

the south flank and minor, if any, displacement of the 

north flank. In general, these displacements correlate 

in time and space with nearby eruptions, a relation 

also found true for the horizontal deformation. We are 

not concerned here with deformation in the summit 

area, although it is well known that this area undergoes uplift between eruptions and, generally, subsidence during eruptions (Wilson, 1936; Fiske and 

Kinoshita, 1969). We also do not examine vertical deformation within the rift zones themselves, for several 

workers have previously shown that the area near an 

eruption site is uplifted, commonly with the development of a keystone graben along the linear crest of the 

uplift (Macdonald and Eaton, 1964; Fiske and 

Koyanagi, 1968, fig. 8; Jackson and others, 1975; 

Swanson and others, 1976). 

The leveling data on the flanks were obtained over 

periods of several days during more extensive surveys. 

All available information indicates that no deformation occurred during the periods of leveling. The leveling data were evaluated and tabulated by Okamura 

and Swanson (1975) and Karren (1959). In particular, 

Okamura and Swanson (1975) recomputed data from a 

1921 survey in order to correct for errors in rod length 

and instrument collimation by somewhat more accurate methods than previously used (Wilson, 1935). 

CENTRAL PART OF SOUTH FLANK 

Leveling surveys were completed across the central 

part of the south flank in 1921, 1958, 1965, and 1971 

(fig. 11). The estimated qualities of the surveys are: 

1921, poor third order; 1958, good second order; and 

1965 and 1971, poor second order or good third order 

(see U.S. Geological Survey, 1966, for definitions of 

orders). 

Bench mark (BM) 10 (inset, fig. 11A) is the only station common to all four surveys and is taken as the 

local datum point for comparison purposes. However, it 

is located in the area of deformation and may itself 

have undergone some vertical displacement. In fact, 

surveys suggest that BM 10 was uplifted about 17 em 

relative to a tide g1ge at Hilo between 1921 and 1958 

and 9 em more between 1958 and 1971 (Okamura and 

Swanson, 1975, table 3). Although the leveling data, 

particularly in 1921, are of relatively poor quality, it 

nonetheless seems safe to conclude that BM 10 has 

been progressively uplifted at least several centimetres relative to the tide gage since 1921. Consequently, all displacements given in figure 11 are 

minimum values and would probably be several centimetres larger if cr mputed relative to the tide gage. 

The Hilo tide gage is sinking at a rate of 4.1 mm/yr 

relative to sea level, but this is considered as reflecting 

islandwide isostatic, rather than local, subsidence 

(Moore, 1970). 

Comparison of elevations for the 1921, 1958, and 

1971 surveys, which followed the same route but included more bench marks in the later surveys, indicates uplift and seaward tilting of the south flank, increasing in magnitude toward the east rift zone (fig. 

11A). The amount of uplift near the east rift zone is 

large, amounting to at least 2 m at BM 2728 since 

1921; most of this displacement, more than 1.65 m, 

took place since 1958. The maximum recorded uplift 

since 1958 is 2.1 mat BM YY35, near the south edge of 

the east rift zone; this bench mark was not occupied in 

1921. Most bench marks were displaced much more 

between 1958 and 1971 than between 1921 and 1958, 

indicating an accelerated rate of uplift in recent years, 

during which volcanic activity along the east rift zone 

has been intense (table 1). The rate of horizontal deformation likewise has increased since 1958 (compare 

figs. 5E and 5B). 

The apparent subsidence ofBM 2503 for the 1921-58 

and 1921-71 periods is anomalous (fig. 11A) but could 

be removed if the 1921 elevation were about 0.9 m too 

high. A blunder of this magnitude (about 1 yd) would 

have been easy to make if the leveling rod were misread, as yard rods were used, but should have been 

caught in field computations. We rechecked the 1921 

field notes but found no error, so that, if our suspicion is correct, an entry error must also have been 

made. The 1921 elevation of BM 2728 would also have 

to be lowered by the amount, if any, by which BM 2503 

Is In error. 

An alternative interpretation is that the 1921 elevations are good and the displacements are those indicated in figure 11A. In this regard, the subsidence of 

BM YY37 (2302) between 1958 and 1971 should be 

noted. This area may actually experience local subsidence at times, perhaps in some way related to the 

rather abrupt change in slope 5 km southeast of 

Makaopuhi Crater (inset, fig. 11A), which may reflect 

an unrecognized fault in the Hilina fault system. Regardless of which interpretation is correct, the overall 

increasing uplift toward the rift zones is still the dominant pattern of displacement. 

The 1965 leveling, and part of the 1971 leveling, 

followed a route (inset, fig. 11B) slightly south and 

west of the 1921 and 1958 routes, and none of the older 

bench marks except BM 10 and BM YY35 was occupied. The 1965--71 displacement profile (fig. 11B) indicates uplift much like that shown by other profiles 

(fig. 11A) but at an accelerated rate. For example, BM 

YY35 was uplifted about 1.6 m between 1965 and 1971 

but only 0.5 between 1958 and 1965, and BM HVO 53 

was uplifted about 0.75 m between 1965 and 1971 but 

only about 0.4 m between 1958 and 1965 (extrapolating from BM 2503, very close to BM HVO 53). Volcanic 

activity along the upper east rift zone has been especially intense since 1961 (table 1). 

The displacement profiles of figure 11 (except the 

questionable 1921-58 profile) mimic in a general way 
20 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

2.1 2 .1 

2.0 1000 2 .0 1000 

1.9 1.9 

1.8 900 1.8 900 

en 1.7 

w 

a: 1.6 

1-

w 1.5 ~ 

~ 1.4 

1.7 en 

w 800 800 a: 1.6 1-

w 1.5 ~ 

700 ~ 1.4 700 

6 1.3 6 1.3 

~ 1.2 600 ~ 1.2 600 

co 

0 1.1 

1- 1.0 w 

> .9 i= 

<( .8 ...J 

w .7 a: 

1- .6 z 

w 

~ .5 

w 

u .4 <( 

...J .3 a.. 

en .2 0 

...J .1 

<( 

u 0 

1-

a: -.1 w 

> -.2 

en co en w· 0 1.1 w 

a: 1- 500 a: 500 1- w 1.0 1-

w > w 

~ i= .9 ~ 

400 ~ <( .8 400 ~ ...J z' z' w .7 0 a: 0 

300 i= 1- .6 300 i= 

<( z <( 

> w > w ~ .5 w 

...J w 200 ...J 200 w u .4 w 

<0\ 

<( 

? ......... ...J .3 

I .... ... a.. 

~\ I 100 en .2 100 

~\ 0 

~\ I ...J .1 I <( 

0 Sea \ I 0 ~~~=~ u 0 \ i= level 

\ I 

I a: -.1 

\ I w 

\ 100 > -.2 100 

-.3 

-.4 

\! ·.3 0 A 200 -.4 200 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

DISTANCE FROM BM YY35, IN KILOMETRES 

FIGURE 11.-Vertical displacement and topographic profiles across central part of south flank between 192 

is BM 10.A , Leveling surveys of1921, 1958, and 1971, along Kalapana Trail. B, Leveling surveys of 

of Craters Road. Steep parts of topographic profiles are flow-mantled fault scarps, such as Holei Pali. 

destroyed in 1969, but its 1971 elevation can be estimated to within 0.1 m by comparison with a nearby 

Okamura and Swanson (1975, tables 5 and 7). Inset maps show location of bench marks and leveling 

of topographic profile in B. See figure 2 for location of Makaopuhi Crater. Contour interval in inset 

the shape of the topographic profiles. Both leveling 

routes cross the Hilina fault system, and both the topographic and displacement profiles show abrupt 

changes in slope across the fault system, suggesting 

recent fault movement. This effect is especially noticeable for the 1965-71 profile (fig. 11B), which is nearly 

perpendicular to the trend of the fault scarp. The nature of another abrupt change in the 1965-71 profile, 

between HVO 56 and C/L 21, is not known because 

lava flows covered the area in 1970-71, prohibiting 

reoccupation of intermediate bench marks and obscuring evidence of possible surface rupture. 

EASTERN PART OF SOUTH FLANK 

AND LOWER EAST RIFT ZONE 

The 1921 and 1958 leveling surveys extended north 

of BM 10, across the lower east rift zone and the north 

flank (fig. 12). For comparison purposes, BM 359.3, in 

Keaau, is used as the datum point. Leveling from the 

tide gage established in 1926 at Hilo indicates that BM 

359.3 has been stable within survey error relative to 

the gage (Okamura and SwanSC)P.,I1975, table 3) since 

1926; we cannot document such ity between 1921 

and 1926 but think it reasona The poor quality of 

the 1921 leveling must be in any quantitative interpretation of the displ nt data. 

The 1921-58 profile indicates 

flank and lower east rift zone, a maximum displacement of nearly 25 em near topographic crest of 

the rift zone. The uplift is ''""'TTY'ITY'I li•-t-.... -... , the area south 

of the crest showing substan more displacement 

than that to the north. BM 10, the tum point used in 

figure 11, was uplifted about 1 ~ 

Lower east rift eruptive an~ 

tween 1921 and 1958 include the 

donald and Eaton, 1964), the fi 

crossed by the leveling profile, 

cracking episode centered 10-1 

ing route (Finch, 1925). 

The section between BM 10 

leveled in 1973 (Hawaiian V 

pub. data). Assuming BM 655 

· ve events be955 eruption (Maczone of which is 

the 1924 groundeast of the levelObservatory, unchange elevation 
VERTICAL DISPLACEMENT OF SOUTH AND NORTH FLANKS OF KILAUEA 21 

(/) 

w 

a: 

1-

~ 0 .6 

c-l 

cri 0.5 Ln 

M 

~ 

co 

~ 0.4 

w 

> 

i= 

::5 0.3 

w 

a: 

1-

~ 0.2 

~ 

w 

u 

c:x: 

~ 0 .1 

!!? 

0 

...J 

5 0 

i= 

a: 

w 

0 4 8 12 16 20 24 

EXPLANATION 

• Bench mark 

> -0.1 -+------.-----,--------,-----,--------,-----,----' 

0 4 8 12 16 20 

PROJECTED DISTANCE FROM BM 359.3, 

IN KILOMETRES 

24 

400 (/) 

w 

a: 300 tii 

~ 

z 200 -_ 

z 

Q 

1-

100 ~ 

0 

w 

...J 

w 

FIGURE 12.-Vertical displacement and topographic profiles across 

lower east rift zone and eastern part of south flank between 1921, 

1958, and 1973. Datum is 1958 elevation ofBM 359.3. Inset shows 

location of bench marks, leveling route (dashed line), line of topographic profile (light solid line), and generalized eruptive fissures 

for 1955 eruption (heavier solid lines). Displacement data are projected onto line of topographic profile. Contour interval on inset 

map is 100m. 

between 1958 and 1973, displacements relative to it 

can be used to indicate the total displacement since 

1921. This assumption is dubious but serves to show 

that a striking change in the pattern of uplift took 

place (fig. 12). The crest of the uplift shifted southward, 

with BM 737 being displaced nearly three times as 

much as BM 1065 between 1958 and 1973. Thus the 

overall uplift since 1921 is centered 2 to 3 km south of 

the topographic crest of the rift zone, in an area where 

no past eruptive activity is known. 

The 195~73 uplift can be examined more closely by 

using data from surveys conducted along the same 

route in 1964 and 1969 by R. W. Decker (1965, 1969; 

Dieterich and Decker, 1975) (fig. 13A). The uplift grew 

sporadically between 1958 and 1973. The rate of uplift 

was greatest between 1964 and 1969 (maximum of 3.2 

cm/yr) and least between 1969 and 1973 (maximum of 

1 cm/yr). The crest of the uplift did not shift laterally 

during this time. A grabenlike zone developed on the 

north limb of the uplift, progressively deepening with 

time; new cracks cutting a paved road apparently 

define the south side of the graben (R. W. Decker, oral 

commun., 1969). Both the crest of the uplift and the 

graben are located south of all known eruptive fissures 

and vents. BM YY71, only 100 m from a 1955 eruptive 

fissure (vent R of Macdonald and Eaton, 1964), subsided, possibly owing to thermal contraction during solidification of the 1955 dike and cooling ofits wallrock. 

In contrast to the profile in figure 13A, displacement 

profiles about 8 km farther east show progressive subsidence relative to BM YY80 since 1958 (fig. 13B). The 

subsidence is of much smaller magnitude than the uplift, amounting to a maximum of about 12 em at BM 

YY195. The rate of subsidence was most rapid between 

1958 and 1964 and subsequently slowed. Maximum 

subsidence is centered along the 1955 fissure13 and may 

reflect thermal contraction. Removal of this effect by 

smoothing the profiles leaves a broad basin with a 

small but sharp uplift centered at BM YY198; this uplift persists on all three profiles and therefore must be 

real although of small amplitude . 

WESTERN PART OF SOUTH FLANK 

AND KOAE FAULT SYSTEM 

Leveling surveys in 1958 and 1971 permit construction of a displacement profile across the Koae fault 

system and western part of the south flank (fig. 14). 

The profile is complicated by displacements caused by 

several faulting events in the Koae fault system, principally in 1965 (Fiske and Koyanagi, 1968). The south 

flank shows uplift and southward tilting, increasing in 

magnitude toward the Koae fault system. 

NORTH FLANK 

The north flank of Kilauea and adjacent part of 

Mauna Loa between Hilo and the summit were leveled 

three times between 1926 and 1971. Okamura and 

Swanson (1975, table 3) tabulated the measured elevations and showed that changes are generally unsystematic and close to or within survey error. Changes 

are significant only near the summit, presumably as a 

result of the documented summit tumescence (Okamura and Swanson, 1975, table 3). Displacement on the 

eastern part of the north flank between 1921 and 1958 

was similarly small (fig. 12). 

SUMMARY AND INTERPRETATION 

OF VERTICAL DEFORMATION 

ON THE NORTH AND SOUTH FLANKS 

The following statements characterize vertical deformation of the flanks of Kilauea during the 20th century: 

(1) The north flank is virtually stable, as also indicated by both seismic and horizontal displacement 

data. 

(2) In most places, the south flank has been increasingly uplifted toward the rift zones and Koae fault system, with the maximum displacement within or just 

south of the east rift zone. 
22 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

400 

(/) 

w 

c: 300 1-

w 

~ 

~ 

z 200 

0 

f= 

<( 

> w ...J 100 w 

0 

+.3 

+.25 

(/) 

w 

c: 

1- +.2 w 

~ 

~ 

1-. 

z +.15 w EXPLANATION 

~ 

w 

u Bench mark <( ...J 

~ 

(/) +.1 

0 

...J 

<( 

u 

f= +.05 c: 

w 

> 

0 

-.05 

200 

(/) 

w 

c: 

1-

w 

~ 100 

~ 

z 

0 

f= 0 <( 

> 

w ...J 

w 

0 1-. 

z 

w 

~ 

w EXPLANATION 

u(/) 

<Cw -.05 

...Jc: 

~I-

~w 

Bench mark 

0~ 

...Jz 

<(- -.1 

u 

f= 

c: 

w B > -.15 

2 4 6 8 10 12 

PROJECTED DISTANCE FROM BM YY80, IN KILOMETRES 

FIGURE 13.-Vertical displacement and topographic profiles across lower east rift zone of Kilauea be1twE~eh 1958 and 

1973. A, Profiles between BM YY80 (Pahoa) and BM 10 (Kalapana); B, Profiles between BM and BM 

YY174 (Pohoiki). Datum is 1958 elevation of BM YY80. Inset maps show locations of leveling key bench 

marks, and line of topographic profile. Displacement data are projected onto line of topographic Contour 

interval of inset maps is 150 m. 
RELATIVE MAGNITUDE OF HORIZONTAL AND VERTICAL DISPLACEMENTS 23 

-ro 

~ 

Q) 

c:: ~ 

2 ·;u 

::J 

~ 

·.:: 0 

~ 

"' Koae fault -ro <tl 

w system ~ South flank 

+1.0 1100 

+0.8 

+0.6 1000 en 

w 

a: +0.4 1-

w +0.2 900 ~ 

1-z 0 en z- w w . a: ~w -0.2 800 1- w" w u<t ~ <~:" -0.4 -lW ~ ~0 

en- -0.6 700 z ol- 0 _Jo -0.8 i= <{~ <{ ()J: 

-1.0 600 > i=o w _J 0::1-

Ww w -1.2 >> 

i= -1.4 500 <{ _J 

w -1.6 a: -1.8 400 

-2.0 

-2.2 300 

0 2 4 6 8 10 12 14 

DISTANCE FROM BM YY26, IN KILOMETRES 

EXPLANATION 

• Bench mark 

_.L._ 

Fault 

Bar and ball on down thrown side 

FIGURE 14.-Vertical displacement and topographic profiles across 

Koae fault system and (Western part of south flank between 1958 

and 1971. Datum point is Hilo tide gage. Inset map shows location 

of bench marks along Hilina Pali Road and Kalanaokuaiki Pali, a 

normal fault that forms south boundary of Koae fault system. 

Contour interval is 60 m. Hilina Pali is a prominent fault-line 

scarp. 

(3) The rate and amount of displacement in areas of 

large uplift correlate directly with the number and 

proximity of rift eruptions or related faulting events 

for any given time period. 

The uplift of the south flank can be ascribed to the 

effects of forceful intrusion of magma into the rift 

zones. Such uplift was suggested by Macdonald (1956, 

p. 286) on the basis of geologic arguments. Along the 

upper and middle east rift zone, intrusion has apparently been followed quickly by eruption, so that a direct relation between the timing of uplift and eruption 

is evident. Along the lower east rift zone, intrusion 

appears to have proceeded more slowly or over longer 

periods of time, or both; consequently, uplift during 

noneruptive periods may occur (fig. 13A). Subsidence, 

such as that shown in figure 13B, may indicate migration of magma to another site. 

The displacement profiles that completely cross the 

lower east rift zone (figs. 12 and 13) are particularly 

instructive because of their asymmetry. The profiles 

suggest that either the south flank is virtually decoupled from the north flank along the rift zone or that 

the uplift is the result of intrusion of magma as southdipping dikes into the rift zone (Dieterich and Decker, 

1975). South-dipping dikes would explain the location 

of maximum uplift 1-2 km south ofthe lower rift zone 

(fig. 13) but would not account for the location of 

maximum uplift within the central and western parts 

of the rift zone, where vertical or nearly vertical dikes 

seem most likely. We examine the possibility that 

dikes dip southward in the section. ~~A Structural 

Model for the South and North Flanks of Kilauea," 

concluding that some dikes may dip southward at comparatively low angles where gravity faults of the 

Hilina fault system impinge on the rift zone, but that 

most dikes along the length of the rift zones are steeply 

dipping to vertical. 

The profile across the Koae fault system and adjacent south flank (fig. 14) is more complicated than the 

others. Fiske and Koyanagi (1968, p. 19) suggested 

that the uplift immediately south of the Koae system 

during the December 1965 eruptive and faulting 

episode could be attributed to ~~* * * elastic rebound 

due to the sudden release of accumulated stresses 

along Kalanaokuaiki Pali," the fault that forms the 

southern margin of the fault system. Another possibility is that magma was laterally intruded as a dike into 

the Koae from the site of concurrent eruption near the 

intersection of the fault system and east rift zone, causing uplift similar to that elsewhere along the northern 

edge of the south flank. In May 1973, such a dike apparently was intruded several kilometres into the Koae 

system from its source along the upper east rift zone 

(Duffield, 1975; Koyanagi and others, 1973). 

RELATIVE MAGNITUDE OF 

HORIZONTAL AND VERTICAL DISPLACEMENTS 

The relative magnitude of horizontal and vertical 

deformation can be estimated by comparing measured 

horizontal displacements with measured or extrapolated vertical displacements for approximately the 

same time intervals at six south-flank triangulation 

stations (table 3). The horizontal component of displacement is at least several times larger than the vertical component. This generalization appears valid 

across most of the south flank, judging from comparisons of figures 11-14 with figures 5-8. The relative 

importance of vertical displacement probably increases 

near the rift zone, but no triangulation stations are 

located in the critical area to test this suggestion. 

Within the rift zones themselves, vertical displacements (both uplift and subsidence) are commonly large 
24 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

TABLE 3.--Comparison of horizontal and vertical displacements at six 

triangulation stations on the south flank of Kilauea Volcano 

[Vertical data have been extrapolated from nearest bench mark if the triangulation station 

was not included in the leveling survey.] 

Time interval Displacement, m Ratio of 

Station horizontal to 

Horizontal Vertical Horizontal Vertical' vertical 

Hilina _____ _______ -- 1961-70 1958-71 0.8 +0.02 2 40 Panau ______ ________ 1914-58 1921-58 2.1 + .30 "7 

1958-70 1958-71 2.3 + .30 8 

Kupapau --- ------- 1914-58 1921-58 1.2 + .18 37 

1958-70 1958-71 0.8 + .07 11 

Hakuma - ---- ----- 1914-58 1921-58 1.4 + .17 "8 

1958-70 1958-71 0.7 + .09 8 

Laeapuki 1961-70 1965-71 1.3 + .08 316 Kaena Point_ _______ 1961- 70 1965-71 1.3 + .05 3 26 

'Vertical displacement relative to Hilo tide gage or, for the 1921 survey, BM 359.3. The 

1965 survey was based on BM 10 and has been adjusted by assigning half the uplift of BM 10 between 1958 and 1971 to the 1965-71 interval. 2Probably minimum ratio, because time between horizontal surveys is shorter than that 

between vertical surveys. "Probably maximum ratio, because time between horizontal surveys is longer than that 

between vertical surveys. 

but smaller than horizontal ones at a given location 

(Macdonald and Eaton, 1964, fig. 34; Jackson and 

others, 1975; Swanson and others, 1976), although exceptions associated with the formation of major grabens are known (J aggar, 1924; Swanson and others, 

1972, p. 114). 

The predominance of horizontal displacement on the 

south flank probably results from dikes that are forcefully wedged into the rift zones, causing horizontallydirected dilation that manifests itself by the opening of 

gaping fissures and the corresponding displacement of 

the south flank away from the rift zones. 

A STRUCTURAL MODEL FOR THE 

SOUTH AND NORTH FLANKS OF KILAUEA 

The nature and timing of seismic events and ground 

displacements prompt us to propose a model in which 

the south flank of Kilauea moves in response to forceful intrusion of magma into the rift zones (fig. 15). Intrusions, in the form of dikes, literally shoulder the 

south flank upward and southward and are effectively 

tearing the volcano apart along the two rift zones. 

From the timing of seismic and deformation events, we 

conclude that intrusion is the immediate cause of south 

flank displacement, not the reverse. 

The data further suggest that the north flank is relatively stable and does not respond notably to intrusive 

events. This lack of response may be explained by the 

following two interrelated reasons: (1) the north flank 

is buttressed by the huge mass of Mauna Loa and the 

other volcanoes on the island, whereas the south flank 

is free to move southward; and (2) the development of 

the rift zones of Kilauea is dominated by the gravitational stress field imposed by Mauna Loa, on which 

Kilauea is constructed, and this gravitational field 

favors seaward displacement away from the center of 

the volcanic pile (Fiske and Jackson, 1972). 

Figure 16 illustrates the present gr at difference in 

size between Kilauea and the rest bf the island, particularly Mauna Loa. It is reasona l that this large 

mass of older rock might buttress Ki auea, especially 

since intrusion in the rift zones is k . hallow process, 

probably taking place within 8 km, h efly 5 km, of the 

ground surface (fig. 15). The buttre~s effect would be 

important even if the contact of auna Loa and 

Kilauea were vertical and extends to the old sea floor 

on which the volcanoes are construc~e . In all probability, however, the contact interfin · ers as shown 

schematically in figures 15 and 16. In act, we consider 

it probable that Mauna Loa was already a large volcano when Kilauea first began formin , because of the 

present large difference in size. 

KJ (/) 

w 

a: 

fw 

::2: 

0 

_J 

~ 

~ 

z' 

0 

f= 

<( 

> 

w 

_J 

w 

(/) 

w 

a: 

fw 

::2: 

0 

_J 

~ 

~ 

z' 

0 

f= 

<( 

> 

w 

_J 

w 

(/) 

w 

a: 

fw 

::2 

0 

_J 

~ 

~ 

z' 

0 

f= 

<( 

> 

w 

_J 

w 

South 

A 

Sea 0 level 

·5 

-10 

-15 

B 

Sea 0 level 

-5 

-10 

-15 

c 

Sea 0 level 

-5 

-10 

-15 

~ 

~ 

K1ilauea Mauna Loa 

fault 

system 

~ 

East rift zone 

Active Inactive part part 

~ 

East 

rift' 

zone 

EXPLANATION 

Clastic rocks 

North 

A' 

c' 

Q 

Mauna Loa 

Subaerial shield Pillow lava 

~ 

Direction and relative amount of isp acement caused by forceful intrusio ~ o ' 

magma as dikes. Length proportion I 

to amount of displacement 
A STRUCTURAL MODEL FOR THE SOUTH AND NORTH FLANKS OF KILAUEA 25 

Not so easily visualized, but perhaps of greater importance, is the control exerted by Mauna Loa on the 

stress field in which intrusions at Kilauea take place. 

Fiske and Jackson (1972) demonstrated that gravitational stresses govern the direction of dike propagation 

in gelatin models free of other stresses, whether the 

dikes are injected directly beneath the top of the model 

or at some place on irts flanks. If this modeling can be 

generalized to naturally occurring volcanic edifices, 

then a younger shield built on the sloping flank of an 

older shield should come under the influence of the 

latter's gravitational stress field, which should in turn 

control dike orientation and overall shape. 

Fiske and Jackson (1972, p. 314-317) showed how 

this concept could explain the relation of Kilauea to 

Mauna Loa. Assuming that Kilauea inherited the 

gravitational stress field of Mauna Loa, intrusion of 

magma into the rift zones as dikes should produce dilation in a direction consistent with this stress field, that 

is away from the central part of the edifice and toward 

the free slope. Thus, the Fiske-Jackson model predicts 

that the south flank of Kilauea would be far more 

mobile than its north flank, a relation indicated by the 

seismic and geodetic evidence. 

Much of the east rift zone of Kilauea probably extends beyond the effects of Mauna Loa, however. The 

submarine part of the rift zone extends at least 110 km 

beyond the east tip of the island (Macdonald and Abbott, 1970, p. 313). The submarine rift zone forms a 

prominent ridge standing high above its base (Moore, 

1971; fig. 1), and extends well beyond the bulk of 

Mauna Loa; consequently, it may have its own highFIGURE 15.-Diagrammatic cross sections, with no vertical exaggeration, through Kilauea depicting our interpretation of relation between east rift zone and rest of volcano. Locations of sections 

shown in figure 1. A-A', Summit region. Magma is intruded, 

primarily as dikes, into east rift zone from upper part of magma 

conduit or reservoir complex. Magma only rarely enters Koae fault 

system. B-B ', Middle east rift zone. Magma intruded as dikes from 

beneath summit region wedges south flank seaward and upward. 

North flank moves little if at all. Dikes in inactive part of rift zone 

are older, extend to greater depths, and terminate upward at 

slightly lower elevations than dikes in active part. C-C', Submarine part of east rift zone. Displacements caused by intrusion of 

magma are assumed to be nearly symmetric across rift zone. In 

A-A' and B-B ', gravity sliding is interpreted to take place chiefly 

in layer of hyaloclastic rocks above and interbedded with pillow 

lava, displacing seaward part of south flank downward. Hyaloclastic material is generated at shallow depth but sloughed downward 

by slides and current action (Moore and Fiske, 1969), possibly 

forming a deposit much thicker than indicated inA-A' andB-B'. 

Depths to base of volcanic pile and to mantle from Hill (1969), and 

bathymetry and rock type from Moore and Fiske (1969). Depths to 

contact of Kilauea and Mauna Loa are speculative; rocks from the 

two volcanoes are shown as interfingering only at relatively shallow depths, consistent with our belief that Kilauea did not begin to 

form until Mauna Loa was already a large edifice. Flows of pillow 

lava from Mauna Kea may underlie Mauna Loa pile (fig. 16). 

level gravitational stress field. If so, dike-induced dilation should be directed equally northward and southward as C-C' in figure 15 portrays. The most recent 

intrusion into the submarine part of the rift zone may 

have taken place in 1924 (Finch, 1925), and earthquake activity in this area has been minor since then 

(Koyanagi and others, 1972, and references therein). 

Thus it is unlikely that this part of the rift zone is 

presently undergoing much deformation. The north 

side of the submarine part of the rift zone is steeper 

than the south side (Moore, 1971), possibly because of 

gravity-induced slides, although why slides are concentrated on only one side is not evident. 

The south flank is assumed to be mobile everywhere 

south and southeast of the rift zones, but the depth to 

which it remains mobile is not easily determined. The 

flank is probably mobile to a depth of at least 5 km 

because seismic evidence during eruptions suggesrts 

that some intrusion takes place at such a depth. Almost all earthquakes on rthe south flank have focal 

depths of less than 15 km, and most have depths of 8 

km or less, calculated using a crustal model devised by 

J. P. Eaton and E. T. Endo, based on Hill's (1969) refraction study (Koyanagi and others, 1975). The mobility may therefore extend to a depth of at least 8 km, 

which is almost certainly well within the edifice of 

Mauna Loa and near the level of the old sea floor on 

which Mauna Loa is built (fig. 15; Hill, 1969). Seismic 

evidence suggests that the lithologic break at shallow 

depths between subaerial flows capping the volcano 

and pillow lavas forming the submarine bulk of the 

volcano (fig. 15; Moore and Fiske, 1969) does not influence the overall mobility of the south flank, although 

the Hilina fault system may largely bottom out at this 

level, as discussed in a later section. 

It seems unlikely that the base of the mobile flank 

would be defined by a single plane of dislocation. Instead, we favor a model in which the displacement 

gradually decreases below the depth at which intrusion of magma is concentrated. Thus the base of the 

mobile south flank may actually be a zone several 

kilometres thick, and much of the displacement may be 

taken up by many local adjustments within the pillow 

complex. 

Moore and Krivoy (1964) believed the east rift zone 

to be a southward-dipping structure forming the sole of 

a large landslide block (the south flank). They attributed dilation of the rift zone to((* * *southward movement of the flank of rthe volcano south of the rift zone 

under the influence of gravity and inflation of the dipping rift zone by magma from the summit reservoir" 

(1964, p. 2043). However, the documented uplift of the 

south flank associated with rift dilation and eruption 

during recent years argues against landslide-type 

faulting along the rift zone, for subsidence would be 
26 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

3 

0 

-3 

~ -{) 

:: -9 

~ -12 

0 

_J 3 

~ 0 

~ -3 z 

0 -{) 

~ -9 

> w 

_J 3 w 0 

-3 

-{) 

A 

N 

Volcanic 

Original oceanic 

Kilauea 

pile -9 oceanic crust 

0 6 12 18 24 MILES 

I I 1 1 I I I I I I I I 

0 6 12 18 24 KILOMETRES 

FIGURE 16.-Diagrammatic cross sections, with no vertical exaggeration, across Kilauea, Mauna Loa, H ·, and part of Mauna Kea. 

Depth to base of volcanic pile is from Hill (1969); submarine topography off south flank of Kilauea from and Fiske (1969). We as-

sume that each volcano grew to substantial size before its next younger neighbor began erupting, as i'r tdic:at !d by interfingering only at 

relatively shallow depth. Other volcanoes that never reached above sea level may contribute to volcanic pi Contour inverval in index 

map 600 m. 

expected, at least on the south side of the rift zone, 

from gravity faulting during eruption. In addition, 

seismic studies show that earthquakes associated with 

intrusion emanate from directly below the surface 

trace of the rift zone, not south of the zone (Koyanagi 

and others, 1972). Finally, intrusion and eruption 

along the east rift zone precede seismic activity and 

displacement on the south flank, suggesting that forceful intrusion is the immediate cause, not the effect, of 

the rift dilation and displacement. 

We agree with Moore and Krivoy (1964) that gravity 

faults characterize the Hilina fault system (see later 

section) but believe that these are only relatively 

superficial, secondary structures resulting from instabilities generated by injection of steeply dipping 

dikes into the rift zones. A broad unfaulted area lies 

between the rift zone and the Hilina system, except 

locally along the lower east rift zone, and this alone 

suggests different origins for the rift zones and Hilina 

fault system. 

Gravity plays an important role in our model, just as 

it does in the model of Moore and Krivoy. Were regional tectonic stresses dominant over local gravitational stresses, the rift and . flank structures would 

doubtless be much different, as Fiske and Jackson 

(1972) found them to be in isolated Hawaiian shields. 

However, from the observed timing of events we contend that intrusion, not gravity-inducing sliding, 

opens the rift zones. 

Ryall and Bennett (1968) interpreted the east rift 

zone to be a north-dipping related to a large 

normal fault that displaces the st and upper mantle. This interpretation was lar based on gravity 

data in the east rift area, which, as we show in a later 

section, can better be in as indicating southward growth of the east rift with time. Their 

model presents a host of other ~~,u~,-~~ 

rationalized with seismic, 

largely gathered subsequent 

Dieterich and Decker (1975) "''''-..rr""'"'~£'n 

the lower east rift zone dip 451 

parison of the 1958-64 level· 

with finite-element models of 

an elastic medium. The 

other leveling profiles a 

, on the basis of comprofile in figure 13A 

emplacement into 

feature of this and 

same route that 

suggests such a low dip to the · rkers is its marked 

asymmetry, the south flank coh'"dh Y\rr substantial uplift 

and the north flank almost no In their interpretation, the graben just north crest of the uplift 

formed at the locus of maxim stress, as located by 

the model. 

Dieterich and Decker's (1 interpretation conflicts with the magnitude of ~'~" ~~·"~~tal displacements 

in that a~ea. For example, l horizontal displacements at hlewa and BM YY 1958 and 1970 

are each about 50 em (fig. 5B 

displacement predicted from 

for about the same time peri 

an order of magnitude less. 

placements do not fit any of 

as the maximum 

finite-element model 

958-69) is only 5 em, 

large horizontal disseveral models given 
THE RIFT ZONES AND KOAE FAULT SYSTEM AS ZONES OF DILATION 27 

by Dieterich and Decker (1975). Instead, the large extension may reflect the intrusion of a family of dikes, 

not just one dike. In his helpful review of this paper, 

Decker (oral commun., 1973) suggested that the horizontal displacements resulted from ground rupture, a 

process not covered by the finite-element model. This 

may be true, but no nlew ground cracks were observed 

in the critical area along the rift zone during this time, 

and seismic activity in this area, especially since 1961, 

has been so slight as to suggest no surface rupturing. 

Another problem with the interpretation by 

Dieterich and Decker is that the most definitive data, 

those north of the rift zone, lie 3 to 4 km off the profile 

used in their model comparison, whereas most of the 

other data are much closer to the profile. Whether the 

data can be projected so far and still be subject to quantitative interpretation is open to question, particularly 

in view of the observed rapid change from uplift to 

subsidence within a sr ort distance along the rift zone 

(fig. 13). In the absence of data for the north flank 

closer to the profile, the displacement profile can be 

interpreted to indicate almost any dip desired. 

As a result of these difficulties, we regard as unproven the existence of dikes in the lower east rift zone 

that dip much less than 90°. Nonetheless, we believe it 

is possible that some dikes in the area crossed by the 

leveling line in figure 13A do dip southward at angles 

of 50-70° because of the structural setting of this part 

of the lower east rift zone. In this area, the Hilina fault 

system intersects the rift zone (figs. 2 and 3), and 

magma supplied from steeply dipping dikes farther uprift could conceivably be intruded along some of the 

preexisting south-dipping fault planes. If such intrusion has taken place, we would view it as a perturbation from the otherwise steep dips of the dikes indicated elsewhere along the rift zone by the patterns of 

seismicity and uplift, and found on eroded Hawaiian 

shields (Stearns, 1947; Stearns and Vaksvik, 1935). 

NUMERICAL COMPARISON OF VOLUMES 

OF INTRUSION AND DEFORMATION 

If south flank deformation is caused by injection of 

dikes, then there shoJ ld be a specific relation between 

the volume of deformation (defined principally by the 

amount of uplift) and the volume of intrusion. This 

relation will not be one-to-one, as the deformed rocks 

should decrease in internal volume to some degree. 

Still, a rough balance between the volume of intrusion 

and the volume of deformation should be maintained. 

For the period 195&-71, a cross section of the south 

flank southeast of Makaopuhi Crater shows an increase in volume of about 107m3fkm owing to uplift, 

computed from data in figure 11A assuming negligible 

deformation of the north flank. A dike 0.5 m wide, a 

width consistent with many observed Hawaiian dikes, 

has a volume of 5 x 105m3/km if it extends from the 

surface to a depth of 1 km, 1.5 x 106m3fkm to a 3-km 

depth, and 2.5 x 106m3fkm to a 5-km depth. The calculated volume of deformation could thus be balanced by 

4 to 20 such dikes. Table 1 indicates that 8 to 13 intrusive events occurred near Makaopuhi during this time; 

the uncertainty in number reflects the unknown degree to which events near Alae Crater and Mauna Ulu 

could have influenced the deformation. At least one of 

these intrusive events, in August 1968, involved very 

shallow magma injection-possibly only to a depth of 

500 m (Jackson and others, 1975)-and seismic evidence for others suggests that the dikes bottom at 

depths of 5 km or less. Thus, a reasonable agreement 

between the volumes of deformation and intrusion is 

evident. 

Similar calculations for the 1965-71 period (fig. 11B) 

show a volume of deformation of 8 X 106m3/km, which 

could be balanced by 3 to 16 dikes. During this time 

there were five to eight intrusive events. Again, the 

two volumes are in reasonable agreement. 

Quite obviously these calculations are crude, for the 

existing data and theoretical models are inadequate 

for detailed analysis. Nonetheless, the approximate 

balance between the volumes of intrusion and deformation is of some value, as our model predicts that the 

dikes make room for themselves by displacing the 

wallrock up and to the side. 

THE RIFT ZONES AND KOAE FAULT SYSTEM 

AS ZONES OF DILATION 

If the south flank of Kilauea is being displaced 

southeastward because of the forceful intrusion of 

magma as dikes into the rift zones, its contact with the 

less mobile part of the volcano should be marked by a 

zone of dilational opening. We postulate that such a 

zone of dilation is defined by the two rift zones and the 

Koae fault system. 

The east and southwest rift zones have long been 

recognized as containing normal faults, grabens, and 

gaping cracks with matching opposite walls, and in 

recent years such · features have been observed and 

measured in the process of formation. Fissures that 

form during any given rift eruption tend to be arranged in an en echelon pattern, with either a right- or 

left-hand sense of offset depending on their location 

along the length of the rift zone (Macdonald and Eaton, 

1964; Moore and Krivoy, 1964; Duffield and Nakamura, 1973; Jackson and others, 1975). Analysis of 
28 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

these en echelon fissures by the method of Nakamura 

(1970) and Duffield and Nakamura (1973) suggests dilational opening perpendicular to the rift zones. However, new lava erupted along the rift zones repeatedly 

covers the fissures, so that the net amount of dilation 

over long periods of time cannot be estimated 

adequately. Thus we must look elsewhere for pertinent 

quantitative information regarding the amount of 

cumulative dilation. 

The Koae fault system (fig. 3) is not the site of eruptions, except for small, infrequent outbreaks near its 

intersections with the east and southwest rift zones. 

Further, it has, in the recent geologic past, seldom 

been covered by lava erupted from vents farther upslope, as the geologic maps of Peterson (1967) and 

Walker (1969) show. As a result, the Koae fault system 

serves as a window through which the net effect of 

displacements produced by many episodes of deformation are well displayed, and it is the best place to find 

direct structural evidence that displacements such as 

those measured in this century have characterized the 

recent geologic past. 

Duffield (1975) found that the Koae fault system is 

characterized by gaping cracks arranged in en echelon 

fashion, some of which coalesce to form long, sjnuous 

zones of normal faults, generally with north-facing 

scarps, that define the south margins of asymmetric 

grabens (fig. 17). Symmetric grabens are uncommon. 

The fault scarps are as high as 20m, although most are 

less than 5 m, and single cracks are as much as 2 m 

wide. Most cracks and faults dip vertically, strike N. 

7 5° E., and open perpendicular to their strike, as determined by numerous measurements of matching crack 

walls. A similar direction of opening was determined 

by Duffield and Nakamura (1973) from a study of the 

pattern of en echelon cracks, and this direction virtually parallels that of horizontal displacement vectors 

on the south flank (fig. 5). This evidence suggests 

rather definitely that similar intrusive events cause 

both the south-flank displacement and the Koae (and 

rift zone) dilation. 

Knowledge of the amount of dilation recorded by the 

Koae fault system and the time during which this dilation occurred is helpful in evaluating the history of 

past displacement events. The total amount of dilation 

along two profiles across the fault system was measured to be 18.69 m (fig. 17,A-A') and 32.55 m (fig. 17, 

B-B ') (Duffield, 1975). Our field observations suggest 

that the total dilation decreases westward from profile 

A-A'. We interpret the greater amount of opening in 

the eastern part of the fault system to reflect proximity 

to the principal source of dilation-intrusion within 

the east rift zone. 

The measured dilation is that accumulated since the 

12A.s-,-

~ 

Puhimau 

Crater 

RIFT ZONE 

Puu Kane Nui o Hamo 

H luhulu -(r 

-9-: Mauna 

Ulu 

!. ___..,._. 

EXPLANATION 

0 

Pit crater 

0 2 3 4 5 Mi l ES 

Cone 

0 1 2 3 4 5 Kl LOMETR S 

FIGURE 17.-Principal zones of normal faults 

Note that most fault scarps face north. Aalong which total dilation was measured. 

(1975). Kalanaokuaiki Pali is southernmos 

fault system. 

Fault 

Bar and ball on 

downthrown side 

n Koae fault system. 1 and B-B 1

, traverses 

odified from Duffield 

fault scarp in Koae 

last lava flows covered the ar a. Duffield (1975) 

broadly estimated this time to hav been 500-2,500 

years ago from evidence involvin~, respectively, extrapolation of 20th century rates o displacement on 

the south flank and the interprete<;l ge of the summit 

caldera (Powers, 1948; Rubin a9d Suess, 1956), in 

which lava erupted at the summit e ds to pool rather 

than flooding areas farther dof slope. Hawaiian 

legends concerning the latest flow n the Koae area 

suggest that the 500-year age may jb more nearly correct (H. A. Powers, written commu ' ., 1974). An ancestral Koae fault system apparently existed before the 

latest flows because there is field e ti.dence that these 

flows poured into cracks along the trace of present 

faults, particularly Kalanaokuaiki Pali. Thus, the 

available data suggest that displJ c ments similar to 

those during this century have oceu red over at least 

the past few hundreds to thousands f years. 

Fault-bounded blocks in the Koae system appear to 

have been tilted south-southeastw r during deformation, important evidence bearing b the origin of the 

faults. Such tilting was directly mJa ured over the December 1965 deformation event, w en one tiltimeter 

on a fault block showed more then 2 Ot-trad of seaward 

tilt (Fiske and Koyanagi, 1968, fig. 3 . Unusually steep 

southward slopes on some fault b]o iks suggest a substantial cumulative effect of su 1 h tilting, as least 

locally. 
SOUTHWARD MIGRATION OF THE RIFT ZONES AND SUMMIT RESERVOIR 29 

The tilting and general configuration of the fault 

•locks are remarkably similar to structures obtained 

n modeling clay by Cloos (1968, fig. 18) during experments that produced asymmetric grabens. Such a gra-

•en forms when one side of a clay cake resting on two 

~verlapping metal sheets is pulled laterally while the 

~ther side remains stationary. The major fault scarps 

ace toward the stationary block, and the part of the 

nodel near the zone of separation sags and tilts in the 

lirection of movement. It is interesting that the direcion of tilt is opposite to that which generally accom-

•anies landsliding, whEen slopes are rotated toward the 

:tationary block, not away from it. The observed strucures in the Koae fault system are just those expected 

rom Cloos' experiment, if movement of the south flank 

Lway from the less mobile part of the volcano is analo-

~ous to pulling apart the clay cake. Duffield (1975, fig. 

i) discussed in more detail the relation of Cloos' model 

o structural problems I at Kilauea. 

The interpretation pf the Koae fault system as a 

~omplex asymmetric graben produced by seawardlirected dilation in the rift zones accords with our 

Jostulate that the system is part of the tear-away zone 

;;eparating the south flank from the rest of the volcano. 

[ntrusion in the rift zones causes dilation, which then 

Jropagates into the Koae system as the south flank 

moves seaward, gradually dying out away from the site 

Jfintrusion. In this se:o.se, the Koae is a relatively passive area, reacting to events in the rift zones without 

generating activity itself. Intrusion within the Koae 

seldom takes place, pliesumably because it lacks a direct connection to the summit reservoir complex. Eruptions that do occur are thought to be fed by magma that 

~nters the fault system during rift eruptions near the 

[ntersection with the Koae. Such an eruption occurred 

m 1972, when the migration of earthquake foci into the 

Koae indicated that a dike was being intruded from the 

;;ite of initial eruption on the east rift zone (Koyanagi 

:tnd others, 1973). Such a rift eruption apparently 

~a uses dilation, which propagates into the Koae, creat-

_ng stress conditions favorable for magma to leave the 

~ift zone and intrude the fault system. 

In summary, the Koae fault system and rift zones 

together define the zo e of dilation that, according to 

Jur interpretation, necessarily bounds the north edge 

Jf the mobile south flank. 

RELATIVE IMPORTANCE OF THE 

TWO RIFT ZONES IN GOVERNING MOBILITY 

OF THE SOUTH FLANK 

The horizontal disp[acements from the September 

1971 eruption (fig. 8B) indicate that the south flank 

responds to dike intrusion in the southwest rift zone, 

just as in the east rift zone. However, the southwest rift 

zone has apparently been of subordinate importance in 

governing south-flank structures. It is much shorter, 

lower, and narrower than the east rift zone (fig. 1, 2) 

presumably because it contains fewer dikes and thus 

has fewer eruptions. Furthermore, Duffield (1975) 

found that both width and total dilation of the Koae 

fault system decrease westward away from the east rift 

zone, a pattern suggesting dominance of the east rift 

zone in the development of the Koae. Duffield (1975) 

and Duffield and Nakamura (1973) also showed that 

the direction of dilation across the Koae parallels that 

across the east rift zone and the south flank but is in 

most places oblique to the direction of dilation along 

the southwest rift zone. Additional evidence suggesting dominance of the east rift zone is the east-west 

extent of the Hilina fault system; this system extends 

south of much of the length of the east rift zone and 

Koae fault system but apparently ends, or becomes 

much less conspicuous, near the southwest rift zone 

(fig. 2; Stearns and Macdonald, 1946, pl. 1). From this 

evidence we conclude that events in the east rift zone 

largely govern the mode of deformation of the south 

flank. 

SOUTHWARD MIGRATION OF THE RIFT ZONES 

AND SUMMIT RESERVOIR 

EAST RIFT ZONE 

Several lines of evidence suggest that the active part 

of the east rift zone has migrated southward together 

with the south flank. The topographic crest of the rift 

zone is as much as 4 km north of the recently active 

upper and middle sections of the rift zone (fig. 18A ). 

This crest is probably a constructional ridge built by 

eruptions along it, by flows from a parasitic shield centered near the east end of Kilauea Iki (fig. 2; see also 

the topographic map of the Volcano quadrangle), or 

both. Relics of possible vent areas along the crest are 

hidden beneath dense jungle cover or younger flows 

erupted from the parasite shield. No eruptions have 

taken place along the crest in late prehistoric or historic time, suggesting that, if the ridge was built by rift 

eruptions, the active part of the middle and upper rift 

zone shifted southward from the crest before this time. 

The relation of the complete Bouguer gravity anomaly to the recently active part of the east rift zone 

likewise suggests southward migration (fig. 18B). 

Kinoshita and others (1963; W. T. Kinoshita, oral 

commun., 1973) interpreted this positive anomaly as 

caused by a swarm of dikes of greater bulk density 

than the vesicular flows or loosely packed pillows that 

make up most of Kilauea and underlying Mauna Loa. 

These dikes must be the feeders for lava erupted on the 
30 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

CONTOUR INTERVAL 60 METRES 

A 

B 

COMPLETE BOUGUER GRAVITY ANOMA Y CONTOUR 

INTERVAL 10 MILLIGALS 

0 5 10 15MILES 

0 5 10 15 20 KILOMETRES 

FIGURE 18.-Maps of Kilauea Volcano. A, Topography B, Complete Bouguer gravity anomalies. LO)ca IOlltS 

vent fissures, and pit craters along both rift zones (black areas on both maps) from Stearns and ,M.~~cdon:ald 

additions for later eruptions. Main belt of recent volcanic activity lies along southern part of 

axis of gravity anomaly, taken from B, shown in A by shaded band. Gravity data, in milligals, 

others (1963), with numerous additions along rift zones by W. T. Kinoshita (unpub. data); 

g/cm3 was used in gravity reductions. 
SOUTHWARD MIGRATION OF THE RIFT ZONES AND SUMMIT RESERVOIR 31 

rift zone. However, almost all of the recently active 

vents are along the south side of the gravity anomaly 

(fig. 18B), not along its crest. This suggests that young 

dikes have been added preferentially to the south side 

of the swarm. 

The gravity anomaly on the east rift zone is strongly 

asymmetric, with steeper gradients south of the rift 

zone. Thus the dike swarm may be thought of as defining a wedge-shaped prism, with a gentle north face and 

a steep south face (W. T. Kinoshita, oral commun., 

1973). Such a prism is the expected product of 

southward-directed growth of the rift zone with time, 

because successively younger dikes would on the average be intruded to higher elevations than older dikes to 

the north as the rift zone grew southward and upward 

(fig. 19). This interpretation differs from that of Ryall 

and Bennett (1968), who used the gravity data of 

Kinoshita and others (1963) to infer a north-dipping 

rift zone. 

The gravity axis is located between the topographic 

crest and the active part of the upper east rift zone (fig. 

18A). This is consistent with the hypothesis of southward migration if the rate of migration has recently 

been sufficiently rapid to prevent construction of a 

newer and higher constructional ridge. The 1969--7 4 

eruptions of Mauna Ulu created a new, low ridge along 

the active part of the rift zone. 

If the active part of the east rift zone is moving 

southward, that part of the rift zone nearest the summit should show more displacement because of the 

likelihood of more frequent dike injection near the 

summit magma reservoir. This may account for the 

greater divergence b~tween the topographic and gravity axes, and between the location of young vents along 

the western and eastern parts of the rift zone. The 

gravity data (fig. 18B) support the concept of greater 

dike frequency near the magma reservoir, for the 

Bouguer values are higher along the upper east rift 

zone than along the lower east rift zone. 

The conspicuous bend in the active part of the east 

rift zone, defined by the location of young cones, pit 

craters, and fissures (figs. 1 and 18), may reflect the 

interplay between the greater degree of intrusion near 

the source reservoir,, the southward migration of the 

rift zone, and two stress systems, one radial to and 

confined to the reservoir area, the other the gravitational system favoring south-southeastward displacement. Evidence of an approximately radial stress field 

at the summit comes from studies of ground deformation accompanying episodes of inflation and deflation 

(for example, Fiske and Kinoshita, 1969; Swanson and 

others, 1976). Near the summit area, dikes tend to be 

radial because of the dominance of the radial stress 

field. Away from the summit, however, the gravitational field controls the orientation of dikes. Figure 20 

Rift zone 

South North 

FIGURE 19.-Schematic cross section showing how continued lateral 

growth of a rift zone generally results in successively younger 

dikes intruding to higher elevations. Dike 1 is oldest; dike 5 is still 

erupting lava that may eventually cover flows from dike 4. Arrow, 

direction of lateral growth. In a less schematic diagram, dikes 

would be so closely spaced that they would define a wedge-shaped 

prism of higher density than wallrock, leading to an asymmetric 

gravity anomaly. 

shows schematically the stages in the possible development of the bend in the east rift zone. At an early 

stage in the history of the rift zone before much displacement of the south flank (fig. 20A), dikes near the 

summit area are radial to the source reservoir and tend 

to cluster in a zone perpendicular to the direction of 

south-flank displacement. As the rift zone migrates 

southward because of dike accretion (fig. 20B), dikes 

remain radial to the reservoir near the summit, but 

curve farther downrift to accommodate themselves to 

the gravitational stress system. This accommodation, 

combined with the presence of more dikes near the 

reservoir than far from it, causes the bend in the rift 

zone. Figure 20C shows the present stage, in which the 

bend has been accentuated as more radial dikes were 

intruded near the summit area. Possible southward 

migration of the magma reservoir itself at a slower 

rate than that of the south flank, as discussed later, 

could also augment the development of such a bend. 

During the 1960's and early 1970's, a fairly unobstructed conduit apparently transported magma 

from the summit reservoir down to and beyond the 

bend in the rift zone (Swanson and others, 1976). This 

conduit became periodically plugged, leading to eruptions near the bend and in the summit area. Such a 

conduit is thought to have developed along the trace of 

the rift zone at some unknown, but very recent, time. 

Jackson, Swanson, Koyanagi, and Wright (1975) 

pointed out the infrequency of eruption along the 

southeast-trending part of the east rift zone, between 

the caldera and the conspicuous bend. The southeast 

trend of this part of the rift zone nearly parallels the 

direction of horizontal displacement in the zone of dilation and in the south flank, so that intrusion may be 

more easily accommodated without surface rupturing 

and resultant eruption than elsewhere along the rift 

zone. 
32 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

N 

1 

A 

B 

c \. 

D1kes 

FIGURE 20.-Sketches in plan view showing possible development of 

bend in east rift zone of Kilauea Volcano owing to accretion of 

dikes along south side of east rift zone near summit magma reservoir. A, Early stage. Dikes near summit area are radial to source 

reservoir and tend to cluster in a zone perpendicular to direction of 

south-flank displacement. B, Intermediate stage. As rift zone migrates southward because of dike accretion, dikes remain radial to 

reservoir near summit but curve farther downrift to accommodate 

themselves to gravitational stress system. C, Present stage. Bend 

accentuated as more radial dikes intruded near summit area. Size 

of reservoir, from which dikes are fed , is exaggerated for clarity, 

and shape is greatly simplified from its probable amoeboid form. 

Model assumes a constant stress field favoring dilation in a southsoutheast direction of rift zone, and a radial stress field immediately surrounding reservoir. Trend of dikes near summit reservoir is largely governed by radial stress field, on the east rift zone 

by south- southeast dilational field. 

A slight change in the strike of the fissures within 

the east rift zone may reflect the southward migration 

of the rift zone. Recognizable prehistoric and historic 

fissures north of the main belt of activity trend more 

westerly by 5°-10° than do those in the main belt (fig. 

18A; Stearns and Macdonald, 1946, pl. 1; Moore and 

Krivoy, 1964; Swanson and ot~ rs, 1976, pl. 1); in 

other words, northerly cracks are ' ore nearly radial to 

the summit than those farther o th. Such a pattern 

may be accounted for by the sout ward growth of the 

rift zone (fig. 20B and C). Even fiss res of historic eruptions, such as those in August 196 and February 1969 

(Swanson and others, 1976, pl. 1), have somewhat different trends depending on their ocation, suggesting 

that older dikes may, to some ext nt, act as guides for 

later intrusions, at least beyond distance of several 

kilometres from the source reserv ir. 

Thus, several lines of evidencr, uggest that the east 

rift zone is migrating south-soutlh astward, parallel to 

the direction of south flank disp acement and away 

from the rest of the island, includi g the north flank of 

Kilauea itself. This migration is consistent with our 

model suggesting that rift intrus ·on takes place in a 

predominantly gravitational stress system and that 

Kilauea is buttressed by the rest ?f the island and can 

accommodate intrusion chiefly by moving away from 

this buttress. In this model, each ike injected into the 

rift zone effectively shores up an stablilizes the zone, 

replacing loose pillow lava and h' ghly jointed subaerial flows with relatively dense and coherent dike rock. 

Successively younger dikes then end to seek out the 

contact between dike and flow r cks, accreting along 

the south margin of the rift zone where southsoutheastward dilation is easie t. In this way the active part of the rift zone migrates eaward. This model 

is derived to explain the net long- erm result. Actually 

the active part of the rift zone is 2--l3 km wide, not razor 

thin, but the net result of intrus· on into this zone is 

that predicted by the model. 

SOUTHWEST RIFT ONE 

I 

The southwest rift zone occupie a more complicated 

structural setting than the east ift zone. The southwest rift zone is bounded on the est by scarps in the 

Kaoiki fault system (fig. 2), a sysftem of normal faults 

cutting the southeast flank of M una Loa. This fault 

system is still active, generatip many hundreds or 

thousands of earthquakes yea ly (Koyanagi and 

others, 1972, fig. 2; Koyanagi an others, 1966). One 

interpretation is that the Kao'ki is a system of 

landslide-type faults, analogouslt the Hilina fault system on the south flank of Kila ea (p. 33), that developed during an earlier stage n :the growth of Mauna 

Loa before Kilauea was very larg .. The southwest rift 

zone of Kilauea may overlie part f the Kaoiki system, 

and it is conceivable that at lea t some of the southwest rift fissures merge at de t with Kaoiki faults. 

Further discussion of these possib'lities must await detailed mapping of the south e t rift zone and the 

Kaoiki fault system. 
THE HILINA FAULT SYSTEM 33 

Although little is known about the southwest rift 

zone, available evidence is consistent with its seaward 

migration. The axis of the southwest rift gravity 

anomaly is offset toward Mauna Loa from the active 

part of the rift zone, as it is on the east rift zone (fig. 

18B). The geologic maps of Stearns and Macdonald 

(1946, pl. 1) and Walker (1969) show that young vents 

tend to be located along the seaward side of the southwest rift zone, although this is complicated by vents 

within the western part of the Koae fault system (for 

example, Puu Koae and Cone Crater on the map by 

Walker, 1969). The vents of the September 1971 eruption also lie near the southeast edge of the rift zone. 

Thus we believe thaF the evidence is consistent with 

seaward migration, possibly less pronounced than 

along the east rift zone because of less frequent 

episodes of magma intrusion. 

Figure 6 shows that several stations on the Mauna 

Loa side of the south est rift zone were displaced away 

from the zone between 1970 and 1971. We suggest that 

this displacement was largely absorbed within the 

Kaoiki fault system by adjustment of fault blocks, in a 

manner similar to the way in which the east rift zone 

north of sites of intrusion responds to displacement 

events (figs. 5A, 5C, 7, and SA). 

SUMMIT RESERVOIR 

The summit reservoir system may also be migrating 

seaward, though at a slower rate than the east rift 

zone. Such migration is suggested by: (1) the 

northeast-southeast elongate shape of the caldera (fig. 

2); (2) the location of the present Halemaumau pit crater in the south part of the caldera; (3) the general 

southerly offset of the present caldera from the high 

area along its north margin; ( 4) the location of the 

present-day reservoir system beneath the south part of 

the caldera (Fiske and Kinoshita, 1969); and (5) the 

general southerly migration of the centers of ground 

deformation during periods of inflation and deflation 

related to reservoir filling and emptying (Fiske and 

Kinoshita, 1969; Jackson and others, 1975). Moreover, 

the positive gravity anomaly is centered somewhat 

north of the area of maximum ground deformation, a 

relation explained in a manner analogous to our riftzone argument. 

This migration presumably is closely related to the 

same processes affecting the rift zones, but may be 

more complex. The summit system is fed by conduits 

rooted in the mantle and hence is susceptible to lateral 

movements of the Pacific plate and the asthenosphere. 

Nonetheless, the postulated seaward migration is an 

expectable consequence of intrusion in a gravitational 

stress system favoring southeastward-directed dilation. 

THE HILINA FAULT SYSTEM 

Leveling surveys since 1921 document uplift in the 

Hilina fault system (figs. 2 and 3), yet abundant evidence indicates net subsidence of this area over the 

past few thousand years. Many geologists (for example, 

Stearns and Macdonald, 1946; Moore and Krivoy, 

1964), have commented on the seaward-facing fault- · 

line scarps, which in places are more than 500 m high 

and bounded by gaping cracks, as evidence of normal 

faults. Walker (1969) measured a seaward dip of about 

50 degrees on one exposed fault plane in the Hilina 

system and noted that the sense of displacement on 

most of the faults is down toward the sea. The faults 

have been recently active, for they locally offset the 

youngest prehistoric flows in the area (Macdonald, 

1956). 

Two large subsidence events were recorded along the 

south coast during the 19th century. In 1823, a series 

of strong earthquakes accompanied ground cracking 

and subsidence of at least 46 em at the village of 

Kaimu (Ellis, 1827, p. 195--196), a short distance from 

BM 10 (inset, fig. 13). In 1868, much of the south coastal area subsided 1-2 m or more during severe earthquakes accompanied by a local tsunami (Brigham, 

1909, p. 103-113). These two events occurred far from 

sites of eruption and thus differ from the well-known 

development of a graben along the rift zone in 1924 

near Kapoho, where the east rift zone enters the ocean 

(Finch, 1925; Jaggar, 1924). 

The weight of geologic evidence and the two major 

recorded subsidence events compel us to believe that 

subsidence is the dominant sense of displacement in 

the Hilina fault system. If so, then the measured uplift 

since 1921 within the fault system is acting against the 

long-term trend and consequently may be decreasing 

the stability of the fault system. We suggest that this 

uplift, probably related to tumescence along the rift 

zone, combines with large seaward-directed displacements to produce instability, which ultimately triggers 

faulting along the unbuttressed south coast, such as 

took place in 1823 and 1868. 

We interpret many of these gravity faults to bottom 

out at shallow depth (fig. 15 A-A', B-B '),merging into 

bedding planes within the hyaloclastic deposits and 

underlying loosely packed pillow lava that form the 

basement to the subaerial part of Kilauea (Moore and 

Fiske, 1969). The displaced blocks are therefore similar to large landslide blocks, as suggested by Stearns 

and Macdonald (1946) and Moore and Krivoy (1964). 

Macdonald later objected to the landslide hypothesis, 

primarily because he believed that ((* * *piles of volcanic rocks with average slopes of 4° to 7° should be 

very stable" (Macdonald, 1956, p. 286). More recent 

work by Moore and Fiske (1969, fig. 3), however, 
34 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

suggests that a significant thickness of clastic deposits 

dipping more than 10° seaward underlies the south 

flank, and we think that these deposits form an 

adequate medium in which lateral and downward slippage could occur. 

Macdonald (1956) offered a challenging alternative 

to the landslide hypothesis, suggesting that tumescence in the summit region, and by implication the rift 

zones, causes the formation of nearly vertical fault 

zones paralleling the rift zones with the inland side 

upthrown. If this were true, rather straight, linear 

faults might be expected. Instead, however, faults in 

the Hilina system curve and branch (fig. 2), and some 

are arranged in en echelon sets on the limbs of large, 

arclike master faults (fig. 21). Macdonald's suggestion 

also does not explain the lack of such faults on the 

north flank of Kilauea, whereas landslide-type faults 

would only be expected on the south flank. The seaward dip of 50° that Walker (1969) measured on one 

fault plane is also inconsistent with Macdonald's 

suggestion. Finally, broad areas at the base of the 

Hilina, Holei, and the Poliokeawe fault-line scarps are 

nearly fiat or even slope gently inland (fig. 22), consistent with rotation accompanying landsliding but not 

with faulting related to rift tumescence. 

On the other hand, evidence from the 1965-71leveling profile (fig. 11B) suggests that the inland side of 

Hilina faults can at times be displaced upward relative 

to sea level. That profile shows a sharp offset across 

Holei Pali, with uplift of 9 em relative to BM 10. This 

reference point itself probably moved upward, if at all, 

during the survey period, so the true amount of uplift 

relative to sea level may be more than 9 em. If the fault 

forming Holei Pali dips seaward, as we believe, then 

the indication of reverse movement is clear. We conclude that Macdonald's suggestion of uplift on the inland side of Hilina faults is indeed correct for certain 

periods of strong tumescence of the rift zones, such as 

between 1965 and 1971, but that this uplift is small 

and contrary to the dominant long-term sense of displacement within the fault system. 

We estimate that the depth to the base of the hyaloclastic layer, within which we interpret many of the 

Hilina faults to die out, is between 1 and 3 km below 

sea level. The depth increases with distance from 

Kilauea. This estimate is based on the results of Moore 

and Fiske (1969, fig. 3) and considerations of the effects 

of isostatic subsidence (Moore, 1970), which may have 

lowered the layer several hundred metres since it 

formed. It is also possible that some of the Hilina faults 

bottom out in a deeper hyaloclastic layer formed during growth of Mauna Loa (fig. 15A and B). 

Walker (1969) mapped several thin dikes in the 

western part of the Hilina fault system; others occur 

FIGURE 21.-Aerial view of Hilina fault 

(lower left), highest scarp at coastr 

arrangement of Puu Kapukapu an~ 

dle and upper right). These en echel n , 

limb of a large fault block. En ecHel 

small fault block (lower right corner) 

Puu Kapukapu is 320m high. Sc 

View to north-northwest. 

m near Puu Kapukapu 

(fig. 2). Note en echelon 

scarps to northeast (midt-line scarps define west 

faults also occur around 

theast of Puu Kapukapu. 

(upper left) is Hilina Pali. 

FIGURE 22.-Nearly flat ground may reflect landward 

rotation during faulting, at base of Pali (background) and 

unnamed fault-line scarp in Hilina system. Part of the area 

at immediate base of unnamed shown as virtually flat on 

Makaopuhi Crater 7%-minute qm dr mgle; lava ponded in this 

area in 1970-73 because of the low which in fact seemed 

slightly reversed in places when in 1970. Lava flows in 

1969-7 4 have greatly modified tnrinctr<>nl-•u 

quadrangle map. View to northwest 

elsewhere in the system. On ·s of field relations, 

Walker interpreted these as crack fillings fed 

from overriding flows. The open cracks in 

the fault system make his,·,. l"t"a, ... ,.,,.,.a-r-ation seem likely to 

us, and we add that neither avity data (fig. 18B; 

Kinoshita and others, 1963) n ismic refraction data 

(Hill, 1969) suggest a dike sw in this area. Thus we 

interpret the dikes as bearing 

Hilina fault system. 
APPLICATION TO OTHER OCEANIC VOLCANOES 35 

We interpret the shape of the coastline of the south 

flank to be controlled principally by where and how 

often lava flows erupted from the rift zones reach the 

sea, not by faults of the Hilina system. Only locally, as 

at the horsts near Kaimu (fig. 2) and Puu Kapukapu 

(figs. 2 and 21), do faults define the trend of the 

coastline. Most of the coastline is vulnerable to flows 

from the rift zones, but the area centered near Puu 

Kapukapu lies between the two rift zones and is less 

frequently inundated with lava; consequently the rest 

of the coastline has grown seaward on either side of 

this sheltered area, leaving a broad embayment. The 

area inland of this embayment is higher than most 

other parts of the fault system and includes Puu 

Kapukapu, the largest and most prominent horst in 

the Hilina system (fig. 21). This area may stand high 

because it is pinchetl between two segments of the 

south flank moving kt slightly convergent azimuths 

away from the southlwest and east rift zones, and so 

may be kept from subb ding as drastically as elsewhere 

by this relative compression. 

Some of the data suggest that the Hilina fault system is poised for another episode of subsidence, with 

only minor additional ground displacement needed to 

trigger it. Figure 10 (lower inset) shows that, in 1970, 

every south-flank earthquake of M~3.5 except one (fig. 

10D) was immediately followed by a brief period of 

extension of a geodimeter line that otherwise showed 

strong net contraction in response to rift intrusion. No 

such correlation could be made i~ 1969. Possibly the 

amount of contraction across the fault system was 

reaching a critical level by 1970, so that earthquakes 

of even moderate size could trigger brief periods of 

strain release. Measurements in 1971 (fig. 6) and 197 4 

(Hawaiian Volcano Observatory, unpub. data) showed 

small extensions across the Hilina system, continuing 

the trend shown in 1970 and suggesting a further 

amount of strain release. Most of the strain acquired 

since 1965 (fig. 10) remains, however, and we anticipate a subsidence (strain-release) event of unknown 

magnitude in the not too distant future. 

(A major subsidence event took place in the Hilina 

fault system on November 29, 1975, 15 months after 

this paper was submitted for publication. The subsidence, accompanied by an earthquake of M=7.2, affected much of the south coast of Kilauea and was as 

much as 3.4 m in the Puu Kapukapu area. Large extensions were measured across the Hilina system, and 

many fault scarps showed renewed displacement, with 

seaward side down. Two campers were killed by a 

tsunami generated by the earthquake and subsidence. 

These deaths emphasize the fact that the unstable nature of the south flank of Kilauea has broad social 

as well as geologic implications.) 

APPLICATION TO OTHER 

OCEANIC VOLCANOES 

We doubt that Kilauea is a unique volcano, and 

many of the princi pies gained from this study should 

be applicable to other oceanic shields. In Hawaii, for 

example, large landslide blocks are probably present 

along the west coast of Mauna Loa, bounded by the 

Kealakakua fault and unnamed faults between 

Hookena and Milolii (Stearns and Macdonald, 1946, 

pl. 1). We consider that the Kaoiki fault system on 

Mauna Loa (fig. 2) may also be a gravity-controlled 

landslide area, now partly shored up by Kilauea but 

still influencing the way in which the southwest rift 

zone of Kilauea is deformed. The landslide-type faults 

of Mauna Loa are downslope from a large rift zone and 

may well have developed in response to intrusion in 

that zone. Moore (1964) described possible giant landslide masses from Koola u and East Molokai Volcanoes 

that may owe their origin to dike intrusion in rift zones 

parallel to the coastline. The flanks of many older 

Hawaiian shields, such as Haleakala, West Maui, and 

East Molokai Volcanoes, dip 11 to 18 degrees seaward, 

much steeper than the average shield slope of several 

degrees. It is possible that these steep slopes partly 

reflect the net effect of doming caused by the intrusion 

of numerous dikes, although all of these volcanoes are 

at a more advanced stage of evolution than Kilauea 

and consequently may be . subject to processes not observed during the young, tholeiitic stage. 

Any oceanic shield volcano that grows on the flank of 

an earlier shield in the absence of a dominant tectonic 

stress system should behave similarly to Kilauea. Consequently, there are probably numerous volcanoes in 

the ocean basins that are characterized by asymmetric 

dilation of rift zones, a mobile seaward-facing flank, 

and secondary landslide blocks on the mobile flank. 

Recognition of these features in deeply eroded shields 

may be difficult, but there are possible clues to look for. 

Dikes in rift zones of buttress~d volcanoes should on 

the average become younger from the landward side 

toward the seaward side; this might be detected in 

favorable circumstances by paleomagnetic or 

radiometric age data, temporally related differences in 

chemistry, or simple stratigraphic evidence. Examples 

in the Hawaiian chain where rift zones could be 

examined in this way are East Molokai and Koolau 

Volcanoes; both of these grew adjacent to an older 

shield (Fiske and Jackson, 1972) and are dissected sufficiently to expose dikes. On the other hand, unbuttressed shields such as Niihau show no such tendency 

and when eroded reveal symmetric rift zones in the 

central part of the edifice. We hope that examination of 

eroded shields with the model of Kilauea in mind can 
36 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

shed more light on their internal structure, as well as 

feed back important clues for a better understanding of 

Kilauea itself. 

SUPPLEMENTAL INFORMATION 

PROBLEM OF GROUND DEFORMATION 

OCCURRING DURING THE HORIZONTAL SURVEYS 

The 1971 trilateration was carried out between October 3 and 18; the 1970 trilateration took place chiefly 

between late August and late September, with a few 

supplementary distances measured in November and 

early December. The volcano was quite stable during 

both surveys, as indicated by measurements of tilt and 

strain at the summit and the absence of major earthquake activity elsewhere on the volcano. 

The 1961 triangulation was conducted between 

March 22 and July 25, spanning the last three days of 

the March summit eruption, the entire July summit 

eruption, and the intervening period of substantial 

summit inflation (about 90 microradians at one tiltmeter at the summit) (Richter and others, 1964, fig. 3). 

These events, especially the inflation, almost certainly 

caused some of the poor triangle closures reported in 

the summit region (table 2) but probably did not 

greatly affect the rest of the volcano. 

The 1958 triangulation lasted from February to 

June. Only slight summit deformation took place during this time, as judged from records of tilt (Eaton, 

1962, fig. 13) and seismicity (Eaton and Fraser, 1958a, 

b), and probably little if any deformation occurred on 

the flanks of the volcano. 

The 1949 triangulation spanned the March-July 

interval. A horizontal pendulum seismometer used as 

a tiltmeter at the summit recorded little deformation 

during this time (Volcano Letter, 1949). 

Virtually nothing is known about ground deformation between 1896 and 1914, for no measurements 

were made then. Mild summit eruption in 

Halemaumau Crater continued throughout most of 

this time. Similar, but more recent, Halemaumau 

eruptions have been accompanied by very little ground 

deformation anywhere on the volcano, particularly on 

the flanks, so we infer that little took place between 

1896 and 1914. 

BASELINE SELECTION 

The selection of baselines and stations considered as 

stable is one of the most critical factors affecting the 

study of horizontal displacement. In general, we tried 

to select stations as far from eruptive activity as feasible. Table 4 shows the stations and baselines that we 

assumed to be stable. Keaau, Olaa, Kaloli, and Kaloli 2 

are far removed from areas of known instability (fig. 3), 

TABLE 4.-Baselines used for deriving horizontal displacements in 

figure 5 

Survey interval 

1896(1914)-1970 

1949-70 

1958-70 

1961-70 

1970--71 

Stable stations 

Olaa, Kalolil Kulani, Keaau, Kaloli 2, 

Keakapulu, Kahaha 2. Kulani, Keaau, Kaloli 2 

Keakapulu. 

Kulani, Strip, Kahaha 2 Kulani, Strip, Keakapulu 

Baselines (lengths) 

Olaa-Kalolil (11,745.71 m). Kulani-Keaau (27,984.81 m); Keaau-Kaloli 2 (10,929.93 m). Kulani-Keaau (27,948.81 m) Keaau-Kaloli 2 (10,929.93 m). 

Kulani-Strip (9,559.14 m). Kulani-Strip (9,559.14 m). 

•Kaloli has been destroyed by wave erosion. Distam es were measured to nearby Kaloli 2 and reduced to Kaloli using offset data determined by t !jle U.S. Coast and Geodetic Survey m 

1949 before the destruction of Kaloli. 

and the distance between Keam and Kaiwiki New, a 

station on the lower east slope of Mauna Kea, did not 

measurably change between 1949 and 1970. Thus the 

assumption of their stability is easonable. 

Kulani, on the southeast flank of Mauna Loa, is 

closer to the summit of Kilauea land could conceivably 

have been affected by the great summit subsidence in 

1924 (Wilson, 1935). For this ason, Kulani was not 

used as a fixed point for any inte val spanning 1924. In 

fact, however, Kulani probabl was not measurably 

affected by the great subsidence, for its apparent displacement over the event is negligible (fig. 5E). Kulani 

did not move measurably betwEen 1949 and 1970 because the distance to Kaiwiki N~w and the astronomic 

azimuth to Keaau are unchanged and because displacements derived using Kula1 i-Keaau as a baseline 

agree within probable error wi h those using KeaauKaloli 2. 

Strip, also on the slope of Mauna Loa, was established in 1961. No unusually! l rge structura~ events 

affected the summit of Kilaue~ in the following decade, and Mauna Loa was quie , on the basis of the 

monitored seismicity of the vol ano and leveling and 

geodimeter data from the su~IJ it (Hawaiian Volcano 

Observatory, unpub. data, 1961-73; Decker and 

Wright, 1968). 

Kahaha 2 is on Mauna Loa and is approximately 

along strike of, though soutHwest of and possibly 

higher than, the known limits < f the Kaoiki fault system (fig. 3), a seismically activ1 system on the southeast flank of Mauna Loa. Dis1lacements for nearby 

stations are small and geologically reasonable, assuming stability of Kahaha 2. 

Keakapulu is located on the northwest edge of the 

Kaoiki fault system, and mos~ c).stances from it had to 

be shortened by amounts app oaching triangulation 

error to achieve geologically te~ sonable results. ThesE 

adjustments suggest possible s~ utheastward displacement of the station, although!' ithin probable survey 

error. In any case, the stabilit of Keakapulu is not 

critical if Kahaha 2 has remair ed stable. 

In summary, all available il}f< rmation, including thE 

consistency of our results, inr ( jltes that the base sta· 
REFERENCES CITED 37 

tions have remained stable within the probable limits 

of survey error during the 20th century. 

PROCEDURE FOR TREATING TRIANGULATION DATA 

We used originality observed angles, adjusted only 

for triangle closure to 180°, in this study. No use was 

made of published lengths or angles resulting from 

previous network adjustments, as such adjustments 

were made in various ways for each triangulation survey and involve different baseline lengths, numbers of 

stations, and other constraints. The original angles 

were obtained through the courtesy of B. K. Meade 

(U.S. Coast and Geodetic Survey data) and J. P. 

Church, C. R. Lloyd, and R. F. Thurston (U.S. Geological Survey data). 

Each triangulation survey was compared with the 

1970 trilateration survey by reference to one or more 

baselines (table 4). The 1970 geodimeter length of each 

baseline, reduced to sea level by standard techniques, 

is assumed to have been the length during each preceding triangulation. U ing this length and originally observed angles, the triangulation was then recomputed 

using the law of sines. In this way, distances derived 

from the trilateration and triangulation surveys are 

directly comparable. In general, corresponding sides of 

triangles differ in length for the two surveys, indicating apparent displacement of one or more stations. 

The ground-displacement vectors were derived by a 

graphical method described by Kinoshita, Swanson, 

and Jackson (1974). This method was selected in preference to the standard least-squares approach normally used for triangulation data because it allows 

visual inspection and evaluation of the displacements 

as they are derived. In this way geologic input could be 

made as to what displacements seem reasonable, and 

what distances should be given more weight than 

others on the basis of the displacements they produce. 

This approach seems justified, given the quality of the 

data. After geologic evaluation, distances were then 

changed by trial and error within permissible limits 

(table 2) to give geologically reasonable displacements. 

The graphical derivation was then resumed along the 

chain of triangles until a displacement had been determined at each station. When a geologically preferred, graphically derived length differed significantly from the original computed one, the new length 

was used for subsequent law of sines computations dependent on it. Finally an acceptable degree of internal 

consistency and geologic reasonableness was obtained 

for all triangles in each survey interval. 

This time-consuming approach is partly subjective, 

but the basic data were not violated. All graphical adjustments were mad~ within stated error limits (table 

2), and most were far less than the maximum permissible amount. In essence, then, this approach is 

analogous to a standard least-squares adjustment with 

an overriding geologic monitor on each operation. 

Experience has shown that the basic pattern of displacement could not be changed greatly when ;;idjustments were kept within the limit of survey error, but 

that, with minor adjustments, the displacements could 

be better fitted to known geology. An example is shown 

in figure 5E, in which displacements derived by us are 

compared with displacements calculated by Lloyd 

(1964) using the standard least-squares approach. A 

similar basic pattern is shown by both displacement 

sets, but the progressive clockwise swing of the leastsquares vectors (a pattern typically indicative of survey errors) may be eliminated by small adjustments in 

favor of displacement directly away from open ground 

cracks that formed during the survey interval, a 

reasonable geologic constraint. In short, the basic pattern is there regardless of how the data are manipulated, and it is with this basic pattern that we are most 

concerned in this paper. 

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38 DISPLACEMENT OF THE SOUTH FLANK OF KILAUEA VOLCANO 

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~ GP 0 6 9 1 • 3 0 B -1 9 7 6 

CONTENTS 

Page 

1\bstract -------------------------------------------------- 1 

Introduction ---------------------------------------------- 1 

1\cknovvledgrnents__________________________________________ 2 

Structural subdivisions of Kilauea ----- -------------------- 3 

Earthquake distribution____________________________________ 7 

Long-term horizontal displacements during the 20th century__ 8 

Existing data and their quality ------------------------ 8 

Results________________________________________________ 9 

1949-70 ------------------------------------------ 9 

1958-70 ------------------------------------------ 11 

1949-58 ------------------------------------------ 11 

1896 (1914)-1970 ---------------------------------- 12 

1896 (1914)-1958 ---------------------------------- 12 

1961-70 ------------------------------------------ 12 

1\ugustr-September 1970 to October 1971 ____________ 13 

Summary of long-term horizontal deformation __________ 13 

Horizontal deformation during specific intrusive or extrusive 

events ---------------------------------------------- 14 

February-May 1955 east rift eruption __________________ 14 

February-May 1969 east rift eruptions__________________ 15 

September 1971 eruption ------------------------------ 15 

Specific displacement events on the south flank __________ 15 

Strains betvveen March 6 and 1\ugust 27, 1970 ______ 16 

Linear contraction of a geodimeter line betvveen 1965 

and 1970 ---------------------------------------- 16 

Summary and interpretation of horizontal deformation 

on the south flank ------------------------------ 16 

Page 

Vertical displacement of south and north flanks of Kilauea 17 

Central part of south flank ---------------------------- 19 

Eastern part of south flank and lovver east rift zone ------ 20 

Western part of south flan!t and Koae fault system ______ 21 ~orthflank ____________________________________________ 21 

Summary and interpretation of vertical deformation on the 

flanks ---------------------------------------------- 21 

Relative magnitude of horizontal and vertical displacements__ 23 

l\ structural model for the south and north flanks of Kilauea__ 24 

~ umerical comparison of volumes of intrusion and deformation 27 

The rift zones and Koae fault system as wnes of dilation ____ 27 

Relative importance of the tvvo rift zones in governing mobility 

of the south flank---------------------------------------- 29 

Southvvard migration of the rift zones and summit reservoir __ 29 

East rift zone------------------------------------------ 29 

Southvvest rift zone -----------------------.------------- 32 

Summit reservoir ____________________ ---------- ______ -- 33 

The Hilina fault system------------------------------------ 33 

1\pplication to other oceanic volcanoes ---------------------- 35 

Supplemental information---------------------------------- 36 

Problem of ground deformation occurring during the horizontal surveys ________________________ -------- ____ -- 36 

Baseline selection _______________________________ -~----- 36 

Procedure for treating triangulation data-------------------- 37 

References cited _______________________________________ --- 37 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of Kilauea Volcano and adjacent areas ------------------------------------------------------------ 2 

2. Map of Kilauea and adjacent part of Mauna Loa shovving main structural features and locations of important faults, 

cones, and pit craters ________________________________________________________ ---------------------------- 3 

~9. Maps shovving: 

3. Structural subdivisions of Kilauea Volcano and locations of triangulation stations ------------------------ 7 

4. Location of earthquake epicenters related to eruptions and ground-cracking events, 196~69 -------------- 8 

5. Horizontal ground displacements for several periods betvveen 1896 (1914) and 1970________________________ 11 

6. Horizontal ground displacements betvveen 1\ugust 24 and September 23, 1970 and October ~18, 1971 ------ 13 

7. Horizontal ground displacements during February 28-May 26, 1955 east rift eruption -------------------- 14 

8. Horizontal ground displacements during the February and May 1969 east rift eruptions and the September 

1971 summit and southvvest rift eruption---------------------------------------------------------- 15 

9. Changes in line length, extensional and dilatational strains, and axes of strain ellipse on south flank betvveen 

March 6 and 1\ugust 27, 1970---------------------------------------------------------------------- 17 

10--14. Graphs shovving: 

10. Contraction of a 5.4-km-long geodimeter line on south flank of Kilauea betvveen 1\ugust 1965 and December 

1970 18 

11. Vertical displacement and topographic profiles across central part of south flank betvveen 1921 and 1971____ 20 

12. Vertical displacement and topographic profiles across lovver east rift zone and eastern part of south flank betvveen 1921 and 1973 _____________________ ~-------------------------------------------------------- 21 

13. Vertical displacement and topographic profiles across lovver east rift zone betvveen 1958 and 1973 ---------- 22 

14. Vertical displacement and topographic profiles across Koae fault system and vvestern part of south flank betvveen 1958 and 1971______________________________________________________________________________ 23 

III 
IV 

FIGURE 

TABLE 

CONTENTS 

Page 

15. Diagrammatic cross sections through summit region, miqdle east rift zone, and submarine pa of east rift zone of 

Kilauea, showing interpreted relations between rift zone and rest of volcanic pile ______ ___________________ 24 

16. Diagrammatic cross sections across Kilauea, Mauna Loa, Hualalai, and part of Mauna Kea __ ___________________ 26 

17. Map of principal zones of normal faults in Koae fault system ------------------------------ _____________ ______ 28 

18. Maps showing topography and complete Bouguer gravity anomalies, Kilauea Volcano ______ ___________________ 30 

19. Schematic cross section showing how lateral growth results in younger dikes intruding to hig er elevations______ 31 

20. Sketches showing development of bend in east rift zone owing to accretion of dikes along south side of rift zone near 

summit magma reservoir ------------------------------------------------------------r ------------------ 32 

21. Aerial photograph of Hilina fault system near Puu Kapukapu, showing en echelon arrangeme t of faultline scarps_ 34 

22. Photograph of nearly flat ground surface at base of Holei Pali and an unnamed faultline scarp i Hilina fault system_ 34 

TABLES 

1. Eruptions and inferred intrusive events at Kilauea Volcano, 1900-1974 _________________________________ · ______ _ 

2. Precision of the triangulation surveys _---------------------------------------------------T ------------------ 3. Comparison of horizontal and vertical displacements at six triangulation stations on the soJ th flank of Kilauea 

Volcano------------------------------------------------------------------------------------------------

4. Baselines used for deriving horizontal displacements in figure 5 ____________________________ _1 _________________ _ 

Page 

4 

9 

24 

36