Geophysical observations of Kilauea Volcano, Hawaii, 2. Constraints on the magma supply during November 1975–September 1977

Journal of Volcanology and Geothermal Research, 7 (1980) 241--269

241

© Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium

GEOPHYSICAL OBSERVATIONS OF KILAUEA VOLCANO, HAWAII, 2. CONSTRAINTS ON THE MAGMA SUPPLY DURING NOVEMBER 1975--SEPTEMBER 1977

DANIEL DZURISIN, L E N N A R T A. A N D E R S O N 1, GORDON P. EATON :, ROBERT Y. KOYANAGI, P ETER W. LIPMAN', JOHN P. LOCKWOOD, R E G I N A L D T. OKAMURA, GARY S. PUNIWAI, MAURICE K. SAKO and KENNETH M. YAMASHITA

U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, HI 96718 (U.S.A.) (Received March 16, 1979; revised and accepted July 19, 1979)

ABSTRACT Dzurisin, D., Anderson, L.A., Eaton, G.P., Koyanagi, R.Y., Lipman, P.W., Lockwood, J.P., Okamura, R.T., Puniwal, G.S., Sako, M.K. and Yamashita, K.M., 1980. Geophys!cal observations of Kilauea volcano, Hawaii, 2. Constraints on the magma supply during November 1975--September 1977. J. Volcanol. Geotherm. Res., 7: 241--269. Following a 22-month hiatus in eruptive activity, Kilauea volcano extruded roughly 35 X 106 m 3 of tholeiitic basalt from vents along its middle east rift zone during 13 September--1 October, 1977. The lengthy prelude to this eruption began with a magnitude 7.2 earthquake on 29 November, 1975, and included rapid summit deflation episodes in June, July, and August 1976 and February 1977. Synthesis of seismic, geodetic, gravimetric, and electrical self-potential observations suggests the following model for this atypical Kilauea eruptive cycle. Rapid summit deflation initiated by the November 1975 earthquake reflected substantial migration of magma from beneath the summit region of Kilauea into the east and southwest rift zones. Simultaneous leveling and microgravity observations suggest that 40--90 X l 0 s m 3 of void space was created within the summit magma chamber as a result of the earthquake. If this volume was filled by magma from depth before the east rift zone intrusive event of June 1976, the average rate of supply was 6--13 x l 0 s m3/month, a rate that is consistent with the value of 9 x l 0 s m 3/month suggested from observations of long-duration Kilauea eruptions. Essentially zero net vertical change was recorded at the summit during the 15-month period beginning with the June 1976 intrusion and ending with the September 1977 eruption. This fact suggests that most magma supplied from depth during this interval was eventually delivered to the east rift zone, at least in part during four rapid summit deflation episodes. Microearthquake epicenters migrated downrift to the middle east rift zone for the first time during the later stages of the February 1977 intrusion, an occurrence presumably reflecting movement of magma into the eventual eruptive zone. This observation was confirmed by tilt surveys in May 1977 that revealed a major inflation center roughly 30 km east of the summit in an area of anomalous steaming and forest kill first a ot e d in March 1976. ~Current address: U.S. Geological Survey, Federal Center, Denver, CO. 80225, U.S.A. 2Current address: U.S. Geological Survey, National Center, Reston, VA 22092, U.S.A.

242 Seaward m o v e m e n t of the south flank of the volcano during and i m m e d i a t e l y after the N o v e m b e r 1975 e a r t h q u a k e caused significant dilation of the upper and middle east rift zone. A rough u p p e r estimate o f the net v o l u m e change implied by observed surface extensions across the rift is 100 x 10 ~ m 3. A t the rate of m a g m a supply calculated from s u m m i t leveling and gravity data, 8--18 m o n t h s would be required to fill this newly created volume. The S e p t e m b e r , 1977 e r u p t i o n occurred 15 m o n t h s after the first intrusive event, and roughly 3 m o n t h s supply of m a g m a was extruded. In addition to the magma that presumably filled newly created space within the east rift zone, an u n k n o w n v o l u m e of newly delivered m a g m a was responsible for net uplift of the eruptive zone during the S e p t e m b e r 1977 eruption. We therefore propose that the total v o l u m e of m a g m a supplied to Kilauea volcano f r o m its mantle source during the period N o v e m b e r 1 9 7 5 - - S e p t e m b e r 1977 ( 1 3 0 - - 2 8 0 X 106 m 3) was partitioned as follows: (1) 40---90 x 10 ~ m 3 to refill newly created void space within the s u m m i t m a g m a chamber, (2) 100 x 10 ~ m 3 to occupy new v o l u m e within the east rift zone, (3) 35 × 10 ~ m 3 e x t r u d e d , and (4) 0--55 × 104 m 3 responsible for net uplift of the eruptive zone. A l t h o u g h complications will likely arise, these values suggest that quantitative m o d e l i n g of the m a g m a supply at Kilauea may n o w be feasible, given adequate geophysical surveillance of the s u m m i t region and rift zones.

INTRODUCTION

Scope of the study Kilauea volcano on the Island of Hawaii erupted along its middle east rift zone on 13 September, 1977, after nearly 22 months of quiescence. This outbreak was not preceded by sustained summit inflation typical of recent Kilauea activity (Kinoshita et al., 1974), but rather by five episodes of relatively rapid summit deflation in November--December 1975, June 1976, July 1976, August--September 1976, and February 1977. The first episode was triggered by a magnitude 7.2 earthquake that struck the southeast coast of Hawaii on 29 November, 1975; a minor Kflauea eruption and precipitous summit deflation reflected substantial magma movements within the volcano. In June 1976, July 1976, and February 1977, rapid summit deflations and intense, shallow earthquake swarms along the upper east rift zone signaled movement of magma from the summit reservoir eastward into the rift zone. A fifth major deflation episode during August 1976 was largely aseismic and is therefore unique in recent Kilauea history. These summit deflations, which culminated in the September 1977 eruption, were documented as part of the volcano monitoring program at the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO). In addition to conventional seismic, tilt, leveling, and geodimeter observations, electrical self-potential measurements were made before and immediately after each deflation episode. Precise microgravity traverses in the summit region bracket the November 1975 earthquake, the intrusion sequence, and the September 1977 eruption. This report presents a synthesis of available observations in the form of a quantitative model of the magma supply at Kilauea volcano for the period November 1975--September 1977. Seismic, geodetic, and electrical self-

243

potential data are discussed first to provide a framework for analysis of simultaneous leveling and gravity observations in the summit region. Implications for the rate of magma supply from depth, the existence and importance of temporary magma storage chambers within the east rift zone, and prospects for near-future volcanic activity are discussed in the final section. A more detailed treatment of surface deformation data and their implications for Kilauea dynamics is presented elsewhere (Lipman et al., 1978, 1980) along with a comprehensive chronology of the September 1977 eruption (Moore et al., 1978, 1980).

Geologic setting Kilauea is an active basaltic shield volcano built against the southeast flank of larger Manna Loa volcano on the Island of Hawaii (inset, Fig. 1). Major structural features of the Kilauea shield include a 3 by 5 km summit caldera with its central pit crater Halemaumau, prominent east- and southwesttrending rift zones radiating from the caldera, and the Koae and Hilina systems of normal faults cutting Kilauea's south flank (Fig. 1).

l p u e Pt.

KOHAL&

2G**¢X~'

UALALAI

I

LOA

ILAU

Fig. 1. Sketch map o f Kilauea s u m m i t region, east rift zone, and upper southwest rift zone. September 1977 eruptive fissures are shown schematically. Prominent fractures along upper southwest rift zone are indicated by heavy lines; major escarpments of Hilina system o f normal faults are indicated by lines with bars on downthrown sides.

244 Eruptmns on Kflauea generally occur within the summit caldera or along the rift zones and are seemingly fed by magma from a shallow storage reservoir beneath the caldera (Koyanagi et al., 1974). Seismic data suggest that thi~ reservoir is in turn supplied by magma generated in the upper mantle at a depth of 45--60 km (Eaton, 1962}. Geodetic and seismic evidence indicate that rift eruptions are fed via a well-developed system of lateral conduits. The existence of temporary magma storage reservoirs within the east rift zone has oeen inferred from surface deformation studies (Jackson et a l . 1975; Swanson et al.. 1976) anc~ petroiogic considerations (Wright and Fiske 1971). A schematic representation of the subsurface structure of Kilauea based on a synthesis of dat~ coiled:ted a~ HVO during recent decades is presented in

S~{J:~M[CAND C,EODETI(,~OBSERVATIONS Seismic and tilt records at Kilauea during the 24 months preceding the September 1977 eruption are summarized in Fig. 2. Following an eruption along the upper southwes~ rift zone in December 1974, the summit region began to inflate a~ a relatively constant rate of 8 prad/month. The rate of ~, .,:hi:: strain retease 0uring Lnis period was moderate and essentially constant, with 400--600 small near-summit earthquakes typically recorded per day. Tumescence of this kind, observed to precede many recent Kilauea eruptions, has been interpreted as the surface expression of dilation of the summit magma reservoir owing to filling (Eaton and Murata~ 1960; Macdonald: 1961). Summit inflation ceased abruptly when, at 0448 local standard time (1448 GMT) on 29 November, 1975, a magnitude 7.2 earthquake occurred beneath the southeast flank of Kilauea (see Fig. 1). Geodetic measurements indicate that the south flank subsided as much as 3.5 m and moved 7--8 m seaward. Deflation of the summit area at 2--7 grad/hr started at 0530 and a minor summit eruption began within Kilauea caldera at 0532. The rate of subsidence slackened 48 hours after onset of the eruption, so that a net deflation of 135 ~rad occurred, as measured by the east-west c o m p o n e n t of the Uwekahuna short-base water tube tiltmeter (Fig. 2). The volume change implied by this deflation (computed from summit leveling observations) is 68 X 106 m 3, compared with 0.25 X 106 m 3 of erupted material. Thus less than 0.5% of the magma lost from the summit magma reservoir of Kilauea as a result of the earthquake actually reached the surface, suggesting that most of the deflation was accomplished by the migration of magma into the flanks of the volcano. Following this brief summit eruption, the continuing high rate of deflation and the increasing frequency of moderate-size (M = 1--3) earthquakes within +he .~outhwest n~'t zone of Kllauea suggested movement of magma into that area. Large numbers of presumed tectonic south flank aftershocks complicate the r e c o ~ i t i o n of otheI intrusive areas, including one suspected along r,ue eas~ 1Lzt ZC,n~. GeoOetm informatmr~ is likewise inadeauate because tilt,

245

(A)

UWEKAHUNA TILT

iO00-. , / ~ tao~

,

D(

SUMMIT

I SEISMICITY I 1

il

IB)

!-

I O I N l ~ l J i F l i l l l l l J

JI&,SIOIN,DIJIFIMIAIMIJ

JLAISIO]NID}

Fig. 2. Kilauea tiltand seismic summary for period October 1975--December 1977. A. Summit tiltdata representing dally readings of east-west component of a short-base water tube tiltmeter at Uwekahuna vault (UWE, Fig. 3D). B. Summit earthquake counts from North Pit seismometer (NPT, Fig. 3D) including all shallow (< 5 km) events of M i> -- 1. C. East rift zone counts taken from several instruments along upper and middle east rift zone, generally including events of M > 0.1. Aftershocks associated with 29 November, 1975, M = 7.2 earthquake are included. Six rapid summit deflation events, indicated by arrows and circled numbers, began on: (1) 29 November, 1975 (earthquake-associated deflation); (2) 21 June, 1976 (east rift intrusion); (3) 14 July, 1976 (east rift intrustion); (4) 22 August, 1976 (aseismic deflation and probable east rift intrusion); (5) 8 February, 1977 (east rift intrusion); and (6) 12 September, 1977 (east rift eruption), respectively.

leveling, and horizontal distance measurements by laser geodimeter recorded large-scale ground deformation associated with the M -- 7.2 earthquake and its aftershock sequence, an event that masked possible small scale deformation owing to magma intrusion into the rift zones. The period December 1975--June 1976 at Kilauea was characterized by slow recovery from the effects of the 29 November, 1975, earthquake. Shallow summit earthquakes, which dropped to below 100 per day after deflation of the summit region, gradually increased to roughly 300 per day. Aftershock activity along the east rift zone and south flank gradually decreased but continued at a high level, with 300--1000 microearthquakes typically recorded daily. The east-west component of the Uwekahuna tiltmeter suggested gentle

246 summit inflation, but other observations nearer the center of deformation indicated continuing slow deflation and seaward m o v e m e n t of the south flank. Harmonic tremor increased beneath the summit of Kilauea in early May 1976. On 21 June, the onset of rapid summit deflation was accompanied by an intense shallow earthquake swarm in the Pauahi--Mauna Ulu area of the upper east rift zone, near its intersection with the Koae fault system (Figs. 1 and 3). (A)

KILAUEA

JUNE 21-25, t976

~DERA

KALALUoA

:~:%.-:. o

M 1.5-2.4

o

M ?..5-3.4

MAKAOPUHI

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JULY 14-15, 1976

KILAUEA

DERA

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5

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F E B R U A R Y 8 - 9 , 1977

KILAUEA ERA

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SEPTEMBER

(D)

13-14, 1977

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247 TABLE 1 Summit deflation events, June 1976--September 1977 Net summit (Uwekahuna) deflation (urad)

Duration of intense seismic activity {days )

Number of microearthquakes detected (M ~ 0.1)

Epicentral area (kin)

Energy release from earthquakes of M~>2.5 (1016 ergs)

Maximum selfpotential increase

(mV)

21--22 June 1976

7

1

4000

10

4

75

14--15 July, 1976

7

<1

2000

15

2

75

22 August-15 September 1976 8--18 February 1977

41

no anom . alous seismieity

28

<1

12--16 September 1977 90

.

.

.

.

0

8000

20

7

65

5 12,000 pre-eruption and early eruption only

250

68

70

Roughly 4000 earthquakes (28 of M = 3--4) were recorded in a 10-hour period and summit deflation totaled 7 prad at Uwekahuna (see Table 1). Magma presumably migrated from the summit region, b u t no eruption occurred. Later measurement of horizontal distances along the upper east rift zone revealed large extensional strains across the easternmost Koae fault system (Fig. 4, Fig. 3. Epicenter plots for three east rift zone intrusive events and early stages of September 1977 eruption. A. 21--23 June, 1976; B. 14--15 July, 1976; C. 8--9 February, 1977; D. 13--14 September, 1977. Stippling encloses epicenters of earthquakes interpreted to be generated by subsurface magma movements during intrusive events; other quakes are interpreted to be associated with post-intrusion adjustments within south flank or with response to rapid summit deflation. Rift zone earthquakes near intrusive zones generally occurred in discrete swarms at depths of ~ 7 km; south flank events were more widely distributed spatially and occurred to depths of 10 km. Summit earthquakes associated with September 1977 summit deflation and east rift zone eruption (D) generally occurred at depths of ~ 3 kin. Magnitude c u t o f f for all located events is at M ~ 1.5. Also shown in Fig. 3D are the following seismometer locations: CPK = Cone Peak; UWE = Uwekahuna; NPT = North Pit; OTL = Outlet Vault, ESR ffi Escape Road; A H U = Ahua; KPN = Kipuka Nene; PAU = Pauahi; MPR = Makaopuhi; L U A = Kalalua; WHA = Wahanla; HUL = Heiheiahulu. See Table 1 for additional pertinent information.

248

~

T-2

I

/

VO t43

\\

V O )0

:

H V O ~3~5

GOAT

I

I

i

I

I~

I

Fig. 4. Kilauea summit and upper east rift zone horizontal-strain network. Data for intervals including three intrusive events and September 1977 eruption are presented in Table 2.

Table 2). Tilt measurements indicated as much as 82 prad of inflation of the east rift zone during the period 1 May---22 June, 1976 (Fig. 5A). A similar deflation event occurred on 14 July, 1976, when magma apparently drained from the summit region and again collected in the east rift zone near Mauna Ulu, as indicated by a shallow earthquake swarm. The east-west component of the Uwekahuna tiltmeter recorded a sharp 7-~rad deflation (Fig. 2). Roughly 2000 earthquakes (10 o f M = 3--4) were recorded {Fig. 3, Table 1), and both horizontal distance measurements and tilt stations recorded additional dilation-inflation since the 21 June episode (FiRs. 4 and 5B, Table 2). No lava reached the surface, and the pattern of weak Uwekahuna inflation and moderate east-rift seismicity was reestablished in less than 24 hours. This pattern continued until 22 August, 1976, when the Kilauea summit region entered a period of modezate, nearly aseismic deflation (Fig. 2). Subsidence tot~!ing 41 ~md was recorded at Uwekehuna during the next three weeks, while microe~thquake activity was in no way anomalous. Tilt data for both rift zones ahowed no si$nifi~_nt changes; vertical and horizontal deformation surveys detected only the effects of summit deflation. The August-September 1976 deflation episode is unique in the 20-year period for which

249 I MAY - 22 JUNE 1976

Kiimmu

22 JUNE- 20 JULY 1976

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Hilihlliilh¢Ikl

-'.

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x j

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/t

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Hi4hliohulu = ~

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Fig. 5. Kilauea tilt observations, May 1976--May 1977. Precision of each tilt measurement, made with a level using modified techniques outlined by Kinoshita et al. (1974), is roughly + 5/~rad; measurement sites are shown at tail of each arrow.

modern tilt data are available at Kilauea; previous deflations of comparable magnitude have invariably been associated with eruptions or with pronounced local seismicity within a shallow intrusive zone. Sustained summit inflation at roughly 12/~rad/month and an increase in east rift zone seismicity began in late September 1976, and culminated in another intrusive event beginning 8 February, 1977. Roughly 2500 earthquakes (33 of M = 3--4) were recorded during the first 12 hours of activity, while summit tilt dropped at a peak rate of 2/~rad/hr. Deflation continued at a reduced rate for 17 days and caused a net inward tilt of 28 prad at Uwekahuna (Fig. 2, Table 1). Tilt observations at other sites during the period October 1976--January 1977 (Fig. 5C) confirmed the pattern of net summit inflation b u t revealed no distinct inflation center along the east rift zone. Distance measurements bracketing the intrusive event recorded both summit expansion and east rift zone dilation within the epicentral zone (Fig. 4, Table

2). Hypocentral depths for earthquakes suggest that magma approached to within a few hundred meters of the surface during the February 1977 event.

-I0

+201 +149

+33

110 110

114

HVO HVO HVO HVO

-39 . . . . . +69

HVO HVO HVO HVO HVO HVO HVO . .

. .

132 34

HVO l l 3 - - H V O 114 HVO 1 1 3 - - H V O 43

HVO 119--I-NO HVO 113--~[V0

10 144 118 113 34 10

H V O 1 0 9 - - H V O 114 H V O 1 1 4 - - H V 0 114 HVO 114--HVO 132 H V O 114--68-24 6 8 - 2 4 - - H V O 132 H V O 110--68-24

ll0--HVO 119--HVO 119--HVO 119--HVO llg--HVO 119---HVO 119--68-24

--

+3 +32 . . . -25

119

II7--HVO lll--HVO lll--HVO 111--I~V'O

Puu H u l u h u l u - - H V O 117

-17 -3

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13.1

4.9 -2.3

x 10-'

.

+372 +335

.

+173 . +435 .

. .

--

--

. . +207

+186 +252 +50 +232

+21

-65 +71

(ram)

72.5 185.0

151.3

54.8

--

--

107.8

49.1 63.7 14.7 38.0

8.4

-14.4 12.6

x 10-'

~L/L

--6.2 8.2 --25.1 --9.0 2.3 --2.6 -3.4 --0.9 --4.0 --1.6 -15.3 12.6 22.9 --8.6 3.2 35.3 37.4 9.5 13.6 --6.5 --10.4 66.5 18.9 34.2

X 10 -6

-28 --46 --63 --34 +9 --9 21 -4 13 --3 --15 +20 +52 24 6 +91 +118 --15 +39 -17 -12 +169 +97 +62

ALIL

,',L (ram)

AL

AL/L

AL (ram)

aL (ram)

AL/L x 10-'

29 S e p t e m b e r 1976 M a r c h 1 9 7 6 --9 F e b r u a r y 1977 A u g u s t 1977

J a n u a r y - - 2 9 J u n e , 29 J u n e - - 1 5 July, 1976 1976

Length changes and horizontal strains

Puu H u l u h u l u - - H V O 110 Puu H u l u h ~ l u - - H V O 111

Line

Kilauea g e o d i m e t e r o b s e r v a t i o n s

TABLE 2

+242 --17 +29 +153 --139 +158 273 +88 --223 -237 --90 --203 78 -43 --118 -234 -342 179 -440 383 182 -336 659 212

(ram)

AL

53.8 --3.0 11.5 40.4 -35.1 46.4 --44.7 20.3 -78.6 123.4 -91.6 --127.4 -34.4 15.4 -62.1 -90.7 108.3 113.3 153.0 146.5 -158.3 132.1 128.4 I17.1

× 10 -6

AL[L

1 September 1976-14 O c t o b e r 1977

0

H V O 1 1 3 - - H V O 87 H V O l l 3 - - H V O 143 HVO 8 7 - - H V O 43 HVO 8 7 - - H V O 143 HVO 8 7 - - H V O 10 HVO 1 0 - - H V O 43 T - 2 - - H V O 113 T - 2 - - H V O 118 H V O 1 2 0 - - H V O 118 C o n e P e a k - - H V O 120 Cone P e a k - - H V O 118 C o n e P e a k - - H V O 113 Cone P e a k - - H V O 1 1 9 Cone P e a k - - H V O 144 Cone P e a k - - P u u K o a e C o n e P e a k - - H V O 145 Puu K o a e - - H V O 145 Puu K o a e - - H V O 120 Puu K o a e - - H V O 144 Puu K o a e - - H V O 109 Puu K o a e - - H V O 135 Puu K o a e - - H V O 136 H V O 1 4 5 - - H V O 136 HVO 1 3 6 - - H V O 135

Line

TABLE 2 (continued)

_

_

m

m

D

_

_

_

_

M

_

_

m

m

m

m

m

m

_

m

m

m

m

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.

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.

.

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.

.

X 10 -6

~L/L

.

.

.

.

.

.

AL/L

+9 6

--7.8

30 12 +37 +30

3.3 --1.4

--2.2 4.3 5.9

--6.9 --1.2 5.8 --0.2

16.6 -12.3

13.1 --14.7 26.7 8.4 2.1

--0.9

5.0 34.9 31.6

1.6 15.9 22.1

X 10 -6

30 6 +25 . 1

+27 28

+40 39 +74 +23 +8

3

+9 +87 +40

+5 +33 +53

(mm)

AL

AL

(mm)

(mm)

X 10-'

(mm)

AL/L

AL/L

~L

AL

29 S e p t e m b e r 1 9 7 6 M a r c h 1 9 7 6 - --9 F e b r u a r y 1977 A u g u s t 1977

J a n u a r y - - 2 9 J u n e , 29 J u n e - - 1 5 J u l y , 1976 1976

L e n g t h changes and h o r i z o n t a l strains

+13 +17

--187 +148 +89

--101

--101 +200 --13 --157

--72 --261

+146 +70 +69 --248 --127

+196

--136 --218 --284

4.8 3.9

--26.3 --34.1 17.4 16.8

-23.4 40.1 --3.0 -25.5

--44.3 --114.5

47.9 26.4 24.9 --90.2 --32.6

61.3

--75.4 --87.4 --224.7

--97.6 --64.7 --54.6

X 10 -6

(mm) --307 --134 --131

AL/L

AL

1 September 1976-14 O c t o b e r 1 9 7 7

b~ ¢J1 b.a

252

Unlike previous episodes, this intrusion produced visible cracking of local roadways; strain gauges across preexisting cracks in the area recorded extensions as large as 22 ram. This episode was likewise unusual in that the site of m a x i m u m seismic strain release migrated east of Pauahi crater during the final stages of the event (Fig. 6A).

l (4)

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SEPT, 14

5(PT

15

Fig. 6. Hourly frequency of Kilauea microearthquakes in localities sequentially downrift from summit, determined from P-wave arrival differences measured at key stations of HVO seismic network. A. Migration pattern for February 1977 intrusion; (1) caldera vicinity with first arrivals at station N-PT, (2) upper east rift zone near Puhimau with first arrivals at stations AHU and ESR, (3) upper east rift zone near Kokoolau with first arrivals at station PAU, and (4) upper east rift zone and eaJt of Pauahi. B. Migration pattern for early phases of September 1977 eruption.: (1) upper east rift =one near Mskaopuhi with first arrivals at stations PAU and I ~ R , (2) upper east rift zone east of M~k~opuhi with first arrivals at stations MPR and LLTA, (3) middle east rift zone near KaJalua with first arrivals at station LUA, and (4) middle e u t rift zone between Kal~ua a n d ~ e i a h u l u with first arrivals at stations LUA and HUL. Also shown are south flank earthquakes with first arrivals at WHA.

Records from a borehole tiltmeter at Puu Hutuhulu within the early epicentral zone support the suggestion that magma drained eastward past Mauna Ulu in the later stages of the February 1 9 7 7 intrusive event. After recording a sharp 6-~zrad inflation from 1 8 4 0 through 2 0 0 0 on 8 February, 1977, the Puu

253 Huluhulu instrument indicated the onset of deflation that eventually returned tilt levels in the area to precrisis values. We attribute the early inflation at Puu Huluhulu to the forcible injection of summit magma; after a period of high local seismicity, magma apparently gained access to the middle east rift zone, an event allowing Puu Huluhulu tilt levels to return to normal. Tilt measurements for the period January--May 1977 indicate that summit magma from the February intrusive event eventually caused inflation of the Puu Kauka-Heiheiahulu area, 30 km east of the summit (Fig. 5D). We could n o t establish unambiguously whether summit magma was directly responsible for this inflation or had simply forced magma that was stored within the rift zone to move nearer to the surface in the eventual eruption area. The degree of differentiation of lavas erupted in September 1977 favors the latter hypothesis. Anomalous steaming and plant kill had been observed in the Heiheiahulu area since March 1976. In retrospect, the arrival of new magma at the middle east rift zone during early 1977 seems to have set the stage for the September eruption, and the Heiheiahulu steaming area and eventual eruption site were probably determined in part by structural adjustments to the November 1975 earthquake. Following a six-month period of negligible summit tilt changes during which the east rift zone inflation center migrated westward uprift from Heiheiahulu to Kalalua, Kilauea erupted on 13 September, 1977, along a 5.5-km fissure stretching from Kalalua eastward to Puu Kauka (Fig. 1). Intermittent activity during the next 18 days produced roughly 35 × 106 m 3 of erupted material, 70% of which surfaced during the final, most vigorous phase from 26 September through 1 October. Tilt surveys revealed net uplift of the eruptive zone during the eruption, an observation that suggests subsurface storage of newly delivered magma within the rift. By November, 1978, seismicity in the eruptive area had decreased markedly, and Kilauea had entered a period of moderate summit inflation and increasing summit seismicity. ELECTRICAL SELF-POTENTIAL OBSERVATIONS Electrical self-potential anomalies are large over recently active fissures on Kilauea volcano and increase during periods of activity (Zablocki, 1976). Accordingly, a self-potential eruption m o n i t o r 2.7 km long was established in September 1975 across the east rift zone along Escape and Ainahou roads (Fig. 1). Self-potentials along the traverse have proved to be very stable during volcanically quiet times, b u t significant self-potential increases were recorded near recently active fissures after the June 1976, July 1976, and February 1977 intrusive events, and after the September 1977 eruption. No significant changes were noted after the aseismic deflation episode of August 1976. The self-potential m e t h o d used at Kilauea and related theoretical considerations are summarized b y Anderson and Johnson (1976). The general character of the self-potential field distribution along the east rift zone m o n i t o r is shown by the lines in Fig. 7A; these data were obtained

254

IO'CEMSE1'975

(O)9 FEBRUARY 1977 16 FEBRUARY 1977

25 FEBRUARY 1977

ii v

(E) 16 S E P T E M B E R 1977

0 I

500 ,

,

,

J

!

IO00m I

M a y ~ 1969

NOv 1 9 7 3 AUQ 1 9 6 8

F;suure

Fissure Fissure

255

during a routine survey on 18 November, 1975. A broad, double-peaked 700mV anomaly in the northern part of the traverse correlates with eruptive fissures of August 1968 and November 1973. To the south, a single 400-mV anomaly coincides with the eruptive fissure of May 1969. The effect of the 29 November, 1975, M = 7.2 earthquake on the selfpotential field is shown in the upper part of Fig. 7A by a superposition of the 2 December, 1975, data on the 18 November, 1975, profile. Large self-potential increases at the southern end of the traverse, near the May 1969 eruptive fissure (Fig. 7A), are believed to be direct consequences of structural changes and possible magma movement. Small self-potential differences in the northern part of the traverse, particularly near the November 1973 fissure, only slightly exceed measuring errors. The self-potential field returned to its preearthquake distribution by 8 January, 1976 (Fig. 7A). Reoccupation of the self-potential monitor on 18 May, 1976 showed no substantial differences from the survey of 8 January. On 22 June, less than 24 hours after the 21 June deflation episode, a self-potential increase of roughly 75 mV was detected between the November 1973 and August 1968 fissures (Fig. 7B). By 7 July, the anomaly had disappeared. Additional changes in the self-potential profile were noted on 15 July, immediately after the summit deflation episode of the previous day. The anomaly first detected on 22 June had reappeared and broadened, with potential differences as large as 75 mV at some sites relative to the 18 May profile (Fig. 7C). One week later, on 23 July, the self-potential field had returned to the level of 18 May. No change in the self-potential field was observed on the day after the onset of the 8 February, 1977, deflation episode (Fig. 7D), but by 16 February, the self-potential field had again increased in the zone between August 1968 and November 1973 eruptive fissures, and also in the vicinity of the May 1969 fissure. These increases are above noise levels, and form coherent patterns that follow the shape of the original anomaly. By 25 February, the self-potential field had generally returned to its normal distribution, except between the August 1968 and November 1973 fissures, where self-potentials fell below earlier levels. A self-potential profile obtained approximately 60 hours after onset of the September 1977 eruption (Fig. 7E) shows local increases by as much as 70 Fig. 7. Escape-Ainahou Road electrical self-potential profiles. Data are from repeated surveys of 2.7-kin traverse along Escape-Ainahou Road, which crosses east rift zone -~ 7 km east of Kilauea summit. Electrode spacing was 61 m in each case except in (A), where 30-m spacing has been generalized to match later surveys. Lines are reference profiles measured during volcanically quiet periods; " x "s indicate measurements immediately before, during, or immediately after rapid summit-deflation events. Data for November 1975 earthquake (A) are plotted relative to profile obtained 18 November, 1975; those for June and July 1977 intrusive events (B and C) are compared to profile obtained 18 May, 1976; those for February 1977 intrusion (D) are compared to profile obtained 4 February, 1977; and those for September 1977 eruption (E) are compared to profile obtained 8 July, 1977.

256

mV between the November 1973 and August 1968 fissures. These changes are of the same order of magnitude as those observed during the earlier inlxusive events. Although a larger self-potential response over active fissures might seem likely on the basis of the large volume of mobile magma during early stages of the eruption, seismic observations suggest that conduits were deeper than 5 km in the monitored area, which m a y account for the relatively small amplitude of observed self-potential changes. In summary, self-potential changes observed during summit deflation episodes are relatively small but exceed measurement reproducibility. Anomalies occur near preexisting fissures and are attributed to magma migration through conduits at depths of 1--5 km on the basis of seismic evidence. No significant self-potential changes were measured during the slow aseismic deflation in August 1976, suggesting an alternate route for the migrating magma, or passage at depth beyond the sensitivity of the self-potential method. GRAVIMETRIC OBSERVATIONS

Theoretical considerations

Leveling and microgravity surveys in the Kilauea summit region during September--November 1975, December 1975--January 1976, March--April 1977, and September-October 1977 provide information on changes accompanying the 29 November, 1975, earthquake (Jachens and Eaton, 1980), the sequence of east riftzone intrusions,and the September, 1977, eruption. Data are availablefrom 59 levelingand 21 gravity stations distributed throughout an area of 20 k m 2. All levelingsurveys were of third-order accuracy; standard errors for gravity changes were 5--17/~gai depending on the number of meters and repeat readings in each survey. Pre-eruption and post-eruption differences (Fig. 8) illustratethe systematic nature of the data. In order to interpret these observations in terms of m a g m a redistribution at depth, we assume that the summit of Kilauea is underlain by a spherical m a g m a storage chamber of radius R, centered at depth d (Fig. 9); this chamber is fed by m a g m a from depth at an assumed constant rate (see Swanson, 1972) via a well-developed verticalconduit system (Koyanagi et al., 1974). The simple Mogi (1958) model of elasticsurface deformation has been applied to Kilauea in the past with qualified success (Jackson et al.,1975; Swanson et al.,1976); we adopt it here for its analytic symplicity and because more complex models have yet to demonstrate significantimprovement over it (Swanson et al.,1976). According to this model, elevation change A h owing to inflation-deflationcycles is given by (see Appendix, equation A-7): AR ~h =

3(1 + x2/d~) 3n

(I)

where A R = the change in m a g m a chamber radius owing to inflation-deflation, x = the horizontal distance from the deformation center, and d = the depth to center of the chamber.

257 M I

.2.o

°

o

APRIL-OCTOBER 1977 - - ,,h (ram) Ag {M.gois)

o , I

?"

Fig. 8. Gravity and elevation changes in Kilauea summit region for period including September 1977 eruption. Solid circles indicate measurement sites for both gravity and surface elevation; open circles indicate additional sites for which elevation data alone are available. Stippling delineates summit caldera with its central pit Halemaumau, as well as more prominent pit craters of upper east rift zone. This pattern of gravity increases and elevation decreases, centered near south caldera rim, is characteristic of recent summit deflation events at Kilauea.

Ahx

-Z

T

~X

Fig. 9. Schematic cross section of Kilauea summit region and magma reservoir, used to model elevation and gravity changes measured at surface.

T he mean d e p t h and di am e t e r of the summit magma cham ber of Kilauea can be i n d e p e n d e n t l y estimated f r o m seismic and surface d e f o r m a t i o n observations. E a r t h q u a k e h y p o c e n t e r plots in the summit region reveal the existence of a quasispherical aseismic zone beginning at roughly d = 3 km and extending to d = 6 kin. Koyanagi et al. (1974) have argued t h a t this zone reflects the absence

258 of brittle failure in rock containing a large proportion of partial melt, and that the aseismic zone is therefore coincident with the summit magma chamber. Vertical deformation studies suggest a slightly shallower chamber centered at 2--4 km depth (Swanson et al., 1976). Inversion of three sets of vertical deformation data obtained during the period covered by this report yields depths ranging from 2.6 to 3.3 km, with chamber radii of 0.9--1.1 km. For completeness, we shall discuss the implications of both seismic (d = 4.5 kin, R = 1.5 km) and vertical-deformation (d = 3.0 km, R = 1.0 km) results. However, inversion of deformation data is possible only for discrete deformation episodes, no one of which may involve the entire summit chamber. The seismic result reported by Koyanagi et al. (1974) was intentionally derived for a period that included sustained summit inflation and gradually increasing seismicity culminating in a summit eruption, during which a large part of the summit chamber was likely under stress. For this reason, we suggest that the seismic result m a y be more appropriate for our calculations. Gravity changes measured at the surface during inflation-deflation cycles reflect the combined effects of both subsurface mass changes and simple surface elevation changes (free-air effect); the net change is given by the analytic relation (see Appendix, equation A-10):

112~R2Gp°

~ g (ugais) = L

d~

0.3086

]

~ h (mm)

(2)

where G = the gravitational constant, 6.67 × 10 -s cm3/g s 2, and P0 = the density of magma added to or withdrawn from the summit chamber. Only the ratio of chamber radius to chamber depth (R/d) appears, and this ratio is the same for both the seismic and vertical deformation results discussed above. Thus, in either case, equation (2) yields Ag (~gals) Ah ( m m )

- 0 . 0 2 7 9 p o - 0.3086

(3)

The average pore-free density of molten Hawaiian ba_.~_ltis approximately 2.7 g/cm 3 (Macdonald, 1963). Thus, in the a b s e n c e o f o t h e r effects, simultaneous elevation and gravity changes measured during typical inflation-deflation cycles at Kflauea are expected to plot along a line with slope = - 0 . 2 3 3 ~gal/nnn. In practice, however, inflation-deflation m a y also involve the filling or emptying of voids within the summit magma reservoir, with little or no associated surface deformation. We show in the Appendix that this effect does not destroy the linear relation between gravity and elevationchanges but rather changes the slope of the plot of Ag vs. Ah in a predictable way (see Table 3). The volume V of fracture space involved is given by (see Appendix, equation A-16):

V = Ag~'xd2 Gpo where hgmax = the m a x i m u m gravity change o b s e r v e d a t the surface.

(4)

259 TABLE 3 Relation between gravity and elevation changes Process

Ag

Ah

~g/ ~h

Inflation (deflation)

negative (positive)

positive (negative)

--0.233

Void filling (emptying)

positive (negative)

0

more (less) negative than --0.233

Replacement from depth: constant volume c o n s t a n t mass

positive 0

0 negative

more negative less negative

Observations

Data for three intervals bracketed by leveling and gravity observations are summarized in Fig. 10. For the November 1975 earthquake and associated deflation (circles, Fig. 10), observed changes follow the relation Ag (pgals) = -- 0.175 Ah ( r n m ) - - 9.2, and therefore plot well below the theoretical inflation-deflation line for P0 = 2.7 g/cm 3. In part 1 of this report, Jachens and Eaton (1980) interpret this result in terms of the incomplete collapse of overlying rock in response to the withdrawal of magma from the summit chamber. This implies the creation of voids, presumably newly emptied fractures. We therefore envision the gravity changes observed during this interval to reflect the combined effects of magma chamber deflation (with partial surface collapse) and the withdrawal of magma from pre-existing voids that remained partly open, thereby reducing the bulk density of the chamber. We assume that gravity and elevation changes associated with this magma chamber deflation follow the analytic relation given by equation (3) and represented by the p = 2.7 g/cm 3 line in Fig. 10; departures from this line are interpreted to reflect emptying of voids. The m a x i m u m departure of the preearthquake and post-earthquake data from this theoretical line is Agmax = - 8 0 pgals (Fig. 10). From equation (4), the implied volume of newly emptied fracture space is from 40 (deformation) to 90 (seismic) × 106 m 3. Data for the period December 1975--April 1977 (squares, Fig. 10), which brackets the intrusion sequence, follow the linear relation Ag (pgals) = - 0 . 6 0 7 Ah (mm) + 33.6, and therefore plot significantly above the p = 2.7 g/cm 3 line. Maximum departure from the theoretical curve is roughly +100 pgals, a value suggesting that all the fracture space created beneath the summit had refilled with magma by April 1977. Data for the September 1977 eruption (triangles), Fig. 10) plot near but slightly above the theoretical line. No evidence exists that any process other than simple inflation-deflation occurred at Kilauea over this interval.

260

300

--

250

20O

i50--

&

i

®

<~ IO0 NOVEMBER - DECEIIIER A g , -.175 A h - 9 . 2

50

DECEMBER 1 9 7 5 - APRIL 1977 A g • - . 6 0 7 A h +33.6

&

APRIL - OCTOBER 1977 A g • - . 2 8 0 Zth - 7 . 2

--50

i -1600

1

I -1200

i

In

\

rn 0

\

1975

I -800

I

I -400

i

i 0

i

I 400

A h (mm)

Fig. 10. Plot of gravity and elevation changes in Kilauea summit region. Surveys bracket November 1975 earthquake (ci~J~), e u t rift zone intn~ion sequence (squares), and September 1977 eruption (tziaag~). p = 2.7 line derives t ~ m analytic model developed in text, according to which magma of mean density 2.7 g/cm s is introduced to (inflation), or withdrawn from (deflation), summit magma reservoir. Resulting gravity changes reflect both free-air effects owing to elevaUon changes and ma~-related effects owing to magma redim'ibution. D e p a m u ~ from this theoretical inflation-deflation line are discumed in text. Thin lines repmmnt least-squares fits to data, which are thought to be accurate to within roughly ±10 ~gals and ±5 ram. DISCUSSION T h e f o l l o w i n g is a s y n t h e s i s o f t h e d a t a p r e s e n t e d a b o v e , in t h e f o r m o f an interpretive chronology for the November 1975--September 1977 eruptive c y c l e a t K i l a u e a (Fig. 11). T h i s c y c l e w a s a t y p i c a l in several respects, a n d t h e f o l l o w i n g c h r o n o l o g y m a y t h e r e f o r e b e o f p a r t i c u l a r i n t e r e s t t o t h o s e concemed with long-term phenomenology of Hawaiian volcanism.

Chronological model and magma budget T h e 29 N o v e m b e r , 1 9 7 5 , M = 7.2 e a r t h q u a k e c a u s e d s u b s t a n t i a l c h a n g e s in the standard mode of deformation of Kilauea volcano. Before the earthquake, h o r i z o n t a l a n d vertical d e f o r m a t i o n rates in t h e s u m m i t r e g i o n w e r e c o m p a -

261 Summit

EARTHQUAKE

INTRUSION SEQUENCE

ERUPTION

UERZ

1 •

.-MERZ

(A)

•

•

(8)

1 • 1 • 1 •

•

(e)

1

•

(0)

1

~(E)

l--t·--~~(F)

Fig. 11. Schematic diagram of response of Kilauea to November 1975 earthqaake. A. Prior to earthquake, summit reservoir was charged with magma following sustained inflation, and a significant volume of magma from previous intrusive events was undoubtedly stored within east rift zone. B. Earthquake caused rapid summit deflation (arrow), migration of magma from summit reservoir into rift zones, and new voids in and around summit reservoir (indicated by blank). Magma storage capacity of east rift zone is divided into one hypothetical reservoir along upper east rift zone (UERZ) near Makaopuhi and another along middle east rift zone (MERZ) near Heiheiahulu. C. Gravity observations suggest that fractures in and around summit chamber were refilled by June 1976, when (D) a sequence of intrusions delivering magma to upper east rift zone began. Uplift along rift (indicated by arrow) is in this case hypothetical because deformation measurements are unavailable. E. During final phases of last rift zone intrusion in February 1977, magma gained access to middle east rift zone, as documented by downrift migration of microearthquakes and by rift inflation near Heiheiahulu, first observed in May 1977. F. Magma that finally reached surface on 13 September, 1977 may have been stored within rift zone for some time, as indicated by preliminary petrologic studies.

rable and relatively high; the pattern of deformation was typically symmetric and suggested surface flexing in response to inflation-deflation of a shallow magma reservoir beneath the summit. For many months after the earthquake,

262 measured deformation patterns were extremely complex; the dominant pattern was a continued southward displacement of the south flank in response to the earthquake, with horizontal deformation rates greatly exceeding the vertical rates related to reinflation of the volcano (Lipman et al., 1978, 1980). Typical summit inflation-deflation cycles during November 1975--September 1977 were thus partly masked by continuing structural readjustment to the M = 7.2 earthquake. Gravimetric data for N o v e m b e r - D e c e m b e r 1975 suggest that 40--90 × 106 m 3 of void space were emptied or created within the summit magma reservoir as a result of the earthquake, and refilled by magma from depth before the June 1976 east rift intrusion. This implies a magma supply rate of 6--13 X 106 m3/month, in good agreement with the value of 9 × 106 m3/month proposed by Swanson (1972) from observations of long-duration Kilauea eruptions. The net tilt change at the summit was essentially zero from June 1976 to September 1977, as four east rift zone intrusive events caused significant dilation and inflation of the rift. We propose that essentially all the magma supplied from depth over this interval entered the east rift zone before the September 1977 eruption. From June 1976 to February 1977, this magma presumably filled voids within the upper east rift zone, as evidenced by shallow seismic swarms in the Pauahi--Mauna Ulu area and by disturbances of the selfpotential field along Escape-Ainahou Road. Magma may have accumulated temporarily near Makaopuhi, where rift inflations seemingly related to eventual eruptions have been d o c u m e n t e d in the past (Jackson et al., 1975; Swanson et al., 1976). Observations of possible surface deformation in this area are unfortunately missing for the period in question owing to the destruction of measurement sites by Manna Utu flows during 1969--1974. Migration of microearthquakes east of Pauahi crater during the later stages of the February 1977 event as well as substantial rift inflation near Heiheiahulu, first detected in May 1977, indicate that conduits were open to the middle east rift zone during the period February--September 1977. A rough estimate of the volume of void space created along the east rift zone by the November 1975 earthquake and subsequent adjustments, and then presumably filled before the September 1977 eruption, can be derived from periodic distance measurements across the rift zone during this period. The data show that the Puu Huluhulu- <~oat 2 line (Fig. 4), which trends essentially normal to the rift f o r a distance of 5 kin, extended by a total of 0.7 m during the period November 1975--February 1977 and then remained essentially unchanged until September 1977. Seismic data suggests that the active rift zones on Kilauea extend to a depth of roughly 5 km (Koyanagi et al., 1974). We assume in the absence of other data that this 0.7 m of extension occurred to a depth of 5 km along the entire 30 km of rift between the summit and the eventual eruption site, so that a maximum of (0.7 m) × (5 X 103 m) X (30 × 1 0 3 m) = 100 × 106 m 3 of new void space were created as a result of the November 1975 earthquake.

263

At the magma supply rate calculated above (6--13 × 106 m3/month), 8--18 months should have been required to refill this volume from depth. When the September 1977 eruption occurred 15 months after the June 1976 intrusion, the equivalent of 3--6 m o n t h s ' magma supply (35 × 106 m 3) was extruded. We therefore suggest that the total volume of magma supplied to Kilauea from depth during the period November 1975--September 1977 (130--280 × 106 m 3) was partitioned as follows: (1) 40--90 × 106 m 3 to refill voids within the summit magma reservoir; (2) 100 × 106 m 3 to fill newly created volume within the east rift zone; (3) 35 × 106 m 3 extruded, and (4) 0---55 × 106 m 3 responsible for net uplift of the eruptive zone. Clearly, the magma budget for Kilauea outlined above is subject to large uncertainties. A spherical model of the summit magma reservoir is an obvious oversimplification, and the mean depth and diameter of the reservoir are uncertain by at least 20%. Least-squares fits of the leveling and gravity data are only accurate to within roughly 10% owing to inherent scatter in the observations. By far the largest uncertainties, however, stem from the absence of geodetic and gravity control along the middle east rift zone during the period in question; these observations are essential to estimates of the volume of void space created by the November 1975 earthquake and related adjustments (and presumably refilled before the September 1977 eruption). Quantitative surface deformation observations would also be required to estimate the volume of stored magma implied by rift inflations, such as that which occurred in the eruptive zone before and during the September 1977 eruption. The middle east rift zone of Kilauea consists largely of inaccessible tropical jungle, a circumstance that complicates the task of adequate geophysical surveillance. These difficulties notwithstanding, our data suggest that quantitative modeling of the rate of magma supply to Kilauea may now be feasible and t h a t increased efforts should be made to obtain periodic geodetic and gravimetric observations at the summit and at key points along the rift zones, including all sections of rift zones suspected of functioning as temporary magma storage centers. Our model suggests that only a small fraction of the volume of magma delivered to the east rift zone during June 1976--September 1977 has reached the surface to date. In fact, petrologic considerations favor the hypothesis that no newly delivered summit magma was erupted during the September 1977 outbreak, the products of which may have been derived from a partially differentiated magma remnant from an earlier intrusion into the rift zone.

Prospects for future volcanic activity Our model suggests that the wounds inflicted on Kilauea volcano by the November 1975 earthquake were largely healed by late 1977, at least in the summit region and as far along the east rift zone as Heiheiahulu. The summit accordingly entered a period of sustained inflation at a mean rate of 9 ~rad/ m o n t h ; the upper and middle east rift zone had been charged with fresh

264

magma from depth, and conduits had been established to the Kalalua--Puu Kauka area. Dry-tilt observations immediately after the September 1977 eruption revealed that the eruptive zone was more highly inflated at that time than before the outbreak, a condition that is not unusual because reinflation of the rift zone after eruptions has been d o c u m e n t e d in the past during periods of frequent east rift zone activity (Jackson et al., 1975; Swanson et al., 1976). Summit seismicity gradually increased since the end of the September 1977 eruption, with 200--400 microearthquakes typically recorded per day in June 1978. These facts suggest to us that extrusive activity is likely to occur in the near future along the upper and middle east rift zone~ where it has occurred many times in the recent past. A series of east rift zone eruptions migrating generally (but not necessarily uniquely) uprift, similar in many respects to the series of seven rift zone eruptions that occurred during 1 9 6 0 - 1 9 6 5 , is regarded as a likely prospect for the coming years at Kilauea. Summit outbreaks are not precluded, and indeed three of these outbreaks occurred during the east rift zone sequence of 1960--1965. Nevertheless, the next decade at Kilauea is likely to be characterized primarily by continuing east rift zone activity; following the pattern established during the past two decades and presumably reinforced by the effects of the 29 November, 1975, M = 7.2 earthquake.* ACKNOWLEDGEMENTS

Detailed reviews were provided by R.C. Jachens and D.A. Swanson. Gravity surveys reported here were made possible through equipment loans from R.C. Jachens, U.S. Geological Survey. APPENDIX

A program of periodic microgravity surveys on Kilauea and Mauna Loa volcanoes was initiatedin mid-November 1975 by Gordon P. Eaton and Robert C. Jachens of the U.S. Geological Survey. The goal of this program is two-fold. O n M a u n a Loa the acquisition of detailed vertical deformation data by conventional leveling techniques is a formidable task owing to difficultaccess, the large areal and verticalcoverage required (total elevation range, 4170 m), and the relativelyhigh manpower requirements. Precise gravity observations at selected sites,however, are generally easier to obtain, can by a simple and rapid helicopter flightbe tied to a reference point far removed from the summit, and provide a measurement of the combined effects of vertical deformation and mass redistribution at depth. In the case of Kilauea, gravity measurements can be combined with results from a longstanding leveling program to obtain estimates of the density and volume of mobile m a g m a involved in summit inflation-deflationcycles, intrusive events along the volcano's riftzones, or surface eruptions. *Note added in proof: Kilauea next erupted o n 16--17 N o v e m b e r 1979 from vents in and near Pauahi crater along the upper east rift zone.

265 T h e f o l l o w i n g analytic m o d e l o f t h e s u b s u r f a c e s t r u c t u r e o f Kilauea serves as a f r a m e w o r k f o r i n t e r p r e t a t i o n o f s i m u l t a n e o u s gravity and leveling observations and includes t h r e e processes likely t o o c c u r at Kilauea: (1) inf l a t i o n - d e f l a t i o n o f the s u m m i t m a g m a c h a m b e r , (2) r e p l a c e m e n t o f m a g m a w i t h i n t h e s u m m i t c h a m b e r b y d e n s e r m a g m a f r o m d e p t h ; and (3) filling (or emptying) of fractures within the chamber.

Vertical deformation model T h e Mogi ( 1 9 5 8 ) m o d e l assumes: (1) t h e e a r t h ' s crust is an ideal semiinfinite elastic b o d y , (2) d e f o r m a t i o n is caused b y changes in h y d r o s t a t i c pressure w i t h i n a spherical c h a m b e r o f radius R c e n t e r e d at d e p t h d, and (3) the radius o f this c h a m b e r is small c o m p a r e d t o its d e p t h . Using these assumptions, we can express t h e h o r i z o n t a l and vertical d e f o r m a t i o n at a n y p o i n t b y (Mogi, 1 9 5 8 ; see Fig. 9):

-R3AP Ux . . . . .

x

4p

[(z + 2d) 2 + x 2 ] sn

• (5z 2 + 1 4 d z + 8 d

2 - x 2) (A-l)

+ -R3AP 4p

Uz -

[

x

+

(z 2 + x2) 3n

R3Ap

1

4t~

[(z + 2d) 2 + x 2] sn

+ R3AP 4t~

[

z (z 2 + x2) 3n

x

~n ]

[(z + 2d) 2 + x 2] • (7z 3 + 38dz 2 + 68d2z + 40d 3 + 4dx 2 + zx 2)

z+2d

]

(A-2)

[(z + 2d) 2 + x 2] 3J2

w h e r e Ux = t h e d i s p l a c e m e n t in t h e h o r i z o n t a l (x) d i r e c t i o n , Uz = the displacem e n t in t h e vertical (z) d i r e c t i o n , R = t h e radius o f the m a g m a c h a m b e r , A p = t h e h y d r o s t a t i c pressure change w i t h i n t h e c h a m b e r , p = Lambs c o n s t a n t , and d = the depth to center of the chamber. E l e v a t i o n changes Ah at the surface are given b y e q u a t i o n (A-2), with z = -d:

3R3AP 4ud 2

Ah . . . .

1 (1 + x2/d2) 3n

(A-3)

Gravity models Inflation-deflation. I n f l a t i o n o f t h e s u m m i t m a g m a c h a m b e r can be m o d e l e d as an increase in the radius o f t h e c h a m b e r AR owing t o t h e a d d i t i o n o f m a g m a f r o m d e p t h . We assume t h a t this process does n o t significantly change the bulk d e n s i t y o f the c h a m b e r or o f t h e s u r r o u n d i n g c o u n t r y rock. I n f l a t i o n - d e f l a t i o n

266

will cause both a mass-related gravity change and a free-air change owing to elevation changes at the surface. The mass-related gravity anomaly gs associated with a sphere of radius R at depth d can be shown to be (Telford et al., 1976, p. 57): 4 ~ R 3G (P2 - Pl)

gs =

3d 2

1 (1 + x2/d2) 312

(A-4)

where G = the gravitational constant, 6.67 × 10 -s cm3/g s 2, P2 = the density of the sphere, and p 1 = the density of surrounding rock. Magmatic inflation causes a change in both chamber radius and mass. If the change in radius is small relative to the radius ( ~ R << R), then the change in mass can be approximated by the p r o d u c t of the surface area of the spherical chamber (4~R2), the thickness of the newly added shell (AR), and the density of the newly added magma (p0). The change in mass Am is thus 4 ~ R 2 A R p o , and the change in gravity is given by: Agl =

47rR2ARGpo

1

(1 + x21d2) 3n

d2

(A-5)

Inasmuch as h R cannot be directly measured at the surface, we express this quantity as a function of measured surface-elevation changes by evaluating equation (A-l) at x = R, z = - d . The depth and mean diameter of the Kilauea summit magma chamber have been estimated from seismic observations as d = 4.5 kin, R = 1.5 km (Koyanagi et al., 1974); substitution in equation (A-l) yields: AR ~ 2.99 ~hmax

(A-6)

where hhnmx = the surface elevation change measured at x = 0. This result is only approximate because the chamber will deform assymetrically owing to its p r o x i m i t y to the surface. A similar calculation using equation (A-2) o f the vertical deformation at x = 0, z ffi - R , differs by roughly 20%. This result, which clearly defines the m a x i m u m deformation that occurs, is offset to a large degree by correspondingly smaller changes at the b o t t o m of the chamber (x = 0, z = R). The change in radius for an assumed chamber depth of 3 km and radius of I km, values typical of those obtained b y inversion of surface-deformation data (Swanson et al., 1976), differs from the result above by less than 1%. For our purposes, we therefore a d o p t AR = 3Ahmax; then, from equation (A-3): hh =

AR

3(1 + x21d2) 3n

(A-7)

Equation (A-5) n o w becomes: Ag~ =

12~R2Gpo d2

• Ah

(A-8)

where Ah = the surface-elevation change as derived from equation (A-3).

267 The free-air gravity change Ag associated with a change in elevation Ah is (Telford et al., 1976, p. 17): Ag2 (ugals) = 0.3086 Ah (mm)

(A-9)

Thus the net gravity change (mass change plus free-air elevation change) associated with inflation-deflation cycles is Ag3 (ugals) = Agl + Ag~ =

( 12~R2Gp° d2

0.3086

)

Ah (mm)

(A-10)

For Kilauea, R = 1.5 × l 0 s cm and d = 4.5 × 10 s cm from seismic studies; and R = 1.0 × 10 s cm and d = 3.0 × 10 s cm from deformation studies; in either case, equation (A-10) reduces to: Ag3 (ugals) = ( 0 . 0 2 7 9 p 0 - 0.3086) Ah (mm)

(A-11)

The average pore-free density of Hawaiian magma is roughly 2.7 g/cm 3 (Macdonald, 1963). Simultaneous gravity and elevation changes owing to simple inflation-deflation of the summit magma chamber should therefore define a line with slope A g / A h = 0.233 pgal/mm.

Void filling-emptying. A process likely to accompany inflation-deflation of the summit reservoir that would also produce gravity changes at the surface is the filling of microfractures within the summit chamber by magma supplied from depth. The emptying of microfractures owing to drainage and incomplete collapse of the overlying rock would give a similar but opposite effect. Both effects can be modeled as a change in the bulk density of the magma chamber, with no surface deformation. From equation (A-4), the gravity change at the surface caused by a change in magma chamber density from p to p' is: Ag4 =

4~R3G(p' - p )

1 (1 + x2/d2) 3n

3d:

(A-12)

Now, let P0 = the density of magma filling voids in the chamber, Pr = the density of the rock frame within the chamber, and p = the newly filled-emptied porosity within the chamber; then:

P' = PPo + (1 - P)Pr

(A-13)

and p = (1 -- P)Pr, since the voids are initially empty. Therefore p' - p = PPo~ and equation (A-11) becomes: Ag4 =

4nR3Gppo 3d 2

•

1 ( 1 + x2/d2) 3n

(A-14)

The total volume of fractures involved V is equal to the volume of the magma chamber }~R 3 multiplied by the porosity p as defined above; from equation (A-14) this volume can be expressed as:

4~R3p Ag4d 2 V . . . . . . 3 Gpo

(1 + x2/d2) 3n

(A-15)

268

By definition, the maximum gravity change associated with fracture fillingemptying occurs at x = 0. We can therefore simplify equation (A-15) by considering only the maximum observed gravity change Ag4 (max): V-

Ag4 (max)d 2 -

(A-16)

Gpo

Replacement from depth. The final process to be modeled is the replacement of relatively low density summit m a g m a by denser m a g m a from depth as magm a is delivered to the riftzones during intrusive-extrusive events. Density contrasts between deep and shallow m a g m a are a plausible result of vaporphase separation and mineral fractionation, primarily the crystallization and settling of relatively dense olivine. T w o cases are considered: replacement of (i) an equal volume, and (2) an equal mass of magma, with consequent surface deflation. The firstcase is analytically identical to the void-fillingproblem treated above (equation A-14), and we can write: Ag s = f .

4~R3G(p'o - Po) 3d 2

1 (1 + x2/d2) 3/2

(A-17)

where f = the fractional volume replaced, p~ = the density o f magma from depth, and P0 = the density of summit magma replaced. In the second case, we assume that magma from depth replaces an equal mass within the summit chamber; volume change occurs, the surface deflates, b u t only a free-air gravity change results. If the mean radius of the chamber decreases by AR, conservation of mass requires that: :}~R3p = ( ~ R 3 - 41rR2AR)p '

(A-18)

which, by substitution from equation (A-13) and transposition, yields: AR-

--Rp(po - Po) 3[p(p~ - pr) + pr]

Substituting AR = 3Ah(max) from equation (A-6), the resulting free-air gravity change is given by: [ R p ( p o - Po) Ag,(#gals) = +0.3086 L 9 ~ p ~ - - p ~ + p r ]

]

1 " (1 +x2/d2) 3n (mm)

(A-20)

In summary, the gravity and elevation changes observed at Kilauea during typical inflation-deflation cycles are expected to plot as a line with slope A g / A h = --0.233 #gal/mm, if the mobile magma has a density of 2.7 g/cm 3. Additional changes m a y occur o ~ to filling or emptying of fracture voids, or to replacement of summit magma b y denser magma from depth. Inasmuch as both induced gravity and elevation changes decay with distance from the deformation center according to the same functional dependence, the linear

269

relation between Ag and Ah is preserved. However, the slope of the plot of Ag vs. Ah is altered, as shown quantitatively by the above equations and qualitatively in Table 3. REFERENCES Anderson, L.A. and Johnson, G.R., 1976. Application of the self-potential method to geothermal exploration in Long Valley, California. J. Geophys. Res., 81: 1527--1532. Eaton, J.P., 1962. Crustal structure and volcanism in Hawaii. Am. Geophys. Union, Geophys. Monogr., 6: 13--29. Eaton, J.P. and Murata, K.J., 1960. How volcanoes grow. Science, 132: 925--938. Jachens, R.C. and Eaton, G.P., 1980. Geophysical observations of Kilauea volcano, Hawaii, 1. Temporal gravity variations related to the 29 November, 1975, M = 7.2 earthquake and associated summit collapse. J. Volcanol. Geotherm. Res., 7: 225--240. Jackson, D.B., Swanson, D.A., Koyanagi, R.Y. and Wright, T.L., 1975. The August and October 1968 east rift eruptions of Kilauea Volcano, Hawaii. U.S. Geol. Survey.Prof. Paper, 8 9 0 : 3 3 pp. Kinoshita, W.T., Jackson, D.B. and Swanson, D.A., 1974. The measurement of crustal deformation related to volcanic activity at Kilauea Volcano, Hawaii. In" L. Civetta, P. Gasparini, G. Luongo and A. Rapolla (Editors), Physical Volcanology. Elsevier, Amsterdam, pp. 87--115. Koyanagi, R.Y., Unger, J.D., Endo, E.T. and Okamura, A.T., 1974. Shallow earthquakes associated with inflation episodes at the summit of Kilauea Volcano, Hawaii. In: O. Gonzales-Ferran (Editor), Proc. Syrup. Andean and Atarctic Volcanology Problems, Santiago, Chile, September 1974. Int. Assoc. Volcanol. Chem. Earth's Inter., Spec. Set. Lipman, P.W., Okamura, R.T. and Yamashita, K.M., 1978. Changed deformation mode of Kilauea Volcano since the M = 7.2 earthquake. Cordilleran Sect. Meet. Geol. Soc. Am., Tempe, Ariz., March 1978 (abstract). Lipman, P.W., Lockwood, J.P., Okamura, R.T., Swanson, D.A. and Yamashita, K.M., 1980. Ground deformation associated with the 1975 magnitude 7.2 earthquake and resulting changes in activity of Kilauea volcano, 1975--1977. U.S. Geol. Surv. Prof. Paper (in press). Macdonald, G.A., 1961. Volcanology. Science, 133: 673--679. Macdonald, G.A., 1963. Physical properties of erupting Hawaiian magmas. Geol. Soc. Am. Bull., 74 : 1071--1077. Mogi, K., 1958. Relation of the eruptions of various volcanoes and the deformations of the ground surfaces around them. Bull. Earthquake Res. Inst., T o k y o Univ.. 36: 99--134. Moore, R.B., Dzurisin, D., Eaton, G.P., Koyanagi, R.Y., Lipman, P.W., Lockwood, J.P. and Puniwai, G.S., 1978. The September 1977 eruption of Kilauea Volcano, Hawaii. Cordilleran Sect. Meet., Geol. Soc. Am., Tempe, Ariz., March 1978 (abstract). Moore, R.B., Helz, R.T., Dzurisin, D., Eaton, G.P., Koyanagi, R.Y., Lipmam P.W., Lockwood, J.P. and Puniwai, G.S., 1980. The 1977 eruption of Kilauea volcano, Hawaii. J. Volcanol. Geotherm. Res., 7: 189--210. Swanson, D.A., 1972. Magma supply rate of Kilauea Volcano, 1952--1971. Science, 175: 169--170. Swanson, D.A., Jackson, D.B., Koyanagi, R.Y. and Wright, T.L., 1976. The February 1969 east rift eruption of Kilauea Volcano, Hawaii: U.S. Geol. Surv., Prof. Paper, 891 : 30 pp. Telford, W.M., Geldart, L.P., Sheriff, R.E. and Keys, D.A., 1976. Applied Geophysics. Cambridge University Press, Cambridge, 860 pp. Wright, T.L. and Fiske, R.S., 1971. Origin of differentiated and hybrid lavas of Kilauea Volcano, Hawaii. J. Petrol., 12: 65. Zablocki, C.J., 1976. Mapping thermal anomalies on an active volcano by the self-potential method, Kilauea, Hawaii. Proc., 2nd U.N. Symposium on the Development and Use of Geothermal Resources, San Francisco, Calif., May 1975, 2: 1299--1309.