JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B4, 2219, doi:10.1029/2002JB001989, 2003

Spatiotemporal gravity changes at Miyakejima Volcano, Japan:
Caldera collapse, explosive eruptions and magma movement
Masato Furuya, Shuhei Okubo, Wenke Sun, Yoshiyuki Tanaka, Jun Oikawa,and Hidefumi Watanabe
Earthquake Research Institute, University of Tokyo, Tokyo, Japan

Tokumitsu Maekawa
Graduate School of Science, Hokkaido University, Sapporo, Japan
Received 23 May 2002; revised 13 October 2002; accepted 28 October 2002; published 29 April 2003.

[1] Calderas are ubiquitous topographic features of volcanos, yet caldera formation itself

has not been recorded intensively by modern measurement techniques. Here we report
the spatiotemporal gravity changes before and after caldera collapse at the Miyakejima
volcano, Japan in 2000. A gravity decrease of as much as 145 mGal (1 mGal = 108 m/s2)
at the summit area since June 1998 was detected 2 days prior to the collapse, interpreted
as reflecting the formation of a large void beneath the volcano. Gravity changes detected
after the initiation of collapse can mostly be corrected by the effect of collapsed topography,
from which a rapid rate of collapse of more than 1.6  107 m3/d can be inferred. Correcting
for the effect of topography change, we identified a temporal decrease in gravity fromthe middle of July to late August despite ground subsidence. The gravity decrease isinterpreted as a reduction of the density in a cylindrical conduit, attributed to water inflow
from an ambient aquifer that also promoted intensive magma-water interaction andsubsequent explosive eruptions. From September to at least November 2000, gravity values at
all sites increased significantly by a degree that cannot be explained by ground displacement
alone. We interpret this temporal evolution as primarily due to magma ascent and refilling
INDEX TERMS: 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 8414
of the conduit.
Volcanology: Eruption mechanisms; 8434 Volcanology: Magma migration; 8439 Volcanology: Physics andchemistry of magma bodies; KEYWORDS: caldera collapse, void, hybrid microgravimetry, Miyakejima, Izu islands
Citation: Furuya, M., S. Okubo, W. Sun, Y. Tanaka, J. Oikawa, H. Watanabe, and T. Maekawa, Spatiotemporal gravity changes at
Miyakejima Volcano, Japan: Caldera collapse, explosive eruptions and magma movement, J. Geophys. Res., 108(B4), 2219,
doi:10.1029/2002JB001989, 2003.

1. Introduction[2] Miyakejima is a basaltic stratovolcanic island in the
Izu-Bonin arc, Japan (Figure 1). Historical records showthat the volcano has erupted roughly every 20 years over thelast few centuries [Miyazaki, 1984; Tsukui and Suzuki,
1998]. In the last decade, several institutes in Japan beganmonitoring the Miyakejima volcano, using seismometers[Sakai et al., 2001; Japan Meteorological Agency, 2000;
Ukawa et al., 2000], tiltmeters [Ukawa et al., 2000],
magnetometers [Sasai et al., 2001], and Global Positioning
System (GPS) receivers [Kaidzu et al., 2000; Nishimura etal., 2001] and by levelling [Tokyo Metropolitan Government, 1990]. The activity at Miyakejima in 2000 notably
involved the formation of a collapsed caldera, and this raregeological event was monitored intensively, providing aunique opportunity to investigate the dynamics of caldera
formation and associated eruptions.
[3] The purpose of this paper is to report spatiotemporal
gravity changes during the 2000 Miyakejima unrest and to
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2002JB001989$09.00

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propose a model that accounts for the observed gravity
changes and crustal deformation to give insight into thecaldera collapse mechanism and associated magma movement. The contents of this paper are as follows. In section 2,
we describe the microgravimetry method employed in thisstudy. In section 3, we divide the evolution of the 2000
Miyakejima unrest into four stages, and outline the volcanic
activity and gravity changes in each stage. Subsequent
sections are organized according to these four stages. Eachsection details the observed gravity changes and correlates
the gravity data with the measured crustal deformation so as
to form an unambiguous model and interpretation. Next, we
describe the interpretation and modeling results and conclude with a discussion of the implications with respect tothe mechanisms of caldera collapse, explosive eruptions,and magma movement.

2. Hybrid Microgravimetry[4] Repeated microgravimetry measurement at an active
volcano is a widely used technique that can reveal theredistribution of magma beneath volcanos by sensing

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Figure 1. Map of Miyakejima volcano with contour interval of 100 m. Inset shows location of Tokyo,
Miyakejima and Kozushima. Gravity measurement sites and bench marks are indicated by diamonds.
Absolute gravity measurements were carried out at Miyakejima Weather Station (Japan Meteorological
Agency). Also shown are the locations of four GSI GPS (circles) and five NIED tiltmeter (triangles)
stations. See Figure A1 for temporal evolution of caldera collapse.
directly changes in the density distribution [Jachens andEaton, 1980; Dzurisin et al., 1980; Okubo and Watanabe,
1989; Berrino et al., 1992; Rymer et al., 1995; Battaglia etal., 1999; Gottsmann and Rymer, 2002]. Conventionally,
microgravity measurements have been carried out using
relative gravimeters, which measure spatial changes withrespect to a fixed reference site. To detect temporal changes,
the absolute gravity at the reference site is assumed to beconstant over time. However, on a small volcanic island
such as Miyakejima, there is no guarantee of temporally
constant gravity. For our measurements, we employed astate-of-the-art absolute gravimeter [Niebauer et al., 1995],
installed at the reference station in Figure 1. Other sites in
Figure 1 were tied to the reference station, using two or
three LaCoste-Romberg G-type gravimeters. We refer tothis combined use of absolute and relative gravimeters as
hybrid microgravimetry, and this technique allowed us tounambiguously record spatiotemporal evolution of gravity
associated with the volcanic unrest. The absolute gravimeter

had an accuracy of the order of 1 mGal (1 mGal = 108m/s2). Although there are numerous reports on the gravity
changes associated with earthquakes and volcanic unrest,
Tanaka et al. [2001] were the first to report absolute gravity
changes associated with an earthquake. To the best of ourknowledge, this is the first report of absolute gravity
changes associated with volcanic unrest.

3. Outline of the 2000 Miyakejima
Volcanic Unrest
3.1. Stage 1: 26 June 2000 to Early July 2000
[5] A swarm of volcanic earthquakes in the evening of 26
June 2000 marked the onset of volcanic unrest after 17
years of quiescence [Japan Meteorological Agency, 2000;
Sakai et al., 2001]. The earthquake hypocenters migrated
from the eastern to the western edge of the volcanic island
over 12 hours. On 27 June 2000, changes in the color of thesea surface attributed to a small seafloor eruption were

FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPANobserved on the west side of the island [Nakada et al.,2001]. The hypocenters continued to migrate laterally
toward the northwest for more than 20 km by 1 July
2000. The region beneath the volcano, on the other hand,exhibited virtually no seismicity until 4 July 2000.
[6] Four GPS stations of the Geographical Survey Institute (GSI) of Japan (Figure 1) recorded significant changes
in disposition after the beginning of the earthquake swarm[Kaidzu et al., 2000; Nishimura et al., 2001]. Figure 2
shows the temporal changes in the three components ofeach position; data loss after September 2000 was due to anelectrical outage. The most rapid changes occurred in thefirst week after the onset of unrest.
[7] Tiltmeters of the National Research Institute for Earth
Science and Disaster Prevention (NIED) installed at the fivestations in Figure 1 allowed us to observe the grounddeformation with fine temporal resolution [Ukawa et al.,2000]. The first tilt change occurred at MKT on the eveningof 26 June 2000 with a downward tilt toward the eastnortheast, indicating a swelling of the island. Two hours
later, MKA exhibited the most dramatic downward tilt
toward the south, reaching 0.16 mrad. Although otheramplitudes were less than half that at MKA, the othertiltmeters also recorded changes that suggested a swellingof the island. However, the direction of tilt change at all
meters changed from 28 June 2000, after which the tilt
vectors indicated a subsidence in the western and southernparts of the island.
[8] A gravity-elevation change plot is shown in Figure 3
for the four stages of the 2000 unrest. The gradient of thisplot can be used to interpret the processes occurring underground [Gottsmann and Rymer, 2002]. Since the elevation
changes were not measured simultaneously at all gravity
sites, four gravity sites located as close as possible to the
GSI’s GPS were selected, and the GPS data were used as aproxy of the elevation changes. The data marked withcircles correspond to stage 1. Four other gravity sites that
underwent subsidences of presumably greater than 60 cm
are also plotted. The data for stage 1 deviate significantly
from the Bouguer-corrected free-air gradient, which can beinterpreted as a mass and density decrease [Gottsmann and
Rymer, 2002].
3.2. Stage 2: Early July 2000
[9] Although the seismic swarm continued in the southeast, offshore of Kozushima (Figure 1), there was noseismic activity beneath the summit of the volcano after
28 June 2000 when the dike intruded offshore of Miyakejima. Many researchers considered that on the basis of theirprior knowledge of the Miyakejima volcano, the 2000
volcanic unrest ended at that time. However, the numberof earthquakes increased dramatically below the summit of
the volcano from around 4 July 2000, lasting to the middleof August 2000. The hypocenters were distributed in acylindrical volume with radius of about 500 m and extending 2500 m below sea level [Sakai, 2000].
[10] A small summit eruption occurred on the evening of 8
July, and ceased within 10 min. The collapse of the original
topography at the summit was observed the following
morning. Recognizable shapes such as a scoria cone generated in a 1940 eruption remained the same, and the northern area remained covered with vegetation [Nakada et al.,

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2001]. The appearance strongly suggested that the collapse
was caused by implosion rather than explosion. The eruption
deposits surrounding the collapsed caldera at that time were
estimated to total only 1.5  105 tons [Nakada et al., 2001],
which is an order of magnitude smaller than the collapsedvolume. The summit eruption on the evening of 8 Julymarked the initiation of caldera collapse.
[11] The growth rate of caldera collapse was most rapid in
the initial stage after the first small summit eruption on 8
July 2000 and gradually eased from the middle of August to
September 2000. Remarkably, the caldera growth was notaccompanied by eruptions that could generate eruptive
material comparable to the caldera volume.
[12] Gravity changes detected immediately after the collapse amounted to as much as 1135 mGal, suggesting a
rapid rate of caldera collapse. The data marked withtriangles in Figure 3 correspond to stage 2 and have beencorrected for the effect of topography changes due to thecaldera collapse. As in stage 1, the data suggest a mass anddensity decrease.
3.3. Stage 3: Middle July 2000 to September 2000
[13] Explosive summit eruptions first occurred on 14 and
15 July 2000 followed later by eruptions on 10 August andexplosive phreatomagmatic summit eruptions on 18 and 29
August 2000. The eruption on 18 August 2000 was thelargest; the plume top reached as high as 15 km. Such
explosive summit eruptions have been uncommon for
Miyakejima over the past few centuries, raising the question
as to what controls the explosivity at Miyakejima.
[14] Notable observations during this stage are a longperiod seismic pulse [Kumagai et al., 2001] and step-wisechanges in both tilt [Ukawa et al., 2000] and electric field[Sasai et al., 2001], which began after the onset of caldera
collapse. All these changes occurred simultaneously once or
twice per day and were shown to correlate with the tides[Kasahara et al., 2001]. These characteristic signals, however, have not been observed since the largest eruption on
18 August 2000. Besides these long-period signals, normal
shorter-period earthquakes also occurred beneath the collapsed caldera. The hypocenter distribution was cylindrical
as noted above. The migration of earthquake hypocentersfrom Miyakejima toward Kozushima detected in late June
2000 was not detected from July to August, although theearthquake swarm to the east and southeast offshore of
Kozushima continued.
[15] The crustal deformation also continued, and the rateof subsidence and contraction in the island remained almost
the same through to September 2000 (Figure 2). Correcting
for the effect of topography changes due to caldera collapse
on gravity changes, we found that gravity values did indeed
decrease in this stage, as plotted in Figure 3. The deviations
from the Bouguer-corrected free-air gradient suggest massand density decreases as in stages 1 and 2 [Gottsmann and
Rymer, 2002].
3.4. Stage 4: After September 2000
[16] Although a white vapor plume has emitted continuously from the vent, large-scale explosive eruptions have
not occurred since 29 August 2000. The frequency ofearthquakes beneath the volcano have also dramaticallydecreased. Since September 2000, however, volcanic gas,

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Figure 2. Temporal changes in site positions of GSI GPS (locations shown in Figure 1). Largest
coseismic change is indicated by arrow. Each panel is displayed to conform to the location on the
volcanic island. Because of an electrical outage, data continuity is lost from September 2000. (Copyrightof original data is retained by the Geographical Survey Institute of Japan.)FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN

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[18] The gravity changes detected in stage 4 deviate
systematically and significantly from the free-air gradient(Figure 3), which suggests mass and density increases[Gottsmann and Rymer, 2002]. The distinctly different
clusters for each stage in Figure 3 reflect the different
processes as discussed in detail below.

4. Dike Intrusion and Events Contributing to
Caldera Collapse

Figure 3. Gravity and elevation changes for the fourstages; (1) June 1998 to early July 2000 (circles), (2) earlyJuly (triangles), (3) mid-July to late August 2000 (squares),and (4) September – November 2000 (diamonds). For the
first stage, four other gravity measurements are also shown,representing sites that subsided presumably greater than
60 cm. FAG and BCFAG represent the free-air gradient(3.086 mGal/cm) and Bouguer-corrected free-air gradient(2.00 mGal/cm), respectively. The distinctly different
clusters for each stage reflect the different processes as
discussed in the text.

in particular sulfur dioxide (SO2), has been discharging at
unprecedented volumes. The SO2 emission rate gradually
increased from September to December 2000 and exceeded
4.8  104 t/d on average from September 2000 to January
2001 [Kazahaya et al., 2001]. The huge amount of gas
discharge has prevented the evacuated residents of theisland from returning for more than a year. The Japan
Meteorological Agency (JMA) has been monitoring temperatures near the volcanic vent by infrared thermometer froma helicopter, revealing that the temperature increased to
400C by January 2001. The measured temperature is
probably lower than the actual temperature because the ventitself is often masked by the vapor plume and dust. JMA
also reported a volcanic glow at nights from December 2000
to January 2001.
[17] The electrical outage since September 2000 has
prevented the GSI from continuously monitoring crustal
deformation. According to the four single-frequency GPS
receivers temporarily installed by JMA on October 2000,
the ground contraction appeared to be continuing. The
contraction since September 2000, however, seemed tocease around April – May 2001, as indicated by the baselinechanges recorded by JMA’s GPS. On the basis of thisobservation, we linearly interpolated the GSI’s GPS datawith the measurements resumed in April and May 2001
(Figure 2). The rate of contraction and subsidence certainly
slowed from September 2000, as most clearly observed in
the north-south component of MYK1 (Figure 2). The rate ofhorizontal displacement is at most 1 cm/month. The displacement at MYK4 is the smallest recorded by any of the
GPS sites.

4.1. Observation Results[19] Figures 4a and 4b show the cumulative grounddisplacements from 26 June to 6 July 2000. For the northsouth component at MYK1, we have taken the maximum
displacement to 6 July 2000, considered to be caused by dike
intrusion as discussed below and used it to estimate thewidening of the tensile crack. The style of deformation was
spatially inhomogeneous. As suggested from Figure 2, thenorth-south baseline on the western side of the island
extended as much as 20 cm in the first 3 days. Although
not clearly observable in Figure 2, baseline extension of up to
3 cm was also recognized on the eastern side of the island.
From 28 June 2000 on, all four stations began indicating acontraction of the island. This change of deformation style
was also detected by tilt changes as noted above. The
amplitude of ground subsidence also varied between sites(Figure 2). The southern part of the island subsided significantly in comparison to the northern part. MYK2 and MYK4
subsided as much as 40 and 70 cm, respectively, while
MYK3 subsided at most 15 cm. Although MYK1 exhibited
significant changes in horizontal components, the heightchanges were not as significant as at other locations, andthe total subsidence to 6 July 2000 was evaluated to be 10 cm.
The rate of subsidence at MYK1 slowed gradually over time.
[20] A university team carried out a GPS campaign on
August 1999 and 2 July 2000. The station located close to
gravity site 109 in the middle of the flank of the island(Figure 1) recorded a subsidence of 63 ± 10 cm shown in
Figure 3. Unfortunately, the GPS receiver in the summit
caldera was lost in the caldera collapse.
[21] Figure 4c shows the changes in gravity obtained by
the survey for the periods 2 – 5 June 1998 and 2 – 6 July 2000.
The detected gravity changes vary spatially, and are far fromisotropic. A large gravity decrease of as much as 145 ±10 mGal occurred around the summit on 6 July, 2 days before
the onset of collapse. At one site in the southwest of theisland, we detected a localized increase of +110 ± 10 mGal,and at a couple of sites in the southeastern area of the island,
we detected regional increases of as much as +60 ± 10 mGal.
It should also be noted that the absolute gravity at thereference site exhibited a significant increase of +10 ± 1 mGal.
4.2. Interpretation and Modeling[22] As clearly indicated by the tilt changes and earthquake hypocenter locations, the first change associated withthe 2000 unrest was detected on the south side of thesummit. We interpret this as the beginning of a decompression inside a magma chamber in this area, and thegravity increase detected in the southern part of the island is
explained by a point deflation source in this area [Mogi,
1958; Hagiwara, 1977]. The optimum location and depth
are estimated below.

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Figure 4. (a) Horizontal component of GPS displacement field obtained by GSI from 26 June to 6 July
2000. Northward displacement of MYK1 was obtained to 28 June 2000 (see text). (b) Vertical componentof GPS displacement. Horizontal locations of void and deflation source are shown with dashed and solid
circles, respectively. The strike of tensile dislocation is also shown. (c) Gravity changes from 5 June 1998
to 6 July 2000, 2 days prior to summit collapse. Units are in microGals (= 108 m/s2). The star and solid
dots designate measurement points of absolute and relative gravimetry, respectively. (d) Computed
gravity changes based on the optimum model. See also Figure 5 and Table 1.
[23] In view of the migration of earthquake hypocenters,
the lengthening of the north-south GPS baseline in the west,
and the large tilt changes in the southwest site noted above,it is considered that a dike intruded on the southwesternedge of the island on 27– 28 June 2000. The localized
gravity increase at the southwestern site was presumablycaused by this dike intrusion and subsequent ground subsidence. In modeling the detected changes in gravity andFURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN

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Figure 5. Three-element model: void, rectangular dike, and deflation source, accounting for theobserved gravity changes and ground displacements immediately prior to the collapse.

ground displacement, we employ analytical expressions forground displacement [Okada, 1985] and gravity changes[Okubo, 1992] due to a tensile dislocation.
[24] A large decrease in gravity in the summit area was
detected 2 days prior to the summit collapse. Althoughnegative gravity changes are usually regarded as indicating
ground uplift, the GSI GPS measurements revealed acontraction and subsidence at that time and the university
team’s GPS near gravity site 109 on the southwestern flankof the volcano indicated a significant subsidence. Hence it ishighly unlikely that the summit area uplifted locally. Considering the summit collapse that occurred on 8 July, withvery little associated eruption deposits and the appearance
noted above, the source of this negative gravity change isthought to have been an empty space beneath the volcano.
This is further supported by the total magnetic intensitychanges detected from 26 June to 10 July 2000 close to thesummit, which can be modeled by a perfect demagnetized
sphere [Sasai et al., 2001]. A simple empty sphere is used torepresent the decrease in density without exerting stressaround the medium that would cause uplift at the surface.
[25] In order to account for the ground displacements andgravity changes in Figures 4a, 4b, and 4c, we developed asource model consisting of a deflation source, a tensile
dislocation, and an empty sphere (Figure 5) and estimatedsome of the relevant physical parameters. In the joint
inversion, we minimized the following quantity, L:
0 ! !122 X
29  obs
4
obs
cal
X
gi  gical@di  di A ;
L¼
þ
Wgi
Wdi
i¼1
i¼1

ð1Þ

are shown in Figures 4a, 4b, and 4d. The optimized model
adequately explains the overall observed changes. The
deflation source is estimated to be located to the south of
the summit, as speculated above. The estimated depth of theempty cavity is consistent with that estimated from themeasurement of total magnetic intensity changes [Sasai etal., 2001]. In order to explain some discrepancies in thegravity changes in the southeastern area and ground displacements in the southwestern area, a more complex source
model is necessary.

5. Initiation of Caldera Collapse
5.1. Observation Results[27] GSI carried out airborne photogrammetry on 9 July
2000 and constructed a digital terrain model (DTM), bringTable 1. Optimum Model Parameters for Figure 4
Parameter

Value
Deflation Source

Medium density,a kg/m3
Longitude
LatitudeDepth, m
Volume change, m3

2,600
139310900 ± 300
34403000 ± 300
5,300 ± 450
8.7 ± 0.5  107
VoidMedium density,a kg/m3
Longitude
LatitudeDepth, mRadius, m

2,3001393104200 ± 300
34405100 ± 300
1,740 ± 200
240 ± 20
Tensile Dislocation

where Wgi and Wdi are assigned weights for the ith site. The
weight for gravity data was set at 10 mGal and 2 mGal for theabsolute gravimeter, while the weight for the GPS data wasset at 2 cm. Optimum parameters were obtained by thesimplex algorithm [Press et al., 1992].
[26] The optimized and prescribed source parameters arelisted in Table 1, and the horizontal locations are shown in
Figure 4b. The height and strike of the dike were fixed inaccordance with the hypocenter distributions [Sakai et al.,2001]. Computed ground displacement and gravity changes

Longitudeb
Latitudeb
Strike directionb
Dip angleb
Open dislocation, mLength, m
Bottom depth,b m
Depth to the top,b m
Density contrast,b kg/m3a1392201500
34704900
N60W90
1.6 ± 1.0
13,500 ± 1,000
10,000500
0Density inside the medium is fixed [Fujii and Kushiro, 1977].
Fixed parameters. For tensile dislocation, the position of the westernedge is shown.
b

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Figure 6. (a) Gravity changes at Miyakejima from 1 – 6 July to 11 – 14 July 2000, before and after thesummit collapse. (b) Computed gravity changes based on the collapsed topography model. See Figure 7.

ing to a total of 8 the DTMs constructed for different
periods to January 2001 [Hasegawa et al., 2001]. We madeuse of these to synthesize a digital elevation map for thewhole island as discussed in Appendix A. Our estimates of
the collapsed volumes on 9 and 22 July are 5.77  107 m3
and 28.0  107 m3, respectively. A simple linear interpolation gives a collapse rate of 1.7  107 m3/d, but the rate is
expected to be much higher in the initial stages. It should be
noted, moreover, that the volume of erupted material
surrounding the caldera to 15 July 2000 is estimated to be
2.2  106 m3 [Nakada et al., 2001], which is an order ofmagnitude smaller than the caldera volume. In other words,
caldera growth proceeded with a very small amount of
erupted material. Therefore the mass of the original topography must have disappeared beneath the ground. We canregard the gravity changes due to the eruption deposits
outside the caldera as negligible.
[28] Gravity changes after the initiation of caldera collapse are shown in Figure 6a. These changes are derived
from measurements taken between 2 –6 July and 11 – 14
July 2000. An extraordinary large decrease exceeding1000 mGal near the caldera rim and a significant increasein the middle of the flank were recorded on 11 July.
Measurements at most of the sites near the coast wereconducted on 14 July 2000.
[29] Figure 2 shows that the rate of ground subsidence
and contraction was almost constant from 1 July to the
middle August 2000 except for coseismic changes [Nishimura et al., 2001]: the subsidence rate of MYK1 andMYK3 was 0.25 ± 0.02 cm/d, while that for MYK2 and
MYK4 was 0.68 ± 0.02 cm/d. According to the GPS dataobtained by the university team, ground displacements on
the flank of the island between the two periods, 2 – 6 Julyand 11 – 14 July 2000, were within measurement uncertainty, and subsidence was less than 3 cm (T. Matsushima,
personal communication, 2000), which is roughly consistent
with the rate inferred from GSI’s GPS measurements.
5.2. Interpretation and Modeling[30] Since the direct attraction of the topographic mass
was lost due to the dramatic change of topography duringinitiation of caldera collapse, gravity values will haveincreased at some sites and decreased at others, dependingon the relative height of each gravity point with respect tothe caldera volume. Although many effects contributed inreality as argued below, Figure 6a suggests that this massloss effect is dominant in the measurement results. Becauseof the disappearance of the original topography, downward
attraction was lost near the summit, causing a decrease ingravity (1135 mGal). In contrast, at the sites on the
middle of the flank of the volcano, upward attraction is
lost, and we can expect an increase in gravity of as muchas 140 mGal.
[31] For the moment, we suppose that the gravity changes
in Figure 6a are explained entirely by the loss of topographic mass, and ignore the effects of crustal deformationand density changes. These additional effects are discussed
below. Under this assumption, we first examined how theFURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN

Figure 7. Collapsed topography model for 11 July 2000
accounting for the gravity changes immediately following
summit collapse. The difference between the radii of theupper and lower edges is inferred from photographs of thesummit collapse.
DTM on 9 July 2000 accounts for the gravity changes,
using the original DTM before the collapse as a reference(see Appendix A). We found that the DTM on 9 July 2000
explains only 90 mGal at the largest negative change in
Figure 6a, assuming a density of 2300 kg/m3 for thecollapsed topography. For the site where the largest positivechange was detected in Figure 6a, the DTM accounted for
70 mGal. The DTM on 9 July 2000 obviously fails to
explain the results in Figure 6a.
[32] We can attempt to explain the largest negative andpositive changes in Figure 6a most simply by supposing amuch larger collapsed volume. Considering that the collapse
proceeded most rapidly in the initial stages, it is plausiblethat the volume collapsed by 9 July 2000 when the DTM
was obtained is smaller than the actual volume on 11 July
2000. Gravity change data near the coastline were acquired
on 14 July, and therefore we may have to develop anothermodel to explain those data. However, in order to explain
the large gravity changes near the caldera rim and in the
middle of the flank and the rate of collapse in the very initialstage, we developed a collapsed topography model based
primarily on the larger gravity changes obtained on 11 July
2000 (Figure 7).
[33] Computed gravity changes are shown in Figure 6b. In
the computation, we employed an analytical formula for thegravity of a cylindrical body [Singh, 1977] and approximatedthe shape in Figure 7 as stacked thin circular disks. The
difference in the radius of the upper and lower edges isinferred from the photographs of summit collapse. The model
collapse volume is 11.0  107 m3. Comparing the estimate
with the collapsed volume as of 9 July 2000, the collapse rate
appears to be 2.6  107 m3/d, which is higher than the simple
estimate in section 5.1 and is consistent with the expectationthat the collapse was most rapid in the first stages.
[34] We then consider reevaluation of this model taking
into account the other effects. The effect of crustal deformation will increase the gravity values at all sites because
the entire island was undergoing ground contraction andsubsidence at that time. Thus the collapsed topography
should induce a large negative gravity change near thecaldera rim and a smaller positive change in the middle ofthe flank. Therefore we will have to suppose a larger radiusat the top and a smaller radius at the bottom of the collapsed
topography model (Figure 7). However, the volume of thecollapsed topography is not expected to differ significantly

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from the estimate above because the ground displacement
between the measurement periods was rather small.
[35] If the density changes occur above sea level, gravity
values at some sites will increase and those at others will
decrease. There is no evidence in the period immediately
following the initiation of caldera collapse to suggest theascent of magma above sea level. Thus it is unlikely that thedensity increase occurred somewhere above sea level. In
contrast, in view of the ongoing collapse it is likely thatnegative density changes occurred above sea level. However, it is difficult to distinguish this effect from that ofcollapsed topography, and thus we will suppose that thiseffect, if any, is included in the collapsed topography model
derived above. As discussed in the next section, it is also
likely that some part of the density decrease occurred below
sea level. If so, we will have to suppose a smaller radius at
the top and a larger radius at the bottom of the collapsed
topography model. There is no evidence to suggest, however, that the radius at the bottom of the collapsed caldera
was greater than that at the top. If the density increased
below sea level, it would cause all gravity values toincrease, and it would be necessary to reexamine theestimate. However, we will discuss later that magma flowed
out laterally at depths below sea level in this period, and we
consider that it is unlikely that the effect of density increase
below sea level dominates other effects.

6. Caldera Growth and Explosive Summit
Eruptions6.1. Observation Results[36] Figure 8 shows temporal gravity changes at thereference absolute gravity point on 6 June 2001. Also
shown are the effects of topography change due to caldera
collapse. All other sites exhibited similar behavior (seeauxiliary material1). The effects of mass loss due to caldera
collapse were estimated using the synthetic DTM as discussed in Appendix A. As crustal deformation was notmeasured at all gravity points, those data are not shown.
The observed gravity corrected for topography change in
Figure 8 therefore reveals the effects of both crustal deformation and density changes above and below sea level.
[37] Despite the ground contraction and subsidence, thecorrected gravity changes exhibited a significant temporaldecrease from July to the middle of August, particularly at
northern sites (Figures 3 and 8); the larger the assigned
density of topography, the greater the gravity decrease. It
should also be noted that the period of temporal decrease
coincides with the period of intermittent explosive eruptions
and slowing caldera growth. Figure 9 shows the grounddisplacements (Figures 9a and 9b) and corrected gravity
changes (Figure 9c) from 11 July to 12 August 2000 assuming a density of 2300 kg/m3 for the collapsed topography. A
number of measurement sites were inaccessible due to theintermittent explosive eruptions. As expected from Figure 8,
a temporal gravity decrease can be seen, most notably over1
Auxiliary material is available via Web browser or Anonymous FTP
from ftp://ftp.agu.org, directory ‘‘append’’ (Username = ‘‘anonymous’’,Password = ‘‘guest’’); subdirectories in the ftp site are arranged by paper
number. Information on searching and submitting electronic supplements isfound at http://www.agu.org/pubs/esupp_about.html.

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8 - 10FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN

Figure 8. Temporal changes of observed absolute gravity (thick solid line), effect of collapsed
topography (dashed line) and the corrected gravity (dash-dotted line) since June 1998. Gravity changes
from 979,799,740 mGal are shown. Measurement error of absolute gravity is 1 mGal, while that for
relative gravity is 10 mGal. The collapsed topography model is based on our synthetic digital elevation
maps originally developed by GSI for the summit area at several dates denoted by open circles[Hasegawa et al., 2001]; see also Appendix A. The effects of collapsed topography are temporarily
interpolated to estimate the expected gravity changes on the dates of gravity measurements, denoted bydiamonds. We assumed two values of density, 1900 and 2300 kg/m3 as end-members. The larger thedensity value, the larger the predicted changes.
the northeastern area where ground subsidence was less than
10 cm (Figure 9b). Meanwhile, the southern region undergoes a larger crustal subsidence of as much as 30 cm (Figure9b), causing an increase in gravity. If the actual density ofcollapsed topography is less than 2300 kg/m3, the temporal
gravity decrease will not be so large. As shown in Figure 8,
however, the temporal decrease in gravity can still beobserved at a number of sites even if the density of collapsed
topography is assumed to be 1900 kg/m3.
6.2. Interpretation and Modeling[38] The gravity changes in Figure 9c should be due tothe effects of crustal deformation as well as density changes
beneath the ground. The crustal deformation causes thegravity values to increase as noted above. Nevertheless,we see a gravity decrease in the northern and northeasternareas. The effect of density decrease therefore appears to
exceed that of crustal deformation, and the results indicatethat a significant reduction in density sufficient to mask any
gravity increase due to ground subsidence occurred duringthat period.
[39] In order to account for the inferred gravity changes
as well as the ongoing crustal deformation, we developed amodel consisting of two point deflation sources and acylindrical body lighter than the surrounding medium(Figure 10). We employ a cylindrical body to conform to

the distribution of hypocenters; Kumagai et al. [2001] also
employed a cylindrical piston to explain the long-period
seismic pulse. The GPS displacement is explained only interms of the point deflation sources. Note that the cylindrical body does not exert any stresses around the medium
and thus causes no ground displacement. To compute thegravity due to the cylindrical body, we used an analyticalformula presented by Singh [1977]. As a model to accountfor the ground contraction and subsidence and consequent
gravity increase, we used the point source model of Mogi
[1958] and Hagiwara [1977]. Figures 9a and 9b show larger
horizontal and vertical movements in the southern part ofthe island that cannot be reproduced well by a single
isotropic point source; this is also true for gravity changes.
We failed to derive a physically plausible estimate when we
assumed one deflation source and a closing tensile dislocation. Two deflation sources could adequately reproduce theobserved GPS displacements.
[40] Joint inversion was carried out in the same way as insection 6.1, by minimizing a quantity such as equation (1).
Here, however, the weight for gravity data is set at 20 and
5 mGal for the absolute gravimeter, considering the errors in
our estimate of collapsed topography corrections in addition
to measurement error. The weight for GPS data is set
at 5 cm, considering the errors in the corrections of thecoseismic effect.

FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPANECV

8 - 11Figure 9. (a) Horizontal component of GPS displacement field derived by GSI from 11 – 14 July to 10–
12 August 2000. (b) Vertical component of GPS displacement. (c) Gravity changes acquired from 11 – 14
July to 10– 12 August 2000. Units are in microGals. The effect of collapsed topography has beencorrected, assuming a density of 2300 kg/m3. The reference site is located at the northernmost point, as
designated by a star. (d) Computed gravity changes based on the optimum model. See Table 2.
[41] Computed ground displacement and gravity changes
are shown in Figures 9a, 9b, and 9d. Optimum model
parameters are listed in Table 2, and the locations of thedeflation sources are indicated in Figure 9b. The estimatedsize of the cylindrical body conforms to the seismicity map
for the region beneath the volcano in this particular period.
We interpret the event as a collapse of the preexisting
cylindrical structure and an inflow of water from an ambient

aquifer, resulting in a significant reduction of the density of
the cylindrical body. Although the larger deflation source
moved upward, its horizontal location was estimated to beclose to that listed in Table 1 (see also Figure 4b), suggesting a more complex source system in reality.
[42] The explosive summit eruption was presumablycaused by the interaction of water with magma or hot rockwithin the central conduit. As discussed later, a number of

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8 - 12FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPANtwo substages; before and after January 2001. From January
2001, there have been occasional small eruptions andtemporal fluctuations in gravity, and the physical state of
the volcano appears to have changed. The interpretation andmodeling results for the period after January 2001 will bereported elsewhere. In the following, we estimate thecumulative crustal displacements (Figures 11a and 11b),show the observed gravity changes (Figure 11c) and interpret these data for the period September to November 2000.
As shown in Figure 3, the gradient of gravity and elevation
changes is significantly greater than that of the free-air
gradient.

Figure 10. Source model accounting for both the observed
gravity changes and ground displacements during explosive
eruptions. Two deflation sources were employed to explain
ground displacements. To explain the gravity decrease, we
assumed a cylindrical body that underwent a density
decrease. We take the cylindrical form to conform to the
hypocenter distribution [Sakai, 2000]. See text for details.
observation suggest the involvement of magma, such as
early gravity increases attributed to a rising magma column,
increasing temperatures near the vent, observations of
volcanic glow in December 2000, and increased SO2
emission from September to December 2000.

7. Discharge of Volcanic Gas
7.1. Observation Results[43] Gravity changes since September 2000 are also
shown in Figure 8. Notably, the corrected gravity values at
all sites began to increase after September 2000 up until atleast November 2000. Large changes in gravity weredetected at sites close to the caldera. The largest increasein gravity to May 2001 was observed at site 107. The rate ofgravity change decreased from November to December 2000
to close to the limit of measurement accuracy, and gravity
decreases were observed at some sites. We observed an
obvious increase in gravity from January to March 2001,
followed by a gravity decrease from March to May 2001.
Overall, while gravity values rose steadily to November
2000, there were temporal fluctuations after December 2000.
[44] According to the volcano observations by JMA, therate of SO2 emission gradually decreased from 2001 to 2002,
and at the time this paper was written remains at 1.0 –2.0 
104 t/d on average. After January 2001, however, small-scale
summit eruptions occurred occasionally, with plume tops
approaching 1000– 2000 m above sea level and each eruption
lasting at most 5 – 6 min. Eruptions occur roughly once or
twice per month. The temperature close to the vent fluctuates
between 100 and 400C, and did not show any significant
decreasing trend until 6 June 2001. Lower-frequency volcanic tremors have also been detected from January 2001.
[45] On the basis of the observations above, we candivide the temporal evolution after September 2000 into

7.2. Interpretation and Modeling[46] Because caldera collapse had finished by September
2000, we can assume that the gravity changes in Figure 11ccan be regarded to consist of the effects of crustal deformation and density changes beneath the ground. According to
Figures 11a and 11b, the effect on gravity should be lessthan 20 mGal near the coastline. Using the subsidence rate at
MYK1 as a proxy for that at the absolute gravity point andassuming 3.0 mGal/cm as the gravity gradient, the effect ofground subsidence for the 2.5 months would be 10.5 mGal.
Considering the measurement accuracy of the absolutegravimeter, this value is significantly smaller than theobserved changes of 19 mGal. It is therefore necessary tosuppose unrealistically large values of either the subsidence
rate or the gravity gradient to explain the changes in termsof ground subsidence alone. Hence the temporal gravity
increase was presumed to be caused by crustal deformation
and an increase in density beneath the volcano. The significant deviation of the relevant plots in Figure 3 from thefree-air gradient also supports this interpretation.
[47] We interpret from the largest eruption on 18 August
2000 that the conduit from the chamber beneath the volcano
was probably connected to the surface vent. Magma would
then be able to rise through the conduit toward the surface

Table 2. Optimum Model Parameters for Figure 9
Parameter

Value
Deflation Source I

Medium density,a kg/m3
Longitude
LatitudeDepth (m)Volume change, m3

2600
1393104700 ± 500
34305300 ± 500
3700 ± 100
1.6 ± 0.5  107
Deflation Source II

Medium density,a kg/m3
Longitude
LatitudeDepth (m)Volume change, m3

2600
139290000 ± 300
34501800 ± 500
1900 ± 100
3.7 ± 0.5  106
Cylindrical Body

Longitudeb
Latitudeb
Bottom depth, mRadius, mVertical length, mDensity contrast, kg/m3a1393105000
3450100
2700 ± 300
235 ± 101000 ± 200
1000 ± 300

Density inside the source is fixed.
Horizontal location of the cylinder is fixed beneath the collapsed
caldera.
bFURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPANECV

8 - 13Figure 11. (a) Estimated horizontal ground displacement from September to November 2000. (b)Estimated vertical ground displacement. (c) Gravity changes acquired from 11 – 14 September to 10– 12
November 2000. Units are in microGals. The reference site is located at the northern most point, as
designated by a star. (d) Computed gravity changes based on optimum model. See Table 3. Numbers inparentheses show the contribution from the deflation source.
vent, causing both the temporal increase in temperatureobserved near the vent and the volcanic glow in December
2000. The upwelling magma is also considered to be
responsible for the period of increased SO2 emission from
September to December 2000 and would have contributed
to the increase in gravity values.
[48] We thus developed a model to explain both gravity
changes and crustal deformation from September toNovember 2000 and carried out a joint inversion as inprevious sections. The weight is set at 10 mGal for thegravity data and 2 mGal for the absolute gravimeter. The
weight for the GPS data is set at 5 cm, considering the errors
in the estimate of ground displacement. The model consistsof a cylindrical body as in Figure 10 and a deflation source.
The latter element alone causes a contraction and a subsidence of the island. In contrast to the previous stages, theECV

8 - 14FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN

Table 3. Optimum Model Parameters for Figure 11
Parameter

Value
Deflation Source

Medium density,a kg/m3
Longitude
LatitudeDepth, m
Volume change, m3

2600
139310500 ± 300
34403000 ± 300
5300 ± 1000
1.1 ± 0.5  107
Cylindrical Body

Longitudeb
Latitudeb
Bottom depth,c mRadius, mDensity contrast,b kg/m3

1393105000
3450100
2500 ± 300
125 ± 50+1000aDensity inside the source is fixed.
Horizontal location and density increase of a cylinder body are fixed.
c
Top depth of cylinder body is assumed to correspond to sea level.
bground displacement after September 2000 indicates rather
uniform contraction, and thus we prescribed a single deflation source. In view of the temporal increase in the corrected
gravity from September 2000 (Figure 8), we interpret thatthe density increase occurred in the same region as thedensity decrease in the previous stage. Therefore the horizontal location of the cylindrical body is assumed to be thesame as that in Table 2. Moreover, assuming that the density
inside the conduit recovered from September 2000, theincrease in density is assumed to be +1000 kg/m3 in lightof the density change estimated in section 7.1 (Table 2). We
also suppose that a magma head was rising up to sea level
and that an increase in density occurred from sea level tosome unknown depth. We thus estimate the depth andvolume change of the deflation source and the radius anddepth of the cylindrical body.
[49] The computed changes for gravity and ground displacement are shown in Figures 11a, 11b, and 11d. The
optimum parameters are listed in Table 3. The estimatedradius and depth of the cylinder is in broad agreement withthat estimated in the previous section (Table 2), suggestingthat the model consistently describes the situation at Miyakejima in 2000. The numbers in parentheses in Figure 11d
are the contribution from the deflation source alone. We seethat the gravity increase after September 2000 was caused
by a density increase in the cylindrical body, which may
represent a magma refilling process.

8. Discussion
8.1. Mechanisms of Caldera Collapse[50] Many calderas are considered to have formed as aresult of collapse of the roof of the magma chamber[Williams, 1941], yet few such events have been witnessedhistorically [Simkin and Howard, 1970]. The caldera formation process at 2000 Miyakejima is the first caldera
collapse to be monitored intensively by modern measurement instruments. In light of the gravity changes in Figure
4c, it is highly probable that an empty space formed prior tothe caldera collapse. Total magnetic intensity data provide
further evidence for the existence of an empty space [Sasai
et al., 2001]. This scenario also explains why the groundsurface of the collapsed caldera on 9 July remained partiallyvegetated and why such a small volume of material was
erupted.

[51] We then have to consider when and how the empty
space was formed. Our speculative scenario is as follows.
During the initial deflation of the magma chamber andsubsequent dike intrusion, magma flowed out laterally fromthe magma chamber beneath Miyakejima to the offshore
crust, causing the earthquake swarms. The direction of dike
intrusion and the strike of the intense seismic swarms areperpendicular to the orientation of maximum tension stressin the oceanic crust [Nakamura et al., 1984], reminiscent of
the lateral magma flow characteristic in rift zone volcanoes[Sigurdsson and Sparks, 1978; Rubin and Pollard, 1987;
Lister and Kerr, 1991; Tilling and Dvorak, 1993]. It isinteresting that such a phenomenon occurred in an island arc
volcano. We interpret that the voluminous lateral flow ofmagma broke the lithostatic balance between the magma
chamber and surface crust, thereby forming a void in the
topmost region of the magma chamber, causing the chamberroof to collapse (Figure 12b). We believe that the formationof this empty space immediately precipitated the collapse.
The negative gravity changes detected on 6 July 2000
around the summit (Figure 4c) would therefore represent asnapshot of the upward propagating void, which eventuallycaused the small eruption on 8 July and collapse of thecaldera. The dramatic increase in seismic activity immediately below the volcano from around 4 July 2000 presumably corresponds to the initiation of internal collapse.
[52] The reason for the large lateral magma flow may be as
follows. It is generally accepted that there exists a level ofneutral buoyancy (LNB) at which ascending magma is in
gravitational equilibrium with the country rock. The highfracture resistance of the country rock allows the magma topool at the LNB to form a magma chamber. Without the effectof fracture resistance, magma will not pool and will instead
flow out laterally. The low viscosity of basaltic magma
allows volatiles formed during crystallization of the magma
to separate easily from the liquid, often triggering eruptions[Tait et al., 1989; Woods and Cardoso, 1997]. The eruptionsat Miyakejima over the past few centuries may have beencaused by such mechanisms (Figure 12a). We conjecture thatthe residual, denser volatile-depleted liquid must have accumulated in the magma chamber, unable to descend because
the liquid is still less dense than the underlying country rock.
In such circumstances, the liquid magma tends to spreadlaterally, similar to dike intrusions in rift zones [Rubin and
Pollard, 1987; Lister and Kerr, 1991], in mechanical balancewith the fracture resistance of the country rock (Figure 12a).
As the eruptions repeat many times, the volume of denser
liquid will increase, and the magma pressure may exceed thecritical fracture resistance of the country rock, allowing a lotof magma to flow out laterally. The recurrence period ofcaldera collapse may therefore be controlled by a balance
between the fracture resistance and magma pressure, with thelatter progressively increasing with repeated eruptions. The
previous caldera formation event at the Miyakejima volcano
is geologically estimated to be about 2500 years [Tsukui and
Suzuki, 1998]. We can therefore expect to develop a morequantitative model that considers the numerous competing
processes involved.
8.2. Explosive Basaltic Eruptions[53] Eruptions of basaltic volcanoes are known to be less
explosive than for silicic volcanoes, because the lower

FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPANECV

8 - 15Figure 12. Speculative scenario for the temporal evolution of unrest at the Miyakejima volcano. (a)Magma is in gravitational equilibrium at the level of neutral buoyancy, and the magma pressure ismechanically balanced with the fracture resistance of the country rock. Repeated episodic eruptions will
progressively increase the volume of denser liquid and cause the magma pressure to exceed the fracture
resistance of the country rock. (b) Lateral magma flow initiated internal collapse of a preexisting
cylindrical structure and caldera collapse on the surface. (c) Explosive eruption due to intense interaction
between magma and water from an ambient aquifer. (d) Magma ascent in the conduit. Volcanic gas
emission rate increased, and night volcanic glow is observed. As time passes, the ascended magma cools
and may eventually reach the initial stage.

viscosity of basaltic magma allows volatiles to segregate
easily. As observed in the 1983 eruption of Miyakejima[Aramaki et al., 1986], fire fountains and lava flows are also
typical in basaltic eruptions. However, the summit eruptions
since 14 July 2000 have been rather explosive. Neither fire
fountains nor lava flows were observed in the 2000 eruptions at Miyakejima.
[54] There are a number of reports of explosive eruptionsof basaltic volcanoes [Decker, 1987; Walker et al., 1984;
Aramaki et al., 1986; Dvorak, 1992]. In each case, theexplosivity is largely governed by interaction between surface water and magma [Wohletz, 1986; Koyaguchi andWoods, 1996]. The material erupted during the 2000 eruptions of Miyakejima was mostly fine ash [Nakada et al.,2001], which was probably generated by intense magmawater interaction and subsequent magma fragmentation.
[55] The optimum cylinder model was developed to
explain the spatiotemporal evolution of gravity (Figure 9c)
and may reveal how the explosive eruptions were generated
during the caldera collapse. In view of the inferred negative
density contrast, we speculate that a preexisting cylindricalstructure collapsed internally, and then began filling withwater from the ambient aquifer. Hot magma then came into
direct contact with the water, causing superheating andexpansion of the liquid water in the collapsed cylinder(Figures 12b and 12c). The collapsed cylinder would eventually become incapable of sustaining high gas pressure,leading to the observed explosive eruptions (Figure 12c).
[56] After the largest eruption on 18 August 2000, an
open conduit is expected to have formed. Magma could
then ascend easily from the chamber, effectively bringing
the lateral outflow to an end and increasing the local gravity

as noted above. As the magma head rose, the magma began
to degas (Figure 12d). This process may be viewed as arefilling of the magma reservoir. Although not related tocaldera collapse or explosive eruptions, Dzurisin et al.
[1980] proposed a similar model to explain gravity changes
associated with an M 7.2 earthquake at Kilauea volcano in
1975. Immediately following the earthquake, a void space
was created beneath the summit area as a result of magma
draining away into the rift zones. During the following
2 years, there were significant gravity increases in theabsence of comparable height changes, interpreted asreflecting the refilling of voids with magma.
[57] Decker [1987] argued that on the basis of observations of Kilauea volcano, major explosive eruptions appear
to be associated with caldera collapse. The 2000 unrest at
Miyakejima could be viewed as such an example. The
postulated mechanisms of caldera collapse and the explosive eruption at Miyakejima may be valid for all basaltic
volcanoes regardless of tectonic environment.

9. Conclusion[58] Employing both absolute and relative gravimeters,
we have been recording the spatiotemporal evolution of thegravity field of the Miyakejima volcano in Japan since June
1998. The observed gravity changes provide us with strong
evidence for the formation of an empty space beneath theisland prior to summit collapse. The appreciable gravity
changes immediately following the initiation of collapse canlargely be explained by a collapsed topography model,
involving an initially rapid rate of collapse without accompanying eruptions.

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8 - 16FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN[59] Correcting for the gravity changes due to the caldera
collapse at Miyakejima, we found a temporal decrease ingravity from July to late August 2000, coinciding with aperiod of intermittent explosive summit eruptions. This ismodeled as a temporal reduction in density caused by water

Table A1. Estimated Volume of Collapsed Caldera
Measurement Date

This Study

GSI

9 July 2000
22 July 2000
2 Aug. 2000
28 Sept. 2000
6 Oct. 2000
8 Nov. 2000
30 Nov. 2000
16 Jan. 2001

5.77
28.0
43.4
61.2
62.1
62.4
62.5
62.7

5.6
26.5
42.6
60.0
-

a
Units are in 107 m3. Estimate by GSI is given for comparison[Hasegawa et al., 2001].

inflow from an aquifer into the internally collapsing cylindrical space. We interpret that intense magma-water interaction caused the explosive summit eruptions. Since
September 2000, gravity values have continued to increase
despite only small ground displacements, interpreted as therefilling of magma into the conduit.

Appendix A

Figure A1. Temporal evolution of collapsed caldera at
Miyakejima volcano. (a) Original topography of the summit
area. Maps after the collapse of caldera are based on thesynthetic DTM, for (b) 9 July 2000, (c) 22 July 2000, (d) 2August 2000, (e) 28 September 2000, (f) 6 October 2000,(g) 8 November 2000, (h) 30 November 2000, and (i) 16
January 2001. (Copyright of original data is retained by the
Geographical Survey Institute (GSI) of Japan; copyright oforiginal data taken on 28 September 2000 is shared by GSI,
NEC corporation, and HONDA Airways.)[60] Here we discuss the derivation of the synthesized
DTM of the entire island (Figure A1). The mesh size of the
DTM is 10 m, and the positioning error of the control pointin the original GSI’s DTM is about 3.5 m horizontally and5.0 m vertically [Hasegawa et al., 2001]. As recognized by
GSI, however, there are spurious data in the vicinity of thecaldera border, which obviously prevents us from using theoriginal data as they are. Moreover, GSI’s original DTMs
for the collapsed area are only available for particularperiods. The area covered therefore depends on the measurement period. In order to correct gravity values for theeffect of temporal changes in topography, we need to use a
DTM that records the temporal changes in topography asaccurately as possible. We first synthesized the DTM for theentire island such that the caldera rim smoothly connects tothe flank of the volcano, performed manually by visualinspection. Next we computed the effect of topographychanges at each gravity site at the time of photogrammetry,the values of which are designated by circles in Figure 8.
Here we used an analytical formula for the gravitational
attraction of a rectangular parallelepiped [Banerjee and Das
Gupta, 1977]. For the end-members of crustal density, we
assumed 1900 and 2300 kg/m3. We modeled the temporal
evolution of caldera growth using an analytical form of (1 
exp(t/t)), and predicted gravity changes on those periodsof gravity measurement. The time constant t was estimatedto be 22 days. For assessment of the accuracy of DTM, weshow two independent estimates of cumulative collapsedvolume in Table A1. The estimated volumes at eachmeasurement date are within 5% of observed values, andthe estimated volume after September 2000 is within 2% of
observed values, demonstrating the accuracy of the model
into the period after cessation of caldera collapse.
[61] Acknowledgments. We are indebted to the staff of the Miyakejima Weather Station, Japan Meteorological Agency, Maritime Safety
Agency, Miyake Village, and the Tokyo Metropolitan Government for
support for gravity measurements. We thank A. Takagi, T. Hirasawa, and
Y. Hagiwara for encouragement, and acknowledge the Geographical Survey
Institute of Japan for GPS data and digital elevation maps that have been

FURUYA ET AL.: GRAVITY CHANGES AT MIYAKEJIMA VOLCANO, JAPAN
made publicly available for research purposes. This work was supported bya Special Coordination Fund for Promoting Science Technology granted in2000 and by the Tokyo Marine Kagami Memorial Foundation. Comments
by Associate Editor P. Roperch regarding early versions of manuscript andreviews by K. Ishihara and D. Dzurisin are greatly appreciated. Some of the
figures in this paper were prepared by GMT [Wessel and Smith, 1998].

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M. Furuya, J. Oikawa, S. Okubo, W. Sun, Y. Tanaka, and H. Watanabe,
Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyoku, Tokyo 113-0032, Japan. (furuya@eri.u-tokyo.ac.jp; oikawa@eri.
u-tokyo.ac.jp; okubo@eri.u-tokyo.ac.jp; sunw@eri.u-tokyo.ac.jp; ytanaka@
eri.u-tokyo.ac.jp; watanabe@eri.u-tokyo.ac.jp)
T. Maekawa, Graduate School of Science, Hokkaido University, Tatsuka
142, Sobetsu-cho, Usu-gun, Hokkaido 052-0106, Japan. (maekawa@
uvo.sci.hokudai.ac.jp)