Science - 06.12.2019

modulus and magma density) to vary to ac-
count for their uncertainties. From the MCMC
results and additional independent informa-
tion we computed other parameters of inter-
est, such as the rate of magma outflow from
the reservoir. Parameter estimates take the
form of probability density functions (PDFs),
which account for uncertainties in data and
prior information. We found that model out-
put is consistent with the withdrawal rate of
Kīlauea’s lava lake and the first-order tempo-
ral and spatial pattern of ground deformation
preceding caldera collapse (Fig. 6). We dis-
cuss our modeling results and implications
throughout the following sections.

Location and geometry of subcaldera
magma storage

Magma reservoir depth, volume, and geom-
etry play a direct role in the onset, style, and
duration of caldera collapse ( 15 , 18 – 21 ), but
magma storage beneath most calderas is
poorly understood and subject to controversy

( 22 , 23 ). Investigations at volcanoes that have

hosted historic caldera-forming eruptions sug-

gestthatstoragezonesmaybecomplexand

occur over a range of depths ( 3 , 24 – 28 ). We

found that geodetic data preceding caldera

collapse at Kīlauea in 2018 are consistent

with evacuation of magma from a storage

reservoir centered at ~2 km depth just east

of Halema‘uma‘u crater (Fig. 5 and table S2).

The estimated magma reservoir is somewhat

vertically elongated, as required to explain

the observed ratio of vertical to horizontal

displacements. The reservoir’s depth implies

an initial (pre-eruptive) magma pressure of

~45 MPa on the basis of the magmastatic lava

lake relationship together with prior con-

straint on magma density ( 8 ). To the extent

that magma density and lithostatic density

were similar, the open lava lake vent precludes

large magmatic overpressures before the onset

of the eruption ( 8 ).

In the past two millennia, two long-lived,

deep calderas have existed at the summit of

Kīlauea: one from ~200 BCE to ~1000 CE,

and the modern caldera, which formed in

~1500 CE and began refilling in ~1800 CE ( 29 ).

Magma storage beneath Kīlauea’s 1500 CE

caldera was inferred in the first written rec-

ords of the volcano nearly two centuries

ago ( 30 ) and explains subsidence associated

with rift zone intrusions and eruptions. At

least two persistent magma reservoirs—the

Halema‘uma‘u reservoir just east of Halem‘uma‘u

crater and another at greater depth beneath

the south part of the 1500 CE caldera—have

been hypothesized on the basis of geodetic and

other observations ( 6 , 31 – 38 ). Several transient

storage zones may also have existed ( 36 ), and

VLPseismic energy frequently emitted from

a source ~1 km beneath the northeast rim of

Halema‘uma‘u( 39 ) has been interpreted as

the intersection of north- and east-trending

dikes ( 11 , 40 ). The geometries and relation-

ships between these various magma storage

regions have been difficult to interpret, and

in some cases appear to change over time.

Andersonet al.,Science 366 , eaaz1822 (2019) 6 December 2019 3of10

Fig. 2. Spatial pattern of subsidence at K ̄laueaı ’ssummitin2018.(A)Ground
tilt overlaid on an ascending-mode COSMO-SkyMed interferogram spanning 6 to
10 May 2018 (table S1). Colored dots show observed tilt, and black arrows show
best-fitting tilt velocities used for modeling. Each complete InSAR color fringe
represents 1.55 cm of displacement in the look direction of the satellite
(T symbol, 26.6° from vertical). Small-scale irregularities in the fringe pattern are
evident in the caldera. Background shaded digital elevation model (DEM) shows

Kīlauea’s summit in 2009, similar to its appearance in April 2018. (B) Observed

GPS displacements (colored dots) and best-fitting velocities (black arrows)

overlaid on the unwrapped interferogram from (A). An active lava lake was nested

within Halema‘uma‘u crater, itself nested in the larger 1500 CE Kīlauea caldera.

LoS, line of sight. (C) West-east profiles of LoS COMSO-SkyMed InSAR velocities

approximately through the center of Halema‘uma‘u crater. Profiles differ because

of different look angles. (D) View of GPS data in (B), looking north.

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