The reservoir location and geometry we esti-
mate here lead us to conclude that magma
withdrawal from the Halema‘uma‘u reser-
voir was responsible for observed ground sub-
sidence in 2018.
Misfits between model predictions and
geodetic data provide additional insight into
magma storage (Fig. 6). Our model closely fits
lava lake withdrawal rate data but cannot ac-
count for small-scale features observed in the
InSAR data (fig. S7), nor can it explain the
very-high-quality GPS data to within formal
uncertainties. Material heterogeneity such as
preexisting faults and altered rocks, localized
shallow magma storage, or irregularities in the
top of the reservoir itself may be responsible
for these features [we scale data uncertainties
to account for these limitations ( 8 )]. The mod-
el also inadequately accounts for subsidence
observed south of the caldera. This likely re-
flects the early stages of magma drainage from
Kīlauea’s deeper and more enigmatic south
caldera reservoir. Ground deformation believed
to be due to magma evacuation from this
reservoir increased in cumulative magnitude
and spatial extent through June and July and
continued after the cessation of the eruption
(presumably as magma drained to refill the
ERZ). However, most of the deformation during
our modeled time period can be attributed to
the Halema‘uma‘u reservoir (predicted defor-
mation from the model reduces variance in
modeled InSAR scenes by 93 to 96%).Volume of magma storage
The volume of magma stored beneath a vol-
cano exerts a primary control on nearly all
aspects of volcanic activity, including limit-
ing the size of an eruption and any possible
caldera collapse. Yet, magma storage volumes
are very poorly known at almost all of Earth’s
volcanoes. Intensive study at Kīlauea overprevious years has yielded estimates for the
Halema‘uma‘u reservoir varying over two
orders of magnitude [from 0.2 to >20 km^3
( 6 , 34 , 41 – 44 )].
In general, geodetic data can be used to re-
solve the quantityVp
=mfor a magma reservoir,
whereVis reservoir volume,p
is pressure
change rate, andmis host rock shear modulus,
but not these terms independently ( 45 ). Our
parameter estimation resolvedVby using con-
straints onp
from the lava lake data (below)
and onmfrom previous studies ( 6 , 41 ). Be-
causep
is much more tightly constrained
thanm,wewereabletoresolvetheratio
V=m≅ 1 : 3 T 0 :15 m^3 =Pa ( 8 )(fig.S16).Thisimplies
that reservoir volume should be of the same
order as the rigidity of the host rock. The
combination of spatially dense geodetic data
with the finite-source model used in our study
provided additional constraint on reservoir
volume ( 45 ), and the maximum size of the
reservoir was geometrically limited by its
depth and shape (both resolved geodetically).
We found that 2.5 to 7.2 km^3 of magma (at
68% credible bounds) was stored beneath the
summit of the volcano in the Halema‘uma‘u
reservoir at the beginning of May 2018. The
upper bound should be considered only ap-
proximate; volumes of 10 km^3 or even larger
cannotstrictlyberuledoutbythedata,par-
ticularlyifwerelaxapriorilimitsonthe
presenceofmagmastorageatveryshallow
depths (<750 m) ( 8 ). On the other hand,
volumes of <1 km^3 are improbable, because
smaller reservoirs cannot explain the high
rate of observed ground deformation with-
out requiring an unreasonably weak host
rock(pressurechangerateistightlyconstrained
by the lava lake data). Precollapse storage vol-
umes for other basaltic calderas are not well
known, but our calculated volume is far
smaller than that of reservoirs inferred to have
supplied large silicic caldera-forming eruptions.Rate of magma depressurization and drainage
Reservoir pressure change ratep
is con-
strained in our parameter estimation by the
observed rate of lava lakewithdrawal, the prior
distribution on lava lake density, and the mag-
mastatic assumption ( 8 ). Thus,p
is insensitive
to geodetic data and modeling. We estimated
that pressure in the reservoir decreased at
1.25 ± 0.09 MPa/day (Fig. 5B) after theMw
6.9 earthquake. At this rate, pressure at the
reservoir’s centroid would have decreased
to atmospheric (an impossibility) by early
June. Continuation of the eruption at a high
rate for 3 months therefore required an in-
crease of reservoir pressure through collapse
of the overlying rock. This mechanism is also
consistent with surges in effusion rate after col-
lapses later in the eruption ( 16 ).
ThevolumetricrateofcontractionV
of the
magma reservoir and the volumetric rateqAndersonet al.,Science 366 , eaaz1822 (2019) 6 December 2019 4of10
Fig. 3. Withdrawal of K ̄laueaı ’s lava lake in early May.(A) Thermal images of the lava lake surface taken
from the south rim of Halema‘uma‘u crater while the lake was draining. (B) Time series of change in lava lake
surface height relative to 26 April, and radially outward low-pass–filtered ground tilt at UWD. Time series
after 5 May are shown in Fig. 4. Numbers correspond to acquisition times of images in (A). (C) Photograph
showing the lava lake on 6 May and the laser rangefinder used to measure its surface height. (D) Relationship
between lava lake surface height and radially outward tilt (withMw6.9 earthquake offset approximately
removed). At all stations, the ratio decreased by ~40% around the time of theMw6.9 earthquake, denoted by
the horizontal gray line. Correlation coefficients are denoted by r.
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