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DOI: 10.1126/science.1180165, 1388 (2009);326Science

et al.Cecily J. WolfeMantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Sp

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Mantle Shear-Wave Velocity StructureBeneath the Hawaiian Hot SpotCecily J. Wolfe,1* Sean C. Solomon,2 Gabi Laske,3 John A. Collins,4 Robert S. Detrick,4

John A. Orcutt,3 David Bercovici,5 Erik H. Hauri2

Defining the mantle structure that lies beneath hot spots is important for revealing their depthof origin. Three-dimensional images of shear-wave velocity beneath the Hawaiian Islands, obtainedfrom a network of sea-floor and land seismometers, show an upper-mantle low-velocity anomalythat is elongated in the direction of the island chain and surrounded by a parabola-shapedhigh-velocity anomaly. Low velocities continue downward to the mantle transition zone between410 and 660 kilometers depth, a result that is in agreement with prior observations of transition-zone thinning. The inclusion of SKS observations extends the resolution downward to a depth of1500 kilometers and reveals a several-hundred-kilometer-wide region of low velocities beneathand southeast of Hawaii. These images suggest that the Hawaiian hot spot is the result of anupwelling high-temperature plume from the lower mantle.

Hawaii is the archetypal hot spot and has

been suggested to be the surface expres-

sion of a mantle plume: a localized up-

welling of hot buoyant material from Earths deep

mantle (1, 2), although such an origin has beendebated. The Hawaiian-Emperor chain of islands

and seamounts spans thousands of kilometers;

records age-progressive volcanism for >75 mil-

lion years (3); and exhibits a broad ~1000-km-

wide region of elevated topography, known as

the Hawaiian Swell (Fig. 1A), that surrounds the

locus of current volcanism. Estimated to transport

a larger buoyancy flux than any other active plume

(4, 5), the Hawaiian hot spot has been the focus

of numerous geochemical and geodynamical stu-

dies [for example, (410)] that attempted to re-

solve its origin. One of the most straightforward

indications of whether a hot spot is the result of a

plume is the presence of a narrow, vertically con-tinuous zone of low seismic velocities in the un-

derlying mantle, indicative of higher-than-normal

temperatures, anomalous mantle composition, or

melt (11, 12).

Global seismic tomography suggests that low

seismic velocities may extend from the upper to

the lower mantle beneath the Hawaiian hot spot

(1315), but there remains doubt about how well,

and to what depth, narrow plumes may be resolved

by such methods (16, 17). Moreover, global mod-

els of mantle structure are limited in their resolu-

tion near Hawaii by the sparseness of wave paths,

given the lack of seismic stations on the ocean

floor and the great distances between the islands

and most circum-Pacific earthquake sources. Pre-

vious investigations of mantle structure nearHawaii

using island stations (1820) or sea-floor deploy-

ments of instruments (21, 22) have been limited

in geographic coverage and numbers of instru-

ments.Such experimentshave not yielded the high-resolution regional tomographic models needed

to settle the debate over even such a basic ques-

tion as whether the upper mantle beneath the

Hawaiian Islands is marked by low seismic ve-

locities and higher-than-normal temperatures, as

suggested by some studies (1315, 19),or not,as

implied by others [for example, (23)].

Here we report results from the Hawaiian

Plume-Lithosphere Undersea Melt Experiment

(PLUME), which was designed to determine at

high resolution the mantle seismic velocity struc-

ture beneath the Hawaiian hot spot, to assess the

hypothesis that the hot spot is the product of an

upwelling plume, and to determine how mantleflow may interact with the overlying lithosphere

to generate the Hawaiian Swell. The experim

featured a dense, large-aperture seismic netwo

consisting of two year-long deployments of th

dozen broadband ocean-bottom seismomet

(OBSs) and the concurrent deployment of

portable broadband seismometers on the Haw

ian Islands (Fig. 1A), with all instruments op

ating continuously to record teleseismic and lo

earthquakes. During the experiment period, 21

S-wave relative arrival times (direct S and S

phases) were collected from 97 earthquakes (2(fig. S1) and corrected for estimated variations

station elevation and crustal thickness. Correc

mean station delay patterns reflect upper-man

heterogeneity and indicate relatively low vel

ities beneath and to the west of the Hawai

Islands and high velocities east of Hawaii and

distant stations around the margins of the sw

(Fig. 1B).

The arrival times were inverted forS-wave

locity heterogeneity, damped earthquake relo

tions, and origin time terms using finite-frequen

(13, 25) methods. Because of the large stati

spacing, crossing wave paths were lacking, a

vertical resolution was limited in the uppermmantle. One strategy to mitigate the effect of sm

ing strong, shallow heterogeneity deeper int

model is to include station terms to partially

sorb the effect of shallow structure, but this a

proach comes at the expense of decreased resolut

at shallow depths, and tests indicate that stat

terms may not successfully absorb structure

depths of 100 to 200 km if such variations

extremely strong. We therefore performed inv

sions both with (Fig. 2 and fig. S6) and witho

(Fig. 2 and figs. S7 and S8) station terms. In bo

cases, a low-velocity anomaly was well resolv

in the upper mantle beneath Hawaii and w

elongated in a southeast-northwest direction pallel to the Hawaiian Islands. As expected,

1Hawaii Institute of Geophysics and Planetology, University ofHawaii at Manoa, Honolulu, HI 96822, USA. 2Department ofTerrestrial Magnetism, Carnegie Institution of Washington,Washington, DC 20015, USA. 3Cecil H. and Ida M. GreenInstitute of Geophysics and Planetary Physics, Scripps Institu-tion of Oceanography, San Diego, CA 92093, USA. 4WoodsHole Oceanographic Institution, Woods Hole, MA 02543, USA.5Department of Geology and Geophysics, Yale University, NewHaven, CT 06520, USA.

*To whom correspondence should be addressed. E-mail:[email protected]

165 160 155 150

15

20

25

+2 s

2 s

B

165 160 155 150

15

20

25

A

5000 2500 0 2500

m

Fig. 1. (A) Locations of seismometers used in S-wave tomography. Land stations are indicated by btriangles and OBSs are indicated by red (first deployment; 2005 to 2006) or yellow (second deployme2006 to 2007) circles. Only stations that successfully recorded two horizontal components are showBathymetry is taken from (30). (B) Corrected mean station delays, adjusted to vertical incidence. Eaarrivals are shown by blue circles and late arrivals by red triangles, with symbol size scaled linearly to tmagnitude of the delay (see scale at lower left).

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amplitude of upper-mantle heterogeneity was larger

in inversions without station terms. A parabola-

shaped region of higher-than-average seismic ve-

locity along the edge of the Hawaiian Swell was

also observed to extend downward to 300 km

depth, although patterns outside its boundaries

were constrained only to the southeast, where sta-

tion coverage extends beyond its edge. Low ve-

locities beneath Hawaii continue into the transition

zone, and inversions resolved a broad region of

low velocities in the lower mantle from 700 to

1500 km depth southeast of Hawaii (Fig. 3).

The vertical extent of upper mantle structure

is not well constrained by ourS-wave data alone,

as illustrated by a two-step inversion (fig. S9). In

the first step, we inverted for a model in which all

structure was squeezedbetween 50 and 250 km

depth. In the second step, we corrected the ob-

servations for the effects of the squeezed model

and inverted for an additional residual model.

The two-step model continued to recover a lo

velocity anomaly in the transition zone, and

model deeper than 500 km remained virtually u

changed, but much of the heterogeneity at 300

depth can be squeezed to shallower depths. Giv

the tradeoffs between the amplitude and the de

interval of heterogeneity, however, lateral v

iations at 100 to 200 km depth in this two-s

model were extremely large atT8%. Although r

onciliation with surface-wave observations

needed to provide firm limits on the lateral vartion in upper-mantle seismic velocities and h

resolve these ambiguities, prior surface-wave o

servations (21) and analysis of PLUME data

date indicate mantle S-wave velocity hetero

neity of no more than T4% in the depth inter

from 60 to 140 km and are thus inconsistent w

the large variations recovered in the two-s

model.

SKSwaves have steeper arrival angles th

direct Swaves do and thus provide wave pa

that sample the lower mantle beneath Hawaii (

S2). To test how the subset ofSKSdata contr

utes to the resolution of lower-mantle structu

we performed inversions in which SKS phawere excluded (figs. S11 and S12). DirectSwav

from the first deployment dominantly constrain

the low-velocity anomaly in the upper mantle

neath and around the main Hawaiian Islands, b

these inversions had less structure in the man

transition zone (410 to 660 km depth) and neg

gible lower mantle structure (fig. S11). Includ

directS-wave data from the second OBS deplo

ment extended the spatial region of the model a

improved resolution of the low-velocity anom

in the upper mantle and transition zone (fig. S1

but lower-mantle heterogeneity remained sm

These tests indicate that low velocities in thelow

mantle beneath Hawaii are not artifacts of vertismearing of strong upper-mantle heterogene

but instead are driven by the resolving power

SKSarrival times. The existence and southeast

position of the lower-mantle anomaly constrain

bySKSdata were affected, however, by the so

tion for shallower upper-mantle structure (2

Heterogeneitydeeperthan 1500 km is not requi

by our data alone, but the eventual incorporat

of PLUME data into global models may exte

the resolution to greater depths. Montelli et

(13, 14) imaged a broad region of low velocit

beneath Hawaii that extended downward to 1900

depth in their global S-wave model and to

core/mantle boundary in theirP-wave model. O

low-velocity anomaly is narrower than the ~1

wide anomaly displayedby Montelli etal. (13, 1

which is probably an indication of the improv

horizontal resolution provided by the PLUM

data. Resolution tests conducted with a wide

riety of cylindrical and checkerboard structu

(figs. S16 to S28) demonstrate that the PLUM

data set resolves both upper- and lower-man

structure at the level of our interpretations. O

possible biasing effect on our solutions in

lower mantle, however, may come from un

termined structure near the core/mantle bound

station terms (s )

-2 -1 1 2

26

24

22

20

18

16

164 162 160 158 156 154 152 150 148 146

depth = 100 km

Longitude West

Latitude

-1.5 - 1.0 - 0.5 0.0 0 .5 1 .0 1.5

S-wave % velocity anomaly

26

24

22

20

18

16

164 162 160 158 156 154 152 150 148 146

depth = 300 km

Longitude West

Latitude

-1.5 - 1.0 - 0.5 0.0 0 .5 1 .0 1.5


26

24

22

20

18

16

164 162 160 158 156 154 152 150 148 146

depth = 400 km

Longitude West

Latitu

de

-1.5 - 1.0 - 0.5 0.0 0 .5 1 .0 1.5


26

24

22

20

18

16

164 162 160 158 156 154 152 150 148 146

depth = 100 km

Longitude West

Latitude

-4 .0 -2.7 -1 .3 0.0 1.3 2.7 4 .0


26

24

22

20

18

16

164 162 160 158 156 154 152 150 148 146

depth = 300 km

Longitude West

Latitude

-2 .0 -1.3 -0 .7 0.0 0.7 1.3 2 .0


26

24

22

20

18

16

164 162 160 158 156 154 152 150 148 146

depth = 400 km

Longitude West

Latitu

de

-2 .0 -1.3 -0 .7 0.0 0.7 1.3 2 .0


Fig. 2. Upper-mantle velocity heterogeneity at (top) 100, (middle) 300, and (bottom) 400 km depth.The scale of heterogeneity is indicated in the upper right corner of each panel. The left column displayssolutions from an inversion with station terms; the right column shows the results of an inversion withoutstation terms.

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(24), where steep lateral velocity gradients have

been observed at the edges of the Pacific large

low-shear-velocity province (26).

Our results improve the fidelity of S-wave

mantle imaging around Hawaii as compared withprevious efforts and suggest that the upper man-

tle beneath the islands is characterized by low

seismic velocities and, by inference, anomalously

high temperatures. The peak-to-peak 3% veloc-

ity variations that we imaged at 300 km depth

in inversions with station terms corresponds to a

temperature variation across the model of 250C,

and the 1.5% contrast near 900 km depth cor-

responds to a temperature variation of 300C

(24). These values are consistent with prior pet-

rologic and geodynamic inferences of a high-

temperature plume beneath the Hawaiian hot

spot (46, 10, 27).

In terms of plume dynamics, the broad, low-velocity anomaly at shallow mantle depths (

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