Tonga Slab Research Articles

Elasticity of phase D and implication for the degree of hydration of deep subducted slabs

Seismic anomalies in subducted slabs, including low velocity zones and shear wave splitting, have often been related to hydrous regions. Phase D (MgSi2H2O6, 10–18 wt.% H2O) may be the ultimate water carrier in hydrous subducted peridotite and its seismic properties are thus essential for interpreting observed anomalies in terms of hydration. Here, we report the sound velocities and elasticity of Mg‐ and Al‐Fe‐bearing phase D single‐crystals measured by Brillouin spectroscopy. The room conditions adiabatic bulk and shear modulus are: KS0 = 154.8(3.2) GPa and μ = 104.3(2.1) GPa for Mg‐phase D and KS0 = 158.4(3.9) GPa and μ = 104.7(2.7) GPa for AlFe‐phase D, suggesting minor effect of cation substitution on the elasticity of phase D. Based on the seismic velocity data, we found that 16 vol.% of AlFe‐phase D in hydrous subducted peridotite with 1.2 wt.% H2O could provide a plausible explanation for the negative velocity anomalies of 3% observed in fragments of the Tonga slab below the transition zone.

Geophysical and mineralogical constraints on the post-spinel transformation for the Tonga slab

The depth of the post-spinel transformation is investigated for the Tonga slab, by using data from short period seismic networks at western United States and Japan for about 100 deep and intermediate-depth earthquakes within a small (∼200km by 200km) area near 20°S. Later phases in a time window ∼3 to 20s after direct P waves are analyzed to search for S-to-P converted waves at the 660km discontinuity, which represents the post-spinel transformation. We find that immediately beneath the foci of the deepest earthquakes the discontinuity is depressed down to the depths of 685±5km on average, and that it dips towards WNW by 10±3km within 70km laterally. We constrain the thermal structure near the S to P conversion points based on a plausible assumption that the deepest earthquakes occur around the coldest core of the Tonga slab. The distribution of the hypocenters relocated in this study as well as previously published tomographic images of the same region suggest that the Tonga slab bends upward when approaching the 660km discontinuity and transiently stagnates around the discontinuity. With these observations as the constraints, we numerically model the thermal structure of the Tonga slab, and estimate the temperature around the conversion points as 1200±100°C, which is 300±100K colder than the surrounding mantle. As the average depression of the discontinuity (down to 685±5 km) corresponds to an pressure excess over the global average (660km) by 1.0±0.2GPa, the assumption of equilibrium post-spinel transformation results in an estimate of the Clapeyron slope (C1) of -3.3-2.7+1.3MPa/K. We also obtain an independent estimate of the Clapeyron slope (C2) of −2.0±1.0MPa/K, based on the observation of the dip of the discontinuity and the computed temperature variation (by about 200K). The discrepancy between C1 and C2 is marginally significant and can be diminished by considering that the slab materials at the conversion points are currently descending across the phase boundary fast enough and thus the depth of the post-spinel transformation is controlled by nucleation kinetics as well as by the temperature.

Emergent and animated COLLADA models of the Tonga Trench and Samoa Archipelago: Implications for geoscience modeling, education, and research

We report on a project aimed at developing emergent animated COLLADA (collaborative design activity) models of the Tonga-Samoa region of the western Pacific for teaching and outreach use with Google Earth. This is an area of historical importance to the development of plate tectonic theory and is important today owing to neotectonic activity, including a 29 September 2009 tsunamigenic earthquake. We created three types of models: an emergent digital elevation model of the Tonga slab with associated magmatic arc and backarc basin based on GeoMapApp (Marine Geoscience Data System, Lamont-Doherty Earth Observatory) data mining; animated models of alternative plate tectonic scenarios; and a large-scale model that permits users to view the subsurface down to lower mantle levels. Our models have been deployed in non-science major laboratory classes, and positive learning outcomes were documented in an independent study by S. Wild and J. Gobert. The models have also been made available to colleagues and the public via the Old Dominion University Pretlow Planetarium and an outreach and dissemination website (http://www.digitalplanet.org). In the process of constructing a complete set of tectonic models for the area of interest, we added cases that have not been described in the research literature. Thus, this study spans the three functions of modern academia, i.e., research, teaching, and outreach, and the multifaceted aspects of creating, using, testing, and disseminating electronic geospatial learning resources.

A Kinematic Thermal Model for Descending Slabs with Velocity Boundary Layers: A Case Study for the Tonga Subducting Slab

Abstract:For the purpose of investigating the influence of metastable olivine (MO) phase transformations on both deep seismicity and stagnation of slabs, we constructed a 2‐dimensional finite element thermal model for a 120 Ma‐old 50° dipping oceanic lithosphere descending at 10 cm/yr with velocity boundary layers, which would mitigate the interference of constant velocity field for the slab. The resulting temperatures show that most of intermediate and deep earthquakes occurring within the Tonga slab are occurring inside the 800°C and 1200°C isotherm, respectively. The elevation of olivine transformation near ∼410 km and respective persistence of metastable olivine and spinel within the transition zone and beneath 660 km would thus result in bimodal positive, zonal, negative density anomalies, respectively. These results together with the resulting pressure anomalies may reflect the stress pattern of the Tonga slab: (i) slab pull force exerts above a depth of ∼230 km; (ii) MO existence changes the buoyancy force within the transition zone and facilitates slab stagnation at a depth of 660 km; (iii) as the subducting materials accumulated over 660 km, deepest earthquakes occur due to MO transformation; (iv) a flattened ‘slab’ may penetrate into the lower mantle due to the density increment of Sp transformation.

Upper and mid-mantle anisotropy beneath the Tonga slab

] Measurements of source‐side splitting in S waves fromevents within the Tonga slab reveal anisotropy in the upperand mid‐mantle beneath the slab. We observed splitting forevents originating at both upper mantle and transition zonedepths. Anisotropic fast directions ( ) are trench parallel orsub‐parallel for both upper mantle and transition zoneevents.Delaytimes(dt)decreasewithdepthforuppermantleevents. The source of anisotropy for the upper mantle eventsis likely in the sub‐slab mantle, and is likely indicative oftrench parallel flow due to slab rollback. The source ofanisotropy for the deeper earthquakes is more difficult toconstrain, but the pattern of splitting measurements arguesfor an uppermost lower mantle anisotropic source, and slabinduced deformation may be responsible for this anisotropyas well. Additional constraints from mineral physics studiesare necessary to interpret the mid‐mantle anisotropy signalin terms of geodynamical processes.

Slab stress and strain rate as constraints on global mantle flow

Dynamically consistent global models of mantle convection with plates are developed that are consistent with detailed constraints on the state of stress and strain rate from deep focus earthquakes. Models that best fit plateness criteria and plate motion data have strong slabs that have high stresses. The regions containing the MW 8.3 Bolivia and MW 7.6 Tonga 1994 events are considered in detail. Modeled stress orientations match stress patterns from earthquake focal mechanisms. A yield stress of at least 100 MPa is required to fit plate motions and matches the minimum stress requirement obtained from the stress drop for the Bolivia 1994 deep focus event. The minimum strain rate determined from seismic moment release in the Tonga slab provides an upper limit of ∼200 MPa on the strength in the slab.

Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10-12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine's high-pressure polymorphs, wadsleyite and ringwoodite, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes) west of and within the subducting Tonga slab and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.

Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab

The mineral akimotoite, ilmenite-structured MgSiO(3), exists at the bottom of the Earth's mantle transition zone and within the uppermost lower mantle, especially under low-temperature conditions. Akimotoite is thought to be a major constituent of the harzburgite layer of subducting slabs, and the most anisotropic mineral in the mantle transition zone. It has been predicted that if akimotoite crystals are preferentially oriented by plastic deformation, a cold subducted slab would be extremely anisotropic. However, there have been no studies of crystallographic preferred orientations and very few reports of plastic deformation experiments for MgSiO(3) ilmenite. Here we present plastic deformation experiments on polycrystalline akimotoite, which were conducted at confining pressures of 20-22 GPa and temperatures of 1,000-1,300 degrees C. We found a change in crystallographic preferred orientation pattern of akimotoite with temperature, where the c-axis maximum parallel to the compression direction develops at high temperature, whereas the c axes are preferentially oriented parallel to the shear direction or perpendicular to the compression direction at lower temperature. The previously reported difference in compressional-wave seismic anisotropy between the northern and southern segments of the Tonga slab at depths of the mantle transition zone can conceivably be attributed to the difference in the crystallographic preferred orientation pattern of akimotoite at varying temperature within the slab.

238U‐230Th‐226Ra‐210Pb‐210Po, 232Th‐228Ra, and 235U‐231Pa constraints on the ages and petrogenesis of Vailulu'u and Malumalu Lavas, Samoa

We report 238U‐230Th‐226Ra‐210Pb‐210Po, 232Th‐228Ra and 235U‐231Pa measurements for a suite of 14 geologically and geochemically well‐characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent‐daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas (<8 Ka) resurfacing both its summit crater and lower flanks. 210Pb and 210Po measurements indicate that several flows have erupted within its summit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half‐lives of 230Th, 231Pa, and 226Ra, we interpret their 238U ‐230Th, 235U‐231Pa and 230Th ‐ 226Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (238U/232Th) and (230Th/232Th) is attributed to long‐term source variation. The Samoan basalts are all alkaline basalts and show significant 230Th and 231Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (230Th/238U), (231Pa/235U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that (230Th/238U) and (231Pa/235U) of many OIBS can be explained by simple time‐independent models. Interpretation of the 226Ra data requires time‐dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U‐Th‐Ra and U‐Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant 210Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these 210Pb deficits. The young coeval volcanism on Malumalu and Vailulu'u suggests the Samoa hot spot is currently migrating to the northeast due to dynamic interaction with the Tonga slab.

Pervasive Seismic Wave Reflectivity and Metasomatism of the Tonga Mantle Wedge

Subduction zones play critical roles in the recycling of oceanic lithosphere and the generation of continental crust. Seismic imaging can reveal structures associated with key dynamic processes occurring in the upper-mantle wedge above the sinking oceanic slab. Three-dimensional images of reflecting interfaces throughout the upper-mantle wedge above the subducting Tonga slab were obtained by migration of teleseismic recordings of underside P- and S-wave reflections. Laterally continuous weak reflectors with tens of kilometers of topography were detected at depths near 90, 125, 200, 250, 300, 330, 390, 410, and 450 kilometers. P- and S-wave impedances decreased at the 330-kilometer and 450-kilometer reflectors, and S-wave impedance decreased near 200 kilometers in the vicinity of the slab and near 390 kilometers, just above the global 410-kilometer increase. The pervasive seismic reflectivity results from phase transitions and compositional zonation associated with extensive metasomatism involving slab-derived fluids rising through the wedge.

Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high‐frequency content for precise mapping of discontinuity topography and sharpness. To enhance the low‐amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant stacked. For the 410‐km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660‐km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660‐km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660‐km discontinuity in terms of local temperatures implies a thermal anomaly of −800°K to −1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660‐km discontinuity region implies that the Tonga slab seems to penetrate the 660‐km discontinuity with little deformation. Waveform modeling suggests that both the 410‐ and 660‐km discontinuities are sharp. The 660‐km discontinuity is at most 2 km thick in many parts of the region, and a first‐order discontinuity cannot be precluded. The 410‐km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab. This suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities.

Stresses along the metastable wedge of olivine in a subducting slab: possible explanation for the Tonga double seismic layer

A numerical calculation of the stresses associated with changes in volume during phase transitions of olivine in a descending slab results in a double layer of high shear stress along the metastable olivine wedge in the depth range 350–460 km. Stress in the upper layer is in-plane tensional and stress in the lower layer is down-dip compressional. The modeled stress field agrees with observations of stress in the Tonga double seismic zone. High shear stress also exists in the slab at depths below the metastable wedge. This stress distribution involves down-dip compression and trench-parallel tension, which agrees with about half of the focal mechanisms in the Tonga slab at depths of 460–690 km. The model supports the idea that at least two possible stress release mechanisms for deep earthquakes may act in the Tonga subducting slab. One, transformational faulting, is restricted to the metastable wedge while the other one acts below the metastable wedge.

Seismological constraints on the mechanism of deep earthquakes: temperature dependence of deep earthquake source properties

Seismological observations are of vital importance for understanding the mechanism of deep earthquakes. Most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes. Deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms. Deep earthquake aftershock sequences show temporal decay rates and magnitude–frequency relations ( b-values) that are similar to shallow earthquakes, with the aftershocks occurring preferentially along the mainshock fault planes. In addition, deep earthquake rupture velocities are similar to those of shallow earthquakes. However, there are also some observations that are clearly distinctive relative to shallow earthquakes. Stress drops of deep earthquakes show a large variation but are larger, on average, than shallow earthquakes. Different deep seismic zones show very different b-values, in contrast to shallow earthquakes, which show similar b-values worldwide. In addition, deep earthquakes show fewer aftershocks than shallow earthquakes. A variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab. Both deep earthquake b-values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs, suggesting that these factors are temperature-controlled to an extent much greater than with shallow earthquakes. Large deep earthquakes in warm slabs show slower rupture velocities, larger stress drops, and lower seismic efficiencies than similar earthquakes in cold slabs. The width of deep seismic zones is also probably temperature controlled, but deep earthquake rupture can propagate outside the normal limits of Benioff zone seismicity. Simple thermal models for the Tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200°C warmer than the temperature that limits smaller earthquakes in the slab. These observations can be used to evaluate physical models for deep earthquakes, including brittle slip along fluid-weakened faults, transformational faulting, and thermal (and perhaps melt lubricated) shear instabilities. It is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model, since hydrated materials are expected in only a narrow depth zone at the top of the slab, and the fault planes do not have the expected orientations for reactivated faults. The lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model, in which events should be confined within a narrow metastable wedge. Seismological studies have also failed to find evidence for the existence of metastable olivine in slabs. The temperature dependence of deep earthquakes argues in favor of a temperature-activated phenomenon, such as thermal shear instabilities and perhaps fault zone melting.

Repeating deep earthquakes: evidence for fault reactivation at great depth.

We have identified three groups of deep earthquakes showing nearly identical waveforms in the Tonga slab. Relocation with a cross-correlation method shows that each cluster is composed of 10 to 30 earthquakes along a plane 10 to 30 kilometers in length. Some of the earthquakes are colocated, demonstrating repeated rupture of the same fault, and one pair of events shows identical rupture complexity, suggesting that the temporal and spatial rupture pattern was repeated. Recurrence intervals show an inverse time distribution, indicating a strong temporal control over fault reactivation. Runaway thermal shear instabilities may explain temporally clustered earthquakes with similar waveforms located along slip zones weakened by shear heating. Earthquake doublets that occur within a few hours are consistent with events recurring before the thermal energy of the initial rupture can diffuse away.

Upper mantle discontinuity structure in the region of the Tonga Subduction Zone

We study the mantle structure below the southwest Pacific in order to examine the geometry of the Tonga slab at depth and its interaction with the 410‐ and 660‐km discontinuities (hereafter called the 410 and the 660). We utilize data from stations of both the Lau Basin Ocean Bottom Seismogram experiment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zone and the associated Lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions. We produce a high‐resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subducting slab. Our results show the 410 uplifted by 30 km near the Tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions.

Tonga slab deformation: The influence of a lower mantle upwelling on a slab in a young subduction zone

There are fundamental geographic variations in the deformation of slabs in the transition zone. The seismic energy release and morphology of the Tonga slab show that it is deforming faster and has accumulated more deformation than any other slab. We show that Tonga overlies the edge of the large‐scale Pacific superplume. There is no substantial aseismic penetration into the lower mantle beneath Tonga, consistent with initiation of subduction during the Eocene. Other major subduction systems overlay seismically fast structures. For long‐lived subduction systems, the lower mantle tends to pull down on slabs while in Tonga the lower mantle pushes upward, partially accounting for the intense deformation. The perturbation to the state of slab stress due to large‐scale mantle flow is 10 to 40 MPa — nearly as large as that expected from slab pull.

Defocusing of teleseismic P‐waves by the Tonga‐Kermadec Slab

We analyze P‐wave amplitude reduction for Tonga earthquakes observed at the SKIPPY broadband seismograph network in Australia. The negative amplitude anomaly diminishes with a focal depth, suggesting that the anomaly is due to defocusing caused by the fast Tonga slab. The SKIPPY network covers the distance range where the effect of the Tonga slab is most pronounced. We calculate theoretical amplitudes using simplified slab models based on previous delay time tomography, and show that the theoretical amplitude anomaly reproduces the observations reasonably well. A 100‐km thick slab provides a better fit to the data than a 50‐km thick slab. A thin slab penetrating deep into the lower mantle is inconsistent with the pattern of amplitude variation with distance.

Constraints on the origin of slab and mantle wedge anomalies in Tonga from the ratio of S to P velocities

We examine two prominent upper mantle velocity anomalies in the southwest Pacific, the Tonga slab anomaly and the corresponding overlying mantle wedge anomaly, using data collected during a combined land‐sea deployment of temporary seismometers. The linear geometry and small interstation spacing of the instruments yield high‐resolution data along a cross section of the Tonga subduction zone, including the actively spreading Lau back arc basin. We estimate the relative variation of P and S velocity, often described as ν = δlnVs/δlnVp, for the slab and mantle wedge anomalies using two distinct methods: a linear regression of the P and S travel time residuals, and detailed modeling of the velocity structure using a three‐dimensional finite difference travel time algorithm. The two methods yield similar results, with ν of the slab being 1.1–1.5 and ν of the mantle wedge being 1.2–1.3. These values are consistent with experimental data concerning the effect of temperature on P and S wave velocities in the upper mantle and are lower than what is expected for velocity anomalies generated by the presence of partial melt. These observations imply that either the theoretical estimates of ν for partial melt are too large or very little partial melt is present beneath the Lau basin. In the latter case, melt must be quickly removed from the rock matrix, such that the velocity anomalies are due to increased temperature, and not melt. The bulk of the velocity anomaly in the mantle wedge can be explained by temperature anomalies of 400–600°C because of the amplification of temperature derivatives of seismic velocity by anelastic effects. Such large thermal anomalies, generated by decreased lithospheric thickness and mantle upwelling beneath the fast spreading Lau back arc basin, can still leave the mantle near the solidus, even after accounting for the effect of increased volatile content in the mantle wedge. The lower‐amplitude velocity reductions in the deeper wedge are likely related to an increased concentration of volatiles from the subducting slab.

Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

The occurrence of the March 9, 1994 (Mw 7.6) deep Tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab. Near vertical ScS reverberations from the 410, 520, and 660 discontinuities are visible at 10 stations in the South Pacific at frequencies between 0.003–0.033 Hz without stacking. We use a regional velocity model and ray theory to model the interval from ScS to ScS3. A grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections. Stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab, and 410 km, 500 km, and 640 km to the east of the slab. Analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the Tonga slab, in a region of possible near horizontal slab. These results are consistent with a large cool region just to the west of the intersection of the Tonga slab with the 660.

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S‐P method measures the difference in spectral decay between P and S waves arriving at the same station. The P‐P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two‐dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S‐P and P‐P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S‐P and P‐P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.