elasticity and density

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B12, PAGES 30,809-30,820, DECEMBER 10, 2001 A broadband seismic study of the lowermost mantle beneath Mexico- Constraints on ultralow velocity zone elasticity and density Emily Havens and Justin Revenaugh Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz California, USA Abstract. Broadband seismic stacking is performed for PcP waves sampling the lowermost mantle and core-mantle boundary beneath central Mexico and the western Gulf of Mexico. The data contain evidence of a 10to 20-km-thick ultralow velocity zone on the western edge of the study area, tapering (or vanishing) to the east to less than 5-kin thickness over a horizontal distance of 200 kin. Epicentral distance varies from 20 ø to 60 ø, a range over which the amplitude of reflections from the top of the ultralow velocity zone responds strongly to changes in the ratio of shear to compressional velocity reduction (r = 51n v,/sln vp) and to density within the zone. Where the zone is present and thick enough to separate PcP and the precursory reflection, our modeling is consistent with a value of r near 3 and 51n vp near 10%, assuming only minor density increase (1 to 2%). The v lue of r is reduced to 2 for a - 14% density increase, but 51n vp remains large (7%). Very low amplitude multiply converted phases are synthetically well reproduced with extreme attenuation within the zone, suggesting the presence of a melt component. 1. Introduction The recent discovery of thin lenses or zones of ultralow velocity (ULV) material at the base of the mantle [e.g., Garnero and Helmberger, 1995; Mori and Helmberger, 1995; Garnero and Helmberger, 1996; Revenaugh and Meyer, 1997; Wen and Helmberger, 1998] is driving a rethinking of the dynamics of the mantle and core and their geochemical evolution. Whether this rethinking swells into a sea change or passes as a ripple hinges on the origin and extent of ULV mantle. Current thought is concentrated on three nonexclusive origin hypotheses: partial melt produced in situ by eutectic melting or accumulated as mantle "rain" [Williams and Garnero, 1996]; silicate-iron reaction products produced at the core-mantle boundary (CMB) [Knittle and Jeanloz, 1991; Manga and Jeanloz, 1996]; or enrichment in slabderived material. Of these, partial melt is the most frequently invoked origin in seismological modeling studies and, as such, has the appearance of being the favored origin of ULV mantle despite a paucity of discriminating observations in the literature. The primary constraint on ULV mantle is a 5 to 15% vp decrement inferred from SPdiffKS delays [e.g., Gatnero and Helmberger, 1996] and the amplitudes of PKP [ Vidale and Hedlin, 1998] and PcP precursors [Mori and Copyfight 2001 by the American Geophysical Union. Paper number 2000JB /01/2000JB Helmberger, 1995; Revenaugh and Meyer, 1997]. Partial melt can readily account for this reduction with melt fractions between 5 and 40%, depending on the geometry of melt distribution [Williams and Garnero, 1996]. Silicate-iron reaction products appear limited to smaller vp decrements (_<4%) unless seismically slow FeO and FeSi are segregated from the seismically fast silicate products (MgSiO3 perovskite and SiO2), in which case decrements as large as 10% are possible [Garnero et al., 1998]. Slab-derived material (former basaltic crust and harzburgite) appears to have elastic properties quite similar to mean lowermost mantle [Garnero et al., 1998]. However, greater calcium, aluminum, and volatile component concentrations may predispose slab-derived material to partial melting, such that it remains a candidate. An alternative hypothesis, one which obviates the need for any ULV mantle material, posits a transitional CMB zone, rather than a sharp boundary, and/or a thin layer of finite rigidity material topping the outer core [Garnero and Jeanloz, 2000; Buffett et al., 2000]. Given the overlap in 5 In vp predicted by the different ULV mantle hypotheses, additional seismic constraints are needed. A fundamental characteristic of partial melt is high r = 5 In v,/5 In vp, with theoretical predictions suggesting a value of r near 3 that exhibits little dependence on the distribution (grain boundary film or tubes) of melt or melt fraction [Williams and Garnero, 1996; Berryman, 2000]. By comparison, silicate-iron reaction products are predicted to produce r values near 2 [Garnero et al., 1998]. Because density p (which ex- 30,809

2 30,810 HAVENS AND REVENAUGH: BROADBAND REFLECTIVITY BENEATH MEXICO hibits greater contrast between these two end-members) in this catalog were kept on the basis of source simplicis more difficult to constrain seismically, r is the primary discriminant. There have been few attempts to constrain r of ULV mantle. Revenaugh and Meyer [1997] model PcP precursors produced by topside reflection from a thin ULV ity, the signal quality of direct P, and spatial density of recording. PcP core bounce points, computed by using the iasp91 model of Kennett and Engdahl [1991], populate a thin band measuring about 200 in longitude and 50 in latilayer for three patches of the CMB beneath the Pacific tude and are situated beneath central Mexico and the and favor values of r between 3 and 5. That study, however, examines precursors at long offsets ( ) western Gulf of Mexico (Plate 1). We split this band into six contiguous bins, each measuring in longiwhere precursor amplitude dependence on $ In v is sec- tude by 50 in latitude, in an attempt to image possible ond order in comparison with $ In vp. As a result, the data admit other solutions, including high-density layers with r values near unity and small $ In vp. The overwhelming majority of observations of ULV mantle come via the core diffracted phase SPdiffKS east-west variations in basal mantle structure. Bin size is chosen to be commensurate with the first Fresnel zone at the CMB (reckoned at 0.5 Hz) but large enough to insure high data density (/,25 traces) in all bins. Earthquakes in the data set were recorded on up to [Garnero and Helmberger, 1995, 1996, 1998; Wen and 19 of 22 broadband stations. The average event gather Helmberger, 1998; Helmberger et al., 2000]. SPdiffKS spans a distance range of 100 and has 12 traces after travel time is highly sensitive to basal mantle structure traces with unusual noise levels or recording glitches and can be a much less subtle indicator of ULV man- in the time window of interest are winnowed. Followtle than low-amplitude PcP precursors. While the data ing instrument deconvolution, the vertical component are consistent with r = 3 [Wen and Helmberger, 1998], traces in each event gather are aligned on P by serial cross correlation and summed to obtain an aver- extensive trade-offs between velocity, density, and ULV zone geometry and thickness make it difficult to bound r without independent constraints [Garnero and Helmberger, 1998]. Given the highly heterogeneous nature of ULV mantle and the limited inventory and coverage of body waves sensitive to ULV structure, such additional constraints are hard to obtain. Here we present evidence of a ULV layer beneath Mexico gathered from a broadband stacking study of PcP waveforms recorded at epicentral distances between 200 and 600 where sensitivity to r is strong and trade-offs between $1n vp, density, and ULV zone geometry are lessened. The primary trade-off occurs between $1n v and density. Based on stacked waveform modeling alone, r m 2 or 3 is equally likely with $ In p m 14% or 1%, respectively; $ In vp is large (_>7%) in either case. While any constraint on density is valuable, the r-p trade-off curve unfortunately passes through both the partial melt and silicate-iron reaction prod- mantle. age source time function waveform that is subsequently deconvolved to reduce source time function complexity. Ideally, deconvolution spikes the P arrival. In practice, local and range-dependent propagation effects and finite source bandwidth impart some ringiness, but this varies from trace to trace and from event to event and sums out quickly to produce a band-limited impulse. Apart from the addition of instrument deconvolution, this align-stack-deconvolve process is identical to Revenaugh and Meyer [1997] and Reasoner and Revenaugh [1999]. To extract robust phase and amplitude information from low-amplitude, low signal-to-noise ratio (SNR) core reflections and precursors, we use the double-array stacking technique of Yamada and Nakani hi [1996] and Revenau#h and Meyer [1997]. For each event, we stack over the predicted arrival time surface for a P-to-P refiection from depth d (hereinaftereferred to as PdP). To eliminate biases due to variable source excitation, we normalize traces to unit P amplitude prior to stacking and adjust the stack by the average ratio of P to PdP source excitation determined from the Harvard Cen- ucts end-members. Other considerations, including apparent severe attenuation within the layer, tomographic studies of the overlying mantle [e.g., Grand et al., 1997; van der Hilst et al., 1997], and geodynamic arguments, offer some constraints and lead us to favor, albeit not troid Moment Tensor catalog [e.g., Dziewonski et al., overwhelmingly, partial melt as the local origin of ULV 1981]. For m stations and n events, the stacking equa- 2. Data and Methods Our data consist of 52 Central American and north- tions are m y i=1 j----1 ernmost South American earthquakes recorded by broadband seismographs in the western United States from rn n -, P d P 1990 through These events were selected from a larger catalog collected from the Incorporated Research '---- j----1 E//J Institutions for Seismology (IRIS) database consisting of all Fast Archive Recovery Method (FARM) format where E/ is the excitation of P, --ij E ra/' (d) is the excidata with mo > 5.5 and correct backazimuth. Events tation of PdP, ro is the arrival time of P on aligned (1) ' (2)

3 HAVENS AND REVENAUGH' BROADBAND REFLECTIVITY BENEATH MEXICO 30,811 dln v s !! 4*4.,j,

4 .. --,,, 30,812 HAVENS AND REVENAUGH' BROADBAND REFLECTIVITY BENEATH MEXICO seismograms, rif the model-predicted travel time of p, "ij _Palp(d) is the model-predicted travel time of PdP, and Sij(t) is the recording of event j at station i. We correct for the mean source excitation ratio of PcP to PdP, rather than individual event ratios, to avoid the amplification of low SNR data when the ratio is large; wi effects a weighting by SNR in the 10-s window preceding P. The result of (1), R(d), is an estimate of the topside P reflection coefficient of a potential reflector at depth d [Revenaugh and Meyer, 1997] biased by the omission of spreading and attenuation terms and by the use of a reference one-dimensional velocity model that neither is "tuned" to data nor has a discontinuity at d. These and other known sources of bias are mod- eled explicitly as discussed below. We note that we exclude data windows contaminated by upper mantle phases such as PnPn and SnSn. 3. Modeling Each of the six bins in Plate i is analyzed separately. For each bin, we produce two waveform stacks. The ( ) # Records: ' ' ' ' t # Stations: 17 o.1 o.o " o.o ø -0.1 ø3-0.1 # Records' 74? # Stations' 14. i t d '". :"; :.:,; ' ' ,,,:f'.;,,, '; ' i,.,.. '...,!.:, I, I I, I, E ( ) [ ' [ #, Records: [, [ 76 '., # Stations' 18 - i (Ci)[ ' [ ' [ ' [ ' - # Records: 28. '. t' '..?; # Stations: 9 _- : ' [ ' [ ' [ ' [ ' (e) # Records: 454 (f) 0.2 / ' [ ' [,,,,. j 0.2.., I, I, I. I # Stations: 154 [. /' # Stations: 15. 'o '". i 'i..:. :. :...? ' -01 0'2' ' ' ' '.o.1 02 I ' ' ' ' ' ' Height above CMB (kin) Height above CMB (kin) Figure 1. Stacks of low-pass-filtered data (thin lines) moving from (a) bin 1 on the west to (f) bin 6 on the eastern edge of sampling evaluated at 2-km-depth intervals. Shaded region represents the 95% confidence interval as estimated by using bootstrap resampling. Values in the upperight corner of the panels indicate the number of seismograms and the number of stations included in the stack at the CMB. Neither number varies significantly for other depths. Also shown are iasp91-prem syntheuc stacks (thick lines). The general west-to-east increase PcP amplitude reflects an increase mean epicentral distance in the bins.

5 HAVENS AND REVENAUGH: BROADBAND REFLECTIVITY BENEATH MEXICO 30,813 (a) 0.15 f ' i ', ' A 0.05 o.oo 0.00 [ I [ I [ O0-50 [ I [ I 0.15, [, [, 0.15 (c) 'ii::ii::i ::i!', I [ I , I, I ' I ' ( 0.05!Q o Figure 2. Full bandwidth (, 0 to 0.75 Hz) stacks (thin lines) for bins (a) 1 through (r) 6; all other conventions as in Figure 1. Synthetic stacks (thick lines) have been shifted in depth to best match the observations. Precursory energy to PeP is indicated by arrows and resembles a positive polarity ramp-like structure superposed on the PeP sidelobe. Note how the synthetics overpredict the amplitude of PeP in all bins and fail to match precursor levels in the three western bins. first uses data low-pass filtered below 0.2 Hz and serves as a guide to overall reflectivity in the lowermost mantie. These low-pass stacks reveal no strong evidence of reflectors in the lowermost 200 km of the mantle apart from the CMB and associated structures (Figure 1), although there is a hint of a positive polarity peak near 120-km depth in most stacks that appears to be separate from the sidelobe of the much stronger PcP peak. With the exception of bin 4, PcP amplitude varies predictably with mean offset which increases from west to east across the six bins. The second stack uses the full data bandwidth, allowing for much better separation of PcP and PdP (Figure 2). Precursory energy is indicated in the figure and is associated with lowermost mantle structure. We model broadband stacks with M6'eller's [1985] reflectivity method. Synthetic seismograms are generated for an explosive source at 40-km depth, the mean source depth of the data catalog, below a simple crust with iasp91 velocities and densities adapted from the Pre- liminary Reference Earth Model (PREM) [Dziewonski and Anderson, 1981]. Synthetics are generated at increments, and synthetic stacks are formed by resampling the synthetics according to the distance distribu-

6 ._,,.. 30,814 HAVENS AND REVENAUGH: BROADBAND REFLECTIVITY BENEATH MEXICO (a) 0.15 f '! '! ' t , o.oo,, I I I I, Figure 3. (a) Synthetic stack (bold line) for r = 2 plotted atop the data stack for bin 2. The synthetic was computed for a discontinuity 20 km above the CMB with In v r = -10% and no change in density. The small positive polarity precursor is almost undetectable. Same as in Figure 3a excepthat the discontinuity has a smaller P wave velocity decrement ( In v r - -8%) and a density increase of 16%. This synthetic better matches the relative amplitude of PcP and the precursor. tion of each bin. The explosive source excites P, PcP, and PdP equally, such that no source corrections are required for the synthetic stack. The resindual influence of extended source time functions and limited data band- width is introduced into the synthetic waveform stacks by convolution of each seismogram with the average deconvolved P waveform of all seismograms in the bounce point bin. To mimic the effects of mantle heterogene- ity, the individual traces are given travel time shifts drawn from a zero-mean Gaussian distribution. We find that a standard deviation of rr < 0.8 s has little effect on the appearance of the synthetic stacks which match the width of peaks in the data stacks. For rr > 0.8 s, the change is more noticeable, causing stack peaks to shorten significantly and become overly broad, prompting us to set rr = 0.4 s. All other details of stacking E (a) 0'15 f ',!,.0.05 o.oo O0 (c) I, I, Figure 4. Best fit synthetic stacks (bold lines) for the bins (a) 1 through (c) 3 computed for r- 2 (see Table i for model details). All other conventions as in Figure 2.

7 HAVENS AND REVENAUGH: BROADBAND REFLECTIVITY BENEATH MEXICO 30,815 are equivalento (1). This approach, in particular the use of resampling rather than explicit computation of all seismograms in the model stack, has the considerable advantage of being fast, allowing us to evaluate many more models than otherwise would be possible. Comparison of model stacks computed in this fashion with stacks exactly mimicking data geometry is highly favorable; stacking efficiently damps variability due to source depth and small distance mismatches. As a first modeling stage, we compute synthetic stacks for the iasp91 velocity model with mantle and core density from PREM. Overpredicting PcP amplitude and failing to match R(d) variation in the lowermost 30 km above the CMB, the iasp91 stacks fall outside the 95% confidence intervals of the westernmost sections (Figure 2). To produce the precursory energy and better match the shape and amplitude of PcP, we introduce a thin layer at the base of the iasp91 mantle with velocities reduced from iasp91 (increased velocities within the layer produce precursors with amplitudes larger than PcP for some distances in the study region). Thickness, d lnvp, r, density, and attenuation are varied to match PcP and precursor amplitude and separation. Although some studies have fixed r at i [e.g., Car- nero and Helmberger, 1996], for the distance range of this study, we find that a r value of unity produces a negative polarity precursor, rather than the positive polarity precursor seen in Figure 2, unless ( lnp is quite large (> 25%). For the three bins with obvious precursory energy (all on the western end of sampling), polarity of the precursor is positive, mandating a greater r value. A positive polarity precursor is obtained for r - 2, dlnvp - -10%, and PREM density, but the modeled precursor is too small (Figure 3a). Increasing ( lnvp and maintaining P REM density and r - 2 fail to match the PcP and precursor wave-forms. To raise the amplitude of the precursor while maintaining PcP amplitude, we increase the density of the ULV layer above PREM. Tuning the drop in P velocity and the increase in density while maintaining r - 2 yields synthetic predictions within the 95% confidence limit for 81nvp - -8% and 81np- +16% (Figure 3b). Data and stacks with r - 2 for the three westernmost bins are shown in Figure 4. A thick layer ( 40 km) with elasticity and density perturbations similar to bin 2 is required to model bin 4 (not shown), a point we return to later. Bins 5 and 6 are not shown in Figure 4 because they have no obvious precursory energy and do not require a ULV layer. Increasing r from 2 to 3 lowers PcP amplitude and increases precursor amplitude, similar to the the effect of increasing density in the ULV layer when r - 2. This obviates the need for a large density increase within the layer, and good fits to data are obtained with density increases _ 4%. Data and preferred synthetic stacks for each of the six bins when r - 3 are shown in Figure 5. The preferred models for r- 2 and r- 3 were determined after examination of several hundred combi- nations of d In vp, density increase, and layer thickness. Our criteria were visual with specific reference to the amplitude ratio of PdP to PcP, the timing of the ULV layer reflection, if present, and the coreside slope of the PcP stack peak, these aspects of the stacked waveforms being judged most robust and informative. Standard numerical measures of fit were too strongly influenced by PcP peak amplitude mismatches. For each value of r, it was common for several combinations of layer thicknesses and elasticity variations to provide roughly the same level of fit. Estimates of parameter uncertainty, conditioned by the assumption of r and taken from the range of acceptable model fits, are of the order of +3 km for layer thickness and +3% for velocity and density. The inferred P wave velocity drops and layer thicknesses are consistent with estimates for other geographic locations obtained from PcP precursors at longer offsets [Mori and Helmberger, 1995; Rcvenaugh and Meyer, 1997] and numerous SPdiffKS measurements [Garnero and Helmberger, 1995, 1996, 1998; Helmberger et al., 2000]. In each of the westernmost three bins, our best fitting synthetic stacks for r - 2 or 3 fail to match early PcP coda (mapped to depths below the CMB by the stacking algorithm). Energy in the coda is the sum of P-to-S-to-P conversions and P reverberations within the layer, the latter of which contribute little to the stack. Additional complexity is added by crustal conversions and reverberations that are incompletely eliminated by P stack deconvolution. To minimize the phase conversions in synthetic seismograms, we damp $ waves in the layer by setting Q/ to 5 and Q to 20. Partial melt typically implies low Q/ [Mavko, 1980]; so we have applied this to models where r - 3. Reaction products models (r - 2) also fit better with low Q/, but high attenuation is not a direct consequence of these models unless melt is present. The improvement in fit to early coda, where the residual effects of crustal structure are minimal, is dramatic (Figure 6), but the attenuation required is extreme and not accurately modeled by the linear attenuation model embedded in the reflectivity algorithm. Lacking a self-consistent synthetic calculation and laboratory evidence for extreme attenuation within highpressure silicate partial melts, we do not advocate Q/ = 5, but we do believe that the lack of multiply converted energy requires, at a minimum, strong attenuation within the ULV layer. Other explanations we have investigated include destructive interference caused by variations in layer thickness between conversion points, finite core rigidity, and gradational boundaries. Of these, core rigidity has the greatest potential to reduce the amplitude of S wave reflections from the CMB. Preliminary experiments suggest that an outermost core vs of i km/s is needed to achieve a 33% diminishment of amplitude. Avoiding the introduction of additional arrivals not seen in the data stacks requires either a thick finite rigidity zone (_>20 km) or severe P wave attenu-

8 ,_, 30,816 HAVENS AND REVENAUGH' BROADBAND REFLECTIVITY BENEATH MEXICO 0.00 (a)' ' ' '.! ' 0.05 o.oo i -5O, I, I,, I, I,! (c) (d).:.:.;..:'.,.; :, 0.15 I, I, I I O..,>,, '" : :'... '" o.oo -5O I, I, Figure 5. Best fit synthetic stacks (bold lines) for bins (a) 1 through (f) 6 computed for r = 3 (see Table 1 for model details). All other conventions as in Figure 2. Bins 5 (Figure 5e) and 6 (Figure 5f) both include a synthetic stack computed for a 5-km-thick ULV layer with 5 In vp = -5% and a 1% density increase. Inclusion of a muted and thin ULV layer reduces peak PcP amplitude but does not produce a distinct precursor. Similar reduction in PcP amplitude is produced by CMB gradient zones, other values of r, and increased attenuation in the lower mantle. ation within it. The former would be difficult to maintain as the thickness of the layer exceeds the amplitude of CMB topography [e.g., Garcia and Souriau, 2000], leaving the layer vulnerable to general core circulation. The attenuation required by the latter, assuming the finite rigidity zone is limited to several kilometers of thickness, is as extreme as the ULV zone attenuation. These constraints limit the ability of finite core rigidity to augment or substitute for strong attenuation within the ULV layer, but they do not completery rule it out. In general, the inclusion of finite core rigidity mandates a small reduction in 5 In vp within the ULV layer and little or no change to density to maintain fit to PcP and its precursor. In addition to sharply delineated ULV layer models, we also attempt to model data with gradient zones. We find that a gradient zone of 5-km thickness at the top of the ULV zone fits the data as well as a sharp boundary (Figure 7). Thicker gradient zones mimic the ef- fect of reducing the velocity decrement, increasing the amplitude of PcP above the 95% confidence limits and decreasing precursor amplitude. This can be offset in part by further decreasing velocities in the layer. The trade-off is not complete, however, and we find that

9 HAVENS AND REVENAUGH: BROADBAND REFLECTIVITY BENEATH MEXICO 30,817 gradients _20 km are incompatible with observations. stacks nearly identical to the iasp91 synthetic stacks, Persh et al. [2001] found no evidence of a ULV layer in and near bins 1 through 4 in short-period PcP recordings for event/station geometriesimilar to ours. This suggests that the upper boundary of the ULV layer is diffuse, damping short-period reflections, but less than 20 km in thickness (assuming a linear gradient). For the sake of simplicity in our subsequent modeling we providing a limit on our resolution. Although not explicitly modeled, synthetics with r = 2 or 3 could include a small (<2 km) CMB gradient without altering the shape of the waveforms. Table I details the best fitting models for the first four bins. The three westernmost bins are broadly similar, all featuring significant wave speed decrements and will assume a sharp discontinuity, recognizing that the a ULV zone some 10 to 20 km thick. The two eastvelocity decrements we infer are conservative. As was mentioned in the introduction, a gradational CMB has been suggested as an alternative to a ULV ernmost bins, although mutually similar, differ greatly from the westernmost three. No ULV zone is required for these bins (Figure 2), although we have chosen to layer. Recent research based on $PdiffK$ data admits the possibility that the CMB may be spread over several kilometers, creating a sort of core-mantle zone rather than a core-mantle boundary [Garnero and Jeanloz, 2000]. We test this in our modeling by creating a linear gradient between lower mantle and core values of vr, v, and p and find that even the best fit models, a 3-km gradient CMB in each case, fall outside of the 95% confidence limits (Figure 8). It is possible that by tuning the individual forms of the velocity and density transitions, we could obtain fits to data comparable to Figures 5 and 6, but to fully model the multiple arrivals seen in data stacks would require gradient zones com- parable in thickness to our r- 2 and 3 layer models. Gradient zones <2 km in thickness produce synthetic model them with thin, muted ULV layers in order to best match PcP amplitude in Figure 5. Note that they can be equally well matched by thin layers with other r values or by a thin CMB transition zone or simply by reduced Q in the lower mantle and no basal mantle structure. We note that Castle and van der Hilst [2000] found no evidence of a ULV layer beneath bins 5 and 6 using short-period $cp, although their resolution of such structures was limited by low SNR. The fact that the thin ULV layers modeled for bins 5 and 6 in Figure 5 produce no apparent precursors gives an indication of the minimum detectable structure. The bin connecting the western and eastern ends of CMB sampling, bin 4, resembles neither of its neighbors. The r = 3 model providing the best fit features 0.1 I I, o.oo :: %..., iii '0'05f, I, I (c) 0.00 i Figure 6. Synthetic stacks (bold lines) for bins (a) 1 through (c) 3 computed for r = 3 and severe attenuation within the ULV layer. Models for bins 1 (( lnv r = -10% and ( ln v = -30%) and 2 (ULV layer thickness of 20 kin) are changed from Table 1. All other conventions as in Figure 2. The improvement in fit is considerable; compare with Figures 5a through 5c.

10 30,818 HAVENS AND REVENAUGH. BROADBAND REFLECTIVITY BENEATH MEXICO 0.1 '1:3, (a) 2 m Helmberger, 1998]; so we are hesitant to place much emphasis on this result. The effects of three-dimensional structure are doubtless present in the adjoining bins as well, but the consistency of structure from bin to bin and the good fits obtained by simple models afford us much more confidence in our interpretations of these stacks. It would be interesting to model the effects of a sharp transition in ULV layer thickness on PcP and compare that with bin [ -50 0'15 f (c) ' [ [ ' I I, I, ' kin CMB CMB CMB 4. Discussion We find observational evidence of a ULV layer beneath central Mexico fading in thickness and/or severity toward the east over a distance range of 200 km. We have modeled these observations under three different paradigms: partial melt, silicate-iron reaction products, and a gradational CMB. Simple models providing good fits to data are found for the first two, both involving a discrete layer of ULV material atop the CMB. In an attempt to discriminate between these two models, we examine additional lines of evidence. We find that low Q in the ULV layer greatly improves fits to early PcP coda by damping $ wave energy in the layer and thereby eliminating late-arriving P-to-S-to-P converted phases. High attenuation is a logical consequence of partial melt [Mavko, 1980]; solidstate reaction products do not necessarily imply low Q, but it is possible that core-mantle reaction products are themselves more readily melted, resulting in a hybrid melt/reaction product model. Such mixtures would be difficult to distinguish from our end-members. While the implication of severe attenuation is intriguing, it is not diagnostic. Tomographic models of the lowermost mantle provide an important regional context (e.g., Plate 1), and both P and S wave tomographic images of the lower- most mantle display a west to east increase in velocity across the study area [Grand et al., 1997; van der Hilst Figure 7. (a) Bin 2 stack with the synthetic (bold line) computed for a 20-km-thick ULV layer with r = 3 et al., 1997]. This increase mirrors eastward thinning of and In vp = - 10% superposed. Similar compar- the ULV zone by 20 km. This transition occurs over a ison, but synthetic has a 5-km-thick gradient between distance of about 200 km, implying an east-west slope of normal and ULV mantle atop the CMB. (c) Same as about 6 ø. Our modeling shows that a layer of reaction in Figure 7b but the gradient zone is now 10 km thick. products would require a density approximately 15% The midpoint of the transition remains 20 km above higher than ambient lower mantle density. A 60 slope the CMB in all three models. All other conventions as is large and would be difficult to maintain in such highin Figure 2. density material, requiring an energetic mechanism of a 40-km-thick layer with vp and v drops of 15% and 45%, respectively, making it km thicker than the westernmost bins and comparable to the strongest vp reductions reported in the literature [Vidale and Hedlin, 1998]. However, this bin occupies an apparentransi- tion from a thick ULV zone on the western end of sampling to muted or absent ULV material at the eastern end. Strong structural variation at scale lengths below the first Fresnel zone of the data can lead to consid- erable bias in a one-dimensional model [e.g., Wen and Table 1. Best Fit Models. In vr, In v, In p, Thickness, Bin ' % % % km I

11 HAVENS AND REVENAUGH: BROADBAND REFLECTIVITY BENEATH MEXICO 30,819 I ' o.oo I -50 I km A 3km,... 5km I.'... 10km ] Figure 8. (a) Bin 2 with the synthetic stack (bold line) computed for a 3-km-thick linear CMB transition zone. The synthetic fails to produce precursory energy but does model early PcP coda well. All other conventions as in Figure 2. CMB gradient zone models of variable thickness. Arrivals are tightly clustered for transition zones less than 5 km thick. For greater thicknesses the first arriving phase is PcP, followed by weaker phase conversions. piling reaction products. The geography of the ULV layer suggests an east to west sweeping flow across the sampled region. The tomographic models support this direction, with lower velocities implying higher temperstructure inferred for bin 4 is potentially reconcilable with this scenario if advective thickening of the ULV material pile produces a welt at the edge of the subducting slab. This might result in a region of little or atures in the west and possible assembly of upwelling no ULV material beneath the slab on the eastern marflow. However, the velocity gradient is also supportive gin of sampling abutted by a greatly thickened pile just of a partial melt origin, as there is a thick ULV zone outboard of the slab with more modest accumulations on the low-velocity, presumably warmer, end, and little further to the west. or no ULV material on the high-velocity, presumably cooler, end. Williams et al. [1998] document a correlation between the locations of hot spot volcanism on the sur- Tomographic models reveal faster than average ( 1%) face and the locations of ULV mantle at the CMB. velocities throughouthe study area, creating dilemmas The Guadeloupe hot spot, located at 18.7øN, 111.0øW for both candidate hypotheses. A generally cool lowermost mantle limits the ability of local upwelling to advectively thicken a reaction product pile. On the other hand, local impingement of a subducting slab on the CMB [Grand et al., 1997] could push basal mantle ma- [Sleep, 1990], lies just off the western side of the study area where we propose ULV mantle is present, and is thus consistent with the global correlation. The underlying question of source provenance of this low-flux hot spot is, however, beyond the scope of this report. terial outward, causing it to accumulate on the margins of flow. The 14% density increase implied by our r = 2 Acknowledgments. This research was supported by models, however, substantially exceeds that of the de- NSF grant EAR The authors wish to thank Colin Reasoner for considerable assistance in the synthetic modelscending slab. Unless the mass of the slab can act to ing and Steven Persh and Ed Garnero for insightful reviews. push aside the ULV material, slab impingement would Center for the Study of Imaging and Dynamics of the Earth not appear capable of steeply piling ultradense reaction (formerly the Institute of Tectonics) contribution 442. products. If the ULV zone is due to the accumulation of partial References melt, one of several scenarios must be in place. The sim- Berryman, J. G., Seismic velocity decrement ratios for replest invokes compositional heterogeneity to depress the gions of partial melt in the lower mantle, Geophys. Res. eutectic, enabling melt at lower temperatures. A possi- Lett.,, 7, , ble candidate is enhanced water content, although other Bufferr, B., E. Garnero and R. Jeanloz, Sediments at the excursions from mantle mean composition might lower top of Earth's core, Science, œ90, , Castle, J. C. and R. D. van der Hilst, The core-mantle the eutectic temperature. A second scenari once again boundary under the Gulf of Alaska: No ULVZ for shear appeals to the recent encounter of a cold slab with the waves, Earth Planet. Sci. Lett., 176, , CMB, displacing warm material atop the ULV layer and Dziewonski, A.M. and D. L. Anderson, Preliminary referforcing outward the underlying partially molten ULV mantle. In this scenario, advective thickening of the ence Earth model, Phys. Earth Planet. Inter.,, $, , ULV material pile outpaces conductive cooling, produc- Dziewonski, A.M., T. A. Chou, and J. H. Woodhouse, Detemination of earthquake source parameters from waveing a detectable, albeit ephemeral, ULV zone. In broad form data for studies of global and regional seismicity, J. outline, this conceptual model follows the numerical re- Geophys. Res., 86, , suits of Sidorin et al. [1999]. We note that the unusual Garcia, R., and A. Souriau, Amplitude of the core-mantle

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