Evidence of a ULVZ near Vanuatu from Sdiff postcursors

Evidence of a ULVZ near Vanuatu from Sdiff postcursors1 Carl Martin1∗, Lobke Harmsma1, James Atkins2, Arwen Deuss1, Sanne Cottaar2 2 1Department of Geosciences, Utrecht University, 3584 CB, Netherlands3 2Bullard Laboratories, Department of Earth Sciences, University of Cambridge, CB3 0EZ,4 UK5 *Corresponding author ([email protected])6 Key points7 •We present new evidence for an ultra-low velocity zone (ULVZ) on the CMB to the southeast8 of Vanuatu using Sdiff postcursors (Sdiff+) from earthquakes in the South Pacific Rise region9 towards seismic arrays in East Asia.10 •The data are explained by a quasi-cylindrical ULVZ located at 172.2±0.9°E and 22.9±1.1°S11 with height 20 ±5 km, radius 240 ±50 km, and shear wave velocity reduction 30 ±5%.12 •Combined with previous studies using ScP and SPdKS in the area, this is a broad-scale region13 with a patchwork of ULVZs comparable in variety to Hawaii.14 •The Vanuatu ULVZ lies within the Pacific large low velocity province, and might potentially15 be a root to a mantle plume that deflected towards the eastern Australian hotspots around16 the Tonga slab.17 1 1 Abstract18 Thin anomalous structures known as ultra-low velocity zones (ULVZs) have been found on the19 core-mantle boundary (CMB) and have extreme velocity reductions. These features are detected20 due to their effect on seismic waves that travel through them, typically producing precursors or21 postcursors. In this study we use postcursors to shear core-diffracted waves (Sdiff+) that sample22 the CMB near Vanuatu to detect and characterise the properties of a ULVZ. We identified a total23 of 19 earthquakes originating from the South Pacific Rise region detected by stations across East24 Asia – particularly Japan – showing Sdiff+ signals. Of these events, six with the highest quality25 Sdiff+ signals are included in a Bayesian inversion of travel times using the 2D Wavefront Tracker26 we previously developed. A subset of events was selected for further analysis by modelling using27 3D full waveform synthetics for a range of parameters. The comparison of the real data with the28 synthetic waveforms suggests that a ULVZ is located to the southeast of Vanuatu at 172.2±0.9°E29 and 22.9±1.1°S and its broad-scale structure can be approximated as a cylinder with a height of30 20 ±5 km, radius 240 ±50 km, and shear wave velocity reduction of 30 ±5%. These parameters31 are comparable to other ULVZs previously detected and modelled with Sdiff and Sdiff+. There32 are appreciable uncertainties in the location along the NW-SE direction due to the distribution of33 earthquakes and seismic arrays, as well as trade-offs between the height, size and velocity reduction34 of the ULVZ. Other studies using SPdKS, ScP and PcP have reported detections of ULVZs in the35 proximate region, some of which are consistent with the well-fitting parameter space of the ULVZ36 in this study. The Vanuatu ULVZ lies within the southwest edge of the Pacific large low velocity37 province. There is potentially a mantle plume rooted by this ULVZ that has diverted towards38 the hotspots on the eastern Australian plate around the Tonga slab, although most tomographic39 models do not show a continuous plume here.40 2 2 Introduction41 The lowermost mantle above the core-mantle boundary (CMB) contains strong laterally hetero-42 geneous seismic structures not observed in the rest of the lower mantle [e.g. Ritsema et al., 2011;43 French and Romanowicz, 2014]. Around 30% of the CMB is covered by two large regions with44 relatively low shear wave velocities, called large low velocity provinces (LLVPs), which are broadly45 located beneath the Pacific and Africa [e.g. Cottaar and Lekic, 2016; Garnero et al., 2016]. The46 LLVPs have been seen in tomographic models for over 40 years [Dziewonski and Anderson, 1981],47 with their detailed structure increasing with time [e.g. Thrastarson et al., 2024; Cui et al., 2024].48 Whether they are thermal plume clusters or thermochemical structures related to superplumes or49 stable piles is still a subject of debate [e.g. Garnero et al., 2016; Lau et al., 2017; Koelemeijer et al.,50 2017; Talavera-Soza et al., 2025], although a potential consensus image is emerging of a thin higher51 density stable base with bundles of thermochemical plumes above [Richards et al., 2023; Davaille52 and Romanowicz, 2020].53 Besides LLVPs, smaller and thinner structures have been identified on the CMB. These struc-54 tures – known as ultra-low velocity zones (ULVZs) – have much higher shear wave velocity reduc-55 tions, on the order of 10–50% [e.g. Yu and Garnero, 2018; Li, Leng, Jenkins and Cottaar, 2022].56 ULVZs have been detected since the mid-1990s using various seismic phases that interact with the57 CMB [Williams et al., 1998] and are reported to have a broad variation in size (from a few to 100 km58 in height and a few to 1000 km in lateral extent) and velocity reduction [e.g. summary by Yu and59 Garnero, 2018]. Naturally, there are strong trade-offs between height, size and velocity reduction.60 ULVZs detected by bounce point phases (ScS, ScP, PcP) or by scattering (PKP) generally lack61 constraints on their lateral extent or shape, whereas those detected by diffracted phases (Sdiff,62 SPdKS) sample large portions of the CMB but there are ambiguities in where along those paths63 anomalies originate. In cases where there is crossing data and/or broad coverage of an area, 3D64 models of these structures have been proposed. For example, synthetic modelling of rectangular or65 amorphous shapes for the Samoa mega-ULVZ have been effective in reproducing observed SPdKS66 data [Thorne et al., 2013; Jensen et al., 2013; Krier et al., 2021]. For Sdiff waveforms, cylindrical67 shapes are typically assumed as they reproduce the observed out-of-plane postcursor signals; in-68 cluding ULVZs found near Hawaii [Cottaar and Romanowicz, 2012; Li, Leng, Jenkins and Cottaar,69 3 2022], Iceland [Yuan and Romanowicz, 2017], Galapagos [Cottaar et al., 2022], Pitcairn [Li et al.,70 2024], St Helena [Davison et al., 2024] and in the central Pacific [Kim et al., 2020; Wolf and Long,71 2023; Martin et al., 2024]. With good data coverage for the Hawaii ULVZ, an elongated or elliptical72 cylinder has been constrained [Li, Sun and Bower, 2022; Martin et al., 2023b]. These broadest UL-73 VZs of 500–1000 km or more in lateral extent are sometimes referred to as ‘mega-ULVZs’ [Thorne74 et al., 2013].75 Since ULVZs are relatively thin structures that lie on the CMB, studies are typically focussed76 on a target region, rely on dense seismic arrays, and mainly use forward modelling to interpret77 the seismic observations. The first studies to invert for ULVZ morphology investigated layered78 structures in the radial direction using Bayesian methods to explain preand postcursors in ScP79 waveforms [Pachhai et al., 2015, 2023]. For Sdiff postcursors (Sdiff+), Martin et al. [2023a] de-80 veloped a Bayesian method mapping the structure in the 2D horizontal plane, using a wavefront81 tracker as the forward model [Hauser et al., 2008]. These methods illustrate the range of models82 that can fit the data and reduce the exploration required through forward modelling.83 Major questions remain as to what ULVZs are made of and how they form. A chemically84 distinct composition that is enriched in iron-rich ferropericlase could explain the observed velocity85 reductions [e.g. Wicks et al., 2010; Dobrosavljevic et al., 2019; Esdaille and Chen, 2024]. Geody-86 namic modelling suggests ULVZs have a composition with a major increase in density, implying87 that ULVZs have a solid-state origin [Bower et al., 2011]. Chemical anomalies have been suggested88 to be a remnant of a basal magma ocean [Labrosse et al., 2007] or sediments from the outer core89 [Fu et al., 2023]. The presence of partial melt has also been suggested to explain high dVp/dVs90 ratios [e.g. Williams et al., 1998]. Melting of subducted slab material has been proposed to explain91 ULVZs located in subducted slab regions [e.g. Festin et al., 2024; Li, 2023]. However, it is unlikely92 that the melt will remain stable within the ULVZ over long timescales as it will pool on the CMB,93 resulting in a global melt layer [e.g. Hernlund and Tackley, 2007; Dannberg et al., 2021], unless there94 is sufficient internal stirring [Hernlund and Jellinek, 2010]. Such a global melt layer has not yet95 been detected unambiguously due to the seismic resolution limit, but a global layer of up to several96 kilometres may be possible [e.g. Russell et al., 2022; Hansen et al., 2023]. Longer period normal97 modes are also consistent with a thin global layer [Russell et al., 2023] but alternatively this could98 be due to a long wavelength component in the distribution of piled ULVZ material [Koelemeijer99 4 et al., 2012].100 Besides their nature, their relationship with surrounding flow remains unknown. Some studies101 have suggested a relationship between mega-ULVZs and hotspots at the surface and potential102 mantle plumes connecting them [Thorne et al., 2013; Yuan and Romanowicz, 2017; Kim et al.,103 2020; Cottaar and Romanowicz, 2012], which is consistent with geophysical models [e.g. Jellinek104 and Manga, 2002]. ULVZs might therefore represent a reservoir to explain anomalous signatures105 of 3He/4He and µ183W [Mundl-Petermeier et al., 2020; Cottaar et al., 2022]. However, anomalous106 geochemical observation are not always observed at plumes near ULVZs [Herret et al., 2023; Davison107 et al., 2024], nor can all mega-ULVZs be related to a hotspot or mantle plume [Martin et al., 2024].108 The latter is particularly true for the large range of smaller ULVZs that are observed globally [e.g. Yu109 and Garnero, 2018; Hansen et al., 2023]. The largest observed ULVZs are also predominantly located110 at the boundaries of the LLVPs, which is consistent with geodynamical modelling that suggests111 that dense ULVZs that form outside the LLVPs are swept to the boundaries and accumulate there112 [Liu et al., 2024].113 In this study, we focus on the region of the CMB beneath the Southwest Pacific, near Vanuatu114 and Fiji. Positive detections and null observations of ULVZs have been reported here from multiple115 seismic probes [Yu and Garnero, 2018] (summarised in Figure 1). Due to the distribution of116 earthquakes caused by the subduction of the Pacific plate under the Australian plate with respect117 to the Alice Springs and Warramunga seismic arrays in Australia, there are numerous studies118 which have investigated the structure of ULVZs in this region using ScP waveforms [e.g. Rost119 and Revenaugh, 2001; Idehara et al., 2007; Brown et al., 2015; Pachhai et al., 2015]. Using the120 same events, other studies have investigated the possibility of ULVZs here from anomalous SPdKS121 waveforms [e.g. Thorne and Garnero, 2004; Jensen et al., 2013; Thorne et al., 2021]. The wide range122 of reported parameters and locations for a possible ULVZ indicate that substantial uncertainty123 remains regarding the fine-scaled structures on the CMB in the New Caledonia/Vanuatu region.124 Here, we present new observations of Sdiff postcursors (Sdiff+) which sample the CMB and125 provide evidence of a ULVZ in the region beneath Vanuatu and Fiji, from earthquakes in the126 Pacific Rise region towards stations across East Asia. We thus refer to the ULVZ causing these127 Sdiff+ signals as the ‘Vanuatu’ ULVZ. We invert for ULVZ location and parameters from Sdiff+128 travel times with a 2D wavefront tracker for six high quality events. From the ensemble of models129 5 Figure 1. (a) Probability map of the presence of ULVZs based on the 1% misfit map by Thorne et al. [2021]. Note that there is no distinction in this study between low probability and poor data coverage. The dashed black line is the Samoa ULVZ from Thorne et al. [2021] and Krier et al. [2021]. The black dashdotted line gives the proposed ULVZ from Jensen et al. [2013]. The dashed blue line represents the convex hull of our well-fitting parameter space of the ULVZ found in this study. The solid blue line gives the preferred model of the ULVZ in this study, with a radius of 240 km and location 172.2°E and 22.9°S. (b) Zoom-in of the black box in (a) with locations of bounce points on the CMB given by: circles [Rost and Revenaugh, 2001, 2003; Rost et al., 2005, 2006, 2010], stars [Idehara et al., 2007], squares [Thomas et al., 2009], diamonds [Pachhai et al., 2015], pentagons [Brown et al., 2015], triangles [Hansen et al., 2020], and crosses [Pachhai et al., 2022]. The lines are ray paths of SPdKS waveforms on the CMB from Thorne and Garnero [2004]. Their colours indicate the presence (red) and absence (black) of anomalous waveforms; complex waveforms (orange) are not interpreted. The coloured dashdotted lines are proposed ULVZs in the region surrounding Vanuatu from multiple previous studies [Thomas et al., 1999; Thorne et al., 2013; Pachhai et al., 2022, 2023]. 6 which fit the data we compute 3D full waveform synthetic models for several of the highest quality130 events to test constraints on the thickness, lateral extent, and shear wave velocity reduction of131 the ULVZ. Finally, we go on to compare our model with others reported in this region, which are132 largely consistent.133 3 Data and Methods134 3.1 Sdiff postcursors (Sdiff+)135 Shear waves diffract along and propagate parallel to the CMB and can be observed beyond epicentral136 distances of approximately 100°(Sdiff, Figure 2a), although this varies depending on the assumed137 1D velocity model and event depth. Energy of the Sdiff wave gets trapped and propagates as138 a guided wave within the ULVZ, lagging behind energy of the Sdiff waves propagating above or139 around the ULVZ. Due to the velocity contrast at the boundaries of the ULVZ, the guided wave is140 (a) (b) Figure 2. (a) Propagation of ScP, ScS, SPdKS, S, and Sdiff seismic phases from an earthquake (star) to receivers (triangles) through a cross-section of the Earth. ScP and ScS are core-reflected phase that sample bounce points on the CMB. SPdKS and Sdiff are phases that have diffracting legs along the CMB, with diffraction ray paths that attenuate with distance. (b) Wavefront propagation of Sdiff from an earthquake (star), calculated using the 2D wavefront tracker [Hauser et al., 2008; Martin et al., 2023a]. The wavefront propagates through time (sequential grey lines), producing postcursor signals as it passes across a ULVZ (pink circle). A receiver (triangle) detects the direct arrival (Sdiff, green line) and an additional delayed arrival due to refraction from the ULVZ (Sdiff+, blue line) at a different angle to the direct backazimuth. 7 also refracted upon entry and exit of the ULVZ. As a result, the propagation of the wavefront is141 disturbed and creates postcursors to the main Sdiff phase (Figure 2b), here referred to as Sdiff+.142 The time delay of the Sdiff+ compared to the direct Sdiff phase is caused by both the propagation143 within the ULVZ and the longer path taken due to refraction when off-axis. Sdiff+ signals caused144 by a cylindrical ULVZ have an approximately hyperbolic travel time move-out, with the minimum145 arrival time at the on-axis azimuth relative to the ULVZ midpoint from the source. Detection of146 Sdiff+ requires a large and dense seismic array. In particular, most recent studies of ULVZs using147 Sdiff+ have only been possible due to the dense coverage of seismic stations in North America [e.g.148 Cottaar and Romanowicz, 2012; Yuan and Romanowicz, 2017; Cottaar et al., 2022; Wolf and Long,149 2023; Martin et al., 2024], while some Sdiff+ signals have been observed by networks throughout150 Europe [Davison et al., 2024], central Asia [Yuan and Romanowicz, 2017], and South America [Li151 et al., 2024]. Here, we use a new geometry from the South Pacific Rise to stations in East Asia.152 In this study, Sdiff specifically refers to SHdiff, observed on the transverse component, which153 is observable at epicentral distances up to 150°for sufficiently large earthquakes. SVdiff, on the154 other hand, is strongly attenuated in the diffracted portion due to leaking into the core, resulting155 in a large decay in amplitude as a function of epicentral distance [Komatitsch et al., 2010] and is156 not studied further here.157 3.2 Data selection158 We search for evidence of Sdiff+ for events in the South Pacific Rise region using the dense seis-159 mometer distribution in East Asia. We inspected all earthquakes located near the South Pacific160 Rise region at any depth from 1995 to 2024 and with a moment magnitude larger than 5.5. Data161 are obtained for a distance range of 90–150°, although most data in this geometry is observed162 Date Lon (°) Lat (°) Depth (km) Mag ∆t(s) Region 12003/08/28 -115.20 -49.92 15 6.2 -5.0 Southern East Pacific Rise 22005/05/12 -138.91 -57.57 12 6.5 -3.0 Pacific-Antarctic Ridge 32001/09/02 -136.75 -54.31 15 6.3 0.0 Pacific-Antarctic Ridge 4a 2007/03/31 -123.61 -55.94 12 6.2 -9.5 Southern East Pacific Rise 4b 2016/08/18 -123.63 -55.97 13.44 6.0 -9.5 Southern East Pacific Rise 52015/05/19 -132.39 -54.53 14.91 6.6 -1.5 Pacific-Antarctic Ridge Table 1. List of selected events with high quality Sdiff+ signals. Source parameters are from the Global CMT Project [Ekstr¨om et al., 2012]. The full list of events can be found in Table S1. 8 Figure 3. Map of event locations (stars), receivers (triangles) and the Sdiff ray paths sensitive to the CMB (below 2800 km depth). The earthquakes and ray paths are coloured per event: 1–green, 2–purple, 3–red, 4a–blue, 4b–black and 5–orange (Table 1); light grey stars are events identified but not analysed (Table S1). Hotspot locations are shown as orange circles. The dashed blue line represents the convex hull of ensemble of models of ULVZs which fit the Sdiff+ arrival time data in this study. The solid blue line gives the preferred model of the ULVZ in this study, with a radius of 240 km and location 172.2°E and 22.9°S. The models of the Samoa ULVZ are given as black dashed lines: A–irregular shaped [Thorne et al., 2021] and B–rectangular shaped [Krier et al., 2021]. The black dashdotted line shows the inferred C–diamond shaped ULVZ from Jensen et al. [2013]. The background displays the LLVP vote map at 2800 km depth [Cottaar and Lekic, 2016]. The solid red line and greyscale markers denote the cross-section transect in Figure 8. at 115–130°. The seismograms were downloaded from networks available through the EarthScope163 Consortium Data Management Center and the National Research Institute for Earth Science and164 Disaster Prevention (hereafter NIED) F-net, a broadband seismograph network consisting of more165 than 70 stations across Japan [Okada et al., 2004]. After downloading, the data were resampled to166 10 Hz, components checked for missing data, and rotated into the ZRT (vertical, radial, tangential)167 orientation since SHdiff is primarily visible on the tangential component.168 A total of 78 earthquakes located in the South Pacific Rise region were analysed. Most of the169 earthquakes had a low signal-to-noise ratio (SNR) due to their low moment magnitude. There were170 9 However, there is significant variation in the relative amplitudes between Sdiff and Sdiff+, with301 small but strong anomalies resulting in small Sdiff+ amplitudes, and large but weak anomalies302 producing large Sdiff+ amplitudes.303 We also compute synthetics for models away from the trade-off curve to demonstrate the unsuit-304 ability of these models and illustrate uncertainties. A number of tested models are larger or more305 strongly reduced (Models B1, B3 & C1; ‘above’ the curve in Figure S7), resulting in pronounced306 Sdiff+ that are more delayed and have longer periods than observed in the real data (Figures S13307 & S14). For models that are smaller or more weakly reduced (Models B2, B4 & C2; ‘below’ the308 curve in Figure S7), the Sdiff+ are comparable in amplitude when filtered between 10–20 s but are309 too weak to be observed in 20–30 s. As expected, the travel time delays of the Sdiff+ are reduced.310 Overall, there are appreciable trade-offs of the size and velocity reduction with the thickness of the311 ULVZ, which is reflected in the uncertainties in our final preferred model.312 Figure 6. Displacement synthetics for PREM with a ULVZ with height 20 km, radius 240 km, and shear velocity reduction of 30% (Model A3, Table S2) filtered between periods of 10–20 s for the six events listed in Table 1. The same Sdiff+ highlights from Figure 4 are overlaid, and the colours correspond to the event location stars in Figure 3. 16 4.3 Preferred model313 The consistent presence of Sdiff+ arrivals in the observed data is strong evidence for the existence314 of a ULVZ beneath Vanuatu on the CMB. Inversion of the Sdiff+ arrival times with the 2DWT315 suggests a ULVZ located at 172.9±0.9°E and 22.9±1.1°S. Further modelling with 3D full waveform316 synthetics shows the data are consistent with a cylindrical ULVZ with radius 240 ±50 km, height317 20 ±5 km, and shear wave velocity reduction 30 ±5%. Synthetics for all of the events used in this318 study are shown in Figure 6.319 Whilst the ULVZ was modelled using a simplified cylindrical shape, which effectively reproduces320 the hyperbolic move-out of Sdiff+, the limited azimuthal coverage of the region of interest makes it321 difficult to justify to what extent this approximation is reasonable. As Sdiff travels long distances322 along the CMB, anomalous structures anywhere along those ray paths may cause interference and323 produce additional waveforms. The available data set samples the ULVZ along a single azimuth324 (NW–SE), meaning the location is poorly constrained along the ray paths in the NW–SE direction325 but well constrained in the NE–SW direction (Figure 5a).326 5 Discussion327 5.1 Comparison to nearby earlier studies328 The Southwest Pacific has been intensively investigated, with previous studies finding evidence of329 the presence and absence of ULVZs using ScP and SPdKS (summarised in Figure 1). A probability330 map derived from anomalous SPdKS waveforms suggests the likelihood of ULVZs to the west,331 north, and particularly to the east of our preferred model location [Thorne et al., 2021], where332 locally targeted studies have mapped the Samoa ULVZ [Thorne et al., 2013; Krier et al., 2021]. To333 the west, the fortuitous source-receiver distribution has led to a plethora of studies using ScP to334 probe beneath the Coral Sea [e.g. Idehara et al., 2007; Rost et al., 2010; Pachhai et al., 2022], as335 well as a localised study using SPdKS [Jensen et al., 2013]. Since the analyses of these data assume336 an in-plane origin of preor postcursors in the waveforms, it is possible that out-of-plane energy337 from the ULVZ identified in this study could cause these signals [Pachhai et al., 2024] and we note338 that the Vanuatu ULVZ is located in a gap of in-plane data coverage for both phases.339 17 We test if our data would be able to observe the other ULVZs previously proposed in the340 region by computing full waveform synthetics for Events 1 and 4a & 4b. For the two published341 Samoa ULVZ models – an irregular shaped ULVZ [Thorne et al., 2021] and a rectangular shaped342 ULVZ [Krier et al., 2021] – we use a thickness of 26 km and a shear wave velocity reduction of343 20% as implemented by Krier et al. [2021]. To the west, we test a ULVZ in the region of highest344 probability (purple contour in Figure 1), which is comparable in size to our preferred model. For345 this we use parameters from our preferred model with radius 240 km, height 20 km and shear346 velocity reduction of 30% relocated to the midpoint of the local high probability region, 166°E and347 24.5°S. The majority of previous ScP studies in this region therefore fall within the boundaries of348 our trial model, as well as within the upper limit of uncertainty in radius for our preferred model349 (Figure S15). All ULVZ models tested produced Sdiff+ signals (Figures S16 & 7 for Events 1 and 4a350 & 4b, respectively). For the models of ULVZs to the east, Sdiff+ waveforms are offset significantly351 to the larger azimuths, which means that the Sdiff+ in the observed data cannot be caused by the352 Samoa ULVZ (Figure 7d-e). However, this does not exclude the possibility that these contribute to353 the waveform complexities observed, nor would lack of signals necessarily contradict the presence354 of the Samoa ULVZ (Figure S16g), as we have previously noted that the visibility of ULVZs might355 vary by direction [Martin et al., 2023b]. For the model to the west, the waveforms look – perhaps356 unsurprisingly – fairly similar, as we used the properties of our preferred model, but they are offset357 to smaller azimuths. While the offset in azimuth is only slight for Event 1 (Figure S16f), it is quite358 appreciable for Event 4 (Figure 7f). This demonstrates that the location of our preferred model is359 well constrained by the combination of all six events, each with slightly different coverage, at least360 in the SW-NE direction.361 While a mega-ULVZ model further to the west might not be expected in our data, a range of362 ScP studies in this region have observations that fall within the convex hull of our model space363 (Figure 1). These observations from bounce point studies, however, suggest thinner and patchier364 ULVZ coverage than we infer here [e.g. Idehara et al., 2007; Rost et al., 2010; Pachhai et al., 2022],365 which could be the narrower edge of a larger, thicker structure [Jensen et al., 2013]. Given the low366 sensitivity of Sdiff and Sdiff+ waveforms to small-scale structures at relatively long period, this367 study cannot resolve the complex morphology of this regional structure, but supports the presence368 of a large, thick quasi-cylindrical structure (∼500 km across, ∼20 km deep). The possibility of in-369 18 Figure 7. Waveform data and synthetics for the Vanuatu ULVZ in this study and for other models of proximate ULVZs. (a) Observed and (b-f) synthetic displacement waveforms for Events 4a & 4b. Synthetics for PREM with (b) no ULVZ, (c) the preferred ULVZ in this study, (d) the irregularly shaped Samoa ULVZ [Thorne et al., 2021], (e) the rectangular-shaped Samoa ULVZ [Krier et al., 2021], and (f) a ULVZ with parameters of our preferred model located at the proximate probability peak of Thorne et al. [2021] (Figure S15). The Sdiff+ signals are highlighted where visible. Data are filtered between 10–20 s period. plane and out-of-plane postcursors caused by a mega-ULVZ should be considered in future studies370 attempting to reconcile observations of ScP with observations of Sdiff and SPdKS. Overall, this371 region appears to have many complexities observed by different seismic phases, similar to the372 patchier areas found around the mega-ULVZ near Hawaii [Jenkins et al., 2021].373 Future studies using Sdiff+ and SPdKS could also examine the possible internal layering of374 ULVZs by using shorter period postcursors. Identifying Sdiff+ at higher frequencies is challenging375 as they are often obscured by noise, although Li, Leng, Jenkins and Cottaar [2022] have been able376 to identify the internal layering of the Hawaii ULVZ using this method.377 5.2 Comparison to other mega-ULVZs and relation to LLVPs378 The Vanuatu ULVZ has comparable parameters to – but is somewhat smaller in lateral extent than –379 other modelled mega-ULVZs which have been observed using Sdiff+ [e.g. Cottaar and Romanowicz,380 19 Figure 8. Cross-sections of recent whole mantle tomographic models (a) SEMUCB-WM1 [French and Romanowicz, 2014], (b) GLAD-M35 [Cui et al., 2024], and (c) REVEAL [Thrastarson et al., 2024] through the transect on Figure 3, which passes through the Vanuatu and Samoa ULVZs and near the Lord Howe and Samoa hotspots. Velocity deviations are with respect to the (whole Earth) radial average of each tomographic model. Nearby hotspots are projected onto the cross-section: EA = East Australia, T = Tasmantid, LH = Lord Howe, and S = Samoa. ULVZ thickness (solid black lines) is exaggerated for visualisation. 2012; Davison et al., 2024]. Since the events sampling this ULVZ are typically shallow and low381 magnitude, it is difficult to differentiate between weak Sdiff+ signal and noise at shorter periods.382 There is distinct Sdiff+ energy between 10–20 s period and it is visible up to 30 s, suggesting a383 comparable frequency content and thickness to that of the Hawaii ULVZ [20 km in height, Martin384 et al., 2023b] and thicker than that of Iceland [15 km, Yuan and Romanowicz, 2017] or the mid-385 Pacific ULVZ [10 km, Martin et al., 2024].386 At the southwest edge of the Pacific LLVP, the Vanuatu ULVZ is the eighth mega-ULVZ mod-387 elled in 3D. All of these – so far – are located at or near the LLVP boundaries, reinforcing a potential388 correlation between the largest ULVZs and the boundaries of the LLVPs. Previous mega-ULVZs389 have also been interpreted as whole mantle plume roots due to their geochemical signatures [Cot-390 taar et al., 2022]. The Vanuatu ULVZ is almost equidistant to the Samoa and Lord Howe hotspots,391 with roughly ∼15–20°to either. However, the Samoa ULVZ lies closer to that hotspot and has392 previously been suggested as its potential root [e.g. Thorne et al., 2013; Krier et al., 2021].393 The Lord Howe seamount chain lies on the eastern Australian plate. This seamount chain,394 combined with the Tasmantid and Eastern Australia hotspot, tracks further west and has no395 observable topographic swell, implying that the heat flux in the area is very minor [Hoggard et al.,396 2020]. However, the Lord Howe seamount chain has similar geochemical signatures to other Pacific397 ocean island basalts, with Sr-Nd isotope analysis suggesting a deep mantle source [Rogers et al.,398 2023]. These signals are also seen on the Tasmantid seamount chain suggesting a similar source, but399 3He/4He measurements to confirm a primordial reservoir are lacking in the region [Rogers et al.,400 2023].401 20 We show cross-sections for three tomographic models [French and Romanowicz, 2014; Cui et al.,402 2024; Thrastarson et al., 2024] between the Lord Howe and Samoa hotspots, across the location403 of the Vanuatu ULVZ (Figure 8). There are differences between the cross-sections, but all three404 models show the Tonga slab, the Pacific LLVP, and a broad-scale potential plume beneath Samoa,405 underlain by the Samoa ULVZ. Another, older slab has been suggested in this region at a depth406 of ∼1100 km from a fossil subduction zone [Schellart et al., 2009; Van der Meer et al., 2018]. The407 South Loyalty Basin slab is difficult to distinguish from the Tonga slab in the tomographic models408 we show due to resolution. Towards Lord Howe, one model in particular, REVEAL, suggests a409 continuous low velocity anomaly from the region of the Vanuatu ULVZ to the surface. While the410 other two models do not show a continuous plume here, all models show some suggestion of hot411 upwelling material deflected around the South Loyalty Basin slab and down-dipping Tonga slab412 towards the southwest. Potentially, splitting of this plume into thinner plumelets towards the upper413 mantle makes it hard to image these. Alternatively, the plume may have been recently cut off by414 the slab and could be retreating southwards, which is consistent with an age-progressive reduction415 in magmatic flux observed for the Lord Howe seamount chain [Seton et al., 2019] and the lack of416 present-day topographic swell [Hoggard et al., 2020].417 The image of two nearby broad-scale plumes, rooted by ULVZs, aligns with the ‘bundle of418 plumes’ view proposed as an explanation for LLVPs by Davaille and Romanowicz [2020]. Since419 the connection of the ULVZ to the mantle plume and a potential hotspot is tenuous, we stick to420 naming it ‘Vanuatu ULVZ’ and not the ‘Lord Howe ULVZ’. The Vanuatu and Samoa ULVZs thus421 represent two large scale structures at a distance of approximately 20°(∼1200 km at the CMB).422 Future geodynamical models could test if these features can remain stable, or would be expected423 to eventually merge.424 21 6 Conclusions425 Sdiff+ produced by 19 earthquakes located in the South Pacific Rise region and detected by stations426 across East Asia provide evidence for the Vanuatu ULVZ. A combination of inverse modelling of427 travel times for Sdiff+ and forward modelling of Sdiff+ waveforms for six of the highest quality428 events places constraints on the ULVZ parameters. Since all of the earthquakes are co-located, the429 position of the ULVZ is well constrained in the SW-NE orientation but poorly constrained along430 the NW-SE direction. The preferred model of the ULVZ is a cylinder with a height of 20 ±5 km, a431 radius of 240 ±50 km, and a shear wave velocity reduction of 30 ±5% located to the southeast of432 Vanuatu at 172.2±0.9°E and 22.9±1.1°S. There are strong trade-offs between the height, velocity433 reduction and size of the ULVZ.434 Although the region surrounding the proposed location of the Vanuatu ULVZ has been inten-435 sively studied using ScP and SPdKS waveforms, our study of Sdiff+ address a gap of data coverage.436 Full waveform synthetics of nearby previously proposed models do not explain our observations.437 Joint modelling of SPdKS and Sdiff for this region may provide crucial additional constraints on438 the dVp/dVs ratio, and potentially density, of the ULVZ and therefore differentiate between origin439 hypotheses.440 Synthesising our observations with previous studies suggests the CMB here is a complex land-441 scape comprising a ‘mega-ULVZ’ and smaller-scale ULVZs, which is within the southwest edge of442 the Pacific LLVP and relatively close to the Samoa mega-ULVZ. Recent tomographic models hint at443 the possibility that the Vanuatu ULVZ lies at the root of a plume that is deflected to the southwest444 around the Tonga slab. However, tomographic models do not agree whether this plume feeds into445 the Lord Howe and neighbouring hotspots or if it is now a waning plume.446 22 Acknowledgements447 CM and AD were funded by a Vici award (grant number 016.160.310/526) from the Dutch Re-448 search Council (NWO). CM and SC received funding from the European Research Council (ERC)449 under the European Union’s Horizon 2020 research and innovation programme (grant agreement450 No. 804071 -ZoomDeep). SC and JA received funding from the Natural Environment Research451 Council (NE/V018213/1 and NE/S007164/1, respectively). Some of the work in this study was452 performed using resources provided by the Cambridge Service for Data Driven Discovery (CSD3)453 operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk). We454 also acknowledge the cluster facilities, Eejit in Utrecht and Gauss in Cambridge, and thank the455 support staff who maintain them. We would like to thank Stuart Russell, Florian Millet, Douwe van456 Hinsbergen, and the Utrecht seismology group for helpful discussions; and Neil Marjoram, Lukas457 van de Wiel and Theo van Zessen for technical support. We thank the editor, Vernon Cormier,458 and two anonymous reviewers for their thoughtful comments and suggestions which improved the459 manuscript. Finally, we would like to thank NIED – and everyone involved with the deployment460 and maintenance of the seismic network infrastructure – for making such a rich data set freely461 available.462 Data and code availability463 The facilities of IRIS Data Services (www.iris.edu), and specifically the IRIS Data Management464 Center, were used for access to waveforms and related metadata. IRIS Data Services are funded465 through the Seismological Facilities for the Advancement of Geoscience (SAGE) Award of the Na-466 tional Science Foundation under Cooperative Support Agreement EAR-1851048. Data from the F-467 net and Hi-net networks were made freely available by National Research Institute for Earth Science468 and Disaster Resilience [Okada et al., 2004]. Data from NIED was downloaded using HinetPy [Tian,469 2024]. Earthquake parameters were used from the Global CMT Project (www.globalcmt.org).470 Codes used will be made available by CM upon request.471 23 Competing interests472 Authors declare that they have no competing interests.473 24 References474 Bodin, T. and Sambridge, M. [2009], ‘Seismic tomography with the reversible jump algorithm’,475 Geophysical Journal International 178(3), 1411–1436.476 Bower, D. J., Wicks, J. K., Gurnis, M. and Jackson, J. M. [2011], ‘A geodynamic and mineral physics477 model of a solid-state ultralow-velocity zone’, Earth and Planetary Science Letters 303(3-4), 193–478 202.479 Brown, S. P., Thorne, M. S., Miyagi, L. and Rost, S. [2015], ‘A compositional origin to ultralow-480 velocity zones’, Geophysical Research Letters 42(4), 1039–1045.481 Cottaar, S. and Lekic, V. [2016], ‘Morphology of seismically slow lower-mantle structures’, Geophys-482 ical Supplements to the Monthly Notices of the Royal Astronomical Society 207(2), 1122–1136.483 Cottaar, S., Martin, C., Li, Z. and Parai, R. [2022], ‘The root to the Gal´apagos mantle plume on484 the core-mantle boundary’, Seismica 1(1).485 Cottaar, S. and Romanowicz, B. [2012], ‘An unusally large ULVZ at the base of the mantle near486 Hawaii’, Earth and Planetary Science Letters 355, 213–222.487 Crotwell, H. P., Owens, T. J., Ritsema, J. et al. [1999], ‘The TauP Toolkit: Flexible seismic488 travel-time and ray-path utilities’, Seismological Research Letters 70, 154–160.489 Cui, C., Lei, W., Liu, Q., Peter, D., Bozda˘g, E., Tromp, J., Hill, J., Podhorszki, N. and Pugmire, D.490 [2024], ‘GLAD-M35: a joint P and S global tomographic model with uncertainty quantification’,491 Geophysical Journal International 239(1), 478–502.492 Dannberg, J., Myhill, R., Gassm¨oller, R. and Cottaar, S. [2021], ‘The morphology, evolution and493 seismic visibility of partial melt at the core–mantle boundary: implications for ULVZs’, Geophys-494 ical Journal International 227(2), 1028–1059.495 Davaille, A. and Romanowicz, B. [2020], ‘Deflating the LLSVPs: Bundles of mantle thermochemical496 plumes rather than thick stagnant “piles”’, Tectonics 39(10), e2020TC006265. e2020TC006265497 10.1029/2020TC006265.498 URL: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020TC006265499 25 Thrastarson, S., van Herwaarden, D.-P., Noe, S., Josef Schiller, C. and Fichtner, A. [2024], ‘RE-654 VEAL: A global full-waveform inversion model’, Bulletin of the Seismological Society of America655 114(3), 1392–1406.656 Tian, D. [2024], ‘HinetPy: A Python package for accessing and processing NIED Hi-net seismic657 data’, Journal of Open Source Software 9(98), 6840.658 Van der Meer, D. G., Van Hinsbergen, D. J. and Spakman, W. [2018], ‘Atlas of the underworld:659 Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity’,660 Tectonophysics 723, 309–448.661 van Driel, M., Krischer, L., St¨ahler, S. C., Hosseini, K. and Nissen-Meyer, T. [2015], ‘Instaseis:662 instant global seismograms based on a broadband waveform database’, Solid Earth 6(2), 701–663 717.664 Wicks, J., Jackson, J. and Sturhahn, W. [2010], ‘Very low sound velocities in iron-rich (Mg, Fe) O:665 Implications for the core-mantle boundary region’, Geophysical Research Letters 37(15).666 Williams, Q., Revenaugh, J. and Garnero, E. [1998], ‘A correlation between ultra-low basal veloci-667 ties in the mantle and hot spots’, Science 281(5376), 546–549.668 Wolf, J. and Long, M. D. [2023], ‘Lowermost mantle structure beneath the central Pacific669 Ocean: Ultralow velocity zones and seismic anisotropy’, Geochemistry, Geophysics, Geosystems670 24(6), e2022GC010853.671 Yu, S. and Garnero, E. J. [2018], ‘Ultralow velocity zone locations: a global assessment’, Geochem-672 istry, Geophysics, Geosystems 19(2), 396–414.673 Yuan, K. and Romanowicz, B. [2017], ‘Seismic evidence for partial melting at the root of major674 hot spot plumes’, Science 357(6349), 393–397.675 32 Evidence of a ULVZ near Vanuatu from Sdiff postcursors1 Carl Martin1∗, Lobke Harmsma1, James Atkins2, Arwen Deuss1, Sanne Cottaar2 2 1Department of Geosciences, Utrecht University, 3584 CB, Netherlands3 2Bullard Laboratories, Department of Earth Sciences, University of Cambridge, CB3 0EZ,4 UK5 *Corresponding author ([email protected])6 SUPPLEMENTARY7 In this supplementary are:8 •Section S1: Earthquake data used in this study9 •Section S2: Waveforms of synthetics for different parameters to demonstrate trade-offs10 •Section S3: Waveform modelling of ULVZs in earlier studies for comparison11 1 S1 Earthquake data12 S1.1 Earthquake list13 In total we identified 19 earthquakes in the South Pacific Rise region towards seismic arrays East14 Asia – particularly the Japanese F-net [Okada et al., 2004] – which showed Sdiff postcursor evidence15 of a ULVZ (Table S1).16 Date Lon. [°E] Lat. [°N] Depth [km] Mag. Location 2001/08/06 -123.05 -55.67 15 Mw 6.7 Southern East Pacific Rise 32001/09/02 -136.75 -54.31 15 Mw 6.3 Pacific-Antarctic Ridge 12003/08/28 -115.20 -49.92 15 Mw 6.2 Southern East Pacific Rise 2004/01/29 -114.85 -50.03 15 Mw 6.1 Southern East Pacific Rise 22005/05/12 -138.91 -57.57 12 Mw 6.5 Pacific-Antarctic Ridge 2006/10/10 -122.41 -56.18 12 Mw 6.0 Southern East Pacific Rise 4a 2007/03/31 -123.61 -55.94 12 Mw 6.2 Southern East Pacific Rise 2007/11/02 -128.80 -55.40 13.55 Mw 6.2 Pacific-Antarctic Ridge 2011/11/02 -129.08 -55.34 15 Mw 6.2 Pacific-Antarctic Ridge 2012/07/18 -128.96 -55.36 20.34 Mw 5.9 Pacific-Antarctic Ridge 2014/05/12 -115.06 -49.90 13.24 Mw 6.4 Southern East Pacific Rise 52015/05/19 -132.39 -54.53 14.91 Mw 6.6 Pacific-Antarctic Ridge 2015/12/24 -123.11 -56.06 12.13 Mww 6.2 Southern East Pacific Rise 2016/02/16 -124.57 -55.74 20.13 Mww 6.1 Southern East Pacific Rise 4b 2016/08/18 -123.63 -55.97 13.44 Mw 6.0 Southern East Pacific Rise 2017/06/15 -124.44 -55.77 20.03 Mww 5.8 Southern East Pacific Rise 2018/11/15 -122.26 -56.23 12 Mww 6.3 Southern East Pacific Rise 2024/05/18 -123.14 -56.05 12 Mww 5.9 Southern East Pacific Rise 2024/06/09 -133.88 -54.06 12.62 Mww 6.2 Pacific-Antarctic Ridge Table S1. Earthquake data and parameters for events showing Sdiff postcursors that sample the CMB near Vanuatu. This study uses the events labelled 1–5 (Table 1 in main paper). Event locations are plotted in Figure 3. Earthquake parameters and moment tensor solutions are taken from the Global CMT Project [Ekstr¨om et al., 2012]. 2 S1.2 Events used in the study17 We reproduce Figure 4, showing the waveforms for six earthquakes, Events 1–5, filtered between18 7–12, 20–30 and 30–40 s period (Figures S1–S3, respectively). We also plot displacement waveforms19 from the Hi-net short period seismometers filtered between 10–20 and 7–12 s period (Figures S4 &20 S5, respectively) [Okada et al., 2004].21 Figure S1. Same as Figure 4 but filtered between 7–12 s period. 3 Figure S2. Same as Figure 4 but filtered between 20–30 s period. Figure S3. Same as Figure 4 but filtered between 30–40 s period. 4 Figure S4. Displacement waveforms from the short period Hi-net seismometers for Events 1–5 filtered between 10–20 s period. The linear stack of each 1°azimuthal bin (thick black lines) overlay the individual seismograms (thin grey lines). Note the different (y axis) azimuthal range compared to broadband data. Figure S5. Same as Figure S4 but filtered between 7–12 s period. 5 S2 Trade-offs in waveform modelling22 Figure S6 shows the synthetics predicted for the source solutions from the Global CMT Project23 with the depth set to 0 km which best matches the observed Sdiff waveforms. We note that the24 data and synthetics for Event 2 have been flipped to match the waveforms of the other events for25 ease of visual comparison.26 Figure S6. Displacement synthetics for PREM filtered between periods of 10–20 s for the six events listed in Table 1. Earthquake parameters and moment tensor solutions are taken from the Global CMT Project [Ekstr¨om et al., 2012], with depths set to 0 km. 6 We explore the trade-off in model parameters now using full waveform synthetics. The location and27 uncertainties are inferred from the 2DWT inversion (Figure 5a), which are assumed to be fixed for28 the remainder of this uncertainty assessment. We trial a number of models along the velocity-size29 trade-off curve of the 2DWT ensemble to demonstrate that this trade-off is reproduced by full30 waveform synthetics as well as some off-curve parameters (Figure S7 & Table S2). Additionally, we31 constrain the height by comparing the presence or absence of postcursors in synthetics with ULVZs32 of heights 15, 20, 25 and 30 km for different filter bands. We also show predicted travel times from33 the 2DWT for the various models listed in Table S2 (Figure S8).34 Figure S7. Duplicate of Figure 5b, with parameter labels overlaid. Ensemble of models that fit the Sdiff postcursor arrival times, showing the trade-off in velocity and size. The median model is marked with a red cross and histograms of each of the parameters are projected onto the respective axes. Model dVs (%) R (km) H (km) A1 −40 165 20 A2 −35 200 20 A3 −30 240 20+15,25,30 A4 −25 305 20 A5 −20 400 20 B1 −35 240 20 B2 −30 200 20 B3 −30 305 20 B4 −25 240 20 C1 −35 305 20 C2 −25 200 20 Table S2. Parameters used to test 3D cylindrical models of the ULVZ. All models are centred at the median location, 172.16 °E and 22.92 °S. Synthetics are also run for Model A3 with heights 15, 25, and 30 km to demonstrate sensitivity of the frequency content of the waveforms to model height. 7 Figure S8. Predicted travel times from the 2DWT for the various models for which we compute full waveform synthetics (Figure S7 & Table S2). (a) is the same as Figure 5c. Note that the 2DWT predictions do not account for height of the model. 8 Figure S9. (a) Real data and synthetic waveforms for (b) PREM with no ULVZ and PREM with a cylindrical ULVZ of height (c) 15 km, (d) 20 km, (e) 25 km, and (f) 30 km for Events 4a & 4b, filtered between 10–20 s period. All models use dVs of -30% and a radius of 240 km (Model A3, Table S2). Figure S10. Same as Figure S9 but filtered between 20–30 s period. 9