Evidence of a ULVZ near Vanuatu from Sdiff postcursors

E idence o a ULVZ nea Vanua u om Sdi pos cu so s1 Ca l Ma in1∗, Lobke Ha msma1, James A kins2, A wen Deuss1, Sanne Co aa 2 2 1Depa men o Geosciences, U ech Uni e si y, 3584 CB, Ne he lands3 2Bulla d Labo a o ies, Depa men o Ea h Sciences, Uni e si y o Camb idge, CB3 0EZ,4 UK5 *Co esponding au ho ([email p o ec ed])6 Key poin s7 •We p esen new e idence o an ul a-low eloci y zone (ULVZ) on he CMB o he sou heas 8 o Vanua u using Sdi pos cu so s (Sdi +) om ea hquakes in he Sou h Paci ic Rise egion9 owa ds seismic a ays in Eas Asia.10 •The da a a e explained by a quasi-cylind ical ULVZ loca ed a 172.2±0.9°E and 22.9±1.1°S11 wi h heigh 20 ±5 km, adius 240 ±50 km, and shea wa e eloci y educ ion 30 ±5%.12 •Combined wi h p e ious s udies using ScP and SPdKS in he a ea, his is a b oad-scale egion13 wi h a pa chwo k o ULVZs compa able in a ie y o Hawaii.14 •The Vanua u ULVZ lies wi hin he Paci ic la ge low eloci y p o ince, and migh po en ially15 be a oo o a man le plume ha de lec ed owa ds he eas e n Aus alian ho spo s a ound16 he Tonga slab.17 1 1 Abs ac 18 Thin anomalous s uc u es known as ul a-low eloci y zones (ULVZs) ha e been ound on he19 co e-man le bounda y (CMB) and ha e ex eme eloci y educ ions. These ea u es a e de ec ed20 due o hei e ec on seismic wa es ha a el h ough hem, ypically p oducing p ecu so s o 21 pos cu so s. In his s udy we use pos cu so s o shea co e-di ac ed wa es (Sdi +) ha sample22 he CMB nea Vanua u o de ec and cha ac e ise he p ope ies o a ULVZ. We iden i ied a o al23 o 19 ea hquakes o igina ing om he Sou h Paci ic Rise egion de ec ed by s a ions ac oss Eas 24 Asia – pa icula ly Japan – showing Sdi + signals. O hese e en s, six wi h he highes quali y25 Sdi + signals a e included in a Bayesian in e sion o a el imes using he 2D Wa e on T acke 26 we p e iously de eloped. A subse o e en s was selec ed o u he analysis by modelling using27 3D ull wa e o m syn he ics o a ange o pa ame e s. The compa ison o he eal da a wi h he28 syn he ic wa e o ms sugges s ha a ULVZ is loca ed o he sou heas o Vanua u a 172.2±0.9°E29 and 22.9±1.1°S and i s b oad-scale s uc u e can be app oxima ed as a cylinde wi h a heigh o 30 20 ±5 km, adius 240 ±50 km, and shea wa e eloci y educ ion o 30 ±5%. These pa ame e s31 a e compa able o o he ULVZs p e iously de ec ed and modelled wi h Sdi and Sdi +. The e32 a e app eciable unce ain ies in he loca ion along he NW-SE di ec ion due o he dis ibu ion o 33 ea hquakes and seismic a ays, as well as ade-o s be ween he heigh , size and eloci y educ ion34 o he ULVZ. O he s udies using SPdKS, ScP and PcP ha e epo ed de ec ions o ULVZs in he35 p oxima e egion, some o which a e consis en wi h he well- i ing pa ame e space o he ULVZ36 in his s udy. The Vanua u ULVZ lies wi hin he sou hwes edge o he Paci ic la ge low eloci y37 p o ince. The e is po en ially a man le plume oo ed by his ULVZ ha has di e ed owa ds38 he ho spo s on he eas e n Aus alian pla e a ound he Tonga slab, al hough mos omog aphic39 models do no show a con inuous plume he e.40 2 2 In oduc ion41 The lowe mos man le abo e he co e-man le bounda y (CMB) con ains s ong la e ally he e o-42 geneous seismic s uc u es no obse ed in he es o he lowe man le [e.g. Ri sema e al., 2011;43 F ench and Romanowicz, 2014]. A ound 30% o he CMB is co e ed by wo la ge egions wi h44 ela i ely low shea wa e eloci ies, called la ge low eloci y p o inces (LLVPs), which a e b oadly45 loca ed benea h he Paci ic and A ica [e.g. Co aa and Lekic, 2016; Ga ne o e al., 2016]. The46 LLVPs ha e been seen in omog aphic models o o e 40 yea s [Dziewonski and Ande son, 1981],47 wi h hei de ailed s uc u e inc easing wi h ime [e.g. Th as a son e al., 2024; Cui e al., 2024].48 Whe he hey a e he mal plume clus e s o he mochemical s uc u es ela ed o supe plumes o 49 s able piles is s ill a subjec o deba e [e.g. Ga ne o e al., 2016; Lau e al., 2017; Koelemeije e al.,50 2017; Tala e a-Soza e al., 2025], al hough a po en ial consensus image is eme ging o a hin highe 51 densi y s able base wi h bundles o he mochemical plumes abo e [Richa ds e al., 2023; Da aille52 and Romanowicz, 2020].53 Besides LLVPs, smalle and hinne s uc u es ha e been iden i ied on he CMB. These s uc-54 u es – known as ul a-low eloci y zones (ULVZs) – ha e much highe shea wa e eloci y educ-55 ions, on he o de o 10–50% [e.g. Yu and Ga ne o, 2018; Li, Leng, Jenkins and Co aa , 2022].56 ULVZs ha e been de ec ed since he mid-1990s using a ious seismic phases ha in e ac wi h he57 CMB [Williams e al., 1998] and a e epo ed o ha e a b oad a ia ion in size ( om a ew o 100 km58 in heigh and a ew o 1000 km in la e al ex en ) and eloci y educ ion [e.g. summa y by Yu and59 Ga ne o, 2018]. Na u ally, he e a e s ong ade-o s be ween heigh , size and eloci y educ ion.60 ULVZs de ec ed by bounce poin phases (ScS, ScP, PcP) o by sca e ing (PKP) gene ally lack61 cons ain s on hei la e al ex en o shape, whe eas hose de ec ed by di ac ed phases (Sdi ,62 SPdKS) sample la ge po ions o he CMB bu he e a e ambigui ies in whe e along hose pa hs63 anomalies o igina e. In cases whe e he e is c ossing da a and/o b oad co e age o an a ea, 3D64 models o hese s uc u es ha e been p oposed. Fo example, syn he ic modelling o ec angula o 65 amo phous shapes o he Samoa mega-ULVZ ha e been e ec i e in ep oducing obse ed SPdKS66 da a [Tho ne e al., 2013; Jensen e al., 2013; K ie e al., 2021]. Fo Sdi wa e o ms, cylind ical67 shapes a e ypically assumed as hey ep oduce he obse ed ou -o -plane pos cu so signals; in-68 cluding ULVZs ound nea Hawaii [Co aa and Romanowicz, 2012; Li, Leng, Jenkins and Co aa ,69 3 2022], Iceland [Yuan and Romanowicz, 2017], Galapagos [Co aa e al., 2022], Pi cai n [Li e al.,70 2024], S Helena [Da ison e al., 2024] and in he cen al Paci ic [Kim e al., 2020; Wol and Long,71 2023; Ma in e al., 2024]. Wi h good da a co e age o he Hawaii ULVZ, an elonga ed o ellip ical72 cylinde has been cons ained [Li, Sun and Bowe , 2022; Ma in e al., 2023b]. These b oades UL-73 VZs o 500–1000 km o mo e in la e al ex en a e some imes e e ed o as ‘mega-ULVZs’ [Tho ne74 e al., 2013].75 Since ULVZs a e ela i ely hin s uc u es ha lie on he CMB, s udies a e ypically ocussed76 on a a ge egion, ely on dense seismic a ays, and mainly use o wa d modelling o in e p e 77 he seismic obse a ions. The i s s udies o in e o ULVZ mo phology in es iga ed laye ed78 s uc u es in he adial di ec ion using Bayesian me hods o explain p e- and pos cu so s in ScP79 wa e o ms [Pachhai e al., 2015, 2023]. Fo Sdi pos cu so s (Sdi +), Ma in e al. [2023a] de-80 eloped a Bayesian me hod mapping he s uc u e in he 2D ho izon al plane, using a wa e on 81 acke as he o wa d model [Hause e al., 2008]. These me hods illus a e he ange o models82 ha can i he da a and educe he explo a ion equi ed h ough o wa d modelling.83 Majo ques ions emain as o wha ULVZs a e made o and how hey o m. A chemically84 dis inc composi ion ha is en iched in i on- ich e ope iclase could explain he obse ed eloci y85 educ ions [e.g. Wicks e al., 2010; Dob osa lje ic e al., 2019; Esdaille and Chen, 2024]. Geody-86 namic modelling sugges s ULVZs ha e a composi ion wi h a majo inc ease in densi y, implying87 ha ULVZs ha e a solid-s a e o igin [Bowe e al., 2011]. Chemical anomalies ha e been sugges ed88 o be a emnan o a basal magma ocean [Lab osse e al., 2007] o sedimen s om he ou e co e89 [Fu e al., 2023]. The p esence o pa ial mel has also been sugges ed o explain high dVp/dVs90 a ios [e.g. Williams e al., 1998]. Mel ing o subduc ed slab ma e ial has been p oposed o explain91 ULVZs loca ed in subduc ed slab egions [e.g. Fes in e al., 2024; Li, 2023]. Howe e , i is unlikely92 ha he mel will emain s able wi hin he ULVZ o e long imescales as i will pool on he CMB,93 esul ing in a global mel laye [e.g. He nlund and Tackley, 2007; Dannbe g e al., 2021], unless he e94 is su icien in e nal s i ing [He nlund and Jellinek, 2010]. Such a global mel laye has no ye 95 been de ec ed unambiguously due o he seismic esolu ion limi , bu a global laye o up o se e al96 kilome es may be possible [e.g. Russell e al., 2022; Hansen e al., 2023]. Longe pe iod no mal97 modes a e also consis en wi h a hin global laye [Russell e al., 2023] bu al e na i ely his could98 be due o a long wa eleng h componen in he dis ibu ion o piled ULVZ ma e ial [Koelemeije 99 4 e al., 2012].100 Besides hei na u e, hei ela ionship wi h su ounding low emains unknown. Some s udies101 ha e sugges ed a ela ionship be ween mega-ULVZs and ho spo s a he su ace and po en ial102 man le plumes connec ing hem [Tho ne e al., 2013; Yuan and Romanowicz, 2017; Kim e al.,103 2020; Co aa and Romanowicz, 2012], which is consis en wi h geophysical models [e.g. Jellinek104 and Manga, 2002]. ULVZs migh he e o e ep esen a ese oi o explain anomalous signa u es105 o 3He/4He and µ183W [Mundl-Pe e meie e al., 2020; Co aa e al., 2022]. Howe e , anomalous106 geochemical obse a ion a e no always obse ed a plumes nea ULVZs [He e e al., 2023; Da ison107 e al., 2024], no can all mega-ULVZs be ela ed o a ho spo o man le plume [Ma in e al., 2024].108 The la e is pa icula ly ue o he la ge ange o smalle ULVZs ha a e obse ed globally [e.g. Yu109 and Ga ne o, 2018; Hansen e al., 2023]. The la ges obse ed ULVZs a e also p edominan ly loca ed110 a he bounda ies o he LLVPs, which is consis en wi h geodynamical modelling ha sugges s111 ha dense ULVZs ha o m ou side he LLVPs a e swep o he bounda ies and accumula e he e112 [Liu e al., 2024].113 In his s udy, we ocus on he egion o he CMB benea h he Sou hwes Paci ic, nea Vanua u114 and Fiji. Posi i e de ec ions and null obse a ions o ULVZs ha e been epo ed he e om mul iple115 seismic p obes [Yu and Ga ne o, 2018] (summa ised in Figu e 1). Due o he dis ibu ion o 116 ea hquakes caused by he subduc ion o he Paci ic pla e unde he Aus alian pla e wi h espec 117 o he Alice Sp ings and Wa amunga seismic a ays in Aus alia, he e a e nume ous s udies118 which ha e in es iga ed he s uc u e o ULVZs in his egion using ScP wa e o ms [e.g. Ros 119 and Re enaugh, 2001; Ideha a e al., 2007; B own e al., 2015; Pachhai e al., 2015]. Using he120 same e en s, o he s udies ha e in es iga ed he possibili y o ULVZs he e om anomalous SPdKS121 wa e o ms [e.g. Tho ne and Ga ne o, 2004; Jensen e al., 2013; Tho ne e al., 2021]. The wide ange122 o epo ed pa ame e s and loca ions o a possible ULVZ indica e ha subs an ial unce ain y123 emains ega ding he ine-scaled s uc u es on he CMB in he New Caledonia/Vanua u egion.124 He e, we p esen new obse a ions o Sdi pos cu so s (Sdi +) which sample he CMB and125 p o ide e idence o a ULVZ in he egion benea h Vanua u and Fiji, om ea hquakes in he126 Paci ic Rise egion owa ds s a ions ac oss Eas Asia. We hus e e o he ULVZ causing hese127 Sdi + signals as he ‘Vanua u’ ULVZ. We in e o ULVZ loca ion and pa ame e s om Sdi +128 a el imes wi h a 2D wa e on acke o six high quali y e en s. F om he ensemble o models129 5 Figu e 1. (a) P obabili y map o he p esence o ULVZs based on he 1% mis i map by Tho ne e al. [2021]. No e ha he e is no dis inc ion in his s udy be ween low p obabili y and poo da a co e age. The dashed black line is he Samoa ULVZ om Tho ne e al. [2021] and K ie e al. [2021]. The black dashdo ed line gi es he p oposed ULVZ om Jensen e al. [2013]. The dashed blue line ep esen s he con ex hull o ou well- i ing pa ame e space o he ULVZ ound in his s udy. The solid blue line gi es he p e e ed model o he ULVZ in his s udy, wi h a adius o 240 km and loca ion 172.2°E and 22.9°S. (b) Zoom-in o he black box in (a) wi h loca ions o bounce poin s on he CMB gi en by: ci cles [Ros and Re enaugh, 2001, 2003; Ros e al., 2005, 2006, 2010], s a s [Ideha a e al., 2007], squa es [Thomas e al., 2009], diamonds [Pachhai e al., 2015], pen agons [B own e al., 2015], iangles [Hansen e al., 2020], and c osses [Pachhai e al., 2022]. The lines a e ay pa hs o SPdKS wa e o ms on he CMB om Tho ne and Ga ne o [2004]. Thei colou s indica e he p esence ( ed) and absence (black) o anomalous wa e o ms; complex wa e o ms (o ange) a e no in e p e ed. The colou ed dashdo ed lines a e p oposed ULVZs in he egion su ounding Vanua u om mul iple p e ious s udies [Thomas e al., 1999; Tho ne e al., 2013; Pachhai e al., 2022, 2023]. 6 which i he da a we compu e 3D ull wa e o m syn he ic models o se e al o he highes quali y130 e en s o es cons ain s on he hickness, la e al ex en , and shea wa e eloci y educ ion o 131 he ULVZ. Finally, we go on o compa e ou model wi h o he s epo ed in his egion, which a e132 la gely consis en .133 3 Da a and Me hods134 3.1 Sdi pos cu so s (Sdi +)135 Shea wa es di ac along and p opaga e pa allel o he CMB and can be obse ed beyond epicen al136 dis ances o app oxima ely 100°(Sdi , Figu e 2a), al hough his a ies depending on he assumed137 1D eloci y model and e en dep h. Ene gy o he Sdi wa e ge s apped and p opaga es as138 a guided wa e wi hin he ULVZ, lagging behind ene gy o he Sdi wa es p opaga ing abo e o 139 a ound he ULVZ. Due o he eloci y con as a he bounda ies o he ULVZ, he guided wa e is140 (a) (b) Figu e 2. (a) P opaga ion o ScP, ScS, SPdKS, S, and Sdi seismic phases om an ea hquake (s a ) o ecei e s ( iangles) h ough a c oss-sec ion o he Ea h. ScP and ScS a e co e- e lec ed phase ha sample bounce poin s on he CMB. SPdKS and Sdi a e phases ha ha e di ac ing legs along he CMB, wi h di ac ion ay pa hs ha a enua e wi h dis ance. (b) Wa e on p opaga ion o Sdi om an ea hquake (s a ), calcula ed using he 2D wa e on acke [Hause e al., 2008; Ma in e al., 2023a]. The wa e on p opaga es h ough ime (sequen ial g ey lines), p oducing pos cu so signals as i passes ac oss a ULVZ (pink ci cle). A ecei e ( iangle) de ec s he di ec a i al (Sdi , g een line) and an addi ional delayed a i al due o e ac ion om he ULVZ (Sdi +, blue line) a a di e en angle o he di ec backazimu h. 7 also e ac ed upon en y and exi o he ULVZ. As a esul , he p opaga ion o he wa e on is141 dis u bed and c ea es pos cu so s o he main Sdi phase (Figu e 2b), he e e e ed o as Sdi +.142 The ime delay o he Sdi + compa ed o he di ec Sdi phase is caused by bo h he p opaga ion143 wi hin he ULVZ and he longe pa h aken due o e ac ion when o -axis. Sdi + signals caused144 by a cylind ical ULVZ ha e an app oxima ely hype bolic a el ime mo e-ou , wi h he minimum145 a i al ime a he on-axis azimu h ela i e o he ULVZ midpoin om he sou ce. De ec ion o 146 Sdi + equi es a la ge and dense seismic a ay. In pa icula , mos ecen s udies o ULVZs using147 Sdi + ha e only been possible due o he dense co e age o seismic s a ions in No h Ame ica [e.g.148 Co aa and Romanowicz, 2012; Yuan and Romanowicz, 2017; Co aa e al., 2022; Wol and Long,149 2023; Ma in e al., 2024], while some Sdi + signals ha e been obse ed by ne wo ks h oughou 150 Eu ope [Da ison e al., 2024], cen al Asia [Yuan and Romanowicz, 2017], and Sou h Ame ica [Li151 e al., 2024]. He e, we use a new geome y om he Sou h Paci ic Rise o s a ions in Eas Asia.152 In his s udy, Sdi speci ically e e s o SHdi , obse ed on he ans e se componen , which153 is obse able a epicen al dis ances up o 150° o su icien ly la ge ea hquakes. SVdi , on he154 o he hand, is s ongly a enua ed in he di ac ed po ion due o leaking in o he co e, esul ing155 in a la ge decay in ampli ude as a unc ion o epicen al dis ance [Koma i sch e al., 2010] and is156 no s udied u he he e.157 3.2 Da a selec ion158 We sea ch o e idence o Sdi + o e en s in he Sou h Paci ic Rise egion using he dense seis-159 mome e dis ibu ion in Eas Asia. We inspec ed all ea hquakes loca ed nea he Sou h Paci ic160 Rise egion a any dep h om 1995 o 2024 and wi h a momen magni ude la ge han 5.5. Da a161 a e ob ained o a dis ance ange o 90–150°, al hough mos da a in his geome y is obse ed162 Da e Lon (°) La (°) Dep h (km) Mag ∆ (s) Region 12003/08/28 -115.20 -49.92 15 6.2 -5.0 Sou he n Eas Paci ic Rise 22005/05/12 -138.91 -57.57 12 6.5 -3.0 Paci ic-An a c ic Ridge 32001/09/02 -136.75 -54.31 15 6.3 0.0 Paci ic-An a c ic Ridge 4a 2007/03/31 -123.61 -55.94 12 6.2 -9.5 Sou he n Eas Paci ic Rise 4b 2016/08/18 -123.63 -55.97 13.44 6.0 -9.5 Sou he n Eas Paci ic Rise 52015/05/19 -132.39 -54.53 14.91 6.6 -1.5 Paci ic-An a c ic Ridge Table 1. Lis o selec ed e en s wi h high quali y Sdi + signals. Sou ce pa ame e s a e om he Global CMT P ojec [Eks ¨om e al., 2012]. The ull lis o e en s can be ound in Table S1. 8 Figu e 3. Map o e en loca ions (s a s), ecei e s ( iangles) and he Sdi ay pa hs sensi i e o he CMB (below 2800 km dep h). The ea hquakes and ay pa hs a e colou ed pe e en : 1–g een, 2–pu ple, 3– ed, 4a–blue, 4b–black and 5–o ange (Table 1); ligh g ey s a s a e e en s iden i ied bu no analysed (Table S1). Ho spo loca ions a e shown as o ange ci cles. The dashed blue line ep esen s he con ex hull o ensemble o models o ULVZs which i he Sdi + a i al ime da a in his s udy. The solid blue line gi es he p e e ed model o he ULVZ in his s udy, wi h a adius o 240 km and loca ion 172.2°E and 22.9°S. The models o he Samoa ULVZ a e gi en as black dashed lines: A–i egula shaped [Tho ne e al., 2021] and B– ec angula shaped [K ie e al., 2021]. The black dashdo ed line shows he in e ed C–diamond shaped ULVZ om Jensen e al. [2013]. The backg ound displays he LLVP o e map a 2800 km dep h [Co aa and Lekic, 2016]. The solid ed line and g eyscale ma ke s deno e he c oss-sec ion ansec in Figu e 8. a 115–130°. The seismog ams we e downloaded om ne wo ks a ailable h ough he Ea hScope163 Conso ium Da a Managemen Cen e and he Na ional Resea ch Ins i u e o Ea h Science and164 Disas e P e en ion (he ea e NIED) F-ne , a b oadband seismog aph ne wo k consis ing o mo e165 han 70 s a ions ac oss Japan [Okada e al., 2004]. A e downloading, he da a we e esampled o166 10 Hz, componen s checked o missing da a, and o a ed in o he ZRT ( e ical, adial, angen ial)167 o ien a ion since SHdi is p ima ily isible on he angen ial componen .168 A o al o 78 ea hquakes loca ed in he Sou h Paci ic Rise egion we e analysed. Mos o he169 ea hquakes had a low signal- o-noise a io (SNR) due o hei low momen magni ude. The e we e170 9 Howe e , he e is signi ican a ia ion in he ela i e ampli udes be ween Sdi and Sdi +, wi h301 small bu s ong anomalies esul ing in small Sdi + ampli udes, and la ge bu weak anomalies302 p oducing la ge Sdi + ampli udes.303 We also compu e syn he ics o models away om he ade-o cu e o demons a e he unsui -304 abili y o hese models and illus a e unce ain ies. A numbe o es ed models a e la ge o mo e305 s ongly educed (Models B1, B3 & C1; ‘abo e’ he cu e in Figu e S7), esul ing in p onounced306 Sdi + ha a e mo e delayed and ha e longe pe iods han obse ed in he eal da a (Figu es S13307 & S14). Fo models ha a e smalle o mo e weakly educed (Models B2, B4 & C2; ‘below’ he308 cu e in Figu e S7), he Sdi + a e compa able in ampli ude when il e ed be ween 10–20 s bu a e309 oo weak o be obse ed in 20–30 s. As expec ed, he a el ime delays o he Sdi + a e educed.310 O e all, he e a e app eciable ade-o s o he size and eloci y educ ion wi h he hickness o he311 ULVZ, which is e lec ed in he unce ain ies in ou inal p e e ed model.312 Figu e 6. Displacemen syn he ics o PREM wi h a ULVZ wi h heigh 20 km, adius 240 km, and shea eloci y educ ion o 30% (Model A3, Table S2) il e ed be ween pe iods o 10–20 s o he six e en s lis ed in Table 1. The same Sdi + highligh s om Figu e 4 a e o e laid, and he colou s co espond o he e en loca ion s a s in Figu e 3. 16 4.3 P e e ed model313 The consis en p esence o Sdi + a i als in he obse ed da a is s ong e idence o he exis ence314 o a ULVZ benea h Vanua u on he CMB. In e sion o he Sdi + a i al imes wi h he 2DWT315 sugges s a ULVZ loca ed a 172.9±0.9°E and 22.9±1.1°S. Fu he modelling wi h 3D ull wa e o m316 syn he ics shows he da a a e consis en wi h a cylind ical ULVZ wi h adius 240 ±50 km, heigh 317 20 ±5 km, and shea wa e eloci y educ ion 30 ±5%. Syn he ics o all o he e en s used in his318 s udy a e shown in Figu e 6.319 Whils he ULVZ was modelled using a simpli ied cylind ical shape, which e ec i ely ep oduces320 he hype bolic mo e-ou o Sdi +, he limi ed azimu hal co e age o he egion o in e es makes i 321 di icul o jus i y o wha ex en his app oxima ion is easonable. As Sdi a els long dis ances322 along he CMB, anomalous s uc u es anywhe e along hose ay pa hs may cause in e e ence and323 p oduce addi ional wa e o ms. The a ailable da a se samples he ULVZ along a single azimu h324 (NW–SE), meaning he loca ion is poo ly cons ained along he ay pa hs in he NW–SE di ec ion325 bu well cons ained in he NE–SW di ec ion (Figu e 5a).326 5 Discussion327 5.1 Compa ison o nea by ea lie s udies328 The Sou hwes Paci ic has been in ensi ely in es iga ed, wi h p e ious s udies inding e idence o 329 he p esence and absence o ULVZs using ScP and SPdKS (summa ised in Figu e 1). A p obabili y330 map de i ed om anomalous SPdKS wa e o ms sugges s he likelihood o ULVZs o he wes ,331 no h, and pa icula ly o he eas o ou p e e ed model loca ion [Tho ne e al., 2021], whe e332 locally a ge ed s udies ha e mapped he Samoa ULVZ [Tho ne e al., 2013; K ie e al., 2021]. To333 he wes , he o ui ous sou ce- ecei e dis ibu ion has led o a ple ho a o s udies using ScP o334 p obe benea h he Co al Sea [e.g. Ideha a e al., 2007; Ros e al., 2010; Pachhai e al., 2022], as335 well as a localised s udy using SPdKS [Jensen e al., 2013]. Since he analyses o hese da a assume336 an in-plane o igin o p e- o pos cu so s in he wa e o ms, i is possible ha ou -o -plane ene gy337 om he ULVZ iden i ied in his s udy could cause hese signals [Pachhai e al., 2024] and we no e338 ha he Vanua u ULVZ is loca ed in a gap o in-plane da a co e age o bo h phases.339 17 We es i ou da a would be able o obse e he o he ULVZs p e iously p oposed in he340 egion by compu ing ull wa e o m syn he ics o E en s 1 and 4a & 4b. Fo he wo published341 Samoa ULVZ models – an i egula shaped ULVZ [Tho ne e al., 2021] and a ec angula shaped342 ULVZ [K ie e al., 2021] – we use a hickness o 26 km and a shea wa e eloci y educ ion o 343 20% as implemen ed by K ie e al. [2021]. To he wes , we es a ULVZ in he egion o highes 344 p obabili y (pu ple con ou in Figu e 1), which is compa able in size o ou p e e ed model. Fo 345 his we use pa ame e s om ou p e e ed model wi h adius 240 km, heigh 20 km and shea 346 eloci y educ ion o 30% eloca ed o he midpoin o he local high p obabili y egion, 166°E and347 24.5°S. The majo i y o p e ious ScP s udies in his egion he e o e all wi hin he bounda ies o 348 ou ial model, as well as wi hin he uppe limi o unce ain y in adius o ou p e e ed model349 (Figu e S15). All ULVZ models es ed p oduced Sdi + signals (Figu es S16 & 7 o E en s 1 and 4a350 & 4b, espec i ely). Fo he models o ULVZs o he eas , Sdi + wa e o ms a e o se signi ican ly351 o he la ge azimu hs, which means ha he Sdi + in he obse ed da a canno be caused by he352 Samoa ULVZ (Figu e 7d-e). Howe e , his does no exclude he possibili y ha hese con ibu e o353 he wa e o m complexi ies obse ed, no would lack o signals necessa ily con adic he p esence354 o he Samoa ULVZ (Figu e S16g), as we ha e p e iously no ed ha he isibili y o ULVZs migh 355 a y by di ec ion [Ma in e al., 2023b]. Fo he model o he wes , he wa e o ms look – pe haps356 unsu p isingly – ai ly simila , as we used he p ope ies o ou p e e ed model, bu hey a e o se 357 o smalle azimu hs. While he o se in azimu h is only sligh o E en 1 (Figu e S16 ), i is qui e358 app eciable o E en 4 (Figu e 7 ). This demons a es ha he loca ion o ou p e e ed model is359 well cons ained by he combina ion o all six e en s, each wi h sligh ly di e en co e age, a leas 360 in he SW-NE di ec ion.361 While a mega-ULVZ model u he o he wes migh no be expec ed in ou da a, a ange o 362 ScP s udies in his egion ha e obse a ions ha all wi hin he con ex hull o ou model space363 (Figu e 1). These obse a ions om bounce poin s udies, howe e , sugges hinne and pa chie 364 ULVZ co e age han we in e he e [e.g. Ideha a e al., 2007; Ros e al., 2010; Pachhai e al., 2022],365 which could be he na owe edge o a la ge , hicke s uc u e [Jensen e al., 2013]. Gi en he low366 sensi i i y o Sdi and Sdi + wa e o ms o small-scale s uc u es a ela i ely long pe iod, his367 s udy canno esol e he complex mo phology o his egional s uc u e, bu suppo s he p esence368 o a la ge, hick quasi-cylind ical s uc u e (∼500 km ac oss, ∼20 km deep). The possibili y o in-369 18 Figu e 7. Wa e o m da a and syn he ics o he Vanua u ULVZ in his s udy and o o he models o p oxima e ULVZs. (a) Obse ed and (b- ) syn he ic displacemen wa e o ms o E en s 4a & 4b. Syn he ics o PREM wi h (b) no ULVZ, (c) he p e e ed ULVZ in his s udy, (d) he i egula ly shaped Samoa ULVZ [Tho ne e al., 2021], (e) he ec angula -shaped Samoa ULVZ [K ie e al., 2021], and ( ) a ULVZ wi h pa ame e s o ou p e e ed model loca ed a he p oxima e p obabili y peak o Tho ne e al. [2021] (Figu e S15). The Sdi + signals a e highligh ed whe e isible. Da a a e il e ed be ween 10–20 s pe iod. plane and ou -o -plane pos cu so s caused by a mega-ULVZ should be conside ed in u u e s udies370 a emp ing o econcile obse a ions o ScP wi h obse a ions o Sdi and SPdKS. O e all, his371 egion appea s o ha e many complexi ies obse ed by di e en seismic phases, simila o he372 pa chie a eas ound a ound he mega-ULVZ nea Hawaii [Jenkins e al., 2021].373 Fu u e s udies using Sdi + and SPdKS could also examine he possible in e nal laye ing o 374 ULVZs by using sho e pe iod pos cu so s. Iden i ying Sdi + a highe equencies is challenging375 as hey a e o en obscu ed by noise, al hough Li, Leng, Jenkins and Co aa [2022] ha e been able376 o iden i y he in e nal laye ing o he Hawaii ULVZ using his me hod.377 5.2 Compa ison o o he mega-ULVZs and ela ion o LLVPs378 The Vanua u ULVZ has compa able pa ame e s o – bu is somewha smalle in la e al ex en han –379 o he modelled mega-ULVZs which ha e been obse ed using Sdi + [e.g. Co aa and Romanowicz,380 19 Figu e 8. C oss-sec ions o ecen whole man le omog aphic models (a) SEMUCB-WM1 [F ench and Romanowicz, 2014], (b) GLAD-M35 [Cui e al., 2024], and (c) REVEAL [Th as a son e al., 2024] h ough he ansec on Figu e 3, which passes h ough he Vanua u and Samoa ULVZs and nea he Lo d Howe and Samoa ho spo s. Veloci y de ia ions a e wi h espec o he (whole Ea h) adial a e age o each omo- g aphic model. Nea by ho spo s a e p ojec ed on o he c oss-sec ion: EA = Eas Aus alia, T = Tasman id, LH = Lo d Howe, and S = Samoa. ULVZ hickness (solid black lines) is exagge a ed o isualisa ion. 2012; Da ison e al., 2024]. Since he e en s sampling his ULVZ a e ypically shallow and low381 magni ude, i is di icul o di e en ia e be ween weak Sdi + signal and noise a sho e pe iods.382 The e is dis inc Sdi + ene gy be ween 10–20 s pe iod and i is isible up o 30 s, sugges ing a383 compa able equency con en and hickness o ha o he Hawaii ULVZ [20 km in heigh , Ma in384 e al., 2023b] and hicke han ha o Iceland [15 km, Yuan and Romanowicz, 2017] o he mid-385 Paci ic ULVZ [10 km, Ma in e al., 2024].386 A he sou hwes edge o he Paci ic LLVP, he Vanua u ULVZ is he eigh h mega-ULVZ mod-387 elled in 3D. All o hese – so a – a e loca ed a o nea he LLVP bounda ies, ein o cing a po en ial388 co ela ion be ween he la ges ULVZs and he bounda ies o he LLVPs. P e ious mega-ULVZs389 ha e also been in e p e ed as whole man le plume oo s due o hei geochemical signa u es [Co -390 aa e al., 2022]. The Vanua u ULVZ is almos equidis an o he Samoa and Lo d Howe ho spo s,391 wi h oughly ∼15–20° o ei he . Howe e , he Samoa ULVZ lies close o ha ho spo and has392 p e iously been sugges ed as i s po en ial oo [e.g. Tho ne e al., 2013; K ie e al., 2021].393 The Lo d Howe seamoun chain lies on he eas e n Aus alian pla e. This seamoun chain,394 combined wi h he Tasman id and Eas e n Aus alia ho spo , acks u he wes and has no395 obse able opog aphic swell, implying ha he hea lux in he a ea is e y mino [Hogga d e al.,396 2020]. Howe e , he Lo d Howe seamoun chain has simila geochemical signa u es o o he Paci ic397 ocean island basal s, wi h S -Nd iso ope analysis sugges ing a deep man le sou ce [Roge s e al.,398 2023]. These signals a e also seen on he Tasman id seamoun chain sugges ing a simila sou ce, bu 399 3He/4He measu emen s o con i m a p imo dial ese oi a e lacking in he egion [Roge s e al.,400 2023].401 20 We show c oss-sec ions o h ee omog aphic models [F ench and Romanowicz, 2014; Cui e al.,402 2024; Th as a son e al., 2024] be ween he Lo d Howe and Samoa ho spo s, ac oss he loca ion403 o he Vanua u ULVZ (Figu e 8). The e a e di e ences be ween he c oss-sec ions, bu all h ee404 models show he Tonga slab, he Paci ic LLVP, and a b oad-scale po en ial plume benea h Samoa,405 unde lain by he Samoa ULVZ. Ano he , olde slab has been sugges ed in his egion a a dep h406 o ∼1100 km om a ossil subduc ion zone [Schella e al., 2009; Van de Mee e al., 2018]. The407 Sou h Loyal y Basin slab is di icul o dis inguish om he Tonga slab in he omog aphic models408 we show due o esolu ion. Towa ds Lo d Howe, one model in pa icula , REVEAL, sugges s a409 con inuous low eloci y anomaly om he egion o he Vanua u ULVZ o he su ace. While he410 o he wo models do no show a con inuous plume he e, all models show some sugges ion o ho 411 upwelling ma e ial de lec ed a ound he Sou h Loyal y Basin slab and down-dipping Tonga slab412 owa ds he sou hwes . Po en ially, spli ing o his plume in o hinne plumele s owa ds he uppe 413 man le makes i ha d o image hese. Al e na i ely, he plume may ha e been ecen ly cu o by414 he slab and could be e ea ing sou hwa ds, which is consis en wi h an age-p og essi e educ ion415 in magma ic lux obse ed o he Lo d Howe seamoun chain [Se on e al., 2019] and he lack o 416 p esen -day opog aphic swell [Hogga d e al., 2020].417 The image o wo nea by b oad-scale plumes, oo ed by ULVZs, aligns wi h he ‘bundle o 418 plumes’ iew p oposed as an explana ion o LLVPs by Da aille and Romanowicz [2020]. Since419 he connec ion o he ULVZ o he man le plume and a po en ial ho spo is enuous, we s ick o420 naming i ‘Vanua u ULVZ’ and no he ‘Lo d Howe ULVZ’. The Vanua u and Samoa ULVZs hus421 ep esen wo la ge scale s uc u es a a dis ance o app oxima ely 20°(∼1200 km a he CMB).422 Fu u e geodynamical models could es i hese ea u es can emain s able, o would be expec ed423 o e en ually me ge.424 21 6 Conclusions425 Sdi + p oduced by 19 ea hquakes loca ed in he Sou h Paci ic Rise egion and de ec ed by s a ions426 ac oss Eas Asia p o ide e idence o he Vanua u ULVZ. A combina ion o in e se modelling o 427 a el imes o Sdi + and o wa d modelling o Sdi + wa e o ms o six o he highes quali y428 e en s places cons ain s on he ULVZ pa ame e s. Since all o he ea hquakes a e co-loca ed, he429 posi ion o he ULVZ is well cons ained in he SW-NE o ien a ion bu poo ly cons ained along430 he NW-SE di ec ion. The p e e ed model o he ULVZ is a cylinde wi h a heigh o 20 ±5 km, a431 adius o 240 ±50 km, and a shea wa e eloci y educ ion o 30 ±5% loca ed o he sou heas o 432 Vanua u a 172.2±0.9°E and 22.9±1.1°S. The e a e s ong ade-o s be ween he heigh , eloci y433 educ ion and size o he ULVZ.434 Al hough he egion su ounding he p oposed loca ion o he Vanua u ULVZ has been in en-435 si ely s udied using ScP and SPdKS wa e o ms, ou s udy o Sdi + add ess a gap o da a co e age.436 Full wa e o m syn he ics o nea by p e iously p oposed models do no explain ou obse a ions.437 Join modelling o SPdKS and Sdi o his egion may p o ide c ucial addi ional cons ain s on438 he dVp/dVs a io, and po en ially densi y, o he ULVZ and he e o e di e en ia e be ween o igin439 hypo heses.440 Syn hesising ou obse a ions wi h p e ious s udies sugges s he CMB he e is a complex land-441 scape comp ising a ‘mega-ULVZ’ and smalle -scale ULVZs, which is wi hin he sou hwes edge o 442 he Paci ic LLVP and ela i ely close o he Samoa mega-ULVZ. Recen omog aphic models hin a 443 he possibili y ha he Vanua u ULVZ lies a he oo o a plume ha is de lec ed o he sou hwes 444 a ound he Tonga slab. Howe e , omog aphic models do no ag ee whe he his plume eeds in o445 he Lo d Howe and neighbou ing ho spo s o i i is now a waning plume.446 22 Acknowledgemen s447 CM and AD we e unded by a Vici awa d (g an numbe 016.160.310/526) om he Du ch Re-448 sea ch Council (NWO). CM and SC ecei ed unding om he Eu opean Resea ch Council (ERC)449 unde he Eu opean Union’s Ho izon 2020 esea ch and inno a ion p og amme (g an ag eemen 450 No. 804071 -ZoomDeep). SC and JA ecei ed unding om he Na u al En i onmen Resea ch451 Council (NE/V018213/1 and NE/S007164/1, espec i ely). Some o he wo k in his s udy was452 pe o med using esou ces p o ided by he Camb idge Se ice o Da a D i en Disco e y (CSD3)453 ope a ed by he Uni e si y o Camb idge Resea ch Compu ing Se ice (www.csd3.cam.ac.uk). We454 also acknowledge he clus e acili ies, Eeji in U ech and Gauss in Camb idge, and hank he455 suppo s a who main ain hem. We would like o hank S ua Russell, Flo ian Mille , Douwe an456 Hinsbe gen, and he U ech seismology g oup o help ul discussions; and Neil Ma jo am, Lukas457 an de Wiel and Theo an Zessen o echnical suppo . We hank he edi o , Ve non Co mie ,458 and wo anonymous e iewe s o hei hough ul commen s and sugges ions which imp o ed he459 manusc ip . Finally, we would like o hank NIED – and e e yone in ol ed wi h he deploymen 460 and main enance o he seismic ne wo k in as uc u e – o making such a ich da a se eely461 a ailable.462 Da a and code a ailabili y463 The acili ies o IRIS Da a Se ices (www.i is.edu), and speci ically he IRIS Da a Managemen 464 Cen e , we e used o access o wa e o ms and ela ed me ada a. IRIS Da a Se ices a e unded465 h ough he Seismological Facili ies o he Ad ancemen o Geoscience (SAGE) Awa d o he Na-466 ional Science Founda ion unde Coope a i e Suppo Ag eemen EAR-1851048. Da a om he F-467 ne and Hi-ne ne wo ks we e made eely a ailable by Na ional Resea ch Ins i u e o Ea h Science468 and Disas e Resilience [Okada e al., 2004]. Da a om NIED was downloaded using Hine Py [Tian,469 2024]. Ea hquake pa ame e s we e used om he Global CMT P ojec (www.globalcm .o g).470 Codes used will be made a ailable by CM upon eques .471 23 Compe ing in e es s472 Au ho s decla e ha hey ha e no compe ing in e es s.473 24 Re e ences474 Bodin, T. and Samb idge, M. [2009], ‘Seismic omog aphy wi h he e e sible jump algo i hm’,475 Geophysical Jou nal In e na ional 178(3), 1411–1436.476 Bowe , D. J., Wicks, J. K., Gu nis, M. and Jackson, J. M. [2011], ‘A geodynamic and mine al physics477 model o a solid-s a e ul alow- eloci y zone’, Ea h and Plane a y Science Le e s 303(3-4), 193–478 202.479 B own, S. P., Tho ne, M. S., Miyagi, L. and Ros , S. [2015], ‘A composi ional o igin o ul alow-480 eloci y zones’, Geophysical Resea ch Le e s 42(4), 1039–1045.481 Co aa , S. and Lekic, V. [2016], ‘Mo phology o seismically slow lowe -man le s uc u es’, Geophys-482 ical Supplemen s o he Mon hly No ices o he Royal As onomical Socie y 207(2), 1122–1136.483 Co aa , S., Ma in, C., Li, Z. and Pa ai, R. [2022], ‘The oo o he Gal´apagos man le plume on484 he co e-man le bounda y’, Seismica 1(1).485 Co aa , S. and Romanowicz, B. [2012], ‘An unusally la ge ULVZ a he base o he man le nea 486 Hawaii’, Ea h and Plane a y Science Le e s 355, 213–222.487 C o well, H. P., Owens, T. J., Ri sema, J. e al. [1999], ‘The TauP Toolki : Flexible seismic488 a el- ime and ay-pa h u ili ies’, Seismological Resea ch Le e s 70, 154–160.489 Cui, C., Lei, W., Liu, Q., Pe e , D., Bozda˘g, E., T omp, J., Hill, J., Podho szki, N. and Pugmi e, D.490 [2024], ‘GLAD-M35: a join P and S global omog aphic model wi h unce ain y quan i ica ion’,491 Geophysical Jou nal In e na ional 239(1), 478–502.492 Dannbe g, J., Myhill, R., Gassm¨olle , R. and Co aa , S. [2021], ‘The mo phology, e olu ion and493 seismic isibili y o pa ial mel a he co e–man le bounda y: implica ions o ULVZs’, Geophys-494 ical Jou nal In e na ional 227(2), 1028–1059.495 Da aille, A. and Romanowicz, B. [2020], ‘De la ing he LLSVPs: Bundles o man le he mochemical496 plumes a he han hick s agnan “piles”’, Tec onics 39(10), e2020TC006265. e2020TC006265497 10.1029/2020TC006265.498 URL: h ps://agupubs.onlinelib a y.wiley.com/doi/abs/10.1029/2020TC006265499 25 Th as a son, S., an He waa den, D.-P., Noe, S., Jose Schille , C. and Fich ne , A. [2024], ‘RE-654 VEAL: A global ull-wa e o m in e sion model’, Bulle in o he Seismological Socie y o Ame ica655 114(3), 1392–1406.656 Tian, D. [2024], ‘Hine Py: A Py hon package o accessing and p ocessing NIED Hi-ne seismic657 da a’, Jou nal o Open Sou ce So wa e 9(98), 6840.658 Van de Mee , D. G., Van Hinsbe gen, D. J. and Spakman, W. 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[2017], ‘Seismic e idence o pa ial mel ing a he oo o majo 674 ho spo plumes’, Science 357(6349), 393–397.675 32 E idence o a ULVZ nea Vanua u om Sdi pos cu so s1 Ca l Ma in1∗, Lobke Ha msma1, James A kins2, A wen Deuss1, Sanne Co aa 2 2 1Depa men o Geosciences, U ech Uni e si y, 3584 CB, Ne he lands3 2Bulla d Labo a o ies, Depa men o Ea h Sciences, Uni e si y o Camb idge, CB3 0EZ,4 UK5 *Co esponding au ho ([email p o ec ed])6 SUPPLEMENTARY7 In his supplemen a y a e:8 •Sec ion S1: Ea hquake da a used in his s udy9 •Sec ion S2: Wa e o ms o syn he ics o di e en pa ame e s o demons a e ade-o s10 •Sec ion S3: Wa e o m modelling o ULVZs in ea lie s udies o compa ison11 1 S1 Ea hquake da a12 S1.1 Ea hquake lis 13 In o al we iden i ied 19 ea hquakes in he Sou h Paci ic Rise egion owa ds seismic a ays Eas 14 Asia – pa icula ly he Japanese F-ne [Okada e al., 2004] – which showed Sdi pos cu so e idence15 o a ULVZ (Table S1).16 Da e Lon. [°E] La . [°N] Dep h [km] Mag. Loca ion 2001/08/06 -123.05 -55.67 15 Mw 6.7 Sou he n Eas Paci ic Rise 32001/09/02 -136.75 -54.31 15 Mw 6.3 Paci ic-An a c ic Ridge 12003/08/28 -115.20 -49.92 15 Mw 6.2 Sou he n Eas Paci ic Rise 2004/01/29 -114.85 -50.03 15 Mw 6.1 Sou he n Eas Paci ic Rise 22005/05/12 -138.91 -57.57 12 Mw 6.5 Paci ic-An a c ic Ridge 2006/10/10 -122.41 -56.18 12 Mw 6.0 Sou he n Eas Paci ic Rise 4a 2007/03/31 -123.61 -55.94 12 Mw 6.2 Sou he n Eas Paci ic Rise 2007/11/02 -128.80 -55.40 13.55 Mw 6.2 Paci ic-An a c ic Ridge 2011/11/02 -129.08 -55.34 15 Mw 6.2 Paci ic-An a c ic Ridge 2012/07/18 -128.96 -55.36 20.34 Mw 5.9 Paci ic-An a c ic Ridge 2014/05/12 -115.06 -49.90 13.24 Mw 6.4 Sou he n Eas Paci ic Rise 52015/05/19 -132.39 -54.53 14.91 Mw 6.6 Paci ic-An a c ic Ridge 2015/12/24 -123.11 -56.06 12.13 Mww 6.2 Sou he n Eas Paci ic Rise 2016/02/16 -124.57 -55.74 20.13 Mww 6.1 Sou he n Eas Paci ic Rise 4b 2016/08/18 -123.63 -55.97 13.44 Mw 6.0 Sou he n Eas Paci ic Rise 2017/06/15 -124.44 -55.77 20.03 Mww 5.8 Sou he n Eas Paci ic Rise 2018/11/15 -122.26 -56.23 12 Mww 6.3 Sou he n Eas Paci ic Rise 2024/05/18 -123.14 -56.05 12 Mww 5.9 Sou he n Eas Paci ic Rise 2024/06/09 -133.88 -54.06 12.62 Mww 6.2 Paci ic-An a c ic Ridge Table S1. Ea hquake da a and pa ame e s o e en s showing Sdi pos cu so s ha sample he CMB nea Vanua u. This s udy uses he e en s labelled 1–5 (Table 1 in main pape ). E en loca ions a e plo ed in Figu e 3. Ea hquake pa ame e s and momen enso solu ions a e aken om he Global CMT P ojec [Eks ¨om e al., 2012]. 2 S1.2 E en s used in he s udy17 We ep oduce Figu e 4, showing he wa e o ms o six ea hquakes, E en s 1–5, il e ed be ween18 7–12, 20–30 and 30–40 s pe iod (Figu es S1–S3, espec i ely). We also plo displacemen wa e o ms19 om he Hi-ne sho pe iod seismome e s il e ed be ween 10–20 and 7–12 s pe iod (Figu es S4 &20 S5, espec i ely) [Okada e al., 2004].21 Figu e S1. Same as Figu e 4 bu il e ed be ween 7–12 s pe iod. 3 Figu e S2. Same as Figu e 4 bu il e ed be ween 20–30 s pe iod. Figu e S3. Same as Figu e 4 bu il e ed be ween 30–40 s pe iod. 4 Figu e S4. Displacemen wa e o ms om he sho pe iod Hi-ne seismome e s o E en s 1–5 il e ed be ween 10–20 s pe iod. The linea s ack o each 1°azimu hal bin ( hick black lines) o e lay he indi idual seismog ams ( hin g ey lines). No e he di e en (y axis) azimu hal ange compa ed o b oadband da a. Figu e S5. Same as Figu e S4 bu il e ed be ween 7–12 s pe iod. 5 S2 T ade-o s in wa e o m modelling22 Figu e S6 shows he syn he ics p edic ed o he sou ce solu ions om he Global CMT P ojec 23 wi h he dep h se o 0 km which bes ma ches he obse ed Sdi wa e o ms. We no e ha he24 da a and syn he ics o E en 2 ha e been lipped o ma ch he wa e o ms o he o he e en s o 25 ease o isual compa ison.26 Figu e S6. Displacemen syn he ics o PREM il e ed be ween pe iods o 10–20 s o he six e en s lis ed in Table 1. Ea hquake pa ame e s and momen enso solu ions a e aken om he Global CMT P ojec [Eks ¨om e al., 2012], wi h dep hs se o 0 km. 6 We explo e he ade-o in model pa ame e s now using ull wa e o m syn he ics. The loca ion and27 unce ain ies a e in e ed om he 2DWT in e sion (Figu e 5a), which a e assumed o be ixed o 28 he emainde o his unce ain y assessmen . We ial a numbe o models along he eloci y-size29 ade-o cu e o he 2DWT ensemble o demons a e ha his ade-o is ep oduced by ull30 wa e o m syn he ics as well as some o -cu e pa ame e s (Figu e S7 & Table S2). Addi ionally, we31 cons ain he heigh by compa ing he p esence o absence o pos cu so s in syn he ics wi h ULVZs32 o heigh s 15, 20, 25 and 30 km o di e en il e bands. We also show p edic ed a el imes om33 he 2DWT o he a ious models lis ed in Table S2 (Figu e S8).34 Figu e S7. Duplica e o Figu e 5b, wi h pa ame e labels o e laid. Ensemble o models ha i he Sd- i pos cu so a i al imes, showing he ade-o in eloci y and size. The median model is ma ked wi h a ed c oss and his og ams o each o he pa ame e s a e p ojec ed on o he espec i e 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. Pa ame e s used o es 3D cylind ical models o he ULVZ. All models a e cen ed a he median loca ion, 172.16 °E and 22.92 °S. Syn he ics a e also un o Model A3 wi h heigh s 15, 25, and 30 km o demons a e sensi i i y o he equency con en o he wa e o ms o model heigh . 7 Figu e S8. P edic ed a el imes om he 2DWT o he a ious models o which we compu e ull wa e o m syn he ics (Figu e S7 & Table S2). (a) is he same as Figu e 5c. No e ha he 2DWT p edic ions do no accoun o heigh o he model. 8 Figu e S9. (a) Real da a and syn he ic wa e o ms o (b) PREM wi h no ULVZ and PREM wi h a cylind ical ULVZ o heigh (c) 15 km, (d) 20 km, (e) 25 km, and ( ) 30 km o E en s 4a & 4b, il e ed be ween 10–20 s pe iod. All models use dVs o -30% and a adius o 240 km (Model A3, Table S2). Figu e S10. Same as Figu e S9 bu il e ed be ween 20–30 s pe iod. 9