Observations of core-mantle boundary Stoneley modes

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 4, 1 5, doi:1./grl.5514, 213 Obervation of core-mantle boundary Stoneley Paula Koelemeijer, 1 Arwen Deu, 1 and Jeroen Ritema 2 Received 11 March 213; revied 22 April 213; accepted 26 April 213. [1] Core-mantle boundary (CMB) Stoneley repreent a unique cla of normal with extremely trong enitivity to wave peed and denity variation in the D region. We meaure plitting function of eight CMB Stoneley uing modal pectra from 93 event with M w > 7.4 between 1976 and 211. The obtained plitting function map correlate well with the predicted plitting calculated for S2RTS+Crut5.1 tructure and the ditribution of S diff and P diff travel time anomalie, uggeting that they are robut. We illutrate how our new CMB Stoneley plitting function can be ued to etimate denity variation in the Earth lowermot mantle. Citation: Koelemeijer, P., A. Deu, and J. Ritema (213), Obervation of core-mantle boundary Stoneley, Geophy. Re. Lett., 4, doi:1./grl Introduction [2] The D region i the lowet 2 3 km of the mantle, atop the core-mantle boundary (CMB). D i characterized by ultra-low-velocity zone (ULVZ), eimic dicontinuitie, aniotropy, CMB topography, and, mot prominently, by large-low-hear-velocity province (LLSVP) below Africa and the Pacific [e.g., Lay, 27; Garnero and McNamara, 28]. The LLSVP extend hundred of kilometer both laterally and vertically into the lower mantle [Ritema et al., 1999]. To ae their effect on mantle dynamic, it i eential to have information on the denity variation [Forte and Mitrovica, 21]. [3] Obervation of Earth normal have the potential to contrain both wave peed and denity variation in the mantle. Previou normal analye ugget an anticorrelation between variation in the eimic hear velocity and denity, particularly for the LLSVP [e.g., Ihii and Tromp, 1999; Trampert et al., 24; Moca et al., 2]. Thee reult motivated the ling of LLSVP a long-lived pile of intrinically dene material [e.g., Davaille, 1999; McNamara and Zhong, 25]. However, it wa debated whether ome of thee denity l are robut, a they depend on the regularization and a priori contraint [Romanowicz, 21; Kuo and Romanowicz, 22], and the tudied have enitivity to both the upper and lower mantle [Reovky and Ritzwoller, 1999]. Additional upporting information may be found in the online verion of thi article. 1 Bullard Laboratorie, Univerity of Cambridge, Cambridge, UK. 2 Department of Earth and Environmental Science, Univerity of Michigan, Ann Arbor, Michigan, USA. Correponding author: P. J. Koelemeijer, Bullard Laboratorie, Department of Earth Science, Univerity of Cambridge, Madingley Rie, Madingley Road, Cambridge, CB3 EZ, UK. (pjk49@cam.ac.uk) 213. American Geophyical Union. All Right Reerved /13/1./grl.5514 [4] Here we reviit normal contraint on the denity tructure of the lower mantle by focuing on Stoneley [Stoneley, 1924]; a unique cla of that are confined to olid-liquid interface uch a the CMB (Figure 1). CMB Stoneley have extremely focued enitivity to tructure in D and the outermot core and hence do not uffer from trade-off with upper mantle tructure. However, they have o far not been oberved due to inufficient available data. We preent, for the firt time, plitting function meaurement of CMB Stoneley, dicu the robutne of our meaurement, and illutrate how they can be ued in tomographic inverion to contrain denity tructure in the lowermot mantle. 2. Normal Mode [5] Earth normal are tanding wave ariing along the urface and radiu of the Earth. They are oberved a clear peak in the amplitude pectra of everal day long eimic recording of large (M w >7.4) earthquake. Mode only exit at dicrete frequencie, due to the finite ize of the Earth, and are characterized by their radial order n and angular order l. We focu here on pheroidal n S l which involve P-SV motion. Each normal conit of 2l+1 inglet with azimuthal order m in the range l,...,l. Thee inglet are degenerate (i.e., have the ame frequency) for a pherically ymmetric, iotropic, nonrotating Earth l uch a the Preliminary Reference Earth Model (PREM) [Dziewonki and Anderon, 1981]. Significant plitting of the inglet into different frequencie occur by the rotation and ellipticity of the Earth and velocity and denity heterogeneity, aniotropy, and topography on internal boundarie in the Earth. [6] Normal plitting can be completely decribed uing the plitting function approach introduced by Woodhoue and Giardini [1985]. Splitting function coefficient c t are linearly related to the perturbation of the reference Earth l according to: c t = Z a ım t (r)k (r)dr + X d ıh d t Hd (1) where ım t (r) are the pherical harmonic coefficient of angular order and azimuthal order t to decribe Earth tructure, including perturbation in S wave velocity (V ), P wave velocity (V p ), denity (), and aniotropy. ıh d t repreent topography on dicontinuitie d, andk (r), H d are the aociated enitivity kernel [Woodhoue, 198]. Splitting function map F(, ) are ued to viualize plitting function, i.e., X2l F(, ) = X = t= c t Y t (, ) (2) 1

2 (a) 1 S (b) 2 S 16 (c) 3 S 26 (a) 758A SUR (b) 264A INCN Surface D CMB V p enitivity V enitivity enitivity D 1km CMB Figure 1. Senitivity kernel for V p (olid), V (dahed), and (red) for repreentative CMB Stoneley n S l and a zoom of the enitivity in the D region. Note that the Stoneley enitivity become more focued at the CMB with increaing angular order l. where Y t (, ) are the complex pherical harmonic of Edmond [196]. Thee map how the local variation in plitting due to the underlying heterogeneity. 3. Method and Data [7] Splitting function are meaured from the inverion of pectra oberved for large earthquake. We make ue of a recent normal pectra data et of 92 event with M w > 7.4 for the period [Deu et al., 211, 213], with the addition of the 211 Tohoku event (M w = 9.). Following Deu et al. [213], we meaure the plitting function uing nonlinear iterative leat quare inverion [Tarantola and Valette, 1982], tarting from PREM or prediction for mantle and crut tructure. Cro validation i ued to determine the error of our meaured coefficient. [8] Meauring CMB Stoneley i complicated a they generally overlap in frequency with a (high-amplitude) fundamental with n =. Hence, we mut invert for their plitting function jointly a previouly done for 1 S 14 by Reovky and Ritzwoller [1998]. We alo meaure the fundamental eparately to verify that we improve the mifit by including the CMB Stoneley. We account for the coupling between fundamental pheroidal and toroidal due to Earth ellipticity and rotation. The mifit i maller when the CMB Stoneley i added (Table 1), which i alo viible for individual pectra (Figure 2). Table 1. Mifit for the Meaured Splitting Function a Mode PREM S2 m t i m t N N ev 1S 11 - S 15 -( T 16 ) S - S 17 -( T 18 ) S 13 - S 19 -( T 2 ) S 14 - S 21 -( T 22 ) S 15 - S 23 -( T 24 ) S 16 - S 25 -( T 26 ) S 14 - S 22 -( T 23 ) S 15 - S 24 -( T 25 ) S 16 - S 26 -( T 27 ) S 25-3 S S 26-6 S 15-9 S a PREM denote the mifit including only ellipticity and rotation, and S2 denote the mifit for S2RTS+Crut5.1 ynthetic. The final mifit i given for the meaurement without (m t i) and with (m t ) the CMB Stoneley (denoted by ). The number of pectra (N ) and event (N e ) i hown. Bold correpond to new, and in bracket are included for rotation and ellipticity coupling. Phae Normalied amplitude Stoneley 1S Fundamental S Frequency (mhz) Stoneley 2S 16 S 26 Fundamental Frequency (mhz) Data Fundamental Fundamental+Stoneley Figure 2. Amplitude and phae pectra for (a) a deep event (Okhotk, 28, 615 km, M w =7.7) at tation SUR (South Africa) and (b) a hallow event (Sumatra, 24, 28.6 km, M w =9.) at tation INCN (South Korea). [9] We compare our meaurement to prediction for mantle and crut tructure, calculated uing mantle V l S2RTS [Ritema et al., 1999]. We aume caling factor of the form R p = ılnv p /ılnv =.5and R = ıln/ılnv =.3, conitent with previou work [Karato, 1993; Li et al., 1991]. The contribution of crutal thickne, urface topography, and water level are calculated uing l Crut5.1 [Mooney et al., 1998]. 4. Reult 4.1. Splitting Function Obervation [1] We have made plitting function meaurement of 23 in total, including eight CMB Stoneley and four other new along the ame overtone branche ( 1 S 15, 1 S 16, 2 S 14 and 3 S 25 ). In addition, we have meaured the aociated fundamental up to S 26. [11] The oberved plitting function map (Figure 3) how the Ring around the Pacific pattern of high frequencie and pronounced low frequencie at the LLSVP. Within the Ring tructure, iolated patche of elevated frequencie are identified, particularly underneath Southeatern Aia and South America. The plitting function map reemble the prediction for S2RTS+Crut5.1 tructure cloely. However, individual coefficient uch a the c 2 differ ubtantially from the prediction (Figure S1 in the upporting information). In addition, the mifit i ignificantly lower for our meaurement (Table 1). We verify uing F-tet tatitic that the mifit reduction due to including the CMB Stoneley i ignificant (9% confidence level). Detail on the mifit calculation and F-tet can be found in the upplementary online material. Correponding center frequencie and quality factor for our meaurement are in Table 2, and our plitting function coefficient can be found online (Table S1) Comparion to S diff and P diff Data [] CMB Stoneley have imilar enitivity to wave diffracting around the core uch a the S diff and P diff phae. We ue the travel time anomaly data et from 2

3 Oberved plitting (a) 1 S - max =4 S2RTS+Crut5.1 (b) 1 S (c) 2 S 16 - max =6 (d) 2 S (e) 2 S 25 - max =6 (f) 2S (g) 3 S 26 - max =4 (h) 3S Figure 3. Oberved plitting function for four CMB Stoneley and their correponding prediction for mantle l S2RTS [Ritema et al., 1999] and crutal l Crut5.1 [Mooney et al., 1998]. The have been meaured up to angular order max. Ritema and Van Heijt [22] obtained for event between 198 and 2 with M b > 5.9. The binned data how good coverage in the northern hemiphere but le in the outhern hemiphere (Figure 4a 4c). Again, we oberve a characteritic Ring around the Pacific pattern, becoming even clearer in the even pherical harmonic expanion of the diffracted wave data (Figure 4b 4d), though, ome of the outhern hemiphere tructure could be due to inherent ymmetry of the even degree expanion. [13] The expanded S diff and P diff data and the CMB Stoneley plitting function of 2 S 25 and 3 S 26 have a trong reemblance. The correlation between the and S diff data i typically.99 and.75 for degree 2 and 4, repectively, wherea the correlation to P diff data i lower at.98 and.5. Thi imilarity trengthen our confidence in our CMB Stoneley meaurement. The additional advantage of the normal i that they automatically provide coverage in the outhern hemiphere. Table 2. Normal Mode Center Frequencie in Hz and Quality Factor Q for the Mode Meaured in Thi Study Compared With PREM Value a Mode PREM f Meaured f PREM Q Meaured Q S S S S 11 1S 1S 13 1S S S S 15 2S 16 2S 25 a Bold correpond to new. 3

4 (a) Traveltime anomalie S diff Even degree expanion (b) S diff - max = (c) P diff (d) P diff - max = Figure 4. S diff and P diff data for comparion with the CMB Stoneley meaurement: (a c) Travel time anomalie with repect to PREM, for epicentral ditance of 1 14 ı plotted at the midpoint of the diffracted path, binned within a 5 ı cap. (b d) Even degree pherical harmonic expanion of the travel time data Senitivity to Denity [14] The enitivity kernel of the CMB Stoneley (Figure 1) how a trong enitivity to V p at the CMB, wherea the enitivity to V and i imilar and peak R P R P Mifit contour Rρ Degree 2 mifit (a) 1 S 1 (b) 3 S 26 Cro ection R P =.5 R P = Rρ Purely thermal Figure 5. (left) Contour plot of normalized degree 2 mifit between obervation and ynthetic plitting function with varying R P and R for 1 S 1 and 3 S 26. The right panel how a cro ection of the mifit veru R ratio along R P =.25 and.5. The red box denote ratio that would be conitent with purely thermal variation [Karato, 1993; Moca et al., 2] Degree 2 mifit Degree 2 mifit in the D above the CMB. Thu, Stoneley are ueful to contrain R in the D which play an important role in determining the nature of the LLSVP. We calculate plitting function ynthetic uing S2RTS and Crut5.1 in which we vary R p and R between 1 and 2 for a 3 km thick D layer. We compute the mifit between the oberved and calculated plitting function coefficient for individual. [15] Contour plot of the mifit for = 2 are hown for 1 S 1 (previouly oberved, e.g., Reovky and Ritzwoller [1998] and Deu et al. [213]) and CMB Stoneley 3 S 26 (Figure 5). 1 S 1 can be ued to put ome contraint on R P but cannot contrain denity variation even though the enitivity in D i nonzero. However, 3 S 26 ha a trong enitivity to both R P and R,andtheame i oberved for other CMB Stoneley. Auming value for R P of.5 and.25 a a range of poible value [Karato, 1993; Ritema and Van Heijt, 22], we oberve for 3 S 26 a bet fitting R of.1 and.5, repectively, cloe to the range of ratio that would be conitent with purely thermal variation [e.g., Karato, 1993; Moca et al., 2]. Thi ugget that the anticorrelation between denity and hear wave velocity might not be required by our new data. However, without good contraint on R P, we cannot contrain R accurately. In addition, R P and R trade-off with other tructure in D uch a CMB topography and ULVZ [Koelemeijer et al., 2], and therefore, a proper inverion i required to draw any firm concluion regarding thermal veru thermochemical LLSVP. 5. Concluding Remark [16] Uing a data et of 93 large earthquake, we make robut plitting function obervation of eight CMB Stoneley. Their plitting function map correlate well with expanded S diff and P diff data uggeting they are robut. We demontrate the enitivity of the Stoneley to denity variation in the lowermot mantle and illutrate 4

5 the trade-off with P wave velocity tructure. Thi trade-off can be partially removed when we conider thinner layer (1 km thick) due to the nature of the enitivity kernel (Figure 1). In addition, a large number of P wave enitive normal obervation i available [Deu et al., 213], and body wave data alo provide contraint on R P. Therefore, when our new meaurement are included with thee in tomographic inverion, they will help to provide tighter contraint on the denity variation in the lowermot mantle. [17] Acknowledgment. We thank the Editor (Michael Wyeion), Caroline Beghein, and Joeph Reovky for their detailed comment, which greatly improved the manucript. Data were provided by the IRIS/DMC. PJK and AD are funded by the European Reearch Council under the European Community 7th Framework Programme (FP7/27-213)/ERC grant agreement PJK i alo upported by the Nahum Scholarhip in Phyic and a Graduate Studenthip, both from Pembroke College, Cambridge. 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