The Earth's outer core has been considered to be chemically well mixed, and seismological models for the outer core have been obtained by assuming a homogeneous shell composed of liquid iron. However, dynamical analyses have proposed the existence of light materials at the top of the outer core, and seismic studies have been inconclusive with evidence both for and against the existence of chemical stratification below them CMB.
The current study is an attempt at resolving this issue with seismological observations. We obtained 606 high-quality measurements of S3KS-SKKS differential traveltimes measurements using broadband records at globally distributed stations from 78 deep earthquakes in the distance range 120º-140º. The long propagation distance and interference with neighboring phases require us to make the measurements of the differential traveltimes by waveform cross correlation. Therefore, for comparison with the observations, we use DSM-calculated waveforms to derive model-predicted differential traveltimes. Modeling experiments using PREM and several PREM-like models suggest that the actual wave speed at the top of the outer core is likely to be about 0.1% slower that PREM.
Taking the modeling results of PREM-like models as guides for determining the a priori probability distribution function, we carried out a Bayesian inversion using the
observed S3KS-SKKS differential traveltime measurements for the velocity structure at the top of the outer core. The Metropolis-Hastings Monte Carlo algorithm was adopted to enable an efficient sampling of the model space. In addition, to further reduce the time needed for computing model-predicted data, we used Taup Toolkit to calculate the
S3KS-SKKS differential traveltimes in the sampling models and correct for the finite-frequency effect using the difference between TauP and DSM results observed for PREM. The resulting Bayesian inversion based on the MHMC algorithm enabled us to achieve a successful sampling of the model space and reach a stable model expectation after 5092 model samples with 109 accepted models.
The final expected model PREM-SOC is on average slower than PREM by about 0.11% in the 550-km layer below the CMB 550 km. The expected model has a maximum perturbation of -0.31% from PREM at about 60 km below the CMB, and shows a clear variation of velocity perturbation with depth. The amount of reduction in wave speed in our expected model in the top 300 km region at the top of the outer core is similar to that proposed by some previous studies (e.g. Helffrich & Kaneshima, 2010).
In conclusion, our result strongly supports the argument for the existence of chemical stratification of light materials at the top of the Earth's outer core.
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Appendix A. Earthquake Information
Table A.1. List of deep earthquakes used in this study.
No. Event ID Lon.
(º)
Lat.
(º)
Depth
(km) Mw Geographic Location
Focal Mech.
# of Data 1 200910072141A 122.51 4.11 579.5 6.8 Celebes Sea 13 2 201003201400A 152.33 -3.32 413.5 6.6 New Ireland 0 3 201007232208A 123.59 6.54 596.8 7.3 Philippines 5 4 201007232251A 123.90 6.62 576.9 7.6 Philippines 19 5 201007232315A 123.48 6.83 641.4 7.4 Philippines 10 6 201007240535A 123.56 6.23 556.4 6.6 Philippines 14 7 201007290731A 123.41 6.69 614.0 6.6 Philippines 16 8 201011300324A 139.26 28.69 460.6 6.8 Japan 0 9 201101010956A -63.21 -27.02 586.0 7.0 Santiago
Argentina 9
10 201102101441A 123.28 4.22 517.6 6.5 Celebes Sea 20 11 201103101708A 116.71 -6.95 517.0 6.6 Bali Sea 22
12 201107290742A 179.92 -23.78 539.0 6.7 Fiji 18
13 201102211057A 178.47 -25.95 567.5 6.5 Fiji 3
14 201005241618A -71.64 -8.08 591.4 6.4 NW Brazil 3 15 201109021347A -63.08 -28.56 597.3 6.7 Santiago
Argentina 6
16 201111221848A -65.18 -15.40 553.8 6.6 Bolivia 3 17 201205280507A -63.07 -28.25 591.6 6.7 Santiago
Argentina 9
18 201108300657A 126.68 -6.47 468.7 6.9 Banda Sea 5
19 072504B 104.38 -2.68 600.5 7.3 Sumatra 54
20 200910041058A 123.56 6.77 625.8 6.6 Philippines 11 21 200911091044A 178.53 -17.11 603.9 7.3 Fiji 0 22 201002180113A 130.66 42.48 579.0 6.9 NE China 0 23 201302221201A -63.00 -27.89 585.7 6.1 Santiago
Argentina 2
24 201302261957A -179.31 -21.48 626.1 5.8 Fiji 1
25 201102101439A 122.91 4.14 520.9 6.5 Celebes Sea 8 26 200908280151A 123.46 -7.09 634.4 6.9 Banda Sea 1 27 201304051300A 131.02 42.77 571.9 6.3 NE China 0
28 201304210322A 138.88 30.02 429.8 6.1 Japan 1
29 201305140032A 145.35 18.67 604.7 6.8 Mariana 2 30 201305241456A 151.48 52.36 642.4 6.7 Okhotsk 3
31 201306071254A 179.07 -23.83 557.1 5.9 Fiji 1
32 200811240902A 154.71 54.27 502.3 7.3 Okhotsk 3 33 200807050212A 153.37 54.12 610.8 7.7 Okhotsk 1
34 200801151752A -179.34 -22.05 603.3 6.5 Fiji 1
35 201308280254A 179.84 -27.73 492.0 6.2 Kermadec 9
36 201308291352A -178.94 -19.35 684.6 5.8 Fiji 0
37 201309040018A 138.79 30.02 412.0 6.5 Japan 3
38 201309210139A 119.92 -7.32 550.7 6.1 Flores Sea 9 39 201310010338A 152.89 53.13 592.0 6.7 Okhotsk 4
40 200710162105A 179.72 -25.70 512.4 6.6 Fiji 10
41 200710050717A 179.50 -25.27 540.8 6.5 Fiji 9
42 200705062111A -179.04 -19.44 690.8 6.5 Fiji 4
43 200611130126A -63.47 -26.10 573.4 6.8 Santiago
Argentina 8
44 200602021248A -178.13 -17.77 611.6 6.7 Fiji 3
45 200601022213A -177.72 -19.80 589.5 7.2 Fiji 2
46 200503211223A -63.54 -24.88 572.3 6.8 Santiago
Argentina 4
47 200502051223A 123.67 5.47 530.6 7.1 Philippines 26
48 111704G -178.40 -19.87 629.4 6.5 Fiji 4
49 071504C -178.52 -17.68 577.2 7.1 Fiji 5
50 072703C 139.23 46.99 477.2 6.7 Sea of Japan 4 51 062003D -71.89 -7.37 556.2 7.0 NW Brazil 4 52 052603E 123.85 6.90 579.7 6.8 Philippines 23
53 111702C 146.45 47.81 479.8 7.3 Kuril Is. 5
54 101202H -71.66 -8.30 539.4 6.9 Brazil 13
55 081902A -179.08 -21.74 630.9 7.6 Fiji 1
56 081902C 178.49 -24.16 699.3 7.7 Fiji 3
57 063002F 179.43 -22.13 631.6 6.4 Fiji 8
58 080700E 123.53 -6.95 648.3 6.5 Banda Sea 2
59 021601C 117.62 -7.11 537.9 6.0 Bali Sea 12
60 061400C 178.38 -25.45 615.4 6.4 Fiji 6
61 200609090413A 120.27 -7.23 583.2 6.3 Flores Sea 11 62 200704210712A 151.46 -3.46 405.0 6.1 New Ireland 10 63 200709250515A -179.85 -30.69 420.6 6.2 Kermadec 9
64 200804182039A -178.98 -17.26 577.8 6.3 Fiji 4
65 200910250753A -178.97 -23.00 419.3 6.0 Fiji 2
66 201006300430A 179.26 -23.19 581.6 6.4 Fiji 8
67 201008161935A -178.67 -20.74 604.0 6.2 Fiji 3
68 201012280834A -179.73 -23.49 571.4 6.3 Fiji 4
69 201102071953A 155.34 -7.04 425.5 6.5 Solomon Is. 1 70 042300B -63.04 -28.41 607.9 6.9 Santiago
Argentina 6
71 121800A -178.98 -21.11 655.7 6.5 Fiji 5
72 091201A -178.90 -20.84 634.1 6.4 Fiji 4
73 201201240052A 178.72 -25.00 581.8 6.3 Fiji 14
74 201108190354A -176.73 -16.52 415.0 6.2 Fiji 7
75 201110270015A -179.40 -17.98 608.7 6.0 Fiji 4
76 201104031407A -178.45 -17.65 562.3 6.4 Fiji 4
77 032998D -178.85 -17.57 553.7 7.1 Fiji 41
78 012798E 179.29 -22.39 629.4 6.4 Fiji 24
79 051698B -179.35 -22.27 608.8 6.8 Fiji 0
80 082098B 139.47 28.99 425.5 7.0 Bonin 0
81 050400K -178.31 -17.72 539.8 6.4 Fiji 0
82 050800C 150.14 -4.38 496.3 6.1 New Britain 0
83 062802B 130.45 43.74 581.5 7.3 NE China 0
84 200606270259A -178.04 -19.77 596.7 6.3 Fiji 0 85 200701082052A -177.68 -18.46 423.7 6.3 Fiji 0 86 200705062201A -179.05 -19.31 691.6 6.0 Fiji 0 87 200911220748A -178.36 -17.72 546.4 6.3 Fiji 0 88 201004112208A -3.69 37.10 616.5 6.3 Gibraltar 0 89 201208140259A 145.70 49.97 598.2 7.7 Okhotsk 0 90 201305240544A 153.77 54.61 611.0 8.3 Okhotsk 0 91 201309020251A 133.65 42.19 454.7 5.7 Vladivostok
Russia 0
92 201311031103A 123.28 4.76 543.1 6.0 Celebes Sea 0 93 201311191700A 145.18 18.54 515.2 6.0 Mariana 0 94 201205262148A 140.17 26.87 472.1 6.0 Japan 0 95 201203050746A -63.27 -28.19 565.1 6.1 Santiago
Argentina 0
96 201108190354A -176.73 -16.52 415.0 6.2 Fiji 0 97 201101122132A 139.94 26.94 511.5 6.5 Japan 0 98 201003080947A 144.67 19.26 432.2 6.1 Mariana 0 99 200912100230A 152.77 53.44 655.7 6.3 Okhotsk 0 100 200911222247A 179.66 -31.47 437.1 6.2 Kermadec 0 101 200809031125A -63.30 -26.85 571.3 6.3 Santiago
Argentina 0
102 200807030302A -179.69 -23.41 589.9 6.2 Fiji 0 103 200804182039A -178.98 -17.26 577.8 6.3 Fiji 0 104 200802011210A -179.27 -21.53 629.5 6.0 Fiji 0 105 200711190052A -178.63 -21.05 562.5 6.3 Fiji 0 106 200707211327A -71.30 -8.08 637.5 6.0 NW Brazil 0 107 200703090322A 133.65 43.22 451.1 6.0 Vladivostok
Russia 0
108 200609220232A -63.05 -26.85 602.4 6.0 Santiago
Argentina 0
109 200606090558A -178.62 -17.36 585.9 6.1 Fiji 0 110 200606020731A -178.54 -20.77 584.6 6.0 Fiji 0 111 200602260308A -179.82 -23.59 553.9 6.4 Fiji 0 112 200602241415A -179.42 -17.94 640.9 6.1 Fiji 0 113 200509042358A 123.32 3.05 454.4 6.0 Celebes Sea 0 114 200503301741A -179.64 -22.31 585.1 6.1 Fiji 0 115 200503211243A -63.56 -24.71 572.2 6.4 Santiago
Argentina 0
116 200503191734A -179.27 -21.88 609.2 6.3 Fiji 0
117 120604B -178.27 -18.84 465.6 6.1 Fiji 0 118 111204H -63.18 -26.85 583.1 6.0 Santiago
Argentina 0
119 110704A 144.52 47.93 493.0 6.1 Okhotsk 0
120 011104C -179.20 -20.21 682.5 6.0 Fiji 0
121 083103C 132.37 43.38 493.0 6.1 Vladivostok
Russia 0
122 070103B 122.67 4.69 604.6 6.0 Celebes Sea 0
123 121002C 179.28 -24.02 538.8 6.0 Fiji 0
124 102202C -178.30 -20.50 560.5 6.1 Fiji 0
125 101702C -178.23 -19.80 621.9 6.1 Fiji 0
126 100402E -178.74 -20.86 650.8 6.3 Fiji 0
127 091502B 130.04 44.77 589.4 6.4 NE China 0
128 080202D 139.25 29.35 441.5 6.2 Japan 0
129 010202E 178.84 -17.63 680.8 6.1 Fiji 0
130 110501D -178.96 -17.12 579.7 6.2 Fiji 0
131 060900Q 137.79 30.47 492.1 6.2 Japan 0
132 021300B 131.64 42.77 524.3 6.0 NE China 0
133 011500C -179.10 -21.11 650.7 6.0 Fiji 0
134 011300B -178.63 -17.38 564.6 6.2 Fiji 0
135 040999B 178.28 -26.37 635.6 6.1 Fiji 0
136 040899B 130.47 43.66 575.4 7.1 NE China 0
137 041498B -179.81 -23.73 509.6 6.1 Fiji 0
138 020798B 141.87 24.92 533.6 6.4 Northern
Mariana 0
139 012798D 179.29 -22.53 616.9 6.3 Fiji 0
Note: We assign a number to each earthquake (first column). The second column is the event ID used in the GCMT catalog. The event longitudes, latitudes, depths and focal mechanisms are all from the GCMT catalog. and their information for location, depth, magnitude and focal mechanism. The last column shows the number of S3KS-SKKS differential travel time data obtained from the records of each earthquake. Out of the 139 deep earthquakes, 61 have not been processed (gray background). Out of the 78 processed events, 6 of them didn't yield any data (yellow background).
Appendix B. Station Information
Table B.1. List of 374 stations yielding data.
Station
Name Lon. (°) Lat. (°) Geographic Locations Data
Central North USA Kyrgyzstan
NE USA Kazakhstan Turkmenistan
NE USA Adak Is.
SW USA Argentina Kazakhstan
NE USA Kyrgyzstan
SE USA Greenland North Atlantic Ocean
SE Australia SW Russia Central Australia
Aleutian Is.
Iran
Central North USA Iran
Chile Barbados SW USA Central USA
Russia Central East USA Central East USA
Zimbabwe South Africa
NW USA SE USA Central East USA
Kazakhstan Central East USA Central East Brazil
SE USA Central North USA
NE USA SE Australia
Antarctica Central East USA
1
CCM
Central USA Kazakhstan New South Wales North Queensland
NE USA Central East USA
SW USA Indian Ocean NE Queensland Central North USA Central North USA
SE Canada SE Canada SW USA SW USA Greenland West Africa Central North USA
SE Canada SE USA Central North USA Central North USA
SE Canada SE Canada Central South Canada
SW USA SW USA Kyrgyzstan SE Canada
Japan NE USA Central North USA
NE USA SE Canada Central Canada North Atlantic Ocean
Australia SW USA NE Canada Switzerland SW USA Central USA
NE USA NE USA Western Australia
ES USA
GRM
South Africa SW USA Central North USA
NE USA SW USA NE USA Costa Rica Central USA
NW USA SW USA SE USA NW USA
NE USA Solomon Is.
NE USA Central USA
Antarctica SE Canada Mediterranean
SW USA Central USA Central USA Central West USA Central South USA
NE USA Central USA
SE USA Arctic Ocean
Finland SW Russia Kazakhstan Western Australia
Kenya North Australia
Greenland SW USA SW USA French Guiana
Central USA Kazakhstan Marshall Is.
Kyrgyzstan Central USA Central North USA
NE USA South Africa
SW USA
LRAL
Zambia SE USA Russia NE USA North Atlantic Ocean
Antarctica NE USA North USA
SW Russia Central USA
Iraq SW USA Western Australia
Central USA Central USA Central USA
SE USA Greenland Greenland Central USA
Moscow Central USA
West USA Central USA Central USA Central USA
NE USA NE USA North Pacific Ocean
Central USA SW USA NW USA NE USA Central USA
Argentina SW USA Papua New Guinea
SW Canada South Africa Central West USA
NE USA
PSUB Central USA Central USA Central USA Central USA Central USA New Britain Is.
NE Brazil SW USA SW USA SW USA Saudi Arabia Saudi Arabia Central USA Central USA South Africa South Africa South Africa South Africa South Africa South Africa NE South Africa
South Africa South Africa South Africa South Africa South Africa South Africa South Africa South Africa South Africa NE South Africa NE South Africa NE South Africa NE South Africa NE South Africa NE South Africa
Botswana Botswana Botswana Botswana Botswana Botswana Botswana Zimbabwe Zimbabwe Zimbabwe Zimbabwe
1
SA76
Zimbabwe Zimbabwe Zimbabwe Zimbabwe Zimbabwe South Africa South Africa SE Canada
Antarctica SW USA Caribbean Sea
SE USA Central USA
Greenland Central USA
NE USA NW USA Caribbean Sea
SW USA Central USA
SW USA Central USA Central USA SW USA North Brazil
Antarctica Arctic Ocean Arctic Ocean Arctic Ocean
NE USA NE USA Greenland South Africa Central USA Central USA NW USA
Zambia SW USA
Italy Italy Kazakhstan
NW USA SW USA SW USA South Atlantic Ocean
SW USA Namibia NW USA
TX31 Central USA Central USA Kyrgyzstan Kyrgyzstan NE USA Central USA Central USA SW USA SW USA Central USA Central USA Central USA
SE USA Antarctica Kazakhstan
Romania SW USA SW USA Central USA Central USA Northern Territory
Central USA Central West USA Mediterranean Sea
NE USA SE USA Namibia SW USA Central USA Northern Territory
Western Australia NE USA Central USA Central USA Central USA Central USA Central USA Central USA SW USA Central USA Central USA Central Russia Central West USA
Central USA Central USA Central USA
Malawi Kazakhstan
1
115A Central USA Central USA Central USA Central USA SW USA Central USA Central USA Central USA Central USA Central South USA Central South USA Central South USA
SE USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA Central South USA
1
Note: List of 374 stations locations and number of data for using in this study. In our searching range, we get 1519 data from 744 stations, but we need to ensure the measurement is precision, we set criteria and finally get 606 differential traveltime measurements from 374 stations. The last column is the number data used in the station.
61
Appendix C. S3KS-SKKS Differential Traveltime Measurements
Table C.1 List of 606 observed S3KS-SKKS differential traveltimes and predictions of PREM and PREM-like models.
Event No.
Station Name
Distance (º)
S3KS-SKKS Differential Times TauP
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
Note: The first and second columns are the event ID numbers in Table A.1 and the station names in Table B.1, respectively. Columns
89
for p and hT are ray parameters and depths of turning points below the CMB, respectively, both calculated in PREM by the TauP Toolkit. The last 7 columns are respectively the S3KS-SKKS differential traveltimes obtained by TauP in PREM, from records (OBS, gray ones are not used in the Bayesian inversion), and from DSM-calculated synthetic seismograms for 5 different models. Model PREM-SOC is the model whose top 550 km of the outer core is obtained by the inversion of the observed differential traveltimes (Column OBS).