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Wannamaker, P. E., Hohmann, G. W., and Ward, S. H. (1984). Magnetotelluric responses

of three-dimensional bodies in layered earths. Geophysics, 49, 1517-1533

996

Wessel, P., Smith, W. H. F., Scharroo, R., Luis, J., and Wobbe, F. (2013). Generic Mapping

997

Tools: Improved version released. Eos, trans. AGU, 94(45), 409-420.

998

https://doi.org/10.1002/2013EO450001

999

Wiens, D. A., Kelley, K. A., and Plank, T. (2006). Mantle temperature variations beneath

1000

back-arc spreading center inferred from seismology, petrology, and bathymetry.

1001

Earth Planet. Sci. Lett., 248, 30-42. https://doi.org/10.1016/j.epsl.2006.04.011

1002

Wilson, C. R., Spiegelman, M., van Keken, P. E., and Hacker, B. R. (2014). Fluid flow in

1003

subduction zones: The role of solid rheology and compaction pressure. Earth Planet.

1004

Sci. Lett., 401, 261-274. https://doi.org/10.1016/j.epsl.2014.05.052

1005

Yoshino, T., Matsuzaki, T., Shatskiy, A., and Katsura, T. (2009), The effect of water on

1006

the electrical conductivity of olivine aggregates and its implications for the

1007

electrical structure of the upper mantle. Earth Planet. Sci. Lett., 288, 291-300.

1008

https://doi.org/10.1016/j.epsl.2009.09.032

33

1009

Table

1010

Station

Latitude

Longitude

Water

E-field

Remote B-

ID

(N)

(E)

Depth [m]

EM1

13°19.25’

143°02.80’

3924

Available

EM6

EM2

13°10.92’

143°15.09’

3749

Available

EM10

EM3

13°06.69’

143°21.56’

3567

Available

EM7

EM4

13°02.37’

143°28.05’

3255

EM5

12°58.00’

143°34.53’

3086

EM8

N/A

EM6

12°56.74’

143°36.41’

2868

Available

EM5

EM7

12°55.43’

143°38.18’

3123

Available

EM5

EM8

12°53.47’

143°41.21’

3316

Available

EM5

EM9

12°49.13’

143°47.57’

2569

++

++

EM10

12°45.09’

143°53.96’

3685

Available

N/A

EM11

12°40.82’

144°00.47’

3751

EM10

EM6

field

1011

1012

Table 1

1013

Station information. Station ID is numbered from the northwesternmost station to the

1014

southeasternmost one; see also Figure 1. Latitude and longitude is the location of the ship

1015

at the time of deployment of the instrument, and water depth is derived from the multi-

1016

narrow beam bathymetric data (Kitada et al., 2006). The symbol “+” for EM4 means that

1017

EM field data were obtained by the experiment, but the MT response estimated from the

1018

data had a low squared coherency between the electric field observed and that predicted

1019

from the MT response estimated and the magnetic field observed, and were not used in

1020

the inversion. The symbol “++” for EM9 means that this instrument has not yet been

1021

recovered.

34

1022

Figures

1023

1024

1025

Figure 1

1026

(a) Bathymetric map, which derives from multi-narrow beam data (Kitada et al., 2006)

1027

and the ETOPO1 data (Amante and Eakins, 2009), with depth contours of the surface of

1028

the subducted Pacific slab (colored dotted lines; Hayes et al., 2012) and the location of

1029

the Mariana Trench (light green dotted line; Bird, 2003), as well as ridge centers of the

1030

back-arc spreading in the Mariana Trough (black dash-dotted line; Kitada et al., 2006).

1031

The white box represents the range of the map in Figure 1b, and the range of this map is

1032

shown by the black rectangle in the right-top inset showing the plate boundaries (Bird,

1033

2003). White dots at around 18°N indicate MT stations used for obtaining an electrical

1034

resistivity structure in the central Marianas (Matsuno et al., 2010).

1035

(b) Bathymetric map with marine MT observational stations (symbols). Circles and

1036

crosses indicate locations of magnetic field data and electric field data, respectively. The

1037

colors red, black, and gray for the symbols indicate stations from which the data were

1038

used in the inversion, stations from which data were obtained but not used in inversion,

35

1039

and stations at which the instrument was not recovered, respectively. The station names

1040

are numbered from northwest to southeast (the northwesternmost station is called EM1,

1041

and the southeasternmost one is called EM11); see also Table 1. The abbreviations in this

1042

map are as follows: MGR: Malaguana-Gadau Ridge, PMVC: Patgon-Masala Volcanic

1043

Chain, FNVC: Fina Nagu Volcanic Chain, ASVP: Alphabet Seamount Volcano Province,

1044

WSRBF: West Santa Rosa Bank Fault, SEMFR: Southeast Mariana Forearc Rift (Stern

1045

et al., 2013; Masuda and Fryer, 2015).

36

1046

1047

Figure 2

1048

Apparent resistivities for all four elements and all stations before and after the correction

1049

of topographic distortions (black circle and red diamond, respectively). Error bars show

1050

one standard error of the observations. An annotation for the vertical axis is shown only

1051

in the upper-left-most panel but is common to all the other panels. The station names are

1052

shown in the upper-left corner in the leftmost panels.

37

1053

1054

Figure 3

1055

Phase values for all four elements and all stations before and after the correction of

1056

topographic distortions. The symbols and the error bars are the same as in Figure 2. Note

1057

that ranges of the phase values for off-diagonal elements and diagonal elements are

1058

different. Annotations for the vertical axis are shown only in the top panels but are

1059

common to each MT impedance element panel. The station names are shown in the upper-

1060

left corner in the leftmost panels.

38

1061

1062

Figure 4

1063

(a) RMS misfits and model roughness values for 2-D electrical resistivity models in the

1064

robust inversion processing. The model constraint in the inversion is only model

1065

smoothness. The robust inversion processing was applied two times, and the resulting

1066

values are shown by triangles, squares, and circles for the 0th, 1st, and 2nd robust inversion

1067

runs. At each run, 9 values for the regularization parameter of model smoothness (τs) were

1068

used: 300, 100, 30, 10, 3, 1, 0.3, 0.1, and 0.03. The optimal value of the regularization

1069

parameter is 0.3 for all robust runs, as shown by the filled symbols.

1070

(b) Quantile-quantile plot with 95% confidence limits for the results of the final inversion

1071

(the 2nd run).

39

1072

1073

Figure 5

1074

Optimal 2-D electrical resistivity inversion models (a) with constraints on only model

1075

smoothness and (b) with constraints on model smoothness and allowance for resistivity

1076

jumps surrounding the subducted Pacific slab. The tip of the subducted slab, which was

1077

taken into account for the resistivity jump in Figure 5b, terminates at 200 km depth.

1078

Stations are represented by inverted triangles with numbers near the top of each figure.

1079

Note that the seafloor spreading center is located at 0 km distance, and that the station

1080

located at the spreading center is EM6.

40

1081

1082

Figure 6

1083

TM mode MT responses observed (circles, with error bar representing one standard error)

1084

and predicted from two types of electrical resistivity inversion models (red and blue lines,

1085

which correspond to the Figure 5a model and Figure 5b model, respectively). The two

1086

lines are almost consistent. The station names are shown in the upper-left corner in each

1087

panel. The filled squares seen only in the EM10 response represent outliers that were

1088

excluded from the data set by the robust inversion processing. RMS misfits for each site

1089

and those for each period are tabulated in Tables S1 and S2, respectively, in the supporting

1090

information.

41

1091

1092

Figure 7

1093

Electrical resistivity as a function of temperature, component, and the amount of melt

1094

interconnected in solid phase and water (or hydrogen) dissolved in solid phase or melt.

1095

Comparing this figure and the inversion model (Figure 5) with an assumption for

1096

temperature of a focusing area, the amount of melt and water (or hydrogen) can be

1097

estimated. See details in text.

1098

(a) Electrical resistivity for several types of minerals and materials as a function of

1099

temperature, overlying the resistivity color scale used for drawing the inversion models

1100

in Figure 5. Black solid lines indicate dry olivine (1a: Yoshino et al., 2009; 1b: Gardés et

1101

al., 2014; 1c: Constable et al., 1992; 1d: Constable , 2006). Blue lines indicate wet olivine

1102

(2a and 2a’: Yoshino et al., 2009 for 0.01 wt.% and 0.1 wt.% water, respectively; 2b, 2b’’,

1103

and 2b’: Gardés et al., 2014 for 0.01 wt.%, 0.03 wt.%, and 0.1 wt.% water, respectively;

1104

2c and 2c’: Wang et al., 2006 for 0.01 wt.% and 0.1 wt.% water, respectively). Red line

1105

indicates basaltic melt (3) (Tyburczy and Waff, 1983 for tholeiite melt at 4.3 kbar). Purple

1106

line indicates hydrous basaltic melt with 1 wt.% water (4) (Sifré et al., 2014). For clarity,

1107

the hydrous melt line, 4, is cut at cross-point by the dry silicate melt line, ...