Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical <i>ScS</i> reverberations

Roth, Erich G.; Wiens, Douglas A.

doi:10.1029/1999gl900203

Cited by 12 publications

(5 citation statements)

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“…The depth range for the 410 observed in this study agrees well with the range of 394 to 432 km inferred for the region by Flanagan and Shearer [1998b] from precursors to pP , sP and sS phases, given the difference in the achieved spatial resolution between the two studies. However, our results show no systematic variations with distance to the slab, consistent with the observation by Roth and Wiens [1999]. Using near‐vertical ScS reverberations, these authors found little evidence for a systematic change in the 410 topography in the Tonga subduction zone.…”

Section: Resultssupporting

confidence: 91%

“…Most of the variations seems to occur along the slab strike. In agreement with Roth and Wiens [1999], we found no systematic variations of the 410 depth as function of distance to the slab. On the basis of a teleseismic study, Gilbert et al [2001] reported that in a 500 km wide region near the Tonga slab, the 410 appears to be uplifted to 390 km depth.…”

Section: Discussionsupporting

confidence: 91%

“…Many seismological studies have noted changes in the elevation of the 410‐km and 660‐km discontinuities near subduction zones [ Bina , 1991; Revenaugh and Jordan , 1991; Shearer and Masters , 1992; Vidale and Benz , 1992; Wicks and Richards , 1993; Shearer , 1993; Niu and Kawakatsu , 1995; Castle and Creager , 1997, 1998; Collier and Helffrich , 1997; Flanagan and Shearer , 1998a, 1998b; Thirot et al , 1998; Roth and Wiens , 1999; Li et al , 2000] (see Collier et al [2001] for a review). These studies generally show an uplift of the 410 and a depression of the 660, which agree with mineral physics data for the phase transition origin of the discontinuities.…”

Section: Introductionmentioning

confidence: 99%

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Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Tibi

Wiens

2005

J. Geophys. Res.

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[1] Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high-frequency content for precise mapping of discontinuity topography and sharpness. To enhance the low-amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660-km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660-km discontinuity in terms of local temperatures implies a thermal anomaly of À800°K to À1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660-km discontinuity region implies that the Tonga slab seems to penetrate the 660-km discontinuity with little deformation. Waveform modeling suggests that both the 410-and 660-km discontinuities are sharp. The 660-km discontinuity is at most 2 km thick in many parts of the region, and a first-order discontinuity cannot be precluded. The 410-km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab. This suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities.Citation: Tibi, R., and D. A. Wiens (2005), Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays,

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Section: Resultssupporting

confidence: 91%

Section: Discussionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

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Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Tibi

Wiens

2005

J. Geophys. Res.

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“…The observed value of 242 km for the transition-zone thickness in Group A, to the east of the deep seismicity (_> 500 kin), is almost same as the global average of 241 km obtained from the SS precursor study [Flanagan and Shearer, 1998]. This result is in good agreement with the recent study of Roth and Wiens [1999]. It is, however, about 5 km less than that estimated from the SS precursor study [Flanagan and Shearer, 1998] for the same region.…”

Section: Discussionsupporting

confidence: 88%

Seismic evidence for a thinner mantle transition zone beneath the South Pacific Superswell

Niu

Inoue

Suetsugu

et al. 2000

Geophysical Research Letters

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“…With the exception of the lithosphere, the fact that a single mantle shear structure fits the EPR SH data propagating north into California indicates a very high degree of homgeneity in the upper mantle and transition zone. Coherence of the triplication branches across any particular one of the record sections shown in Figure 7, 12, or 14 is fragile and easily destroyed by lateral variation along the discontinuities with magnitudes routinely observed beneath continents [ Dueker and Sheehan , 1997; Burdick and Helmberger , 1978], in most subduction zones globally [ Niu et al , 2000; Tajima and Grand , 1995, 1998; Roth and Wiens , 1999; Clarke , 1995; Li et al , 2000; Krishna and Kaila , 1995; Shito and Shibutani , 2001; Kato et al , 2001], or near active rifts like East Africa [ Owens et al , 2000; Nyblade et al , 2000; Ritsema et al , 1998]. The coherence that we observe would be lost with even a single lower mantle upwelling feeding any of the four proposed hot spots along the EPR [ Richards et al , 1988].…”

Section: Discussionmentioning

confidence: 99%

Whole mantle shear structure beneath the East Pacific Rise

Melbourne

Helmberger

2002

J. Geophys. Res.

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[1] We model broadband seismograms containing triplicated S, S 2 , and S 3 along with ScS to produce a pure path one-dimensional model extending from the crust to the core-mantle boundary beneath the East Pacific Rise. We simultaneously model all body wave shapes and amplitudes, thereby eliminating depth-velocity ambiguities. The data consist of western North American broadband recordings of East Pacific Rise (EPR) affiliate transform events that form a continuous record section out to 82°and sample nearly the entire East Pacific Rise. The best fitting synthetics contain attenuation and small changes in lithospheric thickness needed to correct for variation in bounce point ages. The 660-km discontinuity is particularly well resolved and requires a steep gradient (4%), extending down to 745 km. We find no discernible variation in apparent depths of the 405-and 660-km discontinuities over ridge-orthogonal distances on the order of 1000 km (or 20 Ma lithosphere). Body waveform comparisons indicate that we can resolve discontinuity depths to less than ±10 km, providing an upper limit to transition zone topography. These depth estimates, in conjunction with the fan shot nature of the ray paths, lower the detection limit from S 2 precursor analysis of the lateral length scale over which short-wavelength topographic variation could occur and indicate the sub-EPR Transition Zone and upper mantle are remarkably homogeneous. The lower mantle beneath the East Pacific Rise is well modeled by PREM, with the greatest variation occurring in ScS, reflecting strong heterogeneity along the core-mantle boundary. Together, these observations require that the East Pacific Rise spreading ridge cannot be actively supplied from the local lower mantle and that tomographically imaged lateral variation beneath the ridge likely reflects lateral smearing of outlying velocity gradients. Dynamically, the transition zone therefore appears vertically decoupled from the overlying East Pacific Rise spreading system.

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Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

Cited by 12 publications

References 12 publications

Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Seismic evidence for a thinner mantle transition zone beneath the South Pacific Superswell

Whole mantle shear structure beneath the East Pacific Rise

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