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Ultra-low velocity zones beneath the Philippine and Tasman Seas revealed by a trans-dimensional Bayesian waveform inversion

Pachhai, Surya; Dettmer, Jan; Tkalčić, Hrvoje

Description

Ultra-low velocity zones (ULVZs) are small-scale structures in the Earth's lowermost mantle inferred from the analysis of seismological observations. These structures exhibit a strong decrease in compressional (P)-wave velocity, shear (S)-wave velocity, and an increase in density. Quantifying the elastic properties of ULVZs is crucial for understanding their physical origin, which has been hypothesized either as partial melting, iron enrichment, or a combination of the two. Possible...[Show more]

dc.contributor.authorPachhai, Surya
dc.contributor.authorDettmer, Jan
dc.contributor.authorTkalčić, Hrvoje
dc.date.accessioned2016-06-14T23:21:03Z
dc.identifier.issn0956-540X
dc.identifier.urihttp://hdl.handle.net/1885/103690
dc.description.abstractUltra-low velocity zones (ULVZs) are small-scale structures in the Earth's lowermost mantle inferred from the analysis of seismological observations. These structures exhibit a strong decrease in compressional (P)-wave velocity, shear (S)-wave velocity, and an increase in density. Quantifying the elastic properties of ULVZs is crucial for understanding their physical origin, which has been hypothesized either as partial melting, iron enrichment, or a combination of the two. Possible disambiguation of these hypotheses can lead to a better understanding of the dynamic processes of the lowermost mantle, such as, percolation, stirring and thermochemical convection. To date, ULVZs have been predominantly studied by forward waveform modelling of seismic waves that sample the core–mantle boundary region. However, ULVZ parameters (i.e. velocity, density, and vertical and lateral extent) obtained through forward modelling are poorly constrained because inferring Earth structure from seismic observations is a non-linear inverse problem with inherent non-uniqueness. To address these issues, we developed a trans-dimensional hierarchical Bayesian inversion that enables rigorous estimation of ULVZ parameter values and their uncertainties, including the effects of model selection. The model selection includes treating the number of layers and the vertical extent of the ULVZ as unknowns. The posterior probability density (solution to the inverse problem) of the ULVZ parameters is estimated by reversible jump Markov chain Monte Carlo sampling that employs parallel tempering to improve efficiency/convergence. First, we apply our method to study the resolution of complex ULVZ structure (including gradually varying structure) by probabilistically inverting simulated noisy waveforms. Then, two data sets sampling the CMB beneath the Philippine and Tasman Seas are considered in the inversion. Our results indicate that both ULVZs are more complex than previously suggested. For the Philippine Sea data, we find a strong decrease in S-wave velocity, which indicates the presence of iron-rich material, albeit this result is accompanied with larger parameter uncertainties than in a previous study. For the Tasman Sea data, our analysis yields a well-constrained S-wave velocity that gradually decreases with depth. We conclude that this ULVZ represents a partial melt of iron-enriched material with higher melt content near its bottom.
dc.publisherOxford University Press
dc.rightsAuthor/s retain copyright
dc.sourceGeophysical Journal International
dc.titleUltra-low velocity zones beneath the Philippine and Tasman Seas revealed by a trans-dimensional Bayesian waveform inversion
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume203
dc.date.issued2015
local.identifier.absfor040407 - Seismology and Seismic Exploration
local.identifier.ariespublicationu4027924xPUB472
local.type.statusPublished Version
local.contributor.affiliationPachhai, Surya, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationDettmer, Jan, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationTkalčić, Hrvoje, College of Physical and Mathematical Sciences, ANU
local.bibliographicCitation.issue2
local.bibliographicCitation.startpage1302
local.bibliographicCitation.lastpage1318
local.identifier.doi10.1093/gji/ggv368
local.identifier.absseo970102 - Expanding Knowledge in the Physical Sciences
local.identifier.absseo970104 - Expanding Knowledge in the Earth Sciences
local.identifier.absseo970101 - Expanding Knowledge in the Mathematical Sciences
dc.date.updated2016-06-14T08:57:39Z
dcterms.accessRightsOpen Access
CollectionsANU Research Publications

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