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A comparison of predicted and observed ocean tidal loading in Alaska

Martens, H. R. and Simons, M. (2020) A comparison of predicted and observed ocean tidal loading in Alaska. Geophysical Journal International, 223 (1). pp. 454-470. ISSN 0956-540X. doi:10.1093/gji/ggaa323.

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We investigate the elastic and anelastic response of the crust and upper mantle across Alaska to mass loading by ocean tides. GPS-inferred surface displacements recorded by the Plate Boundary Observatory network are compared with predictions of deformation associated with the redistribution of ocean water due to the tides. We process more than 5 yr of GPS data from 131 stations using a kinematic precise point positioning algorithm and estimate tidal contributions using harmonic analysis. We also forward calculate load-induced surface displacements by convolving ocean-tide models with load Green’s functions derived from spherically symmetric Earth models. We make the comparisons for dominant tidal harmonics in three frequency bands: semidiurnal (M₂), diurnal (O₁) and fortnightly (M_f). Vector differences between predicted and observed ocean tidal loading (OTL) displacements are predominantly sub-mm in magnitude in all three frequency bands and spatial components across the network, with larger residuals of up to several mm in some coastal areas. Accounting for the effects of anelastic dispersion in the upper mantle using estimates of Q from standard Earth models reduces the residuals for the M₂ harmonic by an average of 0.1–0.2 mm across the network and by more than 1 mm at some individual stations. For the relatively small M_f tide, the effects of anelastic dispersion (<0.03 mm) are undetectable within current measurement error. Incorporating a local ocean-tide model for the northeastern Pacific Ocean reduces the M₂ vertical residuals by an average of 0.2 mm, with improvements of up to 5 mm at some coastal stations. Estimated RMS observational uncertainties in the vertical component for the M₂ and O₁ tides are approximately ±0.08 mm at the two-sigma level (±0.03 mm in the horizontal components), and ±0.21 mm for the M_f harmonic (±0.07 mm in the horizontal components). For the M₂ harmonic, discrepancies between predicted and observed OTL displacements exceed observational uncertainties by about one order of magnitude. None of the ocean tide and Earth model combinations is found to reduce the M₂ residuals below the observational uncertainty, and no single forward model provides a best fit to the observed displacements across all tidal harmonics and spatial components. For the O₁ harmonic, discrepancies between predicted and observed displacements are generally several-fold larger than the observational uncertainties. For the M_f harmonic, the discrepancies are roughly within a factor of two of the observational uncertainties. We find that discrepancies between predicted and observed OTL displacements can be significantly reduced by removing a network-uniform tidal-harmonic displacement, and that the remaining discrepancies exhibit some regional-scale spatial coherency, particularly for the M₂ harmonic. We suggest that the remaining discrepancies for the M₂, O₁ and M_f tides cannot be fully explained by measurement error and instead convey information about deficiencies in ocean-tide models and deviations from spherically symmetric Earth structure.

Item Type:Article
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https://www.aviso.altimetry.frRelated ItemAviso+ ItemUNAVCO open archive
Martens, H. R.0000-0003-2860-9013
Simons, M.0000-0003-1412-6395
Additional Information:© The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Revision received: 20 September 2019; Received: 24 June 2020; Accepted: 24 June 2020; Published: 27 June 2020. The LoadDef software used to model load-induced deformation is available from Martens et al. (2019). Observed and predicted load-tide displacements for the Alaska PBO network (computed here) are provided in the Supporting Information. FES2014 was produced by Noveltis, Legos, and CLS and distributed by Aviso+, with support from CNES ( Figures were created using the Generic Mapping Tools (Wessel et al. 2013) and Matplotlib (Hunter et al. 2007). GPS data used in our study are available in open archives from UNAVCO ( HRM processed the GPS data, performed the tidal harmonic analysis, generated the forward models of predicted tidal displacements, assessed uncertainties and residuals, produced the figures, and wrote the manuscript. MS provided important feedback on the interpretation of results and the written manuscript. HRM wishes to thank colleagues and staff at ETH-Zürich for their kind hospitality and engaging discussions during the summer and fall of 2019. We are grateful to two anonymous reviewers for their constructive and thorough reviews of the manuscript. We also extend a special thanks to Editor Duncan Agnew for his thoughtful reviews and fruitful discussions on tidal harmonic analysis, as well as to Luis Rivera (Université de Strasbourg) for his generous contributions to the load-tide modelling and interpretation of results. This material is based on work supported by the National Science Foundation under Grant No. 1925267 and the National Aeronautics and Space Administration under Grant No. NNX15AK40A. This material is also based on services provided by the GAGE Facility, operated by UNAVCO, Inc., with support from the National Science Foundation and the National Aeronautics and Space Administration under NSF Cooperative Agreement EAR-11261833.
Group:Seismological Laboratory
Funding AgencyGrant Number
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:Composition and structure of the continental crust, Composition and structure of the mantle, Structure of the Earth, Loading of the Earth, Tides and planetary waves, Planetary interiors
Issue or Number:1
Record Number:CaltechAUTHORS:20201116-123721426
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Official Citation:H R Martens, M Simons, A comparison of predicted and observed ocean tidal loading in Alaska, Geophysical Journal International, Volume 223, Issue 1, October 2020, Pages 454–470,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106679
Deposited By: Tony Diaz
Deposited On:16 Nov 2020 22:32
Last Modified:16 Nov 2021 18:55

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