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Sources of uncertainties and artefacts in back-projection results

Zeng, Hongyu and Wei, Shengji and Wu, Wenbo (2020) Sources of uncertainties and artefacts in back-projection results. Geophysical Journal International, 220 (2). pp. 876-891. ISSN 0956-540X. https://resolver.caltech.edu/CaltechAUTHORS:20200130-140938994

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Abstract

Back-projecting high-frequency (HF) waves is a common procedure for imaging rupture processes of large earthquakes (i.e. M_w > 7.0). However, obtained back-projection (BP) results could suffer from large uncertainties since high-frequency seismic waveforms are strongly affected by factors like source depth, focal mechanisms, and the Earth's 3-D velocity structures. So far, these uncertainties have not been thoroughly investigated. Here, we use synthetic tests to investigate the influencing factors for which scenarios with various source and/or velocity set-ups are designed, using either Tohoku-Oki (Japan), Kaikoura (New Zealand), Java/Wharton Basin (Indonesia) as test areas. For the scenarios, we generate either 1-D or 3-D teleseismic synthetic data, which are then back-projected using a representative BP method, MUltiple SIgnal Classification (MUSIC). We also analyse corresponding real cases to verify the synthetic test results. The Tohoku-Oki scenario shows that depth phases of a point source can be back-projected as artefacts at their bounce points on the earth's surface, with these artefacts located far away from the epicentre if earthquakes occur at large depths, which could significantly contaminate BP images of large intermediate-depth earthquakes. The Kaikoura scenario shows that for complicated earthquakes, composed of multiple subevents with varying focal mechanisms, BP tends to image subevents emanating large amplitude coherent waveforms, while missing subevents whose P nodal directions point to the arrays, leading to discrepancies either between BP images from different arrays, or between BP images and other source models. Using the Java event, we investigate the impact of 3-D source-side velocity structures. The 3-D bathymetry together with a water layer can generate strong and long-lasting coda waves, which are mirrored as artefacts far from the true source location. Finally, we use a Wharton Basin outer-rise event to show that the wavefields generated by 3-D near trench structures contain frequency-dependent coda waves, leading to frequency-dependent BP results. In summary, our analyses indicate that depth phases, focal mechanism variations and 3-D source-side structures can affect various aspects of BP results. Thus, we suggest that target-oriented synthetic tests, for example, synthetic tests for subduction earthquakes using more realistic 3-D source-side velocity structures, should be conducted to understand the uncertainties and artefacts before we interpret detailed BP images to infer earthquake rupture kinematics and dynamics.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/gji/ggz482DOIArticle
ORCID:
AuthorORCID
Wu, Wenbo0000-0002-6249-8065
Additional Information:© 2019 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Accepted 2019 October 19. Received 2019 October 16; in original form 2019 April 24. Published: 21 October 2019. This project is supported by the Earth Observatory of Singapore grant (M4430255). All seismic data and the station codes are collected through the Data Management Center of the Incorporated Research Institutions for Seismology (IRIS), using Wilber 3. All the figures are plotted using the Generic Mapping Tools (GMT), and the Python Matplotlib package. The seismic data are processed using the Python package, Obspy and the waveforms are aligned and selected using the Python package, aimbat (Lou et al.2013). We thank Pavel Adamek for his help in clarifying and improving several aspects of the writing. We appraciate comments and suggestions from Jiuxun Yin, Zengxi Ge and editors, which help to improve the quality and presentation of the manuscript. This work comprises Earth Observatory of Singapore contribution no. 268. This research is partly supported by the National Research Foundation Singapore and the Singapore Ministry of Education under the Resarch Centers of Excellence initiative.
Group:Seismological Laboratory
Funders:
Funding AgencyGrant Number
Earth Observatory of SingaporeM4430255
National Research Foundation (Singapore)UNSPECIFIED
Ministry of Education (Singapore)UNSPECIFIED
Subject Keywords:Body waves; Computational seismology; Earthquake source observation; Wave propagation
Issue or Number:2
Record Number:CaltechAUTHORS:20200130-140938994
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200130-140938994
Official Citation:Hongyu Zeng, Shengji Wei, Wenbo Wu, Sources of uncertainties and artefacts in back-projection results, Geophysical Journal International, Volume 220, Issue 2, February 2020, Pages 876–891, https://doi.org/10.1093/gji/ggz482
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101008
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:30 Jan 2020 23:02
Last Modified:09 Mar 2020 13:19

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