the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Sep 2024
First Report on Coseismic Ionospheric Disturbances Following the Deep-Focus Earthquake (Mw 6.6) in Tarauacá, Acre, Brazil: Ground Uplift and TEC Analysis
Abstract. On January 20, 2024, a deep-focus earthquake of magnitude 6.6 struck near Tarauacá, Brazil, at a depth of 607.0 km. While no surface damage was reported, this event marked a significant seismic occurrence in a region known for deep earthquakes associated with the subducted Nazca Plate. Using Global Navigation Satellite System (GNSS) Total Electron Content (TEC) data from the Brazilian Network for Continuous Monitoring of GNSS Systems (RBMC) and seismic data from the IRIS network, we analyzed the earthquake's impact on both ground surface (in form of uplift) and ionosphere (in form of disturbances). The results show clear ionoquakes characterized by distinct "N-type" wave patterns in TEC data, originating from infrasonic-acoustic waves launched from the earthquake's crustal displacement. The ionoquakes arrived in the ionosphere 8.3 minutes from the mainshock onset and traveled with the net propagation speed of 550 m/s to 743 m/s. This is the first report on coseismic ionospheric disturbances, or ionoquakes, following an earthquake in the Brazilian sector. The spectral analysis shows a maximum TEC amplitude in the frequency range 14 mHz – 16 mHz which suggest that the ionoquakes are signatures of high-frequency infrasonic-acoustic waves dynamics.
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RC1: 'Comment on angeo-2024-16', Anonymous Referee #1, 25 Nov 2024
Comment on the manuscript entitled "First Report on Coseismic Ionospheric Disturbances Following the Deep-Focus Earthquake (Mw 6.6) in Tarauacá, Acre, Brazil: Ground Uplift and TEC Analysis" by Adebayo et al.
This manuscript investigates ionospheric disturbances associated with a magnitude 6.6 deep-focus earthquake in Tarauacá, Brazil, occurred on January 20, 2024. Despite the epicenter being located at a depth of 607 km, and no significant surface damage reported, TEC variations related to this earthquake were detected. One of the most noteworthy aspects of this study is the observation of ionospheric disturbances following such a deep-focus earthquake. However, there are several methodological issues and uncertainties in the analysis and interpretation of the data that require clarification.
The authors assert that Figure 2 illustrates N-shaped disturbances, commonly reported as coseismic ionospheric disturbances in TEC data. While the TEC disturbance in amco_G17 may exhibit N-type characteristics, the variation in amte_G09 may not be recognized as this pattern. Furthermore, the authors state that the onset of the amte_G09 disturbance occurred 330 seconds after the earthquake. Assuming the TEC disturbance occurred at an altitude of 250 km, the propagation speed of acoustic waves is faster than normal speed, as shown in Figure S1. If the wave propagation speed was typical, the disturbance would need to have occurred at a lower altitude. The authors must clarify the altitude at which the TEC disturbances were detected. One of the reasons for these discrepancies may be the determination of the TEC variation onset time. As suggested in Bagiya et al. (2023), the influence of multiple sources of ground motion could explain deviations in the N-shaped TEC disturbance patterns. The disturbances associated with the earthquake need to be precisely identified and their characteristics analysed in detail.
The methods employed for TEC data analysis, as presented in Figures 4–6, raise further concerns. The authors compared the normalized average of dTEC data with 15-second sampled ground motion data, concluding that N-type variations correlate with ground motion. However, it is unclear which specific ground motion peaks correspond to the TEC variations. Using this normalized average of dTEC data, the authors also performed a frequency analysis on the TEC disturbances as shown in Figure 5. Then Figure 6 shows the relationship between ground motion and TEC disturbances.
However, it is necessary to take into account the distance between the epicenter location and the IPP and evaluate the attenuation of the acoustic wave during the propagation from the ground to the IPP in order to compare the ground motion and the ionospheric disturbances quantitatively. Given that the trajectories of the four IPPs vary in distance from the epicenter, the TEC data should be corrected for these distances before making quantitative comparisons with ground motion. The normalization of dTEC data neglects these attenuation effects, leading to potential inaccuracies for the comparison of the intensities of ground motion and the TEC variations.Reviewer Recommendation: Based on the outlined concerns, the reviewer thinks that this manuscript includes serious problems in the interpretation of the observation results and the method of the data analysis. Therefore, the reviewer thinks that this paper should be rejected for publication in Angeo.
Specific Comments:
L86: Replace "slight" with "slant."
L96: Clarify whether "sTEC" refers to the same data as "TEC" mentioned in L99.
Figure 1: Indicate which satellite-receiver combinations correspond to the four IPP trajectories.
L157 "dTEC obtained from the sTEC data is shown in (b1) and (b2). These two receivers observe a clear N-type wave pattern ... " : The TEC variation in (b1) may appear N-type; however, (b2) is unlikely to exhibit an N-type pattern. Additionally, the onset time of (b2) disturbances must be explicitly stated, as this is critical for evaluating propagation delays.
L161"The figure illustrates that ground uplift took off at 21:55 UT": The ground uplift timing inferred from the figure appears to be 21:33, not 21:55 as mentioned.
L162 "This delay could be attributed to the distance between the seismometer and the earthquake epicenter which is 788 km.": Sometimes a technical term "epicenter" uses the following two meanings. One is the position of the hypocenter on the earth’s surface. The other is the hypocenter itself. Which did the authors used for this term?
Figure 2: Verify if the ground motion data were corrected for the seismometer's frequency response. Broadband seismometers often lack sensitivity below 10 mHz (Nakata et al., 2021). When quantitatively comparing ground motions and TEC variations, such as those shown in Figure 6, the sensitivity of the seismometer should be corrected.
L194 "In addition, "AMTE" detected ionoquakes earlier, starting at 330 seconds (5.5 minutes),": If the amte_G09 disturbance onset is 330 seconds after the earthquake, its propagation to the IPP at 250 km altitude seems improbable. Clarify whether the disturbance occurred at a lower altitude and specify the altitude in question.
L206: Detail how the profile of the acoustic wave shown in Figure S1 was determined.
Figure 3: The latitude-based ordering of IPP trajectories does not align with distances from the epicenter. For instance, the easternmost IPP (AMTE?) is closer to the southernmost IPP (CRUZ?) in terms of distance from the epicenter. In the figure, the acoustic wave propagation is indicated by arrows, but it is doubtful whether these arrows represent the correct propagations of the acoustic waves.
References:
Bagiya, M. S., Heki, K., & Gahalaut, V. K. (2023). Anisotropy of the near-field coseismic ionospheric perturbation amplitudes reflecting the source process: The 2023 February Turkey earthquakes. Geophysical Research Letters, 50, e2023GL103931.
Nakata, H., Takaboshi, K., Takano, T., & Tomizawa, I. (2021). Vertical propagation of coseismic ionospheric disturbances associated with the foreshock of the Tohoku earthquake observed using HF Doppler sounding. Journal of Geophysical Research: Space Physics, 126, e2020JA028600. https://doi.org/10.1029/2020JA028600
Citation: https://doi.org/10.5194/angeo-2024-16-RC1 -
AC2: 'Reply on RC1', Oluwasegun Adebayo, 06 Dec 2024
We appreciate the reviewer for this invaluable comments and suggestions. The contributions have helped us to improve the quality of the manuscript. We modified the manuscript based on the reviewer’s comments and suggestions.
Kindly find attached a copy of our response to the reviewer's comments and suggestions.
Thank you.
Adebayo.
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AC2: 'Reply on RC1', Oluwasegun Adebayo, 06 Dec 2024
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RC2: 'Comment on angeo-2024-16', Anonymous Referee #2, 03 Dec 2024
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AC1: 'Reply on RC2', Oluwasegun Adebayo, 06 Dec 2024
We appreciate the reviewer for this invaluable comments and suggestions. The contributions have helped us to improve the quality of the manuscript. We modified the manuscript based on the reviewer’s comments and suggestions.
Kindly find attached a copy of our response to the reviewer's comments and suggestions.
Thank you.
Adebayo.
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AC1: 'Reply on RC2', Oluwasegun Adebayo, 06 Dec 2024
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