In this paper (i.e., Yao et al. (2020)), we make some comments on Yang and Song (2020), which claimed that the conclusions in a series of our studies (Wen, 2006; Yao et al., 2015, Yao et al., 2019) should be rejected because all the studies did not correct for “random clock errors” of the seismic instruments. They also reported that differential inner core (IC) rotation is “the simplest and most reasonable explanation” to the temporal changes of the IC phases. Below is a summary of our comments:
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It is a faulty approach that Yang and Song (2020) used to claim "obvious clock problems" in the seismic data recorded at seismic stations AAK and ARU of the the Global Seismographic Network (GSN) in a repeating earthquake. Actually, there is no evidence for any random clock errors in those GSN data and the “random clock errors” they reported are exactly the effect of relative source depth difference on SKP-DF or SKP-CD differential arrival time that they ignored.
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We do wish to comment on Yang and Song (2020)'s claim that the differential IC rotation is “the simplest and most reasonable explanation” to the temporal changes of the IC phases. Common logic of science has been that, when a proposal is put forward, it is up to the proposer to prove its necessity to the scientific evidence. This is not the case in the seismic studies of IC differential rotation. From its first appearance in the seismological literature to the present day, despite of its constant appearance in article titles such as “Seismological evidence for differential rotation of the Earth's inner core” (Song and Richards, 1996) and “Inner core differential motion confirmed by earthquake waveform doublets” (Zhang et al., 2005), etc., IC differential rotation was never proven to be required to explain the seismic data! It is still up to the proponents of IC differential rotation to provide a line of seismic evidence that requires IC differential rotation as the explanation. Even if unambiguous seismic evidence can be found in the future that some temporal changes come from the IC interior, we still disagree with Yang and Song (2020) on the statement that the IC differential rotation will be “the simplest and most reasonable explanation”. There are many candidate mechanisms that are equally plausible, if not more, for explaining temporal change of seismic properties in the IC interior, to name a few: stress-induced seismic velocity or anisotropy change, or change of partial melt content in the IC.
To summarize the interpretation of IC differential rotation vs. the temporal change of IC surface in the context of the seismic evidence:
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IC differential rotation has never been proven to be required by the seismic data. In fact, it would provide an inconsistent and unreasonable explanation to the seismic data, even if we invoke it in the absence of its requirement by the seismic data and in the presence of the temporal change of IC surface that is required by the seismic data (Yao et al., 2019). We refer the readers to Yao et al. (2019) for the detailed analysis and reasoning.
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On the other hand, temporal change of IC surface is required by the temporal change of CD data and would provide an explanation to all the observed temporal changes of CD and DF data (Wen, 2006; Yao et al., 2015, Yao et al., 2019).
Which constitutes a good explanation to the temporal change of the seismic IC phases, we should let you, the reader, decide.
All the data and scripts used in Yao et al. (2020):
- SSI-D1-199312-200309: doublet 9303 in SSI
- SSI-D2-199807-201306: doublet 9813 in SSI
- Fig1: Figure 1 in Yao et al. (2020)
- Original article: Yi Yang and Xiaodong Song, (2020), Origin of temporal changes of inner-core seismic waves, EPSL, doi:10.1016/j.epsl.2020.116267 | Chinese News
- Comment: Jiayuan Yao, Dongdong Tian, Li Sun, and Lianxing Wen, (2020), Comment on “Origin of temporal changes of inner-core seismic waves” by Yang and Song (2020), doi:10.1016/j.epsl.2020.116640
- Reply: Yi Yang and Xiaodong Song, (2020), Reply to Yao et al.'s comment on “Origin of temporal changes of inner-core seismic waves”, doi:10.1016/j.epsl.2020.116639