Earth’s inner core slowdown confirmed by seismic data

A new scientific study proves that the Earth’s rotation speed has been decreasing throughout the past decade, with data suggesting that the rotational speed of the inner core has gotten 2.5 times slower compared to the past decade.

In the study, published in Nature on June 12, 2024, scientists have leveraged over two decades of seismic data to enhance our understanding of the Earth’s inner core. The research, a collaborative effort by esteemed institutions from China, the United States, and Canada, utilized advanced beamforming techniques and extensive waveform comparisons to scrutinize seismic waves, revealing significant insights about the Earth’s inner mechanisms.

The study predominantly utilized data from two high-quality seismic arrays, ILAR (short for Illinois Long-Range) and YKA (Yellowknife Seismic Array). These arrays have been capturing clear teleseismic P waves at periods near one second, thanks to their well-designed apertures and strategic siting. Spanning a period from 1991 to 2023, the research team compiled data from 121 seismic events. These events occurred in 42 different locations, including 16 distinct multiplets, which are clusters of three to seven repeating seismic events over time.

Beamforming significantly improved the signal-to-noise ratio, allowing the team to conduct precise waveform comparisons. Between the two arrays, a total of 200 waveform pair comparisons were made, focusing on changes in the PKIKP waves over the years. PKIKP waves are a type of seismic wave that travels through the Earth’s inner core, and examining changes in these waves can provide valuable information about the core’s properties.

Interestingly, while many PKIKP waves showed changes over time, other seismic waves, including inner core-reflected phases, did not exhibit noticeable changes in arrival time or waveform. The researchers classified the waveform matches into three categories: similar, somewhat similar, and different. This classification yielded 57, 72, and 71 pairs, respectively, giving a broad view of how PKIKP waveforms have evolved.

One of the intriguing findings of the study involved the analysis of multiplets. Typically, a multiplet involves a triplet of events that repeats over several years. For instance, one triplet labeled as multiplet O repeated in 2003, 2009, and 2020, while another triplet, multiplet J, repeated in 2002, 2009, and 2022.

Visual inspection of these multiplets showed that the middle event often had a waveform that differed from the other two, which were nearly identical. This pattern suggests that PKIKP waveforms can change and then revert to their original form, potentially indicating complexities in the Earth’s inner core dynamics.

The research highlighted broad patterns based on the data from both seismic arrays. Pairs originating from southern locations exhibited less waveform difference at the YKA array. Additionally, most pairs from earlier years showed more significant waveform changes, potentially reflecting temporal variations in the core’s properties.

The study’s authors also noted that the fitting lines, representing the seismic waveforms, indicate a slower rotation rate of the Earth’s inner core after 2008. Specifically, the rotation rate since then is about 2.5 times slower compared to prior years. This finding provides a fresh perspective on inner core dynamics, contributing to the ongoing debate about rotational and other internal processes within the Earth.

This extensive study, drawing from over 20 years of seismic data, marks a significant advancement in our understanding of the Earth’s inner core. By meticulously analyzing waveform changes, especially within PKIKP waves, the research offers critical insights into the core’s dynamic behavior.

References:

¹ Inner core backtracking by seismic waveform change reversals – Wang, W., Vidale, J.E., Pang, G. et al. -Nature – June 12, 2024 – https://doi.org/10.1038/s41586-024-07536-4 – OPEN ACCESS

Featured image credit: University of Southern California