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Abstract:
Probing the Earth’s inner core (IC), accounting for just about 1% of the Earth’s volume, is critical for understanding the planet’s formation and evolution. However, geophysical inferences have been challenging due to the lack of seismological probes sensitive to the IC, especially its shear properties. The utilization of late earthquake records, the coda, has made a profound breakthrough in structural seismology, including studying the Earth’s and deep planetary interiors. More significantly, it has initiated a new theoretical and practical framework, the global correlation wavefield, to better understand the architecture of the late coda and place novel constraints on IC structures.
The global correlation wavefield has resulted in a robust detection of shear waves (i.e., J waves) propagating through the IC, expressed through the I2-PKJKP feature. Here, we will present a new set of IC shear-wave-sensitive correlation features, which have recently become observable thanks to the latest advancement in the correlation wavefield theory. Interestingly, one of the newly studied features is sensitive exclusively to the IC shear-wave speed and minimally influenced by uncertainties in the crust, mantle, and outer core velocity models. Moreover, inspired by the clear expression of late coda correlation features, we scrutinize the late hours of the direct coda wavefield and show unprecedented observation of up-to-fivefold reverberating waves along the Earth’s diameters. The travel time measurements from the observations strengthen the evidence for the ~650-km-radius innermost inner core (IMIC), which has distinct anisotropic properties from the IC’s outer layer.
