Title: Recent insights into Earth’s inner core structures from late earthquake coda

Abstract:

Probing the Earth’s inner core (IC), which accounts 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, including its shear properties. The discovery of spatial correlation in 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 robustly detected shear waves (i.e., J waves) propagating through the IC. Here, we will present new observations of correlation features, which have become possible thanks to the latest advancement in correlation wavefield theory. Interestingly, one of the new features is exclusively sensitive to the IC shear wave speed, which is minimally influenced by inevitable uncertainties in velocity models of the upper major shells, such as the mantle and outer core. This feature suggests even lower J-wave speeds in the IC than our previous estimate.

Moreover, inspired by the clear expression of correlation features, we also 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 innermost inner core (IMIC) possessing distinct anisotropic properties from the outer layer of the IC. The transient interface between the two zones in the IC is likely associated with a shift in the regime of the geomagnetic field in the Earth’s distant past.