The Earth's solid inner core grows from the solidification of the liquid
outer core. The solidificaiton process releases latent heat and expels
light elements, providing driving forces for the outer core convection
and the geodynamo which generates the Earth's magnetic field.
The information of the Earth's inner core boundary (ICB) is important
for our understanding of the thermo-compositional state of the Earth's core.
We use seismic waves reflected from the Earth's ICB to study the spatial and
temporary changes of ICB.
North Korea has conducted six nuclear tests since 2006.
Using seismological techniques, we determine their high-precison locations,
yields, and source characteristics of the six nuclear tests and
associated small seismic events.
Seismic discontinuities in the mantle provide critical insight into thermal
and compositional states of Earth's mantle and its dynamics.
The 410- and 660-km seismic discontinuities are observed globally, and
are generally linked to phase transformations of olivine → wadsleyite and
ringwoodite → bridgmanite + ferropericlase. The 520- and 560-km discontinuities
are usually less observed and studied, and are caused by phase transformation of
wadsleyite → ringwoodite, and exsolution of Ca-pv from garnet. We analyze
waveforms of SS precuros recorded by global stations to investigate global
variations of the 520- and 560-km discontinuities.