An excellent summary can be found in the EOS article by Science Writer Jack Lee.
https://eos.org/research-spotlights/first-report-of-seismicity-that-init...
New study by UArizona Geophysicists reports on the first detection of earthquakes that initiate in the Earth's lower mantle.
Aug. 19, 2021
Photo: Back-projection image (a) and seismic data (b) from a lower mantle earthquake that occurred near 750 km depth.
By using a 4D back-projection method, Kiser et al. traced the path of a 2015, 7.9 magnitude earthquake that took place beneath Japan's remote Ogasawara (Bonin) Islands, and identified, for the first time, seismic activity that initiated in the lower mantle. Their results provide new insights into the mechanisms that can produce earthquakes deep inside the Earth and appear in Geophysical Research Letters, https://doi.org/10.1029/2021GL093111, 2021.