Deep earthquake

Last Saturday May 30 there was a truly deep earthquake, lasting quite long. It even gave 2 JMA earthquake reports, with increasing shindou to 5+ in some areas near Tokyo. According to the JMA it occurred at 590 km depth and had magnitude 8.5. Hopefully it encouraged people without survival packages to prepare.
The USGS however said it occurred at over 660 km depth and had a magnitude of 7.8. Intuitively it would make more sense to trust the JMA in this case, given how many advanced sensors there are in Japan, but why is the difference so great both in depth and magnitude? One publication states that 'Comparisons of real-time USGS epicenters and the Japan Meterological Agency (JMA) locations yield an average difference of 31 km (median 26 km)', however they only compared the accuracy of earthquakes shallower than 50 km.