This would be a good place to mention that the USGS spectrograms for various seismographs are available at USGS.gov, and they are of course in the public domain, for licensing purposes!  If you go to the USGS website you can normally find about 47 of them gong back about two weeks, however - if you know how to hack the URL you can usually find the older ones that are still up; like this July 5th spectrogram from Geyser Peak.

https://earthquake.usgs.gov/monitoring/spectrograms/24hr/23/20190705

Looking at the August 21st spectrum from the same seismograph, it is immediately obvious that there was some activity at around 5.5Hz, with some faint but very chaotic phase noise between 5 and 10 Hertz, and a magnitude 5 aftershock soon followed in the Ridgecrest area.

https://earthquake.usgs.gov/monitoring/spectrograms/24hr/23/20190821

Of course, going back to March 9th it is very interesting to examine the instabilities associated with the 5.5Hz and 7Jz signals that were present at that time.  is this the original premonitory signals?

https://earthquake.usgs.gov/monitoring/spectrograms/24hr/23/20190309

Also take a look at the signals from June, … etc., where that was a massive buildup of phase noise in the 5 to 10Hz range, easily detected from a distance of about 470 miles!

https://earthquake.usgs.gov/monitoring/spectrograms/24hr/23/20190608