Thanks for the advice, Petra
I find it difficult to believe, however that the ear tone had anything to do with a Ml 2.7
three days later 550 km away - it strains my brain to think that. In the past ear tones
for me have been much more commonly associated with changes in the geomagnetic
field. This appears to be the case in this situation. Checking back over the GOES
geomagnetic field for April 23, I find that there was a sudden change in the Hn geomagnetic
field and an equivalent rise in the He field at exactly 00:07 on April 23, the exact time
of my ear tone. For details see:
http://sec.noaa.gov/ftpdir/lists/geomag/20020423_G8mag_1m.txt
This change was, in turn probably due to a sudden burst of Solar X-rays (the strongest
of the day from 00:00 UT to at least 20:00 UT (April 23) which was recorded at satellite
elevations at 00:03 to 00:06 UT resulting in cascading and the geomagnetic field change
at 00:07 UT, the time of my ear tone.
I am satisfied, therefore, that this ear tone was merely my brain reacting to an electron/
X-ray flux change in the ionosphere/atmosphere and was unrelated to an earthquake.