Re solar/lunar gravitational forces with respect to earthquakes:

The gravitational effects on earth caused by the Sun and the Moon
are approximately equal since the moon, although much smaller,
is much closer, and the gravitational force of the massive sun
is dispersed by the square of the distance from earth. This is
why we have approximately equal gravitational tides when the 
sun/moon angle is near 90 degrees. In conjunction (new moon)
the forces add, but an equal tidal force is seen on the opposite
side of the earth because of the conservation of rotational momentum.
At opposition (full moon) the gravitational amplitudes are maximum.

The stress in the earths' crust results in strain and tilts of the
order of 10^-7, and are so predictable that routine calculations of
them are used to calibrate broadband instruments. Over the years
many efforts have been made to correlate earthquake occurrence or
make predictions based on the lunar/solar forces. Some slight statistical
correlation was found with small events on faults optimally oriented
in strike and dip to experience the forces. However, the forces are
quite small compared to the stress in the crust caused by barometric
fronts during storms, and as far as I know, no one has predicted a
quake because of a thunderstorm.

And of course, there is very limited data about the state of stress
of a fault zone, which can range from "locked up" to being on the
edge of rupture, based on the interplay of local and regional 
tectonic structures and other fault zone variables, such as hydrology.

However, from an operational point of view, I have found that the 
probability of earthquake occurrence is directly related to how broken
or inoperable the seismic instrumentation to record it is.

Regards,
Sean-Thomas
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