Ask A Scientist General Science Archive

Question:
Over the years I have noticed that most major earthquakes seem to happen
in the early morning hours between 12:00 midnight and 6:00 a.m.
Is there any truth in this observation?
Could there be a correlation between full and new moon and the frequency 
of earthquakes (land tides)? The Sylmar and the Northridge earthquake
fit this model and the Tehachipe in 1952 occured in the early morning
but I do not know what the phases of the moon was in 1952.
 wayers

Answer:
Ah, so you've noticed that earthquakes tend to occur
in the early morning hours, have you?  You mentioned
the Northridge, Sylmar, and Tehachpi
earthquakes--some recent southern California
earthquakes that fit the pattern are:
 
Earthquake           Magnitude   Time of Day
----------           --------    ----------
1994 Northridge         6.7       4:31 AM
1992 Landers            7.5       3:57 AM
1992 Big Bear           6.5       7:05 AM
1991 Sierra Madre       5.4       6:43 AM
1987 Superstion Hills   6.1       5:15 AM
1987 Whittier Narrows   5.9       6:42 AM
1986 North Palm Springs 5.6       1:20 AM
 
The California Institute of Technology
has a catalog of earthquake times and locations for
southern California that is fairly complete back
to 1933.  Of 44 magnitude six or larger earthquakes
in that catalog, 28 occurred in the morning and
16 occurred in the evening.  The odds of 28 or more
earthquakes occurring in the morning by random
chance is about 5%.

If this is a real effect, it is probably related
to earth tides as you suggest.  I have been planning
to look into it in more detail, but haven't had
the time yet.  Sounds like a good project for
a science fair for an enterprising student.
-Grant