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Name:  wayers
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999


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.


Replies:
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



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