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Bob Rollings                                                                                                          August 1999 
PEPP Workshop Final Project
Possible Gravitational Influence on Earthquake Frequency
From Alignment of Moon and Sun

Objective: Analyze earthquake event archives for possible correlation between earthquake frequency and the gravitational influence of the moon and sun.

Background: The gravitational influence of the moon on Earth is readily noticeable at coastal locations by the daily cycle of oceanic tides. These gravitational forces also create earth or ground tides of lesser height and notice. A visual depiction of this tidal effect is illustrated in the drawing below.


The above drawing shows high tides occurring simultaneously on opposite sides of the planet. As the earth rotates on its daily 24 hour cycle, high tides occur at 12 hour intervals with each location on earth experiencing two tidal cycles each day. This phenomenon is explained in many earth science reference sources as primarily a consequence of variation in lunar gravitational forces with distance between near and far sides of the Earth. The possibility that tidal forces could influence seismic events seems reasonable in view of the planetary strain experienced by the Earth. But 12 hour tidal cycles are so frequent that their influence on seismicity would be seemingly near continuous. However, there is another overlaying tidal cycle. Under a full moon or new moon comparatively higher tides occur, known as "spring" tides, whereas the intervening periods of comparatively lower tides are known as "neap" tides. This cycle repeats on approximately a 27 day interval, as shown on graph to the right.
 

This 27 day tidal cycle is associated with the combined, and therefore increased, gravitational pull experienced by Earth when both the moon and sun are most closely aligned. This is illustrated in the drawing at right. Although the moon has a larger tidal influence than the sun due to its relative closeness to the Earth, the combined influence of both the moon and sun cause the noticeable tidal change shown in the previous graph.

The possibility that this approximately monthly tidal cycle could influence seismic events has been examined by several researchers. Myron L. Fuller noted a monthly seismic frequency pattern for the 1811-1812 New Madrid earthquakes, which he described in his 1912 book titled The New Madrid Earthquake. He noted that, "With one marked exception the groups [of New Madrid seismic events] occur approximately either at times of new or full moon." His tabulation of the New Madrid events by lunar phases counts 283 events for the first and third quarters during February and March of 1812 while 513 events occurred during the second and fourth quarters. A pertinent figure from Fuller's publication is shown below.

Much more recently Dr. Iben Browning analyzed the alignment of the moon and sun to predict, incorrectly, a major earthquake in the New Madrid Seismic Zone for December 3, 1990. Although the earthquake did not occur, Browning's prediction reawakened midwestern U.S. residents to potential future risks associated with New Madrid earthquakes. It appears that contemporary seismologists, perhaps cautious after the unrealized Browning prediction, have largely discounted the possible link between tidal forces and seismic events.

Procedure: To examine possible gravitational influence of the moon and sun on seismic events requires analysis of a comprehensive seismic database. There are several such databases available through the internet which could be easily accessed by students. The National Earthquake Information Center (NEIC) is one recently used by PPNAF students in a preliminary study of this type. The NEIC database allows the user to select earthquakes by geographic area, time interval, and magnitude. Tide cycles can also be accessed on the internet for a specified time interval and location. Two such web sites for tidal data are identified below:

http://tbone.biol.sc.edu/tide
http://www.opsd.nos.noaa.gov/tides
 

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