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Name: Vonisha L.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: March 22, 2004 


Question:
What is the effect of the moon's gravitational pull in producing "land tides" similar to ocean tides? Do seismologists think this could be triggering earthquakes?



Replies:
The short answer is generally not. According the web page of the Seismological Laboratory at University of California at Berkeley:

"The question often arises as to whether astronomical events, such as planetary alignments, can significantly influence the occurrence of earthquakes. The moon, sun, and other planetary bodies in our solar system influence the earth in the form of perturbations to the gravitational field. The relative amount of influence is proportional to the objects mass and inversely proportional to the cube of its distance from the earth. The general idea is that strains in the earths crust caused by perturbations in the gravitational field may influence when an impending earthquake will occur (like the straw that broke the camel's back). If this were indeed the case we would expect to see a correlation between the rate at which earthquakes occur and the perturbations to the gravitational field. The dominant perturbation in the earth's gravitational field generates the semi-diurnal (12 hour) ocean and solid earth tides which are primarily caused by the moon (due to its proximity) and the sun (due to its large mass). No significant correlations have been identified between the rate of earthquake occurrence and the semi-diurnal tides when using large earthquake catalogs. There have, however, been some small but significant correlations reported between the semi-diurnal tides and the rate of occurrence of aftershocks in some volcanic regions, such as Mammoth Lakes."

You can read the entire commentary on this question at www.seismo.berkeley.edu/seismo/faq/planets.html.

Andy Johnson



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