Can Planets cause Earthquakes?

Here is a 2003 paper published by Nitsche in IAMG.

I haven't worked out all the maths that the paper talks about but the main hypothesis is that earthquakes are causes when there is a fluctuation in the combined gravitational field of the planets over the earth. He has taken the example of a month in which there were 11 earthquakes and 4 of them co-occurred with planetary gravitational fluctuations.