I read Core back when, but for some reason all I remembered of the plot was that it featured an antagonist in the form of none other than dear old Saddam Hussein, who hatches a plot to use a deep diagonal borehole and an H-bomb to trigger earthquakes and wipe out Israel, so I never made the connection to the movie. Preuss doesn't even have a story credit on IMDB for the film - I wonder if it's really based on his novel at all.