Your tutor seems unduly inflexible. Both "there is" and "there are" are commonly used in sentences of this type, and no consensus exists as to which is better.
In your example of an increasing number of earthquakes, it's easy to toss off a flippant remark about number being singular. Admittedly "there is" sounds fine, but so does "There are ..."
As far as I know, "main focus" is not a grammatical concept. I just use what sounds euphonious to me, and I advise you to do the same. Or rather, since you are a student, I advise you to listen to what native speakers say, and imitate them. Don't let your tutor's rigid rule interfere with your learning.
Student or Learner