Somewhere there is a crime happening.
In the sentence above from the Robocop films the word somewhere is functioning as a Locative Adjunct. Notice that it can appear either at the beginning or end of the clause:
- There is a crime happening somewhere.
Notice also that the word there cannot move with the word somewhere:
- *Is a crime happening somewhere there.
This suggests that the word there is not part of this Locative Adjunct. If we look at the sentence above, it is easy to see that the main reason it is ungrammatical is because it does not have a Subject. This is because in the original example, the word there IS the Subject of the sentence as the data below indicates:
- There is a crime happening
- Is there a crime happening?
- There is a crime happening, isn't there?
- There is!
The second example above shows there inverting with the auxiliary verb to form a question, as a good Subject should. The third example shows the word there appearing in the question tag isn't there as we would expect (compare with He's happy, isn't he?) The fourth example shows the sentence reduced to a minimal Subject and Predicate. Because the word is constitutes the Predicate, the word there must be the Subject.
In response to the Original Question then, the answer is that somewhere and there do not form a phrase in this sentence, and therefore neither one of them can be the Head of that phrase. Somewhere is functioning as Locative Adjunct, whereas there is functioning as Subject.