Here's the clause in the PA constitution covering this:

Article V, Section 13(b) A vacancy in the office of justice, judge or justice of the peace shall be filled by appointment by the Governor. The appointment shall be with the advice and consent of two-thirds of the members elected to the Senate, except in the case of justices of the peace which shall be by a majority. The person so appointed shall serve for a term ending on the first Monday of January following the next municipal election more than ten months after the vacancy occurs or for the remainder of the unexpired term whichever is less, except in the case of persons selected as additional judges to the Superior Court, where the General Assembly may stagger and fix the length of the initial terms of such additional judges by reference to any of the first, second and third municipal elections more than ten months after the additional judges are selected. The manner by which any additional judges are selected shall be provided by this section for the filling of vacancies in judicial offices.