Concept maps are a self-constructed, pictorial representation of an individual's or group's understanding of major concepts and relationships to important sub-concepts. This study was conducted with San Diego City School (SOCS) K-6 teachers and was designed to test their understanding of earthquakes through the use of concept maps. 60 teachers were asked to break up into mixed-grade level groups of their own choosing. 19 mixed grade level groups were formed ranging from 2 participants up to 6 per group. Groups were then provided instructions which directed them to construct concept maps using the major concept of Earthquakes and a specified list of key sub-concepts of sufficient range to provide a measure of the understanding of Earthquakes held by each group. The completed concept maps were analyzed for overall understanding and continuity, proper linkages between the major concept and the sub-concepts, multiple links between the major concept and sub-concepts, cross links between sub-concepts, and overall construction. The focus of this analysis was on correctness, patterns, and trends observed in the development of the concept maps. The results show that most groups possess an adequate, basic understanding of earthquake processes, but with several identifiable areas where additional focused professional development could help primary teachers reach a more sophisticated level of understanding. More technical sub-concepts such as strain and fault slip tended to be poorly understood by the teachers, demonstrated by the poor and discontinuous development of correctness in those links. Also, a vast majority of the groups were unable to develop multiple links and cross links, both of which demonstrate a deeper understanding of the relationship between the major and sub-concepts. Our results also show that concepts maps are a useful tool to assess the understanding of primary teachers of major geological concepts, and can be used to refine the direction and content of professional development institutes.