  When subjected to a sudden, unanticipated threat, human groups
characteristically self-organize to identify the threat, determine potential
responses, and act to reduce its impact. Central to this process is the
challenge of coordinating information sharing and response activity within a
disrupted environment. In this paper, we consider coordination in the context
of responses to the 2001 World Trade Center disaster. Using records of
communications among 17 organizational units, we examine the mechanisms driving
communication dynamics, with an emphasis on the emergence of coordinating
roles. We employ relational event models (REMs) to identify the mechanisms
shaping communications in each unit, finding a consistent pattern of behavior
across units with very different characteristics. Using a simulation-based
"knock-out" study, we also probe the importance of different mechanisms for hub
formation. Our results suggest that, while preferential attachment and
pre-disaster role structure generally contribute to the emergence of hub
structure, temporally local conversational norms play a much larger role. We
discuss broader implications for the role of microdynamics in driving
macroscopic outcomes, and for the emergence of coordination in other settings.
