  Recent advances on human dynamics have focused on the normal patterns of
human activities, with the quantitative understanding of human behavior under
extreme events remaining a crucial missing chapter. This has a wide array of
potential applications, ranging from emergency response and detection to
traffic control and management. Previous studies have shown that human
communications are both temporally and spatially localized following the onset
of emergencies, indicating that social propagation is a primary means to
propagate situational awareness. We study real anomalous events using
country-wide mobile phone data, finding that information flow during
emergencies is dominated by repeated communications. We further demonstrate
that the observed communication patterns cannot be explained by inherent
reciprocity in social networks, and are universal across different
demographics.
