  Despite recent advances in uncovering the quantitative features of stationary
human activity patterns, many applications, from pandemic prediction to
emergency response, require an understanding of how these patterns change when
the population encounters unfamiliar conditions. To explore societal response
to external perturbations we identified real-time changes in communication and
mobility patterns in the vicinity of eight emergencies, such as bomb attacks
and earthquakes, comparing these with eight non-emergencies, like concerts and
sporting events. We find that communication spikes accompanying emergencies are
both spatially and temporally localized, but information about emergencies
spreads globally, resulting in communication avalanches that engage in a
significant manner the social network of eyewitnesses. These results offer a
quantitative view of behavioral changes in human activity under extreme
conditions, with potential long-term impact on emergency detection and
response.
