The objective of this research is to address a fundamental question in cyber-physical systems: What is the ideal structure of systems that detect critical events such as earthquakes by using data from large numbers of sensors held and managed by ordinary people in the community? The approach is to develop theory about widely-distributed sense and respond systems, using dynamic and possibly unreliable networks using sensors and responders installed and managed by ordinary citizens, and to apply the theory to problems important to society, such as responding to earthquakes. Intellectual Merit: This research develops theory and prototype implementations of community-based sense-and-respond systems that enable people help one another in societal crises. The number of participants in such systems may change rapidly; some participants may be unreliable and some may even deliberately attack systems; and the structures of networks change as crises unfold. Such systems must function in rare critical situations; so designs, analyses and tests of these systems must give confidence that they will function when the crisis hits. The proposed research will show how to design systems with organic growth, unreliable components and connections, security against rogue components, and methods of demonstrating reliability. Broader Impact: People want to help one another in a crisis. Cheap sensors, mobile phones, and laptops enable people to use information technology to help. This research empowers ordinary citizens collaborate to overcome crises. The researchers collaborate with the US Geological Service, Southern California Edison, and Microsoft, and will host 3,000 students at a seismic facility