Terrorism is a result of a process called emotional contagion.
People tend to mimic the facial expressions, postures, and instrumental behaviors of people around them, and thereby "catch" others' emotions as a consequence of such facial, vocal,and postural feedback.
Terror in Resonance (残響のテロル Zankyō no Teroru?, lit. "Echo's Terror"), through this title the authors suggest that terror spread through resonance . This can be seen in the character development of Lisa Mishima. By helping out Sphinx, she is copying their actions and becomes their accomplice which slowly transforms her into a full member of Sphinx. In other words, by doing the same as Nine and Twelve, she become a terrorist like them. Thus. she is helping them to spread terror through the city of Tokyo.
Also, it is important to note that terror is an emotion;
terror is a very strong feelings of fear. This emotion is spread through the population in three steps in this anime: first is a terrorist act itself, second is the broadcast of it by media, and third is through rumors and conversations among residents of Tokyo. More people see the terrorist act; more people are going to talk about it. So there are more people who are going to fear about their life and the life of their loved ones. In other words, terrorism spreads like an infection through the entire population of Tokyo