This map shows Ashqelon, north of Gaza (Map: Perry-Castanada)
Today's escalation in qassam rocket fire makes for an interesting case study. Hamas had been slightly increasing its rocket launchings in the past few days, hitting targets in the western Negev and Sderot that led to casualties. The Israeli air force's operation on Wednesday morning, which seems to have killed at least five members of Hamas's military brigades traveling in a van (New York Times), elicited a massive response from the organization. Hamas is reaching deep into its arsenal, employing those missiles capable of hitting Ashqelon, which suffered several hits today. In response, the Israeli air force struck hard, destroying Palestinian government buildings. One thing that this episode tells us is that Hamas reacts most strongly when senior members of its organization are killed, using its ability to strike Israeli civilian targets as deterrence against Israeli air strikes. For Israel, Ashqelon seems to have become the red line, at which it turns to considering assassinations of Hamas's political leadership.
UPDATE: Amos Harel has a good analysis of the escalation, which I think supports my take.