El Chichón volcano is the most active volcano in the state of Chiapas, México, and experienced its last Plinian eruption (VEI = 5) in 1982. To better assess its volcanic hazard, we studied its readiness to erupt by estimating changes in its internal stress state. These stress changes are difficult to calculate accurately, for example in the absence of focal mechanisms, but their existence can be indirectly revealed by the pres...