Evolution of the seismic activity from the 1982 eruption of El Chichon Volcano, Chiapas, Mexico

We analyzed more than 1700 earthquakes related to the 1982 eruption of El C hichon volcano in southern Mexico. The data were recorded at specific perio ds throughout the whole eruptive interval of March to April 1982, by three different networks. The seismic activity began several months before the fi rst eruption on 28 March. During this period the seismicity consisted of hy brid and long-period shallow earthquakes most likely related to processes o f faulting, fracturing, and fluid movement underneath the volcano. The foci of events occurring before the eruption circumscribe an aseismic zone from approximately 7 to 13 km below the volcano. After the eruption, the seismi c activity consisted of tectonic-type earthquakes that peaked at 1200 event s/h. This later activity occurred over a wide range of depths, mostly betwe en 5 and 20 km, that includes the former aseismic zone and is roughly limit ed by the major tectonic faults in the area.