Comparison of the Seismicity Before and After the 1982 El Chichon Eruption

Abstract

The seismicity recorded at El Chichón Volcano shows significant differences before and after the 1982 eruptive episodes. The analysis of the seismicity was performed using two well-known laws in seismology: the Gutenberg-Richter law, which describes the frequency-magnitude distribution of seismicity and the Omori law, which applies to the temporal decay of the number of earthquakes . Results of the analysis suggest that large quantities of fluids (hydrothermal fluids, water, and/or magma) were involved in the physical processes that generated the precursory seismicity one month before the onset of the eruption; only shallow (2–8 km depth) hybrid earthquakes were located and long-period and volcanic tremors were recorded. The volcanic signature of the seismicity is better described by two slopes instead of only one as is usually the case for tectonic events. Furthermore, the seismicity did not follow the Omori law. In contrast, for one month after the last eruptive phases of the 1982 eruptive events, the seismicity showed a more ‘tectonic’ signature, as evidenced by the occurrence of five large earthquakes (magnitudes ~3.8) on 4 April 1982. These followed both the Gutenberg-Richter law (with a single slope) and the Omori law. The tectonic signature is confirmed further by the single-slope observed in the distribution of the seismicity recorded during the period 1985–1990. Such behavior suggests the absence of abundant fluid at depth after the last plinian event of 4 April 1982, with a slow return to a regular tectonic response of the volcanic system. Seismic analysis of the 1982 eruptive sequence illustrates clear volcano-tectonic feedback interactions.