Microearthquake activity associated to los hume ros caldera, México: preliminary survey

Abstract

From 12 to 24 January 1977 three portable seismic stations were intalled at the interior of Los Humeros Caldera in order to obsenre tlie rilicroearthquake activity associated with it. Seven events of magnitude m~1.0 were located. Three of these events we located using first arrivals, fixing depth at 10 km and taking = 2.5 km/sec. The remaining 4 microearthquakes do not show legible P or S body waves arrivals; we estimated an epicentral location based on the differenci; s of arrival time for the maximum amplitude of surface waves (Airy phase) assuming that the velocity of surface waves is controlled mainly by .the .deposits of unconsolidated pyroclastics t.hat cover the caldera area. Lower magnitude events were recorded at only one or two stations; an average of 3 events/day/station were recorded. such an activity is similar to that observed in other geological structures of geothermal interest. 96 small events were recorded; one half of them occurred in swarm sequences. Microearthquakes of types "A", "B", and "Tremor" were observed; they are tipyca of active or recently active volcanic structures. The data suggest the existence of a zone on western side of the caldera, that strongly attenuates seismic waves; this zone also presents self-potential and temperature anomalies (Alvarez, this issue).