A REASSESSMENT OF CHANGES MICROGRAVITY AND DRY TILT ACCOMPANYING VARIATIONS IN ERUPTIVE ACTIVITY MEASURED AT PACAYA VOLCANO, GUATEMALA, JANUARY 1989

This paper is a re-examination of data obtained in January 1989 at Pacaya Volcano, Guatemala, in an effort to quantify real-time, concurrent variations in eruptive activity, microgravity, and dry tilt. The data consist of 22 hours of discontinuous measurements made during six observation periods of 2.6 to 6.0 hours duration, between Jan. 6 and 17, at a station approximately 100 m. east of an active vent, and 50 m. below the Pacaya summit. The eruptive activity, consisting of mild Strombolean explosions occurring every 50 seconds (on the average), was quantified by counting and recording the times of visible, or audible explosions, and by measuring the flight time of the highest flying bombs from visible explosions using a stop watch. Gravity measurements made (every 8 minutes on the average) using LaCoste Romberg meter 209 were corrected for drift and tidal effects. Dry tilt measurements were made every 5 minutes, on the average, using a Wild NA-2 level with parallel plate micrometer shooting to three 1 m fiberglass stadia rods attached to concrete bases at the corners of a 68x46x30 m. dry tilt triangle. Quantified eruptive activity varied between 0.2 and 2.8 explosions/minute with maximum flight times varying between 4.0 and 11.1 sec. Elevation changes measured during inflation/deflation events consistently produced tilts of up to 50 microradians, all radial to the active vent. Gravity changes up to 50 microgals, measured during observation periods, had maximum rates of change between – 27 and +38 microgals per hour. Temporal comparisons of variations in tilt, microgravity, and quantified eruptive activity are intriguing, but the data are insufficient to establish a consistent relationship between variations in eruptive activity, and gravity changes and tilt (caused by mass redistributions and/or elevation changes). During some observation intervals (1/6,13,1 and 14/89) all three variables seem to change synchronously, suggesting links between changes in microgravity and tilt, with variations in eruptive activity. During other periods (1/7, 11, and 17/89) no relationship is apparent in the timing of measured changes in gravity, tilt, and eruptive behavior.