Families of Long-Period Seismic Signals Observed in Popocatepetl Volcano, Mexico

The most common seismic events at Popocatepetl Volcano are the long-period signals (LP), observed in association with degassing and lava dome building phases since eruptive activity reactivation of the volcano in 1994. We present a detailed description of LP seismicity recorded by a temporal broadband network deployed around the volcano from November, 1999 to July, 2000. In order to investigate the repetitive character of LP events we applied correlation techniques over three months-period (January to March 2000) of continuous data recorded in the nearest stations to the crater. Data were analyzed in the short-period (< 1 s), long-period (0.5-2.5 s) and very-long-period (< 15 s) bands. LP events present distinctive waveforms features in each frequency band. According to their distinctive signatures correlation results differentiate three distinct families of LP signals. Events Type-I exhibit impulsive arrivals between 0.5 and 2.5 s and remarkable waveform pulses (>15s) in the VLP band. LP events Type-II occur in pairs, present emergent arrivals in all bands and pulse-train signatures in the VLP band. LP events type-III, the most energetic signals in the broadband, do not present energy in the VLP band, some events present impulsive arrivals and clear decaying codas. Observed variations in volcanic eruptive activity correlates with defined types of LP events. During the experiment most of LP seismicity occurs in association with degassing and was dominated by LP events Type-I. On February eruptive activity increased in intensity and events Type-II started to occur more frequently. Events Type-III were observed at the end of February and during March, when a lava dome formation was reported and before mild vulcanian eruptions, occurred on April, 23, May, 23 and May, 25. LP seismic variations related to changes in eruptive activity suggest that each family might be associated with different source location or different excitation mechanisms acting on the same source.