Analyses of unusual long‐period earthquakes with extended coda recorded at Katmai National Park, Alaska, USA

A swarm of six long‐period (LP) events with slowly decaying coda wave amplitudes and durations up to 120 s, was recorded by seismic stations located in the proximity of Mt. Griggs, a fumarolically active volcano in the Katmai National Park, Alaska, during December 8–21, 2004. Spectral analyses reveal the quasi‐monochromatic character of the waveforms, dominated by a 2.5 Hz mode frequently accompanied by a weaker high‐frequency onset (6.0–9.0 Hz). Particle motion azimuths and inclination angles show a dominant WNW‐ESE direction of polarization for all the signals, and suggest that seismic energy is radiated by a stable source at shallow depth. Damping coefficients between 0.0014 and 0.0063 are estimated by fitting an exponential decay model to the signal's coda; corresponding quality factors range from 78 to 351. The source of the waveforms is modelled as a resonant cavity filled with a fluid/gas mixture.