Repeating Long‐Period Earthquakes at Shishaldin Volcano (Alaska): Insights Into the Source Mechanism and Relationship to Volcano Dynamics

We investigate repeating earthquakes at Shishaldin Volcano (Alaska) by applying a multi-stage clustering technique to the waveforms of approximately 330,000 Long-Period (LP) earthquakes recorded between October 2003 and July 2004, a period that includes a minor eruption. We find about 5,500 LP earthquakes with at least one repeater in the catalog. These repeaters show time-varying frequency content, which is confirmed by polarization analysis that identifies two distinct wave packets corresponding to different physical processes. The first packet, lasting around 5 s, is a direct source effect, and its polarization dip angle points to the nucleation depth of the pressurization phenomenon that triggers the conduit resonance. The second packet, found in the coda of the first one, is dominated by scattered waves. Repeaters are interpreted as LP events with a common pressurization position and triggering mechanism. The polarization-derived nucleation depths of the overall LP seismicity and of the repeaters show a sharp increase in December 2003–January 2004, coinciding with the onset of eruptive crisis, followed by a slow decrease in nucleation depth from January to May 2004. We interpret this as a fast pressure drop followed by a slow pressure build-up in the feeding system. After January 2004, coda wave interferometry applied to pairs of repeaters indicates an increase in medium velocity. By modeling the feeding system as a homogeneous porous medium and attributing the velocity increase to the squeezing of fluid-filled cracks, we estimate a pressure increase of (Formula presented.) 8 MPa from December 2003 to April 2004.