Volcanism and tectonism in the eastern Aleutian arc are controlled by the subduction of the Pacific plate beneath the North American plate . Subduction of the Pacific plate is occurring at an angle of 45 degrees to a depth of 100 to 150 km which marks t he location of the Benioff zone beneath the volcanic front and Redoubt Volcano. The cold, brittle slab that defines the Benioff seismic zone is the source of multiple earthquakes . This topic will be expl ored further in the following section. The volcanoes of the eastern Aleutian Arc are built on previously existing topographic highs consisting of Mesozoic intermediate volcanic, volcaniclastic and sedimentary rocks.
All stations have single component, vertical, short-period seismometers.
An additiona l five seismographs were emplaced around Redoubt in October 1989 for the purpose of monitoring recently elevated seismic activity within and around the volcano. A few minor long-period seismic events that increased in frequency were recorded prior to Dece mber 13, 1989. These events were interpreted by Chouet et al as the excitation of a pressur ized, fluid-filled crack at shallow depth beneath the summit caused by the interaction of water, steam and magmatic gases. Energetic LP swarms that preceded the initial large phreato-magmatic eruption on December 14 and the powerful dome destroying erupti on on January 2 were detected up to 85 km from the volcanic summit. As the eruption sequence progressed from January to April, the intensity of the LP swarms decreased and identification of the swarms became more difficult.