Tracing The Movement And Storage Of Magma In The Crust Through Seismology: Examples From Alaska And Western Mexico

Abstract

Four studies are presented that examine magma movement and storage in the crust using seismology at three different volcanoes: Fourpeaked volcano in the Cook Inlet region of Alaska, Paricutin volcano in the Michoacan-Guanajato volcanic field in western Mexico, and Colima volcano at the western edge of the Trans-Mexican Volcanic Belt. In 2006, Fourpeaked volcano, Alaska, had a widely-observed phreatic eruption. A modest seismic network was installed in stages following the unrest. The eruption was followed by several months of sustained seismicity punctuated by vigorous swarms and SO2 emissions exceeding a thousand tons/day. Based on the history of Fourpeaked, and observations during and after the phreatic eruption, it is proposed that the activity was caused by a modest injection of new magma beneath the volcano. Also presented are a series of studies from western Mexico, an area of high seismic and volcanic activity. A description of the creation of an automatically generated regional catalog of seismic activity is presented, along with a comparison with existing seismicity studies of the area. From this catalog, a swarm of earthquakes near Parfcutin in May-June 2006 was discovered. This swarm demonstrated a steady upward migration in depth with time. Focal mechanisms during the first part of the swarm reflect the increased stress caused by dike inflation. In early June, the stress orientation changed and became more consistent with the inflation of a horizontal sill-like structure. At Colima volcano, a P-wave tomographic inversion using arrivals from 299 regional earthquakes is presented. The results of the inversion show two distinct low-velocity zones. One is in the upper 10 km under the volcano and may be caused by a magma chamber-type structure. The second anomaly, with peak values of 2.5% slower velocities, was imaged in the crust southeast of the volcano at depths of 15-30 km. This body may be due to partial melt and increased temperatures from a second, deeper area of magma storage.