A continuous record of intereruption velocity change at Mount St. Helens from coda wave interferometry

Journal of Geophysical Research B: Solid Earth
By: , and 

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Abstract

In September 2004, Mount St. Helens volcano erupted after nearly 18 years of quiescence. However, it is unclear from the limited geophysical observations when or if the magma chamber replenished following the 1980–1986 eruptions in the years before the 2004–2008 extrusive eruption. We use coda wave interferometry with repeating earthquakes to measure small changes in the velocity structure of Mount St. Helens volcano that might indicate magmatic intrusion. By combining observations of relative velocity changes from many closely located earthquake sources, we solve for a continuous function of velocity changes with time. We find that seasonal effects dominate the relative velocity changes. Seismicity rates and repeating earthquake occurrence also vary seasonally; therefore, velocity changes and seismicity are likely modulated by snow loading, fluid saturation, and/or changes in groundwater level. We estimate hydrologic effects impart stress changes on the order of tens of kilopascals within the upper 4 km, resulting in annual velocity variations of 0.5 to 1%. The largest nonseasonal change is a decrease in velocity at the time of the deep Mw = 6.8 Nisqually earthquake. We find no systematic velocity changes during the most likely times of intrusions, consistent with a lack of observable surface deformation. We conclude that if replenishing intrusions occurred, they did not alter seismic velocities where this technique is sensitive due to either their small size or the finite compressibility of the magma chamber. We interpret the observed velocity changes and shallow seasonal seismicity as a response to small stress changes in a shallow, pressurized system.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title A continuous record of intereruption velocity change at Mount St. Helens from coda wave interferometry
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1002/2013JB010742
Volume 119
Issue 3
Year Published 2014
Language English
Publisher AGU Publications
Contributing office(s) Earthquake Science Center
Description 16 p.
First page 2199
Last page 2214
Online Only (Y/N) N
Additional Online Files (Y/N) N