A Seismological Comparison of Bezymianny Volcano, Russia, and Mount St. Helens Volcano, Washington Weston Albert Thelen A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2009 Program Authorized to Offer Degree: Department of Earth and Space Sciences 1 University of Washington Graduate School This is to certify that I have examined this copy of a doctoral dissertation by Weston Albert Thelen and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Chair of the Supervisory Committee: Stephen D. Malone Reading Committee: _____________________________________________________________ Stephen D. Malone _____________________________________________________________ John Vidale _____________________________________________________________ Michael West Date: ___________________________________ 2 In presenting this dissertation in partial fulfillment of the requirements for the doctoral degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of the dissertation is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Requests for copying or reproduction of this dissertation may be referred to ProQuest Information and Learning, 300 North Zeeb Road, Ann Arbor, MI 48106-1346, 1-800-521- 0600, to whom the author has granted “the right to reproduce and sell (a) copies of the microform and/or (b) printed copies of the manuscript made from microform.” Signature_________________________ _ Date_____________________________ 3 University of Washington Abstract A Seismologic Comparison of Bezymianny Volcano, Russia and Mount St. Helens, Washington Weston A. Thelen Chair of the Supervisory Committee: Professor Stephen D. Malone Department of Earth and Space Sciences Bezymianny Volcano, Russia and Mount St. Helens, Washington are examples of eruptions involving the sector collapse of a volcano. The study and comparison of Mount St. Helens and Bezymianny has led to a better understanding of the precursory seismicity prior to eruptions. The shallow moment release prior to an eruption is dependent on the time since the last eruption at both volcanoes. This relationship is independent of the type of eruption that ensues, suggesting that overcoming the strength of the plug in shallow conduit must occur prior to the initiation of an eruption. This finding has obvious implications for forecasting eruptions, and more importantly, for forecasting failed eruptions. The comparison of multiplets at Bezymianny and Mount St. Helens suggests that the physical properties of magma in the shallow conduit controlled the occurrence and behavior of multiplets. When there is shallow crystallization and 4 active degassing of the plug within the conduit, multiplets are abundant. Alternatively, when there is no shallow crystallization in a semi-open system, as at Bezymianny, multiplets were less abundant. At both volcanoes prior to an explosion, the multiplet percentage of total seismicity drops, and the average amplitude of each multiplet increases. Multiplet characteristics such as the multiplet percentage of total seismicity, multiplet durations, and the number of contemporaneous multiplets can be tracked prior to an eruption to forecast the type of eruption that will ensue. The future volcanic activity at Mount St. Helens will likely diverge from the current explosive activity at Bezymianny. Without any additional input of mafic material or additional gas content, intermittent viscous dome building at Mount St. Helens will likely continue. In general, the high gas content of the magmas at Bezymianny compared to Mount St. Helens has driven more explosivity during eruptions despite the lower silica content and resultant lower viscosity magma at Bezymianny compared to Mount St. Helens. In future eruptions at Bezymianny and Mount St. Helens, the seismic energy release prior to eruptions will be dictated by the time since the last eruption. Stronger seismicity can be expected in cases where the conduit bends around existing domes. 5 TABLE OF CONTENTS Page LIST OF FIGURES ....................................................................................................... iv LIST OF TABLES .........................................................................................................vii I. Introduction.............................................................................................................1 1.1 Objectives ............................................................................................................1 1.2 Mount St. Helens..................................................................................................2 1.2.1 Eruptive History .............................................................................................3 1.2.2 Magma Plumbing System..............................................................................5 1.3 Bezymianny..........................................................................................................7 1.3.1 Eruptive History .............................................................................................8 1.3.2 Magma Plumbing System............................................................................10 1.4 Structure of the Thesis.......................................................................................11 II. Seismic Characteristics of Bezymianny Volcano, Russia........................................26 2.1 Introduction ........................................................................................................26 2.2 Methods .............................................................................................................29 2.2.1 Earthquake Locations..................................................................................29 2.2.2 Automatic Event Classification Methods .....................................................33 2.2.3 Multiplet Methods ........................................................................................36 2.3 Earthquake Locations ........................................................................................40 2.4 Automated Event Classification Results ............................................................42 2.5 Multiplet Results.................................................................................................44 2.6 Discussion..........................................................................................................48 2.6.1 Earthquake Locations..................................................................................48 2.6.2 Seismological characteristics of volcanic phenomenon ..............................50 i 2.6.3 Conceptual Model........................................................................................53 2.6.4 Application of the Conceptual Model and Event Observations ...................54 III. Seismic Characteristics at Mount St. Helens, Washington.....................................81 3.1 Multiplets during the 1980-1986 eruption at Mount St. Helens ..........................81 3.1.2 Methods.......................................................................................................82 3.1.3 1980-1986 Multiplet Results........................................................................83 3.1.4. Discussion of 1980-1986 Multiplets............................................................84 3.2 Interpretation of Upper Crustal Seismicity..........................................................87 3.3 Automated analysis of 2004 Mount St. Helens eruption ....................................90 3.3.1 Methods.......................................................................................................90 3.3.2 Ratio analysis results...................................................................................92 3.3.2 Ratio analysis discussion ............................................................................93 3.4 Re-analysis of 2004 multiplets ...........................................................................96 3.4.1 Multiplet Methods ........................................................................................96 3.4.2 Multiplet Results ..........................................................................................97 3.4.3 Discussion of Behavior of Multiplets..........................................................101 IV. Comparison of Mount St. Helens, Washington and Bezymianny Volcano, Russia122 4.1 Conduit Structure .............................................................................................122 4.1.1 Mount St. Helens.......................................................................................122 4.1.2 Bezymianny...............................................................................................123 4.1.3 Comparison of Plumbing Systems ............................................................126 4.2 Behavior of Multiplets.......................................................................................127 4.2.1 Multiplet comparison at Bezymianny and Mount St. Helens .....................127 4.2.2 Multiplet comparison to other volcanoes ...................................................131 4.2.3 Generalized Model ....................................................................................136