Journal of Volcanology and Geothermal Research, 2010
The 2004-2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow ... more The 2004-2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow volcano-tectonic earthquakes (VTs) that began on September 23, 2004. We calculated locations and fault-plane solutions (FPS) for shallow VTs recorded during a background period (January 1999 to July 2004) and during the early vent-clearing phase (September 23 to 29, 2004) of the 2004-2008 eruption. FPS show normal and strike-slip faulting during the background period and on September 23; strike-slip and reverse faulting on September 24; and a mixture of strike-slip, reverse, and normal faulting on September 25-29. The orientation of σ1 beneath MSH, as estimated from stress tensor inversions, was found to be sub-horizontal for all periods and oriented NE-SW during the background period, NW-SE on September 24, and NE-SW on September 25-29. We suggest that the ephemeral ~ 90° change in σ1 orientation was due to intrusion and inflation of a NE-SW-oriented dike in the shallow crust prior to the eruption onset.
The reawakening of Mount St. Helens began on September 23, 2004, with a swarm of shallow (<1 km b... more The reawakening of Mount St. Helens began on September 23, 2004, with a swarm of shallow (<1 km below the summit) volcano tectonic (VT) earthquakes. The rate of VT seismicity increased throughout the day, reaching a maximum midday on September 24 before declining early on September 25. The VT seismicity gradually gave way to low frequency and hybrid events through the remainder of the vent-clearing phase, which culminated in a series of short-lived explosions between October 1 and October 5. Systematic changes in the orientation of the deep (4-6 km) stress field as measured by fault-plane solutions were observed at Mount St. Helens during past activity, motivating us to perform a detailed analysis of shallow VT seismicity recorded prior to and during the 2004 eruption. VT earthquakes recorded during a background period between January 1999 and July 2004 and during the period of unrest leading up to the 2004 eruption were repicked and high-quality locations and fault-plane solutions were determined. VT hypocenters for the pre-eruption period form a small cluster centered beneath the 1980-1986 lava dome at depths of 0-1 km beneath the summit, while those for the background period are more scattered beneath the crater and occupy a depth range of 0-9 km. Fault-plane solutions for the background period show primarily normal faulting, which continues through September 23, 2004. Normal faulting ceases on September 24, 2004, when all earthquakes are strike-slip or reverse faulting events, but reappears on September 25, 2004 in addition to continued strike-slip and reverse faulting. The orientation of maximum compressive stress beneath Mount St. Helens was estimated from fault-plane solution p-axis orientations and found to be WNW-ESE on September 24, but p-axis orientations became more heterogeneous by September 25. The results of this analysis suggest that the intrusion of magma into the shallow crust shortly before the initial vent clearing explosion on October 1 significantly affected the shallow stress field orientation at Mount St. Helens. We suggest that the change in the type of faulting from background normal faulting to reverse and strike-slip faulting on September 24 was due to the upward migration of magma breaking through and forcing apart a small seismogenic volume of rock located at roughly 1-2 km beneath the summit. Faulting then transitioned to a mix of normal, reverse, and strike-slip on September 25 as magma moved through the conduit directly below the vent. Additional analysis of fault-plane solutions for shallow background earthquakes recorded since the 1980 eruption of Mount St. Helens may elucidate the nature of the seismogenic volume as well as give a better picture of the shallow stress field during times of quiescence.
Shallow (0-2 km above sea level (ASL)) volcano-tectonic (VT) events have been a persistent featur... more Shallow (0-2 km above sea level (ASL)) volcano-tectonic (VT) events have been a persistent feature of MSH seismicity during quiescent (1986-2004 and 2008-2010) and active (1980-1986 and 2004-2008) periods, albeit at varying rates. In this study we present analyses of shallow MSH VTs occurring between 1990 and 2010, excluding those recorded during the eruption of 2004-2008. During 1990-2010 most VT earthquakes were located within the volcanic edifice at depths of 0-2 km ASL. We repicked and relocated all VTs with magnitudes of MD > -0.9, azimuthal gaps ≤ 135°, closest station < 1 km, and a minimum of seven phase readings, and then determined double-couple fault plane solutions (FPS) for all VTs with at least seven P-wave first motion polarities. From the 702 earthquakes with well-constrained hypocenter locations we were able to calculate 222 FPS. 111 FPS were determined for the time period between 1990 and 1999 and 111 FPS for the time period between 2000 and 2010. FPS show a mixture of normal, reverse, and strike-slip faulting throughout the study period. However, there was a significant increase in the percentage of normal faulting events in 2000-2004 and 2008-2010 (46% normal faulting events) relative to 1990-1999 (25% normal faulting events). FPS P-axis orientations during 1990-1999 have no obvious dominant trend, while strong trends in P-axis orientations for earthquakes from 2000-2004 and 2008-2010 vary between orientations of 320° in 2000 to 250° in 2010, rotating counterclockwise by approximately 7° per year. The increase in normal faulting events, the shift from a random P-axis orientation to an aligned P-axis orientation between the 1990-1999 and 2000-2010 periods, and the systematic rotation of the P-axis orientation between 2000 and 2010 all point to a change in stress conditions at shallow depths starting in 2000 and continuing after the 2004-2008 eruption through to 2010. The timing of the start of this change, ~4 years prior to the 2004 eruption onset, suggests that it could be related to the eventual renewal of eruptive activity. However, the continuation of the stress-field change after the end of the 2004-2008 eruption indicates that it may instead be related to a longer-term systemic change within the edifice due to, for example, changes in the hydrothermal system and/or to loading of the underlying crust by the cooling 1980-86 and 2004-2008 lava domes.
Journal of Volcanology and Geothermal Research, 2010
The 2004-2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow ... more The 2004-2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow volcano-tectonic earthquakes (VTs) that began on September 23, 2004. We calculated locations and fault-plane solutions (FPS) for shallow VTs recorded during a background period (January 1999 to July 2004) and during the early vent-clearing phase (September 23 to 29, 2004) of the 2004-2008 eruption. FPS show normal and strike-slip faulting during the background period and on September 23; strike-slip and reverse faulting on September 24; and a mixture of strike-slip, reverse, and normal faulting on September 25-29. The orientation of σ1 beneath MSH, as estimated from stress tensor inversions, was found to be sub-horizontal for all periods and oriented NE-SW during the background period, NW-SE on September 24, and NE-SW on September 25-29. We suggest that the ephemeral ~ 90° change in σ1 orientation was due to intrusion and inflation of a NE-SW-oriented dike in the shallow crust prior to the eruption onset.
The reawakening of Mount St. Helens began on September 23, 2004, with a swarm of shallow (<1 km b... more The reawakening of Mount St. Helens began on September 23, 2004, with a swarm of shallow (<1 km below the summit) volcano tectonic (VT) earthquakes. The rate of VT seismicity increased throughout the day, reaching a maximum midday on September 24 before declining early on September 25. The VT seismicity gradually gave way to low frequency and hybrid events through the remainder of the vent-clearing phase, which culminated in a series of short-lived explosions between October 1 and October 5. Systematic changes in the orientation of the deep (4-6 km) stress field as measured by fault-plane solutions were observed at Mount St. Helens during past activity, motivating us to perform a detailed analysis of shallow VT seismicity recorded prior to and during the 2004 eruption. VT earthquakes recorded during a background period between January 1999 and July 2004 and during the period of unrest leading up to the 2004 eruption were repicked and high-quality locations and fault-plane solutions were determined. VT hypocenters for the pre-eruption period form a small cluster centered beneath the 1980-1986 lava dome at depths of 0-1 km beneath the summit, while those for the background period are more scattered beneath the crater and occupy a depth range of 0-9 km. Fault-plane solutions for the background period show primarily normal faulting, which continues through September 23, 2004. Normal faulting ceases on September 24, 2004, when all earthquakes are strike-slip or reverse faulting events, but reappears on September 25, 2004 in addition to continued strike-slip and reverse faulting. The orientation of maximum compressive stress beneath Mount St. Helens was estimated from fault-plane solution p-axis orientations and found to be WNW-ESE on September 24, but p-axis orientations became more heterogeneous by September 25. The results of this analysis suggest that the intrusion of magma into the shallow crust shortly before the initial vent clearing explosion on October 1 significantly affected the shallow stress field orientation at Mount St. Helens. We suggest that the change in the type of faulting from background normal faulting to reverse and strike-slip faulting on September 24 was due to the upward migration of magma breaking through and forcing apart a small seismogenic volume of rock located at roughly 1-2 km beneath the summit. Faulting then transitioned to a mix of normal, reverse, and strike-slip on September 25 as magma moved through the conduit directly below the vent. Additional analysis of fault-plane solutions for shallow background earthquakes recorded since the 1980 eruption of Mount St. Helens may elucidate the nature of the seismogenic volume as well as give a better picture of the shallow stress field during times of quiescence.
Shallow (0-2 km above sea level (ASL)) volcano-tectonic (VT) events have been a persistent featur... more Shallow (0-2 km above sea level (ASL)) volcano-tectonic (VT) events have been a persistent feature of MSH seismicity during quiescent (1986-2004 and 2008-2010) and active (1980-1986 and 2004-2008) periods, albeit at varying rates. In this study we present analyses of shallow MSH VTs occurring between 1990 and 2010, excluding those recorded during the eruption of 2004-2008. During 1990-2010 most VT earthquakes were located within the volcanic edifice at depths of 0-2 km ASL. We repicked and relocated all VTs with magnitudes of MD > -0.9, azimuthal gaps ≤ 135°, closest station < 1 km, and a minimum of seven phase readings, and then determined double-couple fault plane solutions (FPS) for all VTs with at least seven P-wave first motion polarities. From the 702 earthquakes with well-constrained hypocenter locations we were able to calculate 222 FPS. 111 FPS were determined for the time period between 1990 and 1999 and 111 FPS for the time period between 2000 and 2010. FPS show a mixture of normal, reverse, and strike-slip faulting throughout the study period. However, there was a significant increase in the percentage of normal faulting events in 2000-2004 and 2008-2010 (46% normal faulting events) relative to 1990-1999 (25% normal faulting events). FPS P-axis orientations during 1990-1999 have no obvious dominant trend, while strong trends in P-axis orientations for earthquakes from 2000-2004 and 2008-2010 vary between orientations of 320° in 2000 to 250° in 2010, rotating counterclockwise by approximately 7° per year. The increase in normal faulting events, the shift from a random P-axis orientation to an aligned P-axis orientation between the 1990-1999 and 2000-2010 periods, and the systematic rotation of the P-axis orientation between 2000 and 2010 all point to a change in stress conditions at shallow depths starting in 2000 and continuing after the 2004-2008 eruption through to 2010. The timing of the start of this change, ~4 years prior to the 2004 eruption onset, suggests that it could be related to the eventual renewal of eruptive activity. However, the continuation of the stress-field change after the end of the 2004-2008 eruption indicates that it may instead be related to a longer-term systemic change within the edifice due to, for example, changes in the hydrothermal system and/or to loading of the underlying crust by the cooling 1980-86 and 2004-2008 lava domes.
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