Imaging northern Cascadia wave speed structure and slow slip

Thesis (Ph.D.)--University of Washington, 2018I calculate tremor source amplitudes for the northern Cascadia episodic tremor and slip (ETS) events from 2007-2010 and find they exhibit similar spatiotemporal patterns of radiated energy from tectonic tremor. In the initiation phase of each event, during which tremor starts downdip and moves updip over ~8 days, the tremor area and tremor amplitudes increase quasi-linearly, implying a constant radiated energy rate per unit area and a diffusional process for tremor migration. During this time, tremor amplitudes do not exhibit a strong sensitivity to tidal stress fluctuations. Once the tremor fills the downdip width of the tremoring region, the ETS events begin to propagate to the north and south at a constant rate, with the amplitudes being strongly modulated by tidal stresses. This implies a generally low effective normal stress or low effective friction along the plate interface, and that stress or friction begins higher during the initiation of an ETS event and decreases as the ETS grows to the point where small tidal stress fluctuations can modulate the energy released during slow slip. Using a 2-year deployment of 70 broadband seismometers, and several other seismic data sets, I invert local earthquake travel times to obtain 3-D P- and S-wave velocity models of the Mount St. Helens (MSH) region. Principal features of the 3-D models include: (1) Low P- and S-wave velocities along the St. Helens seismic Zone (SHZ), striking NNW-SSE north of MSH from near the surface to where we lose resolution at 15–20 km depth. This anomaly corresponds to high conductivity as imaged by magnetotelluric studies. The SHZ could represent a zone of crustal weakness with the presence of fluids, fractured rock, and/or sediments from the accretion of the Siletzia terrane; (2) A 4-5% negative P- and S-wave velocity anomaly beneath MSH at depths of 6-15 km with a quasi-cylindrical geometry and a diameter of 5 km, probably indicating a magma storage region. Based on resolution testing of similar-sized features, it is possible that this velocity anomaly is narrower and slower. Assuming approximately 1% partial melt per % velocity variation, this region could contain up to 5-10 km3 of partial melt; (3) A broad, very low P-wave velocity region below 10-km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high-temperature arc crust and the possible presence of melt; (4) Several anomalies associated with surface-mapped features, including high-velocity igneous units such as the Spud Mountain, Spirit Lake, McCoy Creek, Silver Star, and Tatoosh plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field. This dissertation includes two sets of supplementary files: (1) a set of 3-D P- and S-wave velocity models; and (2) a catalog of earthquakes relocated using 3-D velocity models

Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA

[EN]The size, frequency, and intensity of volcanic eruptions are strongly controlled by the volume and connectivity of magma within the crust. Several geophysical and geochemical studies have produced a comprehensive model of the magmatic system to depths near 7 km beneath Mount St. Helens (Washington State, USA), currently the most active volcano in the Cascade Range. Data limitations have precluded imaging below this depth to observe the entire primary shallow magma reservoir, as well as its connection to deeper zones of magma accumulation in the crust. The inversion of P and S wave traveltime data collected during the active-source component of the iMUSH (Imaging Magma Under St. Helens) project reveals a high P-wave (Vp)/S-wave (Vs) velocity anomaly beneath Mount St. Helens between depths of 4 and 13 km, which we interpret as the primary upper–middle crustal magma reservoir. Beneath and southeast of this shallow reservoir, a low Vp velocity column extends from 15 km depth to the Moho. Deep long-period events near the boundary of this column indicate that this anomaly is associated with the injection of magmatic fluids. Southeast of Mount St. Helens, an upper–middle crustal high Vp/Vs body beneath the Indian Heaven Volcanic Field may also have a magmatic origin. Both of these high Vp/Vs bodies are at the boundaries of the low Vp middle–lower crustal column and both are directly above high Vp middle–lower crustal regions that may represent cumulates associated with recent Quaternary or Paleogene–Neogene Cascade magmatism. Seismicity immediately following the 18 May 1980 eruption terminates near the top of the inferred middle–lower crustal cumulates and directly adjacent to the inferred middle–lower crustal magma reservoir. These spatial relationships suggest that the boundaries of these high-density cumulates play an important role in both vertical and lateral transport of magma through the crust

Imaging Subduction Beneath Mount St. Helens: Implications for Slab Dehydration and Magma Transport

Mount St. Helens (MSH) is anomalously 35–50 km trenchward of the main Cascade arc. To elucidate the source of this anomalous forearc volcanism, the teleseismic‐scattered wavefield is used to image beneath MSH with a dense broadband seismic array. Two‐dimensional migration shows the subducting Juan de Fuca crust to at least 80‐km depth, with its surface only 68 ± 2 km deep beneath MSH. Migration and three‐dimensional stacking reveal a clear upper‐plate Moho east of MSH that disappears west of it. This disappearance is a result of both hydration of the mantle wedge and a westward change in overlying crust. Migration images also show that the subducting plate continues without break along strike. Combined with low temperatures inferred for the mantle wedge, this geometry greatly limits possible source regions for mantle melts that contribute to MSH magmas and requires lateral migration over large distances

Imaging Subduction Beneath Mount St. Helens: Implications for Slab Dehydration and Magma Transport

Mount St. Helens (MSH) is anomalously 35–50 km trenchward of the main Cascade arc. To elucidate the source of this anomalous forearc volcanism, the teleseismic‐scattered wavefield is used to image beneath MSH with a dense broadband seismic array. Two‐dimensional migration shows the subducting Juan de Fuca crust to at least 80‐km depth, with its surface only 68 ± 2 km deep beneath MSH. Migration and three‐dimensional stacking reveal a clear upper‐plate Moho east of MSH that disappears west of it. This disappearance is a result of both hydration of the mantle wedge and a westward change in overlying crust. Migration images also show that the subducting plate continues without break along strike. Combined with low temperatures inferred for the mantle wedge, this geometry greatly limits possible source regions for mantle melts that contribute to MSH magmas and requires lateral migration over large distances

Data from: Shear velocity structure from ambient noise and teleseismic surface wave tomography in the Cascades around Mount St. Helens

Mount St. Helens (MSH) lies in the forearc of the Cascades where conditions should be too cold for volcanism. To better understand thermal conditions and magma pathways beneath MSH, data from a dense broadband array are used to produce high-resolution tomographic images of the crust and upper mantle. Rayleigh-wave phase-velocity maps and three-dimensional images of shear velocity (Vs), generated from ambient noise and earthquake surface waves, show that west of MSH the mid-lower crust is anomalously fast (3.95 ± 0.1 km/s), overlying an anomalously slow uppermost mantle (4.0-4.2 km/s). This combination renders the forearc Moho weak to invisible, with crustal velocity variations being a primary cause; fast crust is necessary to explain the absent Moho. Comparison with predicted rock velocities indicates that the fast crust likely consists of gabbros and basalts of the Siletzia terrane, an accreted oceanic plateau. East of MSH where magmatism is abundant, mid-lower crust Vs is low (3.45-3.6 km/s), consistent with hot and potentially partly molten crust of more intermediate to felsic composition. This crust overlies mantle with more typical wavespeeds, producing a strong Moho. The sharp boundary in crust and mantle Vs within a few km of the MSH edifice correlates with a sharp boundary from low heat flow in the forearc to high arc heat flow, and demonstrates that the crustal terrane boundary here couples with thermal structure to focus lateral melt transport from the lower crust westward to arc volcanoes. This dataset supports the research described here.NSF grant EAR-144427

Local Source Vp and Vs Tomography in the Mount St. Helens Region With the iMUSH Broadband Array

We present new 3-D P wave and S wave velocity models of the upper 20 km of the Mount St. Helens (MSH) region. These were obtained using local-source arrival time tomography from earthquakes and explosions recorded at 70 broadband stations deployed as part of the imaging Magma Under St. Helens (iMUSH) project and augmented by several data sets. Principal features of our models include (1) low P wave and S wave velocities along the St. Helens seismic zone to depths of at least 20 km corresponding to high conductivity imaged by iMUSH magnetotelluric studies. This delineates a zone of weakness that magma can exploit at the location of MSH; (2) a 5- to 7-km diameter, 6-15 km deep, 3-6% negative P wave and S wave velocity anomaly beneath MSH, consistent with previous estimates of the source region for recent eruptions. We interpret this as a magma storage region containing up to 15-20 km(3) of partial melt, which is about 5 times more than the largest documented eruption at MSH; (3) a broad region of low P wave velocity below 10-km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high-temperature arc crust and possible presence of fluids or melt; (4) several anomalies associated with surface-mapped features, including high-velocity igneous units such as the Spud Mountain and Spirit Lake plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field. Our results place further constraints on the geometry of these features at depth. Plain Language Summary We deployed 70 seismometers around Mount St. Helens volcano from 2014 to 2016, which measured the surface ground motion from hundreds of small earthquakes, as well as from 23 explosions that were set off in 2014. We recorded the onset time of shaking from these sources and used a specialized computer code to model how quickly seismic waves travel through the subsurface. Seismic wave speed can be influenced by several factors, including rock type, presence of magma/fluids, temperature, pressure, and how fractured the rock is. Based on the seismic wave speeds in our model, we make several geological interpretations, including (1) increased fluids or fractures, or presence of sedimentary rocks corresponding to elevated earthquake activity to the NNW of Mount St. Helens; (2) a magma storage region beneath the volcano similar to results from previous studies. Our model places further constraints on the orientation and size of the region; (3) a large zone of high temperatures and possible fluids or magma related to regional volcanism between and to the east of Mount Adams and Mount Rainier; (4) more detailed size and depth constraints on geological features seen at the surface, including sedimentary basins and rock units related to previous regional volcanism. Key Points New high-resolution P wave and S wave velocity models are calculated for the Mount St. Helens region Velocity models place further constraints on size and location of magma storage regions, seismic zones, sedimentary basins, and plutons These shed light on the accretionary history of the Siletzia terrane, with a transitional upper crustal boundary near Mount St. HelensNational Science Foundation of Sri Lanka6 month embargo; first published online 19 February 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]