S. M. Hansen
University of Wyoming
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Featured researches published by S. M. Hansen.
Lithosphere | 2012
Karl E. Karlstrom; David Coblentz; Kenneth G. Dueker; W. Ouimet; Eric Kirby; J. W. van Wijk; Brandon Schmandt; Shari A. Kelley; Greg Lazear; Laura J. Crossey; Ryan S. Crow; Andres Aslan; Andy Darling; Richard C. Aster; J. K. MacCarthy; S. M. Hansen; Josh Stachnik; Daniel F. Stockli; R.V. Garcia; M. Hoffman; R. McKeon; J. Feldman; Matthew T. Heizler; Magdalena S. Donahue
The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.
Bulletin of the Seismological Society of America | 2009
S. M. Hansen; Kenneth G. Dueker
We extend the receiver function deconvolution methodology of Bostock (2004) to S-wave receiver functions and develop a method of source function spec- trum estimation to constrain the crustal structure across the Archean-Proterozoic Cheyenne belt suture in southeast Wyoming using data from a dense deployment of seismic stations. S-wave receiver functions are particularly useful because free- surface reverberations do not contaminated the direct Sdp arrivals, and the S-wave receiver function image is able to validate our P-wave receiver function image. P- and S-wave receiver function images and a teleseismic P-wave tomogram find a structure consistent with the imbrication of Proterozoic lower crust across the Chey- enne belt. Both P and S-wave receiver function images delineate a double Moho north of the Cheyenne belt: the Archean Moho is imaged at 41-43 km depth with a deeper velocity step at 60-62 km depth. South of the Cheyenne belt, the P-wave receiver function image finds the Proterozoic Moho dipping ∼7° northwest consistent with observed back-azimuth dependent Pms amplitudes. Given the lateral continuity with the northwest dipping Proterozoic Moho, the deeper velocity step of the double Moho is interpreted as the imbricated Proterozoic Moho. Modeling of P-wave re- ceiver function amplitudes suggests a 6:4-7:4 km=sec velocity step across the shal- lower Archean Moho and a 7:4-7:9 km=sec velocity step across the deeper imbricated Proterozoic Moho. We speculate that imbrication of the Proterozoic lower crust was contemporaneous with the 1.76 Ga uplift and deformation of the 50 km-wide Palmer Canyon block immediately north of the Cheyenne belt exposed in the Laramie Mountains.
Geochemistry Geophysics Geosystems | 2011
Brandon Schmandt; Kenneth G. Dueker; S. M. Hansen; John J. Jasbinsek; Zhongfei Zhang
Teleseismic receiver function analysis of data from six dense arrays in the western U.S. is used to investigate mantle transition zone (MTZ) discontinuities and the prevalence of a low-velocity layer atop the 410 km discontinuity (410-LVL). Negative polarity Ps arrivals indicative of a low-velocity layer with a top 25–60 km above the 410 are identified in 8–11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors. The 410-LVL is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410. The 669 km mean depth of the 660 km discontinuity (660) and the magnitude of 660 topography suggest variable hydration, locally approaching saturation, in addition to >150 K lateral temperature variations beneath five arrays. Mean amplitudes of P410s and P660s increase monotonically with period from 2 to 10 s; however, greater variations are observed in the frequency dependence of P410s compared to P660s implying 410 thickness is more heterogeneous. Variable 410 thickness is attributed to changes in hydration modulating the width of the olivine-to-wadsleyite transition interval. Frequency dependence of P660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems. Sporadic detection of the 410-LVL, the magnitude and length scales of MTZ discontinuity topography, and inferred variations in hydration support the occurrence of vigorous small-scale convection in the western U.S. mantle. Comparison of receiver functions with body wave tomography suggests small-scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410-LVL.
Geology | 2016
Eric Kiser; Imma Palomeras; Alan R. Levander; C. A. Zelt; Steven H. Harder; Brandon Schmandt; S. M. Hansen; Kenneth C. Creager; Carl Ulberg
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.
Geophysical Research Letters | 2015
S. M. Hansen; Brandon Schmandt
In the summer of 2014 a dense array of 904 geophones was deployed at Mount St. Helens along the road and trail system within 15 km distance of the summit crater. The array recorded continuous data for approximately 2 weeks and presents an unprecedented seismic observation of an active volcano. A reverse-time imaging method is applied to short-term-average over long-term-average time series data to automatically detect and locate microseismicity. These efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the Pacific Northwest Seismic Network. Earthquake locations resolve a narrow, ≤1 km wide, vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater. This feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface.
Nature Communications | 2016
S. M. Hansen; Brandon Schmandt; Alan R. Levander; Eric Kiser; J. E. Vidale; Geoffrey A. Abers; Kenneth C. Creager
Mount St Helens is the most active volcano within the Cascade arc; however, its location is unusual because it lies 50 km west of the main axis of arc volcanism. Subduction zone thermal models indicate that the down-going slab is decoupled from the overriding mantle wedge beneath the forearc, resulting in a cold mantle wedge that is unlikely to generate melt. Consequently, the forearc location of Mount St Helens raises questions regarding the extent of the cold mantle wedge and the source region of melts that are responsible for volcanism. Here using, high-resolution active-source seismic data, we show that Mount St Helens sits atop a sharp lateral boundary in Moho reflectivity. Weak-to-absent PmP reflections to the west are attributed to serpentinite in the mantle-wedge, which requires a cold hydrated mantle wedge beneath Mount St Helens (<∼700 °C). These results suggest that the melt source region lies east towards Mount Adams.
Geology | 2017
Brandon Schmandt; D. Gaeuman; R. Stewart; S. M. Hansen; Victor C. Tsai; J. Smith
Measurements and mechanical models of heterogeneous bedload transport in rivers remain basic challenges for studies of landscape evolution and watershed management. A 700 m reach of the Trinity River (northern California, USA), a large gravel-bed river, was instrumented with an array of 76 seismographs during a dam-controlled flood and gravel augmentation to investigate the potential for out-of-stream monitoring. The temporal response to gravel augmentation during constant discharge provides strong evidence of seismic sensitivity to bedload transport and aids in identification of the seismic frequencies most sensitive to bedload in the study area. Following gravel augmentations, the seismic array reveals a period of enhanced transport that spans most or all of the reach for ∼7–10 h. Neither the duration nor the downstream extent of enhanced transport would have been constrained without the seismic array. Sensitivity to along-stream transport variations is further demonstrated by seismic amplitudes that decrease between the upper and lower halves of the reach consistent with decreased bedload flux constrained by time-lapse bathymetry. Insight into the magnitude of impact energy that reaches the bed is also gained from the seismic array. Observed peak seismic power is ∼1%–5% of that predicted by a model of saltation over exposed bedrock. Our results suggest that dissipation of impact energy due to cover effects needs to be considered to seismically constrain bedload transport rates, and that noninvasive constraints from seismology can be used to test and refine mechanical models of bedload transport.
Geochemistry Geophysics Geosystems | 2017
S. M. Hansen; Brandon Schmandt
A method for scattered wave imaging in 3D with both teleseismic P- and S-wave receiver function data is introduced. The approach relies on body-wave scattering kernels that are derived from the adjoint data sensitivity kernels which are typically used for full waveform tomography. The forward problem is approximated using ray theory, yielding a computationally efficient imaging algorithm that can resolve dipping and discontinuous interfaces using both P- and S-wave receiver functions. Travel-time fields for the incident teleseismic arrivals and the receiver point-sources are obtained by solving the Eikonal equation using a Fast Marching code which can handle a 3D reference velocity model. An energy stable finite-difference method is used to simulate elastic wave propagation in a 2D hypothetical subduction zone model. The resulting synthetic P- and S-wave receiver function datasets are used to validate the imaging method. The kernel images are compared with those generated by the Generalized Radon Transform and Common Conversion Point Stacking methods. These results demonstrate the potential of the kernel imaging approach for constraining lithospheric structure in complex geologic environments with sufficiently dense recordings of teleseismic data. Potential imaging targets include short-wavelength compositional variations in the mantle wedge and the slabs lithosphere-asthenosphere boundary.
Earth and Planetary Science Letters | 2012
Brandon Schmandt; Kenneth G. Dueker; Eugene D. Humphreys; S. M. Hansen
Geochemistry Geophysics Geosystems | 2013
S. M. Hansen; Kenneth G. Dueker; Joshua C. Stachnik; Richard C. Aster; Karl E. Karlstrom