Anne F. Sheehan

Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track

Analysis of a deployment of broadband sensors along a 500-km-long line crossing the Yellowstone hotspot track (YHT) has provided 423 in-plane receiver functions with which to image lateral variations in mantle discontinuity structure. Imaging is accomplished by performing the converted wave equivalent of a common midpoint stack, which significantly improves resolution of mantle discontinuity structure with respect to single-station stacks. Timing corrections are calculated from locally derived tomographic P and S wave velocity images and applied to the Pds (where d is the depth of the conversion) ray set in order to isolate true discontinuity topography. Using the one-dimensional TNA velocity model and a Vp/Vs ratio of 1.82 to map our Pds times to depth, the average depths of the 410- and 660-km discontinuities are 423 and 664 km, respectively, giving an average transition zone thickness of 241 km. Our most robust observation is provided by comparing the stack of all NW back-azimuth arrivals versus all SE back-azimuth arrivals. This shows that the transition zone thickness varies between 261 and 232 km, between the NW and SE portions of our line. More spatially resolved images show that this transition zone thickness variation results from the occurrence of 20–30 km of topography over 200–300 lateral scale lengths on the 410- and 660-km discontinuities. The topography on the 410- and 660-km discontinuities is not correlated either positively or negatively beneath the 600-km-long transect, albeit correlation could be present for wavelengths larger than the length of our transect. If this discontinuity topography is controlled exclusively by thermal effects, then uncorrelated 250° lateral temperature variations are required at the 410- and 660-km discontinuities. However, other sources of discontinuity topography such as the effects of garnet-pyroxene phase transformations, chemical layering, or variations in mantle hydration may contribute. The most obvious correlation between the discontinuity structure and the track of the Yellowstone hotspot is the downward dip of the 410-km discontinuity from 415 km beneath the NW margin of the YHT to 435 km beneath the easternmost extent of Basin and Range faulting. Assuming this topography is thermally controlled, the warmest mantle resides not beneath the Yellowstone hotspot track, but 150 km to the SE along the easternmost edge of the active Basin and Range faulting.

Imaging the Indian subcontinent beneath the Himalaya

The rocks of the Indian subcontinent are last seen south of the Ganges before they plunge beneath the Himalaya and the Tibetan plateau. They are next glimpsed in seismic reflection profiles deep beneath southern Tibet, yet the surface seen there has been modified by processes within the Himalaya that have consumed parts of the upper Indian crust and converted them into Himalayan rocks. The geometry of the partly dismantled Indian plate as it passes through the Himalayan process zone has hitherto eluded imaging. Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths. North of the Himalaya, the lower Indian crust is characterized by a high-velocity region consistent with the formation of eclogite, a high-density material whose presence affects the dynamics of the Tibetan plateau.

Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions

Variations in crustal thickness from the Great Plains of Kansas, across the Colorado Rocky Mountains, and into the eastern Colorado Plateau are determined by receiver function analysis of broadband teleseismic P waveforms recorded during the 1992 Rocky Mountain Front Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiment. The receiver functions are calculated using a time domain deconvolution approach and are interpreted in terms of a single crustal layer, with thickness determined by a grid-search comparison of observed receiver functions with synthetics. The average crustal thicknesses determined by these methods are Kansas Great Plains, 43.8±0.4 km; Colorado Great Plains, 49.9±1.2 km; Colorado Rocky Mountains, 50.1±1.3 km; and northeast Colorado Plateau, 43.1±0.9 at latitudes of 38°–40°N. The main variations in crustal thickness that we observe are between the Kansas Great Plains and the Colorado Great Plains and between the Rocky Mountains and the Colorado Plateau. There is not a significant crustal thickness difference between the Colorado Great Plains and the Colorado Rocky Mountains. Together with gravity data and mass balance calculations, these results are incompatible with the hypothesis that the compensation of the Rocky Mountains relative to the Great Plains is accommodated purely by an Airy-type crustal root or any other mechanism that restricts compensation solely to the crust and requires significant support for the excess topography of the Rocky Mountains to come from the mantle. Models with a rigid elastic plate may match receiver function estimates of crustal thickness but underpredict the amplitude of the gravity low over the Rockies. Our favored model includes lateral variations in crustal velocities obtained from refraction studies and crustal thickness variations constrained by the receiver functions. These models indicate that there is a profound transition in mantle density structure near the eastern range front.

Mantle discontinuity structure beneath the Colorado Rocky Mountains and High Plains

Analysis of mantle discontinuity structure using converted P to S (PaS) phases beneath Colorado from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Rocky Mountain Front (RMF) experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations. A stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km, respectively. Imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the Pds conversion points and then stacking the receiver functions in each bin to form spatial images, analogous to common depth point stacking. Corrections for lateral velocity heterogeneity are calculated using the local S wave tomographic model of Lee and Grand [1996] and a constant ∂lnVs/∂lnVP scaling of 1.3. This scaling value is determined from the relative scaling between teleseismic P and S wave travel time residuals measured from the Rocky Mountain Front deployment. Mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography, 30 km of 660 km discontinuity topography, and up to 40 km of transition zone thickness variation. Features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western Colorado and a gradual 10 km dip of the 410 km discontinuity beneath the High Plains. The thickening of the transition zone beneath southwest Colorado is consistent with the presence of the subducted Farallon slab in this region as imaged by Van der Lee and Nolet [1997]. In general, our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies.

Coping with earthquakes induced by fluid injection

Hazard may be reduced by managing injection activities Large areas of the United States long considered geologically stable with little or no detected seismicity have recently become seismically active. The increase in earthquake activity began in the mid-continent starting in 2001 (1) and has continued to rise. In 2014, the rate of occurrence of earthquakes with magnitudes (M) of 3 and greater in Oklahoma exceeded that in California (see the figure). This elevated activity includes larger earthquakes, several with M > 5, that have caused significant damage (2, 3). To a large extent, the increasing rate of earthquakes in the mid-continent is due to fluid-injection activities used in modern energy production (1, 4, 5). We explore potential avenues for mitigating effects of induced seismicity. Although the United States is our focus here, Canada, China, the UK, and others confront similar problems associated with oil and gas production, whereas quakes induced by geothermal activities affect Switzerland, Germany, and others.

Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States

Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau-Great Basin Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) portable broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recordings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Basin is marked by moderate shear wave splitting (1.0 s) and unusually late teleseismic phase arrivals. This suggests material with a higher content of mantle melt or volatiles than regions to either side. Splitting in the transition between the Colorado Plateau and Great Basin is part of a pattern of fast polarizations that align in a semicircle, surrounding a central Great Basin region of null (no splitting) measurements. Away from the California plate boundary, splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the North American plate. No simple spatial relation of splitting with geological and geophysical features such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitting in the Great Basin is compatible with asthenospheric flow. The smallest shear wave splitting delay times coincide with the Eureka Low in heat flow, also having low S velocity at 300 km depth. We suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the Great Basin is caused by mantle flow, by the interaction of mantle up welling and the absolute motion of the North American plate.

Distributed deformation in the lower crust and upper mantle beneath a continental strike-slip fault zone: Marlborough fault system, South Island, New Zealand

Converted phases from teleseisms recorded by a seismic array spanning the northern half of the Marlborough fault system, South Island, New Zealand, show a continuous unbroken Moho underlying a seismically anisotropic lower crust beneath the two north- ernmost faults of the fault system. These observations suggest that distributed deforma- tion, not slip on a narrow vertical fault, accommodates displacement in the lower crust below the 120-480 km of right-lateral slip across the Wairau fault, one splay of the Marl- borough fault system, and the northward continuation of the Alpine fault. Beneath the Wairau fault, the Moho dips 258-308 southeast from a depth of ;26 km northwest of the fault to a depth of ;34 km southeast of the fault. Farther to the southeast, Ps conversions from the Moho continue under the Awatere fault (34 6 10 km of slip) with a constant amplitude and depth of ;34 km. Across the two faults, converted energy from 16-20 km depth varies with back-azimuth in a manner suggesting the presence of anisotropy in the lower crust. These observations imply that one of the tenets of plate tectonics, that faults defining plate boundaries pass through both crust and upper mantle, does not apply to New Zealand, or to continents in general.

Seismic migration processing of P-SV converted phases for mantle discontinuity structure beneath the Snake River Plain, western United States

We experiment with backprojection migration processing of teleseismic receiver functions from the Snake River Plain (SRP) broadband seismic experiment. Previous analyses of data from this experiment have used a common midpoint (CMP) stacking approach, a method widely applied for analysis of P-SV converted phases (receiver functions) to obtain high-resolution imaging of upper mantle discontinuities. The CMP technique assumes that all P-SV conversions are produced by flat-lying structures and may not properly image dipping, curved, or laterally discontinuous interfaces. In this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions. We first perform synthetic experiments that illustrate the potential improvement of migration processing over CMP stacks. Application of the migration processing to the SRP data set shows most of the major features as in the original CMP work, but with a weaker 410-km discontinuity and a more intermittent discontinuity at 250 km apparent depth. Random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images. These tests show that a 20-km elevation of the 660-km discontinuity directly beneath the Snake River Plain is robust, but that the variations in 410-km discontinuity topography that we observe are not stable upon resampling. “Bright spots” near 250 km apparent depth are robust upon resampling, but interpretation of these features is complicated by possible sidelobe artifacts from topside Moho reverberations.

Slow slip near the trench at the Hikurangi subduction zone, New Zealand

Applying pressure to plate tectonics The full range of deformation behavior of subduction zone faults that are responsible for great earthquakes and tsunamis is now clearer. Wallace et al. observed the heave of the ocean floor near the Hikurangi trench, off the east coast of New Zealand, with a network of absolute pressure gauges (see the Perspective by Tréhu). The gauges sit on the ocean floor and detect changes in pressure generated from slow-slip deformation events. Detailed geodetic observation of deformation events will finally clarify the role that such aseismic events play at major plate boundaries. Science, this issue p. 701; see also p. 654 Absolute pressure gauges detect a slow-slip event near the Hikurangi trench. The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world’s largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist.

Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long‐wavelength variations in upper mantle temperature and composition along the Mid‐Atlantic Ridge

We report measurements of SS-S differential travel time residuals for nearly 500 paths crossing the northern Mid-Atlantic Ridge. Differential travel times of such phases as SS and S with identical source and receiver have the advantage that residuals are likely to be dominated by contributions from the upper mantle near the surface bounce point of the reflected phase (SS). Under this assumption, differential SS-S travel time residuals are mapped at the SS bounce points as a means of delineating lateral variations in mantle structure. After removing the signature of lithosphere age, we find evidence for variations in SS-S residuals along the ridge at wavelengths of 1000–7000 km. These travel time anomalies correlate qualitatively with along-axis variations in bathymetry and geoid height. We formulate a joint inversion of travel time residual, geoid height, and bathymetry under the assumption that all arise from variations in upper mantle temperature or bulk composition (parameterized in terms of Mg #). The inversion employs geoid and topography kernels which depend on the mantle viscosity structure. Inversion for thermal perturbations alone provides good fits to travel time and geoid data. The fit to topography, which is likely dominated by unmodeled crustal thickness variations, is not as good. The inversions for temperature favor the presence of a thin low-viscosity layer in the upper mantle and temperature perturbations concentrated at depths less than 300 km. Compositional variations alone are unable to match the travel time and geoid or bathymetry data simultaneously. A joint inversion for temperature and composition provides good fits to both geoid and travel time anomalies. Temperature variations are ±50 K and compositional variations are ±0.5–3% Mg # for models with the temperature variations uniformly distributed over the uppermost 300 km and the compositional variations either distributed uniformly over the same interval or concentrated at shallower depths. The magnitudes of these variations are consistent with the chemistry and geothermometry of dredged peridotites along the Mid-Atlantic Ridge.