A comparison of approaches to the prediction of surface wave amplitude

A controlled experiment is performed to investigate how assumptions and simplifications in the measurement and analysis of surface wave amplitudes affect inferred attenuation variations in the mantle. Synthetic seismograms are generated using a spectral-element method for 42 earthquakes, 134 receiver locations and two earth models, both of which contain 3-D elastic properties and 1-D anelastic properties. Fundamental-mode Rayleigh-wave amplitudes are measured at periods of 50, 75 and 125 s for 4749 paths. The amplitudes are measured with respect to a reference waveform based on 1-D Earth structure, and thus amplitude observations that are not equal to unity can be attributed to differences in the computation of the spectral-element and reference waveforms or to uncertainties in the amplitude measurements themselves. Calculation of earthquake source excitation in the 3-D earth model versus the 1-D earth model has a significant effect on the amplitudes, especially at shorter periods, and variations in the average amplitude for each event are well explained by the effect of Earth structure at the event location on the source excitation. The effect of local Earth structure at the receiver location on the amplitude is, for most paths, much smaller than for the source amplitude. After correcting for source and receiver effects on amplitude, the remaining signal is compared to predictions of elastic focusing effects using the great-circle ray approximation, exact ray theory (ERT) and finite-frequency theory (FFT). We find that, for the earth models we have tested, ERT provides the best fit at 50 s, and FFT is most successful at 75 and 125 s, indicating that the broad zone of surface wave sensitivity cannot be neglected for the longer periods in our experiment. The bias introduced into attenuation models by focusing effects, which is assessed by inverting the measured amplitudes for 2-D attenuation maps, is most important at high spherical-harmonic degrees. Unaccounted-for scattering of seismic energy may slightly (<5 per cent) raise average global attenuation values at short periods but has no detectable effect at longer periods. The findings of this study also provide a set of guidelines for handling source, receiver and focusing effects that can be applied to surface wave amplitudes measured for the real Earth

The mean composition of ocean ridge basalts

The mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major ele- ments, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ALL MORB” is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D-MORB the depleted end-member. ALL MORB and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petro- genesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ` 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of ALL MORB and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.Earth and Planetary Science

Global seismological shear velocity and attenuation: a comparison with experimental observations, Earth planet

We present a comparison of seismologically observed shear velocity and attenuation on a global scale. These observations are also compared with laboratory measurements of the same quantities made on fine-grained olivine and extrapolated to upper-mantle conditions. The analysis is motivated by recent developments in global attenuation tomography and in laboratory measurements of velocity and attenuation at seismic frequencies and upper-mantle temperatures. The new attenuation model QRFSI12 is found to be strongly anti-correlated with global velocity models throughout the upper mantle, and individual tectonic regions are each characterized by a distinct range of attenuation and velocity values in the shallow upper mantle. Overall, lateral temperature variations can explain much of the observed variability in velocity and attenuation. The seismological velocity-attenuation relationship for oceanic regions agrees with the experimental observations at depths N 100 km and indicates lateral temperature variations of 150°-200°C at 150 and 200 km beneath the seafloor. The seismic properties of cratonic regions deviate from the experimental trends at depths b 250 km, suggesting differences between oceanic and cratonic composition or water content at these depths. Globally, seismic properties shift into better agreement with the mineral-physics data at depths of 125 km and~225 km beneath oceans and cratons, respectively, which may indicate the base of a compositional boundary layer

An adaptive Bayesian inversion for upper-mantle structure using surface waves and scattered body waves

Summary: We present a methodology for 1-D imaging of upper-mantle structure using a Bayesian approach that incorporates a novel combination of seismic data types and an adaptive parametrization based on piecewise discontinuous splines. Our inversion algorithm lays the groundwork for improved seismic velocity models of the lithosphere and asthenosphere by harnessing the recent expansion of large seismic arrays and computational power alongside sophisticated data analysis. Careful processing of P- and S-wave arrivals isolates converted phases generated at velocity gradients between the mid-crust and 300 km depth. This data is allied with ambient noise and earthquake Rayleigh wave phase velocities to obtain detailed V &nbsp;S and V &nbsp;P velocity models. Synthetic tests demonstrate that converted phases are necessary to accurately constrain velocity gradients, and S–p phases are particularly important for resolving mantle structure, while surface waves are necessary for capturing absolute velocities. We apply the method to several stations in the northwest and north-central United States, finding that the imaged structure improves upon existing models by sharpening the vertical resolution of absolute velocity profiles, offering robust uncertainty estimates, and revealing mid-lithospheric velocity gradients indicative of thermochemical cratonic layering. This flexible method holds promise for increasingly detailed understanding of the upper mantle

A comparison of oceanic and continental mantle lithosphere

Over the last decade, seismological studies have shed new light on the properties of the mantle lithosphere and their physical and chemical origins. This paper synthesizes recent work to draw comparisons between oceanic and continental lithosphere, with a particular focus on isotropic velocity structure and its implications for mantle temperature and partial melt. In the oceans, many observations of scattered and reflected body waves indicate velocity contrasts whose depths follow an age-dependent trend. New modeling of fundamental mode Rayleigh waves from the Pacific ocean indicates that cooling plate models with asymptotic plate thicknesses of 85-95 km provide the best overall fits to phase velocities at periods of 25 s to 250 s. These thermal models are broadly consistent with the depths of scattered and reflected body wave observations, and with oceanic heat flow data. However, the lithosphere-asthenosphere velocity gradients for 85-95 km asymptotic plate thicknesses are too gradual to generate observable Sp phases, both at ages less than 30 Ma and at ages of 80 Ma or more. To jointly explain Rayleigh wave, scattered and reflected body waves and heat flow data, we propose that oceanic lithosphere can be characterized as a thermal boundary layer with an asymptotic thickness of 85-95 km, but that this layer contains other features, such as zones of partial melt from hydrated or carbonated asthenosphere, that enhance the lithosphere-asthenosphere velocity gradient. Beneath young continental lithosphere, surface wave constraints on lithospheric thickness are also compatible with the depths of lithosphere-asthenosphere velocity gradients implied by converted and scattered body waves. However, typical steady-state conductive models consistent with continental heat flow produce thermal and velocity gradients that are too gradual in depth to produce observed converted and scattered body waves. Unless lithospheric isotherms are concentrated in depth by mantle upwelling or convective removal, the presence of an additional factor, such as partial melt at the base of the thermal lithosphere, is needed to sharpen lithosphere-asthenosphere velocity gradients in many young continental regions. Beneath cratons, numerous body wave conversions and reflections are observed within the thick mantle lithosphere, but the velocity layering they imply appears to be laterally discontinuous. The nature of cratonic lithosphere-asthenosphere velocity gradients remains uncertain, with some studies indicating gradual transitions that are consistent with steady-state thermal models, and other studies inferring more vertically localized velocity gradients

Table_1_Rates and pathways of iodine speciation transformations at the Bermuda Atlantic Time Series.xlsx

The distribution of iodine in the surface ocean – of which iodide-iodine is a large destructor of tropospheric ozone (O3) – can be attributed to both in situ (i.e., biological) and ex situ (i.e., mixing) drivers. Currently, uncertainty regarding the rates and mechanisms of iodide (I-) oxidation render it difficult to distinguish the importance of in situ reactions vs ex situ mixing in driving iodine’s distribution, thus leading to uncertainty in climatological ozone atmospheric models. It has been hypothesized that reactive oxygen species (ROS), such as superoxide (O2•−) or hydrogen peroxide (H2O2), may be needed for I- oxidation to occur at the sea surface, but this has yet to be demonstrated in natural marine waters. To test the role of ROS in iodine redox transformations, shipboard isotope tracer incubations were conducted as part of the Bermuda Atlantic Time Series (BATS) in the Sargasso Sea in September of 2018. Incubation trials evaluated the effects of ROS (O2•−, H2O2) on iodine redox transformations over time and at euphotic and sub-photic depths. Rates of I- oxidation were assessed using a 129I- tracer (t1/2 ~15.7 Myr) added to all incubations, and 129I/127I ratios of individual iodine species (I-, IO3-). Our results show a lack of I- oxidation to IO3- within the resolution of our tracer approach – i.e., <2.99 nM/day, or <1091.4 nM/yr. In addition, we present new ROS data from BATS and compare our iodine speciation profiles to that from two previous studies conducted at BATS, which demonstrate long-term iodine stability. These results indicate that ex situ processes, such as vertical mixing, may play an important role in broader iodine species’ distribution in this and similar regions.</p

DataSheet_1_Rates and pathways of iodine speciation transformations at the Bermuda Atlantic Time Series.pdf

The distribution of iodine in the surface ocean – of which iodide-iodine is a large destructor of tropospheric ozone (O3) – can be attributed to both in situ (i.e., biological) and ex situ (i.e., mixing) drivers. Currently, uncertainty regarding the rates and mechanisms of iodide (I-) oxidation render it difficult to distinguish the importance of in situ reactions vs ex situ mixing in driving iodine’s distribution, thus leading to uncertainty in climatological ozone atmospheric models. It has been hypothesized that reactive oxygen species (ROS), such as superoxide (O2•−) or hydrogen peroxide (H2O2), may be needed for I- oxidation to occur at the sea surface, but this has yet to be demonstrated in natural marine waters. To test the role of ROS in iodine redox transformations, shipboard isotope tracer incubations were conducted as part of the Bermuda Atlantic Time Series (BATS) in the Sargasso Sea in September of 2018. Incubation trials evaluated the effects of ROS (O2•−, H2O2) on iodine redox transformations over time and at euphotic and sub-photic depths. Rates of I- oxidation were assessed using a 129I- tracer (t1/2 ~15.7 Myr) added to all incubations, and 129I/127I ratios of individual iodine species (I-, IO3-). Our results show a lack of I- oxidation to IO3- within the resolution of our tracer approach – i.e., <2.99 nM/day, or <1091.4 nM/yr. In addition, we present new ROS data from BATS and compare our iodine speciation profiles to that from two previous studies conducted at BATS, which demonstrate long-term iodine stability. These results indicate that ex situ processes, such as vertical mixing, may play an important role in broader iodine species’ distribution in this and similar regions.</p

Constraints on shear velocity in the cratonic upper mantle from Rayleigh wave phase velocity

Seismic models provide constraints on the thermal and chemical properties of the cratonic upper mantle. Depth profiles of shear velocity from global and regional studies contain positive velocity gradients in the uppermost mantle and often lack a low‐velocity zone, features that are difficult to reconcile with the temperature structures inferred from surface heat flow data and mantle‐xenolith thermobarometry. Furthermore, the magnitude and shape of the velocity profiles vary between different studies, impacting the inferences drawn about mantle temperature and composition. In this study, forward modeling is used to identify the suite of one‐dimensional shear‐velocity profiles that are consistent with phase‐velocity observations made for Rayleigh waves traversing Precambrian cratons. Two approaches to the generation of 1‐D models are considered. First, depth profiles of shear velocity are predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the lithosphere. Second, shear velocity‐depth profiles are randomly generated. In both cases, Rayleigh wave phase velocity is calculated from the Earth models, and acceptable models are identified on the basis of comparison to observed phase velocity. The results show that it is difficult but not impossible to find acceptable Earth models that contain a low‐velocity zone in the upper mantle and that temperature structures that are consistent with constraints from mantle xenoliths yield phase‐velocity predictions lower than observed. For most acceptable randomly generated Earth models, shear velocity merges with the global average at approximately 300 km.Key Points:Low‐velocity zones produce dispersion curves with different shape than observed phase velocityHigh‐velocity lid constrained to 200 km satisfies observations; without constraint, lid is 300 kmFor cratonic peridotite, Rayleigh waves require colder temperatures than xenolith thermobarometryPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134092/1/ggge20883.pd