Estimation of the effective elastic thickness of the lithosphere using inverse spectral methods: the state of the art

The effective elastic thickness (Te) is a geometric measure of the flexural rigidity of the lithosphere, which describes the resistance to bending under the application of applied, vertical loads. As such, it is likely that its magnitude has a major role in governing the tectonic evolution of both continental and oceanic plates. Of the several ways to estimate Te, one has gained popularity in the 40 years since its development because it only requires gravity and topography data, both of which are now readily available and provide excellent coverage over the Earth and even the rocky planets and moons of the solar system. This method, the ‘inverse spectral method’, develops measures of the relationship between observed gravity and topography data in the spatial frequency (wavenumber) domain, namely the admittance and coherence. The observed measures are subsequently inverted against the predictions of thin, elastic plate models, giving estimates of Te and other lithospheric parameters. This article provides a review of inverse spectral methodology and the studies that have used it. It is not, however, concerned with the geological or geodynamic significance or interpretation of Te, nor does it discuss and compare Te results from different methods in different provinces. Since the three main aspects of the subject are thin elastic plate flexure, spectral analysis, and inversion methods, the article broadly follows developments in these. The review also covers synthetic plate modelling, and concludes with a summary of the controversy currently surrounding inverse spectral methods, whether or not the large Te values returned in cratonic regions are artefacts of the method, or genuine observations

On the pitfalls of Airy isostasy and the isostatic gravity anomaly in general

Isostatic gravity anomalies provide a measure of the Earth's gravity field free from the gravitational attractions of the topography and its isostatic compensation, most commonly represented by a variation in the depth of a compensating density contrast, for example the Moho. They are used by both geodesists and geophysicists alike, though often for different purposes. Unfortunately though, the effect of subsurface loading on the lithosphere renders transfer function (admittance) methods unusable when surface and subsurface loads coexist. Where they exist, subsurface loads are often expressed in the Bouguer anomaly but not in the topography, and it is shown here that this phase disconnect cannot be faithfully represented by either realor complex-valued analytic admittance functions. Additionally, many studies that employ the isostatic anomaly ignore the effects of the flexural rigidity of the lithosphere, most often represented as an effective elastic thickness (Te), and assume only Airy isostasy, i.e. surface loading of a plate with zero elastic thickness. The consequences of such an omission are studied here, finding that failure to account for flexural rigidity and subsurface loading can result in (1) over- or underestimates of both inverted Moho depths and dynamic topography amplitude, and (2) underestimates of the size of topographic load that can be supported by the plate without flexure. An example of the latter is shown over Europe. Finally, it is demonstrated how low values of the isostatic anomaly variance can actually be biased by these anomalies having low power at the long wavelengths while still possessing high power at middle to short wavelengths, compared to the corresponding Bouguer anomaly power spectrum. This will influence the choice of best-fitting isostatic model if the model is chosen by minimization of the isostatic anomaly standard deviation

A Comparison of Techniques for the Integration of Satellite Altimeter and Surface Gravity Data for Geoid Determination

Two methods are tested whereby satellite altimeter measurements of the geoid height are combined with surface measurements of the free-air gravity anomaly. The study area comprises the oceans around the Australian continent. The first method involves draping a grid of the free-air anomaly from satellite data onto a grid of the ship and land data. The second method utilises grids of the altimeter-derived geoid height, combining these with the surface data in an iterative superposition. Preliminary results show that the draping method yields a fit of 5.4 mGal between the satellite and marine data, while the iterative procedure returns 8.1 mGal. Further work can be done, however, to improve these results. The impact of the combined marine gravity data sets is illustrated by comparing the effects on an Australia-wide spherical-FFT geoid solution

Mapping the Mechanical Anisotropy of the Lithosphere using a 2D Wavelet Coherence, and its Applicaton to Australia

We develop a new method for imaging the spatial variations of the anisotropy of the flexural response of the lithosphere, and apply it to recent topographic and gravity data sets over Australia. The method uses two-dimensional Morlet wavelet transforms, superposed in a strictly controlled geometry, to estimate the auto- and cross-spectra of the two data sets in a number of different directions. The resulting wavelet coherence is a function of scale, or wavelength, as well as orientation, and is inverted, at each spatial location, for the three parameters of an anisotropic, thin elastic plate model, i.e., maximum and minimum flexural rigidities and the orientation of the maximum. Extensive tests of the method on synthetic anisotropic, but uniform, data sets, show that it retrieves the amplitude and orientation of the anisotropy with useful accuracy. The results for Australia west of 143oE show a strong correlation with the shallower layers (75-175 km) of a recent model of seismic SV wave azimuthal anisotropy. The 'weak' axes (i.e., of minimum flexural rigidity) in most cases are approximately at right angles to the fast axes of the seismic anisotropy, implying that, for Precambrian Australia, they arise from the same source. This is most likely deformation resulting from the most recent episode of orogeny

Cultural evolution leads to vocal iconicity in an experimental iterated learning task

Experimental and cross-linguistic studies have shown that vocal iconicity is prevalent in words that carry meanings related to size and shape. Although these studies demonstrate the importance of vocal iconicity and reveal the cognitive biases underpinning it, there is less work demonstrating how these biases lead to the evolution of a sound symbolic lexicon in the first place. In this study, we show how words can be shaped by cognitive biases through cultural evolution. Using a simple experimental setup resembling the game telephone, we examined how a single word form changed as it was passed from one participant to the next by a process of immediate iterated learning. About 1,500 naïve participants were recruited online and divided into five condition groups. The participants in the control-group received no information about the meaning of the word they were about to hear, while the participants in the remaining four groups were informed that the word meant either big or small (with the meaning being presented in text), or round or pointy (with the meaning being presented as a picture). The first participant in a transmission chain was presented with a phonetically diverse word and asked to repeat it. Thereafter, the recording of the repeated word was played for the next participant in the same chain. The sounds of the audio recordings were then transcribed and categorized according to six binary sound parameters. By modelling the proportion of vowels or consonants for each sound parameter, the small-condition showed increases of front unrounded vowels and the pointy-condition increases of acute consonants. The results show that linguistic transmission is sufficient for vocal iconicity to emerge, which demonstrates the role non-arbitrary associations play in the evolution of language

The effect of dynamic topography and gravity on lithospheric effective elastic thickness estimation: A case study

Lithospheric effective elastic thickness (T e ), a proxy for plIate strength, is helpful for the understanding of subduction characteristics. Affected by curvature, faulting and magma activity, lithospheric strength near trenches should be weakened but some regional inversion studies have shown much higher T e values along some trenches than in their surroundings. In order to improve T e -estimation accuracy, here we discuss the long-wavelength effect of dynamic topography and gravity on T e estimation by taking the Izu-Bonin-Mariana (IBM) Trench as a case study area. We estimate the long-wavelength influence of the density and negative buoyancy of the subducting slab on observed gravity anomalies and seafloor topography. The residual topography and gravity are used to map T e using the fan-wavelet coherence method. Maps of T e , both with and without the effects of dynamic topography and slab gravity anomaly, contain a band of high-T e values along the IBM Trench, though these values and their errors are lower when slab effects are accounted for. Nevertheless, tests show that the T e map is relatively insensitive to the choice of slab-density modelling method, even though the dynamic topography and slab-induced gravity anomaly vary considerably when the slab density is modelled by different methods. The continued presence of a high-T e band along the trench after application of dynamic corrections shows that, before using 2-D inversion methods to estimate T e variations in subduction zones, there are other factors that should be considered besides the slab dynamic effects on the overriding plate

Spatial variations in the effective elastic thickness of the lithosphere in Southeast Asia

As a proxy for long-term lithospheric strength, detailed information on lateral effective elastic thickness (Te) variations can aid in understanding the distribution pattern of surface deformation and its response to long-term forces. Here we present high-resolution maps of spatial variations of Te for the complex SE Asian region by analyzing the coherence of topography and Bouguer gravity anomaly data. We find that after considering the gravity deficit of less dense sediment, the recovered Te maps are more representative of the geology, particularly in elongated rift basins. The results show that the Te variation pattern in SE Asia, in general, agrees well with its tectonic provinces and major tectonic boundaries. The oceanic basins, the Indosinian suture zones between the Indochina and Sibumasu blocks, and the Makassar Strait are characterized by low Te, while moderate and high Te values are recovered in the Khorat plateau, West Burma, the Singapore Ridge, the Con Song Swell, Borneo, the northern Australian margin and the Molucca Sea. The Te pattern in the south Indonesian margin is complicated by the approach and collision of oceanic plateaus and seamounts with the fore-arc region. The heterogeneous strength features are consistent with the complex assemblage of different tectonic units, and significant deformation during Cenozoic tectonic events. In the Indochina Peninsula, the extruded displacement during the India-Eurasia collision might have been partitioned and absorbed by the combined mechanism of the extrusion and viscous tectonic models. As a result, the offshore displacements of the major strike-slip faults in the South China Sea are much smaller than originally assumed, thus having less effect on the development of the South China Sea than other mechanisms such as the slab pull of the proto-South China Sea. Since the displacement driven by the boundary tectonic forces has been greatly absorbed and decreased by subduction and deformation in the active margins and adjacent weak regions, the motion velocity of the interior regions is greatly lower than the boundary active margins, and they are largely free of seismicity and volcanism. Our results suggest that East Borneo might share a similar crustal basement, and represent a broad tectonic zone of the destroyed Meso-Tethys Ocean extending from West-Middle Java, through East Borneo to northern Borneo of the Sarawak and Sabah. The Indosinian zones between the Indochina and Sibumasu blocks might extend further southeastward across Billiton Island to offshore of southern Borneo, and the Singapore platform and SW Borneo might belong to the same block. The results also show that the internal load fraction F is high in the coastal area of South China, the northern margin of the South China Sea, and the coastal area of Indochina, which, in general, agrees with the distribution of a high-velocity lower crustal layer and Late Cenozoic basaltic rocks

Association Between Empathy and Burnout Among Emergency Medicine Physicians

Background: The association between physician self-reported empathy and burnout has been studied in the past with diverse findings. We aimed to determine the association between empathy and burnout among United States emergency medicine (EM) physicians using a novel combination of tools for validation. Methods: This was a prospective single-center observational study. Data were collected from EM physicians. From December 1, 2018 to January 31, 2019, we used the Jefferson scale of empathy (JSE) to assess physician empathy and the Copenhagen burnout inventory (CBI) to assess burnout. We divided EM physicians into different groups (residents in each year of training, junior/senior attendings). Empathy, burnout scores and their association were analyzed and compared among these groups. Results: A total of 33 attending physicians and 35 EM residents participated in this study. Median self-reported empathy scores were 113 (interquartile range (IQR): 105 - 117) in post-graduate year (PGY)-1, 112 (90 - 115) in PGY-2, 106 (93 - 118) in PGY-3 EM residents, 112 (105 - 116) in junior and 114 (101 - 125) in senior attending physicians. Overall burnout scores were 43 (33 - 50) in PGY-1, 51 (29 - 56) in PGY-2, 43 (42 - 53) in PGY-3 EM residents, 33 (24 - 47) in junior attending and 25 (22 - 53) in senior attending physicians separately. The Spearman correlation (ρ) was -0.11 and β-weight was -0.23 between empathy and patient-related burnout scores. Conclusion: Self-reported empathy declines over the course of EM residency training and improves after graduation. Overall high burnout occurs among EM residents and improves after graduation. Our analysis showed a weak negative correlation between self-reported empathy and patient-related burnout among EM physicians