Recent studies of the focal depths of earthquakes in old continental lithosphere have shown that they are almost entirely confined to the crust. Except where recent subduction of oceanic lithosphere ...is likely to have occurred, no earthquakes with a magnitude of
>
5.5 have yet been located beneath the Moho. In contrast, in oceanic lithosphere earthquakes commonly occur within the mantle. The principal control on whether or not deformation occurs by brittle failure has long been believed to be temperature. We re-examine the thermal models of both oceans and shields. Taking account of the temperature dependence of the thermal conductivity lowers the temperature within the oceanic lithosphere. Except beneath the outer rises of trenches, where the strain rates are large, intraplate oceanic earthquakes are confined to regions cooler than 600 °C. In continental regions most earthquakes occur in the mobile belts that surround Archaean cratons, where the crust is as thick as 50–60 km. Recent studies, of the Canadian Shield in particular, have shown that radiogenic heating is not as concentrated at shallow depths as was previously believed. Taking account of both these effects and the temperature dependence of the thermal conductivity increases the Moho temperatures, which can exceed 600 °C, and produces geotherms that agree well with pressure and temperature estimates from nodule suites from kimberlites. Therefore the mechanical behaviour of oceanic and continental upper mantle appears to depend on temperature alone, and there is as yet no convincing evidence for any compositional effects.
Sea-level rise resulting from climate change is impacting coasts around the planet. There is strong scientific consensus about the amount of sea-level rise to 2050 (0.24–0.32 m) and a range of ...projections to 2100, which vary depending on the approach used and the mitigation measures taken to reduce carbon emissions. Despite this strong scientific consensus regarding the reality of climate change-related sea-level rise, and the associated need to engage publics in adaptation and mitigation efforts, there is a lack of empirical evidence regarding people’s understanding of the issue. Here we investigate public understanding of the amount, rate and causes of sea-level rise. Data from a representative sample of New Zealand adults showed a suprising tendency for the public to overestimate the scientifically plausible amount of sea-level rise by 2100 and to identify melting sea ice as its primary causal mechanism. These findings will be valuable for scientists communicating about sea-level rise, communicators seeking to engage publics on the issue of sea-level rise, and media reporting on sea-level rise.
European windstorms cause socioeconomic losses due to wind damage. Projections of future losses from such storms are subject to uncertainties from the frequency and tracks of the storms, their ...intensities and definitions thereof, and socio-economic scenarios. We use two storm severity indices applied to objectively identified extratropical cyclone footprints from a multi-model ensemble of state-of-the-art climate models under different future socio-economic scenarios. Here we show storm frequency increases across northern and central Europe, where the meteorological storm severity index more than doubles. The population-weighted storm severity index more than triples, due to projected population increases. Adapting to the increasing wind speeds using future damage thresholds, the population weighted storm severity index increases are only partially offset, despite a reduction in the meteorological storm severity through adaptation. Through following lower emissions scenarios, the future increase in risk is reduced, with the population-weighted storm severity index increase more than halved.
Taking advantage of the Polar Amplification Model Intercomparison Project simulations and using a Lagrangian objective feature tracking algorithm, we determine the response of extratropical cyclones ...to sea‐ice loss and consequent weakening of the equator‐to‐pole near‐surface temperature gradient. The wintertime storm tracks are found to shift equatorward in the North Atlantic and over Europe, and eastward in the North Pacific. In both regions, cyclones become weaker and slower, particularly on the poleward flank of the storm tracks. On average, there are fewer individual cyclones in the extratropics each winter, they last longer, are weaker, and travel more slowly. These changes are greatest over the Arctic, but still statistically significant in midlatitudes despite being small compared to internal variability. Inter‐model spread in cyclone responses are not strongly correlated with that in Arctic warming or Arctic amplification. Little change in summertime cyclones is found.
Plain Language Summary
Using identically performed experiments from eight different climate models, we study how low pressure systems, known as cyclones or storms, may change as Arctic sea ice diminishes. An algorithm is used to identify the weather systems and track them across their lifecycle. In the wintertime, the typical path of storms over the Atlantic Ocean shifts southward toward the equator. Over the Pacific, the storm tracks shifts toward North America. Arctic sea‐ice loss causes there to be fewer storms each winter across the northern midlatitudes, and these tend to be weaker, slower moving, but longer lasting. These changes are largest over the Arctic itself but also found over mid‐latitudes. These changes may be important because the number, location, and character of storms control the winter weather over Europe, North America and Asia. In the summertime, we did not find any detectable impact of Arctic sea‐ice loss on storms.
Key Points
The Northern Hemisphere winter storm tracks shift south and/or east in response to Arctic sea‐ice loss
Sea ice loss leads to fewer, slower, longer lasting, and weaker wintertime cyclones
Responses are small compared to internal variability, but larger in winter than summer
The low shear wave velocities
V
s observed beneath spreading ridges and in the low velocity zone beneath plates are commonly attributed to the presence of melt. But geochemical observations suggest ...that the amount of melt present in those parts of the mantle that are melting is about 0.1%, which is too small to produce a major decrease in
V
s. Furthermore laboratory measurements of
V
s at seismic frequencies, 1–10
−
2
Hz, show that
V
s is more strongly affected by temperature than by the presence of a few percentage of melt. It is, however, not straightforward to use laboratory experiments to relate
V
s to the temperature
T because the grain size of most laboratory experiments is 100–1000 times smaller than that of the mantle. We combine thermal models of the Pacific lithosphere and pressure and temperature estimates from mantle nodules brought up by kimberlites with three dimensional models of
V
s from surface wave tomography to obtain an empirical relation for
V
s(
P,
T), where
P is the pressure. This expression is then used to convert regional variations of
V
s as a function of depth to lithospheric thickness. The accuracy of the resulting maps is tested by comparison with the location of diamond-bearing kimberlites, which are in most places restricted to regions where the lower part of the lithosphere is in the diamond stability field.
We have created a performance-based assessment of CMIP6 models for Europe that can be used to inform the sub-selection of models for this region. Our assessment covers criteria indicative of the ...ability of individual models to capture a range of large-scale processes that are important for the representation of present-day European climate. We use this study to provide examples of how this performance-based assessment may be applied to a multi-model ensemble of CMIP6 models to (a) filter the ensemble for performance against these climatological and processed-based criteria and (b) create a smaller subset of models based on performance that also maintains model diversity and the filtered projection range as far as possible.
Summary
We use teleseismic waveform inversion, along with depth phase analysis, to constrain the centroid depths and source parameters of large African earthquakes. The majority of seismic activity ...is concentrated along the East African Rift System, with additional active regions along stretches of the continental margins in north and east Africa, and in the Congo Basin. We examine variations in the seismogenic thickness across Africa, based on a total of 227 well-determined earthquake depths, 112 of which are new to this study. Seismogenic thickness varies in correspondence with lithospheric thickness, as determined from surface wave tomography, with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km. In regions of thin lithosphere, the seismogenic thickness is typically limited to ≤20 km. Larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of ≥1500 Ma, where the East African Rift passes around the Archean cratons of Africa, through the older Proterozoic mobile belts. These correlations are likely to be related to the production, affected by method and age of basement formation, and preservation, affected by lithospheric thickness, of a strong, anhydrous lower crust. The Congo Basin contains the only compressional earthquakes in the continental interior. Simple modelling of the forces induced by convective support of the African plate, based on long-wavelength free-air gravity anomalies, indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in Africa. Seismicity along the margins of Africa reflects a mixture between oceanic and continental seismogenic characteristics, with earthquakes in places extending to 40 km depth.
SUMMARY
Analysis of data from nine, temporary broadband seismic stations operated across West Bengal and Sikkim, along with publicly available data from seismographs in the surrounding region, ...provides the first image of the descending Indian Plate beneath the Darjeeling–Sikkim Himalaya. The down‐going Indian crust is imaged by receiver function common conversion point stacking using data from 32 sites in combination with more detailed analyses from simultaneous modelling of receiver function data and Rayleigh wave group velocity dispersion at 13 stations. Compared to locations farther south on the Indian Shield our southernmost station shows evidence for thickened crust beneath the Mahanadi Rift basin with a possible mafic basal layer. North of the Mahanadi Rift the Indian Moho is ∼38 km deep below the Archean terranes of the northeast part of the Indian Shield. The Moho then dips gently northward beneath the Himalayan foreland basin reaching a depth of 44–48 km below the Himalayan foothills. Below Sikkim the Moho continues to deepen but there are indications of secondary structures in the receiver function image and modelling results suggesting some imbrication of the crust as it flexes downward. The crust thickens further beneath the Greater Himalaya and southern Tibet reaching depths of ∼65–70 km below the Southern Tibet Detachment (STD). Below the Lhasa Terrane north of the STD a double discontinuity exists with interfaces at 55–60 km and ∼80 km depth. There is a significant reduction in the average shear wave velocity of the crystalline crust between sites to the south of and on the Himalayan foreland basin and sites in the Himalaya and to the north. Below the Himalaya and southern Tibet the P‐to‐S conversion (Ps) has a lower amplitude compared to that observed at sites on the undeformed Indian Shield. This decrease in amplitude of the Moho Ps phase could arise from a lower impedance contrast across the crust–mantle boundary or from scattering due to deformation of the crust and Moho. A coherent negative arrival beneath the Darjeeling–Sikkim Himalaya indicate the presence of a low velocity zone (LVZ), possibly associated with the Main Himalayan Thrust. This LVZ can be traced beneath the Darjeeling–Sikkim Himalaya but disappears beneath the Greater Himalaya.
Summary
This paper examines the relationship between seismogenic thickness, lithosphere structure and rheology in central and northeastern Asia. We accurately determine earthquake depth distributions ...which reveal important rheological variations in the lower crust. These variations exert a fundamental control on the active tectonics and the morphological evolution of the continents. We consider 323 earthquakes across the Tibetan Plateau, the Tien Shan and their forelands as well as the Baikal Rift, NE Siberia and the Laptev Sea and present the source parameters of 94 of these here for the first time. These parameters have been determined through body wave inversion, the identification of depth phases or the modelling of regional waveforms. Lower crustal earthquakes are found to be restricted to the forelands in areas undergoing shortening, and to locations where rifting coincides with abrupt changes in lithosphere thickness, such as the NE Baikal Rift and W Laptev Sea. The lower crust in these areas is seismogenic at temperatures that may be as high as 600°C, suggesting that it is anhydrous, and is likely to have great long-term strength. Lower crustal earthquakes are therefore a useful proxy indicating strong lithosphere in places that are too small in areal extent for this to be confirmed independently by estimating effective elastic thickness from gravity-topography relations. The variation in crustal rheology indicated by the distribution of lower-crustal earthquakes has many implications ranging from the support of mountain belts and the formation of steep mountain fronts, to the localization and orientation of rifting. In combination, these processes can also be responsible for the separation of the front of the thin-skinned mountain belts from their hinterlands when continents separate.
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