ALBERT

Description: Abstract Studies of mechanical responses of the Earth crust to large earthquakes can provide us with unique insights into the processes of stress buildup and release. As a complement to geodetic methods that derive crustal strain dynamics from surface observations (e.g., GPS, InSAR), noise‐based seismic velocity monitoring directly probes the mechanical state of the crust, at depth and continuously in time. We investigate the responses of the crust to the Mw 9.0, 2011 Tohoku‐oki earthquake. In addition to the Hi‐net short‐period sensors, we use Hi‐net tiltmeters as long‐period seismometers (8–50 s) to sample the crust below 5 km in depth. The spatial distribution of the strong velocity decreases at short periods appears to be limited to the region of strong ground shaking induced by the 2011 Tohoku‐oki earthquake, while the long‐period velocity changes correlate well with the modeled static strain induced by viscoelastic relaxation and afterslip at depth. Amplitudes of coseismic velocity changes decrease with increasing depth. The temporal evolution of velocity changes in different period bands shows that the maximum drops in the velocity at long periods are delayed in time with respect to the occurrence of the Tohoku‐oki earthquake. The inversion of seismic velocity changes at depth illustrates how S wave velocities evolve down to 40 km at a regional scale after a major earthquake.

Description: Abstract The observation of a transient slip one month before the rupture of the 2011 Tohoku earthquake is a conandrum since the area was supposedly fully coupled. A better understanding of the mechanisms at work during the preseismic phase is thus fundamental. However, the configuration of the Pacific plate and the location of the Tohoku rupture zone 200km from the coast make it difficult to detect microseismic events. In this study, we use a Multi‐Dimensionnal Template Matching (MDTM) technique to detect earthquakes that are hidden in the noise. The temporal distribution of these 395 newly detected earthquakes provide new insights on the slip history of the megathrust earthquake epicentral zone. The detected events can be separated into two groups: 187 low frequency detections (below 5 Hz) that well‐recorded the episodes of earthquake migration prior to the Tohoku earthquake and 208 high frequency detections (above 10 Hz) that occurred close to the rupture zones of the M≥4.8‐6 earthquakes that struck between the 2011/3/9 M7.3 foreshock and the 2011/3/11 Tohoku‐Oki earthquake. The seismic rate of these high frequency detection events starts to increase on the 2010/11/30 until the Tohoku earthquake.

Description: [1]  All climate models predict a freshening of the North Atlantic at high latitude that may induce an abrupt change of the Atlantic Meridional Overturning Circulation (hereafter AMOC) if it resides in the bistable regime, where both a strong and a weak state coexist. The latter remains uncertain as there is no consensus among observations and ocean reanalyses, where the AMOC is bistable, vs most climate models that reproduce a mono-stable strong AMOC. A series of four hindcast simulations of the global ocean at 1/12º resolution, which is presently unique, is used to diagnose freshwater transport by the AMOC in the South Atlantic, an indicator of AMOC bistability. In all simulations, the AMOC resides in the bistable regime: it exports freshwater southward in the South Atlantic, implying a positive salt advection feedback that would act to amplify a decreasing trend in subarctic deep water formation as projected in climate scenarios.

Description: Noise-based crustal seismic velocity changes are known to be affected by environmental perturbations, such as rainfall, atmospheric pressure loading, and temperature changes. Similar to geodetic observations, these external perturbations can mask the effects of tectonic and volcanic processes. In this study, we benefit from the dense Hi-net short-period seismic network that covers the entire Japan to measure continuous changes in seismic velocities over a few years, using noise-based seismic monitoring. Some strong seasonal seismic velocity changes are observed in both southern Japan (Kyushu Island) and northern Japan (Hokkaido Island). Decreasing of seismic velocities in summer in southern Japan can be clearly explained by a model of increased crustal fluid pore pressure associated with high rainfall. In northern Japan, it is necessary to adopt a more complex model to explain the observed seismic velocity variations, which takes into account precipitation, snow depth, and sea-level changes. Moreover, western and eastern Hokkaido Island show very different responses to these different external perturbations. The models developed are used to remove the seasonal components of the seismic velocity changes. The minimum remaining detectable seismic velocity change reduces to 10 −5 , which allows detection of crustal responses to small earthquakes that are previously hidden in the strong seasonal perturbations.

Description: All climate models predict a freshening of the North Atlantic at high latitude that may induce an abrupt change of the Atlantic Meridional Overturning Circulation (hereafter AMOC) if it resides in the bistable regime, where both a strong and a weak state coexist. The latter remains uncertain as there is no consensus among observations and ocean reanalyses, where the AMOC is bistable, versus most climate models that reproduce a mono-stable strong AMOC. A series of four hindcast simulations of the global ocean at 1/12° resolution, which is presently unique, are used to diagnose freshwater transport by the AMOC in the South Atlantic, an indicator of AMOC bistability. In all simulations, the AMOC resides in the bistable regime: it exports freshwater southward in the South Atlantic, implying a positive salt advection feedback that would act to amplify a decreasing trend in subarctic deep water formation as projected in climate scenarios.

Description: Species flocks (SFs) fascinate evolutionary biologists who wonder whether such striking diversification can be driven by normal evolutionary processes. Multiple definitions of SFs have hindered the study of their origins. Previous studies identified a monophyletic taxon as a SF if it displays high speciosity in an area in which it is endemic (criterion 1), high ecological diversity among species (criterion 2), and if it dominates the habitat in terms of biomass (criterion 3); we used these criteria in our analyses. Our starting hypothesis is that normal evolutionary processes may provide a sufficient explanation for most SFs. We thus clearly separate each criterion and identify which biological (intrinsic) and environmental (extrinsic) traits are most favourable to their realization. The first part focuses on evolutionary processes. We highlight that some popular putative causes of SFs, such as key innovations or ecological speciation, are neither necessary nor sufficient to fulfill some or all of the three criteria. Initial differentiation mechanisms are diverse and difficult to identify a posteriori because a primary differentiation of one type (genetic, ecological or geographical) often promotes other types of differentiation. Furthermore, the criteria are not independent: positive feedbacks between speciosity and ecological diversity among species are expected whatever the initial cause of differentiation, and ecological diversity should enhance habitat dominance at the clade level. We then identify intrinsic and extrinsic factors that favour each criterion. Low dispersal emerges as a convincing driver of speciosity. Except for a genomic architecture favouring ecological speciation, for which assessment is difficult, high effective population sizes are the single intrinsic factor that directly enhances speciosity, ecological diversity and habitat dominance. No extrinsic factor appeared to enhance all criteria simultaneously but a combination of factors (insularity, fragmentation and environmental stability) may favour the three criteria, although the effect is indirect for habitat dominance. We then apply this analytical framework to Antarctic marine environments by analysing data from 18 speciose clades belonging to echinoderms (five unrelated clades), notothenioid fishes (five clades) and peracarid crustaceans (eight clades). Antarctic shelf environments and history appear favourable to endemicity and speciosity, but not to ecological specialization. Two main patterns are distinguished among taxa. (i) In echinoderms, many brooding, species‐rich and endemic clades are reported, but without remarkable ecological diversity or habitat dominance. In these taxa, loss of the larval stage is probably a consequence of past Antarctic environmental factors, and brooding is suggested to be responsible for enhanced allopatric speciation (via dispersal limitation). (ii) In notothenioids and peracarids, many clades fulfill all three SF criteria. This could result from unusual features in fish and crustaceans: chromosome instability and key innovations (antifreeze proteins) in notothenioids, ecological opportunity in peracarids, and a genomic architecture favouring ecological speciation in both groups. Therefore, the data do not support our starting point that normal evolutionary factors or processes drive SFs because in these two groups uncommon intrinsic features or ecological opportunity provide the best explanation. The utility of the three‐criterion SF concept is therefore questioned and guidelines are given for future studies.