ALBERT

Description: To identify greater detail in the seismicity pattern preceding the 24 August 2016 Mw6.0 earthquake in Central Italy, we apply waveform matching using 1,028 events as templates. In the 8 months before the mainshock, we find ~2,000 additional earthquakes mostly located along a subhorizontal shear zone (SZ) bounding at depth the extensional fault system. Asynchrony is observed in the occurrence of events nucleating along the SZ compared to the ones on fault portions embedded in the shallower upper crust, with the former anticipating the latter. Within the SZ, we also observe along-strike seismic migration episodes with earthquakes pointing toward the Mw6.0 mainshock nucleation zone. These episodes are followed by an apparent quiescence within the main fault area. We suggest that the variations in the seismic activity along the SZ represent the brittle signature of the tectonic loading process enabling portions of the overlaying normal faults to become unlocked.

Description: Published

Description: 12170–12180

Description: 6T. Variazioni delle caratteristiche crostali e precursori

Description: JCR Journal

Description: Earthquake interaction and stress change on nearby faults by afterslip, static and dynamic triggering play an important role in the activation of major events. We apply a matched filter technique to augment the detected events in the time window between the two main shocks of 20 (Mw 6.1) and 29 (Mw 6.0) May 2012, during the Emilia seismic sequence (Italy). From 1,727 well-located templates, we increase the number of detections to 7,616 lowering the completeness magnitude of approximately 0.6 degrees. This greater detail allows evidencing migrations of seismic events from the nucleation point of the first shock to the second. The seismicity pattern suggests a transient aseismic slip acting immediately after the shallow first event, weakening and loading the volume around the deep nucleation point of the 29 May 2012 Mw6.0 earthquake. Repeating earthquakes are also found between the two main-shocks. The released cumulative slip, estimated from the earliest repeating earthquake amounts to approximately 27 cm at a depth of about 7 km within the first 5 hours after the 20 May Mw6.1. Migrations and repeaters could represent the fingerprint of an early afterslip triggered by the first mainshock.

Description: Published

Description: 625-635

Description: 2T. Deformazione crostale attiva

Description: JCR Journal

Description: There was a typographical error in [1, eq. (18)]. Instead of (Formula Presented). The equation describes the second-order statistics of the interferometric phases. Its significance lies in the fact that from it, one can derive the statistical properties of a wide range of quantities estimated from an interferometric stack, including the closure phases that we addressed in the paper. The other equations, numerical results, and the conclusions remain unaffected, because the error was of a purely typographical nature.

Description: Coastal erosion and flooding transform terrestrial landscapes into marine environments. In the Arctic, these processes inundate terrestrial permafrost with seawater and create submarine permafrost. Permafrost begins to warm under marine conditions, which can destabilize the sea floor and may release greenhouse gases. We report on the transition of terrestrial to submarine permafrost at a site where the timing of inundation can be inferred from the rate of coastline retreat. On Muostakh Island in the central Laptev Sea, East Siberia, changes in annual coastline position have been measured for decades and vary highly spatially. We hypothesize that these rates are inversely related to the inclination of the upper surface of submarine ice-bonded permafrost (IBP) based on the consequent duration of inundation with increasing distance from the shoreline. We compared rapidly eroding and stable coastal sections of Muostakh Island and find permafrost-table inclinations, determined using direct current resistivity, of 1 and 5 %, respectively. Determinations of submarine IBP depth from a drilling transect in the early 1980s were compared to resistivity profiles from 2011. Based on borehole observations, the thickness of unfrozen sediment overlying the IBP increased from 0 to 14m below sea level with increasing distance from the shoreline. The geoelectrical profiles showed thickening of the unfrozen sediment overlying ice-bonded permafrost over the 28 years since drilling took place. We use geoelectrical estimates of IBP depth to estimate permafrost degradation rates since inundation. Degradation rates decreased from over 0.4ma-1 following inundation to around 0.1ma-1 at the latest after 60 to 110 years and remained constant at this level as the duration of inundation increased to 250 years. We suggest that long-term rates are lower than these values, as the depth to the IBP increases and thermal and porewater solute concentration gradients over depth decrease. For the study region, recent increases in coastal erosion rate and changes in benthic temperature and salinity regimes are expected to affect the depth to submarine permafrost, leading to coastal regions with shallower IBP.

Description: Diatoms account for a large proportion of primary productivity in Antarctic coastal and continental shelf zones. Diatoms, which have been used for a long time to infer past sea surface conditions in the Southern Ocean, have recently been associated with diatom-specific biomarkers (highly branched isoprenoids, HBI). Our study is one of the few sedimentary research projects on diatom ecology and associated biomarkers in the Antarctic seasonal sea ice zone. To date, the Adélie Land region has received little attention, despite evidence for the presence of high accumulation of laminated sediment, allowing for finer climate reconstructions and sedimentary process studies. Here we provide a sequence of seasonally to annually laminated diatomaceous sediment from a 72.5 cm interface core retrieved on the continental shelf off Adélie Land, covering the 1970-2010 CE period. Investigations through statistical analyses of diatom communities, diatom-specific biomarkers and major element abundances document the relationships between these proxies at an unprecedented resolution. Additionally, comparison of sedimentary records to meteorological data monitored by automatic weather station and satellite derived sea ice concentrations help to refine the relationships between our proxies and environmental conditions over the last decades. Our results suggest a coupled interaction of the atmospheric and sea surface variability on sea ice seasonality, which acts as the proximal forcing of siliceous productivity at that scale.

Description: The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910±12, 1812±18, 1725±25 and 1580±30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Description: Magnetic properties coupled with sortable silt are investigated for Holocene marine sedimentary sequences located in the subpolar North Altantic, in the Charlie– Gibbs fracture zone (53°N) and in central (57°N) and southern Gardar drift (59°N). All the cores are located at water depths bathed by the Iceland–Scotland Overflow Water, mixed at the southernmost locality with southern sourced water masses. The goal of the multi-proxy study is the changes in the dynamics and the properties of bottom water mass during Holocene. After checking that the magnetic minerals is magnetite of uniform grain size, the low field magnetic susceptibility is used as a magnetic concentration parameter and as a tracer of the transport efficiency by the bottom current from the northern basaltic-derived source. The mean sortable silt size is used as a tracer of bottom current strength whatever the detrital source.

Description: Satellite techniques allow a nearly global coverage of elevation data, even in polar regions. However, in situ observations are crucial to validate these measurements. This dataset contains 20 elevation profiles, observed by kinematic GNSS in central East Antarctica. They have been measured during scientific traverses of the Russian Antarctic Expedition between the Antarctic research stations Vostok, Progress and Mirny, spanning distances between 80 and 2900km each. The given elevations have been corrected for the height offset of the antenna above the snow surface and are thus surface elevation profiles refering to the WGS84 ellipsoid. For consistency with the measurements of satellite altimetry, they have been converted to the mean tidal system. For each epoch, estimates for the standard deviation are given, which are in the order of magnitude of 2 to 5 cm. Crossover analysis shows that independent profiles of the same season differ by slightly more (4 to 9 cm) as a consequence of vehicle dynamics while driving. Further details are given in the related article.

Description: Continental and marine paleoclimate archives from northwestern and northeastern South America recorded positive precipitation anomalies during Heinrich Stadials (HS). These anomalies have been classically attributed to enhanced austral summer (monsoon) precipitation. However, the lack of marine paleoclimate records off eastern South America as well as inconsistencies between southeastern South American continental and marine records hamper a comprehensive understanding of the mechanism responsible for (sub-) tropical South American hydroclimate response to HS. Here we investigate piston core M125-95-3 collected off eastern South America (10.94°S) and simulate South American HS conditions with a high-resolution version of an atmosphere-ocean general circulation model. Further, meridional changes in precipitation over (sub-) tropical South America were assessed with a thorough compilation of previously available marine paleorecords. Our ln(Ti/Ca) and ln(Fe/K) data show increases during HS6-Younger Dryas. It is the first core off eastern South America and the southernmost from the Atlantic continental margin of South America that unequivocally records HS-related positive precipitation anomalies. Based on our new data, model results and the compilation of available marine records, we propose a new mechanism for the positive precipitation anomalies over tropical South America during HS. The new mechanism involves austral summer precipitation increases only over eastern South America while the rest of tropical South America experienced precipitation increases during the winter, challenging the widely held assumption of a strengthened monsoon. South American precipitation changes were triggered by dynamic and thermodynamic processes including a stronger moisture supply from the equatorial North Atlantic (tropical South Atlantic) in austral winter (summer).

Description: Abrupt millennial-scale climate change events of the last deglaciation (i.e. Heinrich Stadial 1 and the Younger Dryas) were accompanied by marked increases in atmospheric CO2 (CO2atm) and decreases in its stable carbon isotopic ratios (d13C), i.e. d13CO2atm, presumably due to outgassing from the ocean. However, information on the preceding Heinrich Stadials during the last glacial period is scarce. Here we present d13C records from two species of planktonic foraminifera from the western South Atlantic that reveal major decreases (up to 1 per mil) during Heinrich Stadials 3 and 2. These d13C decreases are most likely related to millennialscale periods of weakening of the Atlantic meridional overturning circulation and the consequent increase (decrease) in CO2atm (d13CO2atm). We hypothesise two mechanisms that could account for the decreases observed in our records, namely strengthening of Southern Ocean deep-water ventilation and weakening of the biological pump. Additionally, we suggest that air?sea gas exchange could have contributed to the observed d13C decreases. Together with other lines of evidence, our data are consistent with the hypothesis that the CO2 added to the atmosphere during abrupt millennial-scale climate change events of the last glacial period also originated in the ocean and reached the atmosphere by outgassing. The temporal evolution of d13C during Heinrich Stadials 3 and 2 in our records is characterized by two relative minima separated by a relative maximum. This ?w structure? is also found in North Atlantic and South American records, further suggesting that such a structure is a pervasive feature of Heinrich Stadial 2 and, possibly, also Heinrich Stadial 3.