资源环境科技发展态势分析平台

Observed reversals in GNSS surface motions suggests greatly enhanced slab pull in the months preceding the great subduction earthquakes in Maule (Chile, 2010) and Tohoku-oki (Japan, 2011) of moment magnitudes 8.8 and 9.0.

Megathrust earthquakes are responsible for some of the most devastating natural disasters(1). To better understand the physical mechanisms of earthquake generation, subduction zones worldwide are continuously monitored with geophysical instrumentation. One key strategy is to install stations that record signals from Global Navigation Satellite Systems(2,3) (GNSS), enabling us to track the non-steady surface motion of the subducting and overriding plates before, during and after the largest events(4-6). Here we use a recently developed trajectory modelling approach(7) that is designed to isolate secular tectonic motions from the daily GNSS time series to show that the 2010 Maule, Chile (moment magnitude 8.8) and 2011 Tohoku-oki, Japan (moment magnitude 9.0) earthquakes were preceded by reversals of 4-8 millimetres in surface displacement that lasted several months and spanned thousands of kilometres. Modelling of the surface displacement reversal that occurred before the Tohoku-oki earthquake suggests an initial slow slip followed by a sudden pulldown of the Philippine Sea slab so rapid that it caused a viscoelastic rebound across the whole of Japan. Therefore, to understand better when large earthquakes are imminent, we must consider not only the evolution of plate interface frictional processes but also the dynamic boundary conditions from deeper subduction processes, such as sudden densification of metastable slab.

Reverse genetics has been an indispensable tool to gain insights into viral pathogenesis and vaccine development. The genomes of large RNA viruses, such as those from coronaviruses, are cumbersome to clone and manipulate inEscherichia coliowing to the size and occasional instability of the genome(1-3). Therefore, an alternative rapid and robust reverse-genetics platform for RNA viruses would benefit the research community. Here we show the full functionality of a yeast-based synthetic genomics platform to genetically reconstruct diverse RNA viruses, including members of theCoronaviridae,FlaviviridaeandPneumoviridaefamilies. Viral subgenomic fragments were generated using viral isolates, cloned viral DNA, clinical samples or synthetic DNA, and these fragments were then reassembled in one step inSaccharomyces cerevisiaeusing transformation-associated recombination cloning to maintain the genome as a yeast artificial chromosome. T7 RNA polymerase was then used to generate infectious RNA to rescue viable virus. Using this platform, we were able to engineer and generate chemically synthesized clones of the virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)(4), which has caused the recent pandemic of coronavirus disease (COVID-19), in only a week after receipt of the synthetic DNA fragments. The technical advance that we describe here facilitates rapid responses to emerging viruses as it enables the real-time generation and functional characterization of evolving RNA virus variants during an outbreak.

Generation of intense attosecond waveforms with independently controllable amplitude and phase is performed by using a seeded free-electron laser.

Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales(1-3). The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation(4-7). Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters(8,9), multilayer mirrors(10) and manipulation of the driving field(11). However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules(12,13). Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot(14-16). Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser(17). We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers.