Earthquakes have been studied by means of seismometers recording the elastic waves travelling through the interior of our planet. Global Navigation Satellite System and Synthetic Aperture Radar surveys, measuring surface displacements, have provided additional information on earthquakes, as well as on those solid Earth processes responsible for them, such as subduction, collision and extension and the inter-seismic strain accumulation. This instrumentation is deployed over land and thus misses the seas, often surrounding regions where large earthquakes occur. This limitation is nowadays overcome by space gravity missions, thanks to their uniform coverage of the Earth, both inland and offshore. In this perspective, Gravitational Seismology has been identified as a new application of the Next-Generation Gravity Mission (NGGM), with the aim of evaluating its overall performance and of assessing the detectability of earthquake gravity signatures, as well as of those from active tectonics and inter-seismic deformation. Within the framework of self-gravitating viscoelastic Earth models, we have simulated the co- and post-seismic gravity signatures of 291 scenario earthquakes, with different occurrence times and geographical locations, focal mechanisms, depths and lines of strike, and included into the background gravity feeding the NGGM closed-loop simulation which provides observables of multiple pairs of GRACE-like satellites, given the instrument noise. NGGM earthquake detectability is herein defined on the possibility of estimating the amplitude of the original gravity signature of each earthquake by inversion of synthetic NGGM gravity data, consisting of 156 28-day gravity field solutions (about 11 years). For about two thirds of earthquakes of magnitude as low as 7, comparable with the 1980 Irpinia intraplate earthquake, the amplitudes have been estimated with a relative error less than 10% (and less than 50% for almost all the earthquakes), assuming as known the time variable contributions from atmosphere, oceans, hydrology, continental ice and glacial isostatic adjustment. When these contributions are inverted simultaneously with the earthquake ones, instead, we have had to increase the earthquake magnitude to 7.8 in order to estimate more than half of their amplitudes with a relative error less than 10%. We thus have shown that the NGGM will be able to detect, in most cases, the co- and post-seismic signatures of earthquakes of at least magnitude 7.8 and that this lower magnitude threshold can decrease down to magnitude 7 by improving the modelling of the background gravity field.

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下一代重力任务的地震可探测性

地震已通过地震仪记录在我们星球内部传播的弹性波进行了研究。全球导航卫星系统和合成孔径雷达勘测，测量地表位移，提供了关于地震的额外信息，以及那些造成地震的固体地球过程，如俯冲、碰撞和伸展以及地震间应变积累。这种仪器部署在陆地上，因此错过了海洋，通常是发生大地震的地区。由于太空重力任务对地球内陆和近海的均匀覆盖，如今这一限制已被空间重力任务所克服。从这个角度来看，重力地震学已被确定为下一代重力任务（NGGM）的新应用，目的是评估其整体性能并评估地震重力特征以及来自活动构造和地震间变形的特征的可探测性。在自引力粘弹性地球模型框架内，我们模拟了291次情景地震的同震和震后重力特征，不同的发生时间和地理位置、震源机制、深度和走向线，并包含在背景中重力馈送 NGGM 闭环模拟，该模拟提供多对类似 GRACE 的卫星的观测值，给定仪器噪声。NGGM地震可探测性在这里定义为通过合成NGGM重力数据反演估计每个地震原始重力特征幅度的可能性，由 156 个 28 天重力场解（约 11 年）组成。对于大约三分之二的震级低至 7 级的地震，与 1980 年 Irpinia 板内地震相比，估计振幅的相对误差小于 10%（几乎所有地震都小于 50%），假设已知来自大气、海洋、水文、大陆冰和冰川均衡调整的时间变量贡献。相反，当这些贡献与地震同时反演时，我们不得不将地震震级增加到 7.8，以便以小于 10% 的相对误差估计其一半以上的振幅。因此，我们已经表明，在大多数情况下，NGGM 将能够检测到至少 7 级地震的同震和震后特征。