image: Observation and origin of the elastogravity signal preceding direct seismic waves. The map shows the location of the seismometers (triangles) detecting the prompt signals just after the beginning of the Tohoku earthquake (Japan, 11 March 2011, magnitude 9.1), indicated by the black star. We focus here on one of the stations (MDJ), located in north-east China, 1280km away from the Tohoku earthquake. At such distances, direct seismic waves arrive about 165 s after the earthquake starts, as shown in the inset reproducing the MDJ vertical seismogram. However, a clear, even if much weaker, acceleration signal is detected by the seismometer before the direct waves arrival. The origin of such signal can be understood by considering a time after the earthquake onset but before the arrival of the direct seismic waves. For example, about 55 s after origin time, direct waves have propagated inside the volume shown by the grey area, but are still far from arriving at MDJ station. However, inside this volume, seismic waves have caused compressions and dilations of the medium (as further indicated in the bottom cross-section), and the global contribution of all such elements whose mass has changed gives rise to a gravity perturbation, immediately detected by the seismometer (direct effect). The gravitational field is also modified everywhere in the Earth, and each of the elements affected by these perturbations is a secondary source of seismic waves (induced effect). In the green volume around the seismometer, this secondary seismic wavefield arrives before the direct waves. The seismometer therefore records a prompt elastogravity signal, due to the direct and induced effects of the gravity perturbations. This material relates to a paper that appeared in the 1 December 2017, issue of <i>Science</i>, published by AAAS. The paper, by M. Vallée at Institut de Physique du Globe de Paris in Paris, France, and colleagues was titled, "Observations and modeling of the elastogravity signals preceding direct seismic waves." view more
Credit: IPGP, 2017
据研究人员报告，在2011年日本东北大地震之后，尽管地震波仍然还在向各地震监测站传输信号并为该地震的震级提供线索，但地震仪却立刻记录到了反映震级数值的重力改变。他们说，展望未来，这些及时的“弹性重力”信号或能被用来提前做出大型地震震级的估测，这一观念代表了一种检查撕裂地球巨型地震的新方法。由地震所产生的弹性波是一种广为人知的现象，它带有地震特征的宝贵信息。弹性波中的P波尤其被认为是传输地震信息的最快方式，但也有一些地震学家推测是其它的情况。像任何造成板块物质重大破裂一样，地震及其弹性波场会引起地球重力场扰动，在了解这一点的情况下，Martin Vallée和同事在此希望能观测到由重力诱发的这些信号，后者会以光速传播（比P波传播速度快得多）。通过仔细评估在东北大地震时记录的地震数据，他们终于如愿以偿地发现，这些信号始终在其对应地震波信号传到地震监测站之前就率先到达那些监测站。作者们说，如果这种方法在2011年就能使用，矩震级为9.1的东北大地震的震级超过9级的这一事实或能在几分钟内就被检测到。事实上，由日本气象厅提供的近乎实时的震级为7.9级，但在3小时后它被校正为8.8级（不过又被低估）。
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Journal
Science