Lecoulant Jean, Guennou Claude, Guillon Laurent, Royer Jean-Yves
Laboratoire Géosciences Océan, University of Brest & CNRS, 1 rue Dumont d'Urville, 29280 Plouzané, France.
Institut de Recherche de l'École Navale, CC 600, 29240 Brest Cedex 9, France.
J Acoust Soc Am. 2022 May;151(5):3437. doi: 10.1121/10.0010529.
This paper discusses the nature of the low-frequency seismo-acoustic waves generated by submarine earthquakes in the ocean. In a finite-depth homogeneous ocean over a semi-infinite solid crust, the derivation of the acoustic equations shows that waves propagate as modes. The waves propagating with the speed of sound in water (T waves) are preceded by waves with frequencies below the Airy phase. Furthermore, the group speeds of these modes are sensitive to the environmental setting. As a test, we applied the spectral finite-element code SPECFEM2D in a simplified configuration with an ocean layer overlaying a solid crust, and a seismic source below a Gaussian seamount surrounded by a flat seafloor. The simulations confirm that the generated T waves and their precursors follow the theoretical dispersion curves. A more realistic environment with a seismically-layered crust and a sound-speed profile in the ocean is then used to predict the expected acoustic modes. Although noisy, recordings by ocean bottom seismometers from the southwest Indian Ocean show T waves preceded by ultra-low frequency waves, which display two modes comparable to the theoretical ones. They are in good agreement for mode 1, whereas, for mode 0, a slight offset in frequency has yet to be explained.
本文讨论了海洋中海底地震产生的低频地震声波的性质。在半无限固体地壳之上的有限深度均匀海洋中，声学方程的推导表明波以模式传播。以水中声速传播的波（T波）之前存在频率低于艾里相的波。此外，这些模式的群速度对环境设置很敏感。作为一项测试，我们在一个简化配置中应用了谱有限元代码SPECFEM2D，该配置中有一个覆盖固体地壳的海洋层，以及一个位于被平坦海底环绕的高斯海山下方的地震源。模拟结果证实，产生的T波及其前驱波遵循理论频散曲线。然后使用一个具有地震分层地壳和海洋中声速剖面的更现实环境来预测预期的声学模式。尽管存在噪声，但印度洋西南部海底地震仪的记录显示，T波之前存在超低频波，这些超低频波显示出与理论模式相当的两种模式。它们在模式1上吻合得很好，而对于模式0，频率上的轻微偏移尚待解释。
