Free Gravity Waves in the Ocean Excited by Seismic Surface Waves: Observations and Numerical Simulations

Abstract

The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) is a submarine cabled real-time seafloor observatory network for precise earthquake and tsunami monitoring. Ten DONET observatories were in operation during the 2011 Tohoku-Oki event near the Pacific coast of Honshu Island. Each observatory was equipped with an ocean bottom pressure gauge (PG) and a threecomponent ocean bottom seismometer (OBS). A comparative analysis of the PG and OBS records revealed that shortly after seismic surface waves traversed the DONET region, free gravity waves were observed within the water layer. The period of these gravity waves was approximately 170 s, the peak-to-peak amplitude was approximately 3.5 cm, the length was on the order of 22 km, and the phase velocity was 134 m/s. We performed numerical simulations of the observed gravity waves using a combined 2D/3D numerical model. The ground motions required for the simulation were reconstructed from records provided by the DONET OBSs and the nearest ground-based GPS stations. The synthetic bottom pressure variations are in good agreement with the DONET PG records. The synthetic displacements of the ocean surface throughout the simulation domain showed that the observed gravity waves were excited directly above the submarine slopes. Theoretical estimates and numerical experiments revealed the generation mechanism of the observed gravity waves.Plain Language Summary During submarine earthquakes, the ocean floor shifts from its original position. Due to this displacement, a huge volume of water is moved, which can generate a destructive tsunami wave. However, earthquakes are accompanied not only by displacement of the ocean bottom in the epicentral area but also by the emission of seismic waves. These seismic waves are also capable of exciting waves in the ocean. For our study of this phenomenon, we used unique data from the Japanese Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories that were recorded during the Great East Japan Earthquake on 11 March 2011. Based on the records of the DONET seismometers, we reconstructed the movement of the bottom during the passage of a surface seismic wave. Then, using the obtained information, we simulated the movement of the water in the entire DONET area. The results of our simulation are in good agreement with the records of the DONET pressure gauges. The results also reveal that waves are only generated directly above steep submarine slopes during the passage of seismic waves. This study can help to identify tsunami wave signals in the records of bottom pressure gauges.

Arctic-Type Seismoacoustic Waveguide: Theoretical Foundations and Experimental Results

“…In our work, we consider the case where the velocities of longitudinal and transverse waves in a seafloor half-space c l ′ and c t ′ exceed the speed of sound in water c 0 . In the general case, several families of wave modes are distinguished: (I) flexural-gravity wave, note that in [19], in contrast to Equation (1), the gravitational force, which leads to a change in dispersion properties to the gravity wave [16,24], was not taken into account; (II) Scholte-Stonely-type wave corresponds to the so-called bottom surface wave, which has no critical frequency and propagates along the boundary between the water layer and the seafloor geological structures; (III) normal waves of a plate formed by a layer of ice -Lamb modes; (IV) hydroacoustic modes. The study of (III) and (IV) modes is beyond the scope of this paper.…”

Interpretation of Signals Recorded by Ocean-Bottom Pressure Gauges during the Passage of Atmospheric Lamb Wave on 15 January 2022

“…It is important to understand that in a boundless ocean of constant depth, a running atmospheric disturbance forms an exclusively forced perturbation of the water layer. Free gravity waves can only arise in an ocean of variable depth [6,43,44]. When the depth changes, the forced perturbation, which represents a response of the water layer to the atmospheric wave, must undergo reformation, and this process is accompanied by the generation of free gravity waves.…”

Detection of Earthquake Infragravity and Tsunami Waves With Underwater Distributed Acoustic Sensing

“…In this study, we propose an effective DAS signal processing method and report the first observation of infragravity (high‐frequency tsunamis) and tsunami waves induced by earthquakes from ocean bottom DAS data (Bromirski & Stephen, 2012; Ito et al., 2020; Sementsov et al., 2019; Wei et al., 2018). In general, infragravity waves are surface gravity waves with frequencies lower than wind waves (Webb et al., 1991).…”