Body wave microseisms from a distant storm revealed by Hi-net data

Abstract:

Observations of microseisms were firmly established in 1940's. Their excitation sources are ocean swell activities. Microseisms from 0.1 to 0.5 Hz, which double the frequency, indicating the generation of the former through nonlinear wave-wave interaction of the latter. Because the swell activities are their sources, surface wave excitation is dominant.Recently body wave microseisms from a distant storm, however, have been focussed [e.g. Gerstoft et al. 2006]. They show clear teleseismic P waves excited by distant storms. A back-projection method could constrain the source distribution, which gives clues to their excitation mechanisms. Most studies focused, however, only vertical components.
In this study, in order to constrain excitation mechanisms of microseisms, we conducted a 3-component array analysis using Hi-net operated by NIED, when a strong weather bomb hit the Atlantic ocean on Dec. 9th, 2014. We analyzed 3-component velocity-meters with a natural frequency of 1 Hz at 775 stations. The instrumental response was deconvolved by using the inverse filtering technique after reduction of common logger noise Their frequency-wavenumber spectra were calculated at 0.15 Hz. The spectra of a vertical component and a radial component show that clear teleseismic P-wave. The slowness of about 0.05 [s/km] and the back azimuth of -5 degrees are consistent with that of the P wave traveled from the weather bomb in the Atlantic ocean. The radial component shows SV wave with about 20% of P wave amplitudes. These observations could be explained by a single force source at the sea surface in the 1-D medium in the first order approximation. The transverse component also shows SH waves with about 10% of the P wave amplitude. The excitation source of P, SV and SH wave were located in the same small area by a backprojection analysis. These observations suggest that the steep topography and the thick sediment beneath the source area also affected the excitations.Observations of microseisms were firmly established in 1940's. Their excitation sources are ocean swell activities. Microseisms from 0.1 to 0.5 Hz, which double the frequency, indicating the generation of the former through nonlinear wave-wave interaction of the latter. Because the swell activities are their sources, surface wave excitation is dominant.Recently body wave microseisms from a distant storm, however, have been focussed [e.g. Gerstoft et al. 2006]. They show clear teleseismic P waves excited by distant storms. A back-projection method could constrain the source distribution, which gives clues to their excitation mechanisms. Most studies focused, however, only vertical components.
In this study, in order to constrain excitation mechanisms of microseisms, we conducted a 3-component array analysis using Hi-net operated by NIED, when a strong weather bomb hit the Atlantic ocean on Dec. 9th, 2014. We analyzed 3-component velocity-meters with a natural frequency of 1 Hz at 775 stations. The instrumental response was deconvolved by using the inverse filtering technique after reduction of common logger noise. Their frequency-wavenumber spectra were calculated at 0.15 Hz. The spectra of a vertical component and a radial component show that clear teleseismic P-wave. The slowness of about 0.05 [s/km] and the back azimuth of -5 degrees are consistent with that of the P wave traveled from the weather bomb in the Atlantic ocean. The radial component shows SV wave with about 20% of P wave amplitudes. These observations can be explained by a single force source at the sea surface in the 1-D medium in the first order approximation. The transverse component shows SH waves with about 10% of the P wave amplitude. The excitation source of P, SV and SH wave were located in the same small area by a backprojection analysis. These observations suggest that the steep topography and the thick sediment beneath the source area also affected the excitations.