Where do ocean microseisms come from? A study of Love‐to‐Rayleigh wave ratios

Abstract

Our knowledge of the origin of Love waves in the ambient seismic noise is extremely limited. This applies in particular to constraints on source locations and source mechanisms for Love waves in the secondary microseism. Here three‐component beamforming is used to distinguish between the differently polarized wave types in the primary and secondary microseismic noise fields, recorded at several arrays across Europe. We compare characteristics of Love and Rayleigh wave noise, such as source directions and frequency content, measure Love to Rayleigh wave ratios for different back azimuths, and look at the seasonal behavior of our measurements by using a full year of data in 2013. The beamforming results confirm previous observations that back azimuths for Rayleigh and Love waves in both microseismic bands mainly coincide. However, we observe differences in relative directional noise strength between both wave types for the primary microseism. At those frequencies, Love waves dominate on average, with kinetic Love‐to‐Rayleigh energy ratios ranging from 0.6 to 2.0. In the secondary microseism, the ratios are lower, between 0.4 and 1.2. The wave type ratio is directionally homogeneous, except for locations far from the coast. In the primary microseism, our results support the existence of different generation mechanisms. The contribution of a shear traction‐type source mechanism is likely.

Full wavefield decomposition of high‐frequency secondary microseisms reveals distinct arrival azimuths for Rayleigh and Love waves

“…Previously reported directional similarities between R g and L Q energy in the secondary microseism band [e.g., Nishida et al , ; Hadziioannou et al , ; Behr et al , ] are partially present in our results, but deviations occur, especially for directions where R g is weak. At the lowest frequency ( f 1 = 0.35 Hz), we observe the Z component energy to be almost twice the strength of the T component; however, it is likely that Z / T ratios are regionally dependent especially for seismometers sited in sedimentary basins [ Koper and Burlacu , ; Juretzek and Hadziioannou , ].…”

“…L g waves show an increase in the second and predominantly the third quarters, at times where more energetic swells are to be expected. The Z / T ratio remains relatively constant over the course of the year at 0.35 Hz, displayed in Figure , which is in agreement with recent finding for multiple European arrays [ Juretzek and Hadziioannou , ]. For higher frequencies, the ratio decreases in the second and third quarters due to the increased southern Indian Ocean activity combined with the increased scattering/attenuation of R g from southern directions.…”

Ambient Seismic Source Inversion in a Heterogeneous Earth: Theory and Application to the Earth's Hum

“…Previously developed localization methods for ambient sources include spectral analysis (e.g., Anthony et al, ; Bromirski, ; Bromirski & Duennebier, ), polarization analysis (e.g., Chevrot et al, ; Schimmel et al, ; Schulte‐Pelkum et al, ), cross correlation‐based imaging (e.g., Ermert et al, ; Stehly et al, ; Tian & Ritzwoller, ; Yang & Ritzwoller, ), beamforming (e.g., Behr et al, ; Gal et al, ; Gerstoft & Tanimoto, ; Juretzek & Hadziioannou, ; Kurrle & Widmer‐Schnidrig, ; Landès et al, ; Reading et al, ; Rhie & Romanowicz, ), migration of cross‐correlation signals (Brzak et al, ; Dales et al, ; Retailleau et al, ), grid searches for fitting particular observables (Gaudot et al, ; Rhie & Romanowicz, ; Shapiro et al, ), and inversion based on simplified plane‐wave models of noise cross correlations (Harmon et al, ; Lehujeur et al, ; Sadeghisorkhani et al, ; Yao & van der Hilst, ). Nishida and Fukao () inverted for the seasonal source of the Earth's hum using a model of cross correlations based on normal modes and a spherically symmetric Earth model.…”

Large‐Scale Forces Under Surface Gravity Waves at a Wavy Bottom: A Mechanism for the Generation of Primary Microseisms

“…For k D > 0.23, | α |<1 and the horizontal force is larger than the vertical force. This can explain the higher amplitudes of Love waves compared to Rayleigh waves that are often observed for primary microseisms (Juretzek and Hadziioannou, , ).…”