The frequency dependence and locations of short‐period microseisms generated in the Southern Ocean and West Pacific

Abstract

The origin of the microseismic wavefield is associated with deep ocean and coastal regions where, under certain conditions, ocean waves can excite seismic waves that propagate as surface and body waves. Given that the characteristics of seismic signals generally vary with frequency, here we explore the frequency‐ and azimuth‐dependent properties of microseisms recorded at a medium aperture (25 km) array in Australia. We examine the frequency‐dependent properties of the wavefield, and its temporal variation, over two decades (1991–2012), with a focus on relatively high‐frequency microseisms (0.325–0.725 Hz) recorded at the Warramunga Array, which has good slowness resolution capabilities in this frequency range. The analysis is carried out using the incoherently averaged signal Capon beamforming, which gives robust estimates of slowness and back azimuth and is able to resolve multiple wave arrivals within a single time window. For surface waves, we find that fundamental mode Rayleigh waves (Rg) dominate for lower frequencies (<0.55 Hz) while higher frequencies (>0.55 Hz) show a transition to higher mode surface waves (Lg). For body waves, source locations are identified in deep ocean regions for lower frequencies and in shallow waters for higher frequencies. We further examine the association between surface wave arrivals and a WAVEWATCH III ocean wave hindcast. Correlations with the ocean wave hindcast show that secondary microseisms in the lower‐frequency band are generated mainly by ocean swell, while higher‐frequency bands are generated by the wind sea, i.e., local wind conditions.

Source locations of teleseismic P, SV, and SH waves observed in microseisms recorded by a large aperture seismic array in China

“…This can be achieved by using backprojection grid spacing finer than the 400 km used here, and by focusing the array beams with travel time corrections computed from 3D mantle velocity models or with empirical travel time corrections derived from earthquakes near microseism source regions. Using data from a more permanent, non-rolling array with aperture similar to or larger than WTA would also lead to better locations, as might a multi-array backprojection location approach (e.g., Roessler et al, 2010;Pyle et al, 2015;Zhang et al, 2016) and new, sophisticated methods of deconvolving array response functions such as CLEAN-PSF (Gal et al, 2016).…”

“…To verify these results, we use a new implementation of a conventional three-component f-k (frequency-wavenumber) method (Gal et al, 2016), which is partly based on the work of Wagner (1996). In contrast to backprojection, this method implicitly assumes plane wave propagation across the array and does not rely on a reference velocity model.…”

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.…”

Ray-theoretical modeling of secondary microseismPwaves

“…SchultePelkum et al 2004;Gerstoft et al 2006;Koper & de Foy 2008;Zhang et al 2010a,b;Davy et al 2014). Gal et al (2015) further showed that P waves recorded in Australia are mostly generated close to the coast at periods shorter than 2 s and in deep water at longer periods.…”