Short Note Seasonal Anisotropy in Short-Period Seismic Noise Recorded in South Asia

We present an analysis of seismic noise recorded during 1995–2004 by a medium-aperture, short-period seismic array located in Chiang Mai, Thailand (CMAR). We calculated frequency-wavenumber spectra for nearly 1000 randomly selected time windows, each with a length of 160 sec. At frequencies above about 1.4 Hz the noise is unorganized and the wavenumber spectra are isotropic and diffuse; however, at lower frequencies three robust wavenumber peaks exist. Two of the peaks have phase velocities centered near 4:0 km=sec, consistent with highermode Rayleigh waves, while the third peak has much higher apparent velocity (>25 km=sec), consistent with body waves that have interacted with the Earth’s core (PKP, PcP). All three peaks are strongly seasonal with annual power variations of 10– 20 dB, and all show excellent correlation in their putative source regions with ocean wave heights derived from TOPEX/POSEIDON satellite tracks. To the best of our knowledge, this is the first time such a high-velocity component of seismic noise has been consistently observed. The presence of this high-velocity peak raises the possibility of using ambient noise to image the Earth’s lower mantle and core.