Effects of seasonal changes in ambient noise sources on monitoring temporal variations in crustal properties

Continuous data recorded at 39 broadband stations near the Longmen Shan Fault operated by the China Earthquake Administration from 1 January 2008 to 30 September 2010 are used to study temporal variability in direct surface wave arrivals extracted from ambient noise. We use a cross-correlation technique to compute Empirical green functions (EGFs) for all available station pairs at the frequency range of 0.1 to 0.5Hz. Delay times are measured by cross-correlating reference empirical green functions and moving 60-day stacks of EGFs. By comparing the temporal changes with and without the correction for seasonal variations, our results show that for some station pairs temporal variations were strongly affected by the seasonal variation. After correction for seasonal variations, we measure a 0.5-% maximum velocity drop after the 2008 Ms8.0 earthquake in Sichuan, China.We find that the Sichuan Basin exhibited a larger relative velocity drop than the Tibetan plateau area. Our results suggest that correction for seasonal variation is an important procedure for monitoring temporal variations in crustal properties using the direct arrival surface waves extracted from ambient noise.