Pre-seismic changes of noise correlation function (NCF) before the Wenchuan earthquake?

摘要

We calculated noise correlation function (NCF) around the Longmenshan fault zone which is responsible for the Wenchuan M_S8.0 earthquake on May 12, 2008. We used 58 station pairs in the investigation. The time period considered was from April 21, 12:00 a.m. local time, to May 12, 2008, 12:00 a.m. local time, 22 days before the occurrence of the earthquake. Correlation coefficient between NCF of each day and its previous day (the 'second cross-correlation') was calculated. Four hundred second long waveform is used in the 'second cross-correlation'. Before the earthquake, there seemed to be an apparent variation of such correlation coefficient, and such a pattern seemed coherent for almost all the station pairs. From the scattering around the periodical variations of the 'second cross-correlation', a seemingly precursory change can be observed. However, at the present stage, it is still hard to draw any conclusion about the precursory changes before the earthquake.