Passive Imaging Of Ambient Seismic Noise And Its Application In North China

Recently, one of the hotly discussed problems in seismology is using ambient noise to extract the Green’s function. The Green’s functions between pairs of stations can be estimated from the time derivative of the long-time cross-correlation of ambient seismic noise. These Gfs reveal velocity dispersion at relatively short periods, which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface-wave tomography. In this paper, the developing background and process of this method will be introduced firstly. And then its physical principle will be also shown from four different viewpoints. We present the Capital area study of surface wave dispersion, using ambient noise observed at133broad-band stations in the Digital Seismic Network in Capital Circle. Three month long vertical-component time-series recorded between Jan,2007and Mar,2007have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green’s Functions. We filter these Green’s functions to compute Rayleigh wave group dispersion curves at periods of10-20s, using frequency-time analysis technique. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from10-20s. Group velocity maps with a grid spacing of0.5°×0.5°at7S,12S,16S, and23S are reconstructed. The resulting ambient noise group velocity maps reveal the lateral velocity changes in the crust, especially in upper crust very well. Our results reveal an evident lateral heterogeneity of the curst in North China and demonstrate significant agreement with known geological and tectonic features at short periods. Orogenic belt and uplift areas, such as Yanshan uplift and Taihangshan uplift have high velocities, while depressions and sedimentary basins, such as North China basin, have low velocities. These results are evidence that surface wave tomography based on cross-correlations of long time-series of ambient noise data can be achieved over a broad period band on nearly a continental scale and yield higher resolution and more reliable group speed maps than based on traditional earthquake-based measurements.