Preface to the special issue on ambient noise seismology∗

Not too long ago, seismic imaging of the Earth’s interior relied almost exclusively on the illumination from energetic sources, such as earthquakes or artificial sources. However, recent theoretical and laboratory studies have shown that the impulse response (Green’s function) of a structure between two receivers can be obtained from the cross-correlation of “random” noise wavefields recorded at the two receivers. The basic idea is that a random field contains coherent signals traveling between the receivers, which can be stacked and amplified while all other arrivals are canceled out in the cross-correlation. The idea has now found rapid applications in seismology. In particular, surface waves have been found to be most easily retrievable from the cross-correlations of the ambient seismic noise in the Earth. The new area of research is expanding rapidly and has quickly become an important branch of seismological research. This field, which I use a general term ambient noise seismology here, is known as Green’s function retrieval and is often called seismic interferometry in the exploration geophysics community (because of the cross-correlation involved). Major developments include new theories, new data processing techniques, imaging of Earth structures from ambient noise tomography, monitoring of subsurface processes, and characteristics of ambient noise sources. The special issue contains 13 papers on this timely topic from researchers from China, France, Germany, and the U.S.. We hope that the collection can serve as a small window into this exciting field for the vast readership of the Chinese seismological community, while promoting scientific exchanges and communication between researchers in China and western countries. For readers who are interested in learning more about the