Abstract: Focal depths of moderate to small earthquakes and their temporal changes are critical for understanding seismogenic structures, fault characteristics and evolution of seismic stress fields. However, traditional locating methods such as double difference method (HypoDD) require high accurate crustal structure, which is usually hard to be fulfilled. Furthermore, the precisions of these methods can be also seriously affected by seismic station distribution. On the contrary, coda wave interferometry method (CWI) doesn't need to know accurate crustal velocity structure, the only requirement for the crustal model is that there should exist relatively evenly distributed scatters, which is a very common phenomenon in shallow crust. Another advantage of CWI method is that the requirement of seismic stations distribution is also very low. Thus, the CWI method is quite suitable for studying the variation of relative distance between earthquakes. In this paper we collected the data of the 2008 Wenchuan earthquake aftershock sequence, analyzed the possibility and stability of locating earthquakes by CWI method, and finally compared with the results obtained by double difference method. Our result shows that when the distribution of seismic stations are good, the result obtained by double difference method is close to that by the CWI method, however, when the distribution pattern of seismic stations are not good, the CWI method can provide much more robust results than that of double difference method.

Key words: Coda wave interferometry, Similar earthquakes, Location variation of earthquakes, Aftershock sequence of the Wenchuan earthquake