Imaging of seismic sources using local earthquake data

The purpose of seismic imaging methods is in general to make images of the crust, the mantle, and the core. Recently, a new way of seismic imaging has been introduced: time-reversal of seismic waves which imaged the complex rupture mechanism of the 26 Dec. 2004 Sumatra Mw 9.3 earthquake with very long period seismograms (T>200s). The method is based on the time-reversal invariance and the spatial reciprocity of the wave equation, which has been widely used to image anomalous features of a medium in medical imaging, non destructive testing and petroleum exploration. Seismogram is the record of waves whose energy propagated from the seismic source. If time-reversed seismogram propagates back into the medium, seismic energy converges into the source location at the origin time of the event. With local earthquake data, however, we get incomplete images of the reversed wave and thus it is hard to get clear focusing. Thus, in this study, we use only the travel time information of the P wave for time-reversal seismic imaging. Impulsive wavelets at seismic stations propagate back into the medium according to reversed P wave travel time. When it comes to the origin time of an earthquake, we get clear focusing at the location of the earthquake. As the heterogeneity and 3-D property of local structure may affect to location a local earthquake, we feel that this technique has the potential to become a cooperative part of such a system.