OSCILLATORY TENDENCY IN AFTERSHOCKS MIGRATION PATTERN

We have analyzed the angular changes in inter-event direction of seven aftershock sequences and four seismic catalogs as well as a set of acoustic emission data from two triaxial compression tests. For each of the datasets, an inter-event direction was calculated from the coordinates of two successive events; the angular change in inter-event direction is given by the difference in the azimuths of two consecutive inter-event directions. A probability distribution of the angular changes of inter-event direction was derived for each of the datasets. The results from all of the studied data reveal a strong tendency for successive events to occur in a direction opposite to the previous inter-event direction, corroborating the observation reported by Zhou et al. (in review). This oscillatory tendency may be statistical in nature or attributable to the dynamics of faulting events. For the first conjecture, the occurrence of an event on a fault of a finite length essentially dissects the fault into two segments of uneven length. The next event thus has a greater probability to occur within the longer segment, yielding an interevent direction opposite to the previous one. This explanation, however, necessarily assumes the aftershock locations are randomly distributed along a fault. For the second conjecture, the occurrence of an event at a certain point along a fault could modify the stress configuration, causing a greater likelihood for the subsequent event to occur on the other end of the fault. The oscillatory tendency in the aftershock locations is thus essentially governed by the Coulomb stress of fault rupture. In either case, the study has unraveled a pattern of aftershock migration with information pertinent to seismo-dynamics as well as earthquake prediction.  

Zhou, Y., Leung, Y. & Chan, L. S., Oscillatory tendency of interevent direction in earthquake sequences. Submitted to Physical Review E, in review.