Spatial distribution of early aftershock decay rate following the 2008 Mw7.9 Wenchuan earthquake
The time-dependent decay of aftershocks and their statistical behaviors are important for unraveling the physical mechanisms of aftershocks and guiding rapid post-mainshock response and rescue efforts.However,the seismicity rate immediately following a mainshock is very high,resulting in overlapping arrivals of different earthquakes and undercounting of early aftershocks.To mitigate the bias introduced by missing aftershocks,we apply the matched filter technique(MFT)to enhance the catalog by a factor of～5 and improve the magnitude of completeness(Me)from 3.0 to 2.3 within～50 days following the 2008 Mw7.9 Wenchuan earthquake.According to depth distribution of detected aftershocks along the Yingxiu—Beichuan Fault,the seismicity can be roughly divided into 6 areas.Using the maximum likelihood method,we calculate the aftershock decay rate,represented by the p-value in the modified Omori's law.The results indicate that,delineated by Maoxian and Qingchuan,the p-value is relatively large(＞1)in the southwest section,close to 1 in the middle section and relatively small(p＜1)in northeast section.By comparing the spatial distribution of the decay rate with the fault types,aftershocks,co-seismic and post-seismic deformation,the results reveal that:(1)The southwest section of the fault,characterized by the larger p-value,is dominated by the thrust type;the northeast section of the fault,with the smaller p-value,is dominated by the strike-slip type;while the middle section of the fault with medium p-value serves as a transition zone featuring various focal mechanism types;(2)The areas exhibiting larger p-value correlates well with the regions experiencing more post-seismic deformation;(3)The aftershock decay rate in the area is unrelated to the co-seismic slip of the Wenchuan mainshock.Considering the characteristics of fewer aftershocks distributed in the areas with significant co-seismic slip and the consistency in spatial distribution between the number of aftershocks and post-seismic deformation in some regions,we suggest that the early aftershocks following the Wenchuan earthquake may be related to the afterslip.The variation in post-seismic deformation and aftershock decay rate in different regions may primarily stem from different interseismic slip rates among different fault types,regions with high interseismic slip rates exhibit elevated aftershock decay rates and substantial post-seismic deformation.
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