2008年M_(W)7.9汶川地震早期余震频度衰减速率空间分布特征分析

Spatial distribution of early aftershock decay rate following the 2008 M_(W)7.9 Wenchuan earthquake

深入认识余震频度衰减及其影响因素对理解余震触发机理和震后应急救援具有重要意义.然而,由于强震后余震数量庞大,波形相互叠加导致了目录中余震缺失,进而影响了余震频度衰减速率的估计.本研究采用模板匹配技术对2008年M_(W)7.9汶川地震后约50天内连续数据进行扫描,检测余震数量约为模板地震数量的5倍,完备震级由M3.0降低到M2.3.根据余震沿映秀—北川断裂走向优势深度分布的变化,以映秀、汶川、茂县、北川、南坝为界,余震活动大致可分为6个区域.本研究利用最大似然法分别计算了各个区域的余震频度衰减速率,结果显示:以茂县、青川为界,余震频度衰减速率表现出映秀—北川断裂西南段较快、中间段居中、东北段较慢的特征.将余震频度衰减速率和余震空间分布、断裂类型、震后形变、同震形变等进行对比分析,本研究得到以下结论:1、余震频度衰减速率分区与断裂类型分区吻合.其中,映秀—北川断裂西南段余震频度较快衰减速率区域断裂以逆冲为主,中间段为经过各种震源机制类型交替出现的过渡带,映秀—北川断裂东北段余震频度较慢衰减速率区域断裂以右旋走滑为主;2、余震频度衰减速率西南段较快、东北段较慢的空间分布特征与震后形变特征相一致;3、余震频度衰减速率与同震形变相关性不强.结合本研究观测到的同震滑移量较大区域余震分布较少、部分区域余震数目和震后形变空间分布一致性等特征,我们推测汶川地震早期余震可能主要与震后余滑相关,余震频度衰减速率的空间分布差异主要与不同类型断裂震间滑移速率不同相关,震间滑移速率较快的区域余震频度衰减速率较快及震后形变较大.

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(Mc)from 3.0 to 2.3 within~50 days following the 2008 M_(W)7.9 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.