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A shallow aftershock sequence in the north-eastern end of the Wenchuan earthquake aftershock zone

A shallow aftershock sequence in the north-eastern end of the Wenchuan earthquake aftershock zone
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摘要 Previous studies show that mature faults are filled with fault gouge in the shallow part and thus cannot accumulate enough strain energy for earthquakes. Therefore most earthquakes are deeper than 5 km, except those events occurring on new faults or in intact rocks. From field observation, Wenchuan earthquake is found to rupture the free surface about 200 km, but the rupture may extend underground much further from teleseismic body waves inversion and aftershocks distribution. In the northeastern end of the rupture zone, deep rupture may induce stress increase near the free surface, and trigger shallow earthquakes. An Ms 5.7 aftershock occurred at Qingchuan, northeast end of Wenchuan earthquake fault on July 24, 2008, featuring thrust mechanism with a 3 km source centroid depth. The shallow focal depth is confirmed with the sPL phase recorded at station L0205. As Rayleigh wave is well only developed for source depth less than 1/5 of epicentral distance, the observed large amplitude of Rg at a distance of 15 km implied depth of 3 km or less. Dozens of aftershocks’ sPL waveforms are also analyzed to confirm the source depths less than 3 km. On the other hand, no surface ruptures are found by geological survey or InSAR studies. It is strongly suggested that these aftershock sequences initiate fresh rupture in intact rocks triggered by stress increase from the deep co-seismic rupture of the Wenchuan mainshock. Previous studies show that mature faults are filled with fault gouge in the shallow part and thus cannot accumulate enough strain energy for earthquakes. Therefore most earthquakes are deeper than 5 km, except those events occurring on new faults or in intact rocks. From field observation, Wenchuan earthquake is found to rupture the free surface about 200 km, but the rupture may extend underground much further from teleseismic body waves inversion and aftershocks distribution. In the northeastern end of the rupture zone, deep rupture may induce stress increase near the free surface, and trigger shallow earthquakes. An Ms 5.7 aftershock occurred at Qingchuan, northeast end of Wenchuan earthquake fault on July 24, 2008, featuring thrust mechanism with a 3 km source centroid depth. The shallow focal depth is confirmed with the sPL phase recorded at station L0205. As Rayleigh wave is well only developed for source depth less than 1/5 of epicentral distance, the observed large amplitude of Rg at a distance of 15 km implied depth of 3 km or less. Dozens of aftershocks’ sPL waveforms are also analyzed to confirm the source depths less than 3 km. On the other hand, no surface ruptures are found by geological survey or InSAR studies. It is strongly suggested that these aftershock sequences initiate fresh rupture in intact rocks triggered by stress increase from the deep co-seismic rupture of the Wenchuan mainshock.
作者 Luo Yan Ni SiDao Zeng XiangFang Zheng Yong Chen QiFu Chen Yong LUO Yan1,2, NI SiDao1, ZENG XiangFang1, ZHENG Yong3, CHEN QiFu2 & CHEN Yong2 1 School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 2 Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China; 3 Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China
出处 《中国科学:地球科学英文版》 SCIE EI CAS 2010年第11期1655-1664,共10页 SCIENCE CHINA Earth Sciences
基金 supported by Institute of Earthquake Science China Earthquake Adminis-trator (Grant No.02092410) Knowledge Innovation Program of the Chi-nese Academy of Sciences (Grant Nos.KZCX2-YW-116-1, KZCX2-YW-142) National Natural Science Foundation of China (Grant Nos.40674027,40604004) Earthquake Hazard Reduction Program (Grant No.200808078)
关键词 WENCHUAN EARTHQUAKE SHALLOW EARTHQUAKE FOCAL DEPTH wave modeling Wenchuan earthquake shallow earthquake focal depth wave modeling
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