Source mechanism of strong aftershocks (M s⩾5.6) of the 2008/05/12 Wenchuan earthquake and the implication for seismotectonics

Abstract

Citation: Zheng Y, Ma H S, Lü J, et al. Source mechanism of strong aftershocks (M s ≥5.6) of the 2008/05/12 Wenchuan earthquake and the implication for seismotectonics.Dozens of >M5, hundreds of >M4, and much more >M3 aftershocks occurred after the 2008/05/12 Wenchuan earthquake, which were well recorded by permanent and portable seismic stations. After relocated with P arrival, the >M3 aftershocks show two trends of distribution, with most of the aftershocks located along the north-east strike consistent with Longmenshan fault system, yet there is a north-west trend around the epicenter. It seems that substantially more aftershocks occur in regions with crystalline bedrocks. Then we collected waveform data from National Digital Seismograph Network and regional seismograph network of China, and employed "Cut and Paste" method to obtain focal mechanisms and depths of the big aftershocks (M≥5.6). While most of those aftershocks show thrust mechanism, there are some strike slip earthquakes in the northern-most end of the rupture. Focal mechanisms show that the events located on the southern part of central Beichuan-Yingxiu Fault (BY) are mainly thrust earthquakes, which is consistent with initial mechanism of the main shock rupture. In the north part the aftershocks along the BY are also dominated by thrust slip, which is quite different from the right slip rupture of the main shock. Around Qingchuan-Pingwu Fault, the focal mechanisms are dominated by right-slip rupture with large depths (~18 km). So we suspected that in the north part the main shock might rupture on two faults: Beichuan Fault and Qingchuan-Pingwu Fault. The complex pattern of aftershock mechanisms argues for presence of a complicated fault system in the Longmenshan area.focal mechanism, seismotectonics, Wenchuan earthquake, cut and paste.

Relocation of the mainshock and aftershock sequences of M S7.0 Sichuan Lushan earthquake

“…The location accuracy is on an order of hundred meters in a small area. Recently, the method has been widely applied to earthquake relocation at home and abroad [21][22][23][24][25][26][27]. The temporal stations installed after the earthquake are quite intensive, and may cause systematic change to the location results after the temporal stations were set up compared with the results before.…”

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

“…When depth is less than 5 km, mature faults are filled with soft fault gouge in the upper part; thus, it is difficult to accumulate enough strain. Recent studies show that the depths of Wenchuan earthquake aftershocks are mostly in the range of 5-20 km [5][6][7][8][9][10]. But in many regions, especially inside an old stable intraplate such as southwest part of Australia (Merkering) and North America, nearsurface stress measurements in the area show relatively high deviatoric stress.…”

Focal Mechanism Solutions of the 2008 Wenchuan earthquake sequence from P-wave polarities and SH/P amplitude ratios: new results and implications

“…As shown by former results Wang et al 2009;Zheng et al 2009Zheng et al , 2010Cai et al 2011;Yi et al 2012), many strike-slip events with compressional directions roughly in E-W are found in WES. They mainly The lower hemisphere projection is adopted, of which the quadrants filled with colorsare extensional quadrants and the white ones are compressional quadrants.…”

“…Soon later, Wang et al (2009) obtained 88 moment tensor solutions. Zheng et al (2009) also determined the FMSs of 10 biggest aftershocks with magnitude M s C 5.6 employing the Cut-And-Paste (CAP) method. Later on, they further showed FMSs of 18 aftershocks with magnitude M s C 5.0 employing the same technique (Zheng et al 2010).…”