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

Abstract

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 M s 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.Wenchuan earthquake, shallow earthquake, focal depth, wave modeling Citation:Luo Y, Ni S D, Zeng X F, et al. A shallow aftershock sequence in the north-eastern end of the Wenchuan earthquake aftershock zone.On May 12, 2008, an M s 8.0 earthquake occurred at Wenchuan, Sichuan Province, and caused heavy economic loss and casualties. Dozens of strong aftershocks with magnitude ≥5.0 occurred along the Longmenshan faults in the following months. The depth and focal mechanism of the aftershock sequence provided a good constraint on mainshock fault and regional stress field. Focal depth is an important parameter to study regional seismicity and seismic hazard, and also plays a particularly important role in understanding the relationship between seismicity and geological structure. Depths of most intraplate earthquakes are between 5 and 25 km [1][2][3][4]. When depth is larger than 25 km, it is not very easy to accumulate strain because of ductile deformation in the lower crust. 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. Because of strong strength in cold and old

Fault behavior and lower crustal rheology inferred from the first seven years of postseismic GPS data after the 2008 Wenchuan earthquake

“…5d and 5e), which agrees with the near-field postseismic InSAR (Interferometric Synthetic Aperture Radar) observations that reveal short wavelength deformation across this fault segment (Huang et al, 2014). The shallow aftershocks also suggest that the shallow part of this segment was active under the loading of deep coseismic slip (Luo et al, 2010). Combined with this evidence, we infer that the stress on the Qingchuan segment might have been released (Fig.…”

The M5.0 Suining-Tongnan (China) earthquake of 31 January 2010: A destructive earthquake occurring in sedimentary cover

“…Recently, a study of the aftershock sequence of the 2008 Great Wenchuan earthquake also reported events shallower than 5 km in the northeastern tip of the Wenchuan earthquake rupture zone [6]. All these shallow earthquakes took place in the rigid rocks of Paleozoic strata or plutonic bodies, both of which are very strong and accumulate massive strain energy.…”

Determination of focal depth by two waveformbased methods: A case study for the 2008 Panzhihua earthquake

“…Though depth phases such as sPL help to constrain focal depth well, but it is still affected by the velocity model used, though the depth error can be reduced to 1-2 km when a good velocity model is used (Chong et al, 2010;Luo et al, 2010;Wei et al, 2009). Another limitation of using sPL in constraining depth is that sPL can not be observed when SV wave happens to be near the nodal plane.…”