Did the core phase ScS of the Wenchuan earthquake trigger its first <italic>M</italic>6 aftershock?

Abstract

The temporal pattern and generation mechanism of strong aftershocks are important for seismic hazard mitigation purposes, as aftershocks usually hamper rescue efforts and cause further damage to ground and buildings that have been weakened by mainshocks. Previous studies have demonstrated that both static stress change and dynamic stress can trigger aftershocks. Surface waves are thought to be effective in triggering seismicity, and there are limited studies of the triggering capability of far field body waves. About 15.5 min after the 2008 M 7.8 Wenchuan earthquake, an M 6 aftershock occurred, and it has been proposed that it was triggered by the core reflected ScS phase of the mainshock. It is important to study this proposal in detail because it is the first case of a strong aftershock triggered by core reflected body waves. In this paper, we investigate the effects of ScS triggering with ray-base theory. We calculate the geometrical spreading factor of ScS at short distances and derive equations relating moment tensor, focal depth and displacement of ScS. For very short epicentral distances, theoretical analysis demonstrates that the displacement of ScS is almost zero on the vertical component, while the ScS east-west and north-south components are in proportion to the M ze and M zn of the moment tensor respectively. This prediction is consistent with observations of ScS at station ENH. However, the purported ScS signal at station CD2 is very strong on the vertical component, which is inconsistent with the theoretical prediction. From the moment tensor of the Wenchuan mainshock from GCMT, we estimate that the ScS displacement is about 1 mm and particle velocity about 0.01 mm/s, consistent with the observations at station ENH. We also calculate synthetic seismograms of ScS and compare them with observations, and confirm that ScS is observed at station ENH. But the long period signal at station CD2 does not agree with the theoretical ScS waveforms; this could have resulted from the filtering of clipped waveforms of the M 6 aftershock. The M 6 event was relocated by modeling the P wave polarity and the differential travel time between P and S waves. Broadband synthetic seismograms of the M 6 event recorded at CD2 were also computed, assuming a focal depth of 14 km and strike 90 ° , dip 25 ° and rake 110 ° . The synthetic seismograms agree well with the observations in amplitude, polarity and timing of P and S waves, suggesting that the source parameters of this event are reliable. Furthermore, we find that the M 6 aftershock actually occurred before the ScS arrival, thus invalidating the hypothesis that the mainshock ScS triggered the M 6 aftershock. The stress due to ScS for receivers at different periods from 10−50 s was also calculated, and the stress change was found to be less than 1 kPa for periods longer than 30 s. The relatively weak dynamic stress due to ScS results from the free surface boundary condition, which requires that some components of the stress must approach zero for shallow receivers. Further, the boundary condition leads to a difference in the dynamic triggering capability of horizontally propagating surface waves and almost vertically propagating body waves, because the latter involves of direct body waves from deep Earth and surface-reflected waves, which interact to reduce the amplitude of the dynamic stress. Since previous studies demonstrated that seismic waves with periods longer than 30 s are more effective in triggering seismicity, it is probable that the dynamic stress caused by ScS is too weak to trigger aftershocks. In summary, we propose that ScS from the Wenchuan earthquake did not trigger the strong aftershock, and this has been confirmed with analysis of the 2005 Kashmior and the 2015 Nepal earthquakes in the Tibetan Plateau. The core reflected phase ScS of great continental earthquakes may have low potential to trigger strong aftershocks. Further studies of the triggering potential of ScS for subduction earthquakes are needed, as statistics on lag time of strong aftershocks suggest a peak around the ScS arrival time, though the case of ScS triggering for Wenchuan earthquake did not hold to be viable.