Stress Reorientation by Earthquakes Near the Ganzi-Yushu Strike-Slip Fault and Interpretation with Discrete Element Modelling

Earthquakes are generally known to alter the stress field near seismogenic faults. Observations using YRY-four-gauge borehole strainmeters within Yushu (YSH) borehole near the Ganzi-Yushu fault in eastern Tibetan Plateau shows that the azimuth variation of maximum horizontal stress (S-H) first decreased and then increased substantially when the earthquakes occurred during the measurement period from January 1, 2009 to December 31, 2018. In this period, 38 earthquakes (M >= 3) were detected near the fault and the S-H orientation showed a drastic change after the 2010 Ms 7.3 Yushu mainshock. We present a discrete element modelling using Particle Flow Code 2D (PFC2D) to simulate a dynamic fault rupturing process and to use the modelling results for interpretation of the stress reorientation. The modelling reveals that dilatation and compression quadrants are formed around a fault rupturing in strike-slip model, resulting in different spatiotemporal changes of the orientation of maximum horizontal stress (Delta theta). The value of Delta theta in the compression quadrants shows a sharp drop at the time of coseismic slip, then approaches slowly to an asymptotic value. In the dilatation quadrants, Delta theta drops by coseismic slip, then increases sharply and finally reaches a stable value. The modelled Delta theta by coseismic fault slip agrees with in-situ observations at YSH borehole during 2010 Ms 7.3 Yushu mainshock. It is also found that, the value of Delta theta decreases with increasing distance from the rupturing source. We modelled the effect of fault geometry and host rock properties on the Delta theta, and found that structural complexity and off-fault damage by coseismic fault slip have significant impact on the stress field alteration near the rupturing source.

Automated summary

Earthquakes near seismogenic faults can alter the stress field, leading to changes in the orientation of maximum horizontal stress. Modeling dynamic fault rupturing processes reveals that compression and dilatation quadrants are formed around the fault during slip events, causing different spatiotemporal changes in stress orientation.