Annual Variation of Seismicity due to Surface Loads in Normal Fault Systems in Southern Tibet

Abstract:

It had been found that there are seasonal variations of seismicity related to surface hydrology at Main Himalaya Thrust (MHT) fault. In this work, we analyzed the historical micro-earthquakes recorded by China Digital Seismograph Network (CSDN) in normal faulting systems in southern Tibet, to test whether such a phenomenon exist here and to figure out the possible modulation mechanism. There are several N-S striking normal fault systems (e.g. Yadong-Gulu, Shenza-Dingjie rifts) across the southern Tibetan plateau, which are supposed to accommodate the crust extension induced by Indo-Eurasia collision. The quake catalog covers the time span of 2008-2014. All quake events are relocated using the double-differencing method. The catalog was then declustered using CLUSTER2000 (http://earthquake.usgs.gov/research/software/). The declustered catalog was then averaged for one-month period. The monthly catalog shows that the number of earthquake is maximum during the winter months (from January to March), although the maximum values do not agree for individual years (Fig. 1). Such a variation is similar to that found at MHT. Contrary to the situation at MHT (thrust fault), we found it might be explained directly by surface mass redistributions. The contemporary continuous GPS observations confirm that Tibetan plateau crust moves up and down periodically and reaches its lowest position in summer under the surface hydrological load. According to the Coulomb failure criterion (S=τ-μ(σ_n-pf) , where S is Coulomb Stress, σ_n is normal stress, τ is shear stress), an increase of mass load in summer in Tibet will cause an increase in normal stress at the (gently dipping) fault plane and accordingly a decrease in Coulomb stress, which thus inhibits the occurrence of quakes on those normal fault planes.