Rupture Directivity of the 2021 M-W 6.0 Yangbi, Yunnan Earthquake

The 2021 M-W 6.0 Yangbi, Yunnan strike-slip earthquake occurred on an unmapped crustal fault near the Weixi-Qiaoho-Weishan Fault along the southeast margin of the Tibetan Plateau. Using near-source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment-rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (similar to 144 degrees) over a rupture length of similar to 8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast similar to 8.0 km along strike and similar to 5.0 km downdip with a peak slip of similar to 2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as similar to 1.6x1013J $1.6\times {10}<^>{13}\,\mathrm{J}$. The relatively low moment scaled radiated energy ER/M0 ${E}_{R}/{M}_{0}$ of 1.5 x 10(-5) and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest-southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large-slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.