Rupture Directivity of the May 21, 2021 MW 6.0 Yangbi, Yunnan Earthquake

The 2021 Yangbi, Yunnan earthquake (MS 6.4) occurred in the northern portion of the Red River fault zone, along the southeast margin of the Tibetan Plateau in China. The northern segment of the Red River fault zone contains many subfaults. There are several M6.0+ events widely distributed at north and east of the mainshock, whereas no recorded M6.0+ earthquakes in the past near the 2021 mainshock. The local background seismicity increased since 2013 MS 4.8 and 2017 MS 5.1 events in the north. The 2021 sequence began with several M4 foreshocks 3 days ahead and there was intensive aftershock activity, distributed along the northwest-southeast trend (Figure 1). The regional moment tensor inverted by the Cut and Paste (CAP) method has strike 137°, dip 75°, and rake -160°, with seismic moment of 1.08 x 1018 Nm (MW 6.0) and centroid depth of 6.0 km. We apply the empirical Greens function (EGF) method to investigate the rupture finiteness of the mainshock using numerous nearby broadband seismic recordings. Relative moment rate functions obtained by deconvolving three MW 4.1-4.6 EGFs (Figure 1) show: 1) The mainshock rupture propagates along strike toward the southeast at an azimuth of ~135, consistent with aftershock distribution; 2) The average source duration is about 5 s, and the inferred rupture speed is about 1.5 km/s, with a range of 1.3 - 1.8 km/s. The estimated rupture length is about 5.5 - 7.2 km, which is a small portion of the aftershock distribution.