Difference in determination of surface wave magnitude of the M7.8 Kermadec Island earthquake on March 5，2021 in New Zealand

A magnitude M7.8 earthquake struck Kermadec Island, New Zealand on March 5, 2021. In this study we determined surface wave magnitude using 692 stations from USGS global networks and 112 stations from China global earthquake monitoring system, then analyzed the difference. Results reveal that: China’s surface magnitude is always 0.2 larger than America’s due to different formula in calculating surface wave magnitude; Surface wave magnitude determination is effected by the rupture process of the mainshock. As the major slip occurred in the northeast direction, those stations with larger magnitude are mainly in the northeast direction of epicenter due to the seismic Doppler effect. Moreover, there is a small amount of slip in the northwest direction of the epicenter, resulting in the surface wave magnitude of the station in the northwest direction being slightly lower than that in the northeast direction; While weaker vibration is found in the south of the epicenter resulting smaller surface wave magnitude. The earthquake quickly determination by CENC was issued in 27 minutes after mainshock with relatively small surface magnitude since the station that measured the surface wave is basically located south of the epicenter.