Difference in determination of surface wave magnitude of the M7.8 Kermadec Island earthquake on March 5，2021 in New Zealand
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摘要: 2021年3月5日新西兰克马德克群岛发生M7.8地震，本文计算了全球台网692个台站和中国地震台网全球监测台网112个台站的面波震级，并对此震级进行了偏差分析。结果表明：由于面波震级测定公式的不同，我国和美国测震的面波震级偏差为0.2；面波震级的测定受震源破裂过程的影响，由于此次地震主要破裂方向沿北东向，地震的多普勒效应导致震中北东向的台站面波震级偏大，同时在震中北西向存在较小的滑动量，导致北西向的台站面波震级略低于北东向；而震中西南和东南方向的震动较弱，面波震级较小，地震速报受限于时效性，速报中测定面波的台站基本位于震中以南方向，故测定震级偏小。Abstract: 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.
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Key words:
- global networks /
- China seismic networks /
- surface wave magnitude /
- Doppler effect
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图 1 台站分布图
黄色五角星表示震中，蓝色三角表示USGS全球台网，红色三角表示中国地震台网中心全球地震监测台网，绿色三角表示中国地震台网，黑色虚线表示面波在震后27分钟的波前
Figure 1. Distribution of stations
The yellow stars denote the mainshock，the blue triangles denote USGS global network，the red triangles denote global monitoring network using by China Earthquake Networks Center，the green triangles denote China seismic network， the black dash line denotes the wavefront of surface wave 27 min after the origin time
图 2 不同方位角的震级
红色矩形表示USGS台网台站震级，绿色圆圈表示地震速报中测定的单台震级，蓝色四边形表示中国台网台站震级，图中的虚线表示计算的平均震级
Figure 2. The magnitude determined from different azimuth
The red rectangle represents USGS station magnitude，the green circle represents the magnitude of a single station determined in earthquake quickly determination，the blue symbol represents the magnitude of the station on the China network，the dashed line in the figure represents the calculated average magnitude
图 3 震级偏差
红色实线表示地震断层面解，黑色虚线表示震中距范围，红色虚线表示地震速报完成时面波波阵面。圆圈表示震级偏差，不同颜色代表不同的偏差值
Figure 3. Magnitude deviation
The red solid line denotes fault planes from focal mechanism inversion；the black dash line denotes epicenter distance； red dash line denotes the surface wave arrival time when the earthquake rapid report is completed by CENC. Circle represents magnitude deviation, different colors represent different deviation values
表 1 国际地震机构速报的克马德克群岛地震参数
Table 1. Seismic parameters of the Kermadec Islands reported by the International Seismological Agency
序号 发震时刻（UTC） 纬度/°S 经度/°W 深度/km 震级 信息来源 1 19:28:32.2 29.51 177.04 10 7.8（MS） CENC 2 19:28:35.0 29.60 176.00 10 8.0（MWP） USGS 3 19:28:35.6 29.54 177.28 25 7.9（MS） EMSC 4 19:28:35.0 29.65 177.16 33 7.8（MS） RAS 5 19:28:32.0 29.51 177.07 10 7.7（MS） IDC 6 19:28:33.0 29.54 177.25 10 7.9（mb） GFZ 
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