Analysis on magnitude deviation of the 2013 Lushan, Sichuan, M_S7.0 earthquake from Automatic Earthquake Location and Report System and its improvement

The procedure of automatic determination of magnitude for the 20 April 2013, Lushan M_S7.0 earthquake is roughly reproduced by using the waveforms from 40 stations within epicentral distance of 320 km in this paper. The results demonstrate that the magnitude is rising quickly when the parameters of this earthquake were issued by China Earthquake Administration (CEA). At the moment, the measured standard magnitude M5.8 is consistent well with the M5.9 announced by CEA at almost the same time, but it is much smaller than M7.0 revised by the analyst later. The magnitude determined by Automatic Earthquake Location and Report System deviates largely, and the reasons are: ① The usage of clipped records underestimated the magnitude; ② Rapidness is emphasized and the accuracy is neglected. S (or Lg) wave is absent at some stations when the earthquake parameters are issued, resulting in the magnitude smaller. Therefore, by reducing the effect of clipped records within a proper epicentral range and delaying some seconds, the magnitude will be 6.8 at 137 s after occurrence of the earthquake. Additionally, the determination method of M_WP is applied and M_W6.8 is available at 77 s after the earthquake, suggesting the determination method of M_WP can be applied to get a stable moment magnitude rapidly. Finally, we propose the following measures for improving the method for magnitude determination by Automatic Earthquake Location and Report System. ① The determination method of M_L is used for the events with magnitude less than 7.0. In this case, M_L should be determined with seismograms without clipped records, selected from the stations within a proper epicentral range in an area where the number of seismograms without clipped records is absolutely predominant. Meanwhile, the determination method of M_L should be delayed until the furthest station receives the maximum amplitude of S (or Lg) wave. ② The method for the M_WP determination is able to be applied to determine the moment magnitude for large earthquakes.