Discussion on the overestimated magnitude of the 1920 Haiyuan earthquake

The great 1920 Haiyuan earthquake, resulting in tremendous casualties, ranks as one of the largest and most devastating earthquakes in China. This significant event marks the start of investigating earthquakes through modern scientific approaches in China. Recent studies show that the moment magnitude of the 1920 Haiyuan earthquake is M_W（7.9±0.2）, prominently smaller than the widely known and often cited magnitude M8\raise0.5ex\hbox\scriptstyle 1\kern-0.1em/\kern-0.15em\lower0.25ex\hbox\scriptstyle 2 . This paper reviews the re-calibration and conversion of different types of magnitude in the early developing phase of seismometers and analogue seismographs. Similar to the 1920 Haiyuan earthquake, the magnitude of many large shallow earthquakes that occurred in this period are systematically overestimated due to factors such as developing technology, sparse instrumentation and data, and diverse calibration functions. The moment magnitude, linked to physical parameters of earthquake rupture, is the best magnitude scale. For magnitude is the most commonly used parameter in describing an earthquake’s size and energy and is an essential factor in seismic hazard assessment, bias and errors in magnitude conversion have significant consequences in understanding the spatio-temporal pattern of historical seismicity and the reliability of various products of seismic potential and hazard evaluation. We thus advocate citing revised moment magnitude M_W（7.9±0.2） for the 1920 Haiyuan earthquake in future studies and re-evaluating the magnitude of historical earthquakes in general. With a revised magnitude, the 1920 Haiyuan earthquake is similar in size to the 2008 Wenchuan earthquake （M_W7.9, M_S8.0） and the 2001 Kunlun earthquake （M_W7.8, M_S8.1）.