Differences between magnitudes of Taiwan earthquakes from catalogs of Taiwan and Beijing

Taiwan is a region with very high seismicity where hundreds of earthquakes with M≥5 occurred per year and more than 40 with M≥7 since 1900 due to it is located at the boundary of collision between Eurasian and the Philippine Sea plates. The magnitudes for the same event from these two catalogs of Taiwan and Beijing are different, due to different data sources selected and different magnitude scales used. To measure sizes of Taiwan events in uniform magnitude scale, the author adopted a catalog of earthquakes with uniform local magnitude scale (Shin, 1993) compiled by Cheng (1996) in compiling a new catalog of Chinese earthquakes. To measure sizes of Taiwan events, we adopted uniform local magnitude scale for events with 5≤M<6. For event with M≥6 we still adopted IASPEI surface wave magnitude scale. Thus the statistic difference between two magnitude values from Taiwan and Beijing in magnitude interval 5≤M<6 reflects difference between Taiwan uniform local magnitude scale and Beijing surface wave magnitude scale. The statistic difference in magnitude interval M≥6 reflects difference between IASPEI surface wave magnitude scale and Beijing surface wave magnitude scale. Three data sources from “Wang”, “Cheng” and “Chinese mainland” of Taiwan events with M≥7 are given in this paper, in order to obtain a complete catalog. The statistic result indicates that Taiwan magnitude is slightly larger than Beijing magnitude for 998 events with 5≤M<6. The mean difference is about 0.13 of magnitude unit with deviation 0.32. Similarly, the mean difference is about 0.09 of magnitude unit with deviation 0.23 for 298 events with M≥6. We also noted that the difference of magnitude values between Beijing and Taiwan surface wave magnitudes changed in different time period. Before 1971, most Taiwan magnitude values of the events is larger than Beijing’s. In contrast, most Beijing magnitude values of the events are larger than Taiwan magnitude values after 1972. It is probably related to data sources. Three catalogs of events with M≥7 are given in this paper. Wang’s catalog (1995) includes 44 events with M≥7, Cheng’s (1996) 39 events and Chinese catalog, before 1911 compiled by Min (1995), 1912–1990 compiled by Wang, et al (1997) and 1991–1998 compiled by Yang, includes 41 events. The variety of these data sources discussed in this paper can be a useful reference for seismologists who use catalog seismic data do their research. However the statistic results and empirical relations presented in this paper can not be used for converting between magnitude scales, since some of them are not the original observation values. In addition, three catalogs of events with M≥7 given in this paper can be used for seismicity research comprehensively. Contribution No. 99FE2023, Institute of Geophysics, China Seismological Bureau. This study is sponsored by the Project 95-05-03-01 from China Seismological Bureau.