CHINESE NATIONAL NETWORK MAGNITUDES, THEIR RELATION TO NEIC MAGNITUDES, AND RECOMMENDATIONS FOR NEW IASPEI MAGNITUDE STANDARDS

Abstract

We investigate the linear regression relationships between common seismic magnitudes determined by the Chinese Earthquake Network Center (CENC) and compare them with related magnitude determinations for the same events at the U.S. Geological Survey's National Earthquake Information Center (NEIC). Despite their generally good agreement some systematic differences are revealed. These differences are due to differences in seismograph response (shape and bandwidth), the time window for measurement of maximum P-wave amplitudes, the period and distance ranges used, and, in part, also the different calibration functions applied. Chinese broadband body-wave magnitude mB, compared with the NEIC short-period Pwave magnitude mb, is much less prone to magnitude saturation. Thus it is more suitable to assess the size of large earthquakes from P waves. Also, following International Association of Seismology and Physics of the Earth's Interior (IASPEI) recommendations of 1967, Chinese surface-wave magnitude MS is determined in a wider distance (1° < Δ 180°) and period range (3 sec < T < 30 sec) than MS(20) at NEIC (20° ≤ Δ ≤ 160° and 18 sec ≤ T 7le; 22 sec, respectively). Chinese MS for small and medium earthquakes at regional distances between 2° < Δ < 10° scales well with local magnitude ML. In contrast, NEIC MS(20) tends to underestimate the magnitude of regional events when the IASPEI-recommended MS calibration function by Vaněk et al. (1962) is used. These findings support some of the new standards for magnitude measurements from digital data adopted at the IASPEI meeting in 2005. They include, complementary to band-limited mb and MS(20), the determination of mB and MS(BB) measured on unfiltered broadband records.