EmpiricalMw–ML,mb, andMsConversions in Western China

Abstract

Local magnitude ( M L), body‐wave magnitude ( m b), and surface‐wave magnitude ( M s), which are saturated at certain values and may lead to an incorrect energy estimation of a large earthquake, are largely used in quantifying the size of an earthquake in western China. Based on the catalog from the China Earthquake Data Center (CEDC) and moment magnitude ( M w) provided by the Global Centroid Moment Tensor (Global CMT) Project, we test the general orthogonal regression (GOR) and the ordinary least‐squares (OLS) methods in M w– M L, M s, and m b conversions for earthquakes in three different tectonic structures of western China. For M L and m b, linear trends vary according to tectonic structures, which implies different body‐wave attenuations in the three structures. For M s, the results are similar, whether using the GOR or the OLS method, but slopes of regression lines are not close to 1, which indicates routine misestimates of M s in western China. The GOR slopes are uniformly larger than the OLS slopes in all magnitude conversions. Standard deviations are between 0.06 and 0.13 for the GOR method but are between 0.12 and 0.25 for the OLS. Thus, the GOR is found to be superior to the OLS method and its use is recommended. Conversions of different magnitudes to M w not only imply different patterns of seismic‐wave attenuations but will also benefit immediate assessment of seismic damage after occurrence of a destructive earthquake in the future. Online Material: Earthquake catalog and tables showing differences between magnitude estimates for M w, M L, m b, and M s.