A τ c magnitude estimation of the 20 April 2013 Lushan earthquake, Sichuan, China

A crucial part of proposed earthquake early warning systems is a rapid estimate for earthquake magnitude. Most of these methods are focused on the first part of the P-wave train, the earlier and less destructive part of the ground motion that follows an earthquake. A method has been proposed by using the period of the P-wave to determine the magnitude of a large earthquake at local distance, and a specific relation for the Sichuan region was calibrated according to acceleration records of Wenchuan earthquake. The M w 6.6 earthquake hit Lushan County, Sichuan, on April 20, 2013 and the largest aftershocks provide a useful dataset to validate the proposed relation and discuss the risks connected to the extrapolation of magnitude relations with a poor dataset of large earthquake waveforms. A discrepancy between the local magnitude (M L) estimated by means of τ c evaluation and the standard M L (6.4 vs. 7.0) suggests using caution when M L vs. τ c calibrations do not include a relevant dataset of large earthquakes. Effects from large residuals could be mitigated or removed by introducing selection rules on τ c function, by regionalizing the M L vs. τ c function in the presence of significant tectonic or geological heterogeneity, and by using probabilistic and evolutionary methods.