Prediction of aftershock characteristics

A large earthquake can trigger a series of smaller earthquakes (i.e., aftershocks) around the epicenter of the first large earthquake (i.e., mainshock).The ground motion characteristics of aftershocks can have a significant impact on collapse risk of post-mainshock structures.However, most current seismic analysis and design do not take into account aftershock risk.This study investigates the aftershock characteristics based on the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation (NGA) West2 (shallow crustal earthquake in active tectonic regimes) ground motion database.Since the classification of mainshock and aftershock is ambiguous and there is lack of consensus on how to define aftershocks, this study classifies the mainshocks and aftershocks by combining the Gardner and Knopoff time window and the centroid Joyner-Boore distance.To investigate the relationship between mainshock and the subsequent aftershocks with maximum magnitude, this study selected 27 MS-AS sequence events.This study finds that the distribution of magnitude differences between mainshock and the largest aftershock is highly positively skewed, indicating that the median magnitude difference is a better measure than its mean value.By using the bootstrap resampling method, the 95% confidence interval of the median magnitude difference is determined to be [0.340,0.635], indicating that the median largest aftershock magnitude is significantly lower than its mainshock magnitude at the 5% significance level.This finding suggests that the widely accepted Bath's law (the magnitude difference between mainshock and the largest aftershock is independent of the mainshock magnitude and is approximately a constant 1.2) may significantly underestimate the magnitude of aftershocks, leading to inadequate aftershock risk assessment.This study also finds that the median largest aftershock duration may be not less than its mainshock duration at the 5% significance level and the mechanism of mainshock may be very different from those of aftershocks.