Directionality of Arias intensity in the Wenchuan Mw 7.9 earthquake, China

Abstract

There is a strong correlation between Arias intensity and the possibility of earthquake-induced landslides. Since landslides have distinct sliding directions, it is necessary to investigate the directional characteristics of Arias intensity. The free-field recordings from 40 stations within 200 km of fault distance in the Wenchuan Mw 7.9 earthquake were selected for this study. We discovered that the fluctuation of Arias intensity with azimuth exhibits distinct features of cosine (or sine) curve with a period of 180°, and we proposed a theoretical expression (Eq. 3) of Arias intensity considering directionality using the maximum value (Ia100), the minimum value (Ia00), and the polarization direction (Amax). The maximum-to-minimum ratios of Arias intensity (Ia100/Ia00) decrease as fault distance increases and can be represented as a piecewise linear decreasing relationship that declines significantly faster in the near-middle field than in the middle-far field. The polarization directions (Amax) for stations near the epicenter are consistent with the coseismic displacement directions, which should be further validated by testing various faulting styles; an estimated equation for the azimuth of the projection of the trace of fault movement on the ground surface was employed to predict Amax using the fault's strike, dip, and rake. Finally, we provided a framework for constructing empirical directional model of Arias intensity using the traditional attenuation relationship, the empirical relationship of Ia100/Ia00, and the estimated equation of Amax, as well as thereby a method for predicting Newmark displacement considering directionality. There is an imperfect solution for Amax estimation, other than our work would still serve as a point of reference for future research into the regional directionality of Arias intensity.