Easy detection for the high-pass filter cut-off frequency of digital ground motion record based on STA/LTA method: a case study in the 2008 Wenchuan mainshock

The goal of this work is to propose a new strategy for automatically quantifying the high-pass cut-off frequency of digital ground motion (GM) records. In previous studies, the high-pass filter cut-off frequency of GM records was often identified by visual inspection. For this, this article proposes a simple and efficient method to detect a correct cut-off frequency where the displacement waveform exactly does not change significantly. The displacement change is delineated as the change rate of the displacement at the ending of padded record with respect to the cut-off frequency. Based on the Short-Term-Average to the Long-Term-Average (STA/LTA) method, detecting the ratio of calculated average energy in two consecutive moving-time windows above a threshold, a critical frequency where the ending displacement starts deviating from zero level can be identified. Nevertheless, the cut-offs may be affected by GM intensities even though meticulous setting for window length or trigger threshold is adopted. In this paper, a new characteristic function (CF) on the basis of a modified cumulative envelope function is proposed. It has an appealing advantage to detect the critical cut-off frequency based on the CF shape and free of considering the intensity difference among GM records. Finally, the comparison of results with the cut-off frequencies of GMs from the 2008 Wenchuan mainshock calculated by the proposed method and those derived from two traditional methods is presented. Influence of the cut-off frequency on ground motion displacement, elastic, and inelastic spectrum is also studied.