Both keeping and removing the appended zero pads after acausal low-cut (high-pass) filtering of earthquake acceleration data by infinite impulse response (IIR) filters (e.g., Butterworth) cause some problems. By applying acausal filtering with a new and a simple technique (backward-forward filtering), the need to append zero pads before filtering is eliminated, and similar results are obtained. However, for some cases, the displacements may not be zero, and, more importantly, they may not have a tendency to go to zero at the end of the record regardless of using or not using zero padding before filtering. IIR filter initialization techniques used in radar and ultrasound technologies for digital signal processing to eliminate the need to append zero pads before filtering are applied to solve this problem. Because those techniques yielded insufficient results, a new initialization technique tailored to low-cut IIR filtering of earthquake acceleration data is proposed. Because processing by filtering is more appropriate for far-field records than for near-fault records, examples are given mainly from far-field records. However, the proposed techniques can be applied to near-fault records equally well, and some examples are also provided from near-fault records in the last part of the study.