Command: bp p 1 n 4 c 0.02 0.1 & bp p 2 n 4 c 0.02 0.1  

      Some caution must be exercised in applying these filters. First, all recursive filters have non-linear phase, which can result in some dispersion of filtered waveforms. For applications where the phase of the resulting filtered waveform is important, a zero-phase implementation of the recursive filters is provided. Zero-phase filtering is possible by running the filter forward and backward over the data, instead of just forward over the data. This two-pass operation results in a effective filter magnitude response which is the square of the original magnitude response (Here is what the p 2 means). It also results in a non-causal filter impulse response, which can leave a signal containing a sharp time onset with a ringing precursor. For this reason, you should not measure arrival times of data that has been filtered using this two-pass option. For cases where signal precursors cannot be tolerated, such as onset picking operations, it may not be a good idea to do two-pass filtering (p 1 is then preferable in this case). Second, the filters can become numerically unstable if the width of the filter pass band is very small compared to the folding frequency of the data. The problem is only aggravated by increasing the number of poles in the filter. Situations that seem to require an exceptionally narrow band filter can be handled more reliably by decimation, filtering with a filter of more moderate band width, and interpolation to the original sampling rate. Recourse to this resampling strategy should be made when the filter band width drops below a few percent of the folding frequency (Excerpts from the SAC Command Reference Manual)

      In most cases the p 1 is the preferable option to use, except the studies that particularly care about the phase shift, such as ambient noise cross-correlation, for example.