The symmetric impulse response of linear phase Finite Impulse Response (FIR) filters which are most commonly used in modern seismic recording systems produces precursory signals to impulsive arrivals. These acausal filter generated artifacts may cause misinterpretations of various onset properties. Prior to any onset interpretation, these effects have to be removed from the seismic record. This can be achieved without loss of bandwidth by post-filtration of the digital seismograms if the filter coefficients and the decimation ratios are known. We have analyzed numerous signals from different instruments and sampling rates for precursory phases and found that - in contrast to common belief - FIR filter related precursory signals are not always easy to recognize visually based on their waveform signature. Furthermore, they can exhibit surprisingly similar properties as those reported for nucleation phases although the majority of nucleation phases reported in the past have been obtained on instruments with a causal response. We demonstrate examples of filter related precursory signals for events scanning nine orders of moment, from 10 10 Nm to 10 19 Nm. Surprisingly, the lower bound of the artifact durations as a function of seismic moment scales close to the cube root of the seismic moment. We interpret this as being caused by the fact that above a certain seismic moment, the attenuated source signal acts as a causal low pass filter of smaller bandwith than the FIR filter. Assuming a source model, constant stress drop, and an empirical relationship between maximum artifact duration and cutoff f requency of the FIR filter, the artifact durations are expected to scale proportional to the 1/2.5 power of the seismic moment in comparison to 1/3 as proposed for nucleation phases.
Key words: waveform analysis, onset properties, instrumentation