TIME/FREQUENCY ANALYSIS OF TERRESTRIAL IMPACT CRATER RECORDS

The terrestrial impact cratering record recently has been examined in the time domain by Chang & Moon (2005). It was found that the ${\sim}26$ Myr periodicity in the impact cratering rate exists over the last ${\sim}250$ Myrs. Such a periodicity can be found regardless of the lower limit of the diameter up to D ${\sim}35km$. It immediately called pros and cons. The aim of this paper is two-fold: (1) to test if reported periodicities can be obtained with an independent method, (2) to see, as attempted earlier, if the phase is modulated. To achieve these goals we employ the time/frequency analysis and for the first time apply this method to the terrestrial impact cratering records. We have confirmed that without exceptions noticeable peaks appear around ${\sim}25$ Myr, corresponding to a frequency of ${\sim}0.04(Myr)^{-1}$. We also find periodicities in the data base including small impact craters, which are longer. Though the time/frequency analysis allows us to observe directly phase variations, we cannot find any indications of such changes. Instead, modes display slow variations of power in time. The time/frequency analysis shows a nonstationary behavior of the modes. The power can grow from just above the noise level and then decrease back to its initial level in a time of order of 10 Myrs.