32 An Automated Despiking Algorithm for Seismic Normal Mode Data

The normal modes spectra of the Earth’s free oscillations contain valuable information on the longest wavelength three dimensional structure of the Earth’s interior. Dahlen (1982) first showed that the optimum record length for measuring eigenfrequencies and decay rates of normal modes using a Hanning taper is 1.1 Q cycles. Since typical modal Q values lie in the range 10 of to 10, optimum window lengths are on the order of days to weeks. However, it is difficult even today to retrieve clean continuous time series spanning days or weeks following a large earthquake: in addition to aftershocks and other seismic events, data spikes due to transient disturbances at the station or temporary data storage failures are often unavoidable. Therefore, it is necessary to edit seismograms prior to the spectral analysis. Spikes are usually diffficult to identify by existing algorithms because they have features which often are quite similar to those of real earthquake signals. The majority of conventional despiking algorithms require empirical tuning of associated parameters. The required parameter tuning often degrades the efficiency of automated algorithms, which in the end do not save much time and effort when compared with manual approaches. Hence, most despiking is done by direct manual editing, which can be a rather monotonous task, especially when one deals with a large dataset. Ideally, to be helpful, an automated despiking algorithm should not require users to manually intervene.