Bias in reported seismic arrival times deduced from the ISC Bulletin

The accurate timing of seismological data is crucial for most quantitative examinations in seismology. We present evidence that traveltime data from m any stations contain systematic variations in timing which can be identifie d by checking the median of station delay times as a function of time. This function is expected to be constant but many deviations are found. Several hundred stations that report arrival times to the ISC have been examined. The median station delay times of almost 8 per cent of these stations show changes of more than 1 s and thus exceed the structural signal in the data. Temporal variations of,0.5-1 s are common. Changes in the distribution of observed earthquakes and other possible explanations of such variations hav e been tested and fail to explain most of the observations. Therefore, the bulk of the observed changes must be caused by flaws in the timing of the d ata or by biased picking of arrival times. For instance, at one station wit h a strong annual variation of noise level, the arrival times are on averag e picked several tenths of a second later during months with a high noise l evel. Because of their systematic nature, these errors will not necessarily cance l out by using the large number of traveltimes in the ISC Bulletin and may therefore introduce a bias in many investigations. If the observed timing v ariations are due to the recording equipment at the stations, the errors wi ll be present in the digital waveform data as well. Tomographic studies cou ld potentially be affected, but in particular studies of temporal variation s of Earth structure based on traveltime data, e.g. inner core rotation, ne ed to be looked at with caution as results might be influenced by station e ffects. The exact nature of the bias is study-dependent and needs thorough investigation in each individual case.