The effects of data quality in local earthquake tomography: Application to the Alpine region

Abstract Despite the increase in quality and number of seismic stations in many parts of the world, accurate timing of individual arrival times remains crucial for many tomographic applications. To achieve a data set of high quality, arrival times need to be picked with high accuracy, including a proper assessment of the uncertainty of timing and phase identification, and a high level of consistency. We have investigated the effects of data quantity and quality on the solution quality in local earthquake tomography. We have compared tomographic results obtained with synthetic and real data of two very different data sets. The first data set consisted of a large set of arrival times of low precision and unknown accuracy taken from the International Seismological Centre (ISC) Bulletin for the greater Alpine region. The second high-quality data set for the same region was seven times smaller and was obtained by automated quality-weighted repicking. During a first series of inversions, synthetic data resembling the two data sets were inverted with the same amount of Gaussian distributed noise added. Subsequently, during a second series of inversions, the noise level was increased successively for ISC data to study the effect of larger Gaussian distributed error on the solution quality. Finally, the real data for both data sets were inverted. These investigations showed that, for Gaussian distributed error, a smaller data set of high quality could achieve a similar or better solution quality than a data set seven times larger but about four times lower in quality. Our results further suggest that the quality of the ISC Bulletin is degraded significantly by inconsistencies, strongly limiting the use of this large data set for local earthquake tomography studies.