Probabilistic Completeness Assessment of the Past 30 Years of Seismic Monitoring in Northeastern Italy

Stefania Gentili, Monica Sugan, Laura Peruzza, & Danijel Schorlemmer

Submitted 2010, SCEC Contribution #1326

We investigate the detection probabilities and recording completeness of the seismic network in northeastern Italy, operated by the OGS (Istituto Nazionale di Oceanografia e Geofisica Sperimentale) during the last 30 years (6 May 1977–31 December 2007). This regional seismic network detected more than 39,000 local events, among which about 16,500 have been located and magnitudes have been computed. Completeness of this dataset is varying in space and time due to the evolution of network geometry, instrumental characteristics, and monitoring and processing strategies during earthquake sequences. We therefore subdivided the whole period into four time windows, roughly corresponding to the main changes in the acquisition system. The Probability-based Magnitude of Completeness (PMC) method by Schorlemmer & Woessner (2008) is based on empirical data and requires, besides the earthquake catalog containing phase picks, knowledge of the locations and on-/off-times of the stations, and of the attenuation relation used to compute event magnitudes; because on- and off-times were not systematically reported by the network, we reconstructed them by merging the available information on instruments with the analysis of inter-pick times at each station. We further revised the relationship between duration and local magnitudes. Although magnitudes were computed throughout the 30 years of operation using the same equation based on signal duration (MD), we found inhomogeneities: our regression analysis shows a different MD-ML relation in distinct time periods depending on the quality of data readings (traces read on paper trails or digitally processed). The maps of probability-based magnitude of completeness show large fluctuations in space and time. They are consistent with the network evolution, with the exception of the first period of digital recordings (1988–1994) for which a bias cannot be completely fixed without large reprocessing of the original waveform recordings.
We introduced some constraints on the application of the PMC method to improve its performance for networks with sparse data; however isolated clusters with higher detection probabilities at various stations have been detected and still persist. Such anomalies have been briefly investigated suggesting azimuthal effects. They are beyond the scope of this study. In addition, we show the role of missing events and of temporary networks on regional completeness maps, by a comparison with a catalog of a local and dense network.
The results demonstrate that within its authoritative region, as of 31 December 2007, the OGS network is detecting earthquakes of magnitude 1.5 completely for a large part of the Friuli-Venezia-Giulia region. The completeness magnitude increases towards the Veneto region and the coastline. In northeastern Italy, the OGS regional network is able to match the best performance of the Italian National Seismic Network. In some places, this high quality of recordings lasts since more than 30 years.

Gentili, S., Sugan, M., Peruzza, L., & Schorlemmer, D. (2010). Probabilistic Completeness Assessment of the Past 30 Years of Seismic Monitoring in Northeastern Italy. Bulletin of the Seismological Society of America, (submitted).