A High-Resolution Seismic Catalog for the Southern Apennines (Italy) Built Through Template-Matching

The incompleteness of earthquake catalogs is a well-known issue caused by our technical limitation in detecting the small-to very small-magnitude seismicity falling near or below the background seismic noise. The detection of small-magnitude events is fundamental for improving our knowledge of geometry and kinematics of seismogenic sources and the spatio-temporal characteristics of seismicity, thus leading to better models for seismic hazard. Template-matching (TM) is a powerful technique that, based on similarity measure (cross-correlation) of seismic waveforms, allows to detect hidden earthquakes that are similar to known events (called templates). The high computational effort often limits such technique to small areas and for short time frames (less than 1 year). In this work, we present the first application of template-matching at regional scale for the Italian Peninsula, focusing on the Southern Apennines. We use about 3,600 high-quality events as templates, scanning 6-year long continuous recordings (2009-2014), at more than 180 stations of the INGV network. About 20,000 new events are found, showing a comparable quality to the template catalog in terms of hypocentral solution, reaching a decrease of the magnitude of completeness of about one unit. To highlight the improved quality of the TM catalog, we report two main examples regarding the Sannio-Matese area, where TM allowed us to unravel relevant details on the spatio-temporal distribution of the local seismicity. The natural and man-made seismic noise is commonly higher than the signal of small-size earthquakes, which represents the vast majority of the seismic activity. Identifying these hidden events is crucial for gaining a better understanding of fault structures, their location, and overall characteristics. A powerful technique for the retrieval of hidden seismicity is template-matching, based on the comparison of known earthquakes with the continuous recording of a network of seismic stations. However, due to the high computational demand, the use of template-matching is usually limited to small areas (e.g., volcanoes) and/or small time periods. In this study, we provide the first application of template-matching at regional scale for the Italian Peninsula, specifically in the area of Southern Apennines. We analyze 6 years of continuous recording from more than 180 stations. Starting from about 3,600 templates, we detected over 20,000 previously hidden earthquakes with uncertainties on the earthquake location that are low and comparable to those of the initial templates. To demonstrate the potential of such an enriched earthquake catalog for future research, we focused on the Sannio-Matese region. For the earthquake sequence in the Matese area that occurred between 2013 and 2014, template-matching revealed a consistent level of background seismicity before the onset of the sequence, whereas no activity was reported in the official catalog. In the nearby Sannio area, we identified several earthquake clusters at unusual depths, indicating a sudden increase in the depth of seismic activity. These observations could significantly contribute to improving the seismic hazard model for the area. First application of template-matching at regional scale in the Southern Apennines (Italy) for the retrieval of hidden seismicity The study results in 20,000 new detections, a five-fold increase with respect to the starting catalog Focusing on the Sannio-Matese area, we show how such a high-resolution catalog enhances the spatio-temporal characterization of seismicity