An automatic modular procedure to generate high-resolution earthquake catalogues: application to the Alto Tiberina Near Fault Observatory (TABOO), Italy.

The Alto Tiberina Near Fault Observatory (TABOO) in the upper Tiber Valley (northern Appennines) is a INGV research infrastructure devoted to the study of preparatory processes and deformation characteristics of the Alto Tiberina Fault (ATF), a 60 km long, low-angle normal fault active since the Quaternary. The TABOO seismic network, covering an area of 120 × 120 km, consists of 60 permanent surface and 250 m deep borehole stations equipped with 3-components, 0.5s to 120s velocimeters, and strong motion sensors. Continuous seismic recordings are transmitted in real-time to the INGV, where we set up an automatic procedure that produces high-resolution earthquakes catalogues (location, magnitudes, 1st motion polarities) in near-real-time. A sensitive event detection engine running on the continuous data stream is followed by advanced phase identification, arrival-time picking, and quality assessment algorithms (MPX). Pick weights are determined from a statistical analysis of a set of predictors designed to correctly apply an a-priori chosen weighting scheme. The MPX results are used to routinely update earthquakes catalogues based on a variety of (1D and 3D) velocity models and location techniques. We are also applying the DD-RT procedure which uses cross-correlation and double-difference methods in real-time to relocate events with high precision relative to a high-resolution background catalog. P- and S-onset and location information are used to automatically compute focal mechanisms, VP/VS variations in space and time, and periodically update 3D VP and VP/VS tomographic models. We present results from four years of operation, during which this monitoring system analyzed over 1.2 million detections and recovered ~60,000 earthquakes at a detection threshold of ML 0.5. The high-resolution information is being used to study changes in seismicity patterns and fault and rock properties along the ATF in space and time, and to elaborate ground shaking scenarios adopting diverse slip distributions and rupture directivity models.