Reliability of the automatic procedures for locating earthquakes in southwestern Alps and northern Apennines (Italy)

International audienceReliable automatic procedure for locating earthquake in quasi-real time is strongly needed for seismic warning system, earthquake preparedness, and producing shaking maps. The reliability of an automatic location algorithm is influenced by several factors such as errors in picking seismic phases, network geometry, and velocity model uncertainties. The main purpose of this work is to investigate the performances of different automatic procedures to choose the most suitable one to be applied for the quasi-real-time earthquake locations in northwestern Italy. The reliability of two automatic-picking algorithms (one based on the Characteristic Function (CF) analysis, CF picker, and the other one based on the Akaike's information criterion (AIC), AIC picker) and two location methods (“Hypoellipse” and “NonLinLoc” codes) is analysed by comparing the automatically determined hypocentral coordinates with reference ones. Reference locations are computed by the “Hypoellipse” code considering manually revised data and tested using quarry blasts. The comparison is made on a dataset composed by 575 seismic events for the period 2000–2007 as recorded by the Regional Seismic network of Northwestern Italy. For P phases, similar results, in terms of both amount of detected picks and magnitude of travel time differences with respect to manual picks, are obtained applying the AIC and the CF picker; on the contrary, for S phases, the AIC picker seems to provide a significant greater number of readings than the CF picker. Furthermore, the “NonLinLoc” software (applied to a 3D velocity model) is proved to be more reliable than the “Hypoellipse” code (applied to layered 1D velocity models), leading to more reliable automatic locations also when outliers (wrong picks) are present

Evaluation of earthquake stress parameters and its scaling during the 2016-2017 Amatrice-Norcia-Visso sequence—Part I

This article has been accepted for publication in Geophysical Journal International ©: The Authors 2019. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Uploaded in accordance with the publisher's self-archiving policy.The Amatrice–Norcia–Visso sequence is characterized by complex behaviour that is somewhat atypical of main-shock–aftershock sequences, as there were multiple large main shocks that continued for months. In this study we focus on the Amatrice sequence (main shock 2016 August 24, Mw = 5.97) to evaluate the apparent stress values and magnitude-dependent scaling in order to improve our knowledge of processes that control small and large earthquakes within this active region of Italy. Apparent stress is proportional to the ratio of radiated seismic energy and seismic moment, and as such, these stress parameters play an important role in hazard prediction as they have a strong effect on the observed and predicted ground shaking. We analyse 83 events of the sequence from 2016 August 24 to October 16, within a radius of 20 km from the main shock and with an Mw ranging between 5.97 and 2.72. Taking advantage of the averaging nature of coda waves, we analyse coda-envelope-based spectral ratios between neighbouring event pairs.We use equations proposed byWalter et al. to consider stable, low-frequency and high-frequency spectral ratio levelswhich provide measures of the corner frequency and apparent stress ratios of the events within the sequence. The results demonstrate non-self-similar behaviour within the sequence, suggesting a change in dynamics between the largest events and the smaller aftershocks. The apparent stress and corner frequency estimates are compared to those obtained by Malagnini and Munaf`o who utilized hundreds of direct S-wave spectral ratio measurements to obtain their results. Although our analysis is based only on 83 events, our results are in very good agreement, demonstrating once more that the use of coda waves is very stable and provides lower variance measures than those using direct waves. A comparison with recent Central Apennines source scaling models derived from various seismic sequences (1997–1998 Colfiorito, 2002 San Giuliano di Puglia, 2009 L’Aquila) shows that the Amatrice sequence source scaling in this study is well represented by the models proposed by Pacor et al. and Malagnini and Mayeda.Published446-4553T. Sorgente sismicaJCR Journa

Seismo-stratigraphic model of the Po Plain (Italy)

The aim of this study is to provide a seismo-stratigraphic model of the Po Plain sedimentary basin (Northern Italy), to be implemented in soil hazard studies at regional scale. The proposed model characterizes the subsoil up to the seismic bedrock depth. Mascandola et al. (2018) identifies the seismic bedrock of the Po Plain in correspondence with a marked increase in the mechanical properties of the subsoil materials, which produces a measurable resonance effect at the surface in the medium-to-long-period range. To map the seismic bedrock depth we relies on an extensive collection of both existing and newly acquired ambient vibration measurements, with the aim of defining the soil resonance frequencies and the shear-wave velocity gradients within the soft sediments above seismic bedrock. Based on the collected data, an empirical regression model that relates the thickness of the soil deposits above the seismic bedrock to their resonant frequency is defined and applied to map the seismic bedrock depth in the Po Plain area. The resultant seismic bedrock map is correlated with depth of the main unconformities recognized inside the Quaternary succession (Regione Emilia-Romagna,ENI–AGIP, 1998; Regione Lombardia, Eni Divisione Agip,2002). The shear-wave velocity model above seismic bedrock is derived through the interpolation of 51 S-wave velocity profiles selected after a quality check on the available data. The velocity gradients highlights two different zones inside the study area: one at Northwest and another at East-Southeast with higher and lower velocity gradients respectively. To compute the soil amplification functions, the velocity model is discretized into a grid. For each grid node, a 1D soil model is defined and a numerical ground response analysis is carried out. The gridded soil amplification model is checked at those sites with both borehole and surface seismic sensors by comparing the theoretical and empirical soil amplification functions. These results will be included in regional seismic hazard studies, to account for soil amplification in seismic hazard estimates.PublishedParma5T. Sismologia, geofisica e geologia per l'ingegneria sismic