ALBERT

Description: The impact on probabilistic ground-motion hazard of different definitions of the horizontal component of ground shaking is examined. The scope is to highlight how such a minor detail within the complex computation chain of a probabilistic seismic-hazard assessment can play a crucial role on final results. This is achieved by comparing hazard maps produced for Italy considering different definitions of the ground-motion component at different spectral periods. In our exercise, special attention is paid to the treatment of the aleatory variability of ground motion (sigma) when one switches from one metric to another. The results show that differences in the definition of the horizontal component could imply differences in the hazard results as large as 40%. Online Material: Color maps of geographical distributions of hazard value ratios.

Description: 〈span〉〈div〉ABSTRACT〈/div〉The compilation of reliable and complete seismic catalogs represents a fundamental issue for most studies in seismology. Nowadays, the availability of an ever‐increasing number of stations and, therefore, the huge amount of recordings to be processed and analyzed require a lot of effort in terms of man‐hours. In the present work, we present a fully automatic procedure for compiling seismic catalogs starting from continuous recordings. The procedure relies on a multistep algorithm that includes event detection tool, automatic 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐phase picker, hypocenter locator, and magnitude and strong‐motion parameter calculator. This automatic procedure is applied for compiling seismic catalogs for two real‐world usage scenarios starting from the open‐access waveform database provided by European Integrated Data Archive. The first scenario concerns the monitoring of the seismicity of northwestern Italy; the second one concerns the analysis of the data recorded during the first month of the 2016 sequence in central Italy. The comparison between reference manually revised and automatic seismic catalogs points out negligible differences in terms of both 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐phase pickings, hypocentral coordinates, and local magnitude values, thus showing the overall reliability of the procedure. The ability of the proposed automatic procedure in detecting and locating very low‐magnitude events is prominent to compile automatic catalogs characterized by a magnitude of completeness significantly lower than that of reference manual catalogs.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The compilation of reliable and complete seismic catalogs represents a fundamental issue for most studies in seismology. Nowadays, the availability of an ever‐increasing number of stations and, therefore, the huge amount of recordings to be processed and analyzed require a lot of effort in terms of man‐hours. In the present work, we present a fully automatic procedure for compiling seismic catalogs starting from continuous recordings. The procedure relies on a multistep algorithm that includes event detection tool, automatic 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐phase picker, hypocenter locator, and magnitude and strong‐motion parameter calculator. This automatic procedure is applied for compiling seismic catalogs for two real‐world usage scenarios starting from the open‐access waveform database provided by European Integrated Data Archive. The first scenario concerns the monitoring of the seismicity of northwestern Italy; the second one concerns the analysis of the data recorded during the first month of the 2016 sequence in central Italy. The comparison between reference manually revised and automatic seismic catalogs points out negligible differences in terms of both 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐phase pickings, hypocentral coordinates, and local magnitude values, thus showing the overall reliability of the procedure. The ability of the proposed automatic procedure in detecting and locating very low‐magnitude events is prominent to compile automatic catalogs characterized by a magnitude of completeness significantly lower than that of reference manual catalogs.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The size of an earthquake can be defined either from the seismic moment (M〈sub〉0〈/sub〉) or in terms of radiated seismic energy (E〈sub〉r〈/sub〉). These two parameters look at the source complexity from different perspectives: M〈sub〉0〈/sub〉 is a static measure of the earthquake size, whereas E〈sub〉r〈/sub〉 is related to the rupture kinematics and dynamics. For practical applications and for dissemination purposes, the logarithms of M〈sub〉0〈/sub〉 and E〈sub〉r〈/sub〉 are used to define the moment magnitude M〈sub〉w〈/sub〉 and the energy magnitude M〈sub〉E〈/sub〉, respectively. The introduction of M〈sub〉w〈/sub〉 and M〈sub〉E〈/sub〉 partially obscure the complementarity of M〈sub〉0〈/sub〉 and E〈sub〉r〈/sub〉. The reason is due to the assumptions needed to define any magnitude scale. For example, in defining M〈sub〉w〈/sub〉, the apparent stress (i.e. the ratio between M〈sub〉0〈/sub〉 and E〈sub〉r〈/sub〉 multiplied by the rigidity) was assumed to be constant, and under this condition, M〈sub〉w〈/sub〉 and M〈sub〉E〈/sub〉 values would only differ by an off-set which, in turn, depends on the average apparent stress of the analysed dataset. In any case, when the apparent stress is variable and, for example, scales with M〈sub〉0〈/sub〉, the value of M〈sub〉E〈/sub〉 derived from M〈sub〉w〈/sub〉 cannot be used to infer E〈sub〉r〈/sub〉.In this study, we investigate the similarities and differences between M〈sub〉w〈/sub〉 and M〈sub〉E〈/sub〉 in connection with the scaling of the source parameters using a dataset of around 4700 earthquakes recorded at both global and regional scales and belonging to four datasets. These cover different geographical areas and extensions and are composed by either natural or induced earthquakes in the magnitude range 1.5 ≤ M〈sub〉w〈/sub〉 ≤ 9.0. Our results show that M〈sub〉E〈/sub〉 is better than M〈sub〉w〈/sub〉 in capturing the high-frequency ground shaking variability whenever the stress drop differs from the reference value adopted to define M〈sub〉w〈/sub〉. We show that M〈sub〉E〈/sub〉 accounts for variations in the rupture processes, introducing systematic event-dependent deviations from the mean regional peak ground motion velocity scaling. Therefore, M〈sub〉E〈/sub〉 might be a valid alternative to M〈sub〉w〈/sub〉 for deriving ground motion prediction equations for seismic hazard studies in areas where strong systematic stress drop scaling with M〈sub〉0〈/sub〉 are found, such as observed for induced earthquakes in geothermal regions. Furthermore, we analyse the different datasets in terms of their cumulative frequency-magnitude (CFM) distribution, considering both M〈sub〉E〈/sub〉 and M〈sub〉w〈/sub〉. We show that the b values from M〈sub〉w〈/sub〉 (〈span〉b〈/span〉〈sub〉Mw〈/sub〉) and M〈sub〉E〈/sub〉 (〈span〉b〈/span〉〈sub〉ME〈/sub〉) can be significantly different when the stress drop shows a systematic scaling relationship with M〈sub〉0〈/sub〉. We found that 〈span〉b〈/span〉〈sub〉ME〈/sub〉 is nearly constant for all datasets, while 〈span〉b〈/span〉〈sub〉Mw〈/sub〉 shows an inverse linear scaling with apparent stress.〈/span〉

Description: In this work, we test a fully automatic procedure to obtain local earthquake tomography (LET), starting from seismic waveforms and applying the capability of the automatic phase picker and locator engine "RSNI-Picker" ( Spallarossa et al. , 2014 ), which is based on a multistep iterative procedure working on P and S arrival times. This code is currently operating as part of the Earthquake Monitoring System at the University of Genoa (RSNI designates the regional seismic network of northwestern Italy). In particular, we compare P - and S -wave tomographic results obtained using this fully automatic procedure for picking and locations with those based on data from accurate manual picking and revised locations. We use a dataset of 409 earthquakes that occurred in the Trentino region (Northeastern Italy) in the 1994–2007 period. The highly variable waveform qualities (e.g., signal-to-noise ratio), mainly due to recording stations equipped with different types of sensors and digitizers (including both one-component narrowband stations and three-component broadband seismic stations), ensure a severe test for the automatic procedure. The comparison of the two 3D velocity propagation models for the Trentino region (i.e., LET images) from the automatic and manual procedures, shows maximum differences of 0.54 and 0.34 km/s for P and S waves, respectively (if we consider 90% of all the computed absolute velocities, as a reference percentage). The automatic LET shows velocity anomaly distributions and reliability patterns (e.g., resolution diagonal element [RDE] values) similar to those obtained using the manual procedure; 90% of RDE differences are lower than 0.15. The results obtained by testing the RSNI-Picker engine suggest it can be used to automatically process large amounts of seismic recordings in order to identify P and S wavepicks for reliable LET analysis.

Description: In this work, we test a fully automatic procedure to obtain local earthquake tomography (LET), starting from seismic waveforms and applying the capability of the automatic phase picker and locator engine "RSNI-Picker" ( Spallarossa et al. , 2014 ), which is based on a multistep iterative procedure working on P and S arrival times. This code is currently operating as part of the Earthquake Monitoring System at the University of Genoa (RSNI designates the regional seismic network of northwestern Italy). In particular, we compare P - and S -wave tomographic results obtained using this fully automatic procedure for picking and locations with those based on data from accurate manual picking and revised locations. We use a dataset of 409 earthquakes that occurred in the Trentino region (Northeastern Italy) in the 1994–2007 period. The highly variable waveform qualities (e.g., signal-to-noise ratio), mainly due to recording stations equipped with different types of sensors and digitizers (including both one-component narrowband stations and three-component broadband seismic stations), ensure a severe test for the automatic procedure. The comparison of the two 3D velocity propagation models for the Trentino region (i.e., LET images) from the automatic and manual procedures, shows maximum differences of 0.54 and 0.34 km/s for P and S waves, respectively (if we consider 90% of all the computed absolute velocities, as a reference percentage). The automatic LET shows velocity anomaly distributions and reliability patterns (e.g., resolution diagonal element [RDE] values) similar to those obtained using the manual procedure; 90% of RDE differences are lower than 0.15. The results obtained by testing the RSNI-Picker engine suggest it can be used to automatically process large amounts of seismic recordings in order to identify P and S wavepicks for reliable LET analysis.