ALBERT

Description: We present an approach to infer the slip and rupture velocity distributions on the fault plane from the non-linear inversion of the apparent source time functions, obtained from the empirical Green's function deconvolution method. The main advantage of this technique is that it allows overcoming, in the forward modelling, the limitations related to the computation of the Green's function, as the choice of a correct and reliable earth propagation model. We perform a parameter resolution and uncertainty study, which is based on the analysis of the misfit function in the neighbourhood of the best-fitting model. In this paper, we present the results obtained by applying the technique to synthetic and real records from an M w 4 event which occurred during the 2009 L’Aquila (central Italy) aftershock sequence. Results show a heterogeneous slip distribution, characterized by two main high slip patches located NW of the hypocentre and an average slip of 3.7 cm, corresponding to a seismic model of about 0.82  x 10 15 Nm.

Description: The space–time distribution of coseismic slip of the 2011 February 21, M w 6.2, Christchurch earthquake, New Zealand, is explored, differently from all previous studies, through a joint inversion of geodetic and strong-motion data. The geodetic data consist of both global position system (GPS), from campaign and continuous stations, and synthetic aperture radar (SAR) interferograms from two ascending satellite tracks. The strong motion data consist of 10 stations located in the Canterbury plains, these stations offering a good azimuthal coverage of the event. The kinematic rupture model for the analysed event was obtained using the parametrization and non-linear inversion scheme proposed by Delouis et al. In particular, for any subfault we explore for the local source time function (local slip history), slip direction and rupture onset time. The geometry of the fault plane used for the kinematic inversion is inferred from the analysis of the geodetic data. To validate our results we perform a resolution study for both the single and complete data sets, and an errors analysis of our final kinematic rupture model. Considering the complexity highlighted by superficial deformation data, we adopted a fault model consisting of two partially overlapping segments, with dimensions 15  x 11 and 7  x 7 km 2 , corresponding to different faulting types. This two-fault model, instead of single-fault model, is needed to reconstruct the complex shape of the superficial deformation data. The total seismic moment resulting from the joint inversion is 3.0  x 1025 dyne · cm ( M w  = 6.2) with an average rupture velocity of 2.0 km s –1 .

Description: Automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterized by low signal-to-noise ratio, automated location methods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this paper, we propose a picking free earthquake location method based on the use of the short-term-average/long-term-average (STA/LTA) traces at different stations as observed data. For the P phases, we use the STA/LTA traces of the vertical energy function, whereas for the S phases, we use the STA/LTA traces of a second characteristic function, which is obtained using the principal component analysis technique. In order to locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a 3-D grid, we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event. A pilot application was performed in the Campania-Lucania region (southern Italy) using a seismic network (Irpinia Seismic Network) with an aperture of about 150 km. We located 196 crustal earthquakes (depth 〈 20 km) with magnitude range 1.1 〈 M L  〈 2.7. A subset of these locations were compared with accurate manual locations refined by using a double-difference technique. Our results indicate a good agreement with manual locations. Moreover, our method is noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals.

Description: A non-linear, global-search, probabilistic, double-difference earthquake location technique is illustrated. The main advantages of this method are the determination of comprehensive and complete solutions through the probability density function (PDF), the use of differential arrival times as data and the possibility to use a 3-D velocity model both for absolute and double-difference locations, all of which help to obtain accurate differential locations in structurally complex geological media. The joint use of this methodology and an accurate differential time data set allowed us to carry out a high-resolution, earthquake location analysis, which helps to characterize the active fault geometries in the studied region. We investigated the recent microseismicity occurring at the Campanian-Lucanian Apennines in the crustal volume embedding the fault system that generated the 1980 M S 6.9 earthquake in Irpinia. In order to obtain highly accurate seismicity locations, we applied the method to the P and S arrival time data set from 1312 events ( M L 〈 3.1) that occurred from August 2005 to April 2011 and used the 3-D P - and S -wave velocity models optimized for the area under study. Both manually refined and cross-correlation refined absolute arrival times have been used. The refined seismicity locations show that the events occur in a volume delimited by the faults activated during the 1980 M S 6.9 Irpinia earthquake on subparallel, predominantly normal faults. We find an abrupt interruption of the seismicity across an SW–NE oriented structural discontinuity corresponding to a contact zone between different rheology rock formations (carbonate platform and basin residuals). This ‘barrier’ appears to be located in the area bounded by the fault segments activated during the first (0 s) and the second (18 s) rupture episodes of the 1980s Irpinia earthquake. We hypothesize that this geometrical barrier could have played a key role during the 1980 Irpinia event, and possibly controlled the delayed times of activation of the two rupture segments.

Description: Earthquake Early Warning Systems (EEWS) are potentially effective tools for risk mitigation in active seismic regions. The present study explores the possibility of predicting the macroseismic intensity within EEW timeframes using the squared velocity integral (IV2) measured on the early P-wave signals, a proxy for the P-wave radiated energy of earthquakes. This study shows that IV2 correlates better than the peak displacement measured on P-waves (PD) with both the peak ground velocity and the Housner Intensity, with the latter being recognized by engineers as a reliable proxy for damage assessment. Therefore, using the strong motion recordings of the ITalian ACcelerometric Archive (ITACA 2.0), a novel relationship between the parameter IV2 and the macroseismic intensity (IM) has been derived. The validity of this relationship has been assessed using the strong motion recordings of the INGV Strong Motion Data and ‘ Osservatorio Sismico delle Strutture ’ databases, as well as, in the case of the M W 6 May 29 th 2012 Emilia Earthquake (Italy), comparing the predicted intensities with the ones observed after a macroseismic survey. Our results indicate that P-wave IV2 can become a key parameter for the design of on-site EEWS, capable of proving real-time predictions of the IM at target sites.

Description: Earthquake early warning studies are shifting real-time seismology in earthquake science. They provide methods to rapidly assess earthquakes to predict damaging ground shaking. Preventing false alarms from these systems is key. Here, we developed a simple, robust algorithm, A uthorizing GR ound shaking for E arthquake E arly warning S ystems, (AGREEs) to reduce falsely issued alarms. This is a network-threshold-based algorithm, which differs from existing approaches based on apparent velocity of P- and S-waves. AGREEs is designed to function as an external module to support existing earthquake early warning systems (EEWS) and filters out the false events, by evaluating actual shaking near the epicenter. Our retrospective analyses of the 2009 L'Aquila and 2012 Emilia earthquakes show that AGREEs could help an EEWS by confirming the epicentral intensity. Furthermore, AGREEs is able to effectively identify three false events due to a storm, a teleseismic earthquake, and broken sensors in Irpinia Seismic Network, Italy.

Description: A new strategy for a P-wave based, on-site earthquake early warning system has been developed and tested on Japanese strong motion data. The key elements are the real-time, continuous measurement of three peak amplitude parameters and their empirical combination to predict the ensuing peak ground velocity. The observed parameters are compared to threshold values and converted into a single, dimensionless variable. A local alert level is issued as soon as the empirical combination exceeds a given threshold. The performance of the method has been evaluated by applying the approach to the catalog of Japanese earthquake records and counting the relative percentage of successful, missed and false alarms. We show that the joint use of three peak amplitude parameters improves the performance of the system as compared to the use of a single parameter, with a relative increase of successful alarms of about 35%. The proposed methodology provides a more reliable prediction of the expected ground shaking and improves the robustness of a single-station, threshold-based earthquake early warning system.

Description: When accompanied by appropriate training and preparedness of a population, Earthquake Early Warning Systems (EEWS) are effective and viable tools for the real-time reduction of societal exposure to seismic events in metropolitan areas. The Italian Accelerometric Network, RAN, which consists of about five hundred stations installed over all the active seismic zones, as well as many cities and strategic infrastructures in Italy, has the potential to serve as a nation-wide early warning system. In this work, we present a feasibility study for a nation-wide EEWS in Italy obtained by the integration of the RAN and the software platform PRESTo (Satriano et al., 2011). The performance of the RAN-PRESTo EEWS is first assessed by testing it on real strong motion recordings of forty of the largest earthquakes that have occurred during the last ten years in Italy. Furthermore, we extend the analysis to regions that did not experience earthquakes by considering a nation-wide grid of synthetic sources capable of generating Gutenberg-Richter sequences corresponding to the one adopted by the seismic hazard map of the Italian territory. Our results indicate that the RAN-PRESTo EEWS could theoretically provide for higher seismic hazard areas reliable alert messages within about 5 to 10 seconds and maximum lead-times of about 25 seconds. In case of large events (M 〉 6.5), this amount of lead-time would be sufficient for taking basic protective measures (e.g. duck and cover, move away from windows or equipment) in tens to hundreds municipalities affected by large ground shaking.

Description: The wide availability of user-provided content in online social media facilitates the aggregation of people around common interests, worldviews, and narratives. However, the World Wide Web (WWW) also allows for the rapid dissemination of unsubstantiated rumors and conspiracy theories that often elicit rapid, large, but naive social responses such as...