Shear wave splitting measures at six temporary stations in the Southern Apennines are computed analyzing fifteen events recorded during the spring and summer 1996, with magnitude greater than 5.8. The splitting parameters were measured using Silver and Chan's (1991) method only on SKS and SKKS phases. Evidence for strong seismic anisotropy was found at all the stations: delay times dt are generally larger than 1.5 s and fast directions Ø are quite variable in different geodynamic domains. NW-SE Ø average directions are found at stations on the mountain belt while at stations on the foredeep and foreland Ø is on average N-S. Changes in splitting parameters may be related to upper mantle structure, and particularly to the geometry of a fragmented lithosphere subducting beneath the Apennines.
Abstract. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is an Italian research institution, with focus on Earth Sciences. INGV runs the Italian National Seismic Network (Rete Sismica Nazionale, RSN) and other networks at national scale for monitoring earthquakes and tsunami as a part of the National Civil Protection System coordinated by the Italian Department of Civil Protection (Dipartimento di Protezione Civile, DPC). RSN is composed of about 400 stations, mainly broadband, installed in the Country and in the surrounding regions; about 110 stations feature also co-located strong motion instruments, and about 180 have GPS receivers and belong to the National GPS network (Rete Integrata Nazionale GPS, RING). The data acquisition system was designed to accomplish, in near-real-time, automatic earthquake detection, hypocenter and magnitude determination, moment tensors, shake maps and other products of interest for DPC. Database archiving of all parametric results are closely linked to the existing procedures of the INGV seismic monitoring environment and surveillance procedures. INGV is one of the primary nodes of ORFEUS (Observatories & Research Facilities for European Seismology) EIDA (European Integrated Data Archive) for the archiving and distribution of continuous, quality checked seismic data. The strong motion network data are archived and distributed both in EIDA and in event based archives; GPS data, from the RING network are also archived, analyzed and distributed at INGV. Overall, the Italian earthquake surveillance service provides, in quasi real-time, hypocenter parameters to the DPC. These are then revised routinely by the analysts of the Italian Seismic Bulletin (Bollettino Sismico Italiano, BSI). The results are published on the web, these are available to both the scientific community and the general public. The INGV surveillance includes a pre-operational tsunami alert service since INGV is one of the Tsunami Service providers of the North-eastern Atlantic and Mediterranean Tsunami warning System (NEAMTWS).
Abstract We calculate spectral ratios for S waves and codas to evaluate the amplification of 14 sites in the southern San Francisco Bay area relative to a nearby bedrock site in the Coyote Hills. Our data are seismograms written by Loma Prieta aftershocks: the epicentral distances and azimuths to the stations are effectively the same. All the sites in the study are amplified with respect to the reference site at frequencies from 0.5 to 7 Hz. The shapes of the S -wave and coda spectral ratios are similar, but the coda ratios are greater than the S -wave ratios by as much as a factor of 4. The difference is larger for sites on alluvial and bay mud deposits, particularly at frequencies around 1 Hz, suggesting the presence of waves trapped in the alluvial basin. In general, the length of the analysis window affects the S -wave spectral ratios for alluvial sites. Longer windows give ratios similar to coda ratios, apparently because these windows include more of the phases that contribute to the coda. We classify the sites according to their geological characteristics and surficial shear-wave velocities. For the S -wave ratios, the differences between the classes showed no systematic trend; the softest and hardest soil classes we consider have practically identical S -wave amplifications. The average coda ratios for the site classes clearly increase as the soil classes include slower and “softer” materials. After correction for differences in reference sites, the coda amplifications are very similar to the relative amplifications for these site classes estimated by Borcherdt and Glassmoyer (1992) from the strong-motion recordings of the 1989 Loma Prieta earthquake.
Abstract The protracted nature of the 2016-2017 central Italy seismic sequence, with multiple damaging earthquakes spaced over months, presented serious challenges for the duty seismologists and emergency managers as they assimilated the growing sequence to advise the local population. Uncertainty concerning where and when it was safe to occupy vulnerable structures highlighted the need for timely delivery of scientifically based understanding of the evolving hazard and risk. Seismic hazard assessment during complex sequences depends critically on up-to-date earthquake catalogues—i.e., data on locations, magnitudes, and activity of earthquakes—to characterize the ongoing seismicity and fuel earthquake forecasting models. Here we document six earthquake catalogues of this sequence that were developed using a variety of methods. The catalogues possess different levels of resolution and completeness resulting from progressive enhancements in the data availability, detection sensitivity, and hypocentral location accuracy. The catalogues range from real-time to advanced machine-learning procedures and highlight both the promises as well as the challenges of implementing advanced workflows in an operational environment.
We analyze the broadband recordings from a borehole station installed in the bedrock underneath the Po basin (Northern Italy) and from a second station temporarily deployed on the surface at the same site to determine the surface sedimentary layer response and test the robustness of the different empirical techniques used to estimate local site effects. We investigate the site amplification by evaluating the surface-to-downhole spectral ratios and the horizontal-to-vertical spectral ratios on both weak-motion events and microtremors. All the different empirical methods, except the horizontal-to-vertical spectral ratios on weak motions, reveal the main amplification peak at about 0.8 Hz, although the amplification factors are not always in agreement; the 0.8 Hz peak is the fundamental mode of the site, due to the upper 130-m Quaternary alluvium column ( V s ∼ 300 m/sec). We compare these experimental spectral functions with the theoretical 1D site response including and not including the borehole site response: first we evaluate the theoretical transfer function at the surface then at depth and take the smoothed ratio. The shallow velocity and attenuation structure at this site is assumed from previous studies. We attempt to account for the effect of the reflected wave field in the borehole recordings in two different ways: first we include the reflected wave field in the theoretical response function taking the theoretical transfer function9s ratio (Hst/Hbt) and comparing it to the experimental earthquake ratios of horizontal components recorded at surface and at depth (Hs/Hb). Alternatively we multiply the Hs/Hb earthquake ratio by the coherence and compare it to the surface response function. This second approach does not provide a good fit to the amplitude of model response, even if the shape of the ratio is in agreement with the theoretic response, suggesting that multiplying by the coherence is not a good way to correct for the effect of reflected phases. To further understand the goodness of our theoretical 1D model (Hst/Hbt theoretical evaluations), we simulate the propagation through the shallower soil structure of the seismic waveforms recorded by the borehole sensor for a couple of events, obtaining reasonably good agreement with the surface time histories both in phase and amplitude. This test site and the results of this study are useful for understanding the influence of sedimentary layers on the ground shaking in the Po basin, the largest and most populated alluvial basin in Italy.
