The Central Italy 2016–2017 seismic sequence: site response analysis based on seismological data in the Arquata del Tronto–Montegallo municipalities
Giovanna LaurenzanoCarla BarnabaMaria Adelaide RomanoEnrico PrioloMichele BertoniP. L. BragatoPaolo ComelliIlaria DreossiMarco Garbin
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In this paper we evaluate the local seismic response for thirteen sites located in the municipalities of Arquata del Tronto and Montegallo, two areas which suffered heavy damage during the Mw 6.0 and Mw 5.4 earthquakes which struck Central Italy on August 24, 2016. The input dataset is made by ground motion recordings of 348 events occurred during the sequence. The spectral site response is estimated by the Generalized Inversion Technique and makes use of reference sites. The interpretation is further improved through the information provided by a reference-site independent method (i.e., the so called Receiver-Function Technique) and by the Horizontal-to-Vertical Spectral Ratios of ambient noise recordings. We also provide an independent estimate of the local amplification by comparing the Peak Ground Velocity and the Spectral Amplitudes observed at each site to the value estimated by well-established Ground Motion Prediction Equations for a rock-class site. The results obtained by the adopted methodologies are all highly consistent, and they emphasize the different seismic behavior of several sites at local scale. Thus, sites located on Quaternary deposits overlying the bedrock, such as Castro, Pretare, Spelonga, Pescara del Tronto, and Capodacqua feature some relevant amplifications in a medium (2–10 Hz) frequency range; two sites at Spelonga show amplifications also at low frequencies; three sites located on stiff formations, i.e. Uscerno, Balzo and Colle d'Arquata, respectively, feature either nearly neutral response or low amplification level. A probable topographic effect was identified at the rock site of Rocca di Arquata (MZ80).Keywords:
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A probabilistic seismic hazard analysis has been conducted for a potential nuclear power plant site on the coast of South Africa, a country of low-to-moderate seismicity. The hazard study was conducted as a SSHAC Level 3 process, the first application of this approach outside North America. Extensive geological investigations identified five fault sources with a non-zero probability of being seismogenic. Five area sources were defined for distributed seismicity, the least active being the host zone for which the low recurrence rates for earthquakes were substantiated through investigations of historical seismicity. Empirical ground-motion prediction equations were adjusted to a horizon within the bedrock at the site using kappa values inferred from weak-motion analyses. These adjusted models were then scaled to create new equations capturing the range of epistemic uncertainty in this region with no strong motion recordings. Surface motions were obtained by convolving the bedrock motions with site amplification functions calculated using measured shear-wave velocity profiles.
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Abstract Although Singapore is located in a low‐seismicity region, huge but infrequent Sumatran subduction earthquakes might pose structural problems to medium‐ and high‐rise buildings in the city. Based on a series of ground motion simulations of potential earthquakes that may affect Singapore, the 1833 Sumatran subduction earthquake ( M w =9.0) has been identified to be the worst‐case scenario earthquake. Bedrock motions in Singapore due to the hypothesized earthquake are simulated using an extended reflectivity method, taking into account uncertainties in source rupture process. Random rupture models, considering the uncertainties in rupture directivity, slip distribution, presence of asperities, rupture velocity and dislocation rise time, are made based on a range of seismologically possible models. The simulated bedrock motions have a very long duration of about 250 s with a predominant period between 1.8 and 2.5 s, which coincides with the natural periods of medium‐ and high‐rise buildings widely found in Singapore. The 90‐percentile horizontal peak ground acceleration is estimated to be 33 gal and the 90‐percentile horizontal spectral acceleration with 5% damping ratio is 100 gal within the predominant period range. The 90‐percentile bedrock motion would generate base shear force higher than that required by the current design code, where seismic design has yet to be considered. This has not taken into account effects of local soil response that might further amplify the bedrock motion. Copyright © 2002 John Wiley & Sons, Ltd.
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Aftershocks of the 17 August 1999, Mw = 7.4, Kocaeli earthquake were used to investigate the large‐amplitude SmS arrivals in the Sea of Marmara. We produced the event record sections from the transverse components of S and SmS arrivals at a distance of 10 to 200 km for the data recorded towards the east and west in the Sea of Marmara. For the data recorded towards the west, we observed that the amplitude of SmS phases enhances the ground motion by a factor of 2 to 3. The dipping Moho and/or anisotropy in the upper mantle or deep convex structure or deep lens are possible sources for the large‐amplitude SmS phases. Since the study area has a potential of creating damaging earthquakes in the future, the effect of large‐amplitude SmS phases on the ground motion should be considered in the studies of the seismic hazard of the region.
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A shallow bedrock fold imaged by a 1.3‐km long high‐resolution shear‐wave seismic reflection profile in west Seattle focuses seismic waves arriving from the south. This focusing may cause a pocket of amplified ground shaking and the anomalous chimney damage observed in earthquakes of 1949, 1965 and 2001. The 200‐m bedrock fold at ∼300‐m depth is caused by deformation across an inferred fault within the Seattle fault zone. Ground motion simulations, using the imaged geologic structure and northward‐propagating north‐dipping plane wave sources, predict a peak horizontal acceleration pattern that matches that observed in strong motion records of the 2001 Nisqually event. Additionally, a pocket of chimney damage reported for both the 1965 and the 2001 earthquakes generally coincides with a zone of simulated amplification caused by focusing. This study further demonstrates the significant impact shallow (<1km) crustal structures can have on earthquake ground‐motion variability.
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Abstract The Marina District of San Francisco, California, with its artificial fill and a thick section of sand and clay covering a northwest-trending valley in the bedrock, suffered extensive damage during the 18 October 1989 Loma Prieta earthquake. Following the earthquake, the USGS drilled a hole at Winfield Scott School at Beach and Divisadero Streets; the borehole intersects bedrock surface at a 79.5-m depth. Two three-component seismometers, one in bedrock at a 88-m depth and one located at the surface, have been installed at the site; each seismometer consists of one vertical and two orthogonally oriented horizontal geophones having a natural period of 0.5 sec. Between August 1990 and January 1991, more than 50 earthquakes have been recorded digitally. Eight among these, ranging in magnitude between 2.8 and 3.6 and originating on the Calaveras, Franklin, Greenville, and Hayward faults and on faults parallel and close to the San Andreas fault, generated seismograms with high signal-to-noise ratio. Horizontal ground-motion amplification, expressed as spectral ratio between ground motions at the surface and those in the bedrock, has been calculated for motions in two orthogonal directions (along Divisadero and Beach Street); each ground-motion spectrum has been calculated using an entire seismogram consisting of body waves, surface waves, multiply reflected and scattered coda waves, and a short section (∼ 2 sec) of pre-event ambient noise. Before calculating spectral ratio, each spectrum has been smoothed using a truncated Gaussian window 0.61-Hz wide. Except for the lowest-frequency spectral-ratio peak at ∼ 1 Hz, frequency of other peaks depends on earthquake location. Amplitude of spectral-ratio peaks also show variation depending on ground-motion direction and earthquake location. For example, amplitude of the 1-Hz spectral-ratio peak varies from 7.2 to 12.7. The surface-downhole spectral ratio therefore provides only partial information on how ground motions are amplified by sediment deposits. If we choose to use this ratio for earthquake engineering applications, the ratios from the eight earthquakes give an indication of the variation in spectral ratio to be expected from earthquakes with similar magnitudes and epicentral distances on various Bay area faults. Also noteworthy are the observations that the two horizontal-component seismograms recorded by each seismometer have similar coda amplitude and duration regardless of earthquake location and that particle-motion polarization becomes complex shortly after the P-wave and S-wave onset. The complex particle-motion polarization indicates that wave fields in the bedrock and at the surface are three-dimensional; the bedrock topography underlying the site has been delineated previously to be three-dimensional from drill-hole information. We suggest from these observations that three-dimensional effects need to be considered when modeling site amplification in the Marina District. Finally, the eight earthquakes are divided into two groups, comprising those whose epicenters are located east of San Francisco Bay and those whose epicenters are located south of San Francisco Bay. Within each group, spectral-ratio peaks from different earthquakes line up with each other, thus showing consistency in spectral-ratio peaks as a function of earthquake location.
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