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Are Synthetic Accelerograms Suitable for Local Seismic Response Analyses at Near-Field Sites?
Abstract
Local seismic response (LSR) studies are considerably conditioned by the seismic input features due to the nonlinear soil behavior under dynamic loading and the subsurface site conditions (e.g., mechanical properties of soils and rocks and geological setting). The selection of the most suitable seismic input is a key point in LSR. Unfortunately, few recordings data are available at seismic stations in near-field areas. Then, synthetic accelerograms can be helpful in LSR analysis in urbanized near-field territories. Synthetic accelerograms are generated by simulation procedures that consider adequately supported hypotheses about the source mechanism at the seismotectonic region and the wave propagation path toward the surface. Hereafter, mainshocks recorded accelerograms at near-field seismic stations during the 2016–2017 Central Italy seismic sequence have been compared with synthetic accelerograms calculated by an extended finite-fault ground-motion simulation algorithm code. The outcomes show that synthetic seismograms can reproduce the high-frequency content of seismic waves at near-field areas. Then, in urbanized near-field areas, synthetic accelerograms can be fruitfully used in microzonation studies.
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The Mw 6.5 Norcia, Italy, earthquake occurred on 30 October 2016 and caused extensive damage to buildings in the epicentral area. The earthquake was recorded by a network of strong-motion stations, including 14 stations located within a 5 km distance from the two causative faults. We used a numerical approach for generating seismic waves from two hybrid deterministic and stochastic kinematic fault rupture models propagating through a 3D Earth model derived from seismic tomography and local geology. The broadband simulations were performed in the 0–5 Hz frequency range using a physics-based deterministic approach modeling the earthquake rupture and elastic wave propagation. We used SW4, a finite-difference code that uses a conforming curvilinear mesh, designed to model surface topography with high numerical accuracy.
The simulations reproduce the amplitude and duration of observed near-fault ground motions. Our results also suggest that due to the local fault-slip pattern and upward rupture directivity, the spatial pattern of the horizontal near-fault ground motion generated during the earthquake was complex and characterized by several local minima and maxima. Some of these local ground-motion maxima in the near-fault region were not observed because of the sparse station coverage. The simulated peak ground velocity (PGV) is higher than both the recorded PGV and predicted PGV based on empirical models for several areas located above the fault planes. Ground motions calculated with and without surface topography indicate that, on average, the local topography amplifies the ground-motion velocity by 30%. There is correlation between the PGV and local topography, with the PGV being higher at hilltops. In contrast, spatial variations of simulated PGA do not correlate with the surface topography. Simulated ground motions are important for seismic hazard and engineering assessments for areas that lack seismic station coverage and historical recordings from large damaging earthquakes.
During the 2016–2017 Central Italy earthquake sequence, a series of moderate to large earthquakes M > 5 occurred near the Amatrice and Norcia towns. These events are recorded on a dense seismic network, providing relevant observational evidence of complex earthquakes in time and space. In this work, we used this substantial data set to study the ground-motion characteristics of the Norcia earthquake M6.5 on October 30, 2016, through a broadband ground-motion simulation. Three-component broadband seismograms are generated to cover the entire frequency band of engineering interest. Low and high frequencies are computed considering the heterogeneous slip rupture model of Scognamiglio et al. (2018) [1]. High frequencies are calculated using a stochastic approach including P, SV, and SH waves, while low frequencies are obtained through a forward simulation of the kinematic model at the various stations. To predict earthquake-induced ground-motions in the area, we adopted region-specific attenuation and source scaling parameters derived by Malagnini et al. (2011) [2]. Ground-motion parameters, including peak ground acceleration (PGA), peak ground velocity (PGV) and spectral amplitudes, are calculated at the selected sites adopting physics-based parameters to understand better the earthquake fault rupture, the wave propagation, and their impacts on the seismic hazard assessment in the region. We showed that combining the fault rupture history over the entire frequency spectrum of engineering interest, the attenuation characteristics of the seismic wave propagation and the properly defined site responses can improve the prediction of ground-motions and time histories, especially in near seismic sources.
On 24th August 2016 at 01:36 UTC a ML6.0 earthquake struck several villages in central Italy, among which Accumoli, Amatrice and Arquata del Tronto. The earthquake was recorded by about 350 seismic stations, causing 299 fatalities and damage with macroseismic intensities up to 11. The maximum acceleration was observed at Amatrice station (AMT) reaching 916 cm/s2 on E-W component, with epicentral distance of 15 km and Joyner and Boore distance to the fault surface (RJB) of less than a kilometre. Motivated by the high levels of observed ground motion and damage, we generate broadband seismograms for engineering purposes by adopting a hybrid method. To infer the low frequency seismograms, we considered the kinematic slip model by Tinti et al. (2016). The high frequency seismograms were produced using a stochastic finite-fault model approach based on dynamic corner-frequency. Broadband synthetic time series were therefore obtained by merging the low and high frequency seismograms. Simulated hybrid ground motions were compared both with the observed ground motions and the ground-motion prediction equations (GMPEs), to explore their performance and to retrieve the region-specific parameters endorsed for the simulations. In the near-fault area we observed that hybrid simulations have a higher capability to detect near source effects and to reproduce the source complexity than the use of GMPEs. Indeed, the general good consistency found between synthetic and observed ground motion (both in the time and frequency domain), suggests that the use of regional-specific source scaling and attenuation parameters together with the source complexity in hybrid simulations improves ground motion estimations. To include the site effect in stochastic simulations at selected stations, we tested the use of amplification curves derived from HVRSs (horizontal-to-vertical response spectra) and from HVSRs (horizontal-to-vertical spectral ratios) rather than the use of generic curves according to NTC-18 Italian seismic design code. We generally found a further reduction of residuals between observed and simulated both in terms of time histories and spectra.
On 24 January 2020 an Mw 6.8 earthquake occurred at 20:55 local time (17:55 UTC) in eastern Turkey, close to the town of Sivrice in the Elazığ province, causing widespread considerable seismic damage in buildings. In this study, we analyse the main features of the rupture process and the seismic ground shaking during the Elazığ earthquake. We first use Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1 satellites) to constrain the fault geometry and the coseismic slip distribution of the causative fault segment. Then, we utilize this information to analyse the ground motion characteristics of the mainshock in terms of peak ground acceleration (PGA), peak ground velocity (PGV) and spectral accelerations. The absence of seismic registrations in near-field for this earthquake imposes major constraints on the computation of seismic ground motion estimations in the study area. To do this, we have used a stochastic finite-fault simulation method to generate high-frequency ground motions synthetics for the Mw 6.8 Elazığ 2020 earthquake. Finally, we evaluate the potential state of stress of the unruptured portions of the causative fault segment as well as of adjacent segments, using the Coulomb stress failure function variations. Modelling of geodetic data shows that the 2020 Elazığ earthquake ruptured two major slip patches (for a total length of about 40 km) located along the Pütürge segment of the well-known left-lateral strike-slip East Anatolian Fault Zone (EAFZ), with up to 2.3 m of slip and an estimated geodetic moment of 1.70 × 1019 Nm (equivalent to a Mw 6.8). The position of the hypocenter supports the evidence of marked WSW rupture directivity during the mainshock. In terms of ground motion characteristics, we observe that the high-frequency stochastic ground motion simulations have a good capability to reproduce the source complexity and capture the ground motion attenuation decay as a function of distance, up to the 200 km. We also demonstrate that the design spectra corresponding to 475 years return period, provided by the new Turkish building code is not exceeded by the simulated seismograms in the epicentral area where there are no strong motion stations and no recordings available. Finally, based on the Coulomb stress distribution computation, we find that the Elazığ mainshock increased the stress level of the westernmost part of the Pütürge fault and of the adjacent Palu segment and as a result of an off-fault lobe.
The 2016–2017 seismic events that struck central Italy led the Government to carry out a project to produce the third level Seismic Microzonation studies in 138 municipalities. These activities have involved many experts in different disciplines such as geology, geomorphology, geophysics, seismology and geotechnical engineering. This project represented the first opportunity to perform nationally coordinated third level Seismic Microzonation studies over a wide area in a quite short time (6 months). It provided the chance to improve methodological procedures, to test the reliability of methods and models for site response analyses and to produce a huge amount of validated data. This paper focuses on the contribution of geological disciplines and concerns: (a) the definition of the main “morphostructural domains” of the Central-Northern Apennines; (b) the creation of an archive of all the lithostratigraphic units occurring in the study area with their conversion into engineering-geological units and their distribution in the different morphostructural domains; (c) the construction of the reference geological and geotechnical models, which are essential to classify the territory into seismically homogeneous microzones and to perform the successive 1D and 2D numerical analyses of the local site response. The geophysical dataset acquired for the study allowed a first statistical characterization of the Vs values typical of the engineering-geological units identified in this study. Some examples of the recurrent geological and geotechnical models are shown to explain the complexity and variety of the geological and geomorphological features of the investigated area and to highlight the different seismostratigraphic behavior of rocks and cover terrains. The analysis of third level Seismic Microzonation data made it possible to identify recurrent subsoil models and to note the main stratigraphic and morphological control-factors of the ground motion modification in the different morphostructural domains.
This study focuses on two weak points of the present procedure to carry out microzoning study in near-field areas: (1) the Ground Motion Prediction Equations (GMPEs), commonly used in the reference seismic hazard (RSH) assessment; (2) the ambient noise measurements to define the natural frequency of the near surface soils and the bedrock depth. The limitations of these approaches will be discussed throughout the paper based on the worldwide and Italian experiences performed after the 2009 L’Aquila earthquake and then confirmed by the most recent 2012 Emilia Romagna earthquake and the 2016–17 Central Italy seismic sequence. The critical issues faced are (A) the high variability of peak ground acceleration (PGA) values within the first 20–30 km far from the source which are not robustly interpolated by the GMPEs, (B) at the level 1 microzoning activity, the soil seismic response under strong motion shaking is characterized by microtremors’ horizontal to vertical spectral ratios (HVSR) according to Nakamura’s method. This latter technique is commonly applied not being fully compliant with the rules fixed by European scientists in 2004, after a 3-year project named Site EffectS assessment using AMbient Excitations (SESAME). Hereinafter, some “best practices” from recent Italian and International experiences of seismic hazard estimation and microzonation studies are reported in order to put forward two proposals: (a) to formulate site-specific GMPEs in near-field areas in terms of PGA and (b) to record microtremor measurements following accurately the SESAME advice in order to get robust and repeatable HVSR values and to limit their use to those geological contests that are actually horizontally layered.
The 2016–2017 seismic sequence in the Central Apennines (Italy) demonstrated, once again, the relevant role of the differentiated seismic effects related to different features in complex geological contexts at a short distance (i.e. soft sediment-filled valley, hilly relieves, complex buried geometries). The heavy damages of old settlements located on the top of a rocky hill, such as Arquata del Tronto hill, raised the question on the main role of threedimensional movements of asymmetrical isolated rocky reliefs in generating disruptions during the seismic shaking. In order to shed light on these effects, the writing authors analysed the seismic response of a digital 3D model of the Arquata del Tronto hill, that was constructed by considering the topography and the underground geological settings of the area. The seismic response was evaluated through 3D numerical simulations of seismic waves propagation with the spectral element method for the period range 0.1–1 s, that covers the typical resonance frequencies of the buildings involved in the studied case. The obtained results explain the recorded strong motion at Arquata site and are consistent with the observed heterogeneous damage distribution. This study evidences the need for accurate 3D models in the evaluation of the seismic response of ridges with topographic and geological asymmetries and demonstrates that the common usage of 2D simplified simulations to approximate the 3D seismic behaviour is not correct. Additionally, the study points out the paramount role that the characterization in engineering - geological terms plays in the numerical 3D simulation of the seismic site response. Thus, in microzoning studies, simplifications related to both dimensional and geo-litho-seismic characterizations should be avoided.
The great influence of site effects on damage patterns produced by seismic events of the past, stresses out the need to find proper relationships between the site local conditions and the potential damage that existing buildings may experience under earthquakes of different intensity.
This issue proved to be relevant, especially for those masonry buildings in small historic centers, such as the ones belonging to the Apennine areas of Italy that suffered serious damage after the 2009 L'Aquila and the 2016–2017 Central Italy seismic sequences. Some of these centers showed a not regular damage distribution, suggesting that site effects influenced the seismic response of their buildings.
In this perspective, this paper provides an attempt to correlate earthquake damage to site effects, focusing the attention on two historic centers of the Abruzzi region, Campotosto and Cortino, which were damaged by the 2016–2017 Central Italy seismic sequences and for which the amplification factors of the subsoil were carefully identified, resulting more influencing were higher damage was observed.
First, the damage patterns observed after the 18 January 2017 seismic shock are represented in terms of Damage Probability Matrices and maps developed in GIS environment. Moreover, the data collected in terms of stats are interpreted through the application of the binomial probability distribution, in order to check its reliability in reproducing the gathered frequencies.
Finally, in order to justify the damage of some specific buildings, the results of site response analyses are compared with the damage observed punctually on three damaged masonry buildings.
Ambient vibrations spectral ratios at forty-five sites and shear wave velocity profiles obtained by inverting surface waves dispersion curves were used to assess seismic site response properties in the Floridia village located in southeastern Sicily. The results of the horizontal-to-vertical spectral ratios analysis often indicate fundamental in the frequency range 1.5-4.0 Hz. Conversely, where the bedrock outcrop, spectral ratios are flat. The calcarenites outcropping in the north-west sector of the village and the soft sediments in the south-east are characterized by shear wave velocity of about 700 m/s and ranging between 200-600 m/s, respectively. The geological substratum has instead shear wave velocity of about 1500 m/s. The achieved geophysical and geological data help to build a subsoil model of the studied area, classifying the main lithotypes above the bedrock and determining the hidden morphology. Moreover, amplification functions were computed for zones with homogenous geological and geophysical properties. Therefore, these site amplification functions were used to simulate ground motion scenarios corroborating the efficiency of the obtained results.The used strategy, involving geophysical and geological methods, representsan important step to derive ground motion scenarios and seismic risk maps. Moreover, the characterization of amplification due to local site effects could help to understand the pattern and the damage distribution of historical earthquakes