323,150 research outputs found
New relationships between V-s, thickness of sediments, and resonance frequency calculated by the H/V ratio of seismic noise for the Cologne area (Germany)
No full textNoise measurements were carried out in the Cologne area. (Germany), and the resonance frequency of each site was estimated from the main peak in the spectral ratio between the horizontal and vertical component. For 32 of these sites, the thickness of the sedimentary cover was known from boreholes, and a clear correlation between resonance frequency and sediment thickness was observed. A formula that correlates cover thickness with frequency of the main peak in the horizontal-to-vertical spectral ratio was derived. In addition, a best-fitting shear-wave-velocity distribution with depth, v(s)(z), as well as a relationship between average shear-wave velocity (V) over bars and thickness of the sedimentary cover, was calculated. By using all of the noise measurements and applying the derived relationships, we obtained a subsoil classification for the Cologne area
High-Resolution Crustal S-wave Velocity Model and Moho Geometry Beneath the Southeastern Alps: New Insights From the SWATH-D Experiment
No full textWe compiled a dataset of continuous recordings from the temporary and permanent seismic networks to compute the high-resolution 3D S-wave velocity model of the Southeastern Alps, the western part of the external Dinarides, and the Friuli and Venetian plains through ambient noise tomography. Part of the dataset is recorded by the SWATH-D temporary network and permanent networks in Italy, Austria, Slovenia and Croatia between October 2017 and July 2018. We computed 4050 vertical component cross-correlations to obtain the empirical Rayleigh wave Green’s functions. The dataset is complemented by adopting 1804 high-quality correlograms from other studies. The fast-marching method for 2D surface wave tomography is applied to the phase velocity dispersion curves in the 2–30 s period band. The resulting local dispersion curves are inverted for 1D S-wave velocity profiles using the non-perturbational and perturbational inversion methods. We assembled the 1D S-wave velocity profiles into a pseudo-3D S-wave velocity model from the surface down to 60 km depth. A range of iso-velocities, representing the crystalline basement depth and the crustal thickness, are determined. We found the average depth over the 2.8–3.0 and 4.1–4.3 km/s iso-velocity ranges to be reasonable representations of the crystalline basement and Moho depths, respectively. The basement depth map shows that the shallower crystalline basement beneath the Schio-Vicenza fault highlights the boundary between the deeper Venetian and Friuli plains to the east and the Po-plain to the west. The estimated Moho depth map displays a thickened crust along the boundary between the Friuli plain and the external Dinarides. It also reveals a N-S narrow corridor of crustal thinning to the east of the junction of Giudicarie and Periadriatic lines, which was not reported by other seismic imaging studies. This corridor of shallower Moho is located beneath the surface outcrop of the Permian magmatic rocks and seems to be connected to the continuation of the Permian magmatism to the deep-seated crust. We compared the shallow crustal velocities and the hypocentral location of the earthquakes in the Southern foothills of the Alps. It revealed that the seismicity mainly occurs in the S-wave velocity range between ∼3.1 and ∼3.6 km/s
Application of Surface wave methods for seismic site characterization
No full textSurface-wave dispersion analysis is widely used in geophysics to infer a shear wave velocity model of the subsoil for a wide variety of applications. A shear-wave velocity model is obtained from the solution of an inverse problem based on the surface wave dispersive propagation in vertically heterogeneous media. The analysis can be based either on active source measurements or on seismic noise recordings. This paper discusses the most typical choices for collection and interpretation of experimental data, providing a state of the art on the different steps involved in surface wave surveys. In particular, the different strategies for processing experimental data and to solve the inverse problem are presented, along with their advantages and disadvantages. Also, some issues related to the characteristics of passive surface wave data and their use in H/V spectral ratio technique are discussed as additional information to be used independently or in conjunction with dispersion analysis. Finally, some recommendations for the use of surface wave methods are presented, while also outlining future trends in the research of this topic
The 9 October 1963 Vajont Catastrophe from the Point of View of the WWSSN-LP Recordings of the TRI-117 Station, Trieste, Italy
No full textIn this study, we analyze the seismic signal generated by the 1963 Vajont catastrophic landslide recorded at the Worldwide Standardized Seismographic Station Network- Long Period station of Trieste (Italy). The landslide (nearly 260–270 million m3) invaded an artificial reservoir designed for electrical production, and generated a 220 m high wave that flowed over the dam and claimed the lives of approximately 2000 people. The original seismograms have been digitized and analyzed using time–frequency tools and numerical simulations. The results indicate that a seismic signal comparable to that generated by an Ms 3.7 earthquake was generated by the landslide. Furthermore, the calculatednearly2×1014 Joffrictionalenergy,consideringtheknownparameterofthe mass movement, is compatible with a friction coefficient of 0.29, in excellent agreement with the values from previous studies. The seismic efficiency that we calculate (1:12 × 10−4 –4:45 × 10−4 ), also taking into account available data on the landslide, is within the range of values previously noted in literature. Finally, via the numerical sim- ulations and adopting an ad hoc crustal model for the area, the origin time of the event is estimated at 21 hr 41 min 42 s UTC. The results confirm the importance of the re-analy- sis of analog seismograms with modern tools within a multihazard context
Statistical analysis of Horizontal to Vertical Spectral Ratios (HVSR)
No full textStatistical properties of the horizontal to vertical spectral ratios (HVSR) applied to noise recording are analyzed in order to define optimal strategies for numerical processing and identification of possible artifacts. To this purpose, two time series have been analyzed: one constituted by environmental seismic noise in the presence of a genuine physical signal and one relative to pure instrumental noise, both obtained with the same experimental apparatus. By means of suitable statistics, some guidelines for the HVSR analysis are provided. A statistical test proposed by Albarello (2001) for the identification of artifacts in the HVSR function has been analyzed and invalidated
Applying the damage assessment for rapid response approach to the august 24 M6 event of the seismic sequence in central Italy (2016)
No full textSeismic monitoring networks are increasingly being used in urban areas to record and locate earthquakes. Recordings in the proximity of buildings also allow assessing, as a first approximation, the expected building damage. The DARR (Damage Assessment for Rapid Response) method provides local-scale information on expected damage patterns. The potential of this approach is discussed here for the August 24 M6 event of the Central Italy seismic sequence (2016-2017). We focus only on the first event of the sequence because cumulative damage is outside the scope of this study. The earthquake recordings are available from two Italian monitoring networks: the Italian Accelerometric Archive (ITACA) and the OSS (Osservatorio Sismico delle Strutture), which collects data from monitored buildings and bridges in Italy. We selected four target areas (Amatrice, Norcia, Visso and Sulmona) characterized by different epicentral distances and building typologies, that suffered different levels of damage during the M6 event on 24 August 2016. Using recordings either in the free field or in the basement of buildings, the expected relative displacement of building typologies common in the studied areas is calculated with the DARR method. Using predefined damage thresholds from literature, the obtained results allow quantifying the expected damage for dominant building typologies in the surroundings of the recording sites. We investigate and discuss the potential use and applicability of the DARR method in different areas depending on the epicentral distance and building characteristics. The results indicate that the DARR approach is useful for supporting and improving rapid response activities after a seismic event
Direct Estimation of the Source Corner Frequency of Minor to Moderate Earthquakes from Fourier Phase Spectra Fitting
No full textEstimating the corner frequency () of an earthquake is of fundamental importance to improving our knowledge of the physics of the rupture that gives rise to a seismic event, while also having important implications by providing information on the high‐frequency radiation for seismic hazard studies. However, the estimation of through spectral‐fitting methods suffers from trade‐offs with the estimation of seismic wave attenuation, making the obtained values precise but not necessarily accurate. For this reason, after a review of the source model proposed by Brune (1970), a new method of estimating , based on the phase fitting of Fourier spectra of the Brune’s seismic pulse related to S‐waves, called Fourier phase spectra fitting (FPS), is proposed and evaluated in this study. The method can be applied in cases in which Brune’s model may be appropriate, while also considering the effects of propagation on impulse deformation, for which the synthetic tests have been conducted. The results, obtained first using synthetic seismograms generated under controlled conditions and then on a data set of recordings of real seismic events collected at the Groningen gas field in the Netherlands, showed the method to be promising (being accurate and precise) and at the same time pointed out its limitations, with its applicability being restricted to short hypocentral distances of <20 km
The Damage Assessment for Rapid Response (DARR) Method and its Application to Different Ground-Motion Levels and Building Types
No full textSeismic recordings in buildings and on the ground are increasingly available due to the increment and expansion of seismic monitoring networks worldwide. However, most urban strong‐motion networks consist of stations installed at the ground or, less frequently, in selected building’s basement. It is, therefore, of utmost importance to develop methods that can provide estimates of expected structural damage, starting from earthquake recordings at the ground level. Damage Assessment for Rapid Response (DARR) provides first‐level estimates of the expected damage to buildings, based on ground‐motion recordings and simple information on buildings’ characteristics. In this work, we apply DARR using both weak and strong ground‐motion recordings available for different low‐ and mid‐rise building typologies. A total of 9 buildings and 19 earthquake recordings were analyzed. DARR reproduces the shaking at the building’s top, and estimates the peak structural relative displacement or average interstory drift. Results show that the method works well for the considered building types and ground‐motion levels for the estimation of relative and total displacements using first‐order assessments. Comparison with the previously defined thresholds allows the estimation of expected damage. Our results (i.e., no damage for most buildings and events) are consistent with the absence of damaging events in northeastern Italy in the studied period (2019–2021). For a school building in central Italy, which was heavily damaged by the 2016 Central Italian sequence, DARR correctly predicted this fact
Non-parametric spectral modelling of source parameters, path attenuation and site effects from broad-band waveforms of the Alborz earthquakes (2005-2017)
No full textS-wave spectral amplitudes from 312 crustal earthquakes recorded at the Iranian National Broadband Seismic Network in the Alborz region between 2005 and 2017 are analysed in order to evaluate earthquake source parameters, path attenuation and site amplification functions using the non-parametric generalized inversion technique (GIT). We exploit a total number of 1117 seismograms with ML 3-5.6 in the frequency range 0.3-20 Hz. The evaluated non-parametric attenuation functions decay uniformly with distance for the entire frequency range and the estimated S-wave quality factor shows low Q values with relatively strong frequency dependence. We assume the omega-square source model to retrieve earthquake source parameters from the inverted source spectra. The obtained stress drops range from 0.02 to 16 MPa with a mean value of 1.1 MPa. Stress drop and radiated energy show fairly self-similar scaling with seismic moment over the available magnitude range; however, the magnitude range of this study is too narrow to draw a definite conclusion on source scaling characteristics. The obtained moment magnitude M-w and the local magnitude ML are linearly correlated and approximately equivalent in the range of M-w 3-4. For larger events, Mw generally underestimates ML by about 0.1-0.5 magnitude units. The estimated site amplification functions for horizontal component (GIT H) are nearly flat with no obvious pre-dominant frequency peaks for most stations, as expected for the sites of permanent broad-band seismic stations located on rock, though a few stations show amplification peaks from 1 to 8 Hz, with a maximum amplification of about a factor of 7 with respect to the reference site. The evaluated site responses for the vertical components present remarkable amplification or deamplification, leading to differences of the H/V amplitude levels in comparison with the GIT H amplification curves. The results of this study provide a valuable basis for predicting appropriate ground motions in a context of seismic hazard assessment
Evaluation of proxies for seismic site conditions in large urban areas: The example of Santiago de Chile.
No full textCharacterizing the local site response in large cities is an important step towards seismic hazard assessment. To this regard, single station seismic noise measurements were carried out at 146 sites in the northern part of Santiago de Chile. This extensive survey allowed the fundamental resonance frequency of the sedimentary cover, derived from horizontal-to-vertical (H/V) spectral ratios, to be mapped. By inverting the spectral ratios under the constraint of the thickness of the sedimentary cover, known from previous gravimetric measurements, local S-wave velocity profiles have been retrieved. After interpolation between the individual profiles, the resulting high resolution 3D S-wave velocity model allows the entire area, as well as deeper parts of the basin, to be represented in great detail. Since one lithology shows a great scatter in the velocity values only a very general correlation between S-wave velocity in the uppermost 30 m (View the MathML source
) and local geology is found. Local S-wave velocity profiles can serve as a key factor in seismic hazard assessment, since they allow an estimate of the amplification potential of the sedimentary cover. Mapping the intensity distribution of the 27 February 2010 Maule, Chile, event (Mw = 8.8) the results indicate that local amplification of the ground motion might partially explain the damage distribution and encourage the use of the low cost seismic noise techniques for the study of seismic site effects
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