Constraining source terms, regional attenuation models, geometric spreading, and site terms for Eurasia

Earthquake-induced disasters are often more severe over soft soils than over firm soils or rocks owing to the seismic site effects related to the amplification of ground motion. On a regional scale, such differences can be estimated by spatially predicting the subsurface soil thickness over the entire target area. Generally, soil deposits are deeper in coastal or riverside areas than in inland regions. In this study, the seismic site effects in the coastal metropolitan areas of Incheon and Bucheon, South Korea, were assessed to provide information on seismic hazards. Spatial prediction of geotechnical layers was performed for the entire study area within an advanced GIS framework. Approximately 7500 existing borehole records in the Incheon and Bucheon areas were gathered and archived into a GIS database. Surface geotechnical data were acquired from a walk-over survey. Based on the optimized geo-data, spatial zoning maps of site-specific seismic response parameters, based on multiscale geospatial modeling, were created and presented for use in a regional seismic mitigation strategy. Seismic zonation was also performed to determine site coefficients for seismic design over the entire target area and to compare them with each other. We verified that the geotechnical data based spatial zonation would be useful for seismic hazard mitigation.

This paper presents empirical seismic site effect analysis for Tehran city, the capital of Iran. Empirical methods using both spectral ratio of the horizontal components of sedimentary site to rocky site (/) (site/reference) and horizontal to vertical spectral ratio (H / V) are considered. To estimate the seismic site effect parameters using the empirical methods, recorded motions from Firozabad-Kojour (2004) and Kahak-Qom (2007) earthquakes are analysed. The recorded motions in rocky stations are used for the / method. Site amplification ratios in various frequency ranges are extracted from the results of both / and H / V methods. The peak surface accelerations are computed using the site amplification ratios, for different frequency ranges, and compared with the recorded peak accelerations. Based on the values of the least square relative errors, the results from the method for the 1–10 Hz frequency range show a good agreement between the calculated and recorded peak surface accelerations, and thus, this frequency range can be considered a reliable range for the results of this study to be used for seismic design purposes in the study area.

Geometric spreading is an important factor that needs to be taken into account in the analysis of seismic amplitudes. In particular, when using any modification of amplitude variation with offset or amplitude versus azimuth methods, the effect of geometric spreading is crucial to isolate the effect of reflection from a particular interface. The relative geometric spreading controls the amplitude of seismic waves passing through a velocity model. In the case of an anisotropic medium, geometric spreading becomes very complicated. Usually, geometric spreading is computed from ray tracing. I have derived simple analytical formulas to compute the relative geometric spreading of P-waves in a stack of acoustic orthorhombic layers with azimuthal variations in symmetry planes. I also analyzed the kinematic properties of the derived equations and performed sensitivity analysis with respect to three anelliptic parameters. A simple and accurate approximation for the relative geometric spreading is derived and tested against well-known approximation. My approximations give insight into the role that anelliptic parameters play into the azimuthal distribution of amplitudes and can be used for amplitude analysis in multilayered orthorhombic models.

Summary Seismic spectral attributes of apparent attenuation, as explained in the first part of this paper, measures the spectral changes of seismic signals due to absorption and scattering, which can provide quantitative measures of the seismic apparent attenuation effect. Seismic attenuation is sensitive to hydrocarbon accumulation, fluid-saturated fractures, and rugosity. Even in the absence of noise, the constant Q model and the spectral ratio techniques may provide poor estimates of the spectral response, particularly when geometric attenuation plays an important role. Here, we evaluate alternative seismic attenuation measurements (attributes) to characterize the apparent attenuation on field data. The results show the proposed attributes are valuable for seismic interpretation and provide important information about the reservoirs.

Progressive waves in non-ideal gases

Strong ground motion prediction based on finite-fault simulation requires the identification of the fault (strike, dip, length and width), source kinematics parameters (stress drop, rupture velocity and slip distribution), regional crustal properties (geometrical spreading, anelastic structure, and upper crustal amplification and attenuation parameters) and the determination of amplification effects due to the local site geology. The general purpose of this study is to understand source and attenuation properties in the Azores, by the determination of stress drop, quality factor and kappa, through records obtained by the Portuguese digital seismic and accelerometer network. Source Spectra were obtained, for each record, after correcting observed spectra from geometrical spreading and anelastic attenuation effect: quality factor was estimated based on coda decay in the time domain and the kappa parameter was estimated by fitting the high-frequency decay of the acceleration spectrum with a straight line in a log-linear scale. Mean stress drop value was obtained considering that ω-squared model for the source spectra prevails. Parameters kappa, k, and quality factor, Q, have been estimated to be k = (0.075 ± 0.02)s and $$Q(f) = (76 \pm 11)f^{0.69 \pm 0.09}$$ , respectively. A mean value of stress drop was estimated to be around 90–130 bars for the earthquakes in analysis. The knowledge of source and path parameters, in association with other ground motion parameters, allows improving ground-motion estimates for the Azores and, consequently, will lead to more accurate seismic hazard assessment for the Azores and better characterization of seismic scenarios.

Numerical analysis of dispersion, attenuation, and seismic effects in a porous rock saturated with three-phase immiscible fluids

Assessment of subsurface conditions and seismic site effects using the HVSR and MASW techniques at the University of Lagos, Akoka campus southwestern Nigeria

International audience

Dynamic properties of the Quaternary sedimentary rocks and their influence on seismic site effects. Case study in Bucharest City, Romania

Many of the urban settlements in Central Italy are placed nearby active faults and, consequently, the ground motion evaluation and seismic site effects under near-fault earthquakes are noteworthy issues to be investigated. This paper presents the results of site investigations, the seismic site characterization, and the local seismic response for assessing the effects induced by the Mw 6.7 2 February 1703, near-fault earthquake at the Madonna delle Fornaci site (Pizzoli, Central Italy) in which notable ground failure phenomena were observed, as witnessed by several coeval sources. Even though recent papers described these phenomena, the geological characteristics of the site and the failure mechanism have never been assessed through in-situ investigations and numerical modeling. Within a project concerning the assessment of soil liquefaction potential and co-seismic ground failure, deep and shallow continuous core drilling, geophysical investigations and in-hole tests have been carried out. Subsequently, the geotechnical model has been defined and the numerical quantification of the different hypotheses of failure mechanisms has been evaluated. Analyses showed that liquefaction did not occur, and the excess pore water pressure induced by the shaking was not the source of the ground failure. Therefore, it was hypothesized that the sinkhole was likely caused by earthquake-induced gas eruption.

We have recently been made aware of errata and omissions in the introduction section for describing the seismological characteristics of the 2017 Pohang earthquake as stated in the title of this article, which was recently published in ISPRS [...]

The growth that aviation has seen in the last decades has drawn the attention on the environmental impact of aircraft. An important part of this environmental impact is the noise emitted by air vehicles, which is considered rather significant for community annoyance. The generation and propagation of air vehicle noise are two different areas of interest, which require accurate prediction in order to control the emitted noise levels. The present thesis employs numerical methods in order to investigate various air vehicle noise propagation effects. It is divided in two parts: the far field and the near field study. Each of these studies is concentrated on the sound propagation mechanisms that are dominant for each case and uses a numerical method that is best suited for it in terms of mechanisms incorporated and cost effectiveness. The far-field study of this thesis focuses on the nonlinear propagation of helicopter rotor noise using the Burgers equation, a well known one direction propagation method. The Burgers equation incorporates geometrical spreading, atmospheric absorption and nonlinear distortion effects. Towards this study, the HELISHAPE descent case experimental database is used. Blade Vortex Interaction (BVI) noise, the dominant noise contributor during descent, is mainly examined. It is shown that advancing side BVI noise is affected by nonlinear distortion, while retreating side BVI noise is not. For some frequency bands the difference between linear and nonlinear calculations can be as high as 7 dB. Based on signal characteristics at source, two quantities are derived. The first quantity (termed polarity) is based on the pressure gradient of the source signal and can be used to determine whether a BVI signal will evolve as an advancing or a retreating side signal. The second quantity (termed weighted rise time) is a measure of the impulsiveness of the BVI signal and can be used to determine at which frequency nonlinear effects start to appear. Finally, polarity and weighted rise time are shown to be applicable in cases of BVI noise generated from different blade tips, as well as, in cases of non-BVI noise. However, employment of the Burgers equation can be time consuming to be included in routine calculations. It also requires knowledge of the initial pressure time signal. The power spectrum alone, which is usually known, is not sufficient. In order to ii overcome these difficulties, three prediction methods are presented that are based on the Burgers equation. These are: i) a numerically generated database, ii) correlation equations and iii) the phase assignment method. Near field propagation of air vehicle noise requires different treatment than far field. The effects which are mainly affecting the propagation are geometrical spreading, convection and refraction effects due to the flow field, as well as reflections and diffraction on the air vehicle surfaces. Towards these objectives, a new low-order flow/acoustics interaction method for the prediction of sound propagation and diffraction in unsteady compressible flow using adaptive 3-D hybrid grids is investigated. The total field is decomposed into the flow field described by the Euler equations, and the acoustics field described by the Nonlinear Perturbation equations. The method is shown capable of predicting monopole sound propagation, while employment of acoustics-guided adapted grid refinement improves the accuracy of capturing the acoustic field. Interaction of sound with solid boundaries is also examined in terms of reflection and diffraction. Sound propagation through an unsteady flow field is examined using static and dynamic flow/acoustics coupling demonstrating the importance of the latter. Proof of concept for the new method is provided by its application to the case of a conventional jet transport airplane, examining the effect of flow field and wing shielding on the near field noise levels. During the aforementioned noise investigation and analysis, results on Blade Wake Interaction (BWI) noise were also reached. Presently, the mechanism of BWI noise generation, as well as the corresponding prediction model, are still under consideration. Helicopter rotor BWI noise is known to be significant during take-off and level flight, while less attention has been given to descent flight conditions, where BVI noise is dominant. Through signal analysis of the HELISHAPE descent case acoustic database, the rotor azimuthal region responsible for BWI noise is localized and the dominance of BVI noise in the BWI frequency region is shown. Coherence analysis of the blade pressure data indicate significant chordwise coherence in the 3 to 4 Struhal number range and absence of acoustic dipoles in the BWI frequency range. The findings of this study support BWI prediction models based on Amiet’s theory and suggest that BWI noise can be ignored for predictions of rotor noise in descent flight conditions.

We thank Igor B. Morozov for his interest in our article and for his comment (Morozov 2011) regarding the non-parametric attenuation curves for the Irpinia–Basilicata region obtained by generalized spectral inversion (Cantore et al. 2011). Morozov's comment has its root in a new model proposed by Morozov (2008, 2010) for the interpretation of seismic attenuation data, where the author comes to the conclusion that the typically used geometrical spreading terms are oversimplified and argues in favor of a new geometrical spreading model basically including an additional corrective term decaying exponentially with time (respectively distance). Morozov (2008, 2010) reanalyzed a range of datasets presented by other authors in the past using this model and, contrary to previous studies, concludes that all these datasets can be well explained using a frequencyindependent Qe (which he terms as “effective Q”), in contrast to the classical power-law model often implying a significant frequency-dependence of Q. The publication of his new model has led to a major controversy, and other authors provided a range of weighty counterarguments to Morozov's point of view (Xie 2010; Li 2010; Li and Lu 2010). In his comment on our article, Morozov (2011) essentially repeats the arguments that he has already discussed in great detail in his previous papers (Morozov 2008, 2010). In view of this open discussion, we acknowledge that there are certainly different ways of interpreting our non-parametric attenuation results (see also the paper Oth et al. 2011), but also note that a detailed discussion of the differing viewpoints was beyond the scope of our article, for which we refer the interested reader to the cited publications. Nevertheless, we would like to provide a brief discussion of the key points of Morozov's (2011) comment in the following. First of all, we would like to recall that the attenuation results obtained from our spectral inversions are the non-parametric attenuation functions themJ Seismol (2012) 16:91–93 DOI 10.1007/s10950-011-9243-x

A study of the local magnitude scale in the Basin of Mexico is presented using about 1810 synthetic Wood-Anderson peak amplitudes at 13 stations from the Valley of Mexico seismic network (rsvm). Data from all available rsvm earthquakes located in central Mexico were used, and the events within the Basin of Mexico were relocated. Parametric and nonparametric expressions for the log A ( r ) curve were determined and corrected to Richter’s reference level, which was obtained after the linear separation of source, site, and distance terms. The parametric attenuation function is best expressed as log A ( r ) = −0.48 log ( r /100) − 0.0018 ( r − 100) − 3. The results of the attenuation curve are consistent with ground-motion scaling studies (geometrical spreading, Qs , and stochastic simulations): the Basin of Mexico is characterized by a complex attenuation model at short distances, but the general trend is that of a highly attenuative region. We observe that the coda magnitude has been underestimating the excitation levels of local events in the Basin of Mexico and may require station corrections to improve its accuracy. Our study will provide local magnitudes to the seismicity catalog in the Basin of Mexico, thus contributing to more successful earthquake hazard analyses in the most important and heavily populated region of Mexico.