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  • Seismological Society of America (SSA)  (11)
  • 1
    Publication Date: 2015-10-02
    Description: Ground-motion prediction equations (GMPEs) play a crucial role for estimating the seismic hazard in any region using either a deterministic or a probabilistic approach. Indeed, they represent a reliable and fast tool to predict strong ground motion, given source and propagation parameters. In this article, we estimated GMPEs for the South Korea peninsula. GMPEs were computed for peak ground displacement, peak ground velocity, peak ground acceleration, and spectral accelerations (damping at 5%) at 13 different periods from 0.055 to 5 s. We analyzed data from 222 earthquakes recorded at 132 three-component stations of the South Korea Seismic Network, from 2007 to 2012, with local magnitude ranging between 2.0 and 4.9 and epicentral distances varying from 1.4 to ~600 km. A nonlinear mixed effects technique is used to infer the GMPE coefficients. This technique includes both fixed and random effects and accounts for both inter- and intraevent dependencies in the data. Station-specific corrective coefficients were estimated by a statistical approach and were included in the final ground-motion prediction model. Finally, predictions for peak ground acceleration and spectral acceleration are compared with observations recorded for an M L  5.1 earthquake that occurred in 2014, the data for which were not included in the modeling. Online Material: Figures showing final ground-motion prediction equation models versus observations, and intra- and interevent residuals.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2012-12-01
    Description: We present a nonlinear technique for the purpose of estimating the distribution of the final slip and the rupture velocity on the fault plane from the inversion of strong-motion records. In this work, the ground-motion simulation is obtained by evaluating the representation integral in the frequency domain, through a finite-element approach, based on a Delaunay’s triangulation of the fault plane. The slip distribution is parameterized by a linear combination of 2D overlapping Gaussian functions. This choice allows us to relate the maximum frequency in the data to the smallest resolvable wavelength on the fault plane, insuring a smooth representation for the slip function. We investigate the capability of such a representation to describe complex slip maps, and we relate the width of the Gaussian function and the overlapping to the minimum wavelength of the slip function. The inverse problem is solved by a two-step procedure aimed at separating the computation of the rupture velocity from the evaluation of the slip distribution. While a global exploration is maintained for the rupture velocity, for each explored value of this quantity, the slip solution is computed as the best solution approaching the observations in the sense of the L2 norm. The nonlinear step is performed through the neighborhood algorithm (NA), while the linear one uses the nonnegative least-squares (NNLS) method. The technique has been applied to retrieve the rupture history of the 2008 Iwate–Miyagi, Japan, earthquake. The slip distribution is characterized by a large slip patch extending from the hypocenter to the southern shallow part of the fault plane, with a maximum amplitude of 6 m. In addition, a relatively smaller asperity is located in the north shallow part of the fault. We found that the rupture lasted about 12 s with an average rupture velocity of about 2.0 km/s. Online Material: Figures showing synthetic inversion test results.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 3
    Publication Date: 2016-04-28
    Description: Finite-fault earthquake source inversions infer the (time-dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities. To address the uncertainties in earthquake-source inversion methods and to understand strengths and weaknesses of the various approaches used, the Source Inversion Validation (SIV) project conducts a set of forward-modeling exercises and inversion benchmarks. In this article, we describe the SIV strategy, the initial benchmarks, and current SIV results. Furthermore, we apply statistical tools for quantitative waveform comparison and for investigating source-model (dis)similarities that enable us to rank the solutions, and to identify particularly promising source inversion approaches. All SIV exercises (with related data and descriptions) and statistical comparison tools are available via an online collaboration platform, and we encourage source modelers to use the SIV benchmarks for developing and testing new methods. We envision that the SIV efforts will lead to new developments for tackling the earthquake-source imaging problem.
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
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  • 4
    Publication Date: 2012-12-01
    Description: We present a nonlinear technique for the purpose of estimating the distribution of the final slip and the rupture velocity on the fault plane from the inversion of strong-motion records. In this work, the ground-motion simulation is obtained by evaluating the representation integral in the frequency domain, through a finite-element approach, based on a Delaunay's triangulation of the fault plane. The slip distribution is parameterized by a linear combination of 2D overlapping Gaussian functions. This choice allows us to relate the maximum frequency in the data to the smallest resolvable wavelength on the fault plane, insuring a smooth representation for the slip function. We investigate the capability of such a representation to describe complex slip maps, and we relate the width of the Gaussian function and the overlapping to the minimum wavelength of the slip function. The inverse problem is solved by a two-step procedure aimed at separating the computation of the rupture velocity from the evaluation of the slip distribution. While a global exploration is maintained for the rupture velocity, for each explored value of this quantity, the slip solution is computed as the best solution approaching the observations in the sense of the L2 norm. The nonlinear step is performed through the neighborhood algorithm (NA), while the linear one uses the nonnegative least-squares (NNLS) method. The technique has been applied to retrieve the rupture history of the 2008 Iwate-Miyagi, Japan, earthquake. The slip distribution is characterized by a large slip patch extending from the hypocenter to the southern shallow part of the fault plane, with a maximum amplitude of 6 m. In addition, a relatively smaller asperity is located in the north shallow part of the fault. We found that the rupture lasted about 12 s with an average rupture velocity of about 2.0 km/s.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 5
    Publication Date: 2003-04-01
    Description: It has been previously demonstrated that no reflection is generated when elastic (or electromagnetic) waves enter a region with Perfectly Matching Layer (PML) absorbing conditions in a continuous medium. The practical application of PMLs, however, is in numerical modeling, where the medium is discretized by either a finite-element or a finite-difference scheme thus introducing a reduced amount of reflection. In such a case what is the practical and quantitative efficiency of PML absorbing boundaries? Assuming a regular spatial mesh, we start by evaluating analytically the reflection of body waves introduced by the discrete transition toward PML properties, under variable angle of incidence and wavelength. We then extend our evaluation with numerical tests for both body and Rayleigh waves. Surprisingly enough, the absorption remains equally efficient at wavelengths far larger than the PML thickness itself. As a consequence, the PML thickness can be kept minimal even for studies involving relatively low frequencies, and no rescaling with model size is required. Another pleasant feature is that it is all the more efficient at shallow angles of incidence. Finally, we show through numerical examples that a major advantage of using PMLs is their efficiency in absorbing Rayleigh waves at the free surface, a point where more classical methods perform rather poorly. Although previous authors essentially limited the description of their discrete implementation to 2D, we develop to some level of detail a 3D finite-difference scheme for PMLs and provide numerical examples.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 6
    Publication Date: 2009-07-29
    Description: A local magnitude scale has been defined for southern Italy, in the area monitored by the recently installed Irpinia Seismic Network. Waveforms recorded from more than 100 events of small magnitude are processed to extract synthetic Wood-Anderson traces. Assuming a general description of peak-displacement scaling with the distance, by means of linear and logarithmic contributions, a global exploration of the parameter space is performed by a grid-search method with the aim of investigating the correlation between the two decay contributions and seeking for a physical solution of the problem. Assuming an L (super 2) norm, we found M=logA+1.79 log R-0.58 yielding an error on the single estimation smaller than 0.2, at least when the hypocenter location is accurate. Station corrections are investigated through the station residuals, referring to the average value of the magnitude. Using a z test, we found that some stations exhibit a correction term significantly different from 0. The use of the peak acceleration and peak velocity as indicators of the magnitude is also investigated.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 7
    Publication Date: 2012-12-01
    Description: We present a nonlinear technique for the purpose of estimating the distribution of the final slip and the rupture velocity on the fault plane from the inversion of strong-motion records. In this work, the ground-motion simulation is obtained by evaluating the representation integral in the frequency domain, through a finite-element approach, based on a Delaunay's triangulation of the fault plane. The slip distribution is parameterized by a linear combination of 2D overlapping Gaussian functions. This choice allows us to relate the maximum frequency in the data to the smallest resolvable wavelength on the fault plane, insuring a smooth representation for the slip function. We investigate the capability of such a representation to describe complex slip maps, and we relate the width of the Gaussian function and the overlapping to the minimum wavelength of the slip function. The inverse problem is solved by a two-step procedure aimed at separating the computation of the rupture velocity from the evaluation of the slip distribution. While a global exploration is maintained for the rupture velocity, for each explored value of this quantity, the slip solution is computed as the best solution approaching the observations in the sense of the L2 norm. The nonlinear step is performed through the neighborhood algorithm (NA), while the linear one uses the nonnegative least-squares (NNLS) method. The technique has been applied to retrieve the rupture history of the 2008 Iwate-Miyagi, Japan, earthquake. The slip distribution is characterized by a large slip patch extending from the hypocenter to the southern shallow part of the fault plane, with a maximum amplitude of 6 m. In addition, a relatively smaller asperity is located in the north shallow part of the fault. We found that the rupture lasted about 12 s with an average rupture velocity of about 2.0 km/s.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2012-03-29
    Description: We studied the ambient noise recorded at Irpinia Seismic Network (ISNet), a seismic network installed along the Campania-Lucania Apenninic chain (southern Italy), with the aim of characterizing the noise spectrum for each station as a function of time and the detection threshold of the network. For the latter purpose, we proposed a mixed indirect approach based on the signal-to-noise ratio (SNR) in the time domain, with parameterization in the frequency domain. The source signature is represented by the convolution of the Brune source time function with the Azimi attenuation curve. We found that 1.3 is the minimum magnitude an event should have to be detected at least at five stations with an SNR larger than five, wherever it occurs. We observed a space variability of the detection threshold as large as 0.3 units, ascribed to both the geometrical configuration of the network and the differences in the noise levels at the different stations. A sensitivity study indicates that the estimation of the detection threshold is robust for changes in the focal depths and stress drop, while it is strongly affected by the anelastic attenuation. In our case, changes of the reduced time t (super *) in the range 0.015-0.035 s generate changes in the completeness threshold of 0.5 units. Noise levels were obtained by a statistical analysis on the power spectral density curves along almost three years of continuous data from 22 stations. We found that, at short periods, major time variations are generated by diurnal changes in the wind intensity and other meteorological factors. At longer periods, we retrieved the microseismic peak, resulting from the constructive interference of oceanic waves. We also found an additional peak between 2 and 4 s, correlated with the sea wave height along the Tyrrhenian coast.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 9
    Publication Date: 2004-12-01
    Description: Several of the world's active volcanoes are located near densely populated areas, and therefore the seismic hazard associated with preeruptive earthquake activity and its relation to the potential damage of engineering structures should be considered as a part of risk evaluation and management. This is true for Mt. Vesuvius volcano (southern Italy), where several hundred thousand people are exposed to volcanic and related seismic risks. This study investigates the effect of preeruptive seismic activity through the massive simulation of earthquake waveforms in the magnitude, location, and focal-mechanism ranges expected for Mt. Vesuvius seismicity. Synthetics are processed to evaluate the characteristic strong-motion parameters, which are useful for estimating the seismic damage to a built-up environment that would arise from both the maximum expected single event and the cumulative effect of a large number of small events. Synthetic and observed strong-motion parameters for a selected set of recorded earthquakes are compared to validate the modeling approach. The scaling of the simulated peak ground acceleration (PGA) with distance appears to be influenced by earthquake depth, owing to the presence of a sharp velocity discontinuity at shallow depths underneath the Vesuvius area. On the other hand, the hysteretic energy spectrum, related to the plastic behavior of the structures, depends strongly on the b-parameter of the Gutenberg-Richter law (G-R). By varying the G-R law parameters across a reasonable expected range, we observe that the cumulative hysteretic energy is comparable to the values observed at near-source distances during the 1997 Umbria-Marche, Italy, event (M 5.8), which produced serious damage to buildings and infrastructure, although a significant PGA value was not recorded.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 10
    Publication Date: 2003-04-01
    Description: It has been previously demonstrated that no reflection is generated when elastic (or electromagnetic) waves enter a region with Perfectly Matching Layer (PML) absorbing conditions in a continuous medium. The practical application of PMLs, however, is in numerical modeling, where the medium is discretized by either a finite-element or a finite-difference scheme thus introducing a reduced amount of reflection. In such a case what is the practical and quantitative efficiency of PML absorbing boundaries? Assuming a regular spatial mesh, we start by evaluating analytically the reflection of body waves introduced by the discrete transition toward PML properties, under variable angle of incidence and wavelength. We then extend our evaluation with numerical tests for both body and Rayleigh waves. Surprisingly enough, the absorption remains equally efficient at wavelengths far larger than the PML thickness itself. As a consequence, the PML thickness can be kept minimal even for studies involving relatively low frequencies, and no rescaling with model size is required. Another pleasant feature is that it is all the more efficient at shallow angles of incidence. Finally, we show through numerical examples that a major advantage of using PMLs is their efficiency in absorbing Rayleigh waves at the free surface, a point where more classical methods perform rather poorly. Although previous authors essentially limited the description of their discrete implementation to 2D, we develop to some level of detail a 3D finite-difference scheme for PMLs and provide numerical examples.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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