Near Source Research Articles

Analytical modelling of near‐source pulse‐like seismic demand for multi‐linear backbone oscillators

SummaryNonlinear static procedures, which relate the seismic demand of a structure to that of an equivalent single‐degree‐of‐freedom oscillator, are well‐established tools in the performance‐based earthquake engineering paradigm. Initially, such procedures made recourse to inelastic spectra derived for simple elastic–plastic bilinear oscillators, but the request for demand estimates that delve deeper into the inelastic range, motivated investigating the seismic demand of oscillators with more complex backbone curves. Meanwhile, near‐source (NS) pulse‐like ground motions have been receiving increased attention, because they can induce a distinctive type of inelastic demand. Pulse‐like NS ground motions are usually the result of rupture directivity, where seismic waves generated at different points along the rupture front arrive at a site at the same time, leading to a double‐sided velocity pulse, which delivers most of the seismic energy. Recent research has led to a methodology for incorporating this NS effect in the implementation of nonlinear static procedures. Both of the previously mentioned lines of research motivate the present study on the ductility demands imposed by pulse‐like NS ground motions on oscillators that feature pinching hysteretic behaviour with trilinear backbone curves. Incremental dynamic analysis is used considering 130 pulse‐like‐identified ground motions. Median, 16% and 84% fractile incremental dynamic analysis curves are calculated and fitted by an analytical model. Least‐squares estimates are obtained for the model parameters, which importantly include pulse period Tp. The resulting equations effectively constitute an R − μ − T − Tp relation for pulse‐like NS motions. Potential applications of this result towards estimation of NS seismic demand are also briefly discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Estimation of PM<sub>10</sub> concentrations over Seoul using multiple empirical models with AERONET and MODIS data collected during the DRAGON-Asia campaign

Abstract. The performance of various empirical linear models to estimate the concentrations of surface-level particulate matter with a diameter less than 10 μm (PM10) was evaluated using Aerosol Robotic Network (AERONET) sun photometer and Moderate-Resolution Imaging Spectroradiometer (MODIS) data collected in Seoul during the Distributed Regional Aerosol Gridded Observation Network (DRAGON)-Asia campaign from March to May 2012. An observed relationship between the PM10 concentration and the aerosol optical depth (AOD) was accounted for by several parameters in the empirical models, including boundary layer height (BLH), relative humidity (RH), and effective radius of the aerosol size distribution (Reff), which was used here for the first time in empirical modeling. Among various empirical models, the model which incorporates both BLH and Reff showed the highest correlation, which indicates the strong influence of BLH and Reff on the PM10 estimations. Meanwhile, the effect of RH on the relationship between AOD and PM10 appeared to be negligible during the campaign period (spring), when RH is generally low in northeast Asia. A large spatial dependency of the empirical model performance was found by categorizing the locations of the collected data into three different site types, which varied in terms of the distances between instruments and source locations. When both AERONET and MODIS data sets were used in the PM10 estimation, the highest correlations between measured and estimated values (R = 0.76 and 0.76 using AERONET and MODIS data, respectively) were found for the residential area (RA) site type, while the poorest correlations (R = 0.61 and 0.68 using AERONET and MODIS data, respectively) were found for the near-source (NS) site type. Significant seasonal variations of empirical model performances for PM10 estimation were found using the data collected at Yonsei University (one of the DRAGON campaign sites) over a period of 17 months including the DRAGON campaign period. The best correlation between measured and estimated PM10 concentrations (R = 0.81) was found in winter, due to the presence of a stagnant air mass and low BLH conditions, which may have resulted in relatively homogeneous aerosol properties within the BLH. On the other hand, the poorest correlation between measured and estimated PM10 concentrations (R = 0.54) was found in spring, due to the influence of the long-range transport of dust to both within and above the BLH.

The displacement coefficient method in near‐source conditions

SummaryThe use of nonlinear static procedures for performance‐based seismic design (PBSD) and assessment is a well‐established practice, which has found its way into modern codes for quite some time. On the other hand, near‐source (NS) ground motions are receiving increasing attention, because they can carry seismic demand systematically different and larger than that of the so‐called ordinary records. This is due to phenomena such as rupture forward directivity (FD), which can lead to distinct pulses appearing in the velocity time‐history of the ground motion. The framework necessary for taking FD into account in probabilistic seismic hazard analysis (PSHA) has recently been established. The objective of the present study is to discuss the extension of nonlinear static procedures, specifically the displacement coefficient method (DCM), with respect to the inelastic demand associated with FD. In this context, a methodology is presented for the implementation of the DCM toward estimating NS seismic demand, by making use of the results of NS‐PSHA and a semi‐empirical equation for NS‐FD inelastic displacement ratio. An illustrative application of the DCM, with explicit inclusion of NS‐pulse‐like effects, is given for a set of typical plane R/C frames designed under Eurocode provisions. Different scenarios are considered in the application and nonlinear dynamic analysis results are obtained and discussed with respect to the static procedure estimates. Conclusions drawn from the results may help to assess the importance of incorporating NS effects in PBSD. Copyright © 2014 John Wiley & Sons, Ltd.

Evaluating Centroid-Moment-Tensor Uncertainty in the New Version of ISOLA Software

Online Material: Additional figures. Focal mechanism and moment tensor determinations based on regional and local waveforms have become routine tasks in seismology. In recent years, considerable effort has been dedicated to extend software capabilities, in particular towards the inclusion of finite‐extent sources and easy, user‐friendly use. Two examples of these efforts are the FMNEAREG and KIWI software packages. Focal Mechanism from NEAr source to REGional distance records (FMNEAREG; Delouis et al. , 2008; Maercklin et al. , 2011) performs a grid search in order to find pure double‐couple sources, based on a 1D (line) finite‐source representation and complete elastodynamic wave field. The KIWI software (KInematic Waveform Inversion; Cesca et al. , 2010) allows the automatic retrieval of point‐source parameters, and for magnitudes above a given threshold kinematic finite‐fault rupture models can also be obtained. KIWI relies on a pre‐calculated database of Green’s functions. KIWI performs the moment tensor inversion in two steps, the first of which is based on the fit of amplitude spectra. The second step is based on time‐domain waveform fits. The ISOLA, ISOLated Asperities, software package (Sokos and Zahradnik, 2008) also performs waveform inversions to find source parameters. ISOLA is based on FORTRAN codes and offers a user‐friendly MATLAB graphical interface (GUI; www.mathworks.com/products/MATLAB). ISOLA allows for both single‐ and multiple‐point‐source iterative deconvolution (Kikuchi and Kanamori, 1991) inversion of complete regional and local waveforms. The moment tensor is retrieved through a least squares inversion, whereas the position and origin time of the point sources are grid searched. The computation options include inversion to retrieve the full moment tensor (MT), deviatoric MT, and pure double‐couple MT. Finite‐extent source inversions may also be performed by prescribing a priori the double‐couple mechanism (which will remain homogeneous over the fault plane). Green’s functions, including near‐field terms, are calculated using the discrete wavenumber method (Bouchon, …