Source Strong Motion Research Articles

Near-source magnitude scaling of spectral accelerations: analysis and update of Kotha et al. (2020) model

Ground-motion models (GMMs) are often used to predict the random distribution of Spectral accelerations ( $$\mathrm{SAs}$$ ) at a site due to a nearby earthquake. In probabilistic seismic hazard and risk assessment, large earthquakes occurring close to a site are considered as critical scenarios. GMMs are expected to predict realistic $$\mathrm{SAs}$$ with low within-model uncertainty ( $${\upsigma }_{\upmu }$$ ) for such rare scenarios. However, the datasets used to regress GMMs are usually deficient of data from critical scenarios. The (Kotha et al., A Regionally Adaptable Ground-Motion Model for Shallow Crustal Earthquakes in Europe Bulletin of Earthquake Engineering 18:4091–4125, 2020) GMM developed from the Engineering strong motion (ESM) dataset was found to predict decreasing short-period $$\mathrm{SAs}$$ with increasing $${M}_{W}\ge {\mathrm{M}}_{\mathrm{h}}=6.2$$ , and with large $${\upsigma }_{\upmu }$$ at near-source distances $$\le 30\mathrm{km}$$ . In this study, we updated the parametrisation of the GMM based on analyses of ESM and the Near source strong motion (NESS) datasets. With $${\mathrm{M}}_{\mathrm{h}}=5.7$$ , we could rectify the $${M}_{W}$$ scaling issue, while also reducing $${\upsigma }_{\upmu }$$ at $${M}_{W}\ge {\mathrm{M}}_{\mathrm{h}}$$ . We then evaluated the GMM against NESS data, and found that the $$\mathrm{SAs}$$ from a few large, thrust-faulting events in California, New Zealand, Japan, and Mexico are significantly higher than GMM median predictions. However, recordings from these events were mostly made on soft-soil geology, and contain anisotropic pulse-like effects. A more thorough non-ergodic treatment of NESS was not possible because most sites sampled unique events in very diverse tectonic environments. We provide an updated set of GMM coefficients, $${\upsigma }_{\upmu }$$ , and heteroscedastic variance models; while also cautioning against its application for $${M}_{W}\le 4$$ in low-moderate seismicity regions without evaluating the homogeneity of $${M}_{W}$$ estimates between pan-European ESM and regional datasets.

An Empirical Model to Account for Spectral Amplification of Pulse-Like Ground Motion Records

Near-source effects can amplify seismic ground motion, causing large demand to structures and thus their identification and characterization is fundamental for engineering applications. Among the most relevant features, forward-directivity effects may generate near-fault records characterized by a large velocity pulse and unusual response spectral shape amplified in a narrow frequency-band. In this paper, we explore the main statistical features of acceleration and displacement response spectra of a suite of 230 pulse-like signals (impulsive waveforms) contained in the NESS1 (NEar Source Strong-motion) flat-file. These collected pulse-like signals are analyzed in terms of pulse period and pulse azimuthal orientation. We highlight the most relevant differences of the pulse-like spectra compared to the ordinary (i.e., no-pulse) ones, and quantify the contribution of the pulse through a corrective factor of the spectral ordinates. Results show that the proposed empirical factors are able to capture the amplification effect induced by near-fault directivity, and thus they could be usefully included in the framework of probabilistic seismic hazard analysis to adjust ground-motion model (GMM) predictions.

Source Process of the October 2016 Tottori-Chubu Earthquake(<i>M</i>j 6.6) and Relation with Near Source Strong Motions

A source process inversion for an Mj 6.6 earthquake which occurred on October 21, 2016 in central Tottori Prefecture, was performed using near-field strong motion waveforms. The results showed that a left-lateral strike-slip component was dominant on the fault plane, the strike of which was in a NNW-SSE direction and the dip almost vertical. The moment magnitude was estimated as 6.2. A large slip area (asperity) was determined around the hypocenter, including its northern shallow part, in which the maximum slip was nearly 1.2 m. Another small asperity was estimated also near the northern edge of the fault plane. These two asperities corresponded to the two distinct wave packets observed at the stations near the source region. Furthermore, the results suggested that a discrepancy in frequency characteristics between these asperities existed, by comparing them with waveforms obtained just above the source region. We also estimated the stress drop on the fault plane by the final slip distribution, and the maximum value was about 16 MPa. This result meant that the strong ground motion excitation of this earthquake was of an average level compared to past crustal earthquakes in Japan.

Identification of High Frequency Pulses from Earthquake Asperities Along Chilean Subduction Zone Using Strong Motion

The Chilean subduction zone is one of the most active of the world with M = 8 or larger interplate thrust earthquakes occurring every 10 years or so on the average. The identification and characterization of pulses propagated from dominant asperities that control the rupture of these earthquakes is an important problem for seismology and especially for seismic hazard assessment since it can reduce the earthquake destructiveness potential. A number of studies of large Chilean earthquakes have revealed that the source time functions of these events are composed of a number of distinct energy arrivals. In this paper, we identify and characterize the high frequency pulses of dominant asperities using near source strong motion records. Two very well recorded interplate earthquakes, the 1985 Central Chile (Ms = 7.8) and the 2007 Tocopilla (Mw = 7.7), are considered. In particular, the 2007 Tocopilla earthquake was recorded by a network with absolute time and continuos recording. From the study of these strong motion data it is possible to identify the arrival of large pulses coming from different dominant asperities. The recognition of the key role of dominant asperities in seismic hazard assessment can reduce overestimations due to scattering of attenuation formulas that consider epicentral distance or shortest distance to the fault rather than the asperity distance. The location and number of dominant asperities, their shape, the amplitude and arrival time of pulses can be one of the principal factors influencing Chilean seismic hazard assessment and seismic design. The high frequency pulses identified in this paper have permitted us to extend the range of frequency in which the 1985 Central Chile and 2007 Tocopilla earthquakes were studied. This should allow in the future the introduction of this seismological result in the seismic design of earthquake engineering.

Estimating coseismic deformations from near source strong motion records: methods and case studies

Digital strong-motion accelerographs have opened up the possibility of extracting ground motion characteristics at much lower frequencies than was offered by analogue instruments. High-quality digital data obtained close to the faults have tempted several efforts to retrieve permanent ground displacements after an earthquake. Such attempts have been partly successful, and somewhat subjective, the main reason being the presence of baseline offsets in the accelerometric data. We review existing methods for such applications, discuss their limitations and propose a more objective and improved scheme to make baseline adjustments and obtain permanent displacements. The proposed technique is applied to 26 digital recordings from the 29 May 2008 Olfus Earthquake in South Iceland and 9 recordings from the 1999 Chi-Chi, Taiwan, Earthquake, and the permanent displacements obtained are compared with published results and GPS measurements from nearby stations. Our case studies show that the proposed technique, in addition to being simple and objective, is effective in making adjustments for baseline errors in accelerometric data.

Two‐dimensional numerical modeling for near source strong ground motions of the Chelungpu fault during the 1999 Chi‐Chi Taiwan earthquake

Two‐dimensional numerical modeling based on the finite element method is employed to simulate near fault tip strong motion records of the Chi‐Chi Earthquake on September 21, 1999, in order to examine the kinematics of earthquake faulting and near field wave propagation properties. The modified ‘split‐node’ technique of Melosh and Raefsky is employed to simulate the equivalent body forces system for a double couple without moment. Different source parameters are examined to synthesize ground motions to simulate the observed near source strong motion records. Based on waveform modeling, a low angle thrust fault 30 km in length with a dip angle of 30°?is suggested for the ruptures near its northern end. The rupture is interpreted as bilateral from the center of the fault. The determined source rupture velocity and risetime are 2.0 km/sec and 5 sec, respectively, while the dislocation is about 6 meters. The estimated peak fault slip velocity is about 3.5 km/sec, showing a largest value than previously reported. Results of this study show that there was lower rupture velocity and longer rise‐time of the fault slip on the northern part of the Chi‐Chi Earthquake fault. Numerical modeling of this study indicates that the directivity of source rupture does not show significant low frequency ground motion near the earthquake fault tip. However, the effects of surface breaking of the fault movement have contributed large ground deformations near the fault tip, consequently, inducing large damage.

Simulation of near source ground motion and its characteristics

It is fact that the severe ground motions of shear waves have a strong effect on the dynamic behavior of buildings and civil structures. We simulate near source strong motions of a pure shear wave and synthesize small motions, using the parameters based on the recorded accelerograms at the site that is regarded as a base rock in the Osaka basin, Japan. By making use of a stochastic technique, we can easily introduce higher frequency contents in the motions and apply the technique to the synthesis of small waves regarding as a green function. We also introduce to the analysis the useful relationships among the time duration Td, the seismic moment M0, the corner frequency fc and the high cutoff frequency fmax which were regressed by a simple representation scheme. Considering two active faults that may affect severe damage on buildings and civil structures, we try to predict strong ground motions in Osaka basin and show the characteristics of them.

The 1997 Umbria-Marche (central Italy) Earthquake Sequence: Insights on the mainshock ruptures from near source strong motion records

A small size foreshock and the two mainshocks of the Umbria Marche earthquake sequence which occurred on September 26, 1997 have been recorded by two digital 3C accelerometers located at near source distances. The close epicentral distance and azimuthal location relative to the fault orientation and geometry make these records relevant to look at the detail of the rupture kinematics. S-wave polarizations, apparent source time duration and waveforms from strong motion records are used to constrain the location of the fracture origin point, the fault geometry, the final slip distribution, size and mechanism of the events. The final model shows that the seismic ruptures occurred along two adjacent, sub-parallel, low angle dipping normal faults. The relative timing, location and geometry of the mainshock faults suggest the presence of a transfer zone (barrier) which has probably controlled the amplitude increase of local stress released by the first rupture at its NW edge which triggered about 9 hours later the second rupture.

The 1997 Umbria-Marche (central Italy) earthquake sequence: Insights on the mainshock ruptures from near source strong motion records

The 1997 Umbria-Marche (central Italy) earthquake sequence: Insights on the mainshock ruptures from near source strong motion records

A very broad‐band stochastic source model used for near source strong motion prediction

The simulation of near‐source ground motion requires the calculation of ground motion in a very broad‐band frequency range (typically from 10 s to 0.1 s). However, the calculation of complete Green's function is too time consuming for high‐frequency simulations. We present a method to obtain very broad‐band synthetic seismograms, combining the kinematic spectral source model of Bernard et al., [1996], and an algorithm, based on the wavenumber technique of Bouchon and Aki, [1977]. High‐frequency radiations are evaluated with ray theory and a broad‐band dislocation defined on a finely discretized fault plane. The low‐frequency radiations due to the near‐ and intermediate‐fields are calculated with the complete field algorithm on a low‐wavenumber filtered dislocation. This dislocation is defined on a coarse grid over the fault, hence significantly reducing the computing time. We apply this method to compute the ground motion at a station located near an extended fault, where the near‐, intermediate‐ and far‐field radiations are important.

Estimation of Source Parameters by the Inversion of Near Source Strong Motion Wave Forms

Estimation of Source Parameters by the Inversion of Near Source Strong Motion Wave Forms

S‐wave polarization inversion of the 15 October 1979, 23:19 Imperial Valley Aftershock: Evidence for anisotropy and a simple source mechanism

S polarizations from the 16 near source strong motion stations of the 15 October 1979, 23:19 Imperial Valley aftershock have been analyzed for constraining the source mechanism. They show a significant delay of the East component relative to the North component at most stations. Assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments (where the rays are dominantly vertical), the measured fast S polarizations are mainly oriented between N0°E and N25°E, and show a mean delay of 0.25 s between slow and fast S. This is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it, with a mean fluid filled crack density of 0.1 to 0.2. The records, once corrected for the splitting effect, show S polarizations stable in time, which have been used to estimate the fault mechanism parameters. This is done by applying a new inversion method which is based on a model probability estimate. The error function is related to the polarization misfits, and the associated joint probability function of the data and parameter is computed in the whole model space. The inversion of the corrected S polarizations provides a unique and well constrained solution for the Imperial Valley aftershock mechanism (strike 145±5°, dip 85±10°, slip −155±15°), which is compatible with the available P polarities and is quite similar to the main shock mechanism.