Near-source Effects Research Articles

Integration of rupture directivity models for the US National Seismic Hazard Model

Several rupture directivity models (DMs) have been developed in recent years to describe the near-source spatial variations in ground-motion amplitudes related to propagation of rupture along the fault. We recently organized an effort toward incorporating these directivity effects into the US Geological Survey (USGS) National Seismic Hazard Model (NSHM), by first evaluating the community’s work and potential methods to implement directivity adjustments into probabilistic seismic hazard analysis (PSHA). Guided by this evaluation and comparison among the considered DMs, we selected an approach that can be readily implemented into the USGS hazard software, which provides an azimuthally varying adjustment to the median ground motion and its aleatory variability. This method allows assessment of the impact on hazard levels and provides a platform to test the DM amplification predictions using a generalized coordinate system, necessary for consistent calculation of source-to-site distance terms for complex ruptures. We give examples of the directivity-related impact on hazard, progressing from a simple, hypothetical rupture, to more complex fault systems, composed of multiple rupture segments and sources. The directivity adjustments were constrained to strike–slip faulting, where DMs have good agreement. We find that rupture directivity adjustments using a simple median and aleatory adjustment approach can affect hazard both from a site-specific perspective and on a regional scale, increasing ground motions off the end of the fault trace up to 30%–40% and potentially reducing it for sites along strike. Statewide hazard maps of California show that the change in shaking along major faults can be a factor to consider for assessing long-period [Formula: see text] near-source effects within the USGS NSHM going forward, reaching up to 10%–20%. Finally, we suggest consideration of minimum parameter ranges and baseline requirements as future DMs are developed to minimize single approach adaptations to enable more consistent application within both ground motion and hazard studies.

Near-source effects on DAS recording: implications for tap tests

SUMMARY In the immediate vicinity of a source, there are strong gradients in the seismic wavefield that are tamed and modified in distributed acoustic sensing (DAS) recording due to combined effects of gauge-length averaging and local stacking on the local strain field. Close to a source broadside propagation effects are significant, and produce a characteristic impact on the local DAS channels. In the presence of topography, of surface or cable, additional effects are introduced that modify the expected signal. All these influences mean that the results of tap tests used to calibrate the channel positions along a DAS cable may give a distorted view of the actual geometry. Such effects can be important for detailed mapping of faulting processes and comparable features.

Seismic loss assessment of residential buildings in Karaj, Iran, by considering near-source effects using stochastic finite-fault approach

Seismic loss assessment of residential buildings in Karaj, Iran, by considering near-source effects using stochastic finite-fault approach

Quantification of rupture directivity effects on strong ground motion during the 8 January 2022 MS6.9 Menyuan earthquake in Qinghai, China

On 8 January 2022, an MS6.9 earthquake occurred in the Menyuan county, Qinghai province at the northeastern margin of the Tibetan Plateau. A dense observatory network consisting of accelerographs and intensity meters has obtained many acceleration recordings in this region, which previously had only a few strong motion recordings. Using acceleration data recorded by 141 accelerographs and micro-electro mechanical system (MEMS)-based intensity meters within 100 km of the causative fault, we investigated the spatial distribution and attenuation characteristics of the near-source ground motion during this event. The observed response spectra were compared with the prediction obtained using the NGA-West2 models. It was found that, in the long-period section of >1.0 s, the acceleration response spectra in the rupture forward direction (southeast of the epicenter) were greater than the prediction, while observations on the northwest side were lower than the prediction. Through the spatial distribution of the ground motion and quantitative analysis of the residuals, we found that the rupture directivity effects on the ground motions were controlled by the spatial azimuth relative to the fault and the along-strike projected length of the rupture, which mainly affected the long-period portion of the >1.0 s. In the rupture forward direction, the spectral accelerations (SAs) with periods of 1.0–10.0 s were enhanced to 1.18–1.56 times the average. In the backward direction (northwest of the epicenter), the SAs with periods of 1.0–10.0 s were only 0.63–0.86 times the average. The rupture directivity effects during the MS6.9 (MW6.6) earthquake exhibited an obvious period-dependence and narrowband characteristics, and the amplification effects were the strongest at 5.0 s. The amplification factors in the straight forward directions were underestimated by the empirical model. Two typical velocity pulse records were observed in the rupture forward direction, and their response spectra were significantly amplified at long periods of greater than 1.0 s, with peak amplification values of 4.0–6.0 near the characteristic period of the pulses. This amplification was related to the resonance excited by a specific periodic pulse. Our study provides a reference of quantifying near-source effects for both moderate and major shallow crustal earthquakes by using MEMS recordings in this region.

Consistency Check of ITACAext, the Flatfile of the Italian Accelerometric Archive

We present the results of a consistency check performed over a flatfile of accelerometric data extracted from the ITalian ACcelerometric Archive (ITACA), enriched with velocimetric records of events with magnitude M < 4.0. The flatfile, called ITACAext, includes 31,967 waveforms from 1709 shallow crustal earthquakes, in the magnitude range from 3.0 to 6.9, and occurred in the period of 1972–2019 in Italy. The consistency check is carried out by decomposing the residuals obtained from a reference ground motion model, for the ordinates of the 5% damped acceleration response spectra. The residual components are subsequently analyzed to identify a list of events, stations, and records that significantly deviate from the median trends predicted by the model. The results indicate that about 10% of events and stations are outliers, while only 1% of the waveforms present anomalous amplitudes. The asymmetrical azimuthal coverage of seismic stations around the epicenter is the most common issue that can affect the estimates of the repeatable event residual term. On the other hand, peculiarities in the site-response or wrong estimates of the soil parameters (i.e., the average shear-wave velocity in the first 30 m of the subsoil) are the main issues related to the repeatable station residuals. Finally, single records can show large residuals because of issues related to signal acquisition (e.g., multiple events, noisy records) or possible near-source effects (e.g., rupture directivity).

NESS2.0: An Updated Version of the Worldwide Dataset for Calibrating and Adjusting Ground-Motion Models in Near Source

ABSTRACT We present an extended and updated version of the worldwide NEar-Source Strong-motion (NESS) flat file, which includes an increased number of moderate-to-strong earthquakes recorded in epicentral area, new source metadata and intensity measures, comprising spectral displacements and fling-step amplitudes retrieved from the extended baseline correction processing of velocity time series. The resulting dataset consists of 81 events with moment magnitude≥5.5 and hypocentral depth shallower than 40 km, corresponding to 1189 three-component waveforms, which are selected to have a maximum source-to-site distance within one fault length. Details on the flat files, metadata, and ground-motion parameters, processing scheme, and statistical findings are presented and discussed. The analysis of these data allows recognizing the presence of distinctive features (such as pulse-like waveforms, large vertical components, and hanging-wall effects) that can be exploited to assess their impact on near-source seismic motion. As an example, we use the NESS2.0 dataset for calibrating an empirical correction factor of a regional ground-motion model (GMM) mainly based on far-field records. In this way, we can adjust the median predictions to account for near-source effects not fully captured by the reference model. The final goal of this work is to promote the use of the NESS2 flat file as a tool to disseminate qualified and referenced near-source data and metadata in the light of improving the constraints of GMMs (both empirical and physics-based) close to the source.

Seismic response of large earth dams in near-source areas

The paper focuses on the seismic response of earth dams close to active faults. The engineering implications of such an issue may be crucial since many existing dams, which have been in operation for several years, are located nearby or even on seismogenic faults. Owing to their size, large earth dams could dramatically be compromised by near-source ground motion. The spatial variability of the incoming seismic wave could promote differential settlements in the dam body and consequently fractures in the watertightness elements with an overall decrease of dam safety. The paper refers to the iconic case of Conza Dam in Italy. During its construction, it was seriously damaged by the catastrophic 1980 Irpinia earthquake (M = 6.9), whose epicenter was very close to the dam site. To model the seismic wave propagation from the causative fault to the dam site, a physics-based approach has been adopted by implementing the domain reduction method to simulate the whole boundary value problem. Two models were hence considered: a regional model, containing the source and the propagation medium of the Campania region basin, and a site model containing the upper foundation soils and the dam body.The paper compares the different response of the dam in the hypothesis of contemplating or disregarding near-source effects. Finally, the dam performance to seismic scenarios resembling the source mechanism of the 1980 Irpinia earthquake was analysed to ascertain the resilience of the dam to future earthquake scenarios having source characteristics similar to those of the 1980 event.

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.

Accounting for directivity-induced pulse-like ground motions in building portfolio loss assessment

Earthquake-induced pulse-like ground motions are often observed in near-source conditions due to forward-directivity. Recent worldwide earthquakes have emphasised the severe damage potential of such pulse-like ground motions. This paper introduces a framework to quantify the impact of directivity-induced pulse-like ground motions on the direct economic losses of building portfolios. To this aim, a simulation-based probabilistic risk modelling framework is implemented for various synthetic building portfolios located either in the fault-parallel or fault-normal orientations with respect to a case-study strike–slip fault. Three low-to-mid-rise building typologies representative of distinct vulnerability classes in the Mediterranean region are considered: non-ductile moment-resisting reinforced concrete (RC) frames with masonry infills, mainly designed to only sustain gravity loads (i.e. pre-code frames); moment-resisting RC infilled frames designed considering seismic provisions for high ductility capacity (i.e. special-code frames); special-code steel moment-resisting frames. Monte Carlo-based probabilistic seismic hazard analysis is first performed, considering the relevant modifications to account for the pulse-occurrence probability and the resulting spectral amplification. Hazard curves for sites/buildings located at different distances from the fault are obtained, discussing the spatial distribution of the hazard amplification. A set of pulse-like ground motions and a set of one-to-one spectrally-equivalent ordinary records are used to perform non-linear dynamic analysis and derive fragility relationships for each considered building typology. A vulnerability model is finally built by combining the derived fragility relationships with a (building-level) damage-to-loss model. The results are presented in terms of intensity-based and expected annual loss for synthetic portfolios of different sizes and distribution of building types. It is shown that, for particularly short-period structures (e.g. infilled RC frames), the influence of near-source directivity can be reasonably neglected in the fragility derivation while kept in place in the hazard component. Overall, near-source directivity effects are significant when estimating losses of individual buildings or small portfolios located very close to a fault. Nevertheless, the impact of pulse-like ground motions on losses for larger portfolios can be considered minimal and can be neglected in most of the practical large-scale seismic risk assessment applications.

Strong Ground-Motion Characteristics Observed in the November 12, 2017, Mw7.3 Sarpol-e Zahab, Iran Earthquake

ABSTRACT The November 12, 2017, moment magnitude (Mw) 7.3 Sarpol-e Zahab earthquake, which occurred after a seismic quiescence, close to the Iran–Iraq border, is one of the largest events recorded in the Zagros fold and thrust belt. In this study, we analyzed strong-motion data of this event from 38 stations within an epicentral distance of 36 to 200 km. Spectral analysis of strong-motion data resulted in an Mw estimate of 7.34, an average rupture velocity of 3.03 km/s, and a stress drop of 154 bars. We estimated that shear dislocation propagated over a rectangular fault area with a length of ~97 km and width of ~17 km. Comparison of the horizontal peak ground-motions from this earthquake with the ground-motion model for active crustal regions indicated overall good agreement; however, the model underestimates ground-motions at stations within distances of approximately 30 km or less. The model appears to underestimate the observed strong-motion recordings from the Mw7.4 September 16, 1978, Tabas and the Mw7.4 June 20, 1990, Manjil earthquakes as well. Investigation of the observed accelerograms showed that the Sarpol-e Zahab earthquake was nucleated by a sub-event with a local magnitude (ML) of 4.9 which contributed about 0.02% to the total seismic moment. The empirical data indicated multiple arrivals due to the development of the rupture as two major sub-events. The spatial distribution of stations along with the time difference between the sub-events on the recorded time-series implies their occurrence from the hypocenter toward the S-SE direction which is coincident with the principal directions of the majority of damaged buildings. The estimated Mw of these sub-events is 6.79 and 7.08, respectively. The distinctive characteristics of near-field recordings from Sarpol-e Zahab earthquake (e.g., multiple phase arrivals due to complex source process, forward directivity effect, and blind thrust faulting) make these data a valuable supplement to the recorded ground-motion database. The improved strong-motion database plays a fundamental role in calibrating seismological simulation models, developing ground-motion models that capture near-source effects, and performing nonlinear dynamic structural analyses.

Effective denoising of magnetotelluric (MT) data using a combined wavelet method

Noise interference, especially from human noise, seriously affects the quality of magnetotelluric (MT) data. Strong human noise distorts the apparent resistivity curve, known as the near-source effect, causing poor reliability of MT data inversion. Based on analyzing the frequency characteristics of human noise resulting from the surrounding environment, a new wavelet-based denoising method is proposed for both synthetic and real MT data in this paper. The new technique combines multi-resolution analysis with a wavelet threshold algorithm based on Bayes estimation and has a remarkable effect on denoising at all band frequencies. The multi-resolution analysis method was employed to reduce long-period noise, and a wavelet threshold algorithm was used to eliminate strong high-frequency noise. In this research, the improved algorithm was assessed via simulated experiments and field measurements with regard to the reduction in human noises. This study demonstrates that the new denoising technique can increase the signal-to-noise ratio by at least 112% and provides an extensive analysis method for mineral resource exploration.

Seismic performance and fragility functions of a 3D steel-concrete composite structure made of high-strength steel

This paper provides insight into a probabilistic seismic demand analysis of a steel-concrete composite structure made of a novel type of high-strength steel moment resisting frame, to be used either in a seismic risk assessment or a fully probabilistic Performance-Based Earthquake Engineering (PBEE) framework. The application of the PBEE methodology with a full probabilistic character is able to rigorously evaluate the seismic risk to which a structure may be exposed, as well as to quantify economic losses, including both direct -repair, reconstruction costs, etc.- and indirect costs -downtimes, etc.-. In this respect, the knowledge of seismic fragility functions is paramount. Moreover, due to the dynamic complexity of the examined structure caused by irregularity in elevation and different lateral-force resisting systems in the two main directions -moment resisting frames (MRFs) and concrete shear walls- the seismic behaviour is not straightforward to foresee. Therefore, two separate 2D analyses along the building main directions may not suffice to identify the actual dynamic response and, consequently, a 3D comprehensive probabilistic seismic demand analysis was performed by taking into account the earthquake incident angle. In order to exploit the inherent overstrength of non-dissipative members, consistently with the capacity design philosophy, the structure, that is a representative example of a realistic office building, is characterised by a newly-conceived type of moment resisting frame made of high-strength steel circular columns filled of concrete and of mild steel beams. In this respect, a nonlinear 3D FE model was developed and calibrated on experimental tests performed on both beam-to-column and column-base joints that formed MRFs. A multiple incremental dynamic analysis (MIDA) was then performed with two groups of bespoke accelerograms characterised, on one hand, by large magnitude and large distance and, on the other hand, by near-source effects. The earthquake incidence angle was also considered and, to decrease the number of simulations, the accelerogram-incident angle pairs were selected by means of the Latin hypercube sampling (LHS) method. The relevant seismic analyses highlighted the need to include the incident angle to better characterise its dynamic behaviour. Hence, the seismic fragility functions were built both for damage and collapse limit states considering both the maximum interstorey drift ratio as engineering demand parameter and different intensity measures as well as the incident angle randomness. The results showed that peak ground displacement entails a more efficient probabilistic model because the dominant structural dynamic behaviour was governed by MRFs characterised by fairly long periods.

Near-source effects and non-linear site response at Kashiwazaki-Kariwa Nuclear Power Plant, in the 2007 Chuetsu-Oki earthquake: evidence from surface and downhole records and 1D numerical simulations

The aim of this paper is to clarify the extent of near-source effects and non-linear soil response that took place at the Japanese nuclear site of Kashiwazaki-Kariwa, during the MW 6.6 Niigata-Ken Chūetsu-Oki earthquake (July, 16th 2007). The interest of this case study stems from the relative small source-to-site distance and shallow hypocenter depth, coupled with the consistent seismic record database available from two down-hole arrays of strong motion accelerometers, installed within the nuclear site. Records were processed first to highlight those features related to near-source conditions, such as directivity and polarization, and, subsequently, to identify dependence of site response on the direction of motion and on the non-linear soil behaviour. Moreover, borehole interferometry was used to check the two available velocity profiles (estimated by in situ PS logging) used for 1D linear equivalent numerical simulations. In one case, an improved agreement with the empirical amplification functions was obtained using a smoothed profile. The main non-linear features highlighted by signal processing were globally reproduced by means of linear-equivalent analyses, that pointed out the strong influence of the input motion direction. Among the principal results of this research, it was found that the hanging-wall and directivity effects along with the non-linear site-effects dominate the ground response during the main shock, and that the site amplification features are strongly dependent on the rotation angle of the horizontal components.

Near-source effects on the ground motion occurred at the Conza Dam site (Italy) during the 1980 Irpinia earthquake

The seismic performance of geotechnical structures placed close to active faults is influenced by the peculiarities of the near-source seismic wave propagation. This issue is of paramount importance for large and massive structures, such as earth dams, for which the strong variability of the ground motion at the foundation level and the contribution of the vertical component of the motion could induce high differential settlements in the embankment with an increased risk for the structure. When modelling a structure in near-source conditions, the domain of analysis should be considerably wider than that typically adopted in the engineering field, since the seismic source and the propagation medium up to the reference site should be added to the structure and to the soil volume interacting with it. The paper illustrates the procedure developed to define the input motion at the Conza Dam site (Italy) during the 1980 Irpinia earthquake (November 23, 1980; Mw = 6.9). Due to the short distance of the site from the fault area, the Conza Dam is representative of several large dams placed in near-source areas in Italy and worldwide. The paper focuses on (1) the reconstruction of the regional structure of the Campania–Lucania region on the basis of former literature studies and on a new calibration of the 1D velocity model at regional scale; (2) the definition of the reference input motion at the site of Conza Dam with highlights on the spatial variability of the ground motion below the dam foundation and related effects on the structure.

Seismic response of deep Quaternary sediments in historical center of L’Aquila City (central Italy)

On 6 April 2009 a Mw=6.1 earthquake produced severe destruction and damage over the historic center of L’Aquila City (central Italy), in which the accelerometer stations AQK and AQU recorded a large amount of near-fault ground motion data. This paper analyzes the recorded ground motions and compares the observed peak accelerations and the horizontal to vertical response spectral ratios with those revealed from numerical simulations. The finite element method is considered herein to perform dynamic modeling on the soil profile underlying the seismic station AQU. The subsurface model, which is based on the reviewed surveys that were carried out in previous studies, consists of 200–400m of Quaternary sediments overlying a Meso-Cenozoic carbonate bedrock. The Martin-Finn-Seed's pore-water pressure model is used in the simulations. The horizontal to vertical response spectral ratio that is observed during the weak seismic events shows three predominant frequencies at about 14Hz, 3Hz and 0.6Hz, which may be related to the computed seismic motion amplification occurring at the shallow colluvium, at the top and base of the fluvial-lacustrine sequence, respectively. During the 2009 L’Aquila main shock the predominant frequency of 14Hz shifts to lower values probably due to a peculiar wave-field incidence angle. The predominant frequency of 3Hz shifts to lower values when the earthquake magnitude increases, which may be associated to the progressive softening of soil due to the excess pore-water pressure generation that reaches a maximum value of about 350kPa in the top of fluvial-lacustrine sequence. The computed vertical peak acceleration underestimates the experimental value and the horizontal to vertical peak acceleration ratio that is observed at station AQU decreases when the earthquake magnitude increases, which reveals amplification of the vertical component of ground motion probably due to near-source effects.

The effects of near-source heterogeneity on shear-wave evolution

The Born and Rytov approximation, radiative transfer theory, and other related techniques are commonly used to model features of wave propagation through heterogeneous geologic media such as scattering, attenuation, and pulse-broadening. However, due to the underlying assumptions about the scattering direction and the reference Green’s function, these methods overlook important features of the wavefield such as mode conversion and near-field term coupling. These effects are particularly important within the predicted S-wave nodes of a seismic source, so we analyzed the problem of wave propagation beneath a vertical-point force on the surface of a heterogeneous, elastic half space. To do this, we generated a suite of 3D synthetic heterogeneous geologic models using fractal statistics and simulated the wave propagation using the finite-difference method. We derived an estimate for the effective source radiation patterns, and we used these to compare the results of the models. Our numerical results showed that, due to a combination of mode conversion and near-source coupling effects, S-wave energy on the order of 10% of the P-wave energy is generated within the shear-radiation node. In some cases, this S-wave energy may occur as a coherent pulse and may be used to enhance seismic imaging.

Design energy input spectra for high seismicity regions based on Turkish registers

This work proposes design energy spectra in terms of an equivalent velocity, intended for regions with design peak acceleration 0.3 g or higher. These spectra were derived through linear and nonlinear dynamic analyses on a number of selected Turkish strong ground motion records. In the long and mid period ranges the analyses are linear, given the relative insensitivity of the spectra to structural parameters other than the fundamental period; conversely, in the short period range, the spectra are more sensitive to the structural parameters and, hence, nonlinear analyses are required. The selected records are classified in eight groups with respect to soil type (stiff or soft soil), the severity of the earthquake in terms of surface magnitude $$M _\mathrm{s} (M_\mathrm{s} \le $$ 5.5 and $$M _\mathrm{s} >$$ 5.5) and the relevance of the near-source effects (impulsive or vibratory). For each of these groups, median and characteristic spectra are proposed; such levels would respectively correspond to 50 and 95 % percentiles. These spectra have an initial linear growing branch in the short period range, a horizontal branch in the mid period range and a descending branch in the long period range. Empirical criteria for estimating the hysteretic energy from the input energy are suggested. The proposed design spectra are compared with those obtained from other studies.

Seismic Source Characteristics of Nuclear and Chemical Explosions in Granite from Hydrodynamic Simulations

Seismic source characteristics of low-yield (0.5–5 kt) underground explosions are inferred from hydrodynamic simulations using a granite material model on high-performance (parallel) computers. We use a non-linear rheological model for granite calibrated to historical near-field nuclear test data. Equivalent elastic P-wave source spectra are derived from the simulated hydrodynamic response using reduced velocity potentials. Source spectra and parameters are compared with the models of Mueller and Murphy (Bull Seism Soc Am 61:1675–1692, 1971, hereafter MM71) and Denny and Johnson (Explosion source phenomenology, pp 1–24, 1991, hereafter DJ91). The source spectra inferred from the simulations of different yields at normal scaled depth-of-burial (SDOB) match the MM71 spectra reasonably well. For normally buried nuclear explosions, seismic moments are larger for the hydrodynamic simulations than MM71 (by 25 %) and for DJ91 (by over a factor of 2), however, the scaling of moment with yield across this low-yield range is consistent for our calculations and the two models. Spectra from our simulations show higher corner frequencies at the lower end of the 0.5–5.0 kt yield range and stronger variation with yield than the MM71 and DJ91 models predict. The spectra from our simulations have additional energy above the corner frequency, probably related to non-linear near-source effects, but at high frequencies the spectral slopes agree with the f −2 predictions of MM71. Simulations of nuclear explosions for a range of SDOB from 0.5 to 3.9 show stronger variations in the seismic moment than predicted by the MM71 and DJ91 models. Chemical explosions are found to generate higher moments by a factor of about two compared to nuclear explosions of the same yield in granite and at normal depth-of-burial, broadly consistent with comparisons of nuclear and chemical shots at the US Nevada Test Site (Denny, Proceeding of symposium on the non-proliferation experiment, Rockville, Maryland, 1994). For all buried explosions, the region of permanent deformation and material damage is not spherical but extends along the free surface above and away from the source. The effect of damage induced by a normally buried nuclear explosion on seismic radiation is explored by comparing the motions from hydrodynamic simulations with those for point-source elastic Green’s functions. Results show that radiation emerging at downward takeoff angles appears to be dominated by the expected isotropic source contribution, while at shallower angles the motions are complicated by near-surface damage and cannot be represented with the addition of a simple secondary compensated linear vector dipole point source above the shot point. The agreement and differences of simulated source spectra with the MM71 and DJ91 models motivates the use of numerical simulations to understand observed motions and investigate seismic source features for underground explosions in various emplacement media and conditions, including non-linear rheological effects such as material strength and porosity.

Nonlinear site response evidence of K-NET and KiK-net records from the 2011 off the Pacific coast of Tohoku Earthquake

We analyzed the acceleration time histories recorded by the K-NET and KiK-net stations of the Mw 9 Tohoku Earthquake in order to investigate site response issues related to near-source effects. Time-frequency analysis of K-NET stations in the Miyagi prefecture, closest to the rupture area, show that sites having a Vs30 < 400 m/s present a combination of deamplification at frequencies higher than 5 to 10 Hz and cyclic mobility (high acceleration peaks riding over a low frequency carrier). This suggests strong nonlinear site response at these stations. Furthermore, using KiK-net data we are able to compute borehole transfer functions from the mainshock and events having small PGA values from the local dataset. The ratio between weak-motion and strong-motion borehole transfer functions constitutes an indicator of nonlinear site response. This ratio reveals strong dependence on Vs30 and shows that widespread nonlinear behavior took place during this large earthquake.

Local variability of the ground shaking during the 2009 L’Aquila earthquake (April 6, 2009—Mw 6.3): the case study of Onna and Monticchio villages

The 2009 Mw 6.3 L’Aquila event caused extensive damage in the city of L’Aquila and in some small towns in its vicinity. The most severe damage was recognized SE of L’Aquila town along the Aterno river valley. Although building vulnerability and near-source effects are strongly responsible for the high level of destruction, site effects have been invoked to explain the damage heterogeneities and the similarities between the 2009 macroseismic field with the intensities of historical earthquakes. The small village of Onna is settled on quaternary alluvium and suffered during the L’Aquila event an extremely heavy damage in the masonry structures with intensity IX–X on the Mercalli-Cancani-Sieberg (MCS) scale. The village of Monticchio, far less than 1.3 km from Onna, is mostly situated on Mesozoic limestone and suffered a smaller level of damaging (VI MCS). In the present paper, we analyze the aftershock recordings at seismic stations deployed in a small area of the middle-Aterno valley including Onna and Monticchio. The aim is to investigate local amplification effects caused by the near-surface geology. Because the seismological stations are close together, vulnerability and near-source effects are assumed to be constant. The waveform analysis shows that the ground motion at Onna is systematically characterized by large high-frequency content. The frequency resonance is varying from 2 to 3 Hz and it is related to alluvial sediments with a thickness of about 40 m that overlay a stiffer Pleistocene substrate. The ground motion recordings of Onna are well reproduced by the predictive equation for the Italian territory.