Seismic Source Characterization Research Articles

Development of Model of Seismic Source Characteristics at the Ikata Site Based on Guidelines for SSHAC Level 3

Development of Model of Seismic Source Characteristics at the Ikata Site Based on Guidelines for SSHAC Level 3

Path Attenuation and Source Characteristics during the 2019 Sichuan Changning Earthquake Sequence in China

To give an insight into the path attenuation and source characteristics of the 2019 Sichuan Changning earthquake sequence, 99 strong‐motion recordings at 33 stations were studied. The S‐wave Fourier amplitude spectra were used to determine the path‐attenuation function and key parameters of the seismic source characteristics of this sequence. The results show the following: (1) The geometric spreading effect presents hinged‐trilinear characteristics, and the corresponding geometric spreading rates were −0.97, 0.29, and −0.73 with the crossover distances 90 km and 183 km. (2) The quality factor Q is correlated with frequency and expressed as 217 × f0.82 for the range from 0.1 to 10.0 Hz. (3) The seismic moment was inversely proportional to the cube of the corner frequency, and their product was 2.37 × 1016 N. m/s3. (4) The stress drop varied from 1.18 to 12.44 MPa. Both the attenuation effect and source parameters of this sequence are significantly different from those of the Lushan and the Wenchuan aftershock sequences in the Sichuan region of China.

NGA-East ground-motion characterization model Part II: Implementation and hazard implications

As a companion article to Goulet et al., we describe implementation of the NGA-East ground motion characterization (GMC) model in probabilistic seismic hazard analysis (PSHA) for sites in the Central and Eastern United States (CEUS). We present extensions to the EPRI/DOE/NRC seismic source characterization (SSC) model for the CEUS needed for full implementation of NGA-East. Comparisons are presented to the EPRI GMC, the currently accepted model by the U.S. Nuclear Regulatory Commission for hazard assessment at nuclear facilities. Comparisons are presented both in terms of GMC model components and in the resulting seismic hazard assessments for a range of site locations in the CEUS. Illustrations of the effect of various components of the NGA-East GMC on seismic hazard results are also presented. Finally, we present recommendations for application of the NGA-East GMC in PSHA.

Seismic source characteristics of the intermediate-depth and intraslab 2019 northern Peru earthquake (Mw 8.0)

On May 26, 2019, an unusual depth-intermediate focus earthquake, in the interior of the Nazca plate, shook the northern Peruvian jungle and caused the death of two victims. This earthquake was felt in almost all of Peru as well as Brazil, Ecuador and Colombia. The focal mechanism calculated is of the normal fault type (strike = 353∘, dip = 57∘, rake = − 99∘) for a focal depth of 114 km. The slip distribution was obtained through an inversion process of teleseismic waveforms. The maximum slip was 6.3 m, located around the central side of the fault plane. The moment magnitude calculated was Mw 8.0; the spectral analysis indicated a similar magnitude of Mw 8.0. The source time function indicates a multiple rupture earthquake, with the presence of several pulses of seismic energy release, with a total duration around 80 s. The rupture directivity was from the south to the north; we have assumed a propagation rupture velocity of 2.8 km/s. The modeled fault plane geometry has dimensions of 208×105 km2. The predicted vertical coseismic deformation field shows a pattern of mostly subsidence.

Probabilistic seismic hazard analysis under incomplete data and imperfect source characterization: the Gulf of Mexico case study

A fully probabilistic formulation is proposed to quantify seismic hazard rates in regions where seismic monitoring has been defective, and as a consequence the catalogue is incomplete in a wide range of magnitudes. Catalogue incompleteness is always accompanied by an imperfect seismic source characterization, the wider the region the more defective its description. To overcome these issues, the approach presented herein is associated with three probabilistic assumptions: (1) the total seismicity of the region is figured out from the incomplete seismic catalogue; (2) the probability that the next seismic event might occur in a specific part of the region is quantified from the spatial distribution of recorded earthquakes; (3) schemes 2 and 3 are naturally incorporated in the classical probabilistic seismic hazard analysis and the seismic hazard curve is computed. The approach is applied to the Gulf of Mexico. It is shown that the Gulf of Mexico should not be considered as a homogeneous seismic source, since non uniform spatial distribution of recorded earthquakes significantly impacts on seismic hazard estimates.

The OGS–Northeastern Italy Seismic and Deformation Network: Current Status and Outlook

Abstract In this article, we describe the infrastructure developed and managed by the Italian National Institute of Oceanography and Applied Geophysics – OGS for the seismological and geodetic monitoring of northeastern Italy. The infrastructure was constituted in response to the ML 6.4 Friuli destructive earthquake of 1976, with the main mandate of supporting civil protection emergency activities. The OGS monitoring infrastructure is presently composed of a seismometric and a strong-motion network, complemented by a number of Global Navigation Satellite Systems stations, each delivering observational data in real time, which are collected and processed by the headquarters of the Center for Seismological Research of OGS in Udine. The OGS networks operate in close cooperation with Italian and international networks from neighboring countries, within the framework of the agreements for real-time data exchange, to obtain improved rapid earthquake location and magnitude estimations. Information regarding seismic events is released to the public through a dedicated web portal and, since 2013, through social media. Aside from the standard monitoring activities (>30,000 events have been recorded since 1976), the OGS has progressively increased the number of services to the public and to the Civil Protection of the Friuli Venezia Giulia and Veneto regions. The high availability of good quality data has resulted in the enhancement of scientific products, including advanced seismological studies of the area, spanning broadly from seismic source characterization to engineering seismology. In the future, the OGS networks are expected to further contribute to the development of seismological research and monitoring infrastructures of the Central European region.

Probabilistic seismic hazard assessment for two potential nuclear power plant sites in Tunisia

As part of a feasibility study for a potential electronuclear power plant in Tunisia, a new seismic catalogue and a new seismic source model have been proposed, for Tunisia and adjacent areas. These findings are used in this study to evaluate the seismic hazard for two selected nuclear power plant sites: Marsa-Douiba in the north and Skhira in the southeast. The investigations and the assessment are conducted at a regional scale. The location of these sites and the choice of a regional review extent allow us to consider the entire Tunisian territory as the study area and also to conduct the hazard evaluation at a ‘national level’. In this study, a probabilistic seismic hazard analysis is performed using the R-CRISIS software, and a logic tree is developed to capture uncertainties related to the characterisation of both seismic sources and ground-motion. First, a hazard map of Tunisia for the return period of 475 years is generated to compare it with the ones obtained within previous studies. Then, the 10,000-year return period is considered, and the recommendations of the International Atomic Energy Agency are applied to determine the uniform hazard spectrums and perform disaggregation for the selected sites.

Seismic source characterization of the Arabian Peninsula and Zagros Mountains from regional moment tensor and coda envelopes

Reliable estimates of moment magnitude and source mechanism for seismic events in the Middle East can be challenging due to a small number of openly available stations, the complex tectonic setting, and regions of high attenuation. Access to high-quality waveform data from well-calibrated regional seismic stations is fundamental in producing robust and stable estimates of earthquake source parameters, particularly when measurements of absolute ground motion amplitudes are required. Earthquake source mechanism and moment magnitude are invaluable information in the assessment of seismic hazard, plate motions, and the characterization of faults and regional stress field. The expansion of the Kingdom of Saudi Arabia’s national seismic network in the past several decades provides an opportunity to develop the capabilities of routine focal mechanism and moment magnitude estimations. In this study, we use time-domain full waveform moment tensor inversion and coda envelope–derived amplitude measurements to solve for earthquake source mechanism, moment magnitudes, and their source type. We compared the moment magnitudes calculated from the two methods and publicly available earthquake catalogs and discuss the implications of the obtained source parameters.

A Time–Frequency Representation Model for Seismic Ground Motions

ABSTRACTA probabilistic model of the time–frequency power spectral density (TFPSD) is presented. The model is developed, based on the time–frequency representation of records from strike-slip earthquakes, in which the time–frequency representation is obtained by applying the S-transform (ST). The model for the TFPSD implicitly considers the amplitude modulation and frequency modulation for the nonstationary ground motions; this differs from the commonly used evolutionary PSD model. Predicting models for the model parameters, based on seismic source and site characteristics, are developed. The use of the model to simulate ground motions for scenario seismic events is illustrated, in which the simulation is carried out using a recently developed model that is based on the discrete orthonormal ST and ST. The illustrative example highlights the simplicity of using the proposed model and the physical meaning of some of the model parameters. A model validation analysis is carried out by comparing the statistics of the pseudospectral acceleration obtained from the simulated records to those obtained using a few ground-motion models available in the literature and considered actual records. The comparison indicates the adequacy of the proposed model.

Capturing epistemic uncertainty in site response

The incorporation of local amplification factors (AFs) determined through site response analyses has become standard practice in site-specific probabilistic seismic hazard analysis (PSHA). Another indispensable feature of the current state of practice in site-specific PSHA is the identification and quantification of all epistemic uncertainties that influence the final hazard estimates. Consequently, logic trees are constructed not only for seismic source characteristics and ground-motion models (GMMs) but also for the site AFs, the latter generally characterized by branches for alternative shear-wave velocity ( VS) profiles. However, in the same way that branch weights on alternative GMMs can give rise to unintentionally narrow distributions of predicted ground-motion amplitudes, the distribution of AFs obtained from a small number of weighted VS profiles will often be quite narrow at some oscillator frequencies. We propose an alternative approach to capturing epistemic uncertainty in site response in order to avoid such unintentionally constricted distributions of AFs using more complete logic trees for site response analyses. Nodes are included for all the factors that influence the calculated AFs, which may include shallow VS profiles, deeper VS profiles, depth of impedance contrasts, low-strain soil damping, and choice of modulus reduction and damping curves. Site response analyses are then executed for all branch combinations to generate a large number of frequency-dependent AFs. Finally, these are re-sampled as a discrete distribution with enough branches to capture the underlying distribution of AFs. While this approach improves the representation of epistemic uncertainty in the dynamic site response characteristics, modeling uncertainty in the AFs is not automatically captured in this way, for which reason it is also proposed that a minimum level of epistemic uncertainty should be imposed on the final distribution.

Fault-based probabilistic seismic hazard assessment of the eastern Makran subduction and the Chaman transform fault, Pakistan: Emphasis on the source characterization of megathrust

Abstract Seismic source characterization (SSC) for probabilistic seismic hazard assessment (PSHA) in regions characterized by subduction megathrust involves a considerable ambiguity. Lack of detailed geologic, seismic, and geodetic data increases the uncertainties. The enigma is enhanced in regions where thin-skinned accretionary prism faults are part of active deformation. In this study, a planar SSC model for seismically active eastern Makran subduction zone, its associated accretionary prism faults and Chaman transform fault zone is proposed based on a kinematic block model accounting strain partitioning. Sensitivity tests for various parameters of the SSC model are performed by computing peak ground acceleration (PGA) maps for 475-year return period. Among alternative magnitude distribution models, the truncated exponential model gives ~10% higher PGA values than the composite recurrence model, which is favored by the observed subduction seismicity. Especially in gently dipping subduction zones such as Makran, estimated PGA values and their spatial distribution are highly sensitive (changing up to 0.3 g) to the megathrust interface geometry near the surface, dip amount, and defined depth limits. Gentler and deeper extending subduction interface maximizes the rupture width and results in higher PGA values towards inland, while the inclusion of shallow aseismic portion produces larger PGA values near the trench. Through eastward transition from subduction to transform motion, thrust faults within the Makran accretionary prism bend northward, forming oblique fault systems with higher slip rates and accommodate part of the compression in accordance with the lateral slip rate variations identified on the Chaman transform fault zone. Using the selected SSC model, the PGA values for short return period (475-year) are estimated to be between 0.5 g and 0.7 g within the accretionary prism down to the trench across the Makran subduction and along southern section of Chaman transform fault zone where slip rates are higher relative to the north.

Automated Seismic Source Characterization Using Deep Graph Neural Networks

AbstractMost seismological analysis methods require knowledge of the geographic location of the stations comprising a seismic network. However, common machine learning tools used in seismology do not account for this spatial information, and so there is an underutilized potential for improving the performance of machine learning models. In this work, we propose a graph neural network (GNN) approach that explicitly incorporates and leverages spatial information for the task of seismic source characterization (specifically, location and magnitude estimation), based on multistation waveform recordings. Even using a modestly‐sized GNN, we achieve model prediction accuracy that outperforms methods that are agnostic to station locations. Moreover, the proposed method is flexible to the number of seismic stations included in the analysis and is invariant to the order in which the stations are arranged, which opens up new applications in the automation of seismological tasks and in earthquake early warning systems.

The Mid‐eighth Century CE Surface Faulting Along the Dead Sea Fault at Tiberias (Sea of Galilee, Israel)

AbstractThe Dead Sea Fault (DSF) is a plate‐boundary where large earthquakes are expected, but there is a lack of such events in the instrumental era. Sequences of earthquakes along the DSF are documented by historical evidence, one of the most devastating occurred in the mid‐eighth century CE. Here we describe site‐specific archaeoseismological observations at the ancient Tiberias city, on the western shore of the Sea of Galilee. We map Roman and Byzantine relics faulted in the mid‐eighth century CE by a pure normal fault. We use geophysical, geomorphological, and structural analyses integrated with published data, to assess the seismic hazard of the Jordan Valley Western Boundary Fault (JVWB). We propose that the normal JVWB can rupture the surface along its ~45 km trace running from Tiberias toward the S crossing Bet Shean, Tel Rehov, and Tel Teomim. The JVWB, parallel to the main strike‐slip Jordan Valley Fault segment, might be regarded as a major earthquake source in this region. We test the hypotheses of both single fault and multifaults rupture scenarios, which result in an expected range of Mw from 6.9 (single rupture of the JVWB) to 7.6 (multiple rupture of the JVWB and Jordan Valley Fault). Our results suggest that seismic source characterization in the Sea of Galilee region must include normal faults capable of surface rupturing, despite the absence of such events in the instrumental catalogue.

Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017

In this paper we present the interpretation of the observations of the flare from 6 September 2017 reported in Paper I. These include gamma-ray (GR), hard X-ray (HXR), soft X-rays, Lyαline, extreme ultraviolet (EUV), Hα, and white light (WL) emission, which were recorded during the two flaring events 1 (FE1) and 2 (FE2) that occurred at 11:55:37 UT (FE1) and 12:06:40 UT (FE2). Paper I also reported the first detection of the sunquake with first and second bounces of seismic waves combined with four other sunquakes in different locations supported with the observations of HXR, GR, EUV, Hα, and WL emission with strongly varying spatial resolution and temporal coverage. In the current paper, we propose some likely scenarios for heating of flaring atmospheres in the footpoints with sunquakes which were supported with EUV and Hαemission. We used a range of parameters derived from the HXR, EUV, and Hαline observations to generate hydrodynamic models, which can account for the blueshifts derived from the EUV emission and the redshifts observed with the EUV Imaging Spectrometer in the He II line and by the CRisp Imaging Spectro-Polarimeter in the Swedish Solar Telescope in Hαline emission. The parameters of hydrodynamic shocks produced by different beams in flaring atmospheres were used as the initial conditions for another type of hydrodynamic models that were developed for acoustic wave propagation in the solar interior. These models simulate the sets of acoustic waves produced in the interior by the hydrodynamic shocks from atmospheres above deposited in different footpoints of magnetic loops. The Hαline profiles with large redshifts in three kernels (two in FE1 and one in FE2) were interpreted with the full non-local thermodynamic equilibrium radiative simulations in all optically thick transitions (Lyman lines and continuum Hα, Hβ, and Pα) applied for flaring atmospheres with fast downward motions while considering thermal and non-thermal excitation and ionisation of hydrogen atoms by energetic power-law electron beams. The observed Hαline profiles in three kernels were fit with the simulate blue wing emission of the Hαline profiles shifted significantly (by 4–6 Å) towards the line red wings, because of strong downward motions with velocities about 300 km s−1by the shocks generated in flaring atmospheres by powerful beams. The flaring atmosphere associated with the largest sunquake (seismic source 2 in FE1) is found consistent with being induced by a strong hydrodynamic shock produced by a mixed beam deposited at an angle of −30° from the local vertical. We explain the occurrence of a second bounce in the largest sunquake by a stronger momentum delivered by the shock generated in the flaring atmosphere by a mixed beam and deeper depths of the interior where this shock was deposited. Indeed, the shock with mixed beam parameters is found deposited deeply into the interior beneath the flaring atmosphere under the angle to the local vertical that would allow the acoustic waves generated in the direction closer to the surface to conserve enough energy for the second bounces from the interior layers and from the photosphere. The wave characteristics of seismic sources 1 and 3 (in FE1) were consistent with those produced by the shocks generated by similar mixed beams deposited at the angles −(0 − 10)° (seismic source 1) and +30° (seismic source 3) to the local vertical. The differences of seismic signatures produced in the flares of 6 September 2011 and 2017 are also discussed.

Interaction between fault and off-fault seismic sources in hazard analysis – A case study from Israel

In this study, seismic source characterization (SSC) for purpose of seismic hazard analysis (SHA) is explored, with a focus on the moment balance and interaction between on-fault and off-fault seismicity. For the purpose of this study, a homogenous, uniform, earthquake catalog for Israel is first compiled and de-clustered. Gridded seismicity is calculated based on the de-clustered catalog, using a fixed kernel, as well as an adaptive kernel. The main faults are represented as planar features and their magnitude-frequency distribution is adjusted to match both the geological and the seismological available data. A full probabilistic seismic hazard analysis (PSHA) is then conducted for the entire state of Israel. Results are presented both in terms of the full spatial distribution, in the form of hazard maps, as well as at specific point locations - near and far from the main sources. Our results show that, based on the available data and balancing seismic moment, the characteristic portion of the main seismic sources in Israel is larger than the classical 94% (Suggested by Youngs and Coppersmith 1985) and is about 96%–98%, depending on the fault. This suggests that the exponential model, as currently used in the building code, leads to a significant underestimation of hazard. We further show that as long as the moment is balanced, hazard results are only mildly sensitive to model choices, such as size of kernel, or type of cutoff between on-fault and off-fault sources. Finally, we suggest using the adaptive-kernel with the Fault-extract interaction, which we believe is the best representation of the currently available geological, seismological and geodetic information.

Time domain intrusive probabilistic seismic risk analysis of nonlinear shear frame structure

Presented is a time domain intrusive framework for probabilistic seismic risk analysis. Seismic source characterization is mathematically formulated. Methodology for simulating non-stationary seismic motions for given source, path and site is proposed. Both uncertain motions and uncertain structural parameters are characterized as random process/field and represented with Hermite polynomial chaos. Intrusive modeling of Armstrong-Fredrick kinematic hardening based on Hermite polynomial chaos is formulated and incorporated into Galerkin stochastic elastic-plastic FEM. Time-evolving probabilistic structural response is solved through developed stochastic elastic-plastic FEM. Following that, formulation for seismic risk analysis is derived. The framework is illustrated by seismic risk analysis of an eight-story shear frame structure. Uncertainties are propagated from earthquake source into uncertain structural system. Difficulties of choosing intensity measure in the conventional framework are avoided since all the uncertainties and important characteristics (e.g., spectrum acceleration Sa and peak ground acceleration PGA) of seismic motions are directly carried by the random process excitations in time domain. Stochastic dynamic equations are solved in an intrusive way, circumventing non-intrusive Monte Carlo simulations.

Probabilistic seismic hazard assessment for Iraq

Probabilistic seismic hazard assessments (PSHA) form the basis for calculating seismic loads in most contemporary seismic provisions in building codes around the world. The current building code of Iraq, which was published in 1997, is currently undergoing a significant engineering update. This study was undertaken in order to support the building code update and to satisfy the need in Iraq for a contemporary assessment of seismic hazard in terms of spectral accelerations. Seismic source characterization largely relies on a newly compiled earthquake catalog since sufficiently detailed information is not available on active faulting in the country even though there are numerous known active faults. There is also a lack of locally recorded strong-motion data. As a result, we make use of attenuation tomography studies in the region to compare local attenuation characteristics with that in other parts of the world where ground-motion prediction equations are available for use in PSHA. For most of the country, the attenuation of 1HzLg waves indicates an attenuation rate that is slower than active tectonic regions but faster than stable continental regions. Hence, we use ground-motion prediction equations from active tectonic and stable continental regions, weighted equally. The PSHA results are presented for a 2% chance of being exceeded in 50 years and on a reference ground condition of the National Earthquake Hazards Reduction Program (NEHRP) site class B. The probability level and reference ground conditions were selected to be consistent with the update of Iraq’s building code. The largest hazard, and consequently the design ground motions, is in the northern cities of Sulaymaniyah and Erbil, consistent with the fact that they are the two urban areas closest to major tectonic features to the north and east of Iraq. Additionally, the Badra–Amarah fault zone is a significant contributor to seismic hazard in the country; therefore, urban areas near it exhibit high seismic hazard.

Complex Rupture of an Immature Fault Zone: A Simultaneous Kinematic Model of the 2019 Ridgecrest, CA Earthquakes

AbstractThe 4 July 2019 Mw6.4 and subsequent 6 July 2019 Mw7.1 Ridgecrest sequence earthquakes (CA, USA) ruptured orthogonal fault planes in a low slip rate (1 mm/year) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas Fault. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high‐rate GNSS, strong motion, GNSS static offset, and Interferometric Synthetic Aperture Radar data. We model the Coulomb stress change to evaluate how the Mw6.4 earthquake may have affected the subsequent Mw7.1 event. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models.

The 2018 Mw 6.8 Zakynthos (Ionian Sea, Greece) earthquake: seismic source and local tsunami characterization

SUMMARYWe investigated the kinematic rupture model of the 2018 Mw 6.8 Zakynthos, Ionian Sea (Greece), earthquake by using a non-linear joint inversion of strong motion data, high-rate GPS time-series and static coseismic GPS displacements. We also tested inversion results against tide-gauge recordings of the small tsunami generated in the Ionian Sea. In order to constrain the fault geometry, we performed several preliminary kinematic inversions by assuming the parameter values resulting from different published moment tensor solutions. The lowest cost function values were obtained by using the geometry derived from the United States Geological Survey (USGS) focal solution. Between the two conjugate USGS planes, the rupture model which better fits the data is the one with the N9°E-striking 39°ESE-dipping plane. The rupture history of this model is characterized by a bilateral propagation, featuring two asperities; a main slip patch extending between 14 and 28 km in depth, 9 km northeast from the nucleation and a slightly shallower small patch located 27 km southwest from the nucleation. The maximum energy release occurs between 8 and 12 s, when both patches are breaking simultaneously. The maximum slip is 1.8 m and the total seismic moment is 2.4 × 1019 Nm, corresponding to a Mw value of 6.8. The slip angle shows a dominant right-lateral strike-slip mechanism, with a minor reverse component that increases on the deeper region of the fault. This result, in addition to the observed possibility of similar mechanisms for previous earthquakes occurred in 1959 and 1997, suggests that the tectonic deformation between the Cephalonia Transform Fault Zone and the northern tip of the Hellenic Arc Subduction zone may be accommodated by prevailing right lateral low-dipping faults, occurring on re-activated structures previously experiencing (until Pliocene) compressional regime. Comparison of predicted and observed tsunami data suggests the need of a better characterization of local harbour response for this type of relatively short-wavelength events, which is important in the context of tsunami early warning. However, the suggested dominantly strike-slip character would in turn imply a reduced tsunami hazard as compared to a dominant thrust faulting regime from this source region.

Site Characterization and Evaluation of Seismic Sources for Amaravati Region

After the bifurcation of Telangana from Andhra Pradesh (AP) state in 2014, the Amaravati and adjoining localities has been proposed as a new capital region to the state of AP, India. As per the seismic zonation map of India, the proposed region falls in zone III. However, the prediction of damage level due to earthquakes is highly impossible, as it is influenced by many regional factors. In order to estimate the damage level from the future earthquakes in the present study, the various compositions involved in the regional fields of geological, geomorphological and soil characteristics are evaluated for the selected study region. In addition to this, to understand the seismicity of this region the earthquake catalogue and seismotectonic map have been prepared by collecting the data within 300km radial distance from Velgapudi (16° 52'N, 80° 51'E) as a center. The results of different fields represented in the form of thematic maps for this geographical information system (GIS) software has been used.