Shear Wave Quality Factor Research Articles

Comparison of Near-Surface Attenuation from Surface Array-Based Seismic Noise Data and Borehole Weak-Motion Recordings at the STIN Test Site in Northeastern Italy

Abstract Seismic wave attenuation and the related shear-wave quality factor (QS) in the near surface are crucial parameters for ground motion simulations and seismic hazard assessments. Although recent approaches developed to calculate QS from seismic noise acquired by surface arrays have been accepted for practice, additional testing and comparison of results estimated using various geophysical methods are still necessary to verify the reliability of such techniques. This work presents the results of an experiment conducted at the STIN site in northeastern Italy, which is equipped with a 100 m deep instrumented borehole. A seismic noise campaign was implemented by installing a temporary independent local surface array of seismological stations. The gathered data were used to initially estimate the shear-wave velocity (VS) profile and frequency-dependent Rayleigh-wave attenuation, and subsequently determine the QS factor via a linearized inversion method. The study compares these findings with the VS and QS values derived from analyzing weak-motion events recorded by two permanent seismic sensors positioned at the top and bottom of the well. The results confirm the potential of the inversion procedure used to obtain QS from local-scale seismic noise arrays as a promising approach for conducting attenuation studies at the local level in less geologically complex sites.

High frequency attenuation of S waves in alluvial deposits of the central Po Plain (northern Italy)

SUMMARY Estimation of local seismic response plays a key role in assessing local seismic hazard and particularly in the design of shaking scenarios. Modelling local seismic response involves knowing of the shear wave velocity (VS) and quality factor (QS) profiles for the site in question. The many techniques that have been developed to assess VS in surface deposits produce reliable measurements of VS, but these rarely correspond to direct measurements of QS. The latter is often considered through damping measures from laboratory tests on small-scale soil samples, which can provide information primarily on intrinsic attenuation, neglecting the contribution of scattering effects. In this paper, using seismic recordings obtained at the surface and in boreholes at 100 m depth, we estimate an average value of QS of some characteristic alluvial deposits of the Po Plain (northern Italy). Data come from a microseismic network which sampled an almost uniform lithology in the central Po Plain and consisted of three surface and four borehole stations with an interstation distance of about 2 km. The average value of QS of the shallowest 100 m of the sedimentary strata, QS,100, is estimated by considering: (1) the high-frequency attenuation of seismic waves due to propagation through the corresponding stratigraphy and (2) the interference between incident and surface-reflected waves observed at borehole stations. We parametrize the first through $\Delta k_{0_{ {100}}}$, the difference between the values of the spectral decay parameter kappa (k) estimated at the surface and at the boreholes depth, respectively. We use the second in order to compute VS,100, the time-averaged VS referred to the uppermost 100 m stratigraphy. We obtain: $\Delta k_{0_{ {100}}} = (11 \pm 3)\,$ ms, ${V_{S,100}} \, =\, (309\, \pm \, 11)\,$ m s−1 and ${Q_{S,100}} \, =\, 31 \pm 10$. At the surface, the estimated values of the site-specific kappa, k0, are found to range from 75 to 79 ms. As expected, these results are in good agreement with studies performed in other sites characterized by sandy or clayey lithologies, and can be usefully used in site response analysis at sites where the rigidity is mainly controlled by lithostatic pressure.

Robust estimation of 1D shear-wave quality factor profiles for site response analysis using seismic noise

Ambient seismic noise, particularly useful in urban areas, contains valuable information on seismic wave attenuation and the related shear-wave quality factor (QS) in the near-surface layers. Although the application of ambient seismic noise for attenuation studies is promising, more development and testing are necessary before it can be adopted for standard in situ analysis. In this paper, a development of the approach suggested in Parolai (2014) [58] is proposed and tested to improve the robustness of the results. In particular, the Simultaneous Algebraic Reconstruction Technique (SART) (Andersen and Kak, 1984) [5] is used for implementing an attenuation positivity constraint to the solutions ― to non-get negative QS values ― avoiding the preliminary testing of parameters required in the original procedure. Tests carried out at sites, where previous QS values estimated with the approach proposed by Parolai (2014) [58] were available, show similar results to the former algorithm. Furthermore, for the used data sets, the attenuation positivity constraint was shown not to be mandatory for gaining solutions with a physical meaning.

0–5 Hz deterministic 3-D ground motion simulations for the 2014 La Habra, California, Earthquake

SUMMARYWe have simulated 0–5 Hz deterministic wave propagation for a suite of 17 models of the 2014 Mw 5.1 La Habra, CA, earthquake with the Southern California Earthquake Center Community Velocity Model Version S4.26-M01 using a finite-fault source. Strong motion data at 259 sites within a 148 km × 140 km area are used to validate our simulations. Our simulations quantify the effects of statistical distributions of small-scale crustal heterogeneities (SSHs), frequency-dependent attenuation Q(f), surface topography and near-surface low-velocity material (via a 1-D approximation) on the resulting ground motion synthetics. The shear wave quality factor QS(f) is parametrized as QS, 0 and QS, 0fγ for frequencies less than and higher than 1 Hz, respectively. We find the most favourable fit to data for models using ratios of QS, 0 to shear wave velocity VS of 0.075–1.0 and γ values less than 0.6, with the best-fitting amplitude drop-off for the higher frequencies obtained for γ values of 0.2–0.4. Models including topography and a realistic near-surface weathering layer tend to increase peak velocities at mountain peaks and ridges, with a corresponding decrease behind the peaks and ridges in the direction of wave propagation. We find a clear negative correlation between the effects on peak ground velocity amplification and duration lengthening, suggesting that topography redistributes seismic energy from the large-amplitude first arrivals to the adjacent coda waves. A weathering layer with realistic near-surface low velocities is found to enhance the amplification at mountain peaks and ridges, and may partly explain the underprediction of the effects of topography on ground motions found in models. Our models including topography tend to improve the fit to data, as compared to models with a flat free surface, while our distributions of SSHs with constraints from borehole data fail to significantly improve the fit. Accuracy of the velocity model, particularly the near-surface low velocities, as well as the source description, controls the resolution with which the anelastic attenuation can be determined. Our results demonstrate that it is feasible to use fully deterministic physics-based simulations to estimate ground motions for seismic hazard analysis up to 5 Hz. Here, the effects of, and trade-offs with, near-surface low-velocity material, topography, SSHs and Q(f) become increasingly important as frequencies increase towards 5 Hz, and should be included in the calculations. Future improvement in community velocity models, wider access to computational resources, more efficient numerical codes and guidance from this study are bound to further constrain the ground motion models, leading to more accurate seismic hazard analysis.

A review of near-surface QS estimation methods using active and passive sources

Seismic attenuation and the associated quality factor (Q) have long been studied in various sub-disciplines of seismology, ranging from observational and engineering seismology to near-surface geophysics and soil/rock dynamics with particular emphasis on geotechnical earthquake engineering and engineering seismology. Within the broader framework of seismic site characterization, various experimental techniques have been adopted over the years to measure the near-surface shear-wave quality factor (QS). Common methods include active- and passive-source recording techniques performed at the free surface of soil deposits and within boreholes, as well as laboratory tests. This paper intends to provide an in-depth review of what Q is and, in particular, how QS is estimated in the current practice. After motivating the importance of this parameter in seismology, we proceed by recalling various theoretical definitions of Q and its measurement through laboratory tests, considering various deformation modes, most notably QP and QS. We next provide a review of the literature on QS estimation methods that use data from surface and borehole sensor recordings. We distinguish between active- and passive-source approaches, along with their pros and cons, as well as the state-of-the-practice and state-of-the-art. Finally, we summarize the phenomena associated with the high-frequency shear-wave attenuation factor (kappa) and its relation to Q, as well as other lesser-known attenuation parameters.

Differential Crustal Attenuation in the Garhwal and Kumaon Himalaya Obtained from Seismological and Geophysical Studies

Abstract The Uttarakhand Himalaya which comprises of the Garhwal and Kumaon Himalaya lies in the central seismic gap region of India. Strong motion networks have been installed separately in the Garhwal and Kumaon Himalaya under various sponsored research projects funded by the Department of Science and Technology (DST) and Ministry of Earth Sciences (MoES). These networks have recorded several near field earthquakes in the recent past. Recorded data from these networks have been utilised to study the crustal attenuation characteristics of these two regions. The crustal attenuation characteristics of medium can be directly estimated from the frequency dependent quality factor ‘Q’. Estimation of shear wave quality factor, coda wave quality factor and attenuation relations have been made by using same methodology in the data set obtained separately from these two regions. It has been found that there is a substantial difference in the attenuation characteristics of the crustal rocks in these two regions. Since shear wave attenuation has a close relation with the shear wave velocity, therefore, the shear wave velocity has been estimated at various locations of the Garhwal and Kumaon Himalaya by both active and passive methods. It has been found that the average shear wave velocity of crustal rocks in the Garhwal Himalaya is comparatively higher than that in the Kumaon Himalaya which clearly support the high attenuation property of the crustal rocks in the Kumaon Himalaya compared to the Garhwal Himalaya.

Seismic Attenuation at the Equatorial Mid‐Atlantic Ridge Constrained by Local Rayleigh Wave Analysis From the PI‐LAB Experiment

AbstractThe ocean lithosphere represents a simple realisation of the tectonic plate, offering a unique opportunity to better understand its physical and chemical properties in relationship to those of the underlying asthenosphere. While seismic velocity is frequently used to image the plate, seismic attenuation (Qμ−1) offers an important complimentary observation. We use fundamental mode Rayleigh waves from 17 local, M > 4.2 earthquakes recorded at stations located on 0–80 My old lithosphere near the equatorial Mid‐Atlantic Ridge. We determine the attenuation coefficient () for periods between 15 and 40 s and invert for 1‐D average shear wave quality factor values () and shear wave velocity (Vs). We find values of 175 ± 16 at 50 km depth, decreasing to 90 ± 15 at greater than 60 km. Comparison of our and Vs measurements to previous observations from oceanic settings shows agreement in terms of higher and Vs in the lithosphere in comparison to the asthenosphere. The observations from oceanic settings are in general agreement with the laboratory predictions for ‐Vs relationships for thermal models. However, a small amount of partial melt (1%) is required to explain several previous observations. Our result also compares favorably to previous observations of lithospheric and asthenospheric attenuation with respect to frequency. Melt is not required for the 1‐D average of our study area, consistent with previous electromagnetic and seismic imaging that suggested melt in punctuated and/or thin channel anomalies rather than over broad regions of the mantle.

Seismic radiation from wind turbines: observations and analytical modeling of frequency-dependent amplitude decays

Abstract. In this study, we determine spectral characteristics and amplitude decays of wind turbine induced seismic signals in the far field of a wind farm (WF) close to Uettingen, Germany. Average power spectral densities (PSDs) are calculated from 10 min time segments extracted from (up to) 6 months of continuous recordings at 19 seismic stations, positioned along an 8 km profile starting from the WF. We identify seven distinct PSD peaks in the frequency range between 1 and 8 Hz that can be observed to at least 4 km distance; lower-frequency peaks are detectable up to the end of the profile. At distances between 300 m and 4 km the PSD amplitude decay can be described by a power law with exponent b. The measured b values exhibit a linear frequency dependence and range from b=0.39 at 1.14 Hz to b=3.93 at 7.6 Hz. In a second step, the seismic radiation and amplitude decays are modeled using an analytical approach that approximates the surface wave field. Since we observe temporally varying phase differences between seismograms recorded directly at the base of the individual wind turbines (WTs), source signal phase information is included in the modeling approach. We show that phase differences between source signals have significant effects on the seismic radiation pattern and amplitude decays. Therefore, we develop a phase shift elimination method to handle the challenge of choosing representative source characteristics as an input for the modeling. To optimize the fitting of modeled and observed amplitude decay curves, we perform a grid search to constrain the two model parameters, i.e., the seismic shear wave velocity and quality factor. The comparison of modeled and observed amplitude decays for the seven prominent frequencies shows very good agreement and allows the constraint of shear velocities and quality factors for a two-layer model of the subsurface. The approach is generalized to predict amplitude decays and radiation patterns for WFs of arbitrary geometry.

Three-dimensional shear-wave quality factor, Qs(f), model for south-central Gulf of California, Mexico obtained from inversion of broadband data

Abstract We apply an iterative inversion scheme, initially developed by Hashida and Shimazaki (1984) and later modified by Joshi et al., (2010), to estimate three - dimensional shear - wave quality factor, Qs(f), of south-central Gulf of California, Mexico. An area of 230 km x 288 km in this region is divided into 108 rectangular blocks of different Qs(f). We use 25 well-located earthquakes recorded at three broadband stations of the regional network RESBAN operated by CICESE (Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California) and three Ocean Bottom Seismographs (OBS) of the Sea of Cortez Ocean Bottom Array (SCOOBA) experiment. This dataset permits us to obtain Qs(f) estimates of different blocks using the modified inversion algorithm. Qs(f) is obtained at various frequencies in 0.16 - 7.94 Hz range. We found that the estimated Qs structure correlates with geological and tectonic models of the region proposed in previous studies. A regional frequency-dependent relation using all 1944 values of shear-wave quality factor is obtained at 18 different frequencies in all blocks can be approximated by a function of the form Qs(f) = 20 f 1.2. This relation is typical in a tectonically active region with high S-wave attenuation and is similar to attenuation relations reported by other authors for the Imperial Valley, California region.

Viscoelastic Model and Synthetic Seismic Data of Eastern Rub’Al-Khali

The Rub’ Al-Khali basin in Saudi Arabia remains unexplored and lacks data availability due to its remoteness and the challenging nature of its terrain. Thus far, there are neither digital geologic models nor synthetic seismic data from this specific area accessible for testing research techniques and analysis. In this study, we build a 2D viscoelastic model of the eastern part of the Rub’ Al-Khali basin and generate a corresponding dual-component seismic data set. We compile high-resolution depth models of compressional- and shear-wave velocities, density, as well as compressional- and shear-wave quality factors from published data. The compiled models span Neoproterozoic basement up to Quaternary sand dunes. We then use the finite-difference technique to model the propagation of seismic waves in the compiled viscoelastic medium of eastern Rub’ Al-Khali desert. In particular, we generate vertical and horizontal components of the shot gathers with accuracy to the fourth and second orders in space and time, respectively. The viscoelastic models and synthetic seismic datasets are made available in an open-source site for prospective re-searchers who desire to use them for their research. Users of these datasets are urged to make their findings also accessible to the geoscience community as a way of keeping track of developments related to the Rub’ Al-Khali desert.

Ground motion simulation and validation of the 2008 Chino Hills earthquake in scattering media

SUMMARYWe simulate 0–2.5 Hz deterministic wave propagation in 3-D velocity models for the 2008 Chino Hills, CA, earthquake using a finite-fault source model and frequency-dependent anelastic attenuation. Small-scale heterogeneities are modeled as 3-D random fields defined using an elliptically anisotropic von Kármán autocorrelation function with its parameters constrained using Los Angeles basin borehole data. We superimpose the heterogeneity models on a leading deterministic community velocity model (CVM) of southern California. We find that models of velocity and density perturbations can have significant effects on the wavefield at frequencies as low as 0.5 Hz, with ensemble median values of various ground motion metrics varying up to ±50 per cent compared to those computed using the deterministic CVM only. In addition, we show that frequency-independent values of the shear-wave quality factor (Qs0) parametrized as Qs0 = 150Vs (Vs in km s–1) provides the best agreement with data when assuming the published moment magnitude (Mw) of 5.4 (M0 = 1.6 × 1017 Nm) for the finite-fault source model. This model for Qs0 trades off with Qs0 = 100Vs assuming Mw = 5.5 (M0 = 2.2 × 1017 Nm), which represents an upper bound of the Mw estimates for this event. We find the addition of small-scale heterogeneities provides limited overall improvement to the misfit between simulations and data for the considered ground motion metrics, because the primary sources of misfit originate from the deterministic CVM and/or the finite-fault source description.

Estimation and applicability of attenuation characteristics for source parameters and scaling relations in the Garhwal Kumaun Himalaya region, India

Source parameters of the small to moderate earthquakes are significant for understanding the dynamic rupture process, the scaling relations of the earthquakes and for assessment of seismic hazard potential of a region. In this study, the source parameters were determined for 58 small to moderate size earthquakes (3.0 ≤ Mw ≤ 5.0) occurred during 2007–2015 in the Garhwal-Kumaun region. The estimated shear wave quality factor (Qβ(f)) values for each station at different frequencies have been applied to eliminate any bias in the determination of source parameters. The Qβ(f) values have been estimated by using coda wave normalization method in the frequency range 1.5–16 Hz. A frequency-dependent S wave quality factor relation is obtained as Qβ(f) = (152.9 ± 7) f(0.82±0.005) by fitting a power-law frequency dependence model for the estimated values over the whole study region. The spectral (low-frequency spectral level and corner frequency) and source (static stress drop, seismic moment, apparent stress and radiated energy) parameters are obtained assuming ω−2 source model. The displacement spectra are corrected for estimated frequency-dependent attenuation, site effect using spectral decay parameter “Kappa”. The frequency resolution limit was resolved by quantifying the bias in corner frequencies, stress drop and radiated energy estimates due to finite-bandwidth effect. The data of the region shows shallow focused earthquakes with low stress drop. The estimation of Zúñiga parameter (ε) suggests the partial stress drop mechanism in the region. The observed low stress drop and apparent stress can be explained by partial stress drop and low effective stress model. Presence of subsurface fluid at seismogenic depth certainly manipulates the dynamics of the region. However, the limited event selection may strongly bias the scaling relation even after taking as much as possible precaution in considering effects of finite bandwidth, attenuation and site corrections. Although, the scaling can be improved further with the integration of large dataset of microearthquakes and use of a stable and robust approach.

Seismic attenuation in the African LLSVP estimated from PcS phases

Seismic tomography models have revealed two broad regions in the lowermost mantle marked by ∼3% slower shear velocity than normal beneath the south central Pacific and southern Africa. These two regions are known as large-low-shear-velocity provinces (LLSVP). There is debate over whether the LLSVPs can be explained by purely thermal variations or whether they must be chemically distinct from normal mantle. Elastic properties alone, have been unable to distinguish the thermal from chemical interpretations. Anelastic structure, however, can help discriminate among models of the LLSVPs since intrinsic attenuation is more sensitive to temperature than to chemical variations. Here we estimate Qμ (the shear wave quality factor) in the African LLSVP using PcS waves generated from a Scotia Arc earthquake, recorded by broadband seismometers deployed in Southern Africa during the Kaapvaal experiment. The upward leg of the PcS waves sweeps from normal mantle into the African LLSVP across the array. We use the spectral ratio (SR) and instantaneous frequency matching (IFM) techniques to measure the differential attenuation (Δt⁎) between waves sampling the African LLSVP and the waves that sample normal lower mantle. Using both methods for estimating Δt⁎ we find that PcS waves sampling the LLSVP are more attenuated than the waves that miss the LLSVP yielding a Δt⁎ difference of more than 1 s. Using the Δt⁎ measurements we estimate the average Qμ in the LLSVP to be about 110. Using a range of activation enthalpy (H⁎) estimates, we find an average temperature anomaly within the LLSVP ranging from +250 to +800 K. Our estimated temperature anomaly range overlaps previous isochemical geodynamic studies that explain the LLSVP as a purely thermal structure although the large uncertainties cannot rule out chemical variations as well.

Determination of site effect and anelastic attenuation at Kathmandu, Nepal Himalaya region and its use in estimation of source parameters of 25 April 2015 Nepal earthquake Mw = 7.8 and its aftershocks including the 12 May 2015 Mw = 7.3 event

The destructive Mw = 7.8 Nepal earthquake happened in Nepal Himalaya, 80 km NW of Kathmandu city on 25 April 2015. A number of aftershocks in which one of them is Mw = 7.3 which occurred on 12 May 2015 are observed around the Kathmandu city of Nepal. In this paper, strong motion data of Nepal earthquake and its eight aftershocks having magnitude range 5.3–7.3, recorded at Kathmandu station is used to determine site effects and attenuation factor. Kathmandu city, capital of Nepal, is situated in a valley which consists of sediments of more than 300 m depth. Hence strong motion data recorded at Kathmandu station is strongly affected by site effect and anelastic attenuation. In this work, S-phase spectra recorded at Kathmandu station are corrected for site effect and anelastic attenuation to compute the source parameters of the events. The site effects and anelastic attenuation are estimated from inversion of strong motion data by using the inversion technique suggested by Joshi (Bull Seismol Soc Am 96:2165–2180, 2006a). The shear wave quality factor (Q β (f)) is computed at Kathmandu station by using the inversion scheme as Q β (f) = 68f0.58. The site effects and attenuation factor obtained by inversion technique are used to correct the spectrum for site effect and anelastic attenuation. The corrected source spectrum is compared with theoretical (Brune in J Geophys Res 78:4997–5009, 1970) spectrum to estimate various source parameters. Both horizontal component (North–South and East–West) are utilized to estimate the source parameters of 25 April 2015 Mw = 7.8 Nepal earthquake and its aftershocks. The best-fit theoretical spectrum provides final values of source parameters, i.e., stress drop, seismic moment, and source radius as 48.7 bars, 5.96 × 1027 dyne cm and 37.75 km, respectively, for the 25 April 2015 Mw = 7.8 earthquake and 1.40 × 1027 dyne cm, 44.7 bars, and 23.90 km, respectively, for the 12 May 2015 Mw = 7.3 earthquake.

Seismic High Attenuation Region Observed Beneath Southern New England From Teleseismic Body Wave Spectra: Evidence for High Asthenospheric Temperature Without Melt

AbstractSeismic attenuation exhibits strong geographic variability in northeastern North America, with the highest values associated with the previously recognized Northern Appalachian Anomaly (NAA) in southern New England. The shear wave quality factor at 100 km depth is 14 < QS < 25, the ratio of P wave and S wave quality factors is QP/QS = 1.2 ± 0.03 (95%), and the frequency dependence parameter is α = 0.39 ± 0.025 (95%). The high values of QP/QS and α are compatible with laboratory measurements of unmelted rock and, in the case of α, incompatible with widespread melting. The low QS implies high mantle temperatures (~1,550–1,650°C) at 100 km depth (assuming no melt). Small‐scale variations in attenuation suggest structural heterogeneity within the NAA, possibly due to lithospheric delamination caused by asthenospheric flow.

Shear wave velocity versus quality factor: results from seismic noise recordings

Shear wave velocity versus quality factor: results from seismic noise recordings

Tomography of crustal seismic attenuation in Metropolitan France: implications for seismicity analysis

In this work, we map the absorption properties of the French crust by analyzing the decay properties of coda waves. Estimation of the coda quality factor \(Q_{c}\) in five non-overlapping frequency-bands between 1 and 32 Hz is performed for more than 12,000 high-quality seismograms from about 1700 weak to moderate crustal earthquakes recorded between 1995 and 2013. Based on sensitivity analysis, \(Q_{c}\) is subsequently approximated as an integral of the intrinsic shear wave quality factor \(Q_{i}\) along the ray connecting the source to the station. After discretization of the medium on a 2-D Cartesian grid, this yields a linear inverse problem for the spatial distribution of \(Q_{i}\). The solution is approximated by redistributing \(Q_{c}\) in the pixels connecting the source to the station and averaging over all paths. This simple procedure allows to obtain frequency-dependent maps of apparent absorption that show lateral variations of \(50\%\) at length scales ranging from 50 km to 150 km, in all the frequency bands analyzed. At low frequency, the small-scale geological features of the crust are clearly delineated: the Meso-Cenozoic basins (Aquitaine, Brabant, Southeast) appear as strong absorption regions, while crystalline massifs (Armorican, Central Massif, Alps) appear as low absorption zones. At high frequency, the correlation between the surface geological features and the absorption map disappears, except for the deepest Meso-Cenozoic basins which exhibit a strong absorption signature. Based on the tomographic results, we explore the implications of lateral variations of absorption for the analysis of both instrumental and historical seismicity. The main conclusions are as follows: (1) current local magnitude \(M_{L}\) can be over(resp. under)-estimated when absorption is weaker(resp. stronger) than the nominal value assumed in the amplitude-distance relation; (2) both the forward prediction of the earthquake macroseismic intensity field and the estimation of historical earthquake seismological parameters using macroseismic intensity data are significantly improved by taking into account a realistic 2-D distribution of absorption. In the future, both \(M_{L}\) estimations and macroseismic intensity attenuation models should benefit from high-resolution models of frequency-dependent absorption such as the one produced in this study.

Estimation of Quality Factor (Qβ) Using Accelerograms of Ahar-Varzaghan Earthquakes, Northwestern Iran

High-frequency strong-motion data of two recent major earthquakes in the Ahar-Varzaghan region, Northwestern Iran, have been used to determine shear-wave quality factor ( Q β ( f )) and seismic source parameters. Data from a local array of 12 stations, two main shocks (Ahar-Varzaghan doublet earthquakes ( M w 6.4 and 6.3) on August 11, 2012), and 38 aftershocks of magnitude 4.1 – 5 were analyzed. The classic Brune model is used to predict the shape of the source spectrum and to provide scaling relationships between spectral and source parameters. In order to obtain reliable estimates of the source spectrum, the effects of attenuation need to be estimated and corrected. By using an inversion algorithm in this work, an average relation in the form Q β = (114 ± 21) f (0.90 ± 0.07) is obtained. The best fit theoretical spectrum provides final values of source parameters, i.e. seismic moment M o and corner frequency f c as 3.19 × 10 25 dyne cm and 0.69 Hz, respectively, for the first event. Obtained Q(f) relationship suggests a low Q o value ( 0.8) for high heterogeneous, tectonically and seismically active regions.

Variable anelastic attenuation and site effect in estimating source parameters of various major earthquakes including M w 7.8 Nepal and M w 7.5 Hindu kush earthquake by using far-field strong-motion data

Strong-motion records of recent Gorkha Nepal earthquake (Mw 7.8), its strong aftershocks and seismic events of Hindu kush region have been analysed for estimation of source parameters. The Mw 7.8 Gorkha Nepal earthquake of 25 April 2015 and its six aftershocks of magnitude range 5.3–7.3 are recorded at Multi-Parametric Geophysical Observatory, Ghuttu, Garhwal Himalaya (India) >600 km west from the epicentre of main shock of Gorkha earthquake. The acceleration data of eight earthquakes occurred in the Hindu kush region also recorded at this observatory which is located >1000 km east from the epicentre of Mw 7.5 Hindu kush earthquake on 26 October 2015. The shear wave spectra of acceleration record are corrected for the possible effects of anelastic attenuation at both source and recording site as well as for site amplification. The strong-motion data of six local earthquakes are used to estimate the site amplification and the shear wave quality factor (Qβ) at recording site. The frequency-dependent Qβ(f) = 124f0.98 is computed at Ghuttu station by using inversion technique. The corrected spectrum is compared with theoretical spectrum obtained from Brune’s circular model for the horizontal components using grid search algorithm. Computed seismic moment, stress drop and source radius of the earthquakes used in this work range 8.20 × 1016–5.72 × 1020 Nm, 7.1–50.6 bars and 3.55–36.70 km, respectively. The results match with the available values obtained by other agencies.

Three-Dimensional Attenuation Structure of the Kumaon Himalayas, India, Based on Inversion of Strong Motion Data

Three-dimensional attenuation structure based on frequency-dependent shear wave quality factor, Q β (f), has been determined for the Kumaon region of the Himalayas. An algorithm based on inversion of strong motion data developed by Joshi (Curr Sci 90:581–585, 2006a) and later modified by Kumar et al. (Pure Appl Geophys, doi: 10.1007/s00024-013-0658-x , 2013) was used for determination of three-dimensional attenuation coefficients. The input of this algorithm is the spectral acceleration of the S phase of the accelerogram and the outcome is the attenuation coefficient and the source acceleration spectra. A dense network monitoring strong ground motion in the Kumaon region of the Uttarakhand Himalayas has been operating since 2006. This network recorded 287 earthquakes up to July, 2013, of which 18 were used for this work. Shear-wave quality-factors were estimated for frequencies of 1.0, 5.0, and 10.0 Hz for two rectangular blocks of surface of dimensions 85 × 55 and 90 × 30 km2 in the Kumaon region of the Himalayas. Both blocks were divided into 25 three-dimensional blocks of uniform thickness with different Q β (f) values. The spatial distribution of frequency-dependent shear-wave quality factors in two different blocks reveal the attenuation properties of the region. The profiles of the contours of shear-wave quality factors observed were comparable with those of major tectonic units present in the region.