Attenuation Of Body Waves Research Articles

Attenuation of Coda and Body Waves for Tbilisi and Surrounding Area, Georgia

The attenuation of high-frequency seismic waves was investigated in the crust beneath Tbilisi and the surrounding territory by analysing 225 local earthquakes that occurred from 2008 to 2020 and were recorded by eight seismic stations. The quality factors of coda waves QC and direct P and S waves, QP and QS, were estimated using the single backscattering model and the extended coda normalization method, respectively. The separation of intrinsic quality factors Qi from scattering quality factor QSC was fulfilled by Wennerberg’s method. Observed results show that all evaluated attenuation parameters are frequency-dependent in the frequency range of 1-32 Hz and increase with increasing frequency. Coda QC values increase also with increasing lapse time window from 20 s to 50 s and vary from 91±5 at 1.5 Hz to 1779±108 at 24 Hz, respectively. P waves attenuate slightly faster than S waves, and the ratio of QS/QP is more than unity and varies in a range of 1.5-1.8. The intrinsic and scattering quality factors are expressed by the following power laws: Qi=77±4f0.930±0.046 and QSC=219±6f0.924±0.050. The results show that Qi is close to QC, but QSC is larger than Qi, which means that intrinsic attenuation has a dominant role compared with the scattering effect. Our results were compared with those obtained in two other seismically active regions of Georgia, as well as with regions of the world. In general, the observed quality factors and their frequency-dependent relationships follow a similar trend, characterizing seismically active regions with complex tectonics. The calculated attenuation parameters characterize the entire earth’s crust under Tbilisi and the surrounding area. The results obtained will be useful in future seismological studies since the Q parameters are estimated for the first time for the given region.

Spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region, India

We have investigated the spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region from the datasets of 465 well located earthquakes, recorded at 52 broadband stations between 2017 and 2020. The body wave attenuation parameters QP and QS were estimated at each station by applying the extended coda normalization method for five different central frequencies ranging from 1.5 to 18 Hz. Strong frequency-dependent body wave attenuation was observed at each station over the whole region. Also, we found a significant variation of QP and QS from north to south which is consistent with geotectonic diversity beneath the study region. We have also made separate estimations of frequency-dependent relations for Garhwal and Kumaun Himalaya region in order to investigate the lateral variation in attenuation characteristics, and obtained the frequency-dependent relations as follows: QP=30±3f1.05±0.05, QS=143±20f0.88±0.07 for the Garhwal Himalaya region and QP=31±1f1.09±0.02, QS=121±11f1.00±0.04 for the Kuamun Himalaya region. Obtained Q values indicate strong body wave attenuation for both the region with no significant lateral variation. It may suggest the presence of crustal level folding, faulting, aqueous fluids, and metamorphic fluids below both the regions. Also, the ratios of QS/QP are high (>1) for the entire analyzed frequency range, suggesting a significant level of heterogeneity and tectonic complexities in the crust of the study region.

Forecasting the onset of volcanic eruptions using the increase in seismicity during magma ascent

The Failure Forecast Method (FFM) has been used to forecast the onset of volcanic eruptions with varying degrees of success. The method involves fitting its empirical equation to precursory observables, e.g., seismic data. Current models explaining the empirical equation assume that the seismic observables used (e.g., seismic event-rate, seismic moment-rate or seismic energy-rate) are related to accelerated fracture growth. In these models, such fracture growth results in an apparent increase in seismic activity. In this study, however, we propose an alternative explanation for the increase in seismicity culminating in volcanic eruptions. We suggest that the increase in seismicity, particularly in the seismic amplitudes, can result from ascending seismic sources towards the surface and that the increase can be used for eruption forecasts using the FFM. Our argument is based on the need for magma to migrate towards the surface to produce an eruption.To test our argument, we used synthetic and field/real data and fitted them with the FFM model. The synthetic data is generated using the body wave attenuation equation, and the real data is from the Piton de la Fournaise eruption on January 2nd, 2010. We used seismic amplitudes and seismic amplitude ratios as the precursory observables and used a combination of Simulated Annealing and Bayesian Inversion algorithms to obtain the best-fit parameters of the FFM model and their posterior distributions. For the synthetic cases, we found that the method gave reasonable estimates of failure or eruption time, suggesting that the seismic amplitudes and the seismic amplitude ratios can be used for forecasting using the FFM. For the field/real cases, the results gave information about the timing of magma chamber failure, magma propagation, and the eruption onset. Our results highlight an example of modelling volcano monitoring data with the FFM that gives reasonable estimates of eruption time even if the data may not fully represent failure processes. We suggest that the result may still be reasonable owing to the empirical and asymptotic nature of the FFM combined with the underlying physics of magma and seismic migrations.

Attenuation of P and S Waves in the Javakheti Plateau, Georgia (Sakartvelo)

The frequency-dependent parameters of attenuation of P and S waves in one of the most seismically active regions, of the Javakheti plateau, have been estimated using digital data for the first time. We have analyzed and processed hundred and fifty local shallow earthquakes that occurred from 2006 to 2018 and were recorded by five seismic stations. The quality factors for P waves ( Q p ) and for S waves ( Q s ) were evaluated by means of the extended coda normalization method. The obtained Q p and Q s are strongly frequency dependent in the frequency range of 1.5 to 24 Hz, and increase with frequency according to the following power laws: Q P = 17.4 ± 2.3 f 1.100 ± 0.033 and Q S = 28.8 ± 3.3 f 1.048 ± 0.039 . The observed Q s / Q p ratio was found to be greater than unity over the entire frequency range, suggesting that scattering may play the main role in the attenuation of body waves on the Javakheti plateau. The frequency dependence of the S wave is very similar to the frequency dependence of the shear wave for another seismically active region of Georgia, the Racha area. A comparison of our results to those other regions of the world shows that among the seismically active areas, the Javakheti plateau is characterized by relatively low values of Q p and Q s , but they are more than volcanic regions such as Etna and Qeshm Island, Iran. The observed results characterize the entire earth’s crust in the study area and will be useful for source parameter estimation, ground motion prediction, and hazard assessment of the Javakheti plateau.

Seismic wave attenuation at Valle Medio del Magdalena, Colombia

A study of seismic body wave attenuation was carried out for the Valle Medio del Magdalena (VMM) basin, Colombia. A total of 326 high-quality local earthquakes were used to calculate coda Q (Qc) values, and 1463 high-quality local earthquakes were used to calculate Q scattering (Qs), Q intrinsic (Qi), Q total (QT), and other attenuation parameters. The average Qc ranged from 47 for 2.0 Hz to 904 for 12 Hz. The average Q-frequency relation (Qc -f) for the VMM basin was Qc = 50 ± 6.4 f1.1±0.08. Qi, Qs, and QT ranged from 4, 6, and 2.4 for 2.0 Hz to 420,483, and 224 for 12 Hz, respectively, depending on the region, epicentral distance and depth. Intrinsic absorption was predominant over scattering in the VMM basin at shallow crustal depths, although scattering was also remarkable. Qc value is similar to Qs for low frequencies ( ≤ 2 Hz). For high frequencies ( ≥ 3 Hz), Qc is much higher than Qi, Qs, and QT, indicating that at low frequencies Qc is reflecting the scattering process occurring at VMM. In addition, it means that single-scattering approach is valid to measure attenuation at VMM for low frequencies but not for high frequencies, where the attenuation is underestimated by using Qc. The spatial distribution of the average Qi, Qs, and QT shows that, in general, the VMM region has high attenuation at shallow crustal depths, with higher attenuation to the south and north. This high attenuation is possibly due to the presence of oil and water reservoirs as well as the presence of thick sedimentary deposits compared with the surrounding plutonic rocks. The results of this study will be useful for further seismic source, strong motion and seismic hazard studies in the VMM region.

Frequency-dependent quality factor of body waves in the Kathmandu region, Nepal

The Kathmandu region becomes situated in very close to the major thrust faults such as Main Boundary Thrust (MBT) and Main Central Thrust (MCT) has been significantly affected by a series of several major earthquakes in the past. The current study analyses the attenuation of body waves in the region using the coda normalization method based on the data of two stations (NA430 and NA060) essentially from a temporary seismic network deployed after the occurrence of the 25th April 2015 earthquake. The events considered for the study have a local magnitude of 4–5.5 ​at a depth range of 10–27 ​km. The estimated quality factor of primary and secondary waves are QP = (44 ​± ​12.896)f (0.82±0.173), QS = (62 ​± ​19.265)f (0.94±0.185) for NA430 station and QP = (35 ​± ​9.467)f (0.93±0.155), QS = (68 ​± ​22.510)f (0.91±0.198) for NA060 station. The QS/QP ratio more than one observed for the entire frequency range specifies the highly heterogeneous crust medium beneath the study region. The obtained quality factors could be used in the generation of target ground motion for engineering purposes and in the calculation of source parameters for seismic hazard assessment as risk reduction strategies in the study region.

An attenuating, isotropic and heterogeneous uppermost inner core: evidence from global PKiKP-PKIKP amplitude ratio tomography

SUMMARY The Earth's inner core is solidifying from the liquid outer core, where convection currents power the geodynamo. Constraining properties of the inner core is fundamentally important, yet seismic models do not reconcile body wave and normal mode attenuation properties. Here, we analyse high signal-to-noise ratio core-refracted (PKIKP) and core-reflected (PKiKP) waves generated by earthquakes recorded globally between 1987 and 2021. These phases are excellent indicators of uppermost inner core (UIC) properties due to their low angular separation at the core–mantle boundary and similar ray paths. We analyse their amplitude ratios (ARs) and time delays (DTs) relative to synthetic waveforms and estimate the required attenuation perturbations in a linearized attenuation tomography. The UIC is cylindrically isotropic and heterogeneous in both seismic velocity and attenuation. Contrary to the paradigm that the inner core is seismically hemispherical, we find more complex patterns of attenuation that correlate with seismic velocities, according to both Akaike criterion and Student's t-test. The UIC beneath NE Asia is weakly attenuating and seismically slower, in contrast to the expected properties associated with the quasi-Eastern Hemisphere. Beneath South America, where the core is thought to grow fastest, AR values are more diverse, possibly reflecting interdendritic melt inclusions, and DTs show an E–W gradient. The UIC is seismically slow and weakly attenuating beneath the Atlantic, but strong anomalies emerge near West Africa. Attenuation slightly changes with depth conceivably implying an increase in melt degree or a change in inner core growth rate with time. These 3-D heterogeneities are inconsistent with simple models of core translation or lopsided growth, indicating that more intricate processes are needed to explain inner core structure and evolution.

Azimuthal Path Variations of Regional Body Wave Attenuation via Coda-Source Normalized Method: An Example from Eastern Marmara (NW Turkey)

ABSTRACT This study examines the body wave attenuation characteristics for the eastern section of the Marmara Region (NW Turkey). Qp and Qs values were obtained using the vertical and two horizontal components of the three-component broad-band data, respectively that recorded in a circular areal extent with 120 km radius centred at the ARMT station on the Armutlu Peninsula. The database was prepared according to back-azimuthal distributions, and data-packs representing 36 different back-azimuthal slices were subjected to the Extended-Coda-Normalization-Method to examine the azimuthal path-dependence of the attenuation characteristics of direct body waves in the region. While the region corresponding to the basins in the Marmara Sea with relatively low seismicity is represented by high Q values, low Q values were observed in areas with intense faulting and seismicity in the NE of the study area. Additionally, direction-independent attenuation functions for the region were calculated as Qp = 18 ± 1f0.92±0.2 and Qs = 38 ± 1f0.87±0.01 for P and S phases, respectively.

Structure and QP–QS Relations in the Seattle and Tualatin Basins from Converted Seismic Phases

ABSTRACTWe use converted body-wave phases from local earthquakes to constrain depth to basement and average attenuation relations for the Seattle basin in Washington and the Tualatin basin in Oregon. P-, P-to-S-(Ps), S-to-P-(Sp), and S-wave arrivals are present in three-component recordings of magnitude 2.5–4.0 earthquakes at seismic stations located in these basins. Based on their relative travel times, these phases are attributed to body-wave conversions at the basement-to-basin contact or to high-impedance interfaces within the basins. Depth to basement values are calculated using the differential travel times between direct and converted phases, as well as average P- and S-wave velocity values. We also identify a high-impedance layer in the Tualatin basin that likely represents a laterally extensive deposit of volcanic materials embedded between the basement contact and the Columbia River Basalt Group. In addition, the average QP–QS attenuation relation is calculated for each station by taking the spectral ratio of converted phases to their parent body-wave arrivals. For the Seattle basin, our analysis yields an average QP value of 73 and an average QS value of 60 for seismic waves with frequencies between 2 and 25 Hz. In the Tualatin basin, a much reduced QP–QS relation suggests that average body-wave attenuation is likely higher than in the Seattle basin. The converted phase techniques presented here provide a reliable way to develop estimates of basin depth and attenuation structure for undercharacterized regions using simple passive source seismic records.

Source spectral studies using Lg wave in western Tibet

We investigate the spectral characteristics of Lg wave using 346 waveforms recorded at 24 seismic stations located across Karakoram Fault (KKF) in western Tibet. The tectonic structure of western Tibet is very complex and formed of several blocks, which are separated by distinct suture zones. It is predominated by KKF which is one of the major strike-slip faults in western Tibet. To understand the spatial variation in crustal properties, we subdivide the study region into two parts across the KKF. The analysis of the decay of the spectral amplitudes of Lg waves with epicentral distances allows for the evaluation of the crustal quality factor QLg, which is a direct measurement of attenuation. We have observed a frequency-dependent QLg with strong attenuation in the crust though no variation is observed across the KKF. This is consistent with the reported body wave attenuation study for this region. The corner frequency (fc) is observed to decrease linearly with distance in both sides of the KKF with an effective shift of 2.5 to 1 Hz at 400 km and 1 to 0.7 Hz near 1000 km respectively. The moment magnitude of each earthquake is computed using displacement spectrum and subsequently compared with the reported magnitude.

Determination of the damping ratio by multi-channel spectral analysis of seismic downhole data

Soil dynamic parameters such as shear wave velocity and damping ratio are of major interest in earthquake engineering. While the shear wave velocity, directly linked to the shear modulus, can be determined by a number of laboratory and in-situ tests with satisfying accuracy the damping ratio is much more difficult to obtain. Especially the results of in-situ experiments show often large variations. This is in general due to the troublesome determination of precise signal amplitudes whether in time or frequency domain related with these techniques.The paper presented comes back to a relationship between attenuation and velocity dispersion of body waves which replaces the measurement of the amplitude characteristics of seismic signals by a frequency dependent velocity function. The implementation of this method has previously shown to be difficult because of the very small levels of dispersion observed in seismic data. Our approach aims to overcome the problem by applying a multi-channel spectral analysis which is widely used in surface wave testing to calculate a velocity dispersion. Multi-channel measurements have shown to be more tolerant to erroneous phase characteristics of single seismic traces than the more common two station measurements.The velocity dispersion curve is extracted from a phase velocity–frequency spectrum and the damping ratio is calculated by fitting a theoretical dispersion curve to the extracted curve. The method is demonstrated on correlated data of a seismic downhole test performed using a S-wave vibrator source. The obtained results show a reasonable agreement with damping ratios found in the literature for similar soils.

The relative contributions of scattering and viscoelasticity to the attenuation of S waves in Earth's mantle

Abstract. The relative contributions of scattering and viscoelastic attenuation to the apparent attenuation of seismic body waves are estimated from synthetic and observed S waves multiply reflected from Earth's surface and the core–mantle boundary. The synthetic seismograms include the effects of viscoelasticity and scattering from small-scale heterogeneity predicted from both global tomography and from thermodynamic models of mantle heterogeneity that have been verified from amplitude coherence measurements of body waves observed at dense arrays. Assuming thermodynamic models provide an estimate of the maximum plausible power of heterogeneity measured by elastic velocity and density fluctuations, we predict a maximum scattering contribution of 43 % to the total measured attenuation of mantle S waves having a dominant frequency of 0.05 Hz. The contributions of scattering in the upper and lower mantle to the total apparent attenuation are estimated to be roughly equal. The relative strength of the coda surrounding observed ScSn waves from deep focus earthquakes is not consistent with a mantle having zero intrinsic attenuation.

Attenuation of high frequency body waves in the crust of western Tibet

The attenuation characteristics of western Tibet have been analyzed by estimating the seismic attenuation parameters (Qp−1 and Qs−1) at five different central frequencies (1.5, 3, 6, 12 and 18 Hz). The extended coda normalization method has been used on 469 three component local earthquakes which are recorded at Y2 network of broadband stations deployed in western Tibet during years 2007–2011. We have estimated the frequency dependent Qp−1 and Qs−1 values at each station. The results indicate strong body wave attenuation in the medium below western Tibet. The highest attenuation is observed at WT19 and WT13 stations located near to the Karakoram fault (KKF) and Indus-Yarlung suture (IYS). Across the KKF, we have subdivided the entire study area into two regions to explore the lateral variations of crustal attenuation properties. The first part covers the northeastern of KKF referred as Region 1 while the second part covers the southwestern of KKF referred as Region 2. The frequency dependent relations are found as follows: Qp−1 = (19.4 ± 4.4).10−3f−(0.9±0.2), Qs−1 = (13.3 ± 2.1).10−3f−(0.8±0.1) for Region 1 and Qp−1 = (18.4 ± 1.8).10−3f−(0.8±0.1), Qs−1 = (13.2 ± 2.2).10−3f−(0.8±0.1) for Region 2. Both the regions show strong frequency dependent nature of Q−1 values with no significant lateral variations. It may suggest the presence of similar tectonic complexities and heterogeneities beneath both the regions. We also find that all the estimated values of Qp−1/ Qs−1 ratios are greater than unity at all the frequency range which are likely to be associated with both the intrinsic and scattering phenomenon in the crust of western Tibet.

Investigation of coda and body wave attenuation functions in Central Asia

In this study, we evaluate the body and coda wave attenuation characteristics within Kyrgyzstan and Tajikistan as part of Central Asia. The selected database consists of 354 broadband seismograms from 179 local earthquakes recorded by 24 different stations within the period of 2015 through 2018. First, coda Q has been inferred for different coda window lengths of 20, 30, 40, and 50 s using the single-backscattering interpretation. The coda Q values increase by increasing the coda window length. We show that coda attenuation properties in Central Asia are better modeled by multiple-scattering and surface wave regimes for long-distance records without invoking any depth dependence of the attenuation properties in the crust. Furthermore, standard errors and convergence of different components’ QC indicate that we can fit envelope records of coda waves much better using a coda window length of 50 s. Therefore, we evaluate average coda quality factor functions as QC = 261 f0.601 and QC = 219 f0.633 assuming multiple-scattering and surface wave regimes for a coda window length of 50 s in the frequency range of 1 to 20 Hz for distances up to 200 km. We also show that the source to site distance of records has a significant impact on coda Q estimates. For a shorter distance range up to 100 km, attenuation attributes of Central Asia are better captured by a single-scattering model. We reevaluate the average coda quality factor function as QC = 222 f0.692 assuming a single-backscattering model for a coda window length of 50 s in the frequency range of 1 to 20 Hz for distances up to 100 km. Moreover, we determine QP = 158 f0.706 and QS = 152 f0.856 with a geometrical spreading function of R−1 using the multiple-station coda normalization method.

On the robustness of attenuation measurements on teleseismic P waves: insights from micro-array analysis of the 2017 North Korean nuclear test

SUMMARY Despite their importance as a fundamental constraint on Earth properties, regional-scale measurements of body-wave seismic attenuation are scarce. This is partially a result of the difficulty in producing robust estimates of attenuation. In this paper, we focus on measuring differential attenuation on records of teleseismic P waves. We examine a unique data set of five records of the North Korean nuclear test of 2017 measured at five broad-band seismic stations deployed within a few metres of each other but using different installation procedures. Given their extreme proximity, we expect zero differential intrinsic attenuation between the different records. However, we find that different attenuation measurement methods and implementation parameters in fact produce significant apparent differential attenuation (Δt*). Frequency-domain methods yield a wide range of Δt* estimates between stations, depending on measurement bandwidth and nuances of signal processing. This measurement instability increases for longer time windows. Time domain methods are largely insensitive to the frequency band being considered but are sensitive to the time window that is chosen. We determine that signal-generated noise can affect measurements in both the frequency and time domain. In some cases, the range of results amounts to a significant fraction of the range of differential attenuation across the conterminous United States as determined by a recent study. We suggest some approaches to manage the inherent instability in these measurements and recommend best practices to confidently estimate body wave attenuation.

Frequency-dependent body-Q and coda-Q in Karlıova Triple Junction and its vicinity, Eastern Turkey

A dataset obtained from six broadband stations of 111 small- to moderate-sized local events that occurred between 2010 and 2018 was analyzed to reveal the frequency-dependent seismic body (body-Q) and coda (coda-Q) wave attenuation characteristics of three tectonic branches of the Karlıova Triple Junction: the Varto Fault Zone (VFZ), North Anatolian Fault Zone (NAFZ), and East Anatolian Fault Zone. The magnitude range was between 2.8 and 6.1 with shallow (

Attenuation of short-period body waves in Northwestern Himalayan Region, India

The attenuation of seismic wave is one of the basic physical parameters which is closely related to the seismicity and regional tectonics activity of a particular area. This is also important for seismic hazard measurement. Seismic wave attenuation for local earthquakes is determined from the analysis of direct body waves and coda waves. The dimensionless parameter, Q, is studied in the present work, which is defined as a measure of the rate of decay of the coda waves or body waves within a specific frequency band. Digital seismogram data of 75 earthquakes that occurred in Garhwal Himalaya region during 2004–2006 and recorded at 20 different stations have been analyzed to study the seismic body and coda wave attenuation. Seismic body wave attenuation characteristics are examined by estimating the frequency dependent relationship of quality factors for P-waves, Qα and for S-waves, Qβ in the frequency range 1.5–28 Hz, using 95 seismic observations with hypo central distance less than 100 km. The extended coda normalization method was applied. The average value of Qα is found to be varied from 45.10 at 1.5 Hz to 1400.0 at 28 Hz, while it varies from 109.02 at 1.5 Hz to 3987.0 at 28 Hz for Qs. The estimated frequency dependent relation for quality factors are Qα=(29.077±8.5)f(1.16±0.01) and Qβ=(67.84±13.5)f(1.18±0.02) for P and S-waves, respectively. The rate of increase of Q(f) for P and S waves in this region is comparable with the other regions of the Himalayas. The ratio QβQα is greater than unity in the entire analyzed frequency range. It indicates that scattering is an important factor contributing to the attenuation of body wave in the region. The low Qα,β values or high, attenuation at lower frequencies and high Qα,β values or low attenuation at higher frequency may indicate that the heterogeneity decreases with increasing depth in the study region.

Insights into the lithospheric architecture of Iberia and Morocco from teleseismic body-wave attenuation

The long and often complicated tectonic history of continental lithosphere results in lateral strength heterogeneities which in turn affect the style and localization of deformation. In this study, we produce a model for the attenuation structure of Iberia and northern Morocco using a waveform-matching approach on P-wave data from teleseismic deep-focus earthquakes. We find that attenuation is correlated with zones of intraplate deformation and seismicity, but do not find a consistent relationship between attenuation and recent volcanism. The main features of our model are low to moderate Δt⁎ in the undeformed Tertiary basins of Spain and high Δt⁎ in areas deformed by the Alpine orogeny. Additionally, low Δt⁎ is found in areas where the Alboran slab is thought to be attached to the Iberian and African lithosphere, and high Δt⁎ where it has detached. These features are robust with respect to inversion parameters, and are consistent with independent data. Very mild backazimuthal dependence of the measurements and comparison with previous results suggest that the source of the attenuation is sub-crustal. In line with other recent studies, the range of Δt⁎ we observe is much larger than can be expected from lithospheric thickness or temperature variations.

Attenuation Characteristics of High Frequency Seismic Waves in Southern India

We present a systematic study of seismic attenuation and its related Q structure derived from the spectral analysis of P-, S-waves in the southern India. The study region is separated into parts of EDC (Eastern Dharwar Craton), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT). The study is carried out in the frequency range 1–20 Hz, using a single-station spectral ratio technique. We make use of about 45 earthquakes, recorded in a network of about 32 broadband 3-component seismograph-stations, having magnitudes (M L) varying from 1.6 to 4.5, to estimate the average seismic body wave attenuation quality factors; Q P and Q S. Their estimated average values are observed to be fitting to the power law form of Q = Q 0 f n . The averaged power law relations for Southern Indian region (as a whole) are obtained as Q P = (95 ± 1.12)f (1.32±0.01); Q S = (128 ± 1.84)f (1.49±0.01). Based on the stations and recorded local earthquakes, for parts of EDC, WDC and SGT, the average power law estimates are obtained as: Q P = (97 ± 5)f (1.40±0.03), Q S = (116 ± 1.5)f (1.48±0.01) for EDC region; Q P = (130 ± 7)f (1.20±0.03), Q S = (103 ± 3)f (1.49±0.02) for WDC region; Q P = (68 ± 2)f (1.4±0.02), Q S = (152 ± 6)f (1.48±0.02) for SGT region. These estimates are weighed against coda Q (Q C) estimates, using the coda decay technique, which is based on a weak backscattering of S-waves. A major observation in the study of body wave analysis is the low body wave Q (Q 0 0.5) and Q S/Q P ≫ 1, suggesting lateral stretches of dominant scattering mode of seismic wave propagation. This primarily could be attributed to possible thermal anomalies and spread of partially fluid-saturated rock-masses in the crust and upper mantle of the southern Indian region, which, however, needs further laboratory studies. Such physical conditions might partly be correlated to the active seismicity and intraplate tectonism, especially in SGT and EDC regions, as per the observed low-Q P and Q S values. Additionally, the enrichment of coda waves and significance of scattering mechanisms is evidenced in our observation of Q C > Q S estimates. Lapse time study shows Q C values increasing with lapse time. High Q C values at 40 s lapse times in WDC indicate that it may be a relatively stable region. In the absence of detailed body wave attenuation studies in this region, the frequency dependent Q relationships developed here are useful for the estimation of earthquake source parameters of the region. Also, these relations may be used for the simulation of earthquake strong ground motions which are required for the estimation of seismic hazard, geotechnical and retrofitting analysis of critical structures in the region.

Quantifying Regional Body Wave Attenuation in a Seismic Prone Zone of Northeast India

We evaluated the body wave attenuation parameter in Kopili region of northeast India. Using the modified algorithm of coda normalization method, we delineated frequency-dependent attenuation for both P and S waves. Taking more than 300 seismograms as input, we comprehensively studied microearthquake spectra in the frequency range of 1.5 to 12 Hz. The estimated values of $Q_P^{-1}$ and $Q_S^{-1}$ show strong frequency dependence. The relationships emerge to be $Q_P^{-1}=(23.8\pm 6)\times10^{-3}f^{-1.2\pm 0.008}$ and $Q_S^{-1}=(10.2\pm 2)\times10^{-3}f^{-1.3\pm 0.02}$, respectively. The ratio $Q_P^{-1}/Q_S^{-1}$ is found to be larger than unity for the entire frequency band which implies profound seismic activity and macroscale heterogeneity prevailing in the region. The study may act as the building block towards determination of source parameter and hazard related studies in the region.