Worldwide variations in the attenuative properties of the upper mantle as determined from spectral studies of short-period body waves

A worldwide study of short-period teleseismic body wave spectra shows that the high frequency falloff rates of spectra are correlated with the tectonic type of the source and receiver regions and with source depth. The data indicate, in a consistent manner, that the main cause for such variations is the lateral variation of Q in the upper mantle as well as change of Q with depth. Using the internal consistency checks provided by redundancies in the data set other effects such as crustal, site dependent distortion of the spectra, source effects and instrument non-linearity can be ruled out as significant factors influencing the $\text{t1}$ estimates obtained. The results indicate high attenuation in the upper mantle under tectonic regions and new oceans. Long-period regional attenuation studies indicate similar variations in mantle Q among the types of regions mentioned but yield significantly lower Q estimates in all areas. The short- and long-period attenuation results can be reconciled only by assuming a frequency dependent Q that increases with frequency along all types of paths, such that the relative differences in Q along various types of paths retain the same sign over the short- and long-period bands.     

Attenuation of P- and S-waves in the Chamoli Region, Himalaya, India

The attenuation properties of the crust in the Chamoli region of Himalaya have been examined by estimating the frequency-dependent relationships of quality factors for P waves (Q_α) and for S waves (Q_β) in the frequency range 1.5–24 Hz. The extended coda normalization method has been applied on the waveforms of 25 aftershocks of the 1999 Chamoli earthquake (M 6.4) recorded at five stations. The average value of Q_α is found to be varied from 68 at 1.5 Hz to 588 at 24 Hz while it varies from 126 at 1.5 Hz to 868 at 24 Hz for Q_β. The estimated frequency-dependent relations for quality factors are Q_α = (44 ± 1)f^(0.82±.04) and Q_β = (87 ± 3)f^(0.71±.03). The rate of increase of Q(f) for P and S waves in the Chamoli region is comparable with the other regions of the world. The ratio Q_β/Q_α is greater than one in the region which along with the frequency dependence of quality factors indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of attenuation relation for S wave estimated here (Q_β = 87f^0.71) with that of coda waves (Q_c = 30f^1.21) obtained by Mandal et al. (2001) for the same region shows that Q_c > Q_β for higher frequencies (>8 Hz) in the region. This indicates a possible high frequency coda enrichment which suggests that the scattering attenuation significantly influences the attenuation of S waves at frequencies >8 Hz. This observation may be further investigated using multiple scattering models. The attenuation relations for quality factors obtained here may be used for the estimation of source parameters and near-source simulation of earthquake ground motion of the earthquakes, which in turn are required for the assessment of seismic hazard in the region.     

Stress Drops of the 1997–1998 Colfiorito,Central Italy Earthquakes: Hints for a Common Behaviour of Normal Faults in the Apennines

Stress drop estimates of moderate-magnitude earthquakes in the Umbria–Marche region, in the northern Apennines, exhibit a large scatter. For the two M _w 5.7 and 6.0 main shocks of 26 September 1997 near Colfiorito, several papers resulted in stress drop estimates of 20 MPa, but values as low as 2–3 MPa were proposed as well. Also for the largest aftershocks (M _w > 4), estimates spread from < 1 MPa up to values ten times larger. We have critically revisited methods and data used in the literature. We have specifically faced the trade-off between source and propagation effects, as we believe that it is responsible for a part of the large scatter. To keep this trade-off under control, we have applied a methodology that combines the best fit of both source spectra after Empirical Green’s Function (EGF) deconvolution and observed ground motion spectra, finding that the results of the two different data sets converge independently at the same solution. We have used ground motions observed in the Colfiorito basin, where an accelerograph and a co-located seismological broad-band station recorded three clusters of earthquakes in a broad magnitude interval (1.7 ≤ M _w ≤ 6.0). We have found that the mainshock–aftershock sequences result in stress drops of 2–5 MPa at M _w ≥ 5.6, with an average tendency to decrease at smaller magnitudes where stress drop variability increases. These findings confirm the source scaling recently assessed through Empirical Green’s Function deconvolution for another well-monitored seismic sequence of normal-faulting earthquakes, which struck the city of L’Aquila in the central Apennines in April 2009. The similar scaling law of the two areas suggests common mechanisms of stress release for the shallow normal faults in the Apennines. The propensity of smaller earthquakes to increase in variability, with a tendency toward smaller stress drops, may reflect an effect of fault strength heterogeneities for smaller size ruptures.     

Stress Release and Source Scaling of the 2010–2011 Canterbury,New Zealand Earthquake Sequence from Spectral Inversion of Ground Motion Data

The Canterbury earthquake sequence beginning with the 2010 M _W 7.2 Darfield earthquake is one of the most notable and well-recorded crustal earthquake sequences in a low-strain-rate region worldwide and as such provides a unique opportunity to better understand earthquake source physics and ground motion generation in such a tectonic setting. Ground motions during this sequence ranged up to extreme values of 2.2 g, recorded during the February 2011 M _W 6.2 event beneath the city of Christchurch. A better understanding of the seismic source signature of this sequence, in particular the stress release and its scaling with earthquake size, is crucial for future ground motion prediction and hazard assessment in Canterbury, but also of high interest for other low-to-moderate seismicity regions where high-quality records of large earthquakes are lacking. Here we present a source parameter study of more than 200 events of the Canterbury sequence, covering the magnitude range M _W 3–7.2. Source spectra were derived using a generalized spectral inversion technique and found to be well characterized by the ω ^?2 source model. We find that stress drops range between 1 and 20 MPa with a median value of 5 MPa, which is a factor of 5 larger than the median stress drop previously estimated with the same method for crustal earthquakes in much more seismically active Japan. Stress drop scaling with earthquake size is nearly self-similar, and we identify lateral variations throughout Canterbury, in particular high stress drops at the fault edges of the two major events, the M _W 7.2 Darfield and M _W 6.2 Christchurch earthquakes.     

Site response and source spectra of S waves in the Zagros region, Iran

S wave amplitude spectra from shallow earthquakes with magnitudes ranging between 4.2 and 6.2 in the Zagros region of Iran that occurred between 1998 and 2008 are used to examine source parameters and site response of S waves. A generalized inversion scheme has been used to separate the source, propagation path, and local site effects from S wave spectra. For removing the trade-off between source and site terms and propagation effects (including geometric and anelastic attenuation), the spectral amplitudes of the records used were corrected for attenuation and geometrical spreading function using a path model proposed by Zafarani and Soghrat (Bull Seism Soc Am 102:2031–2045, 2012) for the region. We assume a Brune’s point source model to retrieve source parameters like corner frequency, moment magnitude, and high-frequency fall off coefficient, for each event. When the source spectra are interpreted in terms of Brune’s model, the average stress drops obtained are about 7.1 and 5.9 MPa (71 and 59 bars), respectively for the eastern and western Zagros regions. Stress drops range from 1.4 to 35.0 MPa (14 to 350 bars), with no clear dependence on magnitude. The results in terms of stress drop and S wave seismic energy indicate that the Zagros events are more similar to interplate earthquakes of western North America than to intraplate events of eastern North America. The method also provides us with site responses for all 40 stations individually and is an interesting alternative to other methods, such as the H/V method. A new empirical relationship between body-wave magnitudes and moment magnitude has been proposed for the Iranian plateau using derived seismic moment from the inversion.     

A study of Guptkashi,Uttarakhand earthquake of 6 February 2017 (<Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript>5.3) in the Himalayan arc and implications for ground motion estimation

The 2017 Guptkashi earthquake occurred in a segment of the Himalayan arc with high potential for a strong earthquake in the near future. In this context, a careful analysis of the earthquake is important as it may shed light on source and ground motion characteristics during future earthquakes. Using the earthquake recording on a single broadband strong-motion seismograph installed at the epicenter, we estimate the earthquake’s location (30.546° N, 79.063° E), depth (H?=?19 km), the seismic moment (M₀?=?1.12×10¹⁷ Nm, M _w 5.3), the focal mechanism (φ?=?280°, δ?=?14°, λ?=?84°), the source radius (a?=?1.3 km), and the static stress drop (Δσ _s ~22 MPa). The event occurred just above the Main Himalayan Thrust. S-wave spectra of the earthquake at hard sites in the arc are well approximated (assuming ω^?2 source model) by attenuation parameters Q(f)?=?500f^0.9, κ?=?0.04 s, and f_max?=?infinite, and a stress drop of Δσ?=?70 MPa. Observed and computed peak ground motions, using stochastic method along with parameters inferred from spectral analysis, agree well with each other. These attenuation parameters are also reasonable for the observed spectra and/or peak ground motion parameters in the arc at distances ≤?200 km during five other earthquakes in the region (4.6?≤?M _w ?≤?6.9). The estimated stress drop of the six events ranges from 20 to 120 MPa. Our analysis suggests that attenuation parameters given above may be used for ground motion estimation at hard sites in the Himalayan arc via the stochastic method.     

Attenuation of High-Frequency Seismic Waves in Eastern Iran

We investigated the frequency-dependent attenuation of the crust in Eastern Iran by analysis data from 132 local earthquakes having focal depths in the range of 5–25 km. We estimated the quality factor of coda waves (Q _c) and body waves (Q _p and Q _s) in the frequency band of 1.5–24 Hz by applying the single backscattering theory of S-coda envelopes and the extended coda-normalization method, respectively. Considering records from recent earthquakes (Rigan M _w 6.5, 2010/12/20, Goharan M _w 6.2, 2013/5/11 and Sirch M _w 5.5, 2013/1/21), the estimated values of Q _c, Q _p and Q _s vary from 151 ± 49, 63 ± 6, and 93 ± 14 at 1.5 Hz to 1,994 ± 124, 945 ± 84 and 1,520 ± 123 at 24 Hz, respectively. The average frequency-dependent relationships (Q = Q _o f ⁿ ) estimated for the region are Q _c = (108 ± 10)f ^(0.96±0.01), Q _p = (50 ± 5)f ^(1.01±0.04), and Q _s = (75 ± 6)f ^(1.03±0.06). These results evidenced a frequency dependence of the quality factors Q _c, Q _p, and Q _s, as commonly observed in tectonically active zones characterized by a high degree of heterogeneity, and the low value of Q indicated an attenuative crust beneath the entire region.     

Source parameters and scaling relations for small earthquakes in Kumaon Himalaya, India

We investigate the scaling relationships among earthquake source parameters using more than 300 good quality broad band seismograms from 30 small earthquakes in the Kumaon Himalaya from the spectral analysis of P and S waves. The average ratio of P/S wave corner frequency is found to be 1.13, which is suggestive of shift in the corner frequency. The estimated seismic moment range from 1.6?×?10¹³–5.8?×?10¹⁵ N?m, while the stress drop varies from 0.6 to 16 bars with 80 % of the events below 10 bars. An analysis of stress drop and apparent stress drop indicates the partial stress drop mechanism in the region. The source radii are between 0.17 and 0.88 km. The total seismic energy varies from 1.79?×?10⁸ to 7.30?×?10¹¹ J. We also observe the variation in seismic energy for a given seismic moment. The scaling relation between the seismic moment and stress drop suggests the breakdown of constant stress drop scaling for the range of seismic moments obtained here for the region. This shows the anomalous behavior of small earthquakes in the region. The study indicates that the stress drop is the dominant scaling factor for the moments studied here.     

Attenuation of Coda Waves in the Saurashtra Region, Gujarat (India)

The attenuation characteristics based on coda waves of two areas—Jamnagar and Junagarh of Saurashtra, Gujarat (India)—have been investigated in the present study. The frequency dependent relationships have been developed for both the areas using single back scattering model. The broadband waveforms of the vertical components of 33 earthquakes (Mw 1.5–3.5) recorded at six stations of the Jamnagar area, and broadband waveforms of 68 earthquakes (Mw 1.6–5) recorded at five stations of the Junagarh area have been used for the analysis. The estimated relations for the Junagarh area are: Q _c?=?(158?±?5)f^(0.99±0.04) (lapse time : 20?s), Q _c?=?(170?±?4.4)f^(0.97±0.02) (lapse time : 30?s) and Q _c?=?(229?±?6.6)f^(0.94±0.03) (lapse time : 40?s) and for the Jamnagar area are: Q _c?=?(178?±?3)f^(0.95±0.05) (lapse time : 20?s), Q _c?=?(224?±?6)f^(0.98±0.06) (lapse time : 30?s) and Q _c?=?(282?±?7)f^(0.91±0.03) (lapse time : 40?s). These are the first estimates for the areas under consideration. The Junagarh area appears to be more attenuative as compared to the Jamnagar area. The increase in Q _c values with lapse time found here for both the areas show the depth dependence of Q _c as longer lapse time windows will sample larger area. The rate of decay of attenuation (Q ^?1) with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ. A comparison of the coda-Q estimated for the Saurashtra region with those of the nearby Kachchh region shows that the Saurashtra region is less heterogeneous. The obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the Saurashtra region of Gujarat where no such relations were available earlier. These relations are also important for the simulation of earthquake strong ground motions in the region.     

Attenuation models of the earth

Free oscillation and body wave data are used to construct average Q models for the earth. The data set includes fundamental and overtone observations of the radial, spheroidal and toroidal modes, ScS observations and amplitudes of body waves as a function of distance. The preferred model includes a low-Q zone at both the top and the bottom of the mantle. In these regions the seismic velocities are likely to be frequency dependent in the “seismic” band. Absorption in the mantle is predominantly due to losses in shear. Compressional absorption may be important in the inner core.A grain-boundary relaxation model is proposed that explains the dominance of shear over compressional dissipation, the roughly frequency independent average values for Q and the variation of Q with depth. In the high-Q regions, the lithosphere and the midmantle (200–2000 km), Q is predicted to be frequency dependent. However, the low-Q regions of the earth, where Q is roughly frequency independent, dominate the observations of attenuation.     

Intrinsic Q and its frequency dependence

Various techniques for estimating $\text{t1}$ (travel time/quality factor Q) from short-period seismic-array records of body waves have been investigated. Spectral analysis in the frequency domain seems to be more appropriate for this purpose than time domain methods, because of the relative ease with which source and instrument effects can be removed. Of the techniques available, those based on maximum likelihood and homomorphic deconvolution give estimates of relative power versus frequency which best represent the power contained in a time-domain wavelet of short duration. The latter technique seemed to have better noise-eliminating properties than the former. Therefore, homomorphic deconvolution was used to obtain estimates of $\text{t1}$ values from P, PcP, ScP and S phases recorded at the Warramunga array in the Northern Territory of Australia. The source regions for the event studied were the Sunda, Mariana, New Hebrides, Kermadec and Tonga trench zones.The short-period $\text{t1}$ estimates obtained using the above method were much smaller than estimates from published free-oscillation Q models, indicating that the values of Q for compressional and shear waves are frequently-dependent. It was found that short-period $\text{t1}$ values and free-oscillation Q models could be made consistent with one another by assuming Q = Q₀(1+τω) where Q₀ and τ are constants. The results of this investigation suggest another approach to how the Q structure of the mantle can be investigated.     

Investigation on regional attenuation of Vrancea (Romania) intermediate-depth earthquakes

This paper aims at investigating possible regional attenuation patterns in the case of Vrancea(Romania) intermediate-depth earthquakes.Almost 500 pairs of horizontal components recorded during 13 intermediate-depth Vrancea earthquakes are employed in order to evaluate the regional attenuation patterns.The recordings are grouped according to the azimuth with regard to the Vrancea seismic source and subsequently,Q models are computed for each azimuthal zone assuming similar geometrical spreading.Moreover,the local soil amplification which was disregarded in a previous analysis performed for Vrancea intermediate-depth earthquakes is now clearly evaluated.The results show minor differences between the four regions situated in front of the Carpathian Mountains and considerable differences in attenuation of seismic waves between the forearc and backarc regions(with regard to the Carpathian Mountains).Consequently,an average Q model of the type Q(f) = 115×f~(1.25) is obtained for the four forearc regions,while a separate Q model of the type Q(f) = 70×f~(0.90) is computed for the backarc region.These results highlight the need to evaluate the seismic hazard of Romania by using ground motion models which take into account the different attenuation between the forearc/backarc regions.     

On the Causes of Frequency-Dependent Apparent Seismological Q

Variability of the Earth’s structure makes a first-order impact on attenuation measurements which often does not receive adequate attention. Geometrical spreading (GS) can be used as a simple measure of the effects of such structure. The traditional simplified GS compensation is insufficiently accurate for attenuation measurements, and the residual GS appears as biases in both Q ₀ and η parameters in the frequency-dependent attenuation law Q(f) = Q ₀ f ^η . A new interpretation approach bypassing Q(f) and using the attenuation coefficient χ(f) = γ + πf/Q _e(f) resolves this problem by directly measuring the residual GS, denoted γ, and effective attenuation, Q _e. The approach is illustrated by re-interpreting several published datasets, including nuclear-explosion and local-earthquake codas, Pn, and synthetic 50–300-s surface waves. Some of these examples were key to establishing the Q(f) concept. In all examples considered, χ(f) shows a linear dependence on the frequency, γ ≠ 0, and Q _e can be considered frequency-independent. Short-period crustal body waves are characterized by positive γ _SP values of (0.6–2.0) × 10^?2 s^?1 interpreted as related to the downward upper-crustal reflectivity. Long-period surface waves show negative γ _LP ≈ ?1.9 × 10^?5 s^?1, which could be caused by insufficient modeling accuracy at long periods. The above γ values also provide a simple explanation for the absorption band observed within the Earth. The band is interpreted as apparent and formed by levels of Q _e ≈ 1,100 within the crust decreasing to Q _e ≈ 120 within the uppermost mantle, with frequencies of its flanks corresponding to γ _LP and γ _SP. Therefore, the observed absorption band could be purely geometrical in nature, and relaxation or scattering models may not be necessary for explaining the observed apparent Q(f). Linearity of the attenuation coefficient suggests that at all periods, the attenuation of both Rayleigh and Love waves should be principally accumulated at the sub-crustal depths (~38–100 km).     

Earthquake source parameters and scaling relationships in Hungary (central Pannonian basin)

Fifty earthquakes that occurred in Hungary (central part of the Pannonian basin) with local magnitude $M_\textrm{L}$ ranging from 0.8 to 4.5 have been analyzed. The digital seismograms used in this study were recorded by six permanent broadband stations and 20 short-period ones at hypocentral distances between 10 and 327 km. The displacement spectra for P- and SH-waves were analyzed according to Brune’s source model. Observed spectra were corrected for path-dependent attenuation effects using an independent regional estimate of the quality factor Q _S . To correct spectra for near-surface attenuation, the κ parameter was calculated, obtaining it from waveforms recorded at short epicentral distances. The values of the κ parameter vary between 0.01 and 0.06 s with a mean of 0.03 s for P-waves and between 0.01 and 0.09 s with a mean of 0.04 s for SH-waves. After correction for attenuation effects, spectral parameters (corner frequency and low-frequency spectral level) were estimated by a grid search algorithm. The obtained seismic moments range from 4.21×10¹¹ to 3.41×10¹⁵ Nm (1.7?≤?M _w ?≤?4.3). The source radii are between 125 and 1,343 m. Stress drop values vary between 0.14 and 32.4 bars with a logarithmic mean of 2.59 bars (1 bar = 10⁵ Pa). From the results, a linear relationship between local and moment magnitudes has been established. The obtained scaling relations show slight evidence of self-similarity violation. However, due to the high scatter of our data, the existence of self-similarity cannot be excluded.     

Features of Radiation and Propagation of Seismic Waves in the Northern Caucasus: Manifestations in Regional Coda

Based on the Anapa (ANN) seismic station records of ~40 earthquakes (M_W > 3.9) that occurred within ~300 km of the station since 2002 up to the present time, the source parameters and quality factor of the Earth’s crust (Q(f)) and upper mantle are estimated for the S-waves in the 1–8 Hz frequency band. The regional coda analysis techniques which allow separating the effects associated with seismic source (source effects) and with the propagation path of seismic waves (path effects) are employed. The Q-factor estimates are obtained in the form Q(f) = 90 × f 0.7 for the epicentral distances r < 120 km and in the form Q(f) = 90 × f1.0 for r > 120 km. The established Q(f) and source parameters are close to the estimates for Central Japan, which is probably due to the similar tectonic structure of the regions. The shapes of the source parameters are found to be independent of the magnitude of the earthquakes in the magnitude range 3.9–5.6; however, the radiation of the high-frequency components (f > 4–5 Hz) is enhanced with the depth of the source (down to h ~ 60 km). The estimates Q(f) of the quality factor determined from the records by the Sochi, Anapa, and Kislovodsk seismic stations allowed a more accurate determination of the seismic moments and magnitudes of the Caucasian earthquakes. The studies will be continued for obtaining the Q(f) estimates, geometrical spreading functions, and frequency-dependent amplification of seismic waves in the Earth’s crust in the other regions of the Northern Caucasus.     

Body wave attenuation characteristics in the crust of Alborz region and North Central Iran

Attenuation of P and S waves has been investigated in Alborz and north central part of Iran using the data recorded by two permanent and one temporary networks during October 20, 2009, to December 22, 2010. The dataset consists of 14,000 waveforms from 380 local earthquakes (2 < M _L < 5.6). The extended coda normalization method (CNM) was used to estimate quality factor of P (Q _P) and S waves (Q _S) at seven frequency bands (0.375, 0.75, 1.5, 3, 6, 12, 24 Hz). The Q _P and Q _S values have been estimated at lapse times from 40 to 100 s. It has been observed that the estimated values of Q _P and Q _S are time independent; therefore, the mean values of Q _P and Q _S at different lapse times have been considered. The frequency dependence of quality factor was determined by using a power-law relationship. The frequency-dependent relationship for Q _P was estimated in the form of (62 ± 7)f ^{(1.03 ± 0.07)} and (48 ± 5)f ^{(0.95 ± 0.07)} in Alborz region and North Central Iran, respectively. These relations for Q _S for Alborz region and North Central Iran have estimated as (83 ± 8)f ^{(0.99 ± 0.07)} and (68 ± 5)f ^{(0.96 ± 0.05)}, respectively. The observed low Q values could be the results of thermoelastic effects and/or existing fracture. The estimated frequency-dependent relationships are comparable with tectonically active regions.     

Attenuation coefficients of Rayleigh and <Emphasis Type="Italic">Lg</Emphasis> waves

Analysis of the frequency dependence of the attenuation coefficient leads to significant changes in interpretation of seismic attenuation data. Here, several published surface-wave attenuation studies are revisited from a uniform viewpoint of the temporal attenuation coefficient, denoted by χ. Theoretically, χ( f) is expected to be linear in frequency, with a generally non-zero intercept γ?=?χ(0) related to the variations of geometrical spreading, and slope dχ/df = π/Q _e caused by the effective attenuation of the medium. This phenomenological model allows a simple classification of χ( f) dependences as combinations of linear segments within several frequency bands. Such linear patterns are indeed observed for Rayleigh waves at 500–100-s and 100–10-s periods, and also for Lg from ~2 s to ~1.5 Hz. The Lg χ( f) branch overlaps with similar linear branches of body, Pn, and coda waves, which were described earlier and extend to ~100 Hz. For surface waves shorter than ~100 s, γ values recorded in areas of stable and active tectonics are separated by the levels of \(\gamma _{D} \approx 0.2 \times 10^{-3}\) s^???1 (for Rayleigh waves) and 8 ×10^???3 s^???1 (for Lg). The recently recognized discrepancy between the values of Q measured from long-period surface waves and normal-mode oscillations could also be explained by a slight positive bias in the geometrical spreading of surface waves. Similarly to the apparent χ, the corresponding linear variation with frequency is inferred for the intrinsic attenuation coefficient, χ _i , which combines the effects of geometrical spreading and dissipation within the medium. Frequency-dependent rheological or scattering Q is not required for explaining any of the attenuation observations considered in this study. The often-interpreted increase of Q with frequency may be apparent and caused by using the Q-based model of attenuation and following preferred Q( f) dependences while ignoring the true χ( f) trends within the individual frequency bands.     

Coda Q c Attenuation and Source Parameter Analysis in Friuli (NE Italy) and its Vicinity

—?The digital data acquired by 16 short-period seismic stations of the Friuli-Venezia-Giulia seismic network for 56 earthquakes of magnitude 2.3–4.7 which occurred in and near NE Italy have been used to estimate the coda attenuation Q _c and seismic source parameters. The entire area under study has been divided into five smaller regions, following a criterion of homogeneity in the geological characteristics and the constrains imposed by the distribution of available events. Standard IASPEI routines for coda Q _c determination have been used for the analysis of attenuation in the different regions showing a marked anomaly in the values measured across the NE border between Friuli and Austria for Q ₀ value. A large variation exists in the coda attenuation Q _c for different regions, indicating the presence of great heterogeneities in the crust and upper mantle of the region. The mean value of Q _c (f) increases from 154–203 at 1.5?Hz to 1947–2907 at 48?Hz frequency band with large standard deviation estimates.¶Using the same earthquake data, the seismic-moment, M ₀, source radius, r and stress-drop, Δσ for 54 earthquakes have been estimated from P- and S-wave spectra using the Brune's seismic source model. The earthquakes with higher stress-drop (greater than 1?Kbar) occur at depths ranging from 8 to 14?km.     

L g Coda Q Variation across the Myanmar Arc and its Neighboring Regions

Regional seismograms were collected to image the lateral variations of L _g coda Q at 1 Hz (Q ₀ ) and its frequency dependence η across Burma and its neighboring regions. The data include 660 vertical-component traces recorded at 39 stations. The resulting image indicates that L _g coda Q, at a frequency of 1 Hz, varies between 100 and 500. Lowest Q values (< 200) lie in the Three rivers (the Jinshajiang River, Nujiang River, and Lancangjiang River) area of Southwest China. Relatively low Q values (200–250) are found in the Himalayan region and the eastern Burma highland. Higher L _g coda Q values (> 250) are found in the eastern Indian block. From the L _g coda Q tomography, we found that (1) The Sagain fault acts as a rough boundary between the eastern Indian plate and the Three rivers area of the Eurasia plate; (2) near the eastern Himalayan syntaxis, higher Q value appears in the background of relatively low Q (which may be the consequence of the northward intrusion of the Assam block of the Indian plate into the southern Qinghai-Tibet plateau.     

Estimation of Q p and Q s of Kinnaur Himalaya

The attenuation characteristics of the Kinnaur area of the North West Himalayas were studied using local earthquakes that occurred during 2008–2009. Most of the analyzed events are from the vicinity of the Panjal Thrust (PT) and South Tibetan Detachment Thrust, which are well-defined tectonic discontinuities in the Himalayas. The frequency-dependent attenuation of P and S waves was estimated using the extended coda normalization method. Data from 64 local earthquakes recorded at 10 broadband stations were used. The coda normalization of the spectral amplitudes of P and S waves was done at central frequencies of 1.5, 3, 6, 9, and 12 Hz. Q _p increases from about 58 at 1.5 Hz to 706 at 12 Hz, and Q _s increases from 105 at 1.5 Hz to 1,207 at 12 Hz. The results show that the quality factors for both P and S waves (Q _p and Q _s) increase as a function of frequency according to the relation Q?=?Q _o f ⁿ , where Q _o is the corresponding Q value at 1 Hz frequency and “n” is the frequency relation parameter. We obtained Q _p?=?(47?±?2)f ^(1.04±0.04) and Q _s?=?(86?±?4)f ^(0.96±0.03) by fitting power law dependency model for the estimated values of the entire study region. The Q ₀ and n values show that the region is seismically very active and the crust is highly heterogeneous. There was no systematic variation of values of Q _p and Q _s at different frequencies from one tectonic unit to another. As a consequence, average values of these parameters were obtained for each frequency for the entire region, and these were used for interpretation and for comparison with worldwide data. Q _p values lie within the range of values observed for some tectonically active regions of the world, whereas Q _s values were the lowest among the values compared for different parts of the world. Q _s/Q _p values were >1 for the entire range of frequencies studied. All these factors indicate that the crust is highly heterogeneous in the study region. The high Q _s/Q _p values also indicate that the region is partially saturated with fluids.