Lapse Time Window Research Articles

Attenuation of high-frequency seismic waves in northeast India

SUMMARY We studied attenuation of S and coda waves, their frequency and lapse time dependencies in northeast India in the frequency range of 1–24 Hz. We adopted theories of both single and multiple scattering to bandpass-filtered seismograms to fit coda envelopes to estimate Q for coda waves (QC) and Q for S-waves (QS) at five central frequencies of 1.5, 3, 6, 12 and 24 Hz. The selected data set consists of 182 seismograms recorded at ten seismic stations within epicentral distance of 22–300 km in the local magnitude range of 2.5–5.2. We found that with the increase in lapse time window from 40 to 60 s, Q0 (QC at 1 Hz) increases from 213 to 278, while the frequency dependent coefficient n decreases from 0.89 to 0.79. Both QC and QS increase with frequency. The average value of QS obtained by using coda normalization method for NE India has the power law form of (96.8 ± 21.5)f(1.03±0.04) in 1–24 Hz. We adopted energy flux model (EFM) and diffusion model for the multiple scattered wave energy in three-dimensions. The results show that the contribution of multiple scattering dominates for longer lapse time close to or larger than mean free time of about 60 s. The estimates of QC are overestimated at longer lapse time by neglecting the effects of multiple scattering. Some discrepancies have been observed between the theoretical predictions and the observations, the difference could be due to the approximation of the uniform medium especially at large hypocentral distances. Increase in QC with lapse time can be explained as the result of the depth dependent attenuation properties and multiple scattering effect.

Depth dependent seismic scattering attenuation in the Nuevo Cuyo region (southern central Andes)

In the present work we separated intrinsic from scattering attenuation coefficients both for the crust and the upper mantle in the tectonically highly active areas of the Southern‐Central Andes ‐ Nuevo Cuyo region, analyzing two groups of earthquakes, well separated in depth. This region is characterized by the presence of flat subduction. We apply MLTWA (Multiple Lapse Time Window Analysis), coda normalization and Q‐coda techniques to measure the scattering and intrinsic attenuation coefficient and the total Q for S waves. We find that intrinsic attenuation does not decrease with depth whereas scattering attenuation is higher in the crust than in the upper mantle, and that intrinsic attenuation predominates over scattering attenuation. We interpret this observation in terms of the release of water and other fluids into the overlying lithosphere due to the dynamics of the subduction process, in agreement with most of the prevalent geodynamic models.

Spatial distribution of scattering loss and intrinsic absorption of short-periodSwaves in the lithosphere of Japan on the basis of the Multiple Lapse Time Window Analysis of Hi-net data

SUMMARY Propagation of short period S waves through the crust is strongly controlled by scattering loss and intrinsic absorption. Scattering loss controls the shape of the seismic envelope and the spatial distribution of the energy while intrinsic absorption produces an exponential decay as a function of time which is independent of the coordinates. Since both parameters have a different effect, it is possible to compute them by means of the Multiple Lapse Time Window Analysis (MLTWA). In this paper, high resolution maps of Japan for both seismic S-wave attenuation parameters are obtained by using the MLTWA with data provided by the Hi-net seismic network. The maps show strong regional variation of the parameters. The variations depend mainly on the tectonic setting of each region and volcanism mechanisms. In the scattering loss maps, in the 1–2 Hz map, there is a very different behaviour between the northeast (stronger scattering loss) and the southwest (much weaker scattering loss). In the northeast of Japan, areas with strong scattering loss correspond mainly to the location of the volcanic arcs. In Southwest Japan, Chugoku region and the west side of Kii peninsula show strong scattering levels for the 4–8 Hz and higher frequency bands. In Hokkaido, strong scattering loss and intrinsic absorption can be observed for the lower frequency bands. In the Sendai plain, we find strong scattering loss for all the frequency bands. Under all these areas low velocity anomalies have been detected in previous works. In Tohoku area the east side shows lower intrinsic attenuation than the west side. This characteristic seems to extend to the southern part of Hokkaido and to central Japan. In Kanto area, the values of intrinsic absorption are lower than in the surrounding areas; but in the region around the Itoigawa-Shizuoka tectonic line and in its western side strong absorption areas are observed. The behaviour of each area for intrinsic absorption is similar to the behaviour observed in the Q−1c maps. The MLTWA is based on the hypothesis of multiple isotropic scattering in a medium with the homogeneous distribution of scattering mean free path and intrinsic absorption. Although these hypotheses are very simple, the results are very informative on the characteristics of each region, and show the usefulness of studying the properties of scattered seismic waves in the understanding of the properties of the crust.

Frequency-dependent attenuation of S-waves in the Kanto region, Japan

AbstractApparent, scattering, and intrinsicS-wave attenuations (QS−1,Qscat−1andQint−1) of the upper lithosphere in the Kanto region of Japan were measured in the 1- to 32-Hz frequency range using Multiple Lapse Time Window Analysis (MLTWA) for 115 borehole seismograms of local earthquakes. A new set of time windows for MLTWA, in which multiple isotropic scattering is assumed, was proposed and employed to estimate the frequency dependence ofS-wave attenuation parameters. Scattering attenuation was found to dominate intrinsic attenuation in theS-wave attenuation mechanism at low frequencies (<;2 Hz), whereas the opposite relation was observed at high frequencies. The transition is caused by the different frequency dependences ofQscat−1(∝f−1.5) andQint−1(∝f−0.7) at this frequency. Interestingly,Qscat−1is almost frequency independent at frequencies >8 Hz, which implies the self-similar nature of short-wavelength heterogeneities in the upper lithosphere. In terms of the upper lithosphere of the Kanto region, these results may indicate that the random heterogeneities characterized by the Gaussian autocorrelation function with a fractional fluctuationε≈ 10% and a correlation lengtha≈ 2 km are superimposed on the weak background self-similar heterogeneity.

Intrinsic and scattering attenuation of the Mt Fuji Region, Japan

Mount Fuji is the focus of intense study because of its potential hazard signaled by seismic, geologic and historical activity. Based on extensive seismic data recorded in the vicinity of Mt Fuji, coda quality factor (Q−1C) using a single scattering model hypothesis, and intrinsic and scattering quality factor (Q−1i and Q−1S) using the Multiple Lapse Time Window (MLTW) method were measured. To focus the study on the magmatic structure below Mt Fuji, the data were separated into two groups: a near-Fuji region of rays traversing an area with radius 5 km around the summit (R < 5 km), and a far-Fuji region of rays beyond a radius of 20 km around the summit (R > 20 km). This classification shows the largest discrepancy of Q−1C at a range of sampling volumes corresponding to overlapped sampling depth of about 80 km. Further, the spatial division shows significant difference of Q−1i and Q−1s at hypocentral distance of 80 km. The large difference of Q−1s in bandwidths 2–4, and 4–8 Hz indicates lithospheric heterogeneity beneath Mt Fuji with a characteristic heterogeneity scale length of about 1 km. The results have a small error range due to the large data sample, showing that all Q−1 values in the near-Fuji area are greater than those of the far-Fuji area, and Q−1i for both the near and far-Fuji areas is higher than Q−1s at high frequencies. The Q−1i and Q−1s values for far-Fuji are in the range of values for typical non-volcanic areas. The Q−1i values of the near-Fuji area are lower than those of other volcanic areas considered, where as values of Q−1s are not. The low Q−1i for the volcanic region of near-Fuji suggests that the magmatic activity, indicated by percent partial melt, in the vicinity of Mt Fuji is not as active as hot spot volcanoes, such as Kilauea, Hawaii.

Estimation of Coda Wave Attenuation for the National Capital Region, Delhi, India Using Local Earthquakes

Attenuation of seismic waves is very essential for the study of earthquake source parameters and also for ground-motion simulations, and this is important for the seismic hazard estimation of a region. The digital data acquired by 16 short-period seismic stations of the Delhi Telemetric Network for 55 earthquakes of magnitude 1.5 to 4.2, which occurred within an epicentral distance of 100 km in an area around Delhi, have been used to estimate the coda attenuation Qc. Using the Single Backscattering Model, the seismograms have been analyzed at 10 central frequencies. The frequency dependence average attenuation relationship Qc = 142f1.04 has been attained. Four Lapse-Time windows from 20 to 50 seconds duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of Qc. The Qc values show that frequency dependence (exponent n) remains similar at all the lapse time window lengths. While the change in Q0 values is significant, change in Q0 with larger lapsetime reflects the rate of homogeneity at the depth. The variation of Qc indicates a definitive trend from west to east in accordance with the geology of the region.

Coda Q Estimates in the Andaman Islands Using Local Earthquakes

The attenuation property of Andaman Island has been investigated analyzing coda waves from 57 local earthquakes in the magnitude range of 2.0–4.9, using the single backscattering model. These earthquakes waveforms, recorded on five broadband seismographs sited over the island from north to south during Nov. 2003 to March 2004, have been used to calculate the frequency dependent Coda Q (Q c ) applying the time domain coda-decay method. The Coda Q, computed at central frequencies from (0.5–12) Hz and five-lapse time windows from 40 to 80 s, progressively increases from 105 f 0.88 in the north Andaman to 135 f 0.79 in the south Andaman with an average of 119 f 0.80. The average Q c values vary from 75 ± 42 at 0.5 Hz to 697 ± 54 at 12 Hz central frequency for 40 s lapse time window, while for 80 s lapse time window its variation is from 117 ± 38 at 0.5 Hz to 1256 ± 115 at 12 Hz. The Q c estimated at different lapse times manifests a significant variation from 122f 0.75 to 174f 0.73, corresponding to lapse time window lengths of 40 and 80 s, respectively. The variation of Q c with frequency, lapse time and also with the location of seismograph reflects the marked structural and compositional inhomogeneity with depth along the Andaman Islands. These observations are well correlated with the seismicity pattern and distinct high angle subduction beneath the island.

Attenuation of coda waves in the Northeastern Region of India

Coda wave attenuation quality factor Qc is estimated in the northeastern region of India using 45 local earthquakes recorded by regional seismic network. The quality factor Qc was estimated using the single backscattering model modified by Sato (J Phys Earth 25:27–41, 1977), in the frequency range 1–18 Hz. The attenuation and frequency dependence for different paths and the correlation of the results with geotectonics of the region are described in this paper. A total of 3,890 Qc measurements covering 187 varying paths are made for different lapse time window of 20, 30, 40, 50, 60, 70, 80, and 90 s in coda wave. The magnitudes of the analyzed events range from 1.2 to 3.9 and focal depths range between 7 and 38 km. The source–receiver distances of the selected events range between 16 and 270 km. For 30-s duration, the mean values of the estimated Qc vary from 50 ± 12 (at 1 Hz) to 2,078 ± 211(at 18 Hz) for the Arunachal Himalaya, 49 ± 14 (at 1 Hz) to 2,466 ± 197 (at 18 Hz) for the Indo-Burman, and 45 ± 13 (at 1 Hz) to 2,069 ± 198 (at 18 Hz) for Shillong group of earthquakes. It is observed that Qc increases with frequency portraying an average attenuation relation \(Qc=52.315\pm 1.07f\,^{\left( {1.32\,\pm \,0.036} \right)}\) for the region. Moreover, the pattern of Qc − 1 with frequency is analogous to the estimates obtained in other tectonic areas in the world, except with the observation that the Qc − 1 is much higher at 1 Hz for the northeastern region. The Qc − 1 is about 10 − 1.8 at 1 Hz and decreases to about 10 − 3.6 at 18 Hz indicating clear frequency dependence. Pertaining to the spatial distribution of Qc values, Mikir Hills and western part of Shillong Plateau are characterized by lower attenuation.

Estimation of coda wave attenuation in East Central Iran

The attenuation of coda waves, Qc, has been estimated in Zarand, Jiroft, and Bam regions of east central Iran using a single back-scattering model of S-coda envelopes. For this purpose, the recordings of 97 earthquakes by three seismic networks and a local strong ground motion network have been used. In this research, the frequency-dependent Qc values are estimated at central frequencies of 1.5, 3, 6, 8, 12, 16, and 24 Hz using different lapse time windows from 20 to 60 s. The frequency-dependent relationships obtained are \(Q_c =\left( {102\pm 8} \right)f^{\left( {0.89\pm 0.05} \right)}\) for Zarand, \(Q_c =\left( {113\pm 6} \right)f^{\left( {0.91\pm 0.06} \right)}\) for Jiroft, and \(Q_c =\left( {79\pm 5} \right)f^{\left( {1.01\pm 0.04} \right)}\) for Bam region. From the strong ground motion data, we obtain the relation \(Q_c =\left( {{104}\pm {8}} \right)f^{\left( {{0.90}\pm {0.1}} \right)}\). The Qc frequency-dependent relationship for the entire region of east central Iran from all data (both seismograms and accelerograms) is \(Q_c =\left( {{101}\pm {6}} \right)f^{\left( {{0.94}\pm {0.11}} \right)}\). The average Qc values estimated and their frequency dependent relationships correlate well with a highly heterogeneous and highly tectonically active region. Results also show that the attenuation is higher in Bam region compared to Zarand and Jiroft regions.

Lapse-Time Dependence of Coda Q in the Source Region of the 1999 Chamoli Earthquake

Abstract In the present study the attenuation of seismic-wave energy in and around the source area of the Chamoli Earthquake of 29 March 1999 is estimated using aftershock data. Most of the analyzed events are from the vicinity of the main central thrust (MCT), which is a well-defined tectonic discontinuity in the Himalayas. The method of a single backscattering model is employed to calculate frequency dependent values of coda Q ( Q c ). A total of 30 aftershock events are used for Q c estimation at central frequencies 1.5, 3, 6, 9, 12, 18, and 24 Hz through five lapse-time windows from 10 to 50 sec starting at double the travel time of the S wave. The observed Q c is strongly dependent on frequency, which indicates that the region is seismically and tectonically active with high heterogeneities. The variation of Q c has also been estimated at different lapse times to observe its effect with depth. The variation of Q c with frequency and lapse time shows that the lithosphere becomes more homogeneous with depth. Q c -values for higher frequencies increase very fast with depth within about the top 63 km of the lithosphere and then become more or less constant beyond this depth. This indicates that turbidity at higher frequency decays very fast with depth, and the mantle may be transparent to high-frequency waves. The variation of Q c at 1.5 Hz with lapse time matches quite well with those predicted by Gusev (1995). However, the frequency parameter n in the relation Q c = Q 0 f n , where Q 0 = Q c at 1 Hz, does not follow the expected pattern given in his model. This could be due to faster depth decay of turbidity as mentioned previously.

Lapse time and frequency-dependent attenuation of coda waves in the Zagros continental collision zone in Southwestern Iran

The coda Q, Qc, were estimated for the Zagros continental collision zone in southwestern Iran by analyzing the coda waves of 51 local earthquakes recorded on the three stations of the Iranian National Seismic Network (INSN) with magnitudes of between 3.1 and 4.9 recorded in the region during March and April 2006. Most of the analyzed events are foreshocks and aftershocks of the Darb-e-Astane earthquake which occurred on 31 March 2006 with a magnitude of 6.1 (IIEES). The earthquakes had an epicentral distance of between 120 and 200 km and a focal depth of about 18 km. The Qc values were computed at nine central frequencies of 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 12.0, 16.0 and 20.0 Hz through eight lapse time windows from 25 to 60 s starting at double the time of the primary S-wave from the origin time using the time-domain coda-decay method of a single backscattering model. In this study the collected data were compared between the events that occurred before and after the main event (foreshocks and aftershocks). The analysis showed a significant variation in the value of coda, Q, for the study region in different lapse times and frequencies. The variation of the quality factor, Qc, before and after the main event was estimated at different lapse time windows to observe its effect with depth. The estimated average frequency-dependent relation of Qc for foreshocks varies from Qc = (144 ± 24)f(0.42 ± 0.23) at 25 s to Qc = (85 ± 10)f(0.92 ± 0.11) at 60 s lapse window time length, respectively. For aftershocks, it varies from Qc = (121 ± 55)f(0.97 ± 0.26) at 25 s to Qc = (212 ± 59)f(0.82 ± 0.15) at 60 s. The averages of Qc in all stations and lapse times window are obtained as Qc = 99f0.84 and Qc = 178f0.86 for foreshocks and aftershocks, respectively. The Q frequency relationship for foreshocks is similar to that for the South Carolina, Koyna, western Anatolia and Aleutian earthquakes, whereas for aftershocks it is similar to the Kumaun, NW Himalaya and Parkfield events.

Estimation of shear-wave attenuation characteristics for the Klyuchevskoi volcanic edifice

This paper reports a study of intrinsic attenuation and scattering in the Klyuchevskoi volcanic edifice. The data set consisted of small volcano-tectonic earthquakes occurring as deep as 30 km beneath Klyuchevskoi Volcano and recorded by radio telemetry seismograph stations installed on the edifice and near it. The digital seismograms of small volcano-tectonic earthquakes were processed by the multiple lapse time window analysis (MLTWA) method currently in use in seismology. The method uses experimental normalized integrals of 3D seismic energy density determined from several time windows applied to earthquake records that have been put through bandpass filters. The parameters that characterize the intrinsic attenuation and scattering of seismic energy of small volcano-tectonic earthquakes in Klyuchevskoi Volcano were estimated by adjusting the coefficients to ensure the best fit between experimental and theoretical integrals. An analytical solution to the seismic energy transfer equation was used to calculate the theoretical integrals of 3D seismic energy. Reliable estimates of the parameters that characterize the intrinsic attenuation and scattering in the material of the Klyuchevskoi volcanic edifice have been obtained. Our estimates of the Q for the edifice are below those derived by other workers for the Kamchatka lithosphere.

Variation of intrinsic and scattering attenuation with depth in NW Himalayas

SUMMARY We analysed local earthquake waveforms recorded on a broad-band seismic network in NW Himalayas to compute the variation of intrinsic and scattering attenuation parameters with depth from coda waves. First we estimated lapse time and frequency dependence of coda Q (Qc). Intrinsic (Q−1i) and scattering (Q−1s) attenuations were separated using Qc and direct S-wave Q (Qd). It is observed that for smaller lapse time window estimated Q−1c is close to Q−1i and both of them are much larger than Q−1s suggesting that coda decay is predominantly caused by intrinsic attenuation at shallower depth in the NW Himalayas. As lapse time increases, the Q−1c, Q−1i values decrease and come closer to Q−1i values. As Q values for larger lapse time windows are affected by deeper zones, this indicates that with increasing depth relative effect of intrinsic attenuation with respect to that of scattering attenuation on coda attenuation decreases in the area under study. Both coda and intrinsic attenuation decreases with increasing depth. However, between 111 ± 6.3 and 129.5 ± 7.5 km intrinsic attenuation remains stable indicating possible presence of partial melts and/or fluid filled fractures. On the other hand scattering attenuation increases with increasing depth. This may be due to the fact that continental crustal material of the Indian plate underthrusts the Himalayas. The scattering attenuation characteristics of the crust are very different from those of the upper mantle in the NW Himalayas. It is also observed that at higher frequency energy loss is primarily due to intrinsic dissipation, whereas at lower frequency scattering also plays an important role in attenuating seismic wave energy in the study area.

Intrinsic and Scattering Seismic Attenuation in Southwestern Anatolia

The intrinsic dissipation and scattering attenuation in southwestern (SW) Anatolia, which is a tectonically active region, is studied using the coda waves. First the coda quality factor (Q c ) assuming single scattering is estimated from the slope of the coda-wave amplitude decay. Then the Multiple Lapse Time Window (MLTW) analysis is performed with a uniform earth model. Three non-overlapping temporal data windows are used to calculate the scattered seismic energy densities against the source-receiver distances, which, in turn, are used to calculate separate estimates of the intrinsic and scattering factors. In order to explore the frequency dependency, the observed seismograms are band pass-filtered at the center frequencies of 0.75, 1.5, 3.0, 6.0 and 12.0. The scattering attenuation (Q s −1) is found lower than the intrinsic attenuation (Q i −1) at all frequencies except at 0.75 Hz where the opposite is observed. Overall the intrinsic attenuation dominates over the scattering attenuation in the SW Anatolia region. The integrated energy curves obtained for the first energy window (i.e., 0–15 s) are somewhat irregular with distance while the second (i.e., 15–30 s) and third (i.e., 30–45 s) data windows exhibit more regular change with distance at most frequencies. The seismic albedo B0 is determined as 0.61 at 0.75 Hz and 0.34 at 12.0 Hz while the total attenuation factor denoted by L e −1 changes in the range 0.034–0.017. For the source-station range 20–180 km considered the scattering attenuation is found strongly frequency dependent given by the power law Q s −1 = 0.010*f−1.508. The same relations for Q i −1, Q t −1 (total), Q c −1 and $${\tilde{Q}}^{{- 1}}_{c}$$ (expected) hold as Q i −1 = 0.0090*f−1.17, Q t −1 = 0.019*f−1.31, Q c −1 = 0.008*f−0.84 and $${\tilde{Q}}_{c}^{{- 1}} = 0.014*f^{{- 0.87}},$$ respectively. Compared to the other attenuation factors Q c −1 and $${\tilde{Q}}^{{- 1}}_{c}$$ are less dependent on the frequency.

Intrinsic attenuation and scattering of shear waves in the lithosphere of Kamchatka

Records of small local Kamchatka earthquakes were processed using Multiple Lapse Time Window Analysis (MLTWA). The method makes use of normalized integrals of 3D seismic energy density in several windows applied to an earthquake record that has been put through a bandpass filter. The intrinsic attenuation and scattering properties of the earth were estimated by choosing parameters that provide the best agreement between experimental and theoretical integrals as functions of hypocentral distance. The theoretical normalized integrals as functions of distance were computed using an analytical solution to the equation of seismic energy transport based on a simple model of isotropic scattering for scalar seismic waves excited by an impulse point source in an earth that is uniform as to scattering and intrinsic attenuation. The resulting estimates of attenuation and scattering parameters are similar to those derived in other studies of Kamchatka, as well as of other continents: America (California, US), Europe (the Canary Is., Spain, and Italy), and Asia (Japan).

Intrinsic and scattering attenuation in western India from aftershocks of the 26 January, 2001 Kachchh earthquake

176 vertical-component, short period observations from aftershocks of the M w 7.7, 26 January, 2001 Kachchh earthquake are used to estimate seismic wave attenuation in western India using uniform and two layer models. The magnitudes ( M w ) of the earthquakes are less than 4.5, with depths less than 46 km and hypocentral distances up to 110 km. The studied frequencies are between 1 and 30 Hz. Two seismic wave attenuation factors, intrinsic absorption ( Q i − 1 ) and scattering attenuation ( Q s − 1 ) are estimated using the Multiple Lapse Time Window method which compares time integrated seismic wave energies with synthetic coda wave envelopes for a multiple isotropic scattering model. We first assume spatial uniformity of Q i − 1 , Q s − 1 and S wave velocity ( β). A second approach extends the multiple scattering hypothesis to media consisting of several layers characterized by vertically varying scattering coefficient ( g), intrinsic absorption strength ( h), density of the media ( ρ) and shear wave velocity structure. The predicted coda envelopes are computed using Monte Carlo simulation. Results show that, under the assumption of spatial uniformity, scattering attenuation is greater than intrinsic absorption only for the lowest frequency band (1 to 2 Hz), whereas intrinsic absorption is predominant in the attenuation process at higher frequencies (2 to 30 Hz). The values of Q obtained range from Q t = 118, Q i = 246 and Q s = 227 at 1.5 Hz to Q t ≈ 4000, Q i ≈ 4600 and Q s ≈ 33,300 at 28 Hz center frequencies, being Q t − 1 a measure of total attenuation. Results also show that Q i − 1 , Q s − 1 and Q t − 1 decrease proportional to f − ν . Two rates of decay are clearly observed for the low (1 to 6 Hz) and high (6 to 30 Hz) frequency ranges. Values of ν are estimated as 2.07 ± 0.05 and 0.44 ± 0.09 for total attenuation, 1.52 ± 0.21 and 0.48 ± 0.09 for intrinsic absorption and 3.63 ± 0.07 and 0.06 ± 0.08 for scattering attenuation for the low and high frequency ranges, respectively. Despite the lower resolution in deriving the attenuation parameters for a two layered crust, we find that scattering attenuation is comparable to or smaller than the intrinsic absorption in the crust whereas intrinsic absorption dominates in the mantle. Also, for a crustal layer of thickness 42 km, intrinsic absorption and scattering estimates in the crust are lower and greater than those of the mantle, respectively.

Coda Q in the Kachchh Basin, Western India Using Aftershocks of the Bhuj Earthquake of January 26, 2001

Q C -estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Q c is a function of frequency and increases as frequency increases. The frequency dependent Q c relations obtained for four lapse-time windows are: Q c =82 f1.17 (20–50 sec), Q c =106 f1.11 (30–60 sec), Q c =126f1.03 (40–70 sec) and Q c =122f1.02 (50–80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q0 increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Q c −1 of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low.

Attenuation study in the Straits of Messina area (southern Italy)

The attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquake engineering, which is closely related to the seismicity and regional tectonic activity of a particular area. In this work, the seismic attenuation in the Straits of Messina, affected by several and strong historical earthquakes, was studied using waveforms recorded by a local seismic network composed of seven stations. We measured: the coda quality factor ( Q c) in the Single Scattering model hypothesis; the direct quality factor ( Q d) applying the Coda Normalization method for S-waves; and the intrinsic and scattering quality factor ( Q i and Q s) by the Multiple Lapse Time Window Analysis (MLTWA) method. Coda Q values were obtained using different lapse times (40, 60 and 80 s) for the frequency bands centred at 1.5, 3, 6 and 12 Hz. Our findings indicate that Q c increases with increasing lapse time and that Q c is frequency dependent. This behaviour is usually correlated to the degree of tectonic complexity and to the presence of heterogeneities at several scales. In fact, by using the Coda Normalization method we obtained low Q d values, as expected for a heterogeneous and active zone. Finally, by the MLTWA method we observe that the contribution of the scattering attenuation ( Q s − 1 ) prevails on the intrinsic absorption ( Q i − 1 ) until 3 Hz. Conversely, Q i − 1 and Q s − 1 seem to be of the same order in the higher frequency bands.

S-waves attenuation and separation of scattering and intrinsic absorption of seismic energy in southeastern Sicily (Italy)

Seismic wave attenuation in southeastern Sicily was investigated by using a data set of about 180 local earthquakes (1.5 ≤ML≤ 4.6) recorded in the period 1994–2003. We first estimated the quality factor of S waves (QS) and clarified its frequency dependence by means of the coda-normalization method, applied in the frequency range 1.5–15 Hz. The average QS as function of frequency is given by QS= 49f0.88. A detailed separation of scattering attenuation (Q−1s) from intrinsic absorption (Q−1i) was also attempted by applying the multiple lapse time window analysis (MLTWA), under the hypothesis of multiple isotropic scattering with uniform distribution of scatterers. Intrinsic absorption dominates over scattering in the attenuation process at high frequencies (above 3 Hz). Below 3 Hz scattering is the predominant attenuation effect in the region, at the scale length of these frequencies. However, some discrepancies have been observed between the theoretical model and the observations. This indicates that models with depth-dependent velocity structure and/or non isotropic scattering should be taken into account in order to remove ambiguities in the interpretation of the results.

Characteristics of coda wave attenuation in Yunnan area

The characteristic of seismic coda wave attenuation in Yunnan area in 7 frequency-bands range from 1 Hz to 20 Hz was estimated by using the local earthquake’s waveform data recorded from 22 Yunnan digital seismic stations. Coda attenuation Q c −1 of each station was firstly calculated by single scattering method. Then, mean free path L e and seismic albedo B 0 of each station were calculated, and scattering attenuation Q s −1 and intrinsic attenuation Q i −1 were separated from total attenuation Q t −1 by multiple lapse time window analysis based on the multiple scattering model in uniform random isotropic scattering medium. The attenuating characteristics in Yunnan show that most value of L e are in 10–30 km, with maximal within 2–6 Hz; B 0 are about 0.5 at 1–2 Hz, but less than 0.5 at other frequency-bands, which means Q i −1 is comparable with Q s −1 at 1–2 Hz, and after 1–2 Hz, Q i −1 is greater than Q s −1 and dominates the attenuation process. Q c −1 is close to Q i −1 at other frequency bands except 1–2 Hz. Results show that Q −1 especially Q s −1 varies spatially, Q −1 in eastern Yunnan zone is a bit higher than in north-western Yunnan zone; northwestern Yunnan zone higher than southwestern Yunnan zone. Comparing with other results in global, Q s −1 in Yunnan is lower than the global average value among these results, Q i −1 is higher than the global average value, and Q t −1 lies the middle among these results.