Scattering Quality Factor 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.

Anomalous Attenuation of High‐Frequency Seismic Waves in Taiwan: Observation, Model and Interpretation

AbstractHigh resolution maps of seismic attenuation parameters in Taiwan have been obtained from a modified “Multiple Lapse Time Window Analysis” (MLTWA). At most of the stations in porous sedimentary and highly faulted areas in Taiwan, the conventional modeling of MLTWA based on the scalar theory of radiative transfer in a half‐space with isotropic scattering fails to explain the spatio‐temporal distribution of the whole S‐wave train. Using Monte Carlo simulations of wave transport, we demonstrate that this anomalous energy distribution in the coda may be modeled by multiple anisotropic scattering of seismic waves. In addition to the scattering quality factor , we introduce a parameter (independent of ) which determines the angular redistribution of energy upon scattering (scattering anisotropy). We inverted the attenuation parameters , and g in three frequency bands (1–2, 2–4, and 4–8 Hz). Overall, Taiwan is more attenuating than most orogens with a mean effective scattering loss and a mean intrinsic absorption of 2.5 × 10−3 and 9 × 10−3 at 1.5 Hz, respectively. Scattering loss varies over more than one order of magnitude across Taiwan while absorption variations reach approximately 30%. The more attenuating zones are the Coastal Range and the Coastal Plain where scattering dominates over absorption at low frequency, and inversely at high frequency. These regions are also characterized by strong backscattering ( < −0.85) at 1.5 Hz and rather high ratio. We propose that the observed strong back‐scattering at low frequency, related to large impedance fluctuations in the crust, is induced by the presence of fluids.

Scattering images from autocorrelation functions of P-wave seismic velocity images: the case of Tenerife Island (Canary Islands, Spain)

We present a P-wave scattering image of the volcanic structures under Tenerife Island using the autocorrelation functions of P-wave vertical velocity fluctuations. We have applied a cluster analysis to total quality factor attenuation ( $$ {Q}_t^{-1} $$ ) and scattering quality factor attenuation ( $$ {Q}_{PSc}^{-1} $$ ) images to interpret the structures in terms of intrinsic and scattering attenuation variations on a 2D plane, corresponding to a depth of 2000 m, and check the robustness of the scattering imaging. The results show that scattering patterns are similar to total attenuation patterns in the south of the island. There are two main areas where patterns differ: at Canadas-Teide-Pico Viejo Complex, high total attenuation and average-to-low scattering values are observed. We interpret the difference as induced by intrinsic attenuation. In the Santiago Ridge Zone (SRZ) region, high scattering values correspond to average total attenuation. In our interpretation, the anomaly is induced by an extended scatterer, geometrically related to the surficial traces of Garachico and El Chinyero historical eruptions and the area of highest seismic activity during the 2004–2008 seismic crises.

Scattering attenuation profile of the Moon: Implications for shallow moonquakes and the structure of the megaregolith

We report measurements of the attenuation of short period seismic waves in the Moon based on the quantitative analysis of envelope records of lunar quakes. Our dataset consists of waveforms corresponding to 62 events, including artificial and natural impacts, shallow moonquakes and deep moonquakes, recorded by the four seismometers deployed during Apollo missions 12, 14, 15 and 16. To quantify attenuation and distinguish between elastic (scattering) and inelastic (absorption) mechanisms we measure the time of arrival of the maximum of energy tmax and the coda quality factor Qc. The former is controlled by both scattering and absorption, while the latter is an excellent proxy for absorption. Consistent with the strong broadening of seismogram envelopes in the Moon, we employ diffusion theory in spherical geometry to model the propagation of seismic energy in depth-dependent scattering and absorbing media. To minimize the misfit between predicted and observed tmax for deep moonquakes and impacts, we employ a genetic algorithm and explore a large number of depth-dependent attenuation models quantified by the scattering quality factor Qsc or equivalently the wave diffusivity D, and the absorption quality factor Qi. The scattering and absorption profiles that best fit the data display very strong scattering attenuation (Qsc≤10 ) or equivalently very low wave diffusivity (D≈2km2/s) in the first 10km of the Moon. These values correspond to the most heterogeneous regions on Earth, namely volcanic areas. Below this surficial layer, the diffusivity rises very slowly up to a depth of approximately 80km where Qsc and D exhibit an abrupt increase of about one order of magnitude. Below 100km depth, Qsc increases rapidly up to approximately 2000 at a depth of about 150km, a value similar to the one found in the Earth’s mantle. By contrast, the absorption quality factor on the Moon Qi≈2400 is about one order or magnitude larger than on Earth. Our results suggest the existence of an approximately 100km thick megaregolith, which is much larger than what was previously thought. The rapid decrease of scattering attenuation below this depth is compatible with crack healing through viscoelastic mechanisms. Using our best attenuation model, we invert for the depth of shallow moonquakes based on the observed variation of tmax with epicentral distance. On average, they are found to originate from a depth of about 50km±20km, which suggests that these earthquakes are caused by the failure of deep faults in the brittle part of the Moon.

Separation of intrinsic and scattering attenuation in the crust of central and eastern Alborz region, Iran

In this study, more than 380 local earthquakes (2<ML<4.5) have been used to estimate the direct-shear waves (Qd), coda (Qc), intrinsic (Qi) and scattering quality factor (QSc) in the crust of central and eastern Alborz region. The events were recorded by one temporary and two permanent networks. The quality factors of shear and coda waves have been individually estimated at different frequency bands by using coda normalization (CNM) method and single backscattering (SBS) method, respectively. Average frequency-dependent relationships have been estimated for Qd and Qc as 111±4f0.85±0.04 and 112±8f1.02±0.06, respectively. The intrinsic quality factor, Qi, has been separated from the scattering quality factor QSc by using individually estimated Qc and Qd values. The average frequency-dependent relationships of Qi and QSc have been calculated in the form of 108f1.00 and 784f0.56, respectively. The results of this study suggest that S-wave’s attenuation (Qd−1) is dominated by the intrinsic attenuation. The attenuation of coda waves has been observed similar to the intrinsic attenuation, which indicates, the coda decay is mostly caused by the intrinsic attenuation. It has been observed that the scattering mean free path is frequency independent at frequencies greater than 6Hz. The results of this study are similar to the tectonically active regions.

Sensitivity of coda waves to spatial variations of absorption and scattering: radiative transfer theory and 2-D examples

We investigate the impact of spatial variations of absorption and scattering properties on the energy envelopes of coda waves. To model the spatiotemporal distribution of seismic energy, we employ a scalar version of the radiative transfer equation with spatially dependent absorption and scattering quality factor. The scattering pattern which describes the angular distribution of energy upon scattering is assumed to be statistically isotropic, independent of position, but otherwise arbitrary. Further assuming that the spatial variations of the governing parameters are sufficiently weak, we employ perturbation theory to derive linearized relations between the absorption/scattering properties of the medium and the intensity detected in the coda. These relations take the form of weighted integrals where so-called scattering/absorption sensitivity kernels play the role of weighting function. The kernels depend on the type of perturbation (scattering or absorption), the lapse-time in the coda, and require the knowledge of the complete angular dependence of the specific intensity describing the flow of energy in a given direction at a given location. In the long lapse-time limit, we establish simplified formulae which depend on the first two angular moments of the specific intensity only. As an illustration of the theory, we calculate the absorption and scattering sensitivity kernels in a 2-D isotropically scattering medium at different lapse-times in the coda, and discuss their singularities in detail. The sensitivity kernels are then employed to calculate the relative intensity variations of the coda caused by a localized Gaussian absorption/scattering anomaly. We find that the dominant effect of absorption anomalies is to modify the decay rate of the coda, while scattering anomalies have a more complex signature, causing either positive or negative deflection of the energy envelope, depending on their location and the lapse-time. Our results suggest the possibility to locate and discriminate between scattering and absorption anomalies from the energy envelope of coda waves.

Spatial distribution of intrinsic and scattering seismic attenuation in active volcanic islands – II: Deception Island images

In this work, we present regional maps of the inverse intrinsic quality factor (Qi−1), the inverse scattering quality factor (Qs−1) and total inverse quality factor (Qt−1) for the volcanic environment of Deception Island (Antarctica). Our attenuation study is based on diffusion approximation, which permits us to obtain the attenuation coefficients for every single couple source-receiver separately. The data set used in this research is derived from an active seismic experiment using more than 5200 offshore shots (air guns) recorded at 32 onshore seismic stations and four ocean bottom seismometers. To arrive at a regional distribution of these values, we used a new mapping technique based on a Gaussian space probability function. This approach led us to create ‘2-D probabilistic maps’ of values of intrinsic and scattering seismic attenuation. The 2-D tomographic images confirm the existence of a high attenuation body below an inner bay of Deception Island. This structure, previously observed in 2-D and 3-D velocity tomography of the region, is associated with a massive magma reservoir. Magnetotelluric studies reach a similar interpretation of this strong anomaly. Additionally, we observed areas with lower attenuation effects that bear correlation with consolidated structures described in other studies and associated with the crystalline basement of the area. Our calculations of the transport mean-free path and absorption length for intrinsic attenuation gave respective values of ≈ 950 m and 5 km, which are lower than the values obtained in tectonic regions or volcanic areas such as Tenerife Island. However, as observed in other volcanic regions, our results indicate that scattering effects dominate strongly over the intrinsic attenuation.

Seismic Wave Attenuation in the Greater Cairo Region, Egypt

In the present study, a digital waveform dataset of 216 local earthquakes recorded by the Egyptian National Seismic Network (ENSN) was used to estimate the attenuation of seismic wave energy in the greater Cairo region. The quality factor and the frequency dependence for Coda waves and S-waves were estimated and clarified. The Coda waves (Qc) and S-waves (Qd) quality factor were estimated by applying the single scattering model and Coda Normalization method, respectively, to bandpass-filtered seismograms of frequency bands centering at 1.5, 3, 6, 12, 18 and 24 Hz. Lapse time dependence was also studied for the area, with the Coda waves analyzed through four lapse time windows (10, 20, 30 and 40 s). The average quality factor as function of frequency is found to be Qc = 35 ± 9f0.9±0.02 and Qd = 10 ± 2f0.9±0.02 for Coda and S-waves, respectively. This behavior is usually correlated with the degree of tectonic complexity and the presence of heterogeneities at several scales. The variation of Qc with frequency and lapse time shows that the lithosphere becomes more homogeneous with depth. In fact, by using the Coda Normalization method we obtained low Qd values as expected for a heterogeneous and active zone. The intrinsic quality factor (Qi−1) was separated from the scattering quality factor (Qs−1) by applying the Multiple Lapse Time Domain Window Analysis (MLTWA) method under the assumption of multiple isotropic scattering with uniform distribution of scatters. The obtained results suggest that the contribution of the intrinsic attenuation (Qi−1) prevails on the scattering attenuation (Qs−1) at frequencies higher than 3 Hz.

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.

Volcano topography, structure and intrinsic attenuation: Their relative influences on a simulated 3D visco-elastic wavefield

The seismic wavefield recorded on volcanoes can be significantly influenced by path effects. Scattering from the topography and internal volcanic structure can play a dominant role in the recorded seismograms. Intrinsic attenuation is also thought to play an important role in the characteristics of volcano seismic signals. We use 3D numerical modelling of wave propagation in elastic and visco-elastic media including complex velocity models and topography to investigate the scattering and attenuation characteristics of synthetic seismograms. We generate 5 distinct volcano models and simulate wave propagation through these models using shallow and deep double-couple broadband sources. We then analyse 129 synthetic seismograms calculated on the free surface. The synthetic seismograms resemble VT-A and VT-B events. The scattering effect of the topography alone is capable of producing complex seismograms. The introduction of an internal velocity structure increases the duration of these seismograms while the introduction of intrinsic attenuation decreases the duration but not the complexity. We fitted our synthetic seismograms to the diffusion in a half-space model to quantify the scattering and attenuation coefficients. The scattering quality factor measured on all 129 stations for all 5 volcano models was similar. Therefore, we deduce that the topography appears to be the dominant scatterer. The intrinsic attenuation quality factor for each of the specific models cannot be accurately recovered using the diffusion in a half-space model. The measured quality factor is not only sensitive to the attenuation properties but also to the structural properties. In all cases the scattering is more dominant than the intrinsic attenuation.

Intrinsic and scattering attenuation in the crust of the Abu Dabbab area in the eastern desert of Egypt

Fifty-five microearthquakes recorded by a digital-temporary seismic network in the Abu Dabbab area in the Eastern Desert of Egypt were used to estimate the direct S-wave ( Q β), coda ( Q c), intrinsic ( Q i), and scattering quality factors ( Q s). Sato's [Sato H., 1977] single-scattering assumption was used to fit the amplitude envelopes of the coda at seven central frequency bands (1.5, 3, 6, 9, 12, 18, and 24 Hz), obtaining a Q c varying with frequency as generally observed in tectonically active areas. Lapse time dependence was also studied for the area, with the coda waves analyzed at window lengths ranging from 10 to 40 s starting from the onset of the S-wave arrival. The direct S-wave Q β was estimated using the coda normalization method [Aki, K., 1980a]. The frequency dependence of Q was investigated for the direct S-waves and coda waves. Results show a low quality factor and a high frequency parameter, indicating that the upper lithosphere of the Abu Dabbab area is seismically active and heterogeneous. Using the independent estimates of Q c and Q β, the intrinsic quality factor Q i was separated from the scattering quality factor. The results suggest that intrinsic dissipation plays a predominant role with respect to scattering phenomena in the area; the obtained Q values seem closer to those reported by analyzing volcano-tectonic earthquakes. This finding reflects that the cause of Abu Dabbab earthquake swarms might be igneous activity where the magma is ascending through joints or serpentinized joints that are dewatering.

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.

Seismic Attenuation and Shallow Velocity Structures at Stromboli Volcano, Italy

The surface-wave field associated with the explosive activity at Stromboli volcano is investigated using data recorded by two short-period seismic arrays, deployed on the north and west flanks of the volcano. The group-velocity dispersion curves for Rayleigh waves are derived using the multiple filter technique. The phase-velocity dispersion curves are recovered using a phase match filter and compared with that inferred from zero-lag cross-correlation analysis applied to the array data. These analyses indicate Rayleigh-wave group velocities ranging from 0.29 to 0.24 km/sec in the 1.5- to 8.0-Hz frequency band, and phase velocities ranging from 1 km/sec at 1.5 Hz to about 0.3 km/sec at frequencies above 5 Hz. In addition, the dispersive properties of the attenuation coefficient (γ) for Rayleigh waves are inferred from application of the multiple filter technique to seismograms recorded at different distances from the source. These results are validated through examination of the spectral amplitude decay with distance for both body and Rayleigh waves. The values of the body-wave quality factor thus obtained are Q α = 20 and Q α = 6 for the north and west side of the island, respectively. The velocity and attenuation dispersion curves are inverted for the shear-wave velocity and Q β structures down to a depth of about 200 m. Shear-wave velocities for the west flank range from about 0.3 km/sec for the uppermost 17-m-thick layer to 1.9 km/sec at depths greater than 200 m. Comparison with previous studies indicates a similar velocity structure for the north and west flanks. The attenuation structure for the west flank is described by a shallower, 36-m-thick layer with Q β = 9, underlain by a half-space with Q β = 50. On the north flank, Q β = 40 for the shallower 30-m-thick layer and Q β = 44 for the underlying half-space. Residuals from analysis of the spectral decay with distance are used to quantify site effects affecting the different array elements on the west flank. Local amplifications at that array are interpreted in terms of an edge effect associated with concave topography. Velocity similarities observed at the north and west flanks are compatible with surface geologic data. Discrepancies in attenuation properties at the two sites are interpreted in terms of different degrees of heterogeneity and crack density controlling the scattering quality factor Q s. Manuscript received 15 April 2001.

Attenuation of short-period seismic waves at Mt Vesuvius, Italy

We analysed short-period seismograms from about 250 very local events recorded by a network of high dynamic range short-period seismic stations deployed on Mt Vesuvius to estimate the site-corrected short-period seismic attenuation. We calculated QC− 1, the inverse of the quality factor for coda waves, for short lapse times (12 s), and QP− 1 and QS− 1, the inverse of the direct body wave quality factors for P and S waves follow- ing shallow ray paths. We used the single scattering assumption to fit the amplitude envelopes of the coda at different frequency bands, obtaining a QC− 1 slowly varying with frequency, similar to values measured for other volcanoes at the same lapse time. Site-corrected QP− 1 and QS− 1 were estimated using the frequency decay method for both P and S waves in two frequency bands, 1–6 Hz and 15–24 Hz. Results show that QP− 1 is 0.028 in both frequency bands, and QS− 1 is 0.015 and 0.017 at low and high frequencies respectively, almost independent of the frequency, and that the value of QP− 1 is about twice that of QS− 1, as measured worldwide. A check was made by measuring the broadening of the first P-wave pulse with station–source distance, finding QP− 1=0.024. Using the independent estimates of QC− 1 and QS− 1, we separated the intrinsic from the scattering quality factor at Mt Vesuvius, obtaining a high scattering and a low intrinsi c Q− 1, as already observed for other volcanoes. This result confirms the hypothesis of the predominant role of the scattering phenomena with respect to the intrinsic dissipation in the seismogram formation for volcanic earthquakes.