Multiple Lapse Time Window Analysis Research Articles

Separation of Intrinsic and Scattering Seismic Wave Attenuation in the Crust of Central and South-Central Alaska

ABSTRACT Seismic hazard analysis is essential for evaluating the potential consequences and dangers linked to earthquakes, particularly in areas with regular seismic activity such as central and south-central Alaska. A detailed study of attenuation can help in better defining the wave behavior and so refine the ground-motion prediction. Here, we examined the scattering (Qs−1), intrinsic (Qi−1), and coda-wave (Qc−1) attenuation in central and south-central Alaska. To do so, we performed the multiple lapse time-window analysis (MLTWA) techniques and estimated the coda energy decay. We considered earthquakes that occurred between December 2014 and December 2020, with magnitudes between 2.0 and 6.5. We observed significant spatial variations in scattering loss (Qs−1) up to 3 Hz, which diminish at 6 and 12 Hz. The Wrangell block exhibits the most significant scattering loss at a frequency of 1.5 Hz. Another area of marked scattering loss was identified north of the Alaska Range (AR), where it was pronounced up to 6 Hz. The area around Anchorage registered the lowest intrinsic absorption across all the central frequencies, whereas the highest values were detected north of the AR, particularly at 3 and 6 Hz. The seismic albedo (B0) in central and south-central Alaska varies spatially and is mainly dominated by scattering loss up to 3 Hz. Both the Chugach mountains and Yakutat block (YB) area exhibit lower B0 values at all central frequencies showing the dominance of intrinsic absorption. Low values of Qc (high attenuation) are focused almost on all the frequencies along the Denali fault and YB, showing a strong influence of these structures on the attenuation. The results yield a comprehensive understanding of the unique attenuation characteristics of each region, underscoring the significance of investigating the behavior of seismic wave attenuation for seismic risk purposes.

Depth and Spatial Variation of the Shear Wave Attenuation Parameters in the Shallow Crust and Lower Crust/Upper Mantle of Mt. Etna (Italy)

AbstractSeismic wave attenuation is a key feature of seismic wave propagation that provides constraints on the composition and physical state of the medium within the Earth. We separated intrinsic and scattering attenuation coefficients for the shallow crust and lower crust/upper mantle in the Mt. Etna area. For this purpose, the Multiple Lapse Time Window Analysis (MLTWA) was applied to two groups of earthquakes, well separated in depth. We also studied the spatial variation of the attenuation parameters by dividing the study area into four sectors around Etna. The results show an effective homogeneity of the propagation characteristics inside Etna and, in particular, some lateral variations and minor variations with depth. We observe that structural discontinuities and lithology control scattering losses at all frequencies, with higher scattering in the shallow crust. The intrinsic absorption shows no sensitivity to the presence of these main geological structures and is quite uniform for different depths. Furthermore, compared to the northern sector of the volcano, the southern one shows stronger scattering attenuation at low frequencies. This pattern correlates well with the high seismic activity along most of Etna’s active tectonic structures and ascending magmatic fluids that characterize this sector of the volcano. Although we only discuss the differences in the "average" scattering and inelastic properties of the investigated volumes, the results of this study are very informative about the characteristics of each region. Moreover, they suggest that a future study is necessary, providing a more detailed picture of the spatial distribution of seismic attenuation in the study area, through a 3D inversion of the attenuation parameters estimated along the single source-receiver paths.

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.

Comparison of Multiple Lapse Time Window Analysis and Qopen to determine intrinsic and scattering attenuation

SUMMARYThis study compares the results of Multiple Lapse Time Windows Analysis (MLTWA) and full envelope inversion (Qopen) to determine intrinsic and scattering attenuation of the crust using the region around the central part of the Leipzig–Regensburg fault zone in Germany as an example. We use 18 of the region’s strongest earthquakes from 2008 to 2019 with a magnitude between 1.4 and 3.0 in the frequency band range between 3 and 34 Hz. The determined attenuation values of both methods are similar within their error bars. The inverse quality factors of the shear wave are relatively low compared to other regions, with values of 3.2 × 10−4 to 8.7 × 10−4 for $Q^{-1}_{i}$ and 1.4 × 10−4 to 2.8 × 10−4 for $Q^{-1}_{sc}$, respectively. As a by-product of Qopen, we also obtain the energy site amplification of the stations used in the inversion as well as source displacement spectra and moment magnitudes of the inverted earthquakes. Several combinations of inversion parameters were tested for MLTWA, with $Q^{-1}_{i}$ and $Q^{-1}_{sc}$ providing the lowest trade-off. Likewise, we investigated the influence of window length on the results of Qopen. We found a dependency of the results on the length, if the windows are shorter than 30 s. For longer time windows, the dependence disappears, and the result becomes independent of window length..

Temporal variation in scattering and intrinsic attenuation due to earthquakes in East Asia

Separated attenuation values have not been used in post-seismic variation research, although the scattering attenuation (Qs−1) parameter that can be used to estimate crustal inhomogeneity due to cracks. In this study, three earthquakes that occurred in Kumamoto (M7.3), Tottori (M6.6), and Gyeongju (M5.8) in 2016 were investigated by applying a multiple lapse time window analysis to seismograms recorded before and after the events. At a low frequency, significantly greater variation of the Qs−1 value was observed than the intrinsic attenuation (Qi−1) for the Kumamoto earthquake, whereas similarly large variation was observed for the Gyeongju earthquake. For the surrounding Kumamoto earthquake area of increased attenuation, even higher decreases in Qs–1 and Qi–1 were also observed. The increases occurred within a two year-period after mainshock. The large increases in attenuation, corresponding to regions with high peak ground acceleration, were limited to the basin area with an elevation below 500 m. Furthermore, post-seismic increases in attenuation values were found to correlate with the magnitude and length of the quiet periods of the earthquakes. From this study, Qs–1 and Qi–1 were shown as new parameters that can quantitatively measure the post-seismic deformation due to crustal earthquake.

Seismic attenuation structure across the Karakoram fault in western Tibet

SUMMARY2-D attenuation maps are produced for the crust of western Tibet using local earthquakes which are recorded by an array of 31 broad-band stations operated from 2007 July to 2011 May. Relative contribution of scattering ($Q_{sc}^{-1}$) and intrinsic ($Q_{i}^{-1}$) attenuation have been calculated using Multiple Lapse Time Window Analysis under the assumption of uniform distribution of multiple isotropic scattering and intrinsic absorption in a medium for five different frequency bands centred at 1.5, 3, 6, 12 and 18 Hz, respectively. All the events are selected on the basis of high signal-to-noise ratio having hypocentral distance within 200 km from the respective stations. The obtained Q−1 values show a strong frequency dependent nature which can be correlated to the degree of tectonic complexity and the heterogeneities present in the medium. The intrinsic absorption is found to be the dominant mechanism at all the frequency ranges for all stations except few (WT03, WT07 and WT13) at 18 Hz, which may be correlated with the presence of partial melt, geothermal fluids, hydrothermal springs, mantle-derived fluids and radioactivity in the crust of western Tibet. We have divided the entire area into two regions across the Karakoram fault (KKF) to explore the 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 spatial variations of $Q_{i}^{-1}$ across the region exhibit significant differences between Regions 1 and 2 at all the investigated frequencies. Interestingly, Region 1 exhibits higher $Q_{i}^{-1}$ than Region 2 at lower frequencies, whereas $Q_{i}^{-1}$ shows opposite trends at higher frequencies (> 6 Hz) as it shows higher values in Region 2 than Region 1. We find that the obtained values of Q−1 are also in good agreement with the other segments of Himalaya and Tibet as well as different tectonic regions in the world.

Intrinsic and scattering seismic wave attenuation in the Central Apennines (Italy)

Recently, an earthquake sequence formed by a series of subsequences characterized by large earthquakes (three events M ≥ 6.0 and five events M ≥ 5.0) occurred in the Central Apennines in 6–7 months during 2016–2017. Several studies interpreted this cascading type of earthquake occurrence as associated with fluid movements through an interconnected network of cracks around the active faults that triggered the subsequences in different time and location.In the present study we aim to verify if the presence of fluids is confirmed by the pattern of seismic wave attenuation. To do so we separately estimate the space-averaged parameters intrinsic Qi−1 and scattering Qs−1, proportional to the correspondent attenuation parameters, by applying a modified Multiple Lapse Time Window Analysis (MLTWA), in order to establish reference background values for the study region. MLTWA measures the decay of S-wave energy with distance and lapse time and fit it to the Energy Transport Equation solution for scattering, yielding the seismic albedo, B0 = Qs−1/Qt−1 and the extinction Length, Le,∝Qt−1 parameters as a function of frequency. The knowledge of the separated attenuation parameters allows determining which process of seismic energy dissipation dominates in the region.In the present study B0 results to be around0.6 (at 1.5 Hz) and decreases with increasing frequency down to 0.2 indicating that the attenuation of the seismic energy carried by S-wave is controlled by intrinsic dissipation rather than by scattering. Moreover, the intrinsic attenuation is frequency dependent (f−0.9 for the uniform model and f−0.4 for the crust/Moho model). This behavior well matches with the presence of fluids in the faults and fracture zones in the central Apennines, in agreement with results from other independent studies in the region.Finally, a comparison among the attenuation parameters estimated for this area and those measured in several areas of Italy and other parts of the world is presented together with a discussion on the comparison between the present MLTWA attenuation estimates and homologous results obtained for the same area with different approaches.

Understanding seismic path biases and magmatic activity at Mount St Helens volcano before its 2004 eruption

SUMMARYIn volcanoes, topography, shallow heterogeneity and even shallow morphology can substantially modify seismic coda signals. Coda waves are an essential tool to monitor eruption dynamics and model volcanic structures jointly and independently from velocity anomalies: it is thus fundamental to test their spatial sensitivity to seismic path effects. Here, we apply the Multiple Lapse Time Window Analysis (MLTWA) to measure the relative importance of scattering attenuation vs absorption at Mount St Helens volcano before its 2004 eruption. The results show the characteristic dominance of scattering attenuation in volcanoes at lower frequencies (3–6 Hz), while absorption is the primary attenuation mechanism at 12 and 18 Hz. Scattering attenuation is similar but seismic absorption is one order of magnitude lower than at open-conduit volcanoes, like Etna and Kilauea, a typical behaviour of a (relatively) cool magmatic plumbing system. Still, the seismic albedo (measuring the ratio between seismic energy emitted and received from the area) is anomalously high (0.95) at 3 Hz. A radiative-transfer forward model of far- and near-field envelopes confirms this is due to strong near-receiver scattering enhancing anomalous phases in the intermediate and late coda across the 1980 debris avalanche and central crater. Only above this frequency and in the far-field diffusion onsets at late lapse times. The scattering and absorption parameters derived from MLTWA are used as inputs to construct 2-D frequency-dependent bulk sensitivity kernels for the S-wave coda in the multiple-scattering (using the Energy Transport Equations—ETE) and diffusive (AD, independent of MLTWA results) regimes. At 12 Hz, high coda-attenuation anomalies characterize the eastern side of the volcano using both kernels, in spatial correlation with low-velocity anomalies from literature. At 3 Hz, the anomalous albedo, the forward modelling, and the results of the tomographic imaging confirm that shallow heterogeneity beneath the extended 1980 debris-avalanche and crater enhance anomalous intermediate and late coda phases, mapping shallow geological contrasts. We remark the effect this may have on coda-dependent source inversion and tomography, currently used across the world to image and monitor volcanoes. At Mount St Helens, higher frequencies and deep borehole data are necessary to reconstruct deep volcanic structures with coda waves.

New insights into seismic absorption imaging

In recent years, attenuation has been used as a marker for source and dynamic Earth processes due to its higher sensitivity to small variations of lithospheric properties compared to seismic velocity. From seismic hazard analysis to oil and gas exploration and rock physics, many fields need a better reconstruction of energy absorption, a constituent of seismic attenuation generally considered a reliable marker of fluid saturation in space. Here, we propose absorption tomography (AT), a technique grounded on the principles of scattering tomography and Multiple Lapse Time Window Analysis. We benchmark its efficiency to image absorption in space by comparing its results with those obtained using two of the most common mapping strategies, regionalization and kernel-based inversion for a diffusive regime. We applied these methodologies to three datasets, each characterized by a different tectonic setting and quality of the dataset: the Pollino fault area (Southern Italy), Mount St. Helens Volcano (USA) and Vrancea (Romania). AT overcomes the assignment of a single coda quality factor value between each source-receiver pair by modelling and inverting for the spatial distribution of energy as a function of different lapse times. It can then reconstruct node-dependent envelopes using analytic and computational sensitivity kernels and solve for coda attenuation with a grid-search approach. AT allows for a better reconstruction of the localized absorption anomalies than standard methodologies, even if the efficiency of the technique depends on the quality of the dataset, and identifies outliers in the data that could alter the final result and its interpretation. The implementation of analytical diffusive kernels in AT allows for a fast and highly-resolved imaging of well-sampled seismic faults structures. While slightly reducing resolution on faults and being computationally more expensive, implementing multiple-scattering lapse-time-dependent kernels still provides satisfactory results as well as a more physically-correct forward model.

Depth-dependent seismicity and crustal heterogeneity in South Korea

Depth-dependent seismicity has been suggested in the seismogenic layer, which is divided into upper and lower layers by its seismicity boundary. Since large earthquakes tend to occur in the lower layer, the structure of the seismogenic layer, which includes the seismicity boundary, provides important information for seismic hazard assessment. Using 107 land-based earthquakes, the structure of the seismogenic layer in South Korea was studied with focal depth determination by identifying the minimum residuals between the observations and several models. The same procedure was done for the depth relocation of 138 events associated with the Gyeongju earthquake (ML 5.8), the largest event in South Korea in the last two centuries. The geothermal temperature at the focal depth was also estimated for all earthquakes, including the Gyeongju earthquake cluster and the Pohang earthquake (ML 5.4), which caused more damage than did the Gyeongju earthquake. As observed in a large-scale averaging of mainland China, a 12-km seismicity boundary was suggested due to the frequent seismicity in the upper layer. At 12-km depth at the Gyeongju earthquake site, the geothermal temperature reached 300 °C, and the occurrence of aftershocks was limited below that depth. Based on the rigorously estimated depth, we separated the scattering attenuation, Qs−1, by a multiple lapse time window analysis using individual earthquakes and found higher and lower values for events in the upper and lower layers, respectively. This depth-dependent observation suggests differences in heterogeneity between the upper and lower seismogenic layers and supports the hypothesis that the seismicity of a depth-dependent seismogenic layer is related to cracks, which diminish with depth. Our results suggest that the upper limit of the focal depth can be predicted from the depth of the seismicity boundary for normal large earthquakes.

Estimation of scattering and intrinsic attenuation based on multiple lapse time window analysis in Sikkim Himalayan region, India

Attenuation of seismic wave energy was estimated in and around the source region of the Sikkim earthquake of 18 September, 2011 using aftershocks. The main shock had a moment magnitude 6.9 and after this main shock, approximately 283 aftershocks with a magnitude distribution ranging from 1.02 to 5.0 were recorded by 8 temporary broad band seismometers. In the present study, the relative contributions of scattering attenuation and intrinsic absorption are estimated by multiple lapse time window analysis under the hypothesis of isotropic scattering and a uniform distribution of scatterers in the crustal part of the study area. The coda wave attenuation is also estimated using single backscattering method. All of the attenuation parameters are estimated as a function of frequency in the range 1–16 Hz. The results show that scattering attenuation is greater than intrinsic absorption for all of the frequencies. The relative contribution of scattering attenuation decreases with increasing frequency. Around the 1–4 Hz frequency band, scattering attenuation primarily contributes to seismic wave attenuation in the Sikkim region. The estimated values of seismic albedo ranges from 0.65 to 0.79; as seismic albedo is higher than 0.5 for all frequencies, it is concluded that the medium in the Sikkim Himalayas is highly heterogeneous and tectonically active. Our knowledge of the attenuation mechanism of the Sikkim region is enhanced by the application of the multiple lapse time window analysis method, and results can be useful for the hazard assessment.

Multiple lapse time window analysis using solely single events in South Korea

Scattering (Qs–1) and intrinsic (Qi–1) attenuation, important parameters for inferring both the materials and the physical condition of the regional lithosphere, are generally separated by the method of multiple lapse time window analysis (MLTWA). Recently, depth variation of crustal Qs–1 and Qi–1 by applying single events to MLTWA was first shown; however, this application inevitably combined several events due to insufficient data caused by a window with limited range. In this study, we demonstrated that a flexible range window can be applied successfully to solely single events in MLTWA. In particular, a more reliable constraint was obtained than in the previous study, with a reduced amount of data. This technique may be particularly useful for seismically stable regions due to the limitation of available combinations of similar depth events.

Relative role of intrinsic and scattering attenuation beneath the Andaman Islands, India and tectonic implications

The attenuation characteristics of seismic waves traversing the Andaman Nicobar subduction zone (ANSZ) are investigated using high quality data from a network of broadband stations operational since 2009. We initially studied the Coda wave attenuation (Qc-1) under the assumption of a single isotropic scattering model. Subsequently, following the multiple isotropic scattering hypothesis, we isolated the relative contributions of intrinsic (Qi-1) and scattering (Qsc-1) attenuation employing the Multiple Lapse Time Window Analysis (MLTWA) method within a frequency range 1.5–18Hz. Results reveal a highly attenuative nature of the crust, with the values of Qc being frequency dependent. The intrinsic absorption is mostly found to be predominant compared to scattering attenuation. The dominance of Qi-1 in the crust may be attributed to the presence of fluids associated with the subducted slab. Our results are consistent with the low velocity zone reported for the region. A comparison of our results with those from other regions of the globe shows that the ANSZ falls under the category of high intrinsic attenuation zone. Interestingly, the character of ANSZ is identical to that of eastern Himalaya and southern Tibet, but entirely different from the Garhwal-Kumaun Himalaya and the source zone of Chamoli earthquake, due to the underlying mechanisms causing high attenuation.

Intrinsic and scattering attenuation of high-frequency S-waves in the central part of the External Dinarides

The central part of the External Dinarides (CED) is a geologically and tectonically complex region formed in the collision between the Adriatic microplate and the European plate. In this study, the contributions of intrinsic and scattering attenuation (Qi-1 and Qsc-1, respectively) to the total S-wave attenuation were calculated for the first time. The multiple lapse-time window analysis (MLTWA method), based on the assumptions of multiple isotropic scattering in a homogeneous medium with uniformly distributed scatterers, was applied to seismograms of 450 earthquakes recorded at six seismic stations. Selected events have hypocentral distances between 40 and 90km with local magnitudes between 1.5 and 4.7. The analysis was performed over 11 frequency bands with central frequencies between 1.5 and 16Hz. Results show that the seismic albedo of the studied area is less than 0.5 and Qi-1>Qsc-1 at all central frequencies and for all stations. These imply that the intrinsic attenuation dominates over scattering attenuation in the whole study area. Calculated total S-wave and expected coda wave attenuation for CED are in a very good agreement with the ones measured in previous studies using the coda normalization and the coda-Q methods. All estimated attenuation factors decrease with increasing frequency. The intrinsic attenuation for CED is among the highest observed elsewhere, which could be due to the highly fractured and fluid-filled carbonates in the upper crust. The scattering and the total S-wave attenuation for CED are close to the average values obtained in other studies performed worldwide. In particular, good agreement of frequency dependence of total attenuation in CED and in the regions that contributed most strong-motion records for ground motion prediction equations used in PSHA in Croatia indicates that those were well chosen and applicable to this area as far as their attenuation properties are concerned.

Depth‐Dependent Crustal Scattering Attenuation Revealed Using Single or Few Events in South Korea

Few studies have been conducted on the variance of scattering (![Graphic][1] ) and intrinsic attenuation (![Graphic][2] ) with depth in the crust. Furthermore, these values, routinely estimated using multiple lapse time window analysis (MLTWA), were seriously affected by different earthquake depths. Based on earthquakes recorded by dense seismic networks in South Korea, we used the MLTWA method on single or small clusters of events with similar depths. The depths of 54 earthquakes, relocated using seven velocity models, provide a typical seismogenic layer with the brittle–plastic transition (BPT) depth at 11 km, which correlated well with the seismicity of Korea. The depth was also used for the theoretical model based on the direct‐simulation Monte Carlo method. As the result of MLTWA, events with shallow focal depths exhibit higher ![Graphic][3] correlated with dense scatter, whereas those with deeper focal depths exhibit lower ![Graphic][4] . This depth dependency of the ![Graphic][5] value is indicated by the significant difference between the upper and lower parts of the BPT depth in the seismogenic layer. We also found that ![Graphic][6] values do not show depth dependency, whereas ![Graphic][7] values do. Thus, this study demonstrates for the first time that the crustal depth dependency of total (![Graphic][8] ) attenuation originates from ![Graphic][9] rather than from ![Graphic][10] . The approach using single or few events may be helpful for a regional study of ![Graphic][11] and ![Graphic][12] in seismically inactive regions for which insufficient data exist and may provide insight into the seismogenic layer related to potential seismic hazards. Online Material: Tables of residuals and range of multiple lapse time window analysis (MLTWA) parameters and Q −1 values for studied events. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif [8]: /embed/inline-graphic-8.gif [9]: /embed/inline-graphic-9.gif [10]: /embed/inline-graphic-10.gif [11]: /embed/inline-graphic-11.gif [12]: /embed/inline-graphic-12.gif

Attenuation characteristics in eastern Himalaya and southern Tibetan Plateau: An understanding of the physical state of the medium

Attenuation characteristics of the crust in the eastern Himalaya and the southern Tibetan Plateau are investigated using high quality data recorded by Himalayan Nepal Tibet Seismic Experiment (HIMNT) during 2001–2003. The present study aims to provide an attenuation model that can address the physical mechanism governing the attenuation characteristics in the underlying medium. We have studied the Coda wave attenuation (Qc) in the single isotropic scattering model hypothesis, S wave attenuation (Qs) by using the coda normalization method and intrinsic (Qi-1) and scattering (Qsc-1) quality factors by the multiple Lapse Time Window Analysis (MLTWA) method under the assumption of multiple isotropic scattering in a 3-D half space within the frequency range 2–12Hz. All the values of Q exhibit frequency dependent nature for a seismically active area. At all the frequencies intrinsic absorption is predominant compared to scattering attenuation and seismic albedo (B0) are found to be lower than 0.5. The observed discrepancies between the observed and theoretical models can be corroborated by the depth-dependent velocity and attenuation structure as well as the assumption of a uniform distribution of scatterers. Our results correlate well with the existing geo-tectonic model of the area, which may suggest the possible existence of trapped fluids in the crust or its thermal nature. Surprisingly the underlying cause of high attenuation in the crust of eastern Himalaya and southern Tibet makes this region distinct from its adjacent western Himalayan segment. The results are comparable with the other regions reported globally.

Separation of Intrinsic and Scattering Attenuation Using Single Event Source in South Korea

Abstract Simultaneous use of numerous events in the multiple lapse time window analysis (MLTWA) method is the most effective way to obtain and values. Such simultaneous use of numerous events normally shows a large observational scatter, which may be caused by different radiation pattern and focal depth of each earthquake, and regional alteration of the local structure. To avoid these scattering factors, the current study attempted to use single event source as the input data in MLTWA for South Korea. The direct simulation Monte Carlo (DSMC) method was used to simulate the observations, which showed realistic results when the number of recorded stations was eight or more. The obtained values of seismic albedo ( B 0 ) were less than 0.5, which were in accordance with the same in the previous results. Three envelopes constructed by combination of three regional values of at 1 Hz produced regional variations, of which the ones at high frequencies of and at low frequencies of were remarkable. This result, showing the local variation indicated an advantage of the study using single source event over the previous studies in South Korea that involved extensively distributed data. Because DSMC was known to be applicable to 3D structure due to its simple algorithm, the current study also tried to correct radiation pattern of the observations by inserting focal mechanism algorithm to the code of the DSMC method. Such insertion, however, generated extremely low values of because the geometrically shrunk receiver could not capture most of the scattered particles. This receiver problem is most likely to be a major obstacle of the 3D study of DSMC. In addition, the significant depth effect of Q −1 was identified for the prior MLTWA studies resulting from different measurement range and focal depth.

Scattering and intrinsic attenuation in Cairo metropolitan area using genetic algorithm

A total number of 46 local earthquakes (2.0≤ML≤4.0) recorded in the period 2000–2011 by the Egyptian seismographic network (ENSN) were used to estimate the total (Qt−1), intrinsic (Qi−1) and scattering attenuation (Qsc−1) in Cairo metropolitan area, Egypt. The multiple lapse time window analysis (MLTWA) under the assumption of multiple isotropic scattering with uniform distribution of scatters was firstly applied to estimate the pair of Le−1, the extinction length inverse, and B0, the seismic albedo, in the frequency range 3–24Hz. To take into account the effect of a depth-dependent earth model, the obtained values of B0 and Le−1 were corrected for an earth structure characterized by a transparent upper mantle and a heterogeneous crust. The estimated values of Qt−1, Qsc−1 and Qi−1 exhibited frequency dependences. The average frequency-dependent relationships of attenuation characteristics estimated for the region are found to be: Qt−1=(0.015±0.008)f(−1.02±0.02), Qsc−1=(0.006±0.001)f(−1.01±0.02), and Qi−1=(0.009±0.008)f(−1.03±0.02); showing a predominance of intrinsic absorption over scattering attenuation. This finding implies that the pore-fluid contents may have great effect on the attenuation mechanism in the upper crust where the River Nile is passing through the study area. The obtained results are comparable with those obtained in other tectonic regions.

Seismic wave attenuation in Israel region estimated from the multiple lapse time window analysis and S-wave coda decay rate

Seismic wave attenuation in Israel region estimated from the multiple lapse time window analysis and S-wave coda decay rate

진원 깊이를 고려한 한반도 다중지연시간창 해석

The recent Multiple Lapse Time Window (MLTW) analysis of Korean Peninsula event showed that the focal depth was far greater influence factor than the velocity structure of the model, applying the analysis of the direct simulation Monte Carlo (DSMC) method. Thus, using the events with focal depth of about 10 km, this study considered 330 paths connecting 41 events and 71 stations, and re-examined uniform and depth-dependent velocity models previously studied. As a result, the residual of misfit function greatly decrease from analytic model to DSMC model, reflecting variation of the focal depth from 0 to 10 km. On the other hand, the difference of residuals for each velocity model were relatively small.