Multiple Lapse Time Window Analysis Research Articles

The attenuation mechanism of S-waves in the source zone of the 1999 Chamoli earthquake

In the present study the attenuation mechanism of seismic wave energy in and around the source area of the Chamoli earthquake of 29th March 1999 is estimated using the 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. Separation of intrinsic (Qi-1) and scattering (Qs-1) attenuation coefficient is done over the frequencies 1, 2, 4, 8 and 16Hz using Multiple Lapse Time Window Analysis (MLTWA) method. It is observed that S-waves and their coda are primarily attenuated due to scattering attenuation and seismic albedo is very high at all the frequencies. A comparison of attenuation characteristics obtained using these aftershock data with those obtained using data of general seismicity of this region reveal that at lower frequencies both intrinsic and scattering attenuation for Chamoli was much higher compared to those for Garwhal-Kumaun region using general seismicity data. At higher frequencies intrinsic attenuation for Chamoli is lower than and scattering attenuation is comparable to those obtained using general seismicity data of Garwhal-Kumaun region.

Separation of intrinsic and scattering seismic wave attenuation in Northeast India

SUMMARY We have analysed the local earthquakes (2.0 ≤ ML ≤ 5.5) occurred in northeast (NE) India recorded by a temporary seismic network of 10 stations operated by National Geophysical Research Institute (NGRI), Hyderabad to evaluate the relative contributions of scattering loss (Q −1 ) and intrinsic absorption (Q −1 ) to total attenuation (Q −1 ) using the multiple lapse time window analysis assuming multiple isotropic scattering in a medium of uniformly distributed scatterers. The results show that Q −1 is greater than Q −1 at high frequencies (f > 3Hz), while the opposite is observed at low frequencies (f < 3Hz). The observed frequency dependence of Q −1 corresponds to the scale length of lithospheric heterogeneity beneath NE India, at least comparable with the wavelength for the lowest frequencies analysed, of about 1km. The observed Q −1 for the study region obtained with single scattering theory is close to Q −1 at high frequencies, in agreement with theoretical prediction for an idealized case of uniform distribution of scatterers. However, a discrepancy exists between the two at low frequencies, which can be explained by a depth-dependent velocity and attenuation structure. High value of Q −1 is correlated with the geology and tectonic settings of the region characterized by Himalayan and Burman collision zones with a strong lateral heterogeneity. The Q −1 estimates obtained in this study can be used to infer the average temperature of the lower crust with an upper limit estimate of ∼800 ◦ C assuming a lower crustal gabbroic lithology.

Envelope Inversion Analysis for High-Frequency Seismic Energy Radiation from the 2011 Mw 9.0 Off the Pacific Coast of Tohoku Earthquake

Abstract I study S ‐wave energy radiation in higher frequencies of 1–8 Hz from the 2011 M w 9.0 Off the Pacific Coast of Tohoku, Japan, earthquake (hereafter called the Tohoku‐Oki earthquake) based on a seismogram envelope inversion method. I first evaluate two medium parameters of scattering attenuation and intrinsic absorption for S waves using spatial and temporal distributions of seismic energy from aftershock records, which are necessary for calculating Green’s functions. Next, applying the envelope inversion method to 27 near‐field strong‐motion records from the mainshock, I estimate the S ‐wave energy radiation from 40 subfaults on the fault of 500 km×250 km. Each subfault is allowed to rupture for eight consecutive source time windows. Rupture velocity and the duration of energy radiation for each time window are determined by a grid search to be 2.5 km/s and 8.0 s, respectively. Energy radiated from the entire fault is 5.8×10 16 J for 1–2 Hz, 4.5×10 16 J for 2–4 Hz, 1.5×10 16 J for 4–8 Hz, and totaling 1.2×10 17 J in the 1–8 Hz range. Strong energy emanated twice from greater depths of the off‐Miyagi region, corresponding to two peaks seen at the northern stations. And energy radiation at greater depths off the Fukushima–Ibaraki border is compatible with a single peak found at the southern stations. Strong high‐frequency radiations at greater depths strikingly contrast with large slips at shallower depths estimated in lower frequencies. This frequency‐dependent rupture may accord with structural changes along the dip direction. Online material: Snapshots of high‐frequency energy radiation from the fault plane and results of estimating scattering parameters by the multiple lapse time window analysis in 1–2 and 2–4 Hz.

Depth-dependent intrinsic and scattering seismic attenuation in north central Italy

SUMMARY In this study the attenuation mechanism of seismic wave energy in north central Italy is estimated using low-magnitude earthquake local data recorded at six stations managed by INGV. Most of the analysed events are located along the Alpine chain in the zone of Iseo and Garda lakes, while a minor part in the Po valley. The zone investigated is characterized by the occurrence of significantly intense earthquakes (magnitude up to 6.6) the most recent occurred in 2004 close to the city of Salo on the coast of the Garda lake (Mw= 5.0). Due to the high population density and presence of industrial activity the investigated area is characterized by a high seismic risk. First, the ordinary Multiple Lapse Time Window Analysis (MLTWA) method is applied in the assumption of uniform velocity and scattering and the couple of B0, the seismic albedo and Le−1, the extinction length inverse (corresponding to the total attenuation coefficient) is calculated in the frequency bands of 1.5, 3, 6 and 12 Hz. To retrieve more realistic estimates, the obtained values of B0 and Le−1 are corrected taking into account the effects of a depth-dependent earth model, consisting of an earth structure characterized by a transparent upper mantle and a heterogeneous crust. We find that the corrected intrinsic and scattering attenuation parameters (which are proportional to the inverse of the intrinsic/scattering quality factors, Q−1I and Q−1S) are strongly frequency dependent, with a prevalence of scattering attenuation over the intrinsic dissipation. The corrected and uncorrected values of total Q are in agreement with the total Q values obtained with different approaches for the same area.

Study of attenuation mechanism for Garwhal–Kumaun Himalayas from analysis of coda of local earthquakes

The relative contribution of intrinsic ( Q i − 1 ) and scattering ( Q s − 1 ) attenuation to seismic wave attenuation was estimated for the Garwhal–Kumaun Himalayas using Multiple Lapse Time Window Analysis (MLTWA) method under the assumption of isotropic scattering. Local earthquake data recorded by an array operated by Wadia Institute of Himalayan Geology (WIHG), India was used for this purpose. It is observed that scattering attenuation primarily contributes to seismic wave attenuation in this region and its value is much higher compared to that of intrinsic attenuation at around 1 Hz frequency. As frequency increases the relative contribution of scattering attenuation to total attenuation starts decreasing. However, as seismic albedo is higher than 0.5 for all the frequencies considered, it is concluded that the medium here is highly heterogeneous in nature.

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 (&lt;;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 &gt;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.

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.

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).

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.

Crustal Attenuation Characteristics in Northwestern Turkey in the Range from 1 to 10 Hz

We have analyzed the aftershocks ( M L <4.5) following the 1999 Izmit earthquake ( M w 7.4) to infer the frequency-dependent attenuation characteristics of both P and S waves, in the frequency range from 1 to 10 Hz and in the distance range from 10 to 140 km. A linear-predictive model is assumed to describe the spectral amplitudes in terms of attenuation and source contributions. The results show that both P and S waves undergo a strong attenuation along ray paths shorter than 40 km, while the secondary arrivals significantly contribute to the spectral amplitudes over the distance range from 40 to 60 km, as also confirmed by the computation of synthetic seismograms. For longer ray paths, the decrease in attenuation suggests an increase in the propagation efficiency with depth. Finally, the spectral attenuation curves are flattened, or sloped upward at low frequencies in the range from 100 to 140 km, due to the contemporary arrivals of direct waves and postcritical reflections from the Moho. In terms of geometrical spreading and anelastic attenuation, the attenuation in the range from 10 to 40 km is well described by a spreading coefficient n = 1 for both P and S waves, and the quality factors can be approximated by QS ( f ) = 17 f 0.80 for 1 ≤ f ≤ 10 Hz and QP ( f ) = 56 f 0.25 for 2.5 ≤ f ≤ 10 Hz. For ray paths in the range from 60 to 80 km, the attenuation weakens but the interaction between seismic waves and propagation medium is more complex. The multilapse time window analysis (mltwa) is applied to quantify the amount of scattering loss and intrinsic absorption for S waves. The seismic albedo B decreases from 0.5 at 1 Hz to 0.3 at 10 Hz, while the total quality factor QT increases from about 56 to 408. The multiple lapse time-window analysis (mltwa) results provide only an average estimate of the attenuation properties in the range from 10 to 80 km. In fact, by neglecting the variation of attenuation with depth, the mltwa results underestimate attenuation for distances less than 40 km, and do not capture the significant features caused by the integrated energy of the secondary arrivals observed in the range from 40 to 60 km.

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.

Separation of depth-dependent intrinsic and scattering seismic attenuation in the northeastern sector of the Italian Peninsula

SUMMARY We investigated the intrinsic dissipation and scattering properties of the lithosphere under the Friuli region (northeastern Italy) using two hypotheses: (i) a uniform earth model and (ii) two ‘reasonable’ non-uniform, layered crustal models. For case (i) we measured the coda Q, and used the multiple-lapse time window analysis (MLTWA) technique to obtain separate estimates of intrinsic absorption and scattering attenuation. Results for the uniform earth model show that the lithosphere in northeastern Italy is characterized by a low-scattering attenuation (small scattering Q-inverse, Q −1 ), and by a relatively high intrinsic attenuation (high intrinsic Q-inverse, Q −1 ). A comparison between the investigated region and other areas around the world shows that both Q −1 and Q −1 for the Friuli region are among the lowest values ever measured, with the exception of the southern Apennines, which has the lowest measured Q −1 . For case (ii), numerical simulation of the energy envelopes was performed using two-layered earth models, where the values of the intrinsic and scattering attenuation coefficients are both within ‘reasonable ranges’ when compared with the geological information. The theoretical envelopes calculated for the homogeneous model give a good fit to the synthetic envelopes calculated for the layered models; the best fit is obtained for scattering attenuation coefficients of the uniform model always greater than those of the layered model. The main result is consequently that scattering Q −1 obtained using the MLTWA under the assumption of a uniform medium is overestimated, on average, by a factor 2. Finally, coda Q −1 appears to be closer to the total Q −1 than to the intrinsic Q −1 ,a spredicted

Spatial variation of coda wave attenuation in northwestern Colombia

SUMMARY One thousand seven hundred and eighty-six vertical-component, short-period observations of microearthquake codas from regional earthquakes recorded by 17 stations belonging to the National Seismological Network of Colombia were used to estimate seismic wave attenuation in Colombia. Local magnitudes range from 2.9 to 6.0 and only events occurring at hypocentral distances up to 255 km were considered for the analysis. The frequencies of interest lay between 1 and 19 Hz and the analysis was performed for each seismic station separately. Coda-wave attenuation (Q −1 ) was estimated by means of a single-scattering method whereas the separation of intrinsic absorption (Q −1 ) and scattering attenuation (Q −1 ) from total attenuation (Q −1 )w as performed using a multiple lapse time-window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers. A regionalization of the estimated Q0 (Qc at 1 Hz) values was performed and a contour map of seismic coda attenuation in Colombia is presented, where four zones with significant variations of attenuation related to different geological and tectonic characteristics can be observed. The highest attenuation is linked to the central and western regions (Q0 around 50 and 56) whereas a lower attenuation (Q0 around 69 and 67) is assigned to the northern and eastern regions. Results show that the Q −1 values are frequency dependent in the considered frequency range, and are approximated by a least-square fit to the power law Q −1 ( f ) = Q −1 ( f / f 0) −η . The exponents of the frequency dependence law ranged from η = 0.65 to 1.01 for Q −1 , η = 0.62 to 1.78 for Q −1 , η = 0.28 to 1.49 for Q −1 , and η = 0.53 to 1.67 for Q −1 . On the other hand, intrinsic absorption is found to dominate over scattering in the attenuation process for most of the stations and frequency bands analysed. Some discrepancies have been observed between the theoretical model and the observations for some frequency bands which indicate that it would be necessary to consider models for depth-dependent velocity structure and/or non-isotropic scattering patterns.

Regional estimation of Q from seismic coda observations by the Gauribidanur seismic array (southern India)

Attenuation properties of the lithosphere in southern India are estimated from 1219 vertical-component, short-period observations of microearthquake codas recorded by the Gauribidanur seismic array. The magnitudes of the earthquakes range from 0.3 to 3.7 and have focal depths less than 10 km. Coda-wave attenuation ( Q c −1) is estimated by means of a single isotropic scattering method and a multiple lapse time window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers is used to estimate the contribution of intrinsic absorption ( Q i −1) and scattering ( Q s −1) to total attenuation ( Q t −1). All the attenuation parameters are estimated, as a function of frequency for hypocentral distances up to 255 km. Results show a frequency dependent relation of the Q c −1 values in the range 1–10 Hz that fit the power law Q −1(f)=Q 0 −1(f/f 0) ηA Q 0 −1 value of 0.014 and a decrease of f −1.2 have been found using data from the whole region. On the other hand, scattering attenuation is found to be greater than intrinsic absorption for all the frequency bands. A high value of the seismic albedo (which ranges from 0.68 to 1) is found which indicates that scattering is the dominant effect in the study region. Nevertheless, the attenuation parameters estimated are much lower than the obtained for other regions in the world. On the other hand, the observed energy at 0–15 s from the S-wave arrival time bends significantly downward with decreasing distance. In order to clarify this phenomenon, there is a need to take into account the vertical varying velocity structure in the theoretical model.

Spatial distribution of high-frequency energy radiation on the fault of the 1995 Hyogo-Ken Nanbu, Japan, earthquake (MW6.9) on the basis of the seismogram envelope inversion

AbstractWe studied the generation and propagation of high-frequency (above 1 Hz) S-wave energy from the 1995 Hyogo-Ken Nanbu (Kobe), Japan, earthquake (MW 6.9) by analyzing seismogram envelopes of the mainshock and aftershocks. We first investigated the propagation characteristics of high-frequency S-wave energy in the heterogeneous lithosphere around the source region. By applying the multiple lapse time window analysis method to aftershock records, we estimated two parameters that quantitatively characterize the heterogeneity of the medium: the total scattering coefficient and the intrinsic absorption of the medium for S waves. Observed envelopes of aftershocks were well reproduced by the envelope Green functions synthesized based on the radiative transfer theory with the obtained parameters. Next, we applied the envelope inversion method to 13 strong-motion records of the mainshock. We divided the mainshock fault plane of 49 × 21 km into 21 subfaults of 7 × 7 km square and estimated the spatial distribution of the high-frequency energy radiation on that plane. The average constant rupture velocity and the duration of energy radiation for each subfault were determined by grid searching to be 3.0 km/sec and 5.0 sec, respectively. Energy radiated from the whole fault plane was estimated as 4.9 × 1014 J for 1 to 2 Hz, 3.3 × 1014 J for 2 to 4 Hz, 1.5 × 1014 J for 4 to 8 Hz, 8.9 × 1012 J for 8 to 16 Hz, and 9.8 × 1014 J in all four frequency bands. We found that strong energy was mainly radiated from three regions on the mainshock fault plane: around the initial rupture point, near the surface at Awaji Island, and a shallow portion beneath Kobe. We interpret that energetic portions were associated with rupture acceleration, a fault surface break, and rupture termination, respectively.

Seismic attenuation in underground mines: a comparative evaluation of methods and results

The goal of this study is to evaluate methods of estimating seismic attenuation in underground mining environments, and to relate the attenuation information to rock mass characteristics as determined from geomechanical data. Therefore, four methods — Spectral Ratios, Spectral Decay, Coda- Q, and Multiple Lapse Time Window Analysis (MLTWA) — of measuring attenuation are examined at three underground excavation sites. Each site utilizes microseismic monitoring systems `tuned' to different frequency bandwidths. Considering the MLTWA at the dominant corner frequencies for each site (Site 1, 2.1 to 4.3 kHz; Site 2 and Site 3a, 0.5 to 1.0 kHz; Site 3b, 0.1 to 0.2 kHz), the results indicate that over the entire frequency range examined at all study sites, the intrinsic S-wave Q ( Q β ) is nearly constant at approximately 180, and the scattering Q β follows a frequency dependence of Q β = Q o f 1.4 (where Q o=120). Interestingly, at Site 3 there is a crossover in the dominance of scattering to intrinsic attenuation, and this trend continues through to the Site 1 data. In order for the frequency-independent intrinsic Q β to be substantiated, it is speculated that a fractal distribution of heterogeneities in the rock mass is responsible for intrinsic attenuation (e.g., velocity anomalies, cracks, inclusions). Conversely, the increasing scattering Q β may be attributed to minimum characteristic underground opening dimensions of 3–5 m, causing decreasing wavelengths to encounter lower scattering due to these openings, and vice versa.