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S -wave spectral amplitudes from 312 crustal earthquakes recorded at the Iranian National Broadband Seismic Network in the Alborz region between 2005 and 2017 are analysed in order to evaluate earthquake source parameters, path attenuation and site amplification functions using the non-parametric generalized inversion technique (GIT). We exploit a total number of 1117 seismograms with M L 3–5.6 in the frequency range 0.3–20 Hz. The evaluated non-parametric attenuation functions decay uniformly with distance for the entire frequency range and the estimated S -wave quality factor shows low Q values with relatively strong frequency dependence. We assume the omega-square source model to retrieve earthquake source parameters from the inverted source spectra. The obtained stress drops range from 0.02 to 16 MPa with a mean value of 1.1 MPa. Stress drop and radiated energy show fairly self-similar scaling with seismic moment over the available magnitude range; however, the magnitude range of this study is too narrow to draw a definite conclusion on source scaling characteristics. The obtained moment magnitude M w and the local magnitude M L are linearly correlated and approximately equivalent in the range of M w 3–4. For larger events, M w generally underestimates M L by about 0.1–0.5 magnitude units. The estimated site amplification functions for horizontal component (GIT H) are nearly flat with no obvious pre-dominant frequency peaks for most stations, as expected for the sites of permanent broad-band seismic stations located on rock, though a few stations show amplification peaks from 1 to 8 Hz, with a maximum amplification of about a factor of 7 with respect to the reference site. The evaluated site responses for the vertical components present remarkable amplification or deamplification, leading to differences of the H/V amplitude levels in comparison with the GIT H amplification curves. The results of this study provide a valuable basis for predicting appropriate ground motions in a context of seismic hazard assessment. [ABSTRACT FROM AUTHOR]

We present a new 3-D anelastic attenuation model for the upper mantle of the southern Aegean Sea subduction area, by inverting whole-path attenuation operators (t*) data, for both P - and S - waves. For the attenuation tomography, we have employed 383 intermediate-depth earthquakes, recorded by two temporary networks between 2002 and 2007, namely the EGELADOS seismic monitoring project installed in the broader southern Aegean Sea area, and the smaller local CYCNET network which operated in the central part of southern Aegean Sea. Frequency-independent t * values were computed for both P and S waves from acceleration amplitude spectra using two approaches. Initially, we estimated t * values from the slope of the acceleration spectra above the corner frequency, using an automated procedure based on the signal-to-noise ratio. A more standard approach was also followed, where t* values were estimated from the manual (visual) selection of the linearly decaying segment of the acceleration spectrum by an analyst. The t* value data set was inverted to determine the 3-D Q -structure of Southern Aegean Sea, similar to traveltime tomography. The inversion results show high P - and S -wave attenuation in the upper mantle of the back-arc area, with significant low QP and QS anomalies in the mantle wedge of the subducting eastern Mediterranean slab under the Aegean, and higher QP and QS values in the outer Hellenic arc. The strongest anomalies are observed beneath and to the north of the volcanic arc, mainly at depths between 60 and 80 km. At larger depths, the lowest QP and QS anomalies are observed in the area of highest deep (100–170 km) microseismicity, in the easternmost part of the Hellenic volcanic arc. P -wave attenuation models from both the automated and manual t* determination approach are in good agreement. On the contrary, the automated S -wave attenuation models show significant but random spatial variability, while the manual S -wave attenuation models show a similar spatial pattern with the P -wave results, with stronger attenuation in the mantle wedge area. The final anelastic attenuation models are in good agreement with previous results for the study area (e.g. 3-D velocity models, Ground Motion Prediction Equations proposed for intermediate-depth events), as well as with attenuation models proposed for similar subduction zones. [ABSTRACT FROM AUTHOR]

We estimated the radiated seismic energy (E S) of 65 earthquakes located in the south–central region of the Gulf of California. Most of these events occurred along active transform faults that define the Pacific–North America plate boundary and have magnitudes between M 3.3 and M 5.9. We corrected the spectral records for attenuation using non-parametric S -wave attenuation functions determined with the whole data set. The path effects were isolated from the seismic source using a spectral inversion. We computed radiated seismic energy of the earthquakes by integrating the square velocity source spectrum and estimated their apparent stresses. We found that most events have apparent stress between 3 × 10−4 and 3 MPa. Model independent estimates of the ratio between seismic energy and moment (E S/ M 0) indicate that this ratio is independent of earthquake size. We conclude that in general the apparent stress is low (σa  < 3 MPa) in the south–central and southern Gulf of California. [ABSTRACT FROM AUTHOR]

Comprised of S waves trapped in the Earth's crust, the high-frequency (2–10 Hz) Lg wave is important to discriminating earthquakes from explosions by comparing its amplitude and waveform to those of Pg and Pn waves. Lateral variations in crustal structure, including variations in crustal thickness, intrinsic attenuation and scattering, affect the efficiency of Lg propagation and its consistency as a source discriminant at regional (200–1500 km) distances. To investigate the effects of laterally varying Earth structure on the efficiency of propagation of Lg and Pg, we apply a radiative transport algorithm to model complete, high-frequency (2–4 Hz), regional coda envelopes. The algorithm propagates packets of energy with ray theory through large-scale 3-D structure and includes stochastic effects of multiple scattering by small-scale heterogeneities within the large-scale structure. Source-radiation patterns are described by moment tensors. Seismograms of explosion and earthquake sources are synthesized in canonical models to predict effects on waveforms of paths crossing regions of crustal thinning (pull-apart basins and ocean/continent transitions) and thickening (collisional mountain belts). For paths crossing crustal thinning regions, Lg is amplified at receivers within the thinned region but strongly disrupted and attenuated at receivers beyond the thinned region. For paths crossing regions of crustal thickening, Lg amplitude is attenuated at receivers within the thickened region, but experiences little or no reduction in amplitude at receivers beyond the thickened region. The length of the Lg propagation within a thickened region and the complexity of over- and underthrust crustal layers can produce localized zones of Lg amplification or attenuation. Regions of intense scattering within laterally homogeneous models of the crust increase Lg attenuation but do not disrupt its coda shape. [ABSTRACT FROM AUTHOR]

On 2013 April 13, an inland earthquake of Mw 5.8 occurred in Awaji Island, which forms the western boundary of the Osaka sedimentary basin in western Japan. The strong ground motion data were collected from more than 100 stations within the basin and it was found that in the Osaka Plain, the pseudo velocity response spectra at a period of around 6.5 s were significantly larger than at other stations of similar epicentral distance outside the basin. The ground motion lasted longer than 3 min in the Osaka Plain where its bedrock depth spatially varies from approximately 1 to 2 km. We modelled long-period (higher than 2 s) ground motions excited by this earthquake, using the finite difference method assuming a point source, to validate the present velocity structure model and to obtain better constraint of the attenuation factor of the sedimentary part of the basin. The effect of attenuation in the simulation was included in the form of Q(f) = Q0(f/f0), where Q0 at a reference frequency f0 was given by a function of the S-wave velocity, Q0 = αVS. We searched for appropriate Q0 values by changing α for a fixed value of f0 = 0.2 Hz. It was found that values of α from 0.2 to 0.5 fitted the observations reasonably well, but that the value of α = 0.3 performed best. Good agreement between the observed and simulated velocity waveforms was obtained for most stations within the Osaka Basin in terms of both amplitude and ground motion duration. However, underestimation of the pseudo velocity response spectra in the period range of 5-7 s was recognized in the central part of the Osaka Plain, which was caused by the inadequate modelling of later phases or wave packets in this period range observed approximately 2 min after the direct S-wave arrival.We analysed this observed later phase and concluded that it was a Love wave originating from the direction of the east coast of Awaji Island. [ABSTRACT FROM AUTHOR]

SUMMARY High-resolution 3-D QP and QS models at depths of 0∼14 km in the Longtan reservoir area, which were used to further infer the water infiltration around the reservoir, were constructed by the local earthquake tomography technique. The inversion used t* values measured from velocity spectral amplitudes of 16 134 P-wave and 17 027 S-wave observations of 2828 events recorded by 23 digital seismic stations in the Longtan reservoir area. 3-D QP and QS tomographic images suggest a close relation between the attenuation models and water infiltration. Low QP and QS exist down to 4∼7 km depths beneath the main rivers in the reservoir area, whereas high QP and QS are found both at deep layers and at shallow layers in regions other than the main river area. We inferred that the rocks beneath the main rivers are infiltrated by substantial water and water infiltration reaches 4∼7 km deep, and the ranges of water infiltration vary by region. Specifically, the zone of high water content is estimated to extend up to 10 km away from the Buliu river in the reservoir dam area where water depth in the reservoir reaches maximum, and the extent of water infiltration in rocks can only be about 5 km around from the Hongshui river and the Youla river. Low QP and high QS indicate that the carbonate rocks in the east of the Bala fault area may be saturated with water. Earthquake clusters are mainly concentrated on the low- QP and low- QS zones, indicating that water infiltration beneath the main rivers possibly plays an important role in inducing earthquakes in the Longtan reservoir area. [ABSTRACT FROM AUTHOR]

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 Salò on the coast of the Garda lake ( M= 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 B, the seismic albedo and , 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 and 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, and ) 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. [ABSTRACT FROM AUTHOR]

We investigated the high frequency attenuation of S waves in the southeastern Alps and northern External Dinarides using waveforms from 331 earthquakes (3.0 < M < 6.5). The spectral decay parameter, k, was computed using 1345 three component high quality records, collected by the Italian Strong Motion Network (RAN) and by the Short-Period Seismometric Network of northeastern Italy (NEI) in the period 1976-2007. Weak motion data from 11 stations of the NEI network and strong motion data collected by five accelerometers of the RAN were analysed. The k parameter was estimated in the 0-250 km distance range, in a frequency band extending from the corner frequency of the event up to 25 or 45 Hz, using the amplitude acceleration Fourier spectra of S waves. The observed record-to-record variability of k was modelled by applying a generalized inversion procedure, using both parametric and non-parametric approaches. Our results evidence that k is independent on earthquake size, while it shows both site and distance dependence. Stations of the NEI network present the same increase of k with epicentral distance, R, and show values of the zero-distance k parameter, k( S), between 0.017 and 0.053 s. For the whole region, the k increase with distance can be described through a linear model with slope d k/d R= (1.4 ± 0.1) × 10 s km. Assuming an average S-wave velocity, km s between 5 and 15 km depth, we estimate an average frequency independent quality factor, , for the corresponding crustal layer. The non-parametric approach evidences a weak positive concavity of the curve that describes the k increase with R at about 90 km distance. This result can be approximated through a piecewise linear function with slopes of 1.0 × 10 and 1.7 × 10 s km, in accordance with a three layers model where moving from the intermediate to the bottom layer both and decrease. Two regional dependences were found: data from earthquakes located westward to the NEI network evidence weaker attenuation properties, probably because of S-wave reflections from different parts of the Moho discontinuity under the eastern Po Plain, at about 25-30 km depth, while earthquakes located eastward (in western Slovenia), where the Moho deepens up to 45-50 km, evidence a higher attenuation. Moreover, the k estimates obtained with data from earthquakes located in the area of the 1998 ( M= 5.7) and 2004 ( M= 5.2) Kobarid events are 0.017 s higher than the values predicted for the whole region, probably because of the high level of fracturing that characterizes fault zones. The comparison between measured and theoretical values of k, computed at a few stations with available S-wave velocity profiles, reveals that the major contribution to the total k( S) is due to the sedimentary column (from surface to 800 m depth). The hard rock section contribution is limited to 0.005 s, in accordance with a maximum contribution of 0.010 s predicted by the non-parametric inversion. [ABSTRACT FROM AUTHOR]

Based only on weak-motion data, we carried out a combined study on region-specific source scaling and crustal attenuation in the Central Apennines (Italy). Our goal was to obtain a reappraisal of the existing predictive relationships for the ground motion, and to test them against the strong-motion data [peak ground acceleration (PGA), peak ground velocity (PGV) and spectral acceleration (SA)] gathered during the M 6.15 L'Aquila earthquake (2009 April 6, 01:32 UTC). The L'Aquila main shock was not part of the predictive study, and the validation test was an extrapolation to one magnitude unit above the largest earthquake of the calibration data set. The regional attenuation was determined through a set of regressions on a data set of 12 777 high-quality, high-gain waveforms with excellent S/N ratios (4259 vertical and 8518 horizontal time histories). Seismograms were selected from the recordings of 170 foreshocks and aftershocks of the sequence (the complete set of all earthquakes with M≥ 3.0, from 2008 October 1 to 2010 May 10). All waveforms were downloaded from the ISIDe web page (), a web site maintained by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). Weak-motion data were used to obtain a moment tensor solution, as well as a coda-based moment-rate source spectrum, for each one of the 170 events of the L'Aquila sequence (2.8 ≤ M≤ 6.15). Source spectra were used to verify the good agreement with the source scaling of the Colfiorito seismic sequence of 1997-1998 recently described by . Finally, results on source excitation and crustal attenuation were used to produce the absolute site terms for the 23 stations located within ∼80 km of the epicentral area. The complete set of spectral corrections (crustal attenuation and absolute site effects) was used to implement a fast and accurate tool for the automatic computation of moment magnitudes in the Central Apennines. [ABSTRACT FROM AUTHOR]

We investigate the seismic attenuation along a 160 km profile from the Lower to the High Himalaya in the Garhwal region, India, through the analysis of Lg waveforms from regional earthquakes recorded on 18 broadband seismographs with interstation spacing of 7–10 km. Lateral variability in attenuation is derived through the inversion of 36 two-station measurements using a global optimization scheme. We observe a contrasting attenuation property in the two Himalayan belts: the Lower Himalaya has of 742 ± 235 similar to those observed in the Indian shield, whereas the High Himalaya is characterized by an unusually low value of 30–60. The seismic attenuation is also well correlated with the P-wave teleseismic traveltime residual pattern: faster arrival (approximately 0.2 s) at stations in the Lower Himalaya as compared to azimuthally independent time delay of ∼0.75 s for the High Himalayan stations. The high attenuation and low velocity in the High Himalaya suggests that low viscosity and partial melt in the crust can be correlated to the presence of Miocene leucogranite plutons in this Himalayan belt, a magmatic product of the Indo-Asian collision and presumably evidence of a partial melting event. This shallow low viscosity channel possibly connects the mid-crustal channel beneath Tibet. [ABSTRACT FROM AUTHOR]

We analysed local earthquake waveforms recorded on a broad-band seismic network in northwestern Himalayas to compute the intrinsic and scattered attenuation parameters from coda waves. Similar to other tectonically active and heterogeneous regions, attenuation-frequency relation for western Himalaya is where is the coda Q parameter. Intrinsic and scattering attenuations was separated using and direct S-wave Q data . It is observed that estimated is close to and both of them are much larger than suggesting that coda decay is predominantly caused by intrinsic attenuation. At higher frequencies, both the attenuation parameters and, are similar indicating that coda is predominantly composed of back-scattered S waves at these frequencies. [ABSTRACT FROM AUTHOR]

3-D models of the P-wave velocity ( Vp), the ratio of P- to S-wave velocity ( Vp/Vs), and the P-wave quality factor ( Qp) are determined for the crust in Taranaki, New Zealand, using local tomography and data from a dense network (average spacing 5 km) of 68 three-component, broad-band seismographs. Vp and Vp/Vs models were determined by jointly inverting P traveltimes and S–P traveltime intervals, and a Qp model by inverting t* ( t/ Qp) observations derived from modelling the velocity amplitude spectrum of P wave arrivals. An approximately 5 km diameter region of high Vp and low Vp/Vs beneath the Taranaki volcanoes extends from the surface to a depth of about 10 km and images the roots of the volcanoes formed by successive magmatic intrusions. At Mt Taranaki this will probably be the path through which future magma intrusions will reach the surface. We are unable to image any magma storage within the upper 16 km of the crust as the volumes involved are probably smaller than the 5 km resolution of our models. The volcanoes sit in the Taranaki basin, a large sedimentary basin characterized by low Vp ( c. 4 km s−1) and high Vp/Vs (≥1.9), attributed to unconsolidated, water-saturated sediments, and these are underlain by basement rocks with and . There is a c. 20 per cent contrast in Vp across the Taranaki fault at the eastern boundary of the Taranaki basin, and a small, shallow, high Vp/Vs (≥2.0) anomaly above the seismically active Cape Egmont Fault Zone, west of the Taranaki volcanoes. Although the Qp model is of significantly lower resolution than the Vp and Vp/Vs models, it shows a low Qp anomaly ( c. 100) between 4 and 10 km depth that corresponds to the deepest onshore part of the Taranaki basin. [ABSTRACT FROM AUTHOR]

Intensity assignments for 33 calibration earthquakes were used to develop intensity attenuation models for the Alps, Armorican, Provence, Pyrenees and Rhine regions of France. Intensity decreases with Δ most rapidly in the French Alps, Provence and Pyrenees regions, and least rapidly in the Armorican and Rhine regions. The comparable Armorican and Rhine region attenuation models are aggregated into a French stable continental region model and the comparable Provence and Pyrenees region models are aggregated into a Southern France model. We analyse MSK intensity assignments using the technique of Bakun & Wentworth, which provides an objective method for estimating epicentral location and intensity magnitude MI. MI for the 1356 October 18 earthquake in the French stable continental region is 6.6 for a location near Basle, Switzerland, and moment magnitude M is 5.9–7.2 at the 95 per cent (±2σ) confidence level. MI for the 1909 June 11 Trevaresse (Lambesc) earthquake near Marseilles in the Southern France region is 5.5, and M is 4.9–6.0 at the 95 per cent confidence level. Bootstrap resampling techniques are used to calculate objective, reproducible 67 per cent and 95 per cent confidence regions for the locations of historical earthquakes. These confidence regions for location provide an attractive alternative to the macroseismic epicentre and qualitative location uncertainties used heretofore. [ABSTRACT FROM AUTHOR]