Path-averaged Attenuation Research Articles

Seismic wave attenuation (1/Qp) in the crust underneath the Eastern and eastern Southern Alps (Europe): imaging effects of faults, fractures, and fluids

We present a novel three-dimensional model of compressional wave attenuation (1/QP) for the Eastern and eastern Southern Alps in Europe that includes the eastern part of the Adriatic indenter, termed here the Dolomites Sub-Indenter. Our approach employed waveform data from the SWATH-D network, a dense temporary network operational between 2017 and 2019, as well as selected stations of the larger AlpArray Seismic Network. A spectral inversion method using frequency-independent quality factor QP, was applied to derive 3578 path-averaged attenuation values (t*) from 126 local earthquakes. These were then inverted using the damped least square inversion (local earthquake tomography) for the attenuation structure. The resulting QP model, which builds on and complements a previously calculated 3-D velocity model (VP and VP/VS), exhibits good resolution down to ~ 20 km depth. Several anomalies can be correlated with the distribution of other physical parameters (VP and VP/VS) and regional tectonic features. Notably, the Friuli-Venetian region exhibits the highest attenuation (lowest QP) anomaly, coinciding with low VP values and increased VP/VS. This anomaly is likely associated with a high density of faults and fractures, as well as the presence of fluid-filled sediments along the active thrust front in the eastern segment of the Southern Alps. Another intriguing observation is the low attenuation (high QP) anomaly along the northwestern edge of the Dolomites Sub-Indenter (NWDI), located south of the Periadriatic fault and east of the Giudicarie fault, where seismicity is notably absent. This anomaly coincides with Permian magmatic rocks at the surface and may be a measure of their strength at depth.Graphical

Attenuation tomography in West Bohemia/Vogtland

We present a three-dimensional (3-D) P-wave attenuation (Qp) model for the geodynamically active swarm earthquake area of West Bohemia/Vogtland in the Czech/German border region. Path-averaged attenuation t* is calculated from amplitude spectra of time windows around the P-wave arrivals of local earthquakes. Average t/t* value or Qp for stations close to Nový Kostel are very low (<150) compared to that of stations located further away from the focal zone (increases up to 500 within 80km distance). The SIMUL2000 tomography scheme is used to invert the t* for P-wave attenuation perturbation. Analysis of resolution shows that our model is well-resolved in the vicinity of earthquake swarm hypocenters. The prominent features of the model are located around Nový Kostel focal zone and its northern vicinity. Beneath Nový Kostel a vertically stretched (down to depth of 11km) and a highly attenuating body is observed. We believe that this is due to fracturing and high density of cracks inside the weak earthquake swarm zone in conjunction with presence of free gas/fluid. Further north of Nový Kostel two highly attenuating bodies are imaged which could represent fluid channels toward the surface. The eastern anomaly shows a good correlation with the fluid accumulation area which was suggested in 9HR seismic profile.

Imaging fluid-related subduction processes beneath Central Java (Indonesia) using seismic attenuation tomography

We use local earthquake data observed by the amphibious, temporary seismic MERAMEX array to derive spatial variations of seismic attenuation (Qp) in the crust and upper mantle beneath Central Java. The path-averaged attenuation values (t∗) of a high quality subset of 84 local earthquakes were calculated by a spectral inversion technique. These 1929 t∗-values inverted by a least-squares tomographic inversion yield the 3D distribution of the specific attenuation (Qp). Analysis of the model resolution matrix and synthetic recovery tests were used to investigate the confidence of the Qp-model. We notice a prominent zone of increased attenuation beneath and north of the modern volcanic arc at depths down to 15km. Most of this anomaly seems to be related to the Eocene–Miocene Kendeng Basin (mainly in the eastern part of the study area). Enhanced attenuation is also found in the upper crust in the direct vicinity of recent volcanoes pointing towards zones of partial melts, presence of fluids and increased temperatures in the middle to upper crust. The middle and lower crust seems not to be associated with strong heating and the presence of melts throughout the arc. Enhanced attenuation above the subducting slab beneath the marine forearc seems to be due to the presence of fluids.

Influence of the 3D Distribution of Q and Crustal Structure on Ground Motions from the 2003 Mw 7.2 Fiordland, New Zealand, Earthquake

The M w 7.2 earthquake of 21 August 2003 provides an unprecedented opportunity to evaluate the applicability of the New Zealand acceleration response spectrum attenuation model (McVerry et al. , 2006) in the Fiordland region and to consider effects from 3D Q heterogeneity. This was the largest shallow earthquake in New Zealand in the past 40 yrs. It was well recorded by the GeoNet seismograph and strong-motion network. We compare the observed 5% damped response spectral accelerations (SAs) to the New Zealand model values, for both crustal reverse and subduction interface earthquakes. On average, both models underpredict the observed SAs, by approximately 0.4 in lnSA, for 0–400 km distance; however, the data show great variability. Using the South Island 3D Q model, we compute the path-averaged attenuation rate, CQ for synthetic Fiordland hypocenters to distributed sites. Compared to the standard model, the relatively high Q in South Island produces lower attenuation rates, although the low- Q crustal root of the Southern Alps produces a 10 times greater attenuation rate for certain paths. The average attenuation rate estimated from the SA data is similar to CQ for frequencies <5 Hz and constant above 5 Hz. For an acceleration model for Fiordland earthquakes, 0–60 km depth, an applicable anelastic attenuation rate is the mean CQ from the synthetic hypocenters, which is 0.006 km-1 at 2.5 Hz. For the observed SAs, the most significant regional effect is high relative SAs at some sites that may be due to postcritical Moho reflections from the Southern Alps crustal root.

Shear-wave attenuation tomography of the lithosphere–asthenosphere system beneath the Mediterranean region

3-D tomographic images of the shear-wave attenuation structure of the lithosphere–asthenosphere system beneath the Mediterranean region are derived from the inversion of anelastic attenuation coefficients obtained for the fundamental mode of nearly 3100 Rayleigh wavetrains recorded at very-broad-stations placed in the Mediterranean region. The two-station method is used to compute 124 interstation path-averaged attenuation coefficients within the 15–120 s period range. An adaptation to attenuation coefficients of the Yanovskaya–Ditmar formulation, specifically devoted to path-averaged surface-wave dispersion data and body waves, permits to obtain the local attenuation coefficients and their uncertainties for all the period range, at every node of a grid covering the studied region. Maps of stretching, azimuths and mean radii of the resolution ellipses are also obtained for every period. All of these parameters are strongly related to the spatial path-coverage, data quality and data coherency. A local attenuation dispersion curve is defined then in each grid node and the stochastic inversion of these curves yields 1-D models of inverse shear-wave intrinsic quality factor ( Q β − 1 ). The shear-wave velocity models obtained for every grid node in an elastic tomography developed in a previous work for the same domain are taken as reference models for the inversion process. The kriging contouring algorithm is used to represent the resulting 3-D Q β − 1 structure by means of horizontal and vertical cross-sections down to 160 km depth. The main patterns of this anelastic structure are mostly in agreement with the structural and tectonic features obtained in the reference shear-velocity tomography. The local attenuation maps manifest notable lateral changes for periods of 20 and 40 s, whereas more moderate spatial variation is observed for longer periods. The 3-D anelastic models show that, at shallow levels, the attenuation in the western Mediterranean is usually slightly higher than in the central-eastern part. The area from Corsica–Sardinia Islands to Italy presents high Q β − 1 values of 40.0–70.0 × 10 − 3 down to 75 km depth, whereas Q β − 1 does not exceed 20.0 × 10 − 3 in the rest of the Mediterranean basin, except for the south-eastern part, where slightly higher values are obtained (20.0–30.0 × 10 − 3 ). Those high Q β − 1 values suggest the presence of asthenospheric material at shallow levels, which would be related to distensive tectonic processes responsible for the recent opening of extensional basins, as the Tyrrhenian Sea and the Ligurian–Provençal basin. In the eastern Mediterranean, the vertical cross-sections at different latitudes and longitudes reveal at deep levels some structural patterns consistent with signs of subduction, which are linked to the convergence between the African and Eurasian plates.

Estimating Slab Earthquake Response Spectra from a 3D Q Model

The problem of estimating response spectra for a heterogeneous lithosphere can be addressed by directly computing attenuation from physical models. In a subduction zone, slab earthquakes will have different attenuation through the mantle wedge than the slab. This article is primarily concerned with very high loss paths through the attenuating mantle underlying the volcanic zone of the North Island, New Zealand, where low Q requires modification of the “standard” New Zealand engineering response spectrum model. A lack of strong-motion data prevents a standard regression analysis for paths from deep slab earthquakes through the highly attenuating mantle zone. Instead, modifications have been derived using a 3D frequency-independent Q model that has been developed for the North Island subduction zone from 2-20 Hz local earthquake t * data. By calibrating the t * crustal results to the standard New Zealand model, amplitudes can be compared between the standard model and the 3D Q model. Additional path-averaged attenuation rate coefficients, CQ , for each source and station pair are determined. This results in simple expressions for CQ as a function of centroid depth, for modifying the standard model. This modification reduces the model spectra by a factor of approximately 2-4 for mantle wedge paths below the volcanic region. This reduction is similar to the observed variation in response spectra, at periods below 0.4 sec, for a 160-km-deep M w 6.0 earthquake. For shallow earthquakes propagating through the shallow volcanic region, the new model gives results that are similar to a volcanic-path attenuation term derived by regression analysis. Manuscript received 24 February 2003.

Anelastic structure of the Iberian Peninsula obtained from an automated regionalization algorithm and stochastic inversion

An automated regionalization of a reduced domain of the Earth's surface, without a-priori seismotectonic information, and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis, a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of Rayleigh waves. Our database is formed by path-averaged attenuation coefficients derived from fundamental mode Rayleigh waves crossing the Iberian Peninsula. The wavetrains were recorded at the broad-band stations installed some years ago in the Iberian Peninsula for the ILIHA project. Before the sequential application of the algorithms, the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10–120 s period range. All these maps were obtained by applying Yanovskaya's formulation for laterally heterogeneous media to the set of path-averaged attenuation coefficients mentioned above. After that, we divided the Iberian Peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm. We then obtained, by stochastic inversion, the respective anelastic structure down to a depth of 225 km for the six homogeneous regions. The coefficients vary from 1.0 to 2.0×10 −3 km −1 and the Q −1 β structures, ranging from 110×10 −3 to 40×10 −3, suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth, respectively. A shallow beginning of the asthenosphere is detected for the sixth homogeneous region. This range is very similar to the depths deduced from previous tomographic studies of the Iberian Peninsula based on Rayleigh wave phase and group velocities. Finally, correlations obtained between the regions and some seismotectonic characteristics of the Peninsula are discussed. It is noteworthy that we cannot associate each of the six homogeneous regions with a single Hercynian, Alpine or Neogene domain.