Spatial variations of P wave attenuation in the mantle beneath North America (original) (raw)
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Seismic Research Review, 2015
Ground truth data provide the opportunity to calibrate regional seismic velocity and Q (inverse attenuation) models. However, in many cases, available wave propagation data are too sparse to characterize seismic velocities and Q everywhere. It is therefore of interest to examine on a global basis the relationship between regional geology and heat flow versus the seismic properties (Vs and Qs) of the upper mantle. To better understand the propagation of seismic waves caused by nuclear explosions, we have developed new, theoretical models of the dissipation of energy in the crystalline rocks typical for the Earth's mantle. Using laboratory results, we suggest a temperature dependence of attenuation through the activation energy. We therefore compare maps of the thermal structure of the continental lithosphere with the inverse attenuation of seismic shear waves Qs and seismic velocity Vs as determined from surface wave dispersion and amplitudes. Our study is based on recently available global databases. We compare the values of Qs , Vs, and temperature T at the depths of 50, 100, and 150 km in the continental lithosphere. We find that qualitatively (by the sign of the anomaly) the maps of Qs closely correlate with lithospheric temperatures. The best correlation is observed for the depth of 100 km, where the resolution of the attenuation model is the highest. At this depth, the contour of zero attenuation anomaly approximately corresponds to the 1000 o C contour of lithospheric temperature, in agreement with laboratory data on a sharp change in seismic attenuation and shear velocities in upper mantle rocks at 900-1000 o C. The correlation between Vs and two other parameters (T and Qs), though present, is less distinct. We find that most cratonic regions show high lithospheric Vs, Qs and low T. Several prominent low Qs regions correlate with high lithospheric temperatures. We calculate that even if temperature variations in the lithosphere are the main cause of seismic velocity and attenuation variations, the relation between temperature and seismic properties is non-linear.
Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle
Geophysical Journal International, 2004
Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, Q −1 s , and seismic velocity, V s , we compare global maps of the thermal structure of the continental upper mantle with global Q −1 s and V s maps as determined from Rayleigh waves at periods between 40 and 150 s. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high.
Geophysical Research Letters, 2011
1] We constrain the spatial variation of P-wave (t P *) and S-wave (t S *) attenuation by inverting 190,000 teleseismic P-and S-wave spectra up to 0.8 Hz. These spectra are derived from 250 deep earthquakes recorded at 880 broadband global and regional network stations. The variance and ratios of t P * and t S * values are consistent with PREM's upper mantle velocity and Q structures and conventional t P * and t S * values. High attenuation is resolved beneath stations in tectonically active regions characterized by high heat flow. Low attenuation marks stable continental regions. The maps of t P * and t S * correlate well with the variations of t S * computed and inferred from (1) the most recent surfacewave Q model and (2) a thermal interpretation of shear-wave velocity tomography. This indicates that maps of body-and surface-wave attenuation reflect intrinsic attenuation and variable temperature in the mantle. Citation: Hwang, Y. K., J. Ritsema, and S. Goes (2011), Global variation of body-wave attenuation in the upper mantle from teleseismic P wave and S wave spectra, Geophys. Res. Lett., 38, L08311,
Pure and Applied Geophysics, 2014
We estimated the network-averaged mantle attenuation t*(total) of 0.5 s beneath the North Korea test site (NKTS) by use of P-wave spectra and normalized spectral stacks from the 25 May 2009 declared nuclear test (mb 4.5; IDC). This value was checked using P-waves from seven deep (580-600 km) earthquakes (4.8 \ M w \ 5.5) in the Jilin-Heilongjiang, China region that borders with Russia and North Korea. These earthquakes are 200-300 km from the NKTS, within 200 km of the Global Seismic Network seismic station in Mudanjiang, China (MDJ) and the International Monitoring System primary arrays at Ussuriysk, Russia (USRK) and Wonju, Republic of Korea (KSRS). With the deep earthquakes, we split the t*(total) ray path into two segments: a t*(u), that represents the attenuation of the up-going ray from the deep hypocenters to the local-regional receivers, and t*(d), that represents the attenuation along the down-going ray to teleseismic receivers. The sum of t*(u) and t*(d) should be equal to t*(total), because they both share coincident ray paths. We estimated the upper-mantle attenuation t*(u) of 0.1 s at stations MDJ, USRK, and KSRS from individual and stacks of normalized P-wave spectra. We then estimated the average lower-mantle attenuation t*(d) of 0.4 s using stacked teleseismic P-wave spectra. We finally estimated a network average t*(total) of 0.5 s from the stacked teleseismic P-wave spectra from the 2009 nuclear test, which confirms the equality with the sum of t*(u) and t*(d). We included constraints on seismic moment, depth, and radiation pattern by using results from a moment tensor analysis and corner frequencies from modeling of P-wave spectra recorded at local distances. We also avoided finite-faulting effects by excluding earthquakes with complex source time functions. We assumed x 2 source models for earthquakes and explosions. The mantle attenuation beneath the NKTS is clearly different when compared with the network-averaged t* of 0.75 s for the western US and is similar to values of approximately 0.5 s for the Semipalatinsk test site within the 0.5-2 Hz range.
Pure and Applied Geophysics, 2024
Seismic 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.
Geological Society of America, 2015
A dense seismic array was deployed at a 2 km spacing to record the aftershocks of the M w (moment magnitude) 5.8 Mineral, Virginia (USA), earthquake in 2011. The three-component seismometers, installed on a 60-km-long profi le, recorded 40 aftershocks over 9 days of deployment. Based on manual picking of P-wave (primary, compressional) and S-wave (secondary, shear) arrival times of 15 aftershocks, we fi nd that the P-wave propagates with a velocity of 6.15 km/s through the upper crust, and the direct S-wave travels with a velocity of 3.66 km/s within the fi rst 20 km (Vs <20km ) and decreases slightly to 3.54 km/s (Vs >20km ) for distances >20 km. Hence, the aftershock data show a Vp/Vs ratio of 1.68 within the fi rst 20 km of hypocentral distance, and a ratio of 1.73 for distances >20 km. We attribute the small decrease in Vs with increased distance to the complex geologic setting: the recording array was deployed across the geologic boundary between the Quantico Formation and the Ta River Metamorphic Suite. Near-source attenuation of S-waves (amplitude decay with hypocentral distance R) was measured using ~1200 digital seismograms (northsouth and east-west components) from 40 aftershocks. The decay of amplitude was extracted using a nonlinear least-squares regression for different frequency bands: 1-2, 2-4, 4-8, and 8-16 Hz. For 1-2 Hz the decay can be described as a function of distance (R) as R -0.8 , for 2-4 Hz as R -0.9 , for 4-8 Hz as R -1.05 , and for 8-16 Hz as R -1.15 . The decay exponents, or b values, increase ~9%-15% from a lower to the next higher analyzed frequency band. These values are valid to a distance of as much as ~45 km from the aftershocks.
Journal of Geophysical Research, 2010
1] Surface wave dispersion measurements from ambient seismic noise and array-based measurements from teleseismic earthquakes observed with the EarthScope/USArray Transportable Array (TA) are inverted using a Monte Carlo method for a 3-D V S model of the crust and uppermost mantle beneath the western United States. The combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period, which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km. The high lateral resolution produced by the TA network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3-D model, particularly for the crustal part of the model. We show that a relatively simple model in which V S increases monotonically with depth in the crust can fit the data well across more than 90% of the study region, except in eight discrete areas where greater crustal complexity apparently exists. The regions of greater crustal complexity are the Olympic Peninsula, the MendocinoTriple Junction, the Yakima Fold Belt, the southern Cascadia back arc, the Great Central Valley of California, the Salton Trough, the Snake River Plain, and the Wasatch Mountains. We also show that a strong Rayleigh-Love discrepancy exists across much of the western United States, which can be resolved by introducing radial anisotropy in both the mantle and notably the crust. We focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by Voigt averaging. Model uncertainties from the Monte Carlo inversion are used to identify robust isotropic features in the model. The uppermost mantle beneath the western United States is principally composed of four large-scale shear wave velocity features, but lower crustal velocity structure exhibits far greater heterogeneity. We argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle, including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle. Upper and middle crustal wave speeds are generally correlated, and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions.
Bulletin of the Seismological Society of America, 1987
Strong ground motion attenuation relations are usually described by smoothly decreasing functions of distance. However, consideration of wave propagation in the crust suggests that attenuation relations should be more complex. Such complexity may be present in strong ground motion data for eastern North American earthquakes, which show amplitudes in the distance range of 60 to 150 km that lie above the trends at smaller and greater distances. Using a wavenumber integration method to compute Green's functions and close-in recordings of several earthquakes as empirical source functions, we have generated synthetic seismograms that are in good agreement with regional and strong-motion recordings of eastern North American earthquakes. From these synthetic seismograms, we have shown that the observed interval of relatively high amplitudes may be attributable to postcritically reflected S waves from the Moho. The presence and location of the interval of relatively high amplitudes is h...