Attenuation of multiphase surface waves in the Basin and Range province, part I:LgandLgcoda

Abstract

S U M M A R Y Broad-band data recorded by the Lawrence Livermore Laboratory Network are used to investigate whether or not Lg Q and Lg coda Q have similar values in the Basin and Range province of North America. In order to retrieve spectral estimates of the broad-band Lg and Lg coda signals, a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies. A stochastic approach is used to model the Lg signal and ambient noise, and to analyse results from two-station determinations of Lg Q in the Basin and Range. The effects of ambient noise, frequency-dependent station site response and focusing/defocusing on Lg Q inversion are analysed. It is shown that after averaging spectral ratios determined from repeated observations over four two-station paths, a modest site effect will not cause significant errors in the resulting Lg Q relation. Using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0.2 and 2.5 Hz, we obtain the relation Q L g ( f ) = (267 f 56)f0.37*0.06 for the Basin and Range province. Further constraints on Lg Q were obtained from Lg coda, which, due to the natural averaging effect of scattering, is insensitive to effects of focusing/defocusing. To investigate the validity of the single isotropic scattering model for Lg coda in the Basin and Range province, we calculated ischrons for scatterers corresponding to later Lg coda signals, as required by the single scattering model. It is found that when significant portions of these isochrons are oceanic, Lg coda amplitude decay becomes irregular and abnormally slow, suggesting a strong scattering/reflection effect caused by the rapidly changing crustal structure along the coast. Applying the single isotropic scattering model to Lg coda records with isochrons mostly confined to the continental crust results in Q , ( f ) = (275 f 26) f 0.36*0.03 , which is nearly identical to our QLg relation. This indicates that even in a complicated geological province, a joint interpretation of the dynamic properties of Lg and Lg coda can be successful and fruitful. Inconsistent Lg Q and coda Q relations found in earlier studies in the Basin and Range province may be attributed to different settings of parameter trade-offs, different seismic phases, or different dynamic properties being used.

Analysis of source spectra, attenuation, and site effects from central and eastern United States earthquakes

“…NUREGICR-6564 (Nuttli, 1981;Singh and Herrmann, 1983;ChavCz and Priestley, 1986;Xie and Mitchell, 1990a Nuttli's (1973) results, while Bollinger (1979) found 1 Hz Lg attenuation in the southeastern United States to be consistent with the results of Nuttli (1973).…”

Can We Improve Estimates of Seismological Q Using a New “Geometrical Spreading” Model?

“…Numerous previous studies used high-frequency amplitude datasets to successfully fit a power-law frequency dependence of Q(f) (for summaries, see ANDERSON, 1989;XIE and MITCHELL, 1990;MITCHELL, 1991MITCHELL, , 1995 Qðf…”

“…Individual measurements of path-specific Q, or tomographic inversions of laterally varying Q models, are typically published with discussions of the measurement error or uncertainty (e.g., MITCHELL, 1995;XIE et al, 2004;PHILLIPS et al, 2005;ROMANOWICZ and MITCHELL, 2007;PASYANOS et al, 2009). The vast Q measurements and tomographic Q models collectively reveal two main features of Q in the Earth's crust and mantle: it is highly laterally variable (by a factor of 10 as compared to the velocity variation which is typically less than 20-30%), and often frequency dependent (e.g., DER et al, 1986DER et al, , 1987ANDERSON, 1989;MITCHELL, 1991;XIE and MITCHELL, 1990;MITCHELL and XIE, 1994;WIENS 1994, 1998;SOBOLEV et al, 1996;XIE, 1998;XIE et al, 2004XIE et al, , 2006GUNG and ROMANOWICZ, 2004;SELBY and WOODHOUSE, 2002;WARREN and SHEARER, 2002;SHITO et al, 2004;PHILLIPS et al, 2005;DALTON and EKSTROM, 2006;LEKIC et al, 2009;PASYANOS et al, 2009, also see summaries by ANDERSON, 1989;FLANAGAN and WIENS, 1994;MITCHELL, 1995;ROMANOWICZ, 1998;ROMANOWICZ and MITCHELL, 2007;SATO and FEHLER, 2009). These features have led to inferences of variations of temperature and occurrence of melting in the crust and upper mantle, variations of pore fluid content and other forms of small-scale crustal structure heterogeneities.…”

Regional Attenuation in Northern California: A Comparison of Five 1D Q Methods

“…The TS method takes the ratio of Lg recorded at two different stations along the same narrow path from the same event in order to remove the common source term (e.g., Chavez and Priestley, 1986;Xie and Mitchell, 1990). We implement this method in the frequency domain and take the ratio of two terms with the form of equation (1), which gives…”