Measurement of frequency dependent L<sub>g</sub> attenuation in the Great Basin

S U M M A R YA combined inversion/fonvard modelling procedure, in which the frequency dependence of shear-wave internal friction (Q;') is allowed to vary with depth, was developed and applied t o selected Rayleigh wave and Lg attenuation data in the Basin and Range Province of the south-western United States. Both Q and the frequency dependence of Lg waves were used to constrain the models. Many models can explain Rayleigh wave and Lg data sets within their uncertainties, but at 1 Hz most have low values of Q, (50-80) in the upper 8 km of the crust, rapidly increasing values to about 1000 at mid-crustal depths, and decreasing values at greater depths. Models which include a layer of higher Q values in the upper few kilometres of the crust, overlying a region of lower Q values, cannot be precluded by the attenuation data of this study. Assuming that Q, varies with frequency as f <, models for which the frequency dependence is low ( 5 = 0.0-0.1) in the upper crust best explain the data of this study. In the lower crust that frequency dependence is not well determined, but the models which best explain both the fundamental-mode and Lg data and produce realistic models of Q, are characterized at 1 Hz by high values of both Q, and its frequency dependence. Because of that frequency dependence Q, may be an order of magnitude lower at a period of 100 s (~1 0 0 ) than it is at 1 s (=lOOO). Investigations of the effects of changing crustal velocity on values for Lg Q and its frequency dependence indicate that realistic velocity changes cause only small changes in those values and thus are inconsequential to our results.The low Q, values, and their constancy with frequency, in the uppermost crust can be explained by fluid flow in a network of cracks in brittle rock. Increasing Q, with depth to 10-15 km can be explained by the closing of those cracks due to pressure and their enhanced healing with increasing temperature. Plastic flow at greater depths may contribute further to the dissipation of cracks and to further increases in Q,. Decreasing Q, values at greater depths can be explained as being the result of increasing temperature, increasing content of partial melt, enhanced dislocation motion or some combination of these effects.

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“…which are lower than for stable regions (e.g. Singh & Herrmann 1983;Peseckis & Pomeroy 1984;Chavez & Priestley 1986;Nuttli 1986). …”

Section: Previous Crustal Q Studies Inmentioning

confidence: 95%

Attenuation of multiphase surface waves in the Basin and Range Province-III. Inversion for crustal anelasticity

Mitchell

Xie

1994

Geophysical Journal International

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“…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…”

Section: Two-station (Ts)mentioning

confidence: 99%

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

Ford

Dreger

Mayeda

et al. 2008

Bulletin of the Seismological Society of America

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The determination of regional attenuation Q -1 can depend upon the analysis method employed. The discrepancies between methods are due to differing parameterizations (e.g., geometrical spreading rates), employed datasets (e.g., choice of path lengths and sources), and the methodologies themselves (e.g., measurement in the frequency or time domain). Here we apply five different attenuation methodologies to a Northern California dataset. The methods are: (1) coda normalization (CN), (2) two-station (TS), (3) reverse two-station (RTS), (4) sourcepair/receiver-pair (SPRP), and (5) coda-source normalization (CS). The methods are used to measure Q of the regional phase, Lg (Q Lg ), and its power-law dependence on frequency of the form Q 0 f η with controlled parameterization in the well-studied region of Northern California using a high-quality dataset from the Berkeley Digital Seismic Network. We investigate the difference in power-law Q calculated among the methods by focusing on the San Francisco Bay Area, where knowledge of attenuation is an important part of seismic hazard mitigation. This approximately homogeneous subset of our data lies in a small region along the Franciscan block. All methods return similar power-law parameters, though the range of the joint 95% confidence regions is large (Q 0 = 85 ± 40; η = 0.65 ± 0.35). The RTS and TS methods differ the most from the other methods and from each other. This may be due to the removal of the site term in the RTS method, which is shown to be significant in the San Francisco Bay Area. In order to completely understand the range of power-law Q in a region, it is advisable to use several methods to calculate the model. We also test the sensitivity of each method to changes in geometrical spreading, Lg frequency bandwidth, the distance range of data, and the Lg measurement window. For a given method, there are significant differences in the power-law parameters, Q 0 and η, due to perturbations in the parameterization when evaluated using a conservative pairwise comparison. The CN method is affected most by changes in the distance range, which is most probably due to its fixed coda measurement window. Since, the CS method is best used to calculate the total path attenuation, it is very sensitive to the geometrical spreading assumption. The TS method is most sensitive to the frequency bandwidth, which may be due to its incomplete extraction of the site term. The RTS method is insensitive to parameterization choice, whereas the SPRP method as implemented here in the time-domain for a single path has great error in the power-law model parameters and η is strongly affected by changes in the method parameterization. When presenting results for a given method it is best to calculate Q 0 f η for multiple parameterizations using some a priori distribution.

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“…Studies have been conducted in Nevada [e.g., Chavez and Priestley, 1986] and Marshall et al [1979] have utilized measure ments of the velocity of propagation of P n in the Kazakh region to infer that the upper mantle attenuation is low compared to that in Nevada and is more typical of that in shield areas. It has been suggested by several au thors that because of this low attenuation, the yield versus m b relationship for nuclear ex plosions is considerably different for KTS than it is for NTS [Marshall et al, 1979;Sykes et al, 1983].…”

Section: • Determinationmentioning

confidence: 99%

A New U.S.‐U.S.S.R. Seismological Program

Berger¹,

Brune²,

Bodin³

et al. 1987

EoS Transactions

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On July 9, 1986, a team of researchers from the University of California, San Diego; University of Nevada, Reno; and the University of Colorado, Boulder established the first of three seismic stations to be located in the vicinity of the Soviet nuclear test site in eastern Kazakhstan (KTS) (see cover). Under an agreement reached between the Soviet Academy of Sciences and the Natural Resources Defense Council, a nonprofit U.S. environmental organization, these stations, which are configured to meet the specifications of the proposed new global seismographic network [Incorporated Research Institutions for Seismology (IRIS), 1984], will be complemented by three similarly equipped stations to be installed in the vicinity of the U.S. nuclear test site in southern Nevada (NTS). The stations are to be operated cooperatively by Soviet and U.S. personnel (Figure 1).

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Measurement of frequency dependent L_g attenuation in the Great Basin

Cited by 55 publications

References 8 publications

Attenuation of multiphase surface waves in the Basin and Range Province-III. Inversion for crustal anelasticity

Attenuation of multiphase surface waves in the Basin and Range Province-III. Inversion for crustal anelasticity

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

A New U.S.‐U.S.S.R. Seismological Program

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Measurement of frequency dependent Lg attenuation in the Great Basin

Cited by 55 publications

References 8 publications

Attenuation of multiphase surface waves in the Basin and Range Province-III. Inversion for crustal anelasticity

Attenuation of multiphase surface waves in the Basin and Range Province-III. Inversion for crustal anelasticity

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

A New U.S.‐U.S.S.R. Seismological Program

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Product

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Measurement of frequency dependent L_g attenuation in the Great Basin