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Short Note |
1 Department of Earth and Planetary
Sciences
Washington University
St. Louis, Missouri 63130-4899
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Abstract |
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 Top Abstract IntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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) were estimated
for event-station paths that lie entirely within the western United States,
particularly in the southern Great Basin and surrounding areas. The
Qo
(
at 1 Hz) values
were estimated by fitting synthetic spectra to observed
Lg spectra using a genetic algorithm technique. We
have created a tomographic image of the variations of
Qo for a part of the southwestern United States.
The image shows that Qo varies between about 234
and 312 at 1 Hz, with an average of 267. The lowest
Qo occurs in the northwest part of the Basin and
Range Province, where extensional deformation has occurred since the Mesozoic.
Qo values start to increase toward the Colorado
Plateau to the east and continue to gradually increase northward and decrease
southward. We also find relatively high frequency dependence to the
values, with a 1D
mean of 
= 0.57. |
Introduction |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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Studies of Lg propagation from earthquake and
explosion sources in the Basin and Range Province have reported regional
variations in 
values. In a prior study we calculated the moment and corner frequency for 40
seismic sources
(Al-Eqabi et al., 2001).
In this article we also calculate Qo
(
at 1 Hz) and
(the frequency dependence) and relate the crustal anelastic
properties to the tectonic evolution and geology of the Basin and Range
Province. We further apply the back-projection inversion technique to obtain a
tomographic map of Qo variations in the Basin and
Range Province and its surroundings.
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Data |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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). A genetic algorithm was used to
efficiently search the parameter space and find optimum combinations of the four
parameters to produce a calculated spectrum that best fit the observed
Lg spectrum. The resulting
Qo and values along with event
information are given in
Table 1. While the genetic
algorithm is efficient in finding solutions that provide the least misfit to the
observed spectra, it must be remembered that noise in the signals and effects
from structural heterogeneities can result in a trade-off between the
attenuation values and the corner frequency. There are other factors, such as
geometric spreading, that can also provide a source of contamination for the
individual path attenuation values. However, a consistent technique has been
applied across all of the data, and so contaminating effects have been addressed
as evenly as possible, but they are always a concern.
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Figure 1 shows the ray-path distribution within the area of study. Data coverage across the Basin and Range Province and its surroundings is not uniform because the earthquakes are mostly located in the southern Basin and Range and California coastal region, and the explosions are clustered inside the NTS. The coverage is best in the southern Basin and Range, while the northeastern part of the Basin and Range is poorly sampled.
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QLg Tomography |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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| (1) |
n is a term signifying the error in measurement and
modeling of 
,
Nd is the total number of seismic records, and
Ln is the event-station path length as defined by
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| (2) |
was set to 270,
which is the median value between the lowest and the highest
Qo for all the paths. For the nth record, a
residual term 
is
calculated using
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| (3) |
, is found by back-
projecting 
into
the inverse of Qm. The residual term is redistributed back
to the cells using
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| (4) |
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Results |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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values we obtained
for this area. All values higher than the average lie to the east and north of
the NTS, while the lowest values are concentrated to the south and
southwest of the NTS. The higher
values seem to be
associated with the east-central Basin and Range and western part of the
Colorado Plateau. This pattern of
is consistent with
the distribution of Lg coda Q variations obtained
by Baqer and Mitchell (1998).
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Mitchell (1995) and Mitchell
and Cong (1998) found that
crustal Qo values of any region are directly proportional to
the length of time elapsed since the most recent crustal event that produced
substantial metamorphism and associated fluid release. They considered regions
(including the Basin and Range Province) with Q for
Lg coda
(
) values between
250 and 333 as still tectonically active. Thus the relatively low
Qo values found in this study would characterize a region
that is still tectonically or orogenically active.
While the observed range of Qo values in the Basin and Range Province indicates a region of active deformation, these variations could also be due to other factors such as intrinsic crustal attenuation, increased relative stability and age of volcanism across the Basin and Range Province, and variations in the accumulated sediment thickness, permeability, and age. Two other competing mechanisms may also have caused enhanced attenuation of seismic energy in the Basin and Range Province. One possibility is the presense of interstitial fluids, due to either reactions at depth caused by high geothermal gradients or the circulation of fluids from shallow sources driven by the geothermal gradient (Wyllie, 1988). The second explanation for enhanced attenuation in the Basin and Range Province associates the region's large- scale extension, higher-than-normal heat-flow values, elevated upper-mantle temperatures, and abundant intrusive and extrusive igneous activity during the Cenozoic with the presence of fluid inclusions and partial melt.
Substantial frequency dependence of attenuation is also observed in the area
as indicated by the values
(Table 1). The
values predominantly vary between 0.4 and 0.8 across the region, with an average
of 0.57 and most falling in the range of 0.40 to 0.62. Studies have associated
low values (<0.3) with shields and higher values with
tectonically active regions
(Mitchell, 1995; Baumont et al., 1999).
Our higher values, suggesting a significant frequency-dependent
attenuation of 
,
are therefore expected since the region is tectonically active.
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Discussion and Conclusions |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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and the frequency dependence ) as well as corner frequency and
magnitude
(Al-Eqabi et al., 2001).
These 
values have
been inverted for the crustal attenuation structure of a portion of the Basin
and Range region.
While uncertainties in the attenuation measurements prevent a direct tectonic
interpretation for each anomaly on the map, the major trends of the inversion
are likely robust, and agree well with previous studies. The
values for
individual paths vary between 140 and 350 for frequencies from 0.3 to 10 Hz, and
when inverted, the mean value for our
map (267) agrees
very well with previous estimates, which are listed in
Table 2. For instance, the
study of Benz et al.
(1997) obtained a
Q-value of 235 ± 11 for the Basin and Range Province. Several
studies have determined Q for Lg waves
(
) or coda
(
) in the Basin and
Range Province. Reported
and
values vary
between 138 and 774, but most studies put the Q-values between 200 and
280. In addition, the significant southwest–northeast trend in increasing
Q, ranging between 234 and 312, agrees with the previous work of Baqer
and Mitchell (1998).
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We tested the success of the tomographic attenuation model in fitting the amplitude data by examining the reduction of the variance of the individual data. We first computed the variance of the data relative to a uniform model with Q = 267 everywhere (summing the squares of the differences between the individual path Q–1 values and the averaged Q–1 = 1/267). We then determined Q–1 for each path through the tomographic model, by summing Q–1 for each block segment along a path, and computed the variance of the data path Q–1 values and the tomographic-model Q–1 values. Using the Q–1 values computed from the tomographic model reduced the variance of the data by 11% to a level of 89% of the variance that occurred just using the average Q-value of 267.
The majority of values fall between 0.4 and 0.8
(Table 1), suggesting a strong
frequency-dependent attenuation in the Basin and Range Province. The low
Q-values also indicate a region that is still tectonically active. The
lowest Qo values occur in the south and southwest part of
the Basin and Range Province where active tectonic deformation is occurring
today. Higher values occur in the east-central Basin and Range and the western
margin of the Colorado Plateau. The areas with high Qo
values underwent volcanism between 17 and 55 m.y.a., while volcanism in low
Qo regions occurred less than 17 m.y.a.
(Blackwell, 1978).
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Acknowledgments |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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Manuscript received May 6, 2004
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References |
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TopAbstractIntroductionDataQLg TomographyResultsDiscussion and ConclusionsAcknowledgmentsReferences |
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Al-Eqabi, G. I., K. Koper, and M. E. Wysession (2001).
Source characterization of Nevada Test Site explosions and western U.S.
earthquakes using Lg waves: implications for regional source
discrimination, Bull. Seism. Soc. Am.91
,140
–153.
Atkinson, G. M., and R. F. Mereu (1992). The shape of
ground motion attenuation curves in southeastern Canada, Bull. Seism.
Soc. Am. 82,2014
–2031.
Baqer, S., and B. J. Mitchell (1998). Regional variation of Lg coda Q in the continental United States and its relation to crustal structure and evolution, Pure Appl. Geophys. 153,613 –638.[CrossRef]
Baumont, D., A. Paul, S. Beck, and G. Zandt (1999). Strong crustal heterogeneity in the Bolivian Altiplano as suggested by attenuation of Lg waves, J. Geophys. Res. 104,20287 –20305.[CrossRef]
Benz, H. M., A. Frankel, and D. M. Boore (1997).
Regional Lg attenuation for the continental United States,
Bull. Seism. Soc. Am.87
,606
–619.
Blackwell, H. M. (1978) Heat flow and energy loss in the western United States, in Cenozoic Tectonics and Geophysics of the Western Cordillera, R. B. Smith and G. P. Eaton (Editors), Geol. Soc. Am. Memoir, Vol. 152,175 –208.
Chael, E. (1987). Spectral scaling of earthquakes in the
Miramichi region of New Brunswick, Bull. Seism. Soc. Am.77
,347
–365.
Chavez, D. E., and K. F. Priestley (1986). Measurement of frequency dependent Lg attenuation in the Great Basin, Geophys. Res. Lett.13 ,551 –554.[ISI][GeoRef]
Cong, L., and B. J. Mitchell (1998). Lg coda Q and its relation to the geology and tectonics of the Middle East, Pure Appl. Geophys.153 ,563 –585.[CrossRef]
Mitchell, B. J. (1991). Frequency dependence QLg and its relation to crustal anelasticity in the Basin and Range Province, Geophys. Res. Lett.18 ,621 –624.[ISI][GeoRef]
Mitchell, B. J. (1995). Anelastic structure and evolution of the continental crust and upper mantle from seismic surface wave attenuation, Rev. Geophys.33 ,441 –462.[CrossRef]
Mitchell, B. J., and L. Cong (1998). Lg coda Q and its relation to the structure and evolution of the continents: a global perspective, Pure Appl. Geophys. 153,655 –663.[CrossRef]
Singh, S. K., and R. B. Herrmann (1983). Regionalization of crustal coda Q in the continental United States, J. Geophys. Res. 88,527 –538.[CrossRef][ISI]
Wyllie, P. J. (1988). Magma genesis, plate tectonics, and chemical differentiation of the Earth, Rev. Geophys.26 ,370 –404.
Xie, J., and B. J. Mitchell (1990a). A back-projection method for imaging large-scale lateral variations of Lg coda Q with application to continental Africa, Geophys. J. Int. 100,161 –181.[CrossRef]
Xie, J., and B. J. Mitchell (1990b). Attenuation of multiphase surface waves in the Basin and Range Province, part I, Lg and Lg coda, Geophys. J. Int. 102,121 –137.[CrossRef]
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from
tomographic inversion of Lg amplitudes. Darker shades
represent areas of relatively high
, and lighter
shades show areas of low
, which probably
indicates high crustal temperatures. The shades and
contour lines are
superimposed on a topography gradient map; dashed boundaries distinguish various
tectonic provinces.