Proceedings of the 18 Annual Seismic Research Symposium on Monitoring a comprehensive Test Ban Treaty
Paul M. Davis and Shangxing Gao
Department of Earth and Space Sciences,
University of California, Los Angeles, CA90095
Contract No. F49620-94-1-0161
Sponsored by AFOSR
Teleseismic P- and S-wave attenuation have been measured using the data set that we gathered along a 1280 km profile traversing the Siberian platform, the Baikal rift zone (BRZ), and the Mongolian fold belt. We compared the commonly used spectral ratio method and a common spectrum method for the calculation of relative using synthetic data. The comparison suggests that the common spectrum method is more stable than the spectral ratio method. We used the common spectrum method to calculate and from 37 teleseismic events. It was found that the Baikal rift zone is associated with low and , with corresponding of 0.2 s and of 0.8 s relative to the Siberia platform and the Mongolian fold belt. Under the assumption that the mean is 400 and is 200 beneath the Siberian platform, we obtain a of 130 beneath the BRZ, and a of 60 beneath the BRZ. The best fitting relation between and was found to be . The difference in and implies that the observed low Q in the BRZ is probably caused by intrinsic attenuation in the upper mantle rather than by scattering.
Keywords: Attenuation, Anisotropy, Baikal, Siberia, Mongolia, Continental Rift Zone, Bayesian Inversion
In previous reports we have presented results from our ongoing study of the Baikal rift zone dealing with P wave attenuation, anisotropy, and P wave tomography (Davis and Gao, 1995). We have found the region is underlain by a low velocity zone extending from the Moho beneath the rift to depths of about 200 km and two to three hundred km wide. At almost every station SKS splitting of about 1 second has been observed with a pattern of fast directions that is variable, but shows some spatial correlation, suggesting a relation to the tectonics (Gao et al., 1994, Davis and Gao, 1995). We have reported P wave attenuation measurements along a 570 km east-west profile of seismic stations crossing lake Baikal which exhibit a relative of 0.1 seconds. In this report we present P wave attenuation estimates along a longer 1280 km NW-SE profile and obtain a larger contrast of 0.2 seconds. We have also calculated S wave attenuation along this profile. Characterizing the seismic properties of such a tectonic zone has relevance to CTBT in that nuclear tests in continental zones of high absorption can disguise yield. Also, explosions detonated in highly anisotropic regions may generate S waves making discrimination from earthquakes more difficult if not recognized.
2. Method: A Bayesian Approach for the Calculation of Seismic Body-wave Attenuation Factors
Most of the seismic wave attenuation studies used spectral ratio method (e.g. Teng, 1968; Solomon and Toksoz, 1970; Der and McElfresh, 1976; Taylor et al., 1986). In this method the ratio of spectra between two stations, station i and the reference station j is used to determine , the relative attenuation factor.^M The spectral ratio method uses a single reference record to determine . We have performed synthetic tests that indicate that in the presence of noise the results from this method can be unstable. They are strongly affected by the spectrum of the reference record which, because it appears in the denominator, can cause infinities in the division if there are nulls in the spectrum. Thus the largest values in the fit have the greatest uncertainties and weighted methods should be applied. The method requires careful choice of parameters such as spectral smoothing window, or where to truncate the spectrum when the signal to noise ratio is low. To overcome such problems, Halderman and Davis (1991) proposed a non-linear inversion procedure, called the "Common Spectrum method" (CS), using the routines in Bevington (1969), which makes use of a gradient search method. The CS method uses all the spectra from an event to simultaneously invert for t*, the receiver amplification, and a common spectrum for the event. After the seismograms have been corrected to a standard response, the spectrum recorded by the ith station from the kth event, can be expressed as
where f is frequency, is the common spectrum for the event, and are the near-receiver effects which are assumed to be frequency and source-location independent.
We performed the non-linear inversions using a Bayesian approach (Jackson and Matsu'ura, 1985) to search for , and . The Bayesian approach uses some prior information such as the starting parameters and weights the data and prior data according to their uncertainties. Data processing parameters include:
Note that no smoothing is performed; and no reference record is needed. We have conducted extensive tests on synthetic data of different signal to noise ratios and find that the Common Spectrum approach is significantly more accurate than the spectral ratio method and gives robust estimates for signal to noise ratios as low as 1.25 where the spectral ratio method breaks down.
3. Application to Baikal Data
We applied the common spectrum method to P wave and S wave data from Baikal.^M The stations analyzed were located along a 1280 km long profile traversing the Siberian platform, the BRZ, and the Mongolian fold belt; and in a 300 by 300 km cluster around the southern part of Lake Baikal (Figure 1). All the seismographs were digital and timed by the global Omega clock system. Several types of seismometers were used, most of them were three-component, short period (with free-period 0.5-2 s) sensors. The seismograms were standardized to a uniform response function with free-period 0.5 s, and damping factor 0.707.
Strong teleseismic events with direct P-wave as the first arrival were used. According to the ISC catalog, during the time of the experiment (day 180 and 265, 1992), 48 events with occurred in the distance range of from station 24, which was located on the southern shore of Lake Baikal. Among the 48 events, 37 are found to have sufficient P-wave signal to noise ratio on at least 10 of the stations to be used for P-wave attenuation studies, and 5 of them are found to be suitable for S-wave studies. The locations of the 37 events are shown in Figure 2. The events can roughly be divided into two groups. Events in the southern group are from a backazimuth range of 130-210 , and those in the northern group are in the range of -30-60 . The number of events in the southern and northern groups is 29 ^M and 8 respectively.^M Note that all the 5 events with strong S-waves are in the southern group.
The starting values for are taken as the mean spectrum for all the stations from event k. The standard deviations were set at about 30% of the peak value. The starting parameters for and their standard deviations are taken as those found by the spectral ratio method. All the starting parameters for are taken as 1.0 with . The frequency band of 0.1 - 3.0 Hz is used. Most of the energy recorded by the seismometers, which are short period sensors, is in the frequency band of 0.3 to 2.0 Hz for both P- and S-waves. In this frequency band the Q values vary slowly with period (Anderson and Given, 1982). Therefore the assumption of frequency-independent Q is close to reality.
For the calculation of both and , a time window of 12.8 s was taken starting approximately 6 s before the onset of the signal, and was tapered and Fourier transformed. A noise window of the same length was also selected before the signal window and Fourier transformed. The noise spectrum was subtracted from the signal spectrum. We found that the removal is necessary, especially for stations in the Siberian platform where a 2.09 Hz continuous seismic signal has been detected (Gao et al., 1996 in preparation).
To minimize the effects of stations contaminated by noise, a two-step procedure was used. The first step applies the method to all the stations recording a given event. The second step uses stations with misfit (reduced chi squared) smaller than a cutoff value. The misfits were normalized by the amplitude of the common spectrum and therefore a single cutoff value of the misfit is used for all the events, which is 0.05 for P-waves and 0.2 for S-waves. The reason for a larger cutoff value for S-waves is that S-wave data is nosier than P-wave data and if the same cutoff value were used, there would be only a few qualified S-wave measurements. The number of stations rejected after the first step is about 4, out of typically more than 20 stations recording the event.
4. Results and Discussion
The number of measurements for and is 596 and 82, respectively. The mean values at each station and their standard deviations are shown in Figure 3. The solid curves in Figure 3 are obtained by averaging the measurements in a window of 200 km in width and moving at 10 km each step. The peak to peak anomaly of 0.2 s for , and 0.8 s for is observed in the smoothed mean curves. The mean standard deviation for is 0.06 0.03 s, and for is 0.14 0.08 s. The and anomalies associated with the BRZ are about 6 times the standard deviation. For , the peak values are located in the area of km about Lake Baikal, for , the peak is located at -160 km (i.e., to the Northwest). Because all the events for were from the south and with an S-wave angle of incidence of about 35 , the northward shift may indicate that the depth of the center of the low-Q body is located at about 240 km.
Given that most of the lateral variation of shear wave velocities are found in the top 300 km of the Earth (e.g. Woodhouse and Dziewonski, 1984), we assume that the upper-most 300 km of the Earth beneath the array is responsible for the observed and . Under the assumption that the mean is 400 beneath the Siberian platform, the 0.2 s in results in a of 130 beneath the BRZ, and the 0.8 s in yields a of 60 beneath the BRZ if a of 200 is assumed beneath the Siberian platform.
Figure 4 shows the relation between the smoothed and . The best fitting relation is . The cross correlation coefficient between the two is 0.83. In comparison, similar measurements between the eastern and western United States have / ranges from 2.7 to 3.4 (Der et al., 1980). In addition, using short period core phases, Kanamori (1967) found . This ratio indicates that the observed is the result of difference in purely intrinsic attenuation rather than difference in scattering (Walsh, 1969; Solomon and Toksoz, 1970; Richards and Menke, 1983).
Using synthetic data,
we tested the common spectrum method using a Bayesian non-linear ^M
inversion procedure to calculate relative attenuation
factors for P- and S-waves. We found that it is more stable then the
spectral ratio method.
The common spectrum method was applied to the
recordings from 37 teleseismic events recorded by
about 60 stations in the Siberia, Lake Baikal, and Mongolia
areas in the summer of 1992.
The Baikal rift zone was found to be associated with
a of 0.2 s for P-waves and 0.8 s for S-waves relative
to the Siberian platform.
The best fitting relation between the smoothed
and is
.
The cross correlation coefficient between the two is 0.83.
This difference indicates that the observed is the result
of difference in purely intrinsic attenuation rather than difference in
scattering. The seismologic contrast between the Baikal rift zone and the Siberian craton is
in many regards similar to that between the Basin and Range rift province and east North America.
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Figure 2: An azimuthal equidistant projection map showing epicenters of events, and their back-azimuths and epicentral distances to the array. The center of the map is station 24, which was on the southern shore of the Lake. The four circles represent and epicentral distances.
Figure 3: Results of measurements. The dots are averaged values over all the events for (Figure A) and (Figure B), and the solid lines are the results of spatial averaging in windows of 200 km wide, moving at steps of 10 km. The thickness of the shaded area is 2 times the standard error of the mean.
Figure 4:
Relations between and .
Note that the values used
are from spatial averaging of the mean values (the solid curves
in Figure 3). The best fitting line and the cross correlation
coefficient are shown in the figure.