Frequency dependence of Q in the mantle underlying the shield areas of Eurasia, Part 11: analyses of long period data

Abstract

Long-period multiple S and ScS phases observed in northern Europe were analysed t o determine mantle attenuation in the 0.02 to 0.2 Liz range under the Eurasian shield. Two groups of' events were used: deep Far-Eastern earthquakes and large earthquakes near the edges of the shield areas of Eurasia. The Q of the upper mantle under the Eurasian shield region was estimated in the time domain by taking amplitude ratios and in the frequency domain by taking spectral amplitude ratios among various arrivals. Under shield regions, SS/S amplitude ratios and multiple ScS amplitude ratios give t i -2.5-3.0 s in the 0.02 to 0.20 Hz band. The results show that the uppermantle Q under this shield area is higher than the global average, but less than the Q values inferred from our studies of short-period data in the same area. Preliminary results suggest significantly higher / : under tectonic regions in the same frequency band.

On the Causes of Frequency-Dependent Apparent Seismological Q

“…The bands vary with depth (ANDERSON and GIVEN, 1982) and correlate with tectonic structures McELFRESH, 1976, 1980;DER et al, 1986). Across a broad range of surface-and body-wave frequencies from *0.01 to *100 Hz (e.g., DER et al, 1982DER et al, , 1986LEES et al, 1986;ABERCROMBIE, 1998), Q values consistently increase with frequency in a vast majority of observations. However, attenuation is also always measured indirectly and in the background of strong amplitude variations, and the difficulty of differentiating its apparent attributes from the true in situ properties is well known (DER and LEES, 1985;WHITE, 1992).…”

High-frequency

“…Several studies were performed to delineate the ranges of frequency-dependent models compatible with the available broadband data beneath North America (DER et al, 1982a;LAY and HELMBERGER, 1981;DER and LEES, 1985) and Eurasia (DER et al, 1986;LEES et al, 1986). A worldwide study of frequency dependence (CHAN and DER, 1988) complements the regional variations of Q ScS by SIPKIN and JORDAN (1979) and shows that Q in the short-period band varies in the same sense as the variations of Q ScS worldwide.…”

“…The shear loss mechanism of attenuation translates into the approximate relationship t* s = 4 t* p when the typical mantle velocities for P and S waves are considered. The approximate validity of the relationship between shear wave and P-wave attenuation makes it possible to merge and jointly interpret P-and S-wave atenuation measurements (DER et al, 1986a;LEES et al, 1986) and thus make use of the greater sensitivity of S waves to build attenuation models (DER et al, 1986b).…”

“…Assuming various limiting values of source time functions (such as delta functions, or double delta functions with opposite polarities), it can be shown that dominant periods or rise times measured from the time domain records are limited by the Q along the path (STEWART, 1984). Such arguments were used to put lower limits on Q in several studies (DER et al, 1982a(DER et al, , 1986. The study by CHOY and CORMIER (1986) has shown how such constraints and fitting of spectra can contribute to better Q estimates in time domain studies.…”

“…Note the differences between the actual and apparent t*. This is an illustration of how various types of measurements can be fitted together to construct a frequency dependent t* model (after DER et al, 1986;LEES et al, 1986).…”