NUREG/CR-1655 UCRL-53004 XA04NO347 fNIS-XA-N--048 The Effect of Regional Variation of Seismic Wave Attenuation on the Strong Ground Motion from Earthquakes D. H Chung and D. L. Bernreuter Prepared for U.S. Nuclear Regulatory Commission I-AMIRENCE UVERMORE LABORATORY NOTICE This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party's use, or the results of such use, of any information, apparatus product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. Available from GPO Sales Program Division of Technical Information and Document Control U. S. Nuclear Regulatory Commission Washington, D. C. 20555 Printed copy price: 425 and National Technical Information Service Springfield, Virginia 22161 NUREG/CR-1655 UCRL-53004 RM, RD, RA, R6 The Effect of Regional Variation of Seismic Wave Attenuation on the Strong Ground Motion from Earthquakes Manuscript Completed: July 1981 Date Published: October 1981 Prepared by D. H. Chung and D. L. Bernreuter Lawrence Livermore Laboratory 7000 East Avenue Livermore, CA 94550 Prepared for Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, D.C. 20555 NRC FIN No. A0139 Availability of Reference Materials Cited in NRC Publications Most documents cited in NRC publications will be available from one of the following sources: 1 . The NRC Public Document Room, 1717 H Street., N.W. Washington, DC 20555 2 The NRC/GPO Sales Program, U.S. Nuclear Regulatory Commission, Washington, DC 20555 3. 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ABSTRACT Attenuation is caused by geometric spreading and absorption. Geometric spreading is almost independent of crustal geology and physiographic region, but absorption depends strongly on crustal geology and the state of the earth's upper mantle. Except for very high frequency waves, absorption does not affect ground motion at distances less than about 25 to 50 km. Thus, in the near-field zone, the attenuation in the eastern United States is similar to that in the western United States. Beyond the near field, differences in ground motion can best be accounted for by differences in attenuation caused hy differences in absorption. The stress drop of eastern earthquakes may be higher than for western earthquakes of the same seismic moment, which would affect the high-frequency spectral content. But we believe this factor is of much less significance than differences in absorption in explaining the differences in ground motion between the East and the West. The characteristics of strong ground motion in the conterminous United States are discussed in light of these considerations, and estimates are made of the epicentral qround motions in the central and eastern United States. iii CONTENTS Abstract . Figures . vi Tables . vii Foreword . ix Glossary of Symbols . xi Executive Summary . 1 Introduction . 3 Evidence for Regionalization . 7 Evidence from P- and S-Wave Velocities . 7 Evidence from Surface-Wave Studies . 14 Implications of Regionalization: Discussion . 23 Strong Ground Motion Characteristics in the Conterminous United States . 32 Inferring EUS Attenuations: Four Possible Methods . 33 Method 1 . 36 Method 2 . 36 Method 3 . 37 Method 4 . 38 Examples and Discussion . 39 Estimates of the Epicentral Ground Motion in the Central and Eastern United States . 46 Approach and Results . 47 Comparison with Nuttli's Analysis . 55 Appendix: Published Attenuation Functions . 57 References . 65 v FIGURES 1. Typical ground motion attenuation . 4 2. The Pn-velocity distribution in the conterminous United States . 8 3. Contour map of mean teleseismic P-wave station corrections for the conterminous United States . 9 4. Seismic-wave transmission characteristics in North America . 12 5. Seismograms along three profiles from the Salmon nuclear explosion . 15 6. Comparison of the upper-mantle Q models for the eastern and western United States . 21 7. Contours of upper mantle a from Pn paths in the conterminous United States . 22 8. Peak particle velocity as a function of distance from a salvo-type underground nuclear explosion source (Buggy) . 25 9. Attenuation of vertical ground velocity from the 1971 San Fernando earthquake . 26 10. Comparison of vertical components of velocity from earthquakes in the western United States with magnitudes between 4.5 and 5.8 . 27 11. Variation of amplitude ratio with epicentral distance for three frequencies . 31 12. The intensity attenuation relationships for five historical events in the EUS . 40 13. A comparison between the results of the four attenuation models, for an event with b = 65 . 42 Vi 14. Measured peak accelerations from the Rio Blanco event as a function of distance . 49 15. Estimated peak horizontal acceleration for the 1811-1812 New Madrid earthquake series, based on the 1968 southern Illinois event . 53 TABLES 1. Values for the constants A. in Eq. 14) . 39 1 2. Peak ground accelerations (in cm/s2) calculated for three body-wave magnitudes, using methods 3 and 4 . 44 3. Attenuation coefficients for selected events . 54 vii FOREWORD This report was prepared as part of Phase I of the Seismic Safety Margins Research Program at the Lawrence Livermore National Laboratory. It was supported by the U.S. Nuclear Regulatory Commission under a memorandum of understanding with the U.S. Department of Energy. ix GLOSSARY OF SYMBOLS a Wave amplitude; exponential constant in Eq. 12) A Wave amplitude in Eq. 12) ah Peak horizontal ground acceleration C(z) Factor to account for the variation of Q with depth f Wave frequency GMP Ground motion parameter I0 Epicentral intensity Is Site intensity L Length of salvo explosion array M Generalized earthquake magnitude mb Body-wave magnitude ML Local magnitude PGA Peak ground acceleration PGV Peak ground velocity PSA Peak spectral amplitude Generalized seismic quality factor a Seismic quality factor for P-waves Seismic quality factor for S-waves r Hypocentral distance R Epicentral distance; hypocentral or epicentral distance in Eq. 4) s Ray path coordinate (p. 11) S Factor to account for geometric spreading V Group velocity z Depth a Compressional wave velocity Shear-wave velocity Coefficient of anelastic attenuation Epicentral distance Exponent in attenuation equation on page 5 W Angular frequency xi EXECUTIVE SUMMARY Attenuation is the decay in the amplitude of earthquake motion--motion generated by sudden release of the earth's strain about the earthquake source. The key factors influencing attenuation are the source conditions, the transmission path characteristics, and the local site conditions. The source conditions influence the initial level and frequency content of the motion. The transmission path characteristics influence the path length and the rate of amplitude decay. The local site conditions further influence the level and frequency content of the motion. The attenuation of ground motion from an earthquake is complex in nature, owing in part to the complexity of the ground motion, which consists of a combination of harmonic motions of varying frequency. The frequency dependency of typical ground motion attenuation shows that higher frequency motions attenuate faster than lower frequency motions. The composition of frequencies and the percentage of strong motion caused by different waves i.e., P waves, SH and SV waves, surface waves, etc.) undergo changes along the transmission path. During