Crustal Structure and Surface-wave Dispersion. Part Ii Solomon Islands Earthquake of July

RAYLEIGH waves from the Solomon Islands earthquake of July 29, 1950, recorded at Honolulu, Berkeley, Tucson, and Palisades are analyzed. Both the direct waves and those propagated through the Antipodes were observed for all stations except Honolulu. Application of a correction for land travel results in a dispersion curve for the oceanic portion of the path. It is found that the observed dispersion could be accounted for by propagation through a layer of water 5.57 km. thick overlying simatic rocks having shear velocity 4.56 km/sec. and density 3.0 gm/cc. Basement structure in the Pacific, Indian, South Atlantic, and North Atlantic oceans is ideptical within the limits of accuracy of the method. The sinusoidal nature and duration of the coda is explained by the effect of the oceans on the propagation of Rayleigh waves. The results are compatible with seismic refraction measurements in the Atlantic and Pacific oceans. INTRODUCTION IN A previous paper the authors reinterpreted published readings of travel times of surface waves of various periods in the light of their theoretical treatment of the effect of the ocean water upon Rayleigh wave propagation.2 The data of Wilson and Baykal3 and Bullen and DeLisle were used. It was concluded that the observed Rayleigh-wave dispersion could be explained by a layer of unconsolidated sediments about 0.7-1.3 km. thick, which was considered as an addition to the water depth, underlain by a considerable thickness of rock in which the velocities of shear and compressional waves were about 4.45 and 7.71 km/sec. respectively, assuming Poisson's constant to be 0.25. In Part I it was pointed out that the very small dispersion found by Wilson for suboceanic Love waves indicated that these waves were propagated in a layer with elastic properties differing so slightly from those of the substratum that the rocks could be treated as a unit for Rayleigh-wave propagation, permitting use of theoretical curves already available. Recent seismic refraction measurements in the Atlantic7 and in the Pacific8 give a sediment thickness of the order used in the dispersion study, and give a basement layer about 5 km. thick with compressional wave velocity about 6.8 km/sec. overlying rock having a Manuscript received for publication September 28, 1951. * The research reported in this paper has been sponsored by the Ge'bphysics Research Division of the Air Force Cambridge Research Center under Contract No. AF 19(122)-441. 1 Maurice Ewing and Frank Press, "Crustal Structure and Surface-Wave Dispersion,'' Bull. Seism. Soc. Am., 40: 271-280 (1950). 2 Frank Press, Maurice Ewing, and Ivan Tolstoy, "The Airy Phase of Shallow-Focus Submarine Earthquakes," Bull. Seism. Soc. Am., 40: 111-148 (1950). 8 J. T. Wilson and Orhan Baykal, "Crustal Structure of the North Atlantic Basin as Determined from Rayleigh Wave Dispersion," Bull. SeismSoc. Am., 38: 41-53 (1948). 4 K. E. Bullen, "On Rayleigh Waves across the Pacific Ocean," Mon. Not. Roy. Astron. Soc. Geophys. Suppl., 4: 579-582 (1939); J. F. DeLisle, "On Dispersion of Rayleigh Waves from th~ North Pacific Earthquake of November 10, 1938," Bull. Seism. Soc. Am., 31: 303-307 (1941). 5 See note 1 above. 6 J. T. Wilson, "The Love Waves of the South Atlantic Earthquakes of August 38, 1933," Bull. Seism. Soc. Am., 30: 273-301 (1940). 7 C. B. Officer, Jr., Maurice Ewing, Paul C. Wuensche!, "Seismic Refraction Measurements in the Atlantic Basin. Part II,'' presented at the Los Angeles meeting of the Seism. Soc. Am. 1951. 8 Russel W. Raitt, "Seismic-Refraction Studies of the Pacific Ocean Basin" presented at Los Angeles meeting of the Seism. Soc. Am., 1951. '