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In early 1976, a 35-day cruise aboard R/V Robert D. Conrad of the Lamont-Doherty Geological Observatory was devoted to a geophysical survey of two sites in the Pacific Ocean (Survey Areas PT-4 and PT-5) for the International Phase of Ocean Drilling program (IPOD). PT-5 was surveyed between the Clarion and Molokai fracture zones near magnetic anomaly 13 on fast spreading Pacific crust near 21.5N and 126.5 °W. The geophysical measurements and interpretation of the data for this area are given in Ludwig and Rabinowitz (in press). PT-4 is located near the East Pacific Rise axis between the Siqueiros fracture zone at 8°15'N and the Clipperton fracture zone at 10°N. The evolution of ocean crust in this area has been extensively discussed (Batiza et al., 1977; Rosendahl, 1976; Rosendahl et al., 1976; Orcutt et al., 1975, 1976). The survey was designated to obtain multichannel seismic measurements as well as additional underway geophysical measurements across the East Pacific Rise in the vicinity of proposed drill sites at PT-4. The navigational course of R/V Robert D. Conrad in the vicinity of PT-4 is given in Figure 1. Multichannel seismics and underway geophysical measurements were collected on three lines which traversed the strike of the East Pacific Rise (lines 17, 19 and 20) and on one line which was parallel to it (line 18). Bathymetric, gravity, and magnetic profiles are given in Figures 2, 3, and 4, respectively. These data are shown plotted as profiles along ship tracks on the bathymetric base map given by Rosendahl (this volume). The sonobuoy results are shown as crustal sections in Figure 5. Multichannel seismic measurements processed on line 19 are given in Figure 6 (after Herron et al., 1978; Stoffa et al., in press). ACKNOWLEDGMENTS

Geophysical Measurements across the East Pacific Rise in the Vicinity of the Siqueiros Fracture Zone (Survey Area Pt-4)