432 FISHER AND HESS [CHAP. 17 



deep earthquake foci for island arcs also lie on a plane dipping under the con- 

 cave side at about 45°. 



Hodgson (1957) studied the first motion of large deep-focus earthquakes of 

 the Tonga-Kerinadec and South American regions. From these he deduced 

 that the movement causing the earthquakes occurred as nearly horizontal 

 disi^lacement on nearly vertical planes. Tliis type of analysis results in defining 

 two planes at right angles to each other. Which is the real fault plane is in- 

 determinate. In either case Benioflf's postulated 40-45° tlu"ust motion is 

 ruled out. 



Mclntj^'e and Clii'istie (1957) analyzed Hodgson's results for the Tonga- 

 Kermadec region in an attempt to solve the j)roblem of Axhich was the real 

 plane. They made fabric diagrams of the poles of the fault jilanes and con- 

 cluded that the fabric which paralleled the main linear feature of the topo- 

 graphy, the volcanic ridge and trench, was the more likely. In other words, the 

 fault planes are parallel to the trench. Their reasoning is not compelling, and 

 the present authors would jirefer to choose the set of j)lanes nearly at right 

 angles to the trench. On the basis of tojjography, structural pattern, seismicity 

 and volcanic activity, Hess and Maxwell (1953) postulated a sinistral strike-slip 

 faidt from the southeast end of the New Hebrides to the northern end of the 

 Tonga Trench. Subsequently, Hodgson analyzed an earthquake lying on this 

 line and found sinistral motion : one of his planes paralleled Hess and Maxwell's 

 deduced fault, the other being perpendicular to it. Looking again at Hodgson's 

 data, one finds that sinistral fault motions tend to occur to the north and 

 dextral to the south in the Tonga-Kermadec region. The pattern thus resembles 

 the displacements shown for the line f-f on Fig. 10. For the alternative hypo- 

 thesis of Mclntyre and Christie to have both dextral and sinistral faults 

 paralleling the trench seems mechanically less likely and quite unexplained. 

 On the other hand, many great strike-slip faults of the circum-Pacific do parallel 

 the coast lines and trenches. To mention a few, there are the San Andreas, 

 Kamchatka, 1960 Chilean earthquakes, Luzon to Mindanao in the Philippines, 

 and the Alpine fault of South Island, New Zealand. These have shallow 

 earthquake foci and are perhaps unrelated to the intermediate and deep- 

 focus shocks on Benioff 's 40° plane. In conclusion, it is evident that many of the 

 problems relating seismicity to structure are unresolved. 



References 



Benioff, H., 1949. The fault origin of oceanic deeps. Bull. Qeol. Soc. Amer., 60, 1837-1866. 

 Benioff, H., 1954. Orogenesis and deep crustal structure — additional evidence from 



seismology. Bxdl. Qeol. Soc. Amer., 65, 385-400. 

 Benioff, H. 1955. Seismic evidence for crustal structure and tectonic activity. Qeol. Soc. 



Amer. Spec. Paper 62, 61-73. 

 Berryhill, H. L., Jr., R. P. Briggs and L. Glover, III, 1960. Stratigraphy, sedimentation 



and structure of late Cretaceous rocks in eastern Puerto Rico — Preliminary report. 



Bull. Amer. Assoc. Petrol. Geol., 44, 137-155. 

 Bezrukov, P., 1957. On the sediments of the Idzu-Bonin, Marianas and Ryuku deep-sea 



oceanic trenches. Doklady Akad. Nauk S.S.S.R., 114, 387-390. [In Russian.] 



