Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

STRUCTURAL GEOLOGY. 33 in these upward and downward movements, and this has happened. Great fracture planes cut the rocks and trend mostly northeast-south- west, or approximately parallel with the axis of the land mass, with some minor fractures leading in other directions. Along these frac- ture planes differential movements, some of which measure hundreds of feet, have taken place. The frictional drag of these movements has crushed and broken the softer rocks for several feet on each side of the plane of motion. This faulting has had a most important bearing on engineering, because the faulted zones, especially where the rocks are extensively- crushed and broken, have tended to promote slides. The present faults date from Miocene time or later, and most of them are prob- ably of Pleistocene age. At any rate, so far as the digging of Culebra Cut is concerned, these faults have increased the cost of the canal several millions of dollars. The relation of the faults and sheared zones to the slides is fully discussed elsewhere in this report. The chief faults trending across the canal are shown in figure 1 and Plates XV and XVI, but there are many more than those indicated there. SUBMARINE ESCARPMENT AT PANAMA BAY. Panama Bay is shallow out to its mouth. Beyond that the depth increases from 100 to over 1,000 fathoms in a short distance. Thia deepening is indicative of a great submarine fault escarpment passing just in front of Panama Bay. That there is an important fault escarpment there is established by the following facts: On October 1, 1913, a considerable earthquake shocked Panama. It was especially severe in the southern part of Las Santos Province, where it seriously damaged several buildings. With the beginning of this earthquake the submarine cable from Panama City up the west coast broke. It parted just where it passes from the relatively shallow water of Panama Bay to the deep water beyond. When the break was located by the repair ship it was found that half a mile of one of the broken ends, which lay along near the foot of this sub- marine escarpment, was completely covered by débris and had to be abandoned and a new piece spliced in to take its place. It is, there- fore, evident that the cable broke as the result of differential move- ment along this old fault plane, and that this movement caused the earthquake, and jarred down the submarine material that buried half a mile of the broken end. About 1882 an earthquake shock, accompanied by the breaking of the cable, occurred in about the same way and at approximately the same place. 97348°—Bull. 86—15--3