approximately a half million cubic feet of sand was dislodged as a 

 result of this earthquake . An unusual occurrence was the shoaling of the 

 ridge between the two deepened canyon heads. This shoaling had a maximum 

 of 3 feet along one sounding line. This increase is decidedly above the 

 possible error of surveying . The explanation for this shoaling is un- 

 known but it is hoped in the near future to have a diving operation ■which 

 may throw some light on the subject. A survey a few days after the slide 

 had been detected ^ showed that the intersection, where the water is only 

 about 15 feet deep, had filled to the extent of 2 feet. At this point the 

 canyon depth below its surroundings was 5 feet and became 2 feet. However, 

 ab greater depths only a very slight fill was indicated during this same 

 period. At the time of the landslide no change occurred in the valley, 

 which had been opened up, apparently by an earthquake, in 194-9 » This had 

 not filled previous to the recent earthquakes 



Multi-Sock Sediment Trap 



Sand Movement - Observations of sand movement at different elevations 

 above the bottom in La Jolla Bay were nade using the multi-sock sediment trap 

 described in Progress Report No. 2. As before, the trap was lowered and 

 picked up from a DUKl'i/", Most of the stations were made at 4-0- to 85-foot 

 depths. Swimmers equipped with Aqua -lungs oriented the trap on the bottom, 

 took photographs, and checked for sand movement, ripple marks, and bottom- 

 dT/elling organisms. Repeated observations under different vrave conditions 

 have been nade novr at 4- to 5-foot depth (near breaking), about 20 feet- 

 40 feet, 60 feet, and 85 feet on fairly level sand bottom. In addition a 

 series of four observations was made in the sloping, sand -covered, bowl- 

 like head of South Branch of Scripps submarine canyon. 



The samples collected were analyzed by the settling tube method , 

 Occuri^nce of micaceous minerals vras especially nobed. Mica (bictite and 

 sericite) was found to be especially abundant in sediments from the sub- 

 marine canyon head. Median diameter of the sediments in the trap from the 

 stations on gently sloping shelf show a decrease away from shore. The 

 sands caught by the trap in the canyon head have a decidedly higher mica 

 content than those from like depths on the open shelf, under similar wave 

 conditions. Hov/ever, samples taken in the canyon head during and just 

 after a rainstorm have a mica content very much like normal shelf deposits. 

 No evidence was found of abnormal sediment transport during the rain nor 

 was any mud found in the bags. 



Wave records made by the fathometer on the DUKW were analyzed whenever 

 available to determine wave characteristics. From these values and theore- 

 tical relations, wave orbital velocities at the bottom were calculated. 

 Vftiere such records were not available, as, for instance, near the surf zone, 

 orbital velocities were calculated from observed breaker heights and periods. 

 ■fflien weight of sediment caught per hour at a given bay height above the 

 bottom is plotted against orbital velocity computed from wave theory, the 

 envelope of the point distribution conforms to expectations. HcTever, no 

 quantitative relations have been worked out as yet. 



37 



