Irregularities in recent records have been traced to faulty 

 demodulator chopper^ in the amplifier output circuits. It is planned to 

 replace these parts at an early date. In addition to observations of 

 orbital motion^ laboratory experiments to evaluate more accurately the 

 virtual mass of the system are being planned. 



Repeated surveys have been made of the bottom topography in the 

 vicinity of Scripps Submarine Canyon heads during the past five years. 

 Throughout this period the sand portions of the vShallow canyon heads have 

 exhibited alternate filling and deepening. In general, the changes occur 

 at the heads of the canyon near relatively steep slopes j and show no 

 apparent relationship to the seasonal cut and fill which occurs on and 

 near the local beaches in response to wave actions,* For these reasons 

 it is thought that the periodic deepenings result from mass movement of 

 sand in the form of slides from the adjacent shelf or shallow heads into 

 the deeper parts of 1he canyon, and as such represent a permanent loss of 

 sand from the littoral drift of this area. 



Two years ago a valley developed in an area where none had ex- 

 isted in previous surveys. This new valley formed on the flat shelf just 

 south of the heads of Scripps Canyon and apparently was the result of 

 a landslide which accompanied an earthquake in November 19^9 » The survey 

 following the earthquake showed the new valley to have a maximum depth 

 of approximately 20 feet below the previous shelf level and to represent 

 a displacement of about 3«5 rfiillion cubic feet of sand. 



In the t*ro years since its formation, changes in depth of approx- 

 imately 3 to 10 feet in magnitude have been observed in the new valley, 

 and until 2 January 1952 these had mostly been in the nature of fill in 

 the head of the valley in water 20 to kS feet deep. During the three 

 months since 2 January a new slide seems to have occurred near the head 

 of the valley, producing a deepening of about 6 feet. 



As mentioned in the J^rogress Report Noo 10, an earthquake of moderate 

 intensity on 25 December 195l apparently resulted in a slide in the heads 

 of Scripps Submarine Canyon, A survey on the day following the earthquake 

 indicated a loss of approximately 0,5 million cubic feet of sand from the 

 heads of Scripps Submarine Canyon but showed little change in the new 

 valley. During the past three months the heads of Scripps Canyon have shown 

 a fill, especially South Branch where the fill has amounted to as much as 

 three feet. This shoaling has brought some portions of these canyon heads 

 to the condition that existed prior to the landslide of 25 December 1951 • 



In the future an attempt will be made to estimate the volume of 

 the littxjral drift in the Scripps area from an evaluation of the amounts 

 of sand lost in the canyon and the amount bypassed around the canyon heads, 



*Shepard, F, P,, 1951, Trans, Amer, Geoph, Unxon, Vol, 32, No. 3, PP. i|05 

 I4I8. 



15 



