second mode period (lT.8 minutes) for the entire Valdez embayment ; also 

 that the fundamental period (39 minutes) for the embayment is within 

 response range of the second and third mode periods (respectively U8 and 

 30 minutes) of oscillation of the Sound. These circumstances are con- 

 sidered to have a bearing on the development of waves at Valdez after the 

 earthquake, as portrayed in the inferred marigram for Valdez (Figure 165). 



Our interpretation of what happended at Valdez is based on eyewitness 

 accounts, published literature, and photographic evidence drawn from 

 motion picture films taken by two crew members aboard the S. S. Chena , a 

 cargo vessel of about 11,000 gross tons, which was moored at the north 

 pier of the harbor when the earthquake occurred. 



The suggested explanation for what happened is that during the in- 

 tense shaking of the earthquake in a north-south direction (in which 

 direction there was also a 25-foot displacement of the land southward), 

 a seismic seiche was generated transverse to Port Valdez along the Valdez 

 waterfront. The mechanism for generation of this seiche could have been 

 a submarine slump of the fine sediments off the glacial delta and outwash 

 at the mouth of the Lowe River in the extreme southeast corner of Port 

 Valdez, or it could have been the movement of the land, or both, acting 

 together. 



The Chena responded to the initial northwest riodal surge of water by 

 pulling away from the dock. The docks at the same time began to settle 

 as violent ground shaking produced consolidation of the sediments and 

 incipient slippage of the delta slope. The returning "backlash" wave 

 slammed the Chena on the wreckage of the collapsing docks of the North 

 Arm where she was momentarily caught before being swept into the small- 

 boat harbor (see Figures 166 and 16t). This initial wave is believed to 

 have been, possibly, hO to 50 feet high and of H to 5 minutes periodicity. 

 It carried away canneries and facilities on both the North and South Arms 

 of the harbor in the same southeasterly direction as the Chena. 



The Chena was temporarily aground in the wreckage of the boat harbor 

 before the return flow (in the northwest direction) lifted and freed her, 

 and enabled her to escape under power. The next reversal of the flow to 

 the southeast, however, after the earthquake had ceased, frustrated the 

 Chena' s bid for deep water and carried her, virtually out of control, at 

 jetlike speed in a southeasterly direction alongshore (Figure 166). It 

 is at this stage, according to the photographic evidence (see Figure 173) 

 that the major submarine slide took place south of Valdez, creating a 

 30- to 50-foot high scarp along the shore over which water cascaded into 

 the depression. It was toward this that the Chena herself was being 

 pulled by the drawdown. The temporary mounding of sediments from the 

 submarine slip is presumed to have caused the wall of water, observed to 

 starboard (seaward) of the Chena at this time. As the depression of water 

 level filled, the Chena finally escaped, miraculously unscathed, to the 

 safety of deep water. Presumably also the rapid dispersal of the sub- 

 marine mound of sediment acted to dissipate any serious wave formation. 

 All this occurred within about 6 or 7 minutes of the onset of the 

 earthquake. 



359 



