1946 Aleutian Earthquake & Tsunami - 60 Years Ago Today

[David]

Sixty years ago today (April 1, 1946), an Mw 8.1 earthquake rocked the Aleutian Trench about 90 miles south of Unimak Island, where stood a new U.S. Coast Guard lighthouse, Scotch Cap, & an adjoining radio direction-finding (DF) station.



A tsunami was generated. The Aleutian Shelf is shallow (100 meters) & broad (100 kilometers), so the tsunami had to begin within one minute of the main shock. It was generated directly by the earthquake or by a submarine landslide.

Scotch Cap Lighthouse was destroyed 48 minutes after the main shock. The tsunami travel time from the shelf edge to the lighthouse is 43 minutes. As is typical of most tsunamis, the first crest was nowhere near the biggest. This was noted all around the Pacific Basin as the tsunami spread in various directions: the later waves were the killers. In Hawaii, the first deaths did not occur until wave #3.

At Scotch Cap, the tsunami surged over the coastal cliff to a height of 42 meters (135 feet) above sea level.



Immediately after the 30-second quake, the watchstander from the DF station telephoned down to the lighthouse crew to ensure everything was fine. He reported, "Say, we sure felt that one, boy! I'd swear it raised the deck right under me."

Less than a half hour later, another quake--an aftershock--hit. It was milder than the first quake, & the crews of both units probably weren't unduly surprised by it.

Several minutes after the aftershock, someone noted in the DF station log: "Terrific roaring from the ocean heard followed almost immediately by terrific sea."

All five Coast Guardsmen in the lighthouse were killed as millions of tons of seawater engulfed the cliffs & bluff, among them Chief Boatswain's Mate Anthony Lawrence Petit.



The Coast Guard Unit #368 log makes for chilling reading.

1 APRIL 1946

0130 - Severe earthquake felt. Building rocked severly. Objects shaken from locker shelves. Duration approx. 30-40 seconds. Building creaked and groaned, but no apparent damage. Weather clear and calm.

0157 - Second severe quake felt. Shorter in duration but harder than 0130. Duration 15-20 seconds. Again no apparent damage although buildings were shook severely.

0218 - Terrific roaring from ocean heard, followed almost immediately by terrific sea, top of which rose above cliff and struck station, causing considerable damages. Water in building, crew ordered to higher ground of DP (low frequency direction finder) building. Attempted to send message of condition but no answer to calls. Broadcast following message about 011325 GCT. Exact time not noted on 2072. PP NMJ NNA NNF NXD. Tidal wave-may have to abandon this place. Beleive NNHK lost, inter. RXXX, no answer. (Later found RCVG antenna carried away). Generator running erratically, and not enough power to operate TBL-13.

0245 - Forced to secure power as switchboard is afire. CO2 used and fire extinguished. (Scotch Cap Light Station beleived lost. Light extinguished and horn silent).

0300 - Gathering emergency stores and clothing to DP building. Called crew back as sea is apparently abating, to get more clothing and canned goods. Emergency lights rigged and carrying clothing and supplies to DP building.

0345 - Sea seems to be moderating. Still no wind but clouding up. Heavy roaring from ocean, but seems to be quieting down. Scotch Cap L.S., completely washed away. All hands, total loss.

0346 - Another short sharp tremor.

0400 - Cleaning up generator room and switchboard.

0551 - Power restored one engine, switchboard pumped out and transmitted 011730 to NNA. Rogered for it OK.

0700 - Looking over hill to light station again. Debris strewn along beaches. Found what was apparently human intestines on hill by water tank. Much debris.

Less than five hours after being generated in the Aleutian Trench, the tsunami hit the shores of Hawaii, including Hilo Bay, killing 159 people.



In the photo above, a tsunami advances past Coconut Island onto the Hilo Bay coast. Note that the palm trees to either side of the waves are about 40 feet tall.

Other regions around the Basin were also hit: Washington State, Oregon, California, French Polynesia & Chile. The damage & death from the 1946 tsunami were so widespread that the modern-day warning system was born. On August 12, 1948, the Seismic Sea Wave Warning System was established. The system's name would later be changed to the Pacific Tsunami Warning System (www.prh.noaa.gov/ptwc/).