Stevens Equipment Records Historical Information During TsunamiBy Adam Krumbein One of the longest lived products in water resource monitoring has been the Stevens’ chart recorder. In fact, the chart recorder, originally invented by Stevens’ founder and namesake J.C. Stevens in 1911, is the product that established Stevens’ leadership in the water monitoring business. It's no surprise then that some of these chart recorders have had long service lives and have recorded events of historic value. One of those events is the 1960 tsunami that had it’s origins in Chile and caused damage up the northern California coast. These records, recently discovered, are now providing researchers with valuable knowledge on tsunami characteristics. In order to monitor the effects of seiches on the harbor of Crescent City, California, the US Army Corps of Engineers had set up two gaging stations on the docks of the harbor utilizing the Stevens type A-35 water level recorder. This was a float operated system used in a stilling well 14 inches in diameter. The recorders were set to a speed of one yard per hour, giving maximum resolution of water level activity. This system was maintained by the Crescent City Harbor Master. Orville Magoon, who worked for the Corps of Engineers at the time, was in charge of the seiche project and, because of past experiences with tsunamis, had designed the stilling wells to allow the recording of such an event if it were to happen. Unfortunately his planning paid off on May 22, 1960, when the Great Chilean Earthquake caused massive tsunamis that not only battered Chile but were measurable up to 6,000 miles away in Japan. The earthquake registered 9.5 on the Moment magnitude scale, making it the strongest earthquake ever recorded. The Moment magnitude scale is largely used in place of the Richter magnitude scale for earthquakes greater than 3.5. As the Stevens A-35 recorders were already actively recording information for the seiche project, this system was ready to accurately and continuously record the event as tsunami waves reached Crescent City, which is approximately 20 miles south of the Oregon border. The tsunami was first detected at Crescent City on May 23 at 2:20 am, approximately 15 hours after the quake had occurred. The highest water level recorded was 9 hours later, when the water level reached 12.5 feet at a time where the predicted tide had only been 5.1 feet. During this time, the period of the wave was approximately 20 minutes. The tsunami signal was recorded for five days after the initial reading, leading Mr. Magoon to believe that the Pacific Ocean “rang like a bell”, with the waves being reflected off of the margins of the ocean and reflected back. At the time, most tsunami records were taken from tide gages. Some tide gages of the period would punch a hole in a chart every 15 minutes. This would give an overall measurement of tide but because there was a 15 minute gap between readings, this method could be inaccurate for recording sudden events, such as a tsunami. Due to the fact that the Stevens A-35 records continuously, and because they were already plotting data for the seiche project, Mr. Magoon believes these may be the most complete tsunami levels ever recorded. Amazingly, the records were misplaced and only recently discovered in a San Francisco records repository by Mr. Magoon with the help of Corps of Engineers. According to Mr. Magoon, the records of the 1960 tsunami are in excellent condition and are going to be digitized and analyzed by Dr. Ron Flick of the California Resources Agency. Further analysis will be undertaken by students at the Scripps Institute of Oceanography. This unique set of tsunami records should hopefully allow scientists enhance the understanding of tsunamis and their effects. Many thanks to Mr. Magoon, President of the Coastal Zone Foundation, for providing information that made this article possible. |
Tsunami: A tsunami [(t)su-’nä-mE] is a Japanese word meaning harbor wave. It was adopted into general use in 1963, and refers specifically to waves that have an extremely long wave length and period. Tsunamis are usually triggered by an underwater earthquake, volcano or other disturbance. This is not to be confused with the term “tidal wave”, as tsunamis are not formed by tidal power. Seiche: A seiche (sech) is a phenomenon that takes place in partially or fully enclosed bodies of water, such as a harbor or lake. Atmospheric effects such as wind, or seismic activity can cause the water level on one side of the body of water to rise, and the water level on the other side to drop, forming a standing wave. Because gravity always causes water to find its level, impulses are caused that will travel the length of the body of water.
|
