Our very own Captain Ray Wichmann was on the water at the exact moment the tsunami from the aftermath of the earthquake in Chili made its way into the SF Bay. In his monthly piece for Bay Crossings, Captain Ray describes thenoticeable effect of the tsunami on his sailboat in the Berkeley Marina. Enter Captain Ray…
SHAKE, RATTLE, & ROLL
On Saturday, 27FEB10, a large earthquake — magnitude 8.8 – occurred in Chile. It was the seventh largest earthquake ever recorded. The epicenter was about five miles west of the coastal town of Curanipe and about 22 miles below the surface of the Pacific Ocean. It struck at 3:34 am Chilean time (11:34 pm PST the evening before). Large earthquakes are not unusual in Chile. The largest earthquake ever recorded, with a magnitude of 9.5, occurred there in 1960. Earthquakes with a magnitude of 8.0 occurred in 1985 and 1995.
The cause of all this turmoil is the area’s unique geology. The solid, rocky, outer layer of the Earth is broken into a dozen or so plates that float about (albeit slowly) on the Earth’s semi fluid interior. One of the smaller of these plates – about the size of the United States – is located under the Pacific Ocean, off the coast of Chile. Known as the Nazca plate, it is moving east and pushing itself under the South American plate. As the Nazca plate dives under the edge of the South American plate, the western edge of the South American plate is being wrinkled. The amount of movement each year may only be about one inch, but it has been going on for millions of years. The wrinkles created by plate movement in this part of the world average about 13,000 feet high: the Andes Mountains. As it descends into the heat of the Earth’s core, the edge of the Nazca plate melts, providing a source of magma for the many volcanoes in the area.
In addition, the wrinkling, compressing, and distorting of the rock in both the Nazca and South American plates causes a buildup of stress within the rock. Earthquakes are the release mechanism for this stress. The plate movement thus reshapes the landscape of Chile, through the slow process of wrinkling, and the more spectacular events of volcanoes and earthquakes.
Occasionally, plate movement in Chile can also produce dramatic effects felt much farther away. On 28FEB10, fifteen hours after the most recent big quake in Chile, I was teaching sailing at OCSC SAILING in the Berkeley Marina. The syllabus called for sailing practice in close quarters, so we were repeatedly sailing in and out of the Marina. Because of the full moon that weekend, the tides were significant, about eight feet between the high and the low. The tide had been falling all afternoon, but the movement of the water out of the marina was not especially noticeable. At about 1430, though, the water began to flow rapidly out of the marina, at a speed of about 3 to 3 ½ knots. It was the trough preceding the arrival of the tsunami generated by that earthquake in Chile, almost 5000 miles away! The strong outflow continued for about 15 or 20 minutes, with the speed diminishing until it stopped and began to flow in. The tsunami had arrived! The unusual inflow was at about the same speed as the preceding outflow, and last for about the same amount of time. This pattern of trough and crest repeated once more, and then the disturbance appeared to be over.
But in fact, the earthquake had even more far-reaching, lasting effects: The Earth’s day is 1.26 microseconds shorter, and the Earth’s axis shifted by about 3″. These effects, though, were not noticeable on my boat in the Berkeley Marina.