Gary Griggs, Our Ocean Backyard

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Almost all of us were surprised two weeks ago when the Tonga tsunami hit our doorstep. Unless you were on a boat in the harbor or watching the shore, however, it would probably have gone unnoticed. However, tsunamis, like sharks, strike fear into the minds of many of us, whether they are deserved or not.

NOAA’s warnings for the Tonga tsunami covered the entire California coast, from Crescent City in the far north to Imperial Beach on the Mexican border. While the tsunami arrived at high tide, which is the worst possible time, overall water level increases at tide gauges across the state were generally only in the range of one to two feet.

The highest documented level was at Port San Luis, near Avila in San Luis Obispo County, where seas rose four feet above the predicted high tide. This Tongan tsunami had traveled about 5,300 miles to reach our shore, and still had plenty of momentum to run aground and inundate low shores, such as at Santa Cruz Small Craft Harbor.

While most tsunamis result from large subduction zone earthquakes, such as Japan in 2011 and Sumatra in 2004, our most recent visitor came from a major volcanic explosion in the South Pacific. It’s about the movement of water, whether it’s coming from one edge of a very large tectonic plate that’s been pushed down for hundreds of years and eventually goes back up and bounces, pushing a huge volume of water from sea to top; or the displacement of the surrounding ocean by a catastrophically erupting ocean volcano.

There have been at least three well-studied such eruptions, Santorini in the eastern Mediterranean about 3,600 years ago, Tambora in Indonesia in 1815, and Krakatoa, also in Indonesia, in 1883. Each of them produced major tsunamis. Krakatoa generated waves over a hundred feet high that claimed the lives of approximately 36,000 people. The Tambora tsunami was much smaller and is estimated to have claimed around 4,600 lives.

While the typical wind waves that reach our beaches travel at 25-50 miles per hour in the open ocean, tsunamis travel at the speed of commercial aircraft, 450-500 mph. As a result, the Tonga tsunami arrived in Santa Cruz about 12 hours after the explosive eruption, time for the Pacific Tsunami Warning System to get the word out.

Another big difference to the waves we usually see breaking on our shore is the wavelength, or the distance between two crests. Our typical wind waves or swells have wavelengths of several hundred to perhaps 1,500 feet offshore. Tsunamis, on the other hand, have distances between wave crests in the open ocean of 90 to 100 miles. However, the distance between these wave crests decreases as the waves enter shallow waters and begin to feel or drag on the bottom.

Their speeds are also reduced, but with the much longer wavelengths and higher speeds of tsunamis, the momentum of these waves can carry considerable distances inland where the topography is very low. This happened at the upper end of Santa Cruz Harbor where a flood also occurred two weeks ago. In the 2011 tsunami, water spilled inland up to six miles along the coast of Japan.

The port of Crescent City, near the Oregon border, as well as our own port of Santa Cruz, were the two places where tsunami damage has historically been the greatest. The three recent events with the highest casualties at both locations were the 2011 Japan tsunami and then the 1946 and 1964 Aleutian Trough tsunamis off Alaska. Both of these tsunami source areas are subduction zones where the massive Pacific plate is forced or pushed under an overlying plate producing very large earthquakes and then large tsunamis.

The 1946 April Fools’ Day earthquake in Alaska was an 8.6 magnitude event. The generated tsunami hit Half Moon Bay, 2,000 miles away, which is at low altitude. Fourteen-foot high waves swept a quarter mile inland, damaging homes, boats and docks, and destroying a fishing tackle store in El Granada. Two waves, the first at 10:15 a.m. and the second at 11:51 a.m. were reported in Santa Cruz with maximum heights later documented at around 10 feet. The wave would have pushed the water a considerable distance up the San Lorenzo River.

There was only one tsunami death along the entire California coast from that great tsunami of 1946 and that happened to be in Santa Cruz. It was also the only tsunami-related death the Monterey Bay Area has experienced in recorded history. Around 10.15 a.m. this morning on April 1, 76 years ago, Hugh Patrick, a 73-year-old man walking along the shore near Lighthouse Point, drowned when the water level rose rapidly to 10 feet above normal when the first wave hit. His walking companion, Cephus Smith, 73, was also knocked down by the wave but failed to save his friend. Patrick’s body was found 17 days later in the kelp beds half a mile west of Lighthouse Point.

Men on the municipal wharf reported that the water receded a little after 10 a.m. and suddenly quickly returned and rose high on the beach. There were four surges, the last at 11:50 a.m. which nearly overran the plaza along the main beach.

The message here is that Monterey Bay has only had one tsunami death in its roughly 150 to 200 years of recorded history, and the entire state of California has only had 17 deaths. , including 12 in Crescent City in 1964. With our current warning and only modest exposure to tsunamis, there is a very, very low risk of being caught in a tsunami in Santa Cruz. Still, standing on the shore to watch an approaching tsunami could be fatal.

Gary Griggs is Professor Emeritus of Earth and Planetary Sciences at UC Santa Cruz. He can be contacted at [email protected] For past Ocean Backyard columns, visit http://seymourcenter.ucsc.edu/about-us/news/our-ocean-backyard-archive/.


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