System warned of Tongan tsunami
Nearly all of us were surprised two weeks ago when the tsunami from Tonga arrived at our doorstep. Unless you were on a boat in the harbor, or watching the shoreline, however, it would have likely gone mostly unnoticed. Tsunamis, like sharks, however, inflict fear into the minds of many of us, whether deserved or not
NOAA warnings for the Tonga tsunami covered the entire coast of California from Crescent City in the far north to Imperial Beach at the Mexican border. While the tsunami arrived at high tide, which is the worst possible time, overall increases in water level at the state’s tide gauges were generally only in the one-to-two-foot range.
The highest level documented 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 shoreline, and still had plenty of momentum left to wash up onto and flood low-lying shorelines, such as at the Santa Cruz Small Craft Harbor.
Whereas most tsunamis result from large subduction zone earthquakes, such as the one from Japan in 2011 and from Sumatra in 2004, our most recent visitor was from a major volcanic explosion in the south Pacific. It’s all about the displacement of water, whether from an edge of a very large tectonic plate that has been pulled down for hundreds of years and finally let’s go and rebounds, pushing a huge volume of seawater up; or from the displacement of the surrounding ocean by a catastrophically erupting oceanic 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 these produced major tsunamis. Krakatoa generated waves over one hundred feet high which took the lives of about 36,000 people. The tsunami from Tambora was much smaller and is estimated to have
led to about 4,600 fatalities.
While the typical wind waves which reach our beaches travel at 25-50 mile/hour in the open ocean, tsunamis travel at the speed of a commercial aircraft, 450-500 mph. As a result, the tsunami from Tonga 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 from the waves that we usually see breaking on our shoreline is the wavelength, or the distance between two crests. Our typical wind waves or swell have wavelengths of several hundred to perhaps 1,500 feet at sea. Tsunamis, on the other hand, have distances between wave crests in the open ocean of 90 to 100 miles. The distance between these wave crests decreases, however, as the waves enter shallow water and begin to feel or drag on the bottom.
Their velocities are also reduced, but with the much longer wave lengths and higher velocities of tsunamis, the momentum of these waves can wash considerable distances inland where topography is very low lying. This is what took place at the upper end of the Santa Cruz harbor where flooding too place two weeks ago. In the 2011 tsunami, water washed inland as far as six miles along the coast of Japan.
The harbor at Crescent City near the Oregon border, along with our own Santa Cruz Harbor, have been the two locations where tsunami damage has historically been the greatest. The three recent events with the largest losses at both places were the 2011 tsunami from Japan and then the 1946 and 1964 tsunamis from the Aleutian Trench off Alaska. Both of these tsunami source areas are subduction zones where the massive Pacific Plate is forced or shoved 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 tsunami generated impacted Half Moon Bay, 2,000 miles away, which sits at a low elevation. Fourteen-foot high waves washed a quarter mile inland, damaging houses, boats and docks, and wrecking a fishing tackle shop at El Granada. Two waves, the first at
10:15 a.m. and second at 11:51 a.m. were reported in Santa Cruz with maximum heights later documented at about 10 feet. The wave reportedly pushed water a considerable distance up the San Lorenzo River.
There was only a single tsunami fatality along the entire California coast from that large 1946 tsunami and it just happened to be in Santa Cruz. It was also the only tsunami related death that the Monterey Bay area has experienced in recorded history. About 10:15 am on that April 1 morning, 76 years ago, Hugh Patrick, a 73-yearold man walking along the shoreline near Lighthouse Point was drowned when the water level rose quickly to 10 feet above normal as the first wave hit. His walking companion, 73-yearold Cephus Smith, was also knocked over by the wave but was unsuccessful in attempting to rescue his friend. Patrick’s body was recovered 17 days later in the kelp beds a half mile west of Lighthouse Point.
Men on the municipal wharf reported the water receding a little after 10 a.m. and suddenly returning quickly and surging high on the beach. There were four surges, the last at 11:50 a.m., which nearly topped the Esplanade along Main Beach.
The message here is that Monterey Bay has had only a single tsunami death in its roughly 150-200 years of recorded history, and the entire state of California has only experienced 17 fatalities, 12 of these in Crescent City in 1964. With our present warning system and only modest exposure to tsunamis, there is a very, very low risk of being caught up in a tsunami in Santa Cruz. Nonetheless, standing on the shoreline to watch a tsunami approach, could be fatal.