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Extreme Waves 7 The Southeast Asia Tsunami of December 26, 2004 There was no tsunami warning system in the Indian Ocean in 2004, although experts had recommended that one be installed. When the December 26, 2004, earthquake was detected, some time elapsed before its tsunami potential could be assessed. Once the risk was realized, the Pacific Tsunami Warning Center in Honolulu tried to communicate with Indonesia and other countries thought to be in the path of the tsunami. But there was no preestablished system for notification, no provisions for issuing a warning even if notification had been made. The effort was too little, too late, and thousands of lives were needlessly lost. At the northwest end of Sumatra, 158 miles from the epicenter of the earthquake, the window for notification was perilously short. It took only about 20 minutes for the first tsunami wave to hit land. At that time, on the afternoon of Christmas Day, Hawaii time, the Pacific Tsunami Warning Center was still analyzing the data in an effort to determine just how big the earthquake was. Unfortunately, it was much larger than originally thought. By then, it was too late. The damage reports were already on television around the world. The television images are unforgettable. The waves—not one, but
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Extreme Waves several—roll in from the ocean, across beaches and manicured lawns, sweeping all before them. A jumble of beach umbrellas, rattan lawn chairs, towels, and potted plants are borne by the onrushing water. The water courses through open-air restaurants, flows across hotel lobbies and verandas, and pours in dirty waterfalls down staircases. As riveting and terrible as these images are, they are not the worst. Next we see people running frantically, heads turning, looking back over their shoulders at the churning muddy water that is gaining on them. Then, looking closer, we see someone struggling in the maelstrom, buffeted by the tumultuous seas; we see another man clinging desperately to a palm tree, only to lose his grasp and be sucked from sight by the roiling waters; another, carried toward a building, is eventually rescued. (See Plate 10.) Later, we see more, so many more that our senses are confounded; it does not seem possible, such death and destruction—the swollen corpses; mothers crying over infants; the young, so lifelike in death. It was the worst natural disaster most of us have ever seen or could imagine seeing. It was the Southeast Asia tsunami of December 26, 2004, a definite 6 on the tsunami intensity scale. (Note: Worst is a relative term. Eight months later, almost to the day, the world recoiled to the vision of another terrible disaster: Hurricane Katrina striking New Orleans and the Gulf cities of Louisiana and Mississippi. The cause was different, but the devastation had a terrible familiarity.) The tsunami was triggered by a massive magnitude 9.0 Sumatra-Andaman Islands earthquake in the Indian Ocean near the northwest end of Sumatra. Scientists believe that more than 600 miles of ocean bottom ruptured and heaved upward as much as 66 feet in a process seismologists call subduction when the Indian Plate, moving northeast, slid under and lifted up the Burma Plate. Gradual movement had been proceeding for years in this area, at the rate of about 2.5 inches per year, causing stress to build at the junction of the two tectonic plates, until finally the submarine rock fractured. When this occurred, the locations of nearby islands and the tip of Sumatra were shifted by the earthquake. An earthquake of this size was bound to cause severe structural damage to buildings within a radius of 120 miles, and within seconds, buildings in Sumatra and adjacent areas started collapsing.
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Extreme Waves It is doubtful that any warning could have saved lives in Sumatra and at other points close to the epicenter, since the tsunami took less than an hour to reach them. In deep water, the tsunami traveled at around 384 knots, but slowed to around 174 knots in the shallow waters near the shore. West Sumatra was hit by three waves, the second reportedly the largest. Extreme wave heights reached 90 feet in some locations, but only 40 feet on the north coast. On the other side of Sumatra, seven waves were reported, resulting perhaps from the primary waves striking Malaysia and reflecting back. Traveling east and northeast in the Andaman Sea, the waves struck Java and Malaysia, rolled ashore at beach resorts in southern Thailand, and hit Burma and Bangladesh. The low-lying Andaman and Nicobar Islands close to the epicenter were hit hard. Waves propagating west and northwest crossed the Bay of Bengal in around two hours and hit the east coast of India and ravaged Sri Lanka. Remarkably, a passing satellite happened to catch the image of the tsunami as it crossed the Bay of Bengal on its way to Sri Lanka. It was a fast-moving wave about 28 inches high, with a wave front of ever-broadening circumference. When it reached Sri Lanka, it piled up to a height of 20 to 25 feet. It was not just the height that counted in this tsunami; the earthquake was so large, and the surface rupture so great, that a huge volume of water was set in motion. Consequently, when the waves hit shore, the massive flood of water behind the wave kept coming and coming, rushing far inland. There are many graphic reminders of the volume of water: debris left on second-floor balconies far inland, bodies deposited in the tops of tall trees, and cars and boats left high and dry. Then, as the waters receded, the carnage was doubled or perhaps tripled, as the waves swept walls of debris back toward the ocean. Buildings that might have survived the initial onslaught in a weakened state were torn apart when the waters returned from the opposite direction. In Sri Lanka, a passenger train running along the coast stopped because of the flooding. A number of passengers stayed on the train; other people jumped on the train to escape the water. The next wave rolled the train, and hundreds of persons were trapped and died. Exiting the Bay of Bengal, the tsunami next rolled westward across
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Extreme Waves the Indian Ocean, striking the Maldives three hours later, then Madagascar, and the east coast of Africa—Somalia, Kenya, and Tanzania. Since it took seven to eight hours to cross the Indian Ocean, Africa had some warning. But even here, 3,000 nautical miles from the epicenter, people died. Out of this devastating natural catastrophe have come incredible stories of heroism and survival. Some of the most unique witnesses to the tsunami’s fury were in the ocean—scuba divers underwater off the coasts of Thailand or Sumatra, who felt the turbulence of the passing wave, and in some cases rose to the surface to find their dive boats gone. Or fishermen at sea, who, if they were in deep enough water, survived the passing wave, only to find when they returned to port that they could not recognize anything, so complete was the destruction. DIVING UNDER A KILLER TSUNAMI A minute before 8:00 A.M. on December 26, 2004, guests in the tourist hotels on Patong Beach, Phuket Island, Thailand, were wakened by a tremor from a distant earthquake, followed by a second one about 20 minutes later. No one thought much about it. It was also felt on Koh Phi Phi, a small island about 24 nautical miles east of Phuket. Gene Kim and Faye Wachs, in the process of boarding their dive boat in Ton Sai Bay, Koh Phi Phi Island, did not notice the slight ground movement. They were approaching the end of a two-week vacation in Thailand, a vacation that had taken them from the mountains in the north to the idyllic beaches at Thailand’s southern tip. They came to Phuket on December 24, spent the night in a hotel on Patong Beach, and then caught the first ferry to Koh Phi Phi on the morning of December 25. Upon arrival they rented scuba gear and made an exploratory dive, telling the dive master that if it was a good trip, they’d come back the next day for a longer “three-tank” dive. It was a good trip—close-up looks at leopard sharks and other fish, and Kim and Wachs were ready for more. The dive master promised even more spectacular experiences for the three dive sites to be visited on the next day. Little did they know that the “spectacular experience” would be their salvation. On board the dive boat on December 26 with Kim and Wachs were two Thai boatmen, an Israeli dive master known to them only as
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Extreme Waves Yavit, a Swedish tourist known only as Olsen, and a second dive master known only as Erik. Kim and Wachs had with them the bare essentials for their dive as well as a camera and a wallet containing some cash and credit cards. Their passports, airplane tickets, and other belongings were back in their bungalow at the Koh Phi Phi Cabana Hotel. The dive boat headed northwest to the first dive spot, a wrecked interisland catamaran ferry 280 feet long known as the King Cruiser. It had run aground on Anemone Reef in 1997 and now sat on the reef in water 98 to 115 feet deep. As Kim, Wachs, and the others were boarding their dive boat, approximately 310 nautical miles southwest as the dolphin swims, deep in the ocean along the Sunda trench, the seafloor ruptured—the Burma Plate on the east side rising as the Indian Plate on the west side pushed below it. The fault break propagated rapidly northwest toward the Nicobar Islands, moving at a variable speed later estimated to average 62 miles per minute. Within three minutes it traveled the 180 miles to reach Great Channel, the body of water that separates Banda Aceh, at the northern tip of Sumatra, from Great Nicobar Island. As the dive boat prepared to leave the dock at around 8:15 A.M., the first tsunami waves hit Indonesia. Waves as high as 49 feet virtually obliterated Banda Aceh, at the northern tip of Sumatra. As the fault movement propagated north, new waves were produced. No longer blocked by the Sumatra mainland, these waves had nothing but 300 nautical miles of the open waters of the Andaman Sea between them and Thailand’s west coast and jewel-like offshore islands. The new waves, generated as the fault break propagated north, were accompanied by earlier waves that diffracted around the northern tip of Sumatra to hit Thailand and streamed down the Strait of Malacca to hit Malaysia. In the shallower waters of the Andaman Sea, the waves slowed to perhaps 174 knots. Meanwhile, the dive boat headed northwest, unaware of the cataclysmic sea change bearing down on it. Around 10:00 A.M. the dive boat reached the buoy that marked the site of the wreck and the divers entered the water, dropping beneath the surface along the line attaching the buoy to the wreck. The dive boat—the two crew relaxing on board—drifted on the still waters above the wreck.
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Extreme Waves A few minutes earlier, those on the beach at Phuket were amazed to see the ocean withdraw a long distance from shore, stranding some boats and leaving flopping fish behind. Some curious spectators walked out to investigate; others turned and ran for high ground, screaming warnings to anyone who would listen. Three waves overran the resort areas including Patong Beach. After steamrolling over Phuket Island, the waves continued northeast to Koh Phi Phi Island. The island is shaped like a lowercase backwards letter “h,” with the long part of the h on the right-hand (east) side and the short part on the west. The long and short parts of the h are in reality two islands connected by a narrow, low peninsula. (See Figure 16.) The waves came in from the west, hit the beaches on the top center of the h, roared through Phi Phi village, and crashed into Ton Sai Bay on the other side of the peninsula. Most of the village buildings, including Kim and Wachs’ bungalow, FIGURE 16 Map of Koh Phi Phi Island and vicinity.
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Extreme Waves were ripped apart and carried across the peninsula and into Ton Sai Bay. Anyone caught in the buildings as the two waves carried the rubble first in one direction and then back again either died or suffered frightful injuries. On the dive boat, Kim and Wachs were unaware of the impending catastrophe. What happened next is best described in their own words, as they told me the story at the California Yacht Club in Marina Del Rey, California. Kim: “We descended in two groups—Faye, me, and dive master Yavit as one—and Olsen and Erik as the second group. Yavit had advised us that if strong currents were encountered not to try to swim against them. He gave us hand signals for changing direction, surfacing, and so on. We descended to around 60 feet and proceeded to swim along the side of the wreck. I don’t recall seeing any fish, but didn’t think about it at the time. We swam to the other end of the vessel and there encountered a strong current. The dive master gave us the signal to turn and go back the way we’d come. As we were returning, the water suddenly became quite turbulent. The dive master was next to Faye, and he grabbed her shoulder and gave the hand signal for an emergency ascent. At this point it was around 10:20 and we were about 10 minutes into the dive. They were a short distance from me, but I saw the signal. The water suddenly became very cloudy—I’d describe it as a complete whiteout. As visibility went to inches, I found myself being tumbled violently, as if in a giant washing machine.” Wachs: “The visibility was so bad that I could barely see the dive master, even though he still had a hand on me. I felt myself being tumbled around and at first I did not know which way was up. I inflated my buoyancy vest, but at first nothing happened. I looked at my dive computer and saw that it indicated a depth of 135 feet. I don’t know if this was the real depth or reflected a wave passing above, pressing me down. Finally the vest buoyancy took effect and I started to surface. I went up with the dive master; we stopped at 60 feet for a few minutes, then at 30 feet for a few more minutes, and then surfaced. There was no sign of Gene. I checked my dive computer and saw that 28 minutes had elapsed.” Kim: “I felt myself being sucked downward and tumbled around.
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Extreme Waves I was initially pulled away from the side of the wreck and then slammed against it, hitting my head solidly on the side of the sunken vessel. I was spun around down to about 120 feet, trying to see which way my air bubbles were going so I could figure out how to get to the surface. Finally I worked my way up into clearer water and miraculously found myself right next to the buoy mooring line we’d used in our descent. I went up to 30 feet and hung there a few minutes trying to figure out what was going on. When I looked up I could see two figures hanging on the line. The line was stretched out at a sharp angle and divers’ bodies were stretched out horizontal, indicating the presence of a very strong surface current. I surfaced, hoping to find Faye and Yavik, but discovered it was Olsen and Erik. The dive boat was not in sight. I considered going back down to look for Faye, but then saw that the visibility would be essentially zero once I got down to 30 feet and realized it would be dangerous and probably futile. Meanwhile, one boat appeared in the distance, going away from us, and then a second boat appeared several hundred yards away coming toward us—our boat. We were assisted back in the boat by the crew. The crew spoke no English, so questions were not possible. Reading their body language, the crew did not seem to indicate anything out of the ordinary. I can only assume that the boat was pushed away from the buoy by what appeared to them to be a large swell. A short time later, two heads were seen bobbing some distance away and to my great relief, we got Faye and Yavit into the boat.” Wachs: “I could tell that Yavit was shaken by the experience—said he’d never seen anything like it before. No one had an explanation other than it seemed like a freak current. It was a full moon, high tide, and so we thought it might have had something to do with that. At the moment we had no premonition or thought that it might have been a tsunami.” Kim: “The dive master suggested that we move to the second dive spot, a place on Anemone Reef, where it was more sheltered and there were no currents. We went there, and Yavik went in the water to check it out. He emerged immediately and said the current was too strong, so we aborted the second dive.” Wachs: “We went to the third spot, called Shark Point, supposedly
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Extreme Waves more sheltered. Here again we encountered unusually strong currents. I noticed that the fish behavior seemed strange. Small puffer fish—fish that we usually encountered solitary—were huddled together in groups. The fish seemed unusually skittish and seemed to be schooling tighter. We also cut this dive short. When we were back on the boat, we decided we’d had enough, and elected to return to the harbor early. It was now about 1:30 P.M. The dive master offered us a dive the next day at a location on the other side of the island where we would be sheltered against any turbulence, since this trip was being cut short. We agreed to his plan.” Kim: “It took us about an hour to get back to Ton Sai Bay. As we got closer, we began to see a lot of floating debris that wasn’t there when we’d left the harbor in the morning. Our first reaction was anger that someone had dumped a load of trash into the bay. Then the objects began to take on the appearance of stuff that wasn’t trash—a floating chair, a TV, a six-pack of water in plastic bottles—unopened—other things. Then we thought that maybe there’d been an explosion or a shipwreck. A jet aircraft (fighter plane) came over, flying very low. This was quite unusual and was the first indication that something might be terribly wrong. Then Olsen got a text message on his cell phone from his wife. It said ‘Big catastrophe!’ He called back, and she told him there’d been a tsunami. Suddenly it dawned on us what had happened, but we still had no idea about the earthquake or how extensive the damage was.” Wachs: “Until we started seeing the floating bodies.” For the next several hours the dive boat remained in the bay, unable to dock due to the floating debris and damage to the pier. Meanwhile Kim and Wachs assisted numerous fishing boats in the bay by pointing out locations where bodies were floating so the smaller boats could recover them. Finally ashore at around 5:30 P.M., Kim and Wachs went to their hotel and tried to find their bungalow. Destruction was so complete it was difficult to identify their bungalow among the ruins. Eventually they located what they thought was theirs and confirmed it by the fact that nearby there was a door with their room number. All of their belongings were gone except the few things they’d taken on the boat with
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Extreme Waves them. Passports, tickets, and everything else were lost, although Faye later spotted a pair of her shorts in the mud some distance from the wreckage. Still, they realized their good fortune when they saw their bungalow and the injured people. If they had stayed in the hotel, it is doubtful they would have survived. (See Plate 11.) Kim and Wachs immediately began assisting the victims—Kim helping excavate a man trapped under building debris, Wachs assisting with the injured. Two vacationing doctors set up an improvised treatment area. There were a lot of badly injured people, severe lacerations, compound fractures, saltwater ingestion. Later, Thai helicopters starting flying in. The injured were triaged, and those needing treatment were loaded on improvised stretchers (usually doors) and carried to the helicopters for evacuation to a hospital. This went on until late in the evening. Finally, at 1:00 A.M., Kim and Wachs were exhausted. They went to one of the two hotels still standing, found an empty room on the third floor, and slept for a few hours. In the morning they were able to look around and see the extent of the devastation for the first time. They could see the path where the waves had come in over the beaches on the west side of the island and literally swept the village across the narrow peninsula, grinding up the buildings and dumping the rubble into the bay. Kim said that the waves were 30 to 35 feet high on the basis of the waterline in one of the two hotels (reinforced-concrete construction) that survived. He also saw a small powerboat impaled upside down in the top of a palm tree about 35 feet high. Virtually all of the single-story buildings in the area were demolished; just the two reinforced-concrete hotels remained standing. Kim and Wachs continued to assist the injured. Ferries started arriving to evacuate tourists to the mainland at Krabi. By now they were both suffering from cuts and abrasions on their feet and legs from carrying stretchers through the rubble, and decided it was time to leave. At that point the Thai military had nurses and doctors in place and was managing the evacuation of injured. Kim and Wachs spoke highly of the response of the Thai government in helping the tsunami victims and tourists alike. Still in their swimsuits, they were taken by ferry to Krabi, then flown to Bangkok and finally back to the United States.
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Extreme Waves They told me that they plan to return to Koh Phi Phi at some point, both to pay homage to those that were lost and to reunite with some of the Thai people with whom they worked and who befriended them during the ordeal. During my visit to the Pacific Tsunami Warning Center, I asked Hirshorn what new information had come forth concerning the December 26, 2004, Sumatra-Andaman Islands earthquake. The earliest analyses indicated an earthquake of magnitude 8.0, later revised by the center to 8.5. Hirshorn said that it has now been established that the earthquake magnitude was in the range of 9.0 to 9.3, making it one of the two or three largest earthquakes in recorded history. The length of the rupture zone was in excess of 740 miles, with a width around 30 miles. On the basis of the early data, it appears that the southernmost 250 miles of the fault produced the greatest movement, and it was this section that generated the tsunami. The total earthquake duration was on the order of 10 minutes. The tsunami is unique because the wave was detected in midocean by a satellite. When the Jason 1 satellite passed over the Indian Ocean on December 26, 2004, it recorded a rise in the ocean surface of about 70 centimeters (28 inches), followed by a drop of 30 to 40 centimeters (12 to 16 inches). Other data deduced from the satellite record indicate that the wave was traveling about 404 knots, with a wavelength of about 267 miles and a period of about 37 minutes.1 There is some possibility that the stresses have not been fully relieved and that the northern section of the fault could move and create another tsunami. If this were to occur, it would have a greater effect on the northern portion of the Bay of Bengal than occurred during the December 26, 2004, earthquake and could lead to a huge loss of life in this densely populated region. There is also a chance that a large section of the Sumatra Trench, to the south of the original rupture zone, could break in a magnitude 8 to 9 event that could cause a devastating tsunami for the southern islands of Sumatra and the northwest tip of Australia. This section, and the section of the trench to the north of the December earthquakes, probably have been “loaded” with additional stress at their edges by the December and March events. Due to the lack of seismic sensors and water level instruments in the Indian Ocean, the center did not know whether or not a tsunami
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Extreme Waves had resulted from the December 26, 2004, earthquake. Scientists believed that a locally damaging tsunami was likely, but it was not until four hours later that they realized that an oceanwide destructive tsunami existed and additional warnings were issued for the Seychelles, Diego Garcia, and East African countries. Thanks to the warning, successful evacuations were carried out in Kenya, sparing many lives. But there was no effective system for evacuation in Somalia, and there the death toll was higher. Plans for an Indian Ocean tsunami warning system are being debated. No resolution has been reached at present, although as this book was being written there were indications that India might take the lead in sponsoring an Indian Ocean network. The Japanese have also established a regional warning presence in the Indian Ocean. Both Japan and the Pacific Tsunami Warning Center are now issuing tsunami bulletins for the Indian Ocean. The recent Southeast Asia tsunami with its horrific loss of life and extensive destruction has prompted a review of other areas of the world that may be at risk. Areas that have previously experienced tsunami—such as the west coast of South America, notably Chile, Peru, Ecuador, Nicaragua, and Guatemala—are known risk areas. FUTURE RISK AREAS New research suggests that areas that have not experienced tsunami historically may be at risk. Two areas in particular could lead to huge economic loss and widespread damage if hit by tsunami waves. California experiences damaging earthquakes every decade or so. These are generally associated with the San Andreas Fault zone or tributary faults adjacent to it. Most of these faults are inland and there has been little offshore seismic activity in recent times. However, the Southern California coastal area is crossed with a number of deep submarine canyons between Point Conception and the Mexican border. These canyons are basically extensions of the steep canyons that can be found along the coast. Southern California residents are familiar with the frequent landslides that occur in the coastal areas along Malibu, Palos Verde, and Laguna, especially following a heavy rainfall when the soil becomes saturated and fails. In the Malibu area, virtually no winter
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Extreme Waves goes by without at least one closure of the Pacific Coast Highway due to a landslide. The slopes offshore are composed of thick layers of sedimentary materials that have built up over the ages. These are saturated and characterized by a low shear strength. Offshore mapping indicates that there are areas where sliding and slumping have already occurred. New research indicates that a large enough earthquake—say, a magnitude 7 or greater—could cause a submarine landslide in one of the deep offshore canyons, dislodging a large amount of sedimentary material from the walls of one of these canyons. If the volume of material dislodged is sufficiently large, the effect could be similar to the Papua New Guinea (July 17, 1998) tsunami. Although the earthquake was only a magnitude 7, its proximity to the coast and the steep continental shelf caused waves up to 49 feet high and thousands of fatalities. One place where this might happen is near San Pedro, California, offshore from Point Fermin. Wave heights could reach as much as 66 feet against the Palos Verdes Peninsula bluffs directly opposite the epicenter, decreasing to around 20 feet as the wave moves west and north toward Point Vicente and then 6 feet at Palos Verdes Point. Due to the high bluffs, little damage would occur here. To the south and east, a 13-foot-high wave would impact the Los Angeles and Long Beach harbors, and the low-lying coastal areas near Alamitos Bay, Seal Beach, Huntington Beach, and Newport Beach would be hit with waves 6 feet high.2 Each winter the city of Long Beach erects a sand berm along the beachfront to protect residences from the run-up of water during winter storms, indicating that this area would be susceptible to damage from even a small tsunami wave. Seal Beach is known to undergo extensive flooding in the winter, when there is a combination of several days of rainfall, low atmospheric pressure, and a winter high tide. During the winter of 2005, flooding extended inland to the Pacific Coast Highway, causing it to be closed until the waters receded. The Newport Peninsula sits 9.7 feet above mean sea level. This means that a 10-foot-high tsunami wave would overflow the peninsula. The range of high tides is 4 to 7.2 feet, so if a 10-foot-high wave hit at higher high tide, it would be 7.5 feet (17.2 minus 9.7 feet) above land and the surge would probably reach the upper stories of houses
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Extreme Waves along the peninsula. The water would flow into Newport Harbor; in-undate Lido, Balboa, Bay, Harbor, Linda, and Collins Islands; run up against the cliffs along the Pacific Coast Highway; and then backwash into the harbor and across the peninsula into the ocean. The older single-story structures would be demolished, and it is doubtful that any of the newer structures would survive without major damage. Ironically, a few weeks after I wrote the above paragraph—on June 14, 2005—a magnitude 7.2 earthquake occurred 80 nautical miles off the coast of Northern California, west-southwest from Crescent City, and around 485 nautical miles from Newport. At tsunami speed, it would have taken a fraction of an hour to hit Crescent City, and a little over an hour to hit Southern California. A tsunami alert was issued for the Pacific Coast but was canceled about an hour later. Some communities had no plans to deal with the emergency. Lifeguards were dispatched to clear the beaches at Newport, and a number of people evacuated their homes. My neighbors reported traffic jams trying to get off the peninsula. I was in Washington, D.C., at the time—well out of tsunami range—and was therefore spared the ordeal. However, reading the newspaper accounts the next day, after I returned home, was chilling. It is clear that had this been the real thing, the warning would have been too late to do any good, and many people would not have received the warning or would have ignored it. Those who might have tried to leave the peninsula would have been caught in a huge traffic snarl. The authors of the Palos Verde tsunami scenario used a detailed econometric model to study the cost of such an event to Southern California’s trillion-dollar economy. The estimate included direct losses in the inundated areas, the costs of business interruption and loss of jobs, the economic effects of closure of the ports of Los Angeles and Long Beach for up to a year, and damages to transportation systems and other infrastructure. Four different outcomes, with increasing degrees of severity, were considered. The estimated economic loss was in the range of $7 billion to $40 billion. I’ve personally experienced at least a dozen earthquakes in Southern California and, as a member of the Earthquake Engineering Research Institute, have been involved in the engineering investigation of several of the larger ones and have also worked professionally in the
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Extreme Waves area of seismic testing and improving seismic reliability of equipment and structures. My former residence in West Los Angeles was damaged by both the 1971 magnitude 6.7 San Fernando earthquake and the 1994 magnitude 6.7 Northridge earthquake, in the latter case losing the chimney and requiring $25,000 in miscellaneous repairs. The cost of the 1994 Northridge earthquake, which caused 57 deaths, has been estimated as between $20 billion and $40 billion. If the Palos Verde tsunami scenario earthquake did occur in the postulated location, earthquake damage inland would be substantial. This cost has not been estimated; it would be in addition to the $7 billion to $40 billion cited above. The East Coast of the United States is generally considered a low seismic risk area. However, the entire East Coast of North America—from the Florida Keys to Gander, Newfoundland—faces the open Atlantic in a great arc of densely populated shoreline that extends from latitude 25 degrees north to 50 degrees north. Eastward across the Atlantic, a little more than 100 nautical miles west of southern Morocco, lie the Canary Islands, important waypoints for the early Spanish and Portuguese navigators traveling south along the west coast of Africa. The islands—seven major islands and several smaller islets—were formed by ancient volcanoes. Draw a line due west on the Atlantic Ocean from the westernmost island of La Palma, and after traversing some 3,300 nautical miles of open ocean, you strike land at Melbourne, roughly in the center of Florida’s east coast. La Palma is a pear-shaped island with a large central volcanic caldera called Taburiente. The caldera has a diameter of 5.5 miles and a peak height, known as the Roque de los Muchachois, 7,900 feet above sea level. There is a large canyon, open to the west that leads from the caldera down to the sea. The overall height of the island is 21,320 feet above the ocean floor. The base of the island is made of material called pillow lava that is cut in places by basaltic dykes. Around 1,300 feet above sea level, there is a dividing line between the upper layer of lava and rock. The island last erupted in 1971. The concern about La Palma centers on its unique geology and activity. It appears that there is a rift zone running from the south end of the island north to the center, along the back edge of the caldera and
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Extreme Waves a spot called Cumbre Vieja.3 Rift is a seismologist’s term for a fissure or plane along which rock may most easily split. Since the island has erupted seven times in the historic past (since 1585), and recently in 1949 and 1971, it is not unreasonable to assume it will erupt at some time in the future. The danger is that if molten lava interacts with seawater penetrating the lower level of the island, the resulting steam explosion could dislodge the side of the caldera, or a volcanic eruption itself could blow away the steep side of the island, as happened in Mount St. Helens and Krakatoa. If this occurred, millions of tons of rock, sliding down nearly 2.5 miles to the bottom of the ocean, could create a tsunami. Alternatively, underwater landslides of weak material could occur. No one can predict whether this would be limited to a local tsunami or could affect a wider area. In an extreme scenario, a wave racing west at around 380 knots would wash over Bermuda in seven hours and would reach the East Coast of the United States in around nine hours. As the wave emerged from the deep waters of the Atlantic onto the continental shelf, its height would increase dramatically, potentially towering over buildings along the waterfront from Jacksonville to Boston. While the travel time of the wave would allow some warning to be given, it is doubtful that a mass evacuation of the entire East Coast of the United States could be carried out in a matter of hours. The damage to buildings and infrastructure, as well as the loss of life, would be appalling and unprecedented if this extreme event were to occur. We know, in the case of tsunami, how massive forces and a huge release of energy deep beneath the ocean generate waves that are modest in size as they race across the ocean, but become powerful and extremely high as they reach shallower waters. Also, storms in midocean acting on near-surface waters produce large waves. In the next chapter I want to discuss what happens when several storms occur in different locations, their waves radiating out and dispersing and at some point meeting each other. To a vessel located at the intersection of several wave paths, the seas can be chaotic, with waves of all heights seeming to approach from a bewildering mix of directions. This condition is known as a confused sea.
Representative terms from entire chapter: