The north shore of Orchid Island, situated off Taiwan’s southeastern coast, is strewn with boulders of coral, the largest of which weighs 234 metric tons. The casual observer might suspect they rolled down from the hills above, but that is not possible. The island is volcanic, meaning the coral boulders could only have come from the sea. This caused geologists to ponder the force of nature that transported them to their current locations.
The stretch of beach on the outlying island faces the deep waters of the Ryukyu Trench, a seismically feisty subduction zone. Therefore, scientists theorized that these huge chunks of limestone were ripped from surrounding reefs and deposited onshore by ancient tsunamis.
J. Bruce H. Shyu (徐澔德), an associate professor of geology at National Taiwan University (NTU) in Taipei, was among a team of researchers who conducted a field survey to investigate the origins of these rocks. According to the results of their study, the coral boulders were indeed transported by giant waves. But for the scientists, the trick was working out which ones were moved onshore by tsunamis and which were deposited by storm waves. Powerful typhoons are known to wash smaller rocks onto beaches.
So, in their work on Orchid Island, Shyu and his team eliminated all but the biggest boulders, and focused in particular on those that had been carried tens of meters inland. Their findings indicate that the islet was hit by a tsunami as recently as a century or two ago, as well as a series of older events dating back thousands of years.
The paleotsunami research on Orchid Island was funded in part by Mega-Seismic Risk and Multi-Geological Disasters in Taiwan—a project coordinated by the Institute of Earth Sciences at Academia Sinica, the nation’s foremost scientific institution, that aims to assist public policymaking on hazard mitigation. Geologically speaking, Taiwan is an unstable place. Typhoons drench its soaring peaks, causing landslides in its narrow valleys and floods in the flatlands below. The island also experiences frequent earthquakes. However, until recently, little attention was paid to the potential threat posed by tsunamis. “A decade ago, no geologist would have said Taiwan has a tsunami problem,” Shyu says. “But now there is a feeling that we need to understand this matter more.”
Infrequent Occurrences
Taiwan has not been struck by a deadly tsunami for several generations. Much of the Asia-Pacific region, in contrast, is battered roughly once a decade as quakes along the Pacific Rim send waves traveling at near supersonic speed across thousands of miles of ocean. A case in point is the magnitude-9.5 Great Chilean Earthquake of 1960, the largest tremor ever recorded. Following the quake, the coast of Chile was devastated by up to 25-meter-high waves, which from there radiated outward. Around half a day later, the tsunami struck Australia and New Zealand. Next it reached land in Hawaii, where a 10.7-meter-high wave crashed into Hilo, resulting in the deaths of 61 people. It then continued on to the Philippines, where it killed 32, before the disaster claimed 138 lives in Japan.
The tsunami struck Taiwan as well, but when it hit Hualien County on the island’s eastern coast, the wave was a mere 1.3 meters high and no deaths were recorded. The reason for this is geological. Tsunami waves increase in height only after entering shallow water. This process, called the shoaling effect, is not possible off the majority of Taiwan’s rugged eastern coast, where the depth of the ocean floor often plunges to around 500 meters just a few kilometers from the shore. Therefore, when a tsunami strikes eastern Taiwan, a relatively small wave may form, but most of the energy will bounce out to sea again.
This underwater topography, it was presumed, meant the island had little to fear from tsunamis. But then two major disasters in the region prompted officials and researchers to begin re-examining this question. The first of these catastrophes occurred on December 26, 2004, when a magnitude-9.1 earthquake off the coast of Sumatra, Indonesia unleashed a tsunami that killed roughly 230,000 people in 14 countries. Next, on March 11, 2011, a magnitude-9 earthquake struck northern Japan and generated a massive tsunami. This seismic event claimed at least 15,891 lives and crippled the Fukushima Daiichi Nuclear Power Plant.
Following these disasters, tsunamis were added to the list of potential hazards being investigated by Academia Sinica. Indeed, the research institution was not alone in placing greater focus on this field. During the past decade, paleotsunami studies have become a hot topic among earth scientists around the globe. More than 1,000 papers were being published annually in English by the time the 2011 catastrophe occurred in Japan.
The Japanese, meanwhile, have been studying paleotsunamis for centuries. One of the first recorded in that country’s long written history is the Jogan tsunami of 869, which destroyed the castle town of Tagajo. In 1990, Japanese scientists found confirmation of this event when they uncovered a layer of sand extending more than 4 kilometers inland underlying the now-urbanized Sendai plain.
More recently, when the Ryukyu Islands were struck by the Meiwa tsunami of 1771, witnesses noted that enormous coral boulders were tossed onto the shore. Modern investigations of this tsunami led several Japanese scientists to neighboring Taiwan. Among these researchers were seismologist Masataka Ando, who was a visiting scholar at Academia Sinica and National Central University in northern Taiwan’s Taoyuan City from 2008 to 2011, and geologist Yoko Ota, currently a visiting professor at NTU.
Initial Findings
The search for evidence of paleotsunamis in Taiwan is in its early stages, but preliminary findings from field studies are intriguing. Working with scientists from Academia Sinica and local universities including NTU’s Shyu, Ota has identified probable tsunami boulders on Orchid Island and Green Island, which is also located off Taiwan’s southeastern coast. Meanwhile, another Japanese researcher, Nobuhisa Matta, of Nagoya University, and his team have found peculiar blocks of coral in Jiupeng near Taiwan’s southern tip.
To the Japanese scientists, these boulders and their geological settings eerily recall those found on the Ryukyu Islands. But there is a problem. Taiwan’s east coast has only a century or more of written history, so their research is hampered by a lack of records, which serve as clues as to where to look and can be used correlate field results to past tsunamis.
To overcome this obstacle, Matta, Ota, Ando and local researchers surveyed the folklore of the Amis inhabitants of the eastern coast, and identified a site called Malaulau. In the present-day town of Chenggong in southeastern Taiwan’s Taitung County, some Amis tribespeople recall their elders speaking of a giant wave hitting their village long ago. They renamed the area Malaulau, which means “withered” in the Amis language. Nothing could grow there before the site was irrigated in the 1930s.
That suggested inundation by saltwater. Furthermore, because Malaulau stands 18 meters above sea level, it implied that the area was hit by a powerful wave. Soil cores were drilled, and the results indicate that Malaulau was flooded by seawater not once but three times. The event that lives in tribal memory occurred roughly 700 years ago, while the two earlier inundations both took place more than 2,000 years ago.
Ando and Ota, both now retired, have returned to Japan, although Ota still teaches one class a year at NTU. But research on Taiwan’s paleotsunamis is gaining momentum. In 2013, Ota received funding for a three-year project, supported by Academia Sinica, and since she has now retired and the program involves considerable field work, Shyu has taken over as principal investigator.
Soon a paper will be published about the research conducted on Orchid Island. Meanwhile, Shyu has turned his attention northward to the Yilan alluvial plain, the only low-lying stretch of Taiwan’s eastern coast. Instead of looking for beached tsunami boulders, he is now searching for former swamps where sediments like those found at Malaulau may have been deposited. “If a tsunami hits an inland swamp, it leaves behind a layer of marine sand and fossils,” the geologist notes.
The problem lies in choosing the right location. “Much of the Yilan plain has been settled, and the landscape is highly modified by irrigated paddies and fish ponds,” Shyu explains. “If you do find a promising site, you need to get permission to bring in a backhoe to expose the sediments. Then there is the water table—it is very high, so the hole fills with water.” There are certainly challenges to conducting this type of research, but with luck he might unearth evidence of past events as the Japanese did in their 1990 discovery on the Sendai plain.
Eyewitness Accounts
Taiwan does have a single well-documented case of a historical tsunami. One occurred off the north coast on December 18, 1867, and it was thoroughly recorded by officials and several hundred foreign residents present at the time. On that calm, sunny morning, northern Taiwan was rattled by a strong earthquake. At Jinshan and the surrounding shores, the sea suddenly retreated, leaving the beaches littered with wriggling fish. Women and children rushed out to collect them. Moments later, they faced a 6-meter-high wave. Hundreds were drowned.
Cheng Shih-nan (鄭世楠), a seismologist and earthquake historian at Chien Hsin University of Science and Technology in Taoyuan, has collected around 100 historical documents in a dozen languages describing this event. The tsunami was also accompanied by other odd geological phenomena. For instance, hot springs at Jinshan turned into geysers, indicating that the tsunami was produced by an earthquake not far offshore.
Reconstructing the event from eyewitness accounts, local seismologists estimate that the tsunami was caused by a magnitude-7 quake with an epicenter 30 kilometers off the coast. So, despite its good fortune of being protected from oceanic waves along much of its coastline, Taiwan is not immune to the threat of tsunamis generated by near-shore seismic events. “What we want to know is how often these events recur,” Cheng says, “and you can do that by finding the sediments left behind or by studying historical documents.”
No such records are available for Taiwan’s east coast. But the opposite side of the island faces the South China Sea and the Chinese have been chronicling calamitous wave events in local histories for more than a millennium. Many such waves can be attributed to typhoons. If in the records, waves attacked the coast during storms, they were likely caused by meteorological events. But those that occurred during calm weather could have been caused by tsunamis.
One account that concerns Cheng and other Taiwanese seismologists depicts a possible tsunami in 1781 that struck the southwest of the island. Sources describe a wave of more than 10 meters in height that flooded the low-lying coastline.
This part of Taiwan faces the northern end of the Manila Trench, another of the region’s subduction zones. Though it has been largely quiescent for six centuries, in the view of scientists, including those at the United States Geological Survey, the trench could unleash a magnitude-9 earthquake if a megathrust occurred along its entire length. Such an event would send tsunami waves hurtling toward the Philippines, across to Vietnam, southern China and Hong Kong, and northward to Taiwan.
Seismic sleuths are trying to figure out whether the tsunami of 1781 in southern Taiwan was part of a regional event, but it is tough going. A major challenge is the Babel of languages found along the shores of the South China Sea. However, regional scientists are now using the Internet to overcome this obstacle.
“Ten years ago, if you wanted to compare the records in Taiwan with those in Hong Kong, you’d have to fly there and search the libraries,” Cheng explains. “But now this information is available online, and several research groups are compiling databases.” Right now there are more questions than answers. But the paleotsunami research boom is only a decade old, and the near future will be a time of rich discoveries.
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Glenn Smith is a freelance writer based in Taipei City.
Copyright © 2015 by Glenn Smith