Taipei City residents were recently awoken in the early hours by a shallow, magnitude-4 temblor within city limits. Due to its location on the Pacific Ring of Fire, at a place where tectonic plates collide, Taiwan is an earthquake hotspot, and quakes of this intensity are common enough.
What set alarm bells ringing in local media was that it was the biggest earthquake in 26 years within the perimeter of the Tatun Volcano Group in Yangmingshan National Park in the city’s north.
Comments by Central Weather Bureau Seismological Center Director Kuo Kai-wen on local cable TV, that described the temblor as a normal stress adjustment caused by cooling magma under the mountain, did little to allay fears.
A flurry of speculation followed that the Tatun volcanoes, once considered dormant, could be springing into life. Huge posters around town announcing the imminent release of Hollywood blockbuster “Pompeii” might have fueled fantasies that lava could soon be rolling down the flanks of Qixing Mountain and into central Taipei.
Experts appear divided on how dangerous the Tatun group really is. What is certain is that the group poses a much more serious threat than was thought as recently as 20 years ago, when the prevailing view was that the volcanoes were dormant, if not extinct, with no eruption in the past 200,000 years or so.
“In 2009 or 2010 they found evidence of volcanic activity just 6,000 years ago,” said Chan Yu-chang, an associate research fellow in Taipei City-based Academia Sinica’s Structural Geology and Remote Sensing Laboratory. “That’s the most recent. They found an ash layer in one location, and there was carbon underneath it, so they could date the layer.”
He added that there is also evidence of an eruption 20,000 years ago, based on volcanic ash found under the Taipei basin when drilling core samples.
Although the usefulness of such a division is disputed, academics divide volcanoes into three types: active, dormant and extinct.
“Classification of a volcano as active is based on two sets of criteria: historical evidence and its current physical state,” National Taiwan University Department of Geosciences professor Song Sheng-rong said. “From a historical perspective, to count as active, a volcano must have erupted in the past 10,000 years. In terms of physical activity, the crucial question is whether there is a magma dome underneath the volcano.”
It is this latter point that is generating all the controversy. Some experts claim the recent quake shows the presence of magma, as lava is known in its underground state, while others say the evidence is in doubt. What causes a volcanic eruption is the swelling of a magma dome under the volcano, causing pressure to build up until it becomes too much for the rock to take and results in a catastrophic explosion.
According to Chan, the Feb. 12 quake could have been caused by an active fault, or it could be due to volcanic activity. “To determine what caused the quake, we can monitor the distribution of aftershocks to see if they lie along the fault line or if they are distributed randomly within the Tatun area.”
Chan suggested that only in the latter case could the shocks be ascribed to volcanic activity, “but even then, it is not necessarily due to a magma dome forming.”
One thing the experts do not dispute is that there is a lot of volcanically-fueled fluid activity in the area. Qixing Mountain, the most recently active peak, features several famous fumaroles, or steam vents, on its flanks, such as Siaoyoukeng and Dayoukeng.
These fumaroles are easily noticeable to the least-practiced eye. Each vent is surrounded by a large open gray scar in the mountainside, as the high sulfur content and heat is poisonous to vegetation. Without plant cover, the unstable ash layers are swiftly eroded by heavy rainfall and typhoons.
The many hot springs that dot the lower slopes, popular with tourists and locals alike, are also powered by water heated deep within the earth’s crust. The vents and springs prove useful places for researchers to monitor activity. In 2011 the government established the Tatun Volcano Observatory within the park to help coordinate this work.
Movies about Pompeii may alert people to the dangers of an eruption, but Qixing is no Vesuvius. Both Taipei City and Naples, Italy are at about the same distance, around 10 kilometers, from their respective volcanoes, but Taipei is at much less risk.
It is estimated that around 10,000 people died in 79 A.D., when Pompeii was hit by a huge pyroclastic flow—that is, a superheated cloud of poisonous gas mixed with rocks and ash that shoots out of a volcano during an eruption. The cloud will burn or suffocate anything in its path, and can move hundreds of kilometers per hour.
Vesuvius has been erupting regularly ever since, most recently in 1906, 1929 and 1944. If Naples’ population of around 1 million sees no reason to panic, then people in Taipei, who might suffer an eruption every 10,000 years, instead of every few decades, should keep cooler heads.
In any case, unlike with major earthquakes, volcanoes usually give plenty of warning before they explode.
“There would be plenty of warning of any imminent eruption,” Chan said. “There would be increased seismicity, with 10, 100 or even 1,000 times more earthquakes per month than are occurring now.”
According to Chan, Taipei is subject to temblors all the time, but most of them are too small to be noticed by humans.
“So far there has been no increase, so I wouldn’t worry too much,” he adds. “And there would be plenty of advance warning of an eruption, maybe months before anything happened.”
The intense monitoring that has taken place in recent years has shown no sign of an upturn in activity, Chan said. As well as measurements of such things as the acidity and temperature of hot springs and emissions from vents, the Tatun observatory coordinates measurement from a large array of seismometers in the area. He said researchers are also conducting tomography, which uses seismic waves to image sub-surface characteristics in order to understand the deeper geological structure. “It functions rather like a medical CT scan, but for the earth’s crust.”
So is all the media hype a case of “Qiren you tian,” the man of Qi in ancient China who could not sleep because he thought the sky was falling?
Taiwan does have another volcano that no one disputes is active — Guishan Island, off the coast of Yilan County in northeastern Taiwan. Song said that according to the geological record, Guishan had erupted four times in the past 7,000 years, and the evidence for a magma dome existing underneath it is fairly conclusive, as shown by core drilling studies he has led, for example.
Looking at the various threat scenarios posed by an eruption on the island, Song was quick to dismiss the danger from lava and pyroclastic flows. “There is almost 10 kilometers of ocean between the island and the coast, so there is no way lava would cross that.”
However, he thinks Yilan is potentially vulnerable to a tsunami generated by a massive landslide, triggered by either an earthquake or an eruption, as the plain lies a mere 5 to 10 meters above sea level.
“This is a real possibility because if you look at the northeast part of Guishan Island, you can see that a corner is missing,” Sung said.
Song said that the explosion of Indonesia’s Krakatau Island in 1883 set a worrying precedent as it caused a tsunami 35 meters high, estimated to have killed more than 30,000 people. “Looking at Yilan, we might worry what effect a tsunami 5 or 10 meters high could have.”
An additional possibility is a collapse, such as occurred in 1792 at Japan’s Mount Unzen. Song explained that volcanic temblors caused the entire flank of the mountain to collapse, provoking a tsunami that was 10 meters high when it hit the opposite side of the bay and killed more than 10,000 people in total.
Chan echoed Song’s landslide concerns for the Tatun group. “The mountains in Yangmingshan National Park are very steep and many of them are made of volcanic ash which is loosely consolidated. There’s a risk that a large earthquake could trigger a landslide.”
Even if an eruption at Guishan did not kill anyone in the vicinity, it could still present problems for inhabitants in the rest of Taiwan. Iceland’s Eyjafjallajokull, which exploded in 2010, did not cause any deaths directly, but played havoc for weeks with U.K. air flights hundreds of kilometers away.
Closer to home, the Philippines’ Mount Pinatubo exploded in 1991, leading to the closure of the U.S. air force’s Clark Air Base. Song said he wrote a paper several years ago exploring what might happen to Taiwan if Guishan erupted on a similar scale. The effects would depend very much on wind direction, and hence on seasonal factors, he said, but could prove highly disruptive to Taiwan Taoyuan International Airport.
For the time being at least, it would seem that Taiwan has more to worry about from earthquakes and the annual crop of typhoons than from the Qixing volcano springing back to life. It is comforting to know, however, that experts are keeping a close watch on the situation and working hard to gain a clearer picture of exactly what is going on 10 kilometers beneath the city. (SDH)