Explainer | Recent earthquakes in Hong Kong ‘not a signal of more, or worse, to come’
- City hit by an average of two tremors each year and there have been no casualties since records began
- Observatory sees no trend of quake activity increasing, becoming more intense
Hong Kong has long been regarded as blessed for being affected by few natural disasters, but the recent tremors have prompted questions, not least whether they are a sign of more, or worse, to come.
Victor Ting finds out more about earthquakes in Hong Kong, and whether the city is resilient enough to withstand future occurrences.
Are earthquakes common in Hong Kong?
The first locally felt tremor was recorded in 1905 and since then, 187 tremors of various intensities have been recorded, according to the Hong Kong Observatory. The Hong Kong Short-period Seismograph Network began monitoring local tremors in 1979 and, since then, 78 have been recorded, or an average of about two locally felt tremors each year.
The strongest tremor occurred in 1918 and was caused by an earthquake near Shantou, in Guangdong province, about 300km from Hong Kong. It caused minor damage to a few buildings in Hong Kong – the only event on record to have such an effect.
A slightly more intense tremor was recorded in 1994, following an earthquake in the southern part of the Taiwan Strait.
No locally felt tremor has caused casualties in Hong Kong since records began, and the observatory says the chance of a major local tremor remains small. It monitors earthquakes worldwide, and especially those in the South China Sea.
Sichuan earthquake, 10 years on: how a tragedy changed China
What caused the recent earthquakes?
Most earthquakes occur in the earth’s crust, when there is a sudden fracture or slip along the boundaries of “plates”, or massive sheets of rock, that are always moving slowly relative to each other.
Hong Kong lies within the Eurasian Plate and is located far from the nearest boundary with the Pacific Plate on the Circum-Pacific Seismic Belt that runs through Japan, Taiwan and the Philippines.
Chan Sai-tick, one of the observatory’s senior scientific officer, says the recent earthquakes were not caused by the collision or movement of tectonic plates, but rather by the release of pent-up pressure along small fault lines on the Eurasian Plate.
Earthquakes are measured by their magnitude and intensity. Magnitude refers to the energy unleashed at the earthquake’s epicentre, commonly measured by the Richter scale.
Intensity is a gauge of the shaking effects on human beings, buildings and the environment, and Hong Kong is among many places that use the Modified Mercalli Intensity Scale to rate this on a scale of 1 to 12, where 1 indicates a tremor “not felt by persons”, while 12 indicates the occurrence of major damage.
Should Hongkongers worry?
Last Sunday’s earthquake had an intensity of 3.4 on the Modified Mercalli Intensity Scale, and people said hanging objects swung about, while windows and doors rattled. Some said the tremor woke them up.
Most of the 1,200 people who reported that they felt the earthquake came from the southern part of the city.
Jacky Wong, 35, a Cheung Chau resident, says: “I was in the bathroom brushing my teeth when I felt some movement of the ground below my feet. My window frames were shaking, but I wasn’t too afraid. I went back to my bedroom and found my wife and kids still sleeping all right.”
Chan says many people in Hong Kong felt the tremors, “because of the close proximity of this earthquake”.
Still, he feels Hongkongers need not worry that recent tremors were a signal of more or worse to come. “Statistically, they do not represent an increase in the trend of locally felt tremors,” he says.
Are Hong Kong buildings strong enough?
All buildings in Hong Kong must be designed to withstand winds of 250km/h. The observatory said even in an earthquake with an intensity of 7 on the Modified Mercalli Intensity Scale, little damage would occur to high-rise buildings.
Infrastructure, including bridges, railways, tunnels and subways have been designed to withstand earthquakes of up to Intensity 7, while reservoirs have been built to withstand tremors of Intensity 8.
Veteran structural engineer Ngai Hok-yan says even on outlying islands such as Cheung Chau, where there are more old buildings, the recent tremors would not have caused much damage.
“If the old buildings already had cracks, the earthquakes may have widened them a bit and caused some plaster to fall from the ceiling. But generally speaking, buildings in Hong Kong are strong enough to hold up during quakes.”
