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I. Courses of Action of the Central Weather Bureau
A strong earthquake of magnitude 9.0 struck Sumatra, Indonesia at 8:58 a.m. Taiwan time on December 26, 2004. The Central Weather Bureau (CWB) recorded seismic waves within 6 minutes of the occurrence and, within 15 minutes, received a tsunami warning message issued by the Pacific Tsunami Warning Center (PTWC) located in Hawaii, indicating no possibility of tsunami around the Pacific Rim. With the epicenter being 3,500 km away from Taiwan, the CWB released a message through the media urging people not to panic.
II. Tsunami environment in Taiwan
In the past decades, there were several major tsunamis occurred around the Pacific Rim region. Examples are the 1960 Chile tsunami and the 1964 Alaska tsunami, both having impact in the region. However, these tsunamis had only minor impact on Taiwan because the accompanying energy triggered by distant earthquakes had dissipated long before reaching the island.
As for Taiwan's nearby earthquakes, academic research suggests that the magnitude above which a triggered tsunami needs to be seriously guarded among various regions - 7 for Taitung region, 7.8 for Hualien region and 8 for Lanyu region. It can be concluded that the combined effects of seabed topography, earthquake, and regional coastal configuration make the coastal areas of Keelung, Yilan, and Hualien more prone to strikes of disastrous tsunamis caused by large inshore earthquakes.
Located in the Pacific Rim seismic zone, Taiwan is frequently struck by earthquakes, of which some have had magnitudes over 6 in the past century. Most of them did not trigger tsunamis and, even when they did, the triggered tsunamis were non-disastrous with small wave heights. This is because most inshore earthquakes occurred on the eastern side of Taiwan where the steeply descending offshore seabed prevents energy accumulation for tsunami. As for the western side of Taiwan where the seabed terrain is shallow, there are fewer earthquakes due to limitation in plate conditions, which reduce the probability of tsunami occurrence.
III. Historical tsunami records in Taiwan
According to historical records dating from 1661, Taiwan has been struck by six disastrous tsunamis, occurring in 1661, 1721, 1781, 1792, 1866, and 1867. Except for the one that occurred in 1867, only summary descriptions with no clearly quantifiable scientific evidence were recorded. The 1867 tsunami occurred in the Keelung region of northern Taiwan and was clearly determined as having been triggered by an inshore earthquake. Other than this, over 20 tsunamis in the Taiwan region have been recorded with no reports of damage.
IV. Tsunami warning system in Taiwan
Taiwan's tsunami warnings are issued by the CWB. After a distant earthquake occurs, the CWB will instantly obtain tsunami warnings issued by the Pacific Tsunami Warning Center. It will then estimate the potential impact on Taiwan before sending out a tsunami warning message via SMS or fax to relevant coast guards, central emergency management authorities, and to mass media, in an effort to reduce damage caused on shore by the tsunami.
The PTWC, a United States federal agency located in Hawaii, has contributed substantially to reducing damage caused by natural calamities such as tsunamis. After detecting a major earthquake in the Pacific Rim region, it will begin a comprehensive study on whether or not a tsunami will be triggered and will issue warnings to countries and regions that potentially will be struck by the tsunami, including direct notification to the CWB.
Tsunami related websites
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