Warning issued 9 seconds after the earthquake occurred, the Meteorological Administration revealed the secret of "racing with the earthquake wave" to grab the prime time

After an earthquake occurs, how soon can a national alert be issued? Wu Jianfu, director of the Earthquake Forecasting Center of the Central Meteorological Administration, said that taking the 7.2-magnitude earthquake in Hualien on April 3, 2024, as an example, the Meteorological Administration issued the first wave of national warnings about 9 seconds after the earthquake occurred, racing against strong earthquake waves, giving people in some areas the opportunity to gain a few seconds to tens of seconds to respond before violent shaking. However, areas too close to the epicenter may start shaking before warnings are received.

Wu Jianfu explained at the practical meteorological seminar that after an earthquake occurs, P waves that are faster and usually less destructive will be emitted first, followed by S waves that are slower and usually cause stronger shaking. Earthquake rapid reporting uses the difference in transmission speed of two kinds of shock waves. After detecting the P wave at the measuring station, it quickly analyzes the location, size and estimated earthquake intensity of various places, and issues a warning before the arrival of strong earthquake waves.

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Wu Jianfu emphasized that earthquake warning is not "earthquake prediction", but the use of technology to race against the shock waves after the earthquake has occurred. Since the system needs to collect station data and perform calculations, there may be a "warning blind zone" near the epicenter. By the time people receive the warning, strong shaking may have already begun. In areas farther away from the epicenter, there is an opportunity to obtain a longer warning time.

Taking the 0403 Hualien earthquake as an example, a magnitude 7.2 earthquake occurred at 7:58:9.9 that day. The Meteorological Administration issued the first wave of national warnings at 7:58:18, notifying the four counties and cities of Hualien, Yilan, Nantou and Changhua. It then sent warnings to eight counties and cities including Taichung, Miaoli, Hsinchu County, Yunlin, Chiayi County, Tainan and Taitung at 7:58:24.

However, the 0403 Hualien earthquake also highlighted the technical challenges of immediate warning of strong earthquakes. The Meteorological Administration stated at the time that the earthquake early warning system had estimated magnitudes of 6.2 and 6.8, respectively, which were lower than the subsequent forecast of 7.2. This resulted in the estimated earthquake intensity in some areas being lower than the national warning threshold. The Meteorological Administration will subsequently review and adjust the issuance conditions. Starting from September 2024, new warning issuance conditions will be added with an estimated scale of 6.5 or above and an estimated earthquake intensity of 3 or above, expanding the coverage of the warning.

Wu Jianfu said that Taiwan continues to expand its earthquake observation network and improve earthquake forecasting technology, hoping to shorten the time for rapid reporting operations. Real-time warnings of strong earthquakes are delivered through channels such as Disaster Prevention Warning Cell Broadcast Message (PWS). However, the warning release time is not equal to the actual time each mobile phone receives it, and may still be affected by factors such as message delivery.

He reminded that the purpose of earthquake warning is not to predict when an earthquake will occur, but to provide early warning as soon as possible after an earthquake occurs. Even if it only takes a few seconds, people should immediately take earthquake prevention actions of "get down, cover, and hold on" to reduce the risk of injury.