▲ Korea Meteorological Administration National Earthquake and Volcano Center
This morning (the 7th), a 3.1 magnitude earthquake occurred in Yangseong-myeon, 22 km northwest of the center of Chungju, Chungcheongbuk-do. Fortunately, no significant damage was reported, but since it occurred on land and was the strongest among the seven earthquakes over magnitude 2.0 recorded this year, nearby residents were quite alarmed.
However, the emergency disaster alert that interrupted their sleep immediately after the earthquake reported a magnitude of 4.2. A few minutes later, the Korea Meteorological Administration revised the earthquake’s magnitude to 3.1.
According to the Korea Meteorological Administration, the early warning system automatically analyzed only the faster P-wave (Primary Wave) right after the earthquake at 2:35 a.m. and initially estimated the magnitude to be 4.2. As a result, emergency alerts were sent across the Chungcheong region, as well as Seoul, Incheon, Gangwon, Jeonbuk, Gyeongbuk, and Gyeongnam, indicating a 4.2 magnitude earthquake had occurred. The administration later adjusted the magnitude to 3.1 after a detailed analysis.
It is not uncommon for the estimated magnitude by the early warning system to differ from the magnitude calculated by seismologists conducting a thorough re-analysis of the seismic waves. For example, when a 4.8 magnitude earthquake occurred in Buan-gun, Jeonbuk, on June 12 last year, the early warning system estimated a magnitude of 4.9 by analyzing only the P-wave.
However, the discrepancy between the system’s estimate and the human analysis was quite large for this earthquake. A 1.1 magnitude difference signifies the seismic energy is about 44 times stronger. One reason for the significant difference between the reported magnitude in the emergency alert and the actual magnitude is the earthquake’s proximity of only about 7 km from the seismometer.
Natural earthquakes involve the S-wave (Secondary Wave) reaching later with greater amplitude and more complex waveforms than the P-wave. Hence, the early warning system utilizes the initial P-wave to issue alerts and prompt evacuations to mitigate damage, whereas the actual damaging seismic wave is the S-wave. The speed of the P-wave is about 6-7 km/s, while the S-wave travels at about 3.5-4 km/s. This earthquake was so close to the seismometer that the S-wave was detected just one second after it occurred.
This proximity may have caused the system to struggle in effectively separating the P-wave and S-wave, leading to an overestimated magnitude. The reduction in the number of seismometer observations used to issue alerts from six to four since 2022 might have also contributed, according to the Korea Meteorological Administration. Utilizing fewer seismometer readings can reduce the time required to issue alerts but tend to decrease the accuracy regarding the estimated location and magnitude.
The goal of the Korea Meteorological Administration is to issue early warnings and reports within 5 to 10 seconds of an earthquake being first detected by the seismic network, and alerts have recently been issued faster than this objective. This type of issue may recur in the future as the administration plans to increase the number of seismometers.
Currently, there are 173 seismometers in designated intensive monitoring areas such as the metropolitan area and around nuclear power plants, and 318 in regular monitoring areas. The average distance between seismometers (density) is 11.8 km in intensive monitoring areas and 15.5 km in regular monitoring areas. The Korea Meteorological Administration plans to increase the number of seismometers by 50 in intensive monitoring areas and 23 in regular areas to bring those distances down to 10.5 km and 14.9 km, respectively.
A representative from the Korea Meteorological Administration mentioned that they plan to improve their multi-analysis algorithm for earthquake early warnings, so they can better distinguish the P-wave and S-wave. While some people complain that earthquake warning texts are noisy and inconvenient, modern science and technology cannot predict exactly when earthquakes will occur, so people must tolerate it for their safety and survival. Evacuating to a safe location before the S-wave arrives is essentially the only way to prevent casualties.
One study suggests that if there is no earthquake warning, there could be 100 fatalities. However, if a warning is issued 10 seconds before the seismic wave arrives, fatalities could decrease to 10, with 80 people injured and 10 unharmed. If the epicenter is more than 50 km away, even a 10-second warning can provide 5 seconds for evacuation, considering the speed of the S-wave.
(Photo = Yonhap News Agency)