How to know in advance when an earthquake is coming?

At 12:52 on September 5, 2022, a 6.8-magnitude earthquake occurred in Luding County, Ganzi Prefecture, Sichuan. Many media reported that before the earthquake, residents in Sichuan and Chongqing sent earthquake warnings on their mobile phones. One Chongqing netizen’s warning screenshot showed that "64 seconds later, the earthquake's transverse wave reached Jiangbei District, Chongqing City, with the epicenter in Luding, Sichuan. The warning magnitude was 6.9, and the tremor was strong."

(Photo source: Guangming Daily)

At 22:55 on June 17, 2019, a magnitude 6.0 earthquake occurred in Changning County, Yibin City, Sichuan Province (28.34 degrees north latitude, 104.90 degrees east longitude), with a focal depth of 16 kilometers.

Before the 6.0-magnitude earthquake wave reached the urban area of ​​Chengdu, the "earthquake warning" alarm sounded in many communities and schools in Chengdu. The loudspeakers installed in the communities used a countdown method to inform residents that the earthquake wave was about to arrive. After the earthquake, many Chengdu citizens filmed earthquake warning videos and posted them online. The mainland earthquake warning network jointly built by the Chengdu High-tech Disaster Reduction Institute and the emergency management department successfully warned of this earthquake.

Earthquake warning map

(Image source: self-made by the author)

What is the earthquake early warning system and why can it warn of earthquakes?

Since seismic waves are mechanical waves, their propagation speed is much slower than that of electromagnetic waves. Therefore, when an earthquake occurs, the early warning sensing device will transmit the information to the early warning center after detecting the seismic waves. The early warning center will determine through calculations whether it is necessary to issue early warning information. Once the impact of the earthquake reaches a response level, earthquake early warning information can be issued through television, text messages and other channels. When seismic waves come, there is a certain amount of warning time.

Therefore, a complete earthquake early warning system needs to consist of five parts: sensing device, early warning center computing device, communication device (early warning information release device), early warning information receiving device and supporting emergency disposal and risk avoidance plan. The information received through the APP installed on the mobile phone, text messages, or TV signals belongs to the early warning information receiving end.

Schematic diagram of the working principle of the earthquake early warning system

(Photo source: Chengdu High-tech Disaster Reduction Research Institute official website)

So why can the earthquake early warning system warn of earthquakes? The answer is time lag.

To be precise, there are two time differences: one is that when an earthquake comes, there is a time difference from the epicenter to when we feel the seismic waves - the time difference between the propagation speeds of different seismic waves. There are three types of seismic waves: longitudinal waves, transverse waves, and surface waves. Longitudinal waves are the fastest but less harmful, surface waves are the slowest but most harmful, and transverse waves are in between. This is why a "big firecracker" is set underground (the epicenter emits energy instantly), but what we feel is a short-lived process, because several types of earthquake waves are transmitted in sequence. Waves close to the epicenter last for a short time, and those far away may even feel as if they stopped in the middle and then started to shake again. This is not your illusion.

(Photo source: provided by the author)

Let me tell you an experience that most people don't want to know. When an earthquake strikes, people first feel shaking, then swaying, and then shaking and swaying again. When the third feeling comes, the real harm of the earthquake begins. It's just that the three waves not only have different propagation speeds and propagation capabilities, but people's perception of these three waves is also different. If a person is far away from the epicenter, he may only be able to perceive the more destructive seismic waves such as surface waves.

(Image source: self-made by the author)

Another is the time difference between the propagation of mechanical waves and electromagnetic waves.

This is because the time difference between mechanical waves is relatively limited, because the propagation speed of seismic waves is generally between 5km/s and 15km/s. It is particularly difficult to distinguish shallow-source earthquakes that cause great harm. The propagation speed of electromagnetic waves is 300,000km/s, and the propagation time is almost negligible. This is the key to earthquake early warning.

Netizens still have time to take photos of earthquake warning information before evacuating. Although the earthquake warning information already contains information about the earthquake intensity, we still do not recommend doing so, because every second of an earthquake is precious and every second is snatched by technicians.

The cost of building earthquake early warning devices is high and it is difficult to warn in the epicenter area

Of course, earthquake early warning also has its own shortcomings.

First of all, there is the cost of building earthquake early warning devices. As shown in the previous mechanism schematic diagram, the centralized device does not require additional construction and can be shared with other devices with this function. However, earthquake early warning devices are different. In theory, the more early warning devices are built, the more accurate and timely the response will be, but the cost does not allow us to do so. So as shown in the figure below, the earthquake early warning sensor cannot be located exactly at the epicenter, because this will form an area that is hit by earthquake waves before the early warning sensor. In this area, we can only hope that the epicenter is deep and the time difference between shear waves and surface waves is sufficient to implement the earthquake evacuation plan.

(Image source: self-made by the author)

Secondly, for shallow earthquakes, the earthquake warning time may be almost zero or only a few seconds for the epicenter and surrounding areas where the impact is greatest. This is because the shallower the epicenter, the shorter the time difference between the longitudinal wave and the surface wave, and the later the warning will start; the closer to the epicenter, the earlier the mechanical wave is received, and the time to receive the warning is almost the same, which makes it difficult to ensure that people near the epicenter have enough time to take shelter. In fact, the earthquake near the epicenter suffers the greatest impact, which can be said to be a problem that the earthquake warning mechanism cannot solve.

Earthquake early warning is to create as much time and opportunity as possible for everyone to evacuate. Although there is some truth in the saying "you don't need to run in a small earthquake, but you can't run in a big earthquake", a positive sense of evacuation and various self-help behaviors after the disaster will improve your ability to resist disasters ; moreover, with the promotion of early warning mechanisms and the implementation of various automated plans, the moment the earthquake alarm comes, the computer can take over and implement the plan, such as cutting off the power supply, stopping high-speed trains in an emergency, and automatically closing some places where dangerous materials are stored, etc., to prevent secondary damage caused by earthquakes.

In summary, although earthquake early warning can let people outside the epicenter area know the arrival of earthquake waves in advance, it requires more complete earthquake disaster prevention and mitigation plans and good earthquake avoidance training for the participants, so as to achieve "no panic in the face of natural disasters, and orderly and coordinated survival."

References:

1. Natural Disasters, 6th edition, by Abbott P.

2. Earthquakes, Volcanoes and Other Geological Hazards by Jon Erickson

3. "Natural Disasters" 4th Edition Author: Chen Yong, Shi Peijun

Produced by: Science Popularization China

Produced by: Jing Bo (Liaoning Team of China Building Materials Industry Geological Survey Center)

Producer: China Science Expo