Why was the earthquake in Hualien, Taiwan Province felt strongly in Jiangsu, Zhejiang and Shanghai?

According to the official measurement of the China Earthquake Networks Center, a magnitude 7.3 earthquake occurred at 07:58 on April 3 in the waters of Hualien County, Taiwan (23.81 degrees north latitude, 121.74 degrees east longitude), with a focal depth of 12 kilometers. The earthquake was felt all over the island. The southeastern coastal provinces also felt strong earthquakes, and the earthquakes in Fujian and Guangdong were very obvious. There were also earthquake feedbacks in Zhejiang, Jiangsu, Shanghai and other places. According to the latest reports, the region has a high level of seismic activity in history, and earthquakes of magnitude 6 or above are often accompanied by strong aftershocks of about magnitude 5, but aftershocks of this level will not have a significant impact on the mainland.

The results announced by the China Earthquake Networks Center's Rapid Report Weibo account (Image source: China Earthquake Networks Center Weibo)

The figure below is a statistics of earthquakes of magnitude 5 or above in Taiwan in the past three years that we found from the China Earthquake Networks Center, among which the largest earthquake was this one.

Statistics of earthquakes above magnitude 5 in Taiwan in the past three years

(Photo source: China Earthquake Networks Center)

Part 1

Why does Taiwan Island experience frequent earthquakes?

Distribution map of earthquake events in Taiwan (1900-2022, focal depth ≤40KM)

Image source: Reference 1

Taiwan Island is located at the southeast edge of the Eurasian Plate and the oblique convergence of the Philippine Sea Plate. It is part of the West Pacific Plate Convergence Belt, Volcanic Island Arc Belt, and Earthquake Belt. The Taitung Longitudinal Valley crustal docking zone is the boundary, with the western part belonging to the Eurasian Continental Plate and the eastern part belonging to the Philippine Sea Plate.

In southern Taiwan, the Eurasian Plate subducts eastward under the Philippine Plate with the South China Sea oceanic crust as the lead; in northeastern Taiwan, the Philippine Plate subducts northwestward under the Eurasian Plate, forming a unique double subduction zone - the Manila Trench and the Ryukyu Trench. These two subduction zones are important components of the trench-arc-basin system of the western Pacific formed in the Neogene, and are typical arc-continent collision orogenic belts on the western margin of the Pacific Ocean. The double subduction zones significantly control the geological characteristics and geotectonic structure of Taiwan Island and the surrounding waters, and the geological structure is complex.

Taiwan's geological structural unit division and important fault distribution map

(1) — Taitung Longitudinal Valley Fault; (2) — Lishan Fault; (3) — Laonong (Chaozhou) Fault; (4) — Quchi Fault; (5) — Sanyi Fault

Image source: Reference 2

Hualien County, the epicenter of the earthquake, is located in northeastern Taiwan. The Hualien-Taitung Valley (Hualien) is the boundary between the Philippine Sea Plate and the Eurasian Plate. Since these two plates push and squeeze each other to produce a double subduction zone, the Hualien earthquake basically occurred in this double subduction zone. This type of earthquake is generally a shallow source earthquake (focal depth is less than 70km). For shallow source earthquakes, if the magnitude is large, the damage caused is also relatively large, and the affected area will be relatively wide.

Earthquakes of magnitude 5.0 or above that have occurred near the Hualien area since 2020

Image source: China Earthquake Networks Center

From the earthquakes of magnitude 5.0 or above that occurred near the Hualien area in the past five years, it can be found that earthquakes frequently occur in the Hualien area, among which there are many earthquake swarms, and there have been many destructive earthquake swarms in history. On October 22, 1951, two strong earthquakes of magnitude 7.3 and 7.1 occurred in succession near the coast of Hualien; around February 6, 2018, three earthquakes of magnitude 6.0 or above occurred, namely Ms6.4 on February 4, Ms6.5 on February 6, and Ms6.1 on February 7; on April 20, 2015, three earthquakes of magnitude 6.4, 5.9, and 6.0 occurred. The occurrence of these earthquake swarms is closely related to the tectonic position of Hualien County.

Additional knowledge points about earthquakes:

Earthquake sequence : a general term for a series of earthquakes of different sizes that occur in the same source area within a certain period of time, and whose seismogenic mechanisms have some internal connection or common seismogenic structure. It is generally divided into three types: main shock type, earthquake swarm type, and isolated type .

Main shock type : Generally speaking, the first earthquake with the largest magnitude is called the main shock, and the smaller earthquakes that follow are called aftershocks. And as time goes by, the frequency and magnitude of aftershocks will fluctuate and gradually decrease.

Earthquake swarm type : The main feature is that there is no prominent main shock, and the energy is released through multiple earthquakes of similar magnitude. Within an earthquake sequence, the magnitude difference between each earthquake is very small.

Isolated type : Its main characteristics are that there are almost no foreshocks and almost no aftershocks. If there are foreshocks or aftershocks, their magnitude is very different from that of the main shock.

Part 2

Strong earthquakes were felt in the southeast: What factors are related to the earthquakes?

The epicenter of the earthquake was located in the sea, about 14 kilometers away from Taiwan Island. The earthquake caused strong tremors across the entire island of Taiwan, and the tremors were very obvious in Fujian and Guangdong. There were also tremor feedbacks in Zhejiang, Jiangsu, Shanghai and other places.

The earthquake was so strong that my classmates in Fujian were flooded with messages on their WeChat Moments.

So, is there a certain measurement basis for the shaking people feel during an earthquake?

The answer is yes, but it is not absolutely corresponding. There are many factors that affect our perception of earthquakes, and people in different locations in the same area may feel differently. We can roughly estimate the strength of the earthquake based on the damage to the ground caused by the earthquake, which is what we refer to as intensity in daily life.

Definition of earthquake intensity

Image source: Chen Qifu's courseware

The same earthquake has only one magnitude, and the intensity varies according to the distance from the epicenter or source. Earthquakes of the same magnitude will cause different levels of damage in different regions, and for earthquakes of the same magnitude in the same location, the shallower the epicenter, the greater the damage and the higher the intensity. In addition, due to the uneven geological conditions in different parts of the surface, the degree of damage is also different, so the strength of the earthquake in a region can only be roughly estimated.

We divide the country according to the degree of earthquake risk that each location may suffer in the long term, and use maps to show the differences in potential earthquake risks between regions, thus forming the China Earthquake Intensity Zoning Map.

China earthquake intensity zoning map

(Photo source: Henan Provincial Seismological Bureau)

Part 3

How can we save ourselves when encountering a strong earthquake?

Indoor shockproof method:

1. During an earthquake, find shelter nearby and protect your head with cushions or pillows. The safe posture you should take is: squat or sit down, curl up your body as much as possible, and lower your center of gravity.

2. You should choose a solid object that can protect your body. It is best to choose a place that is easy to form a triangular space. The most ideal place is the bathroom or kitchen, where you may have water and food to sustain your life.

3. When encountering a strong earthquake, remember not to open doors or windows or jump out of a building. You must hide first, and after the earthquake has passed, evacuate quickly when you have the chance.

Earthquake scene in Taipei, Taiwan

(Image source: China Daily)

Outdoor shockproof method:

1. Choose an open area to squat or lie down to avoid falling. Do not run around, avoid crowded places, and do not return to the house.

2. Be careful to avoid tall buildings such as buildings, overpasses, tall chimneys; avoid dangerous objects or hanging objects such as transformers, electric poles, street lights, billboards; also avoid narrow streets, dangerous walls, and piles of bricks, tiles, and wood.

3. If you are on the mountain, you should avoid the foot of the mountain and steep cliffs to prevent landslides, mudslides, mud-rock flows, etc.

The aftermath of the earthquake in the Philippines

(Image source: China Daily)

Shockproofing methods in public places:

1. Avoid rushing out blindly. Follow the instructions of on-site staff, don't panic, and don't crowd at the escape exit.

2. Take different shock-absorbing measures according to different situations. When you are in a shopping mall or subway, squat on a sturdy counter, pillar or corner and protect your head.

3. Avoid glass doors and windows; avoid tall, unstable structures or racks where heavy or fragile objects are placed; avoid hanging objects such as billboards and chandeliers.

Shock absorbers in the car:

1. During an earthquake, drivers of moving cars should not brake suddenly, but should slow down and park by the side of the road.

2. If you are on or near an overpass, viaduct, or highway, leave quickly.

3. When on public transportation, passengers should hold on to the handrails to avoid falling or getting injured; or lower their center of gravity, hide near their seats, and get off the vehicle after the earthquake.

Image source: Pexels

Self-rescue after the earthquake:

1. After a major earthquake, aftershocks may occur from time to time, and the surrounding environment may further deteriorate. Try to change your environment and try to move after it stabilizes.

2. If you are injured, you should bandage it properly to prevent excessive bleeding and loss of life. If you cannot get timely help, you should search for drinkable water, beverages, and edible food around you.

3. Don’t shout randomly, conserve your strength, and when you hear voices outside, knock to call for help.

Mutual rescue after the earthquake:

1. Those who escape danger, if they are not injured or only slightly injured, should actively participate in mutual rescue activities.

2. When digging out buried people, the supporting structures should be protected to prevent further collapse and further injury. If the trapped person cannot climb out by himself, do not pull him or her hard to prevent injury to the body.

3. When carrying a person with spinal injuries, use a door panel or a hard stretcher. If you cannot rescue a survivor immediately, leave a mark for the purpose of rescue.

References:

[1] Lu Donghua. Research on shallow seismicity model in Taiwan[D]. Institute of Geophysics, China Earthquake Administration, 2024. DOI: 10.27487/d.cnki.gzdws.2023.000040.

[2] Zhang Kaibi, Xu Weiguang, Chen Shuhua, et al. Introduction to Taiwan Regional Geology[J]. Fujian Geology, 2017, 36(02): 79-93.

China Earthquake Networks Center: http://news.ceic.ac.cn/

Produced by: Science Popularization China Author: Yang Fan (Institute of Geology and Geophysics, Chinese Academy of Sciences)

Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)