There was an earthquake in Shanghai. Is this strange?

The China Earthquake Networks Center officially determined that at 01:39 on June 15, a magnitude 3.1 earthquake occurred in Qingpu District, Shanghai. Isn't Shanghai located in an earthquake zone? Why are there so many earthquakes?

Written by Li Peng Edited by Chen Yongjie

Interview experts:

Gao Mengtan (Specially appointed expert at the Institute of Geophysics, China Earthquake Administration)

The China Earthquake Networks Center officially determined that at 01:39 on June 15, a magnitude 3.1 earthquake occurred in Qingpu District, Shanghai (31.07 degrees north latitude, 121.09 degrees east longitude), with a focal depth of 8 kilometers.

▲According to the seismic network, a 3.1-magnitude earthquake occurred in Qingpu, Shanghai, in the early morning of June 15, with rumbling sounds and shaking. (Photo source: China Earthquake Networks Center)

The population density within 5 kilometers of the epicenter of this earthquake is about 860 people/square kilometer, with a population of about 68,000. The population density within 10 kilometers of the epicenter is about 1,500 people/square kilometer, with a population of about 480,000. The population density within 20 kilometers of the epicenter is about 1,700 people/square kilometer, with a population of about 2.2 million. It is precisely because of the dense population that the earthquake in Shanghai, although of a small magnitude, has still attracted much attention. Many people wonder, isn't Shanghai not in the earthquake zone? How could an earthquake occur? The earthquake map seen by many netizens is as follows: ▲ (Picture from the Internet)

Gao Mengtan, a specially appointed expert from the Institute of Geophysics of the China Earthquake Administration, said that the above "Distribution Map of Strong Earthquakes and Seismic Belts in China" is not completely reliable and does not truly reflect the areas and levels of earthquakes in China. The following "Distribution Map of Potential Earthquake Sources in China and Neighboring Areas" is much more reliable. (Please watch in horizontal mode)

▲ "Distribution map of potential earthquake source areas in China and neighboring areas" (Photo provided by @高孟潭)

As can be seen from the figure, although Shanghai is not directly on a larger seismic belt, it is also an area with relatively active earthquakes. In the broadest sense, all regions of China are on seismic belts, and earthquakes are possible in any place, it is just a matter of the size of the earthquake. In fact, Shanghai has several earthquakes every year, but most of them are of small magnitude and are not paid attention to. Although Shanghai is not a high-incidence area for earthquakes, it is not surprising that small-scale earthquakes occur. In addition, earthquakes in the surrounding areas and strong earthquakes at a distance will also have an impact on Shanghai. For example, the 6.1-magnitude earthquake in the Yangtze River Estuary in 1996 and the 9.0-magnitude earthquake in East Japan in 2011 were clearly felt in Shanghai. Since the 1970s, there have been as many as 20 earthquakes that have affected Shanghai. Statistics show that Shanghai has experienced more than 150 earthquakes in the past 500 years, including 89 local earthquakes and 61 earthquakes that affected other places. The risk of a large destructive earthquake in Shanghai is very low. According to historical data provided by the Shanghai Earthquake Administration, no earthquakes above magnitude 5.0 have occurred within 50 kilometers of the epicenter of this earthquake. The largest and most recent earthquake with a magnitude of 3.0 or above was the 3.0 magnitude earthquake in Qingpu, Shanghai on February 17, 1991. At that time, it was 4 kilometers away from the epicenter and no casualties were caused. There was one earthquake with a similar magnitude of 3 or above, and none of these earthquakes caused casualties. However, Shanghai is also a city that is vulnerable to earthquakes. First of all, Shanghai is located near the Pacific Rim seismic belt, and its foundation is weak, with a soft soil layer of about 400 meters underground. Secondly, Shanghai has many high-rise buildings and a dense population, and lifeline projects such as transportation, power supply, and water supply are intricate. Once a destructive earthquake occurs in Shanghai, the property loss and social impact will far exceed those in other regions. In addition, Gao Mengtan said that Shanghai has the background for earthquakes of magnitude 5 to 6. Shanghai has also experienced large earthquakes in history. On September 1, 1624, a magnitude 5 earthquake occurred in Shanghai, causing some houses to collapse. There are also some relevant records of large earthquakes in other regions causing damage to Shanghai. For example, on July 25, 1668, an 8.5-magnitude earthquake in Tancheng-Juxian, Shandong, affected Shanghai, causing the water level of the Pujiang River to surge, the ground in Chongming to crack, and the Liuguang Temple in Jiading to collapse. On April 14, 1853 and February 3, 1927, two 6.75-magnitude earthquakes occurred in the South Yellow Sea, causing strong tremors in Shanghai and causing damage.

▲ "Distribution map of destructive earthquake epicenters in mainland China and neighboring areas" (Photo provided by @高孟潭)

Although the risk of a large earthquake in Shanghai is not high, Gao Mengtan reminded that Shanghai also needs to take precautions against some higher-level earthquakes, and improving earthquake-resistant buildings is the most effective measure. He said that "small earthquakes can not be damaged, medium earthquakes can be repaired, and large earthquakes can not collapse" is the basic design principle of earthquake-resistant buildings in my country, and Shanghai is no exception.