I. Moderate and strong earthquakes
Under normal circumstances, earthquakes of magnitude 5 or so can cause different degrees of damage or disasters. According to historical records, there have been two destructive earthquakes in Fuzhou history.
(1) The fourth day of August in the second year of Wanli in Ming Dynasty (1August 574 19) Fuzhou-Lianjiang earthquake.
Epicenter location: 26.2 north latitude and 1 19.4 east longitude.
Magnitude: M=53/4.
Earthquake situation: the epicenter intensity of the earthquake reached seven degrees (Figure 2-2), Fuzhou, Lianjiang and other places suffered different degrees of damage, and the earthquake affected a large area, and most areas in Fujian felt the earthquake.
Fuzhou: During the day in August, there was thunder in the air and a big earthquake. Stones from Fangshan (that is, Wuhu Mountain) fell into the field. The sound goes from northwest to southeast, but not all counties have it.
Lianjiang: In autumn and August, there was a big earthquake with noise. From the northwest, rocks fell and houses collapsed.
Xinghua House: After the earthquake, from southeast to northwest, the sound was like thunder, the houses shook and the ditches overflowed.
Nanping: Earthquakes are audible from the end to the time of applying for the World Heritage.
Youxi: There is a big earthquake, and its sound is like thunder.
Daejeon: The earthquakes in all seven counties are the same.
Quanzhou: Earthquake, Zimaoshan Rift Valley.
Jiang Le: August earthquake.
Huian and Yongchun: Earthquake. (2) Yongtai earthquake in September (1825, 10).
Epicenter location: 25.9 north latitude and 1 19.0 east longitude.
Magnitude: M=43/4.
Earthquake situation: Yongtai earthquake was thunderous and many houses were damaged. It is estimated that the epicenter intensity reached six degrees.
Second, there is a sense of shock.
The magnitude is about 4. When an earthquake happens, most people can feel it near the epicenter. This kind of earthquake is called inductive earthquake, but generally it will not cause damage or only slight damage. As for the degree and scope of feeling, it is related to the depth and time of the earthquake. There are ***9 1 earthquakes recorded in Fuzhou area, including 3 earthquakes of magnitude 4.0 ~ 5.0. Since the establishment of Fujian Seismological Network, three earthquakes of magnitude 3.8 or above have been recorded in Fuzhou, namely14.0 earthquake in the southeast of Fuqing County on May 27, 1996, 3.8 earthquake in the southeast of Pingtan and13.8 earthquake in Baimiao, Fuzhou on September 24, 1999.
(1) In October of the 21st year of Ming Chenghua, the Fuzhou earthquake occurred in Ding Wei (1485 12.6).
Epicenter location: Fuzhou.
Magnitude: M=4.
Earthquake situation: Fuzhou earthquake, starting from the northwest, can be heard, Luoyuan, Lianjiang, Changle also have a sense of shock.
(2) Fuzhou earthquake in June of the 22nd year of Ming Chenghua (1July 6th, 486).
Epicenter location: Fuzhou.
Magnitude: M=4.
Earthquake situation: Fuzhou area: Fuzhou and Xinghua areas have a sense of earthquake; Fuzhou mansion night earthquake. Lianjiang: Earthquake in summer and June. Xinghualou, Putian and Xianyou: Earthquakes can be heard.
(3) The Lianjiang earthquake in Chen Geng on April 6th, 22nd year of Ming Jiajing (1May 9th, 543).
Epicenter location: Lianjiang.
Magnitude: M=4.
Earthquake situation: Lianjiang: Great earthquakes occurred in summer and April. Fuzhou, Xinghua, Quanzhou and Zhangzhou felt the earthquake on the same day.
(4) Fuqing County Southeast Earthquake in June 5438+0988+1October 3 1.
Epicenter location: 25.65 north latitude and 1 19.50 east longitude.
Magnitude: ML=4.0.
Focal depth: 9 km.
Earthquake situation: the macro epicenter is located in the southeast of Fuqing Chengguan, about 4 kilometers away from the micro epicenter. The intensity of the earthquake zone is five degrees. During the earthquake, there was a strong sense of earthquake in the central area and its surrounding areas, and three intensity zones of five to three degrees were defined for the macroscopic investigation of the earthquake site (Figure 2-3).
Fifth-degree zone: The isoseismic line in the fifth-degree zone passes through Longxi, Lu Hong, Youtang, Anmin and Baidu in Fuqing, showing an ellipse with the long axis running northeast, with the long axis about 17km and the short axis about 12km. During the earthquake, most people in the room were awakened. Some people fled in panic, while others felt unstable. Unstable objects in the house were pushed down and knocked down. Many people outside felt it, too. During the earthquake, the water surface of paddy field in Youtang village fluctuated, and fish in Nipu village jumped out of the water in groups.
Houses in the area are slightly damaged, and there is a phenomenon of falling dust and soil. For example, in Anmin village, the top corner of a niche fell off; Wall cracks appeared in Beilin Village, and the plastering layer peeled off about 30 square centimeters. The mortar layer with a thickness of 2 cm on the window sill of a classroom in Yidu Middle School fell off, covering an area of 50×47 cm; There are sporadic tiles falling in Tan Song, Yang Xia, Zhuangdian and other places, including 7 or 8 tiles falling on the roof of Yidu Village; Cracks appeared in the old pool of a well in longxi village.
Four-degree zone: the four-degree zone has a large scope, starting from Shimen area in Changle in the north and reaching the vicinity of Putian estuary in the south. The eastern boundary extends along the first line of Zhanggang, Matsushita, Fuqing Port Head and Jiangyin in Changle, and the western boundary protrudes to the southeast, extending along the first line of Qianying, Fuqing Pass and Putian Zhuangbian in Changle. It is generally shaped like a pig's waist, northeast in the north and southwest in the south. The major axis is about 72 kilometers, the minor axis is about 35 kilometers, and the area is about 2000 square kilometers.
Fourth degree zone: most people feel indoors during the earthquake, only a few people feel outdoors, and people who are moving generally don't feel. People feel the bed shaking, see the hanging electric lights shaking, hear the banging of utensils around them, the windows and doors creak, the copper rings of door knockers and box buckles jingle, the roof tiles rustle, and the floor and wardrobe are shaking.
Third-degree zone: a small number of people who are still indoors feel it. Some people feel the house shaking, see the outdoor wires shaking, and hear the subtle noise of doors and windows.
(5)1September 24, 999 65438+ Fuzhou M = 3.8 earthquake.
Earthquake location: 26. 10 north latitude and 9.45 east longitude1/kloc-0.
Magnitude: ML=3.8.
Focal depth:13km.
Earthquake situation: From 2 1: 23 on September 23, 0999, small earthquake swarms occurred about 13 km northeast of Fuzhou (Guling Baimiao), and by the end of June, there were 5438+0999, 65438+February 3 1 458 times. 33 earthquakes of magnitude 2.0 ~ 2.9; There were 9 earthquakes of magnitude 3 or above, and the largest earthquake was magnitude 3.8 on September 24th (Table 2-2). This earthquake swarm activity is small in magnitude, but high in frequency, which is rare in the history of Fuzhou area. The number of earthquakes in this swarm activity accounts for about 70% of the total number of earthquakes since Fuzhou 197 1. After the earthquake, it has attracted great attention from all sides and strengthened the research work on seismic geological structure and seismic activity in this area. When an earthquake of magnitude 3.8 occurred, it was felt near the epicenter and in Fuzhou.
Table 2-Statistical table of frequency of small earthquakes in Baimiao, Guling from September 23rd, 1999 to February 30th, 2009.
earthquake magnitude
The macro epicenter of the 3.8-magnitude earthquake is located in Baimiao Village, Guling, Fuzhou, and the intensity of the extreme earthquake zone is five degrees.
Fifth-degree zone: The fifth-degree zone is an ellipse running northwest, distributed along the line of Baimei, Xintuling Street Office and Jiguan Mountain, and Jiahu and En Ding Cun are five-degree abnormal zones. People in this area generally hear strong ground sounds, and there is a sense of up and down vibration indoors and outdoors. People in the room fled in panic, doors and windows creaked, roof tiles creaked, the roof was dusty, some eaves tiles fell, and vases and other utensils fell on the table. For example, bricks fell from two chimneys in Baimei Village, cracks appeared in some plastered walls, and a stone-based pigsty was knocked down.
Four-degree zone: The four-degree zone covers a wide range, including Qianyang, Niuxiang, Gongping, Minyi, Nanyang, Guo Lun, Gulingping, Dayang, Liangcuo and Goose Nose. In the earthquake zone, people usually hear the sound of the earth, most of them feel it indoors, some people panic and flee, doors and windows, roof tiles sound, dust falls, and some people feel it outdoors.
Fuzhou and Tingjiang are three zones. During the earthquake, someone felt and heard the voice of the earth indoors.
Thirdly, Shuikou Reservoir induced an earthquake.
The dam site of Shuikou Reservoir is located on the main stream of Minjiang River in dongqiao town, Minqing. 1March 3, 9931day, reservoir storage. On May 23rd, Fuzhou Seismic Telemetry Network recorded the first 0.6-magnitude reservoir-induced earthquake, the epicenter of which was near the estuary of Gutian County. On July 23, two earthquakes of magnitude 3 or above occurred in succession, accompanied by a series of small earthquakes. Since then, seismic activity has fluctuated and gradually increased. Since1March 3, 19931,there have been more than 500 earthquakes in * *, with the largest earthquake being1April 2, 19961with a magnitude of 4. 1. The earthquake induced by Shuikou Reservoir has attracted great attention of provincial and municipal leaders and earthquake departments. The Shuikou reservoir area has strengthened the construction of earthquake monitoring and precursor network, closely monitored the development trend of seismic activity in the reservoir area, and organized relevant experts to conduct in-depth research on the seismic geological structure, historical earthquakes and reservoir seismic activity characteristics around the Shuikou reservoir area. The research results show that the induced earthquake in Shuikou reservoir area has the following characteristics:
(1) Shuikou reservoir area belongs to the area with few earthquakes and weak earthquakes in history. Within 20km of the reservoir area, there was basically no weak earthquake activity before 197 1 impoundment. Only five earthquakes with magnitude ≥3 were recorded in Fuzhou, Yongtai, Changle, Minhou and Lianjiang around the reservoir area. The largest earthquake was Minqing 199 1, and the magnitude was 3.5 on June. There were three moderate earthquakes in the history of 100 km reservoir area, namely Youxi M 53/4 earthquake in the fourth year of Northern Song Dynasty (1067), Fuzhou-Lianjiang M 53/4 34 earthquake in the second year of next year (1574 August 19) and Daoguang earthquake in the fifth year.
(2) The epicenter was concentrated. Mainly concentrated in the upstream area of the dam, covering an area of about 13 square kilometers. The dominant orientation of epicenter distribution is about 30 ~ 35 northeast.
(3) The focal depth of the earthquake is shallow. It is mainly distributed around 1 1 km, basically stable at 4 ~ 10 km, and a few depths reach 14 km, which is obviously shallower than the focal depth of tectonic earthquakes in surrounding areas.
(4) Earthquakes are closely related to water level changes. The sudden change of water level at high water level often induces large earthquakes. The corresponding relationship between Shuikou reservoir earthquake and reservoir water level can be roughly divided into two situations. First, in the initial stage of water storage (roughly before1March 1994), when the water level has not reached the normal operating height, the water level rises and falls sharply, and then an earthquake occurs, which lags behind the time when the water level changes rapidly. Second, after the water level basically reaches the normal height of more than 60 meters, a major earthquake will be induced after the water level rises and falls, but the time of earthquake occurrence lags behind the change of water level for a long time, and this feature has gradually disappeared with the passage of water storage time.
(5) There is little single earthquake activity in the reservoir area, and small earthquakes are mainly grouped. The analysis of the earthquake sequence in the reservoir area shows that the earthquakes in the reservoir area have the nature of precursor swarm or foreshock sequence of tectonic earthquake, whether in different periods or in the whole sequence.
From1June 1993 to1March 1995, several small earthquakes with relatively concentrated strain energy release occurred in the reservoir area, and each event had a large earthquake of magnitude 3 or above. After a year of relative calm, an earthquake of magnitude 4. 1, 1996 occurred on April 2, which is the largest earthquake in the reservoir area so far.
4. 1 The epicenter intensity of the earthquake is five degrees (Figure 2-4), the macro epicenter is located in Wankou-Qianyang area, and the extreme earthquake zone includes natural villages such as Wankou, Qianyang, Kengdou, Songxi and Liucheng.
Fifth-degree zone: the fifth-degree isoseismal line passes through the location of Shuikou Town, extends to Gaozhou in the southwest, turns to Liucheng in the north-north, turns to the east in the west, passes through Houwang Village and Tang Dou Village, and stops at Shuikou, showing an oval shape, and runs northeast, with the major axis of about 12km and the minor axis of about 8km. Most of Wudu District is located in Gutian County, accounting for 75% of the total area, and Nanping and Minqing account for 75%. During the earthquake, most people inside and outside heard the thunder and felt the ground was uneven. Several people fled in panic, beds, doors and windows shook, dishes on tables and cabinets collided with each other, and hanging objects shook. The local houses in this area were slightly damaged, such as mud falling off the walls of brick houses, cracks in the walls, aggravated old cracks, scattered tiles falling from the eaves, plaques falling in front of temples, etc. Some old earth walls that have been in disrepair are inclined to crack.
Grade IV area: The isoseismic line of Grade IV area reaches Minqing East Bridge in the east, Youxi Xibin and Yangzhong in the west, Minqing Baiyun in the south and Gutian County in the north. It is an ellipse extending to the northeast, which deviates from the long axis of the fifth degree zone by about 20 degrees. The major axis is about 33 kilometers, the minor axis is about 24 kilometers, and the total area is about 700 square kilometers. During the earthquake, most people inside felt the house shaking, while only a few people outside felt it.
Third-degree area: The third-degree area is very large, about 2000 square kilometers, which is felt by a few people in the area.