According to the China Earthquake Networks Center, CEA, at 19:17 on August 20th, 2019, Beijing, China, a M4.2 earthquake occurred near Zhanan and Lixin Village in Tianya District, Sanya City, Hainan Province, China (18.51°N, 109.41°E). It is the second largest seismometer-recorded earthquake on the land territory of Hainan since 1969.The focal depth is 12 km. It is not common for such a relatively large earthquake to occur in Sanya, a weak seismic region. The distance between the epicenter and another earthquake which occurred on January 25th, 1982 is approximately 3.9 km. From 10:48 August 20th to 0:56 August 21st, 2019, a total of 7 earthquakes have been recorded in Sanya, including: M≥4.0 earthquake, 1 time, that is Sanya M4.2 earthquake on August 20th, 2019; 3.0≤M < 4.0 earthquake once, 2.0 ≤M < 3.0 earthquake 0 times, 1.0≤M < 2.0 earthquake 5 times (Source: http://www.haindzj.gov.cn/dzml/hnjlqdz/). After the M4.2 earthquake, Hainan Earthquake Agency immediately launched an earthquake emergency response, and dispatched the field team to the earthquake region to conduct the earthquake field work.1 SEISMIC STRUCTURE 1.1 Earthquake Tectonic Setting
The earthquake region belongs to the southern end of the Qiongzhongnan uplift area, which is originated from the neotectonic movement with the characteristics of intermittent arching uplift, multi-level erosion surfaces and terraces, and the deposition of some thinner Late Tertiary and Quaternary strata. The earthquake region has a mountainous and hilly landform and is part of the mountainous area of central Hainan Island. The overall terrain is higher in the northeast and gradually decreases in the southwest. The highest peak in the earthquake region is Jian ridge, which has an elevation of 1 019 m (Fig. 1).
In Fig. 2(b), based on the tectonic units of the area, the region can be divided into Sanjiang Indosinian folds system(Unit I, referred to as SI fold), South China Caledonian folds system(Unit Ⅱ, referred to as SCC folds), Coastal area of South China Variscan folds system(Unit Ⅲ, referred to as CSCV folds) and the South China Sea terrace(Unit Ⅳ, referred to as SCS terrace). From the perspective of the tectonic units, Sanya M4.2 earthquake is basically located at the boundary line between tectonic Units Ⅲ and Ⅳ. In Unit Ⅲ, Proterozoic mixed rocks and early Paleozoic sediments with thickness of over 10 km are exposed, forming a typical geosynclinals deposition. In late Permian, the whole units have been migrated backed into land, accompanied with large area of granite intrusion and regional metamorphism. After Mesozoic, block-faulting is the major movement in the unit. The Unit Ⅳ is a typical terrace unit with the early Cambrian basement and early Paleozoic siliceous carbonate rock and quartz sandstone. The intensity of seismic activities in the north part of Hainan Island where Qiongshan M7
The fracture structure in the earthquake region is relatively developed. The large-scale faults in the vicinity mainly belong to the Changcheng-Ledong-Jiyang and the Jiusuo-Lingshui Fault Zones. The Changcheng-Ledong-Jiyang Fault Zone is from Changcheng, traversing the east Ledong and Zhizhong, and extends to Jiyang, Sanya City. The fault zone is 154 km long and consists of a series of northwest faults in the strike of 30-40. Based on the observation of aerial images, we can see that fractured zones, fault breccia zones and crushed fracture zones distribute intermittently along the fault zone. The fracture has multiple characteristics of activities of which the first intense activity occurred during Mesozoic and the second strong activity occurred in late period. In addition, there are numerous hot springs and small earthquakes along the fault zone. The Jiusuo-Lingshui Fault Zone traverses the cities and counties of Ledong, Sanya and Lingshui, and lies in a EW direction, tending to the south, and consists of several faults with a nearly EW strike (Wu Xiaojiang et al., 2011). In general, the structural belt is a large fault which has a large scale and deep section. The fault may have been formed in Caledonian or earlier, and there is a strong activity in Hercynian and Yanshanian. The epicenter of the earthquake is located near the Changcheng-Ledong-Jiyang and the Jiusuo-Lingshui fault zone (Fig. 2(a)).1.2 Historical Earthquake
Fig. 2(a) also shows a relatively strong earthquake in the vicinity of the earthquake region. In particular, the largest earthquake is the M4.5 earthquake occurred on January 25th, 1982. The epicenter is 18.53°N, 109.38°E, approximately 3.9 km from Sanya M4.2 earthquake, and the focal depth is 5 km. The earthquake has a highest intensity of V degree and has led to a large-scale damage and collapse of the local grass houses and vegetation. The long-axis direction of the extreme earthquake zone is NNW-SSE, and the typical macro seismic damage points are also distributed along this direction.2 SUMMARY OF EARTHQUAKE DISASTER INVESTIGATION
According to "Post-earthquake field works-Part 3: Code for field survey" (GB/T 18208.3-2011), "China Earthquake Intensity Table" (GB/T 17742-2008) and "General ruler for earthquake magnitude" (GB 17740-2017), we mainly investigate and evaluate seismic intensity based on people's feelings in the earthquake zone, utensils response, the degree of house damage, etc. With the support of the local government, three investigation teams have conducted on-the-spot investigations of 25 sites in Fenghuang and Yucai Town and obtained a large amount of basic data and first-hand information in the earthquake region. Except for some uninhabited mountains, inundated reservoirs and impassable roads, the survey area basically covers the residential points in the seismic region.
Through the on-the-spot investigation of the 25 sites, the damage of houses provided by the local government are collected. Among them, 17 sites have an intensity of V, including Jiayou Village of Zhanan Administration Village, Hongxing, Jiayue, Xincun, Nanjin; Mango Village, Lixin Administration Village; Zhachu, Zhayun, Zaka, Xincun, Zhamao, Zhaye, Baoyan Village, Xinhua Team, Nandao Farm and Huixin Ercun Village, Baolong Administration Village, Zhahao, Zhawen Village. These sites are mainly distributed in the northeast and south of the epicenter, and the V-degree area is about 87 km2; the other 8 survey points are rated as Ⅳ degree (Fig. 1).
The earthquake has not resulted in any casualties and obvious damage. Although the epicenter is located in the mountainous area, there are no earthquake-induced geological disasters like landslides along the survey site and along the line. The lifeline project and major engineering earthquake damage are not found in this earthquake. No obvious animals, plants, underground fluids, climate and other macroscopic anomalies are found in the earthquake.3 INVESTIGATION OF EARTHQUAKE DISASTER 3.1 Characteristics of the Earthquake and the Objects of the Epicenter and Analysis
In the light of the field survey of the epicenter area and 25 natural villages nearby, the characteristics of the earthquake and the objects in the epicenter are as follows:
(1) The main building structure of the epicenter is unified, basically single-storey masonry-concrete structure, cast-in-place concrete roofing and the hollow brick wall is loaded. A tin roof canopy is constructed in front of the house (Fig. 4(a)).
(2) The earthquake occurs around the supper time and most of the people are indoor or under canopy. When earthquake occurs, it'll generate two up and down vibrations that lasts for a short period. Some people experience the feeling of electric shock, while other people observe escaping mice.
(3) Most outdoor respondents hear ground sound similar to thunder or explosion, and some people can distinguish the direction of earthquake sound.
(4) The vast majority of residents report that the tin roof of canopy makes a loud sound. Besides, most people report that the doors and windows are ringing, and the iron pots and teacups are vibrating up and down.
There is basically no obvious earthquake damage in buildings, including a small number of old stone huts, brick houses or adobe houses (Figs. 4(b), (d), (f)). A notable feature in the epicentral area of the earthquake is that there are no falling unstable objects or swung suspensions. It seems that the epicenter area is dominated by vertical seismic component that is not likely to cause damage to buildings. Unfortunately, there is no available seismic information in the epicentral area, and the nearest Sanya Seismic Station is about 35 km to the epicenter, unable to study actual vibration characteristics of the epicenter. From the seismic records of Sanya M4.2 earthquake(on bedrock), the energy on horizontal component is stronger than that on the Z-axis, neither vibration amplitude is large, the maximum vibration acceleration(only 1-2 Gal) is eastward and the Z-axis is the smallest(less than 1 Gal). The bedrock acceleration response spectrum decreases rapidly after 0.1 second (Fig. 3).
The buildings in the epicentral area are mainly single-storey of masonry-concrete structure, with cast-in-place concrete roof (Figs. 4(a), (c), (e))which may show a better seismic performance. Detailed house structure types are shown in Table 1.
In the far-off areas from the epicenter, the characteristics of the response of personnel and objects are as follows:
(1) The shaking is mainly in horizon during the earthquake which is consistent with the seismometer record;
(2) Some people outside do not feel the vibration;
(3) Some people outside hear the earthquake sound similar to that generated by planes, cars or thunders.
(4) The doors and windows are ringing, and the small objects are shaking.
Since the earthquake damage of the house is not obvious, the intensity survey is mainly based on personnel response, and the subjective factors are strong.3.2 Investigation and Analysis of House Crack Characteristics
Although houses in the epicentral area are mainly with good anti-damage performance, there are still some old brick or stone auxiliary rooms sporadically. After the earthquake, some residents find cracks in the buildings after the earthquake and we investigate the cracks reported by the Hongxing Group, Jiayou Team and Lixin Group in Zhanan Village(Table 2).
The cracks appear in the brick-concrete structure houses, and no damage of the old auxiliary bungalows is reported. Although some residents report cracks in the walls of houses after the earthquake, from our field investigation, most of them are old horizontal or vertical cracks related to temperature or settlement, indicating that most cracks are existed before the earthquake, and are not directly caused by the earthquake. The uncertainty is that it is difficult to determine whether the cracks expand due to the earthquake.
(1) Hongxing Village. There are some topographic fluctuations of the terrain in Hongxing Village. The reported cracks are almost vertical cracks in both inner and outer walls. In addition, black plaster debris and spider silk appear in the cracks(Fig. 5(a), 5(b)). No wall plaster debris is found on the floor. The vertical cracks can also be seen in the plastering of the exterior wall (similar to Fig. 5(e)), indicating that some cracks are existed before the earthquake, and are not directly caused by the earthquake.
(2) Jiayou Village. In this village, horizontal corner wraps and cracks (Fig. 6(a), 6(b)) are common and are considered as typical temperature-induced cracks along the mortar. One of the villages shows traces of pre-earthquake repairing (Fig. 6(c)), further confirming the non-earthquake damage. In addition, vertical cracks also appear in Jiayou Village (Figs. 5(c), 5(d), 5(g)). The Jiayou Village is located on the mountainside of "Stone Mountain" and the houses are constructed on slopes, which is prone to uneven settlement. Some stone retailing walls partially collapse on the house foundation(Figs. 6(d), 6(e)).
(3) Lixin Village. Following the cracks feedback from 12 households in Lixin Village, the main types of cracks are temperature cracks, horizontal cracks and oblique cracks near the roof due to material shrinkage and uneven settlement, vertical and oblique cracks in the wall, and cracks in the pillars of the shed (Figs. 5(f), 5(g)).
(4) Although the reported earthquake-induced crack is doubtful, the investigation indicates that since the earthquake region is located in the mountainous and hilly areas, the house is prone to cracks due to uneven settlement. With the development of subsidence, some cracks are penetrated, which is enough to affect wall seismic capacity. Relevant authorities are reminded that such houses have a high seismic risk.4 DISCUSSION AND CONCLUSION
(1) Through the on-the-spot investigation of 25 sites, the study indicates that 17 sites have an intensity of V. They are mainly distributed in the northeast and south of the epicenter, and the V-degree area is about 87 km2; the other 8 survey points are rated as Ⅳ degree.
(2) With government support and construction, the buildings in the epicenter area are mainly composed of single-storey brick-concrete structure. The overall cast-in-place concrete roof has good seismic performance and no obvious damage resulted from the earthquake. However, there are still some sporadic old-fashioned simple houses with weak earthquake resistance. Some houses are built with tin roofs. Although the roofs are lighter, they lack the overall integrity of the house dued to the constraint of concrete roofs. Such houses have the potential of collapse when earthquakes occur. Construction columns and ring beams are recommended.
(3) The wall cracks reported by the residents after the earthquake are mostly old horizontal, vertical or oblique cracks related to temperature, material shrinkage or uneven settlement in the mountainous hilly area. No X-type earthquake related cracks are observed. It is difficult to judge whether the cracks expand due to the earthquake. With the development of settlement, some cracks appear in the houses which are built in mountainous and hilly areas have penetrated the walls enough to affect the seismic capacity of the structures. Such houses have potential seismic risks when stronger earthquakes occur. In addition, several cracks in the houses are believed to exist before the earthquake and should be distinguished from others in the seismic intensity assessment.
(4) According to "Code for Seismic Design of Building" (GB 50011-2010)(2016 Edition), it is not necessary to set structural columns for single-storey houses in the Ⅵ degree zone, however, we still recommend that these houses constructed on the mountain should be added with concrete tie columns and ring beams to better maintain the integrity of the house and improve its stability facing uneven settlement and earthquake risks.ACKNOWLEDGMENT
The authors sincerely thanks the editors and reviewers for their help, for the checking and valuable suggestions to the manuscript. Meanwhile, sincere thanks to Sanya Emergency Bureau and related units for their support and assistance in the field investigation. Thanks colleague Li Dongya for providing the Sanya M4.2 seismic waveform data and Forecasting Center Team for offering the focal mechanism solution parameters in Fig. 1.
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