关键词: 陆丰M_L4.3地震, 流动重力测量, 地震重力异常, 地面沉降重力异常

Abstract:

Gravity anomalies occurred at Lufeng, Bomei, Nantang, Jiazi, Xixi and other measuring points in the Guangdong Shantou mobile gravity measurement network before and after the Lufeng M_L4.3 earthquake on September 24^th, 2015, and the area was named as Lufeng gravity anomaly area, where the accumulated change of gravity in two years was greater than three times of the RMS errors of the observations and medium-term decline and reverse turning rise appeared. Meanwhile, gravity anomalies also appeared in the measurement points of Chendian and Heping, so the area was named as Chendian gravity anomaly area, where the gravity showed continuous monotonic increase. The two gravity anomaly areas were adjacent to each other, but the nature of the gravity anomalies might be different, there might be seismic gravity anomalies and ground subsidence gravity anomalies. In order to analyze the development trend of later earthquakes, it is necessary to determine the nature of gravity anomaly. The method of data analysis and field verification was used to distinguish the nature of gravity changes in each gravity anomaly area. The results mainly show that: 1)the Lufeng gravity anomaly area is of seismic gravity anomaly, while the Chendian gravity anomaly area is of ground subsidence gravity anomaly. Through the characteristic analysis of seismic gravity anomalies and ground subsidence gravity anomalies, we had a better understanding of the correlation between seismic gravity field evolution and seismic development, which helped to extract the precursor information of seismic gravity changes and predict earthquake. 2)The gravity changes at the observation points of Lufeng, Bomei, Nantang, Jiazi and Xixi in Lufengthe gravity anomalies area began to decline synchronously in August 2014 and rose synchronously in August 2015, forming a positive gravity anomaly area with regional synchronous decline and reverse turning rise. Through anomaly investigation and verification, no interference source to the observation environment was found, and the Lufeng gravity anomaly area was of seismic gravity anomalies. It was the manifestation of the evolution process of the seismic gravity field during the preparation of the Lufeng M_L4.3 earthquake on September 24, 2015. 3)The gravity changes at Heping and Chendian observation points in Chendian gravity anomalies showed a continuous monotonic increase resulting from the pumping of a large amount of groundwater in Chendian and Heping, which led to ground subsidence, house and ground cracking, so the gravity anomaly was related to land subsidence. It is determined that the nature of the anomaly was land subsidence gravity anomaly. The gravity change caused by ground subsidence at Chendian from March 1995 to July 2016 was 292μgal, and the gravity change caused by ground subsidence at Heping from March 2006 to July 2016 was 137μgal. In the analysis of seismic gravity anomalies, the gravity changes at Heping and Chendian should deduct the gravity changes caused by ground subsidence. 4)For obvious gravity anomalies, the background conditions of the anomalies should be understood in detail, such as geological conditions, the use of domestic water and industrial water, ground subsidence, ground fissures, house fissures, etc., and the source of the anomaly should be found. It is necessary to collect water level observation data and hydrological observation data in gravity anomaly area for trend analysis. 5)According to the comparative analysis of the characteristics of time series gravity variation curve of the gravity anomaly points in the gravity anomaly area, the gravity anomalies of the observation points in Chendian gravity anomaly area showed a long-term continuous monotonic increase, while that in the Lufeng gravity anomaly area showed a medium-term decline and reverse turning rise. By analyzing the gravity variation of the adjacent observation points in the abnormal area, and analyzing the difference of the geological conditions and the field survey data, we can basically judge the nature of the gravity anomalies.

Key words: Lufeng M_L4.3 earthquake, mobile gravity measurement, seismic gravity anomaly, ground subsidence gravity anomaly