Geodesy and Geodynamics 2014,5(4) :34-37 http ://www.jgg09.com Doi: 10.3724/SP .].1246.2014.04034

Characteristics of gravity fields in the Jinggu M6.6 earthquake

1 2 1 2 1 2 Sun Shaoan ' , Hao Hongtao ' and Wei Jin ' 'Key laboratory of Earthquake Geocksy, [,..titute of Seismology, Chi= Earthquake Administration, Wuhan 430071 , Chino 'Wuhan Bcue of lnstituie of Crustal Dynamics, Chi= Earthquake Administration, Wuhan 430071, Chino

Abstract: Based on the study of high-precision gravity data obtained from recent studies and the regional gravi­ ty network for province , a variation in the regional gravity field was identified before the occurrence of the Yunnan Jinggu M6. 6 earthquake. Key words: Jinggu earthqnake; gravity field ; evolution characteristics

2013. [ll Therefore, in this study, we analyzed recent 1 Introduction gravity data obtained from gravity monitoring networks in a seismic area and determined the evolutionary pat­ On October 7,2014, an earthqnake of magnitude 6.6 tern of the regional gravity field before the Jinggu occurred in Jinggu county , Pu' er City, Yunnan prov­ earthqnake. ince ( 23. 4 °N, 100. 5°E). The earthqnake had a focal depth of 5 krn with an epicenter located at the north­ 2 Survey grid and data sketch westward extension of the western Wuliangshan fault. The epicenter was also close to the Lancangjiang and The gravity networks in the area of the Jinggu earth­ Jinggu faults on the northwest side and parallel to the qnake comprised of regional gravity networks in Yun­ middle segment of Wuliangshan and Babianjiang faults nan province and the gravity network for major national on the east side. It was surrounded by several deep projects. These projects primarily consisted of the Crus­ faults , including the southwestward Heihe fault, north­ tal Movement Observation Network of and the In­ westward Nandinghe fault, and northeastward Honghe tegrated Geophysical Observation of China (The east­ fault. The structural features of the seismogenic fault in ern Tibetan Plateau project) ( Fig. 1 ) . The Crustal this region are difficult to observe , and few small-to­ Movement Observation Network of China maiuly aimed moderate earthquakes had occurred there. Conseqnent­ to establish a gravity monitoring network covering the ly, little information was available for predicting the maiuland of China and monitor dynamic changes in the Jinggu earthquake. Despite these constraints, the gravity field. This could provide fundamental informa­ earthqnake was successfully predicted on the basis of tion for studying the dynamic background of strong the relative gravity data and well documented in the re­ earthqnakes and continental dynamics. The Integrated port on seismic tendencies submitted by the First Crust Geophysical Observation of China aimed to optimize Monitoring and Application Center at the end of and modify the mobile gravity earthqnake-monitoring network in the North-South Seismic Belt. This study

Received:2014-10-17; Accepted:2014-11-10 could improve the monitoring of earthquakes of magni­ Corresponding author: Stm Shaoan, E-mail: ssha525@ 163.com tude larger than M6. 0, obtain and process the space­ This study is supported by the National Natural Science Foundation of time images of variations in the North-South seismic China (41304059, 41304059) and the Seismic Industry Research Pro­ Belt's gravity field, and apply the results in the prediction ject ( 201308004). No.4 Sun Shaoan ,et al. Characteristics of gravity fields in the Jinggu M6.6 earthquake 35

100° 105°

Figure 1 Gravity network and structure in Yunnan region of earthquakes. The gravity network (Fig. I) was completed in 2010. 3 Characteristics of variations in the Data from the network is made available once or twice gravity field of the epicentral region a year, with six sets of data provided so far. All meas­ urements were based on quasi-synchronous observations In southwestern Yunnan, there have been seven earth­ conducted by combining absolute and relative gravity quakes of magnitudes above 5. 0. Of these, six were in measurements. The FG5 absolute gravimeter, LCR-G the Yingjiang- area, and one was in Jinggu. gravimeter, and CG-5 relative gravimeter were used in The earthquakes were more concentrated in 2011 and the observations. The inherent quality of the survey da­ 2014. The relative parameters of the seven earthquakes ta was sufficient to obtain information on nontidal gravi­ are listed in table 1. ty caused by tectonic movement. The accuracy of the The evolution of the regional gravity field m recent absolute and relative gravity measurements exceeded years ( Fig. 2) should correlate with the occurrence of 5 X10- 8 ms -2 and 20X 10-8 ms -2, respectively. earthquakes in the Yingjiang, Tengchong, and Jinggu In light of the gravity data obtained from monitoring areas. The variation map for October 2011 revealed that networks in the area of the Jinggu earthquake, it was the gravity field in most of southwestern Yunnan coin­ crucial to select a reasonable space-time datum for cides with the strike in the northwest-southeast direc­ studying the temporal variations in regional gravity a­ tion. The largest increase occurred in the Yingjiang­ nomalies[2l. This study developed a time variation map Tengchong area. The variation map for October 2012 of the regional gravity field, while the first observation showed that the area of increase had narrowed , and the of the network in 2010 was established as the space­ rate had decreased as the center moved toward the Jing­ time datum. Given that anomalies in earthquakes in the gu area. In October 2013, the regional gravity field in range of M7. 0 in the Yunnan region last for about three southwestern Yunnan remained the same as that of Octo­ years , we selected data for the same period in each ber 2012. However, regional gravity field in the Jinggu year since 20 10 and attempted to document the annual area increased dramatically in 2013. By October 2014, activity of the regional gravity field before the Jinggu when the Jinggu earthquake occurred, the area had ex­ earthquake. panded from Jinggu to most of the southwestern Yunnan. 36 Geodesy and Geodynamics Vol.S

Table 1 Earthquakes above MS. 0 in southwestern Yunnan since 2010

Occurrence time Latitude Longitude Focal depth Magnitude Location (YY-MM-DD T HH:MM:SS) (0) (0) (km) (m)

Yingjiang Country, Dehong Dai and 2011-03-10 T 12:58:12.0 24.7 97.9 10 5.8 Jingpo 2011-06-20 T 18:16:49.0 25.1 98.7 10 5.2 Tengchong Country, Baoshan City Border region of Tengchong Country 2011-08-09 T 19:50:16.3 25.0 98.7 11 5.2 and Longyang , Baoshan City

Yingjiang Country, Dehong Dai and 2014-05-24 T 04:49:21.2 25.0 97.8 12.4 5.6 Jingpo Autonomous Prefecture

Yingjiang Country, Dehong Dai and 2014-05-30 T 09:20:12.9 25.0 97.8 12 6.1 Jingpo Autonomous Prefecture

Yingjiang Country, Dehong Dai and 2014-05-30 T 09:20:50.3 25.1 97.8 14 5.1 Jingpo Autonomous Prefecture

Jinggu Dai and Yi Autonomous Coun- 2014-10-07 T 21 :49:39.5 23.4 100.5 5 6.6 try, Jinggu City

~~~~~~~~~~~~~~~uGru ~~~~~~~~~~~~~~uGru -100 -80 -60 -40 -20 0 20 40 60 80 I 00 -100 -80 -60 -40 -20 0 20 40 60 80 100 2011-10 2012-10

97°30' 100°00' 102°30' 105°00' 97°30' I 00°00' I 02°30' I 05°00' ~.. ..;;:: ;::;~:::;:::::;::=;::~~:::;::::;::::;:::;::=;::::j uGal ~-.~~~;=~:::;::::;:::~~~=;~~~~uGal -100 -80 -60 -40 -20 0 20 40 60 80 100 -I 00 -80 -60 -40 -20 0 20 40 60 80 100 2013-10 2014-10 Figure 2 Sequence of regional gravity fields in the epicentral area

in recent years clearly shows the variation m the re­ 4 Conclusions gional gravity field before Yunnan Jinggu M6. 6 earth­ quake. The high-quality data obtained from the gravity network No.4 Sun Shaoan,et al.Characteristics of gravity fields in the Jinggu M6.6 earthquake 37

research on seismic tendency in Sichuan-Yunnan-Guizhou-Guan­ Acknowlegements gxi region. Integrated Geophysical field in Sichuan-Yunnan­ Guizhou-Guangxi. region, 2014. (in Chinese) [ 2 ] Sun Shaoan, et al. Thinking on datum of regional gravity field We would like to thank Euago ( www. enago. en) for variation. Journal of Geodesy and Geodynamics, 2012, 32 ( 1) : The Einglish Language review. 17-20. (m Chin""') [ 3] Sun Shaoan, et al. Research on evolution of western YUiliUlll and References Beijing regional gravity fields and their relationship with earth­ quake. Earthquake, 1999, 19(1), 97-106. (in Chme•e)

[ 1 ] Wang Tongqing, et al. The characteristics of gravity variation and