Geodesy and Geodyoamics 2013,4(3) :1 -6 http :llwww. jgg09. com Doi:10.3724/SP.J.1246.2013.03001 Characteristics of satellite-gravity variations in the North-South Seismic Belt before the 2013 Lushan earthquake 1 2 3 1 2 1 2 1 2 Zou Zhengho " " , Li Hui " , Kang Kaixuan " and Wu Yunlong " 1 &y LolJoratory of Earthquake Geodesy, China Earthquoke Admini.stration, Wuhan 430071 , China 2 Wuhan Base of lnstituJe of Crustal Dynamics, China Earthquake Admini.stration, Wuhan 430071 , China ' School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China Abstract:To study the characteristics of gravity variations in and near the North-South Seismic Belt hefore the 2013 Lushan earthquake, we used the geopotential-field models based on monthly data of the RL05 GRACE satellite to calculate the gravity changes. Here we present the patterns of annually cumulative variation, differentiati.al variation and secular trend , as well as the continuous time-series at 4 characteristic sites during 2004 - 2012. The result shows that the anomalous positive-to-negative transition zone, in which the epicenter of the 2008 Wenchuan earthquake was located , did not show any new gravity change before the Lushan earthquake, though located in the same zone. Key words: satellite gravity; gravity variation; earthquake; time series; North-South Seismic Belt Up to now, GRACE has captured gravity variations re­ 1 Introduction lated to several large earthquakes , such as the 2004 Mw9. 3 Sumatra-Andaman, the 2010 Mw8. 8 Chile, The development of a strong earthquake may be accom­ and the 2011 Mw9. 0 Tohoku-Oki ll-SJ. panied by changes of underground mass density and In China, a previous study1'·'1 using data from some other physical fields in the source and surround­ GRACE has also found an obvious positive-to-negative ing areas, thus it is important to detect such changes anomaly area before the 2008 Mw 7. 9 Wenchuan earth­ by using various monitoring methods in order to study quake in its epicenter area. Since then, the Ms7. 0 earthquake-source mechanism and earthquake predic­ Lushan earthquake occurred(30. 3°N, 103°E) on May tion. Since 2002 , the Gravity Recovery and Climate 23 ,2013, about 80 km to the southwest, near the con­ Experiment (GRACE) has provided a set of stable sat­ verging point of Longmenshan , Xianshuihe and Zemu­ ellite-gravity data. The collection and analysis of such he faults. The focal mechanism is thrust, like the Wen­ data has been found to be useful in studying gravity chuan earthquake , and the slip direction and fault variations over long periods with high spatial resolu­ strike are similar also; however, it is relatively inde­ tion. This method can be applied to studying long-term pendent of the Wenchuan earthquake1101 • Being an ad­ large-scaled background gravity variations and local ditional strong earthquake in recent years in the middle gravity changes associated with large earthquakes. section of the North-South Seismic Belt, the Lushan earthquake may imply that more large earthquakes may Received :2013-05-06; Accepted :2013-05-19 Corresponding author:Zou Zhengbo, E-mail: zouzb@ 126. com. occur in this section, which should thus be closely This work is supported by the China Earthquake Administration Special watched. In this paper, we present and discuss the cal­ Basic Scientific Research Business Expenses ( IS20 1116022 ) and the culated results, based on GRACE gravity data, of the National Natural Science Foundation of China ( 40704009 ,41004030) . cumulative and differential changes and the long-term 2 Geodesy and Geodynamics Vol.4 trend in the North-South Seismic Belt, as well as the and are discussed here. time series at 4 selected sites. 2. 3 Secular trend 2 Data and methodology The gravity variation derived from GRACE was affected by many factors, including land-surface hydrology, earthquake processes, glacial isostatic adjustment, as 2.1 Data well as atmospheric and oceanic processes. Since the We made use of RI1l5 monthly gravity -field models atmosphere and ocean tides had been removed from the from January 2004 to February 2013 released by CSR RL05 monthly gravity field models, the land surface ( Center for Space Research) . The data set has several hydrology became the most significant signal, which is gaps caused by the lack of accelerator observation ( in periodic and can be described using simple mathemati­ January and June 2011; May and October 2012) [lOl. cal functions. Since the aim of this paper is to study The reason we chose the RI1l5 rather than the Rl04 temporal gravity changes related to geodynamic proces­ version was that its signal-to-noise ratio is higher, and ses , the cyclic secular signal had to be removed firstly. its temporal variation in the North-South Seismic Belt is We separated the gravity signals into two parts , period­ 11 2 significantly smaller[ l. Due to the lack of GRACE da­ ic and linear, by using formula 2 [ ] below, and solved ta in C20 , we substituted it with the result of 5 SLR the unknown parameters by the least -squares method. satellites for the same period. Finally we obtained the secular linear rate k and time­ series changes, without the cyclic disturbance : 2. 2 Gravity variation Gravity change, which is the radial derivative of the 3 .&g(t) = L a;cos(w;t + <p,) + kt + b (2) gee-potential variation, at any point on the earth can i=l 2 be calculated by the following formula [ ] : where the unknown parameters a 1 ,a2 ,a3 ,q; 1 ,lp2 ,(/)3 are 1 the magnitudes and initial phases of the periodic func­ GM'- - - .&g = i L (l + 1) L P ... cosO[.&C... cos(mA) + tion ; k is linear slope or the secular trend , and b is the a l=2 m=O (1) intercept. where G is the gravitation constant, M is the mass of 3 Temporal and spatial gravity variatiom the Earth, P1m (cosO) is the fully normalized associated Legendre function of spherical harmonic degree l and 3.1 Cumulative and differentiatial gravity changes order m, lJJUIXis the maximum truncated degree, a is the mean radius of the Earth, A and 0 are longitude and We show our calculated regional gravity changes in two colatitude, and .<1C1m and .<iS'"' are the differences be­ different ways: Cumulative (Fig. 1 ) , which is relative tween Stokes coefficients and background geopotential to the background gravity field, and differentiation coefficients. (Fig. 2) , which is the differences between consecutive In order to remove background noise in the North­ periods. A 20° X 20° window ( 20° - 40°N, 90° - South Seismic Belt, we used the P3 M8 de-correlation ll0°E) is used, with the North-South Seismic Belt in and 450 km Gaussian smoothing methods to estimate the middle. gravity variations during the nine-year period. Then we We chose the mean yearly gravity variation from obtained the long-term gravity changes and time-series GRACE as the background gravity field. Figure 1 shows gravity changes in the North-South Seismic Belt, which the yearly cumulative gravity changes from 2004 to includes both epicenters of the Wenchuan and Lushan 2012. As shown, the gravity changes in the Qinghai-Ti­ earthquakes. Finally, the characteristics of gravity bet Plateau near Lhasa are most prominent during the changes before the Lushan earthquake were examined, study period from 2004 to 2012. In this region, the Zou Zhengbo,et al. Characteristics of satellite-gravity variations in the North-South No.3 Seismic Belt before the 2013 Lushan earthquake 3 20 30° 15 20° 20° N ~...~~... 20°N~~----~--~--~ 90oE 1oooE uoo E 90° E 1ooo E uooE 90°E 1oooE uooE 2004 2005 2006 10 30° 30° 0 20° 20°N 20° 90°E 1oooE uooE 900E 1oooE 110° E 90° E 100° E 110° E -5 2007 2008 2009 40°N 30° 30oN 20° 20° N E~.. ~~~..: ~~~ 20° 90oE 1oooE uoo E 90° E 100° E uoo E 90° E 1oooE uooE 2010 2011 2012 Figure 1 Yearly cumulative gravity changes ( Red points represent epicenter distributions of the Ms > 7 earthquakes) 20 30°N 30°N 15 20°N 20° NL.....:::::.......~.-IIIIilll...._......:::::. 1oooE llOoE 90oE 100° E uooE 90° 1oooE 11ooE 10 2004 2005 2006 30°N 0 20°N 1oooE 100° E uooE 900E 1oooE uooE -5 2007 2008 2009 20oNL-~----~--~--~ 90°E 1ooo E llOoE 90oE lOOoE llOoE 2010 2011 2012 Figure 2 Yearly differentiated gravity changes ( Red points represent the epicenter location of the Ms > 7. 0 earthquakes) gravity decreased gradually from + 20 f.LGal in 2004 to In the North-South Seismic Belt, located in southern - 10 f.LGal in 2007 , and the decreasing trend contin­ Yunnan and Burma, the gravity decreased in 2004 - ued until 2010. The year 2008 is, however, quite dif­ 2006 , increased 2007 - 2009 , decreased greatly in ferent, showing a large gravity increase (from negative 2010, and then increased in 2011 -2012. Altogether to positive) , centered at the North-South Seismic Belt. three significant earthquakes occurred in the North- 4 Geodesy and Geodynamics Vol.4 South Seismic Belt during 2003 -2012: the 2008 Ms8. 0 During the entire 9-yeax period, the secular trend Wenchuan earthquake, the 2011 Mw7. 1 Yushu earth­ ( Fig. 3 ( c ) ) showed a decrease in an abroad area from quake, and the 2013 Ms7. 0 Lushan earthquake. southern Qinghai-Tibet plateau to northeastern North­ The yeaxly differentiatial gravity shows several prom­ South Seismic Belt with a maximum rate of - 0. 3 inent increases and decreases ( Fig. 2 ) : The increases j.LGal/ a , and also an increase in a broader area to the occurred in northeastern Gansu and Ningxia in 2004 north and east with a maximum rate of 0.