Joint inversion of ambient noise and gravity to investigate crustal structure and dynamic processes in the southern Sichuan Basin

Abstract

The southern Sichuan Basin, particularly the Changning area, is characterized by intense tectonic activity and frequent earthquakes. However, crustal structure models derived from different geophysical methods exhibit significant discrepancies, which impede an in-depth understanding of the tectonic evolution and earthquake genesis in this region. In this study, a joint inversion integrating surface-wave dispersion and Bouguer gravity anomaly data is adopted. By employing empirical velocity–density relations, the two datasets are coupled into a unified linearized least-squares inversion framework, in which local optimization is achieved through iterative solving. This approach is utilized to obtain the crustal density structure of the southern Sichuan Basin. The joint inversion results reveal an intracrustal low-density layer in the southern Sichuan Basin, inferred to originate from plastic deformation or partial melting of felsic components at relatively high temperatures. Driven by the topographic pressure gradient, this low-density layer undergoes ductile flow toward the Sichuan Basin. Obstructed by the rigid crust of the Sichuan Basin, the material migrates along both the eastern and western margins of the basin, with a broader eastward migration range. The leading edge of the low-density layer shows a distinct special correspondence with the seismic activity concentration zone in the Changning area, a stress concentration zone jointly shaped by deep material migration and tectonic blocking. This stress accumulation serves as the key controlling factor for the frequent earthquakes in the Changning area.

Keywords

Joint inversion / Seismic tomography / Crustal structure / Changning earthquakes

References

[1] Yao H, van der Hilst RD, Montagner JP. Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography. J Geophys Res Solid Earth. 2010; 115(B12): B12307. doi: 10.1029/2009JB007142

[2] Jiang F, Chen X, Unsworth MJ, et al. Mechanism for the uplift of Gongga Shan in the southeastern Tibetan Plateau constrained by 3D magnetotelluric data. Geophys Res Lett. 2022; 49(9): e2021GL097394. doi: 10.1029/2021GL097394

[3] Hu L, Zhang F, Li Y. Crustal Velocity Structure of the Sichuan—Yunnan Block Revealed by High‐Quality Crustal Phase Travel Time. Seismol Res Lett . 2024; 95(3): 1913-1925. doi: 10.1785/0220230181

[4] Berg EM, Lin F, Allam A, Schulte—Pelkum V, Ward KM, Shen W. Shear velocity model of Alaska via joint inversion of Rayleigh wave ellipticity, phase velocities, and receiver functions across the Alaska transportable Array. J Geophys Res Solid Earth. 2020; 125(2): e2019JB018582. doi: 10.1029/2019jb018582

[5] Liu L, Gao SS, Liu KH, et al. Mantle dynamics of the North China Craton: new insights from mantle transition zone imaging constrained by P—to—S receiver functions. Geophys J Int. 2022; 231(1): 629-637. doi: 10.1093/gji/ggac210

[6] Tarantola A. Inversion of seismic reflection data in the acoustic approximation. Geophysics. 1984; 49(8): 1259-1266. doi: 10.1190/1.1441754

[7] Vozoff K, Jupp DLB. Joint Inversion of Geophysical Data. Geophys J Int . 1975; 42(3): 977-991. doi: 10.1111/j.1365—246X.1975.tb06462.x

[8] Julià J, Ammon CJ, Herrmann RB, Correig AM. Joint inversion of receiver function and surface wave dispersion observations. Geophys J Int. 2000; 143(1): 99-112. doi: 10.1046/j.1365—246x.2000.00217.x

[9] Gallardo LA. Joint two—dimensional DC resistivity and seismic travel time inversion with cross—gradients constraints. J Geophys Res Solid Earth. 2004; 109(B3): B03311. doi: 10.1029/2003jb002716

[10] Maceira M, Ammon CJ. Joint inversion of surface wave velocity and gravity observations and its application to central Asian basins shear velocity structure. J Geophys Res Solid Earth. 2009; 114(B2): B02314. doi: 10.1029/2007jb005157

[11] Chen F, Yu HY, Gao LN. Three—dimensional lithospheric shear wave tomography beneath Sichuan region from joint inversion of gravity anomalies and surface—wave. Comput Tech Geophys Geochem Explor. 2018; 40(3): 277-289. [In Chinese]. doi: 10.3969/j.issn.1001—1749.2018.03.01

[12] Brocher TM. Empirical Relations between Elastic Wavespeeds and Density in the Earth’s Crust. Bull Seismol Soc Am. 2005; 95(6): 2081-2092. doi: 10.1785/0120050077

[13] Wan JK, Luo ZC, Wu YL. Lithospheric density structure beneath the Tibetan Plateau from the joint inversion of gravity and seismic data. Chin J Geophys. 2025; 68(2): 470-482. [In Chinese]. doi: 10.6038/cjg2023R0073

[14] Han SC. Joint Inversion Algorithms of Multiple Types of Seismic Data and Gravity Data and Their Applications in the China Region. Dissertation. University of Science and Technology of China; 2022. doi: 10.27517/d.cnki.gzkju.2022.001743

[15] Liang C, Liu Z, Hua Q, Wang L, Jiang N, Wu J. The 3D seismic azimuthal anisotropies and velocities in the eastern Tibetan Plateau extracted by an azimuth—dependent dispersion curve inversion method. Tectonics. 2020; 39(4): e2019TC005747. doi: 10.1029/2019tc005747

[16] Li M, Zhang S, Wang F, Wu T, Qin W. Crustal and upper—mantle structure of the southeastern Tibetan Plateau from joint analysis of surface wave dispersion and receiver functions. J Asian Earth Sci. 2016; 117: 52-63. doi: 10.1016/j.jseaes.2015.12.002

[17] He D, Lu R, Huang H, Wang X, Jiang H, Zhang W. Tectonic and geological setting of the earthquake hazards in the Changning shale gas development zone, Sichuan Basin, SW China. Pet Explor Dev. 2019; 46(5): 1051-1064. doi: 10.1016/S1876—3804(19)60262—4

[18] Long F, Zhang Z, Qi Y, et al. Three dimensional velocity structure and accurate earthquake location in Changning—Gongxian area of southeast Sichuan. Earth Planet Phys. 2020; 4(2): 163-177. doi: 10.26464/epp2020022

[19] Li D, Zhan Y, Ding Z, et al. Upper crustal velocity and seismogenic environment of the Changning MS6.0 earthquake region in Sichuan, China. Chin J Geophys. 2021; 64(1): 18-35. [In Chinese]. doi: 10.6038/cjg2021O0241

[20] Du N, Li Z, Hao T, Xia X, Shi Y, Xu Y. Joint tomographic inversion of crustal structure beneath the eastern Tibetan Plateau with ambient noise and gravity data. Geophys J Int. 2021; 227(3): 1961-1979. doi: 10.1093/gji/ggab299

[21] Deng Q, Zhang P, Ran Y, Yang X, Min W, Chu Q. Basic characteristics of active tectonics of China. Sci China Ser D—Earth Sci. 2003; 46(4): 356-372. doi: 10.1360/03yd9032

[22] Zhang P, Deng Q, Zhang G, et al. Active tectonic blocks and strong earthquakes in the continent of China. Sci China Ser D—Earth Sci. 2003; 46(2): 13-24. doi: 10.1360/03dz0002

[23] Wu X, Xu X, Yu G, et al. The China Active Faults Database (CAFD) and its web system. Earth Syst Sci Data. 2024; 16(7): 3391-3417. doi: 10.5194/essd—16—3391—2024

[24] Zheng X, Yao Z, Liang J, Zheng J. The Role Played and Opportunities Provided by IGP DMC of China National Seismic Network in Wenchuan Earthquake Disaster Relief and Researches. Bull Seismol Soc Am. 2010; 100(5B): 2866-2872. doi: 10.1785/0120090257

[25] Xia X, Li Z, Bao F, et al. Sedimentary structure of the Sichuan Basin derived from seismic ambient noise tomography. Geophys J Int. 2021; 225(1): 54-67. doi: 10.1093/gji/ggaa578

[26] Pavlis NK, Holmes SA, Kenyon SC, Factor JK. An Earth Gravitational Model to Degree 2160: EGM2008. In: Proceedings of the European Geosciences Union General Assembly 2008; April 13—18, 2008; Vienna, Austria. European Geosciences Union; 2008.

[27] Wapenaar K. Retrieving the Elastodynamic Green’s Function of an Arbitrary Inhomogeneous Medium by Cross Correlation. Phys Rev Lett. 2004; 93(25): 254301. doi: 10.1103/physrevlett.93.254301

[28] Rawlinson N, Sambridge M. Wave front evolution in strongly heterogeneous layered media using the fast marching method. Geophys J Int . 2004; 156(3): 631-647. doi: 10.1111/j.1365—246x.2004.02153.x

[29] Sethian JA. Advances in Fast Marching and Level Set Methods for Propagating Interfaces. In: Jeltsch R, Fey M, eds. Hyperbolic Problems: Theory, Numerics, Applications. Seventh International Conference on Hyperbolic Problems; February 1998; Zürich, Switzerland. Birkhäuser; 1999: 855-864. doi: 10.1007/978—3—0348—8724—3_36

[30] Thomson WT. Transmission of elastic waves through a stratified solid Medium. J Appl Phys. 1950; 21(2): 89-93. doi: 10.1063/1.1699629

[31] Haskell NA. The dispersion of surface waves on multilayered media. Bull Seismol Soc Am . 1953; 43(1): 17-34. doi: 10.1785/BSSA0430010017

[32] Afnimar, Koketsu K, Nakagawa K. Joint inversion of refraction and gravity data for the three—dimensional topography of a sediment—basement interface. Geophys J Int . 2002; 151(1): 243-254. doi: 10.1046/j.1365—246x.2002.01772.x

[33] Li Z, Hao T, Xu Y, Xu Y. An efficient and adaptive approach for modeling gravity effects in spherical coordinates. J Appl Geophys. 2011; 73(3): 221-231. doi: 10.1016/j.jappgeo.2011.01.004

[34] Fang H, Yao H, Zhang H, Huang YC, van der Hilst RD. Direct inversion of surface wave dispersion for three—dimensional shallow crustal structure based on ray tracing: methodology and application. Geophys J Int. 2015; 201(3): 1251-1263. doi: 10.1093/gji/ggv080

[35] Roecker S, Ebinger C, Tiberi C, et al. Subsurface images of the Eastern Rift, Africa, from the joint inversion of body waves, surface waves and gravity: investigating the role of fluids in early—stage continental rifting. Geophys J Int. 2017; 210(2): 931-950. doi: 10.1093/gji/ggx220

[36] Ahmed N, Weibull WW, Grana D. Constrained non—linear AVO inversion based on the adjoint—state optimization. Comput Geosci . 2022; 168: 105214. doi: 10.1016/j.cageo.2022.105214

[37] Fong DCL, Saunders M. LSMR: An Iterative Algorithm for Sparse Least—Squares Problems. SIAM J Sci Comput. 2011; 33(5): 2950-2971. doi: 10.1137/10079687x

[38] Zhang C, Guo C, Chen J, Zhang L, Wang B. EGM 2008 and Its Application Analysis in Chinese Mainland. Acta Geod Cartogr Sin . 2009; 38(4): 283-289. [In Chinese]. doi: 10.3321/j.issn:1001—1595.2009.04.001

[39] Roecker S, Thurber C, McPhee D. Joint inversion of gravity and arrival time data from Parkfield: New constraints on structure and hypocenter locations near the SAFOD drill site. Geophys Res Lett. 2004; 31(12): L12S04. doi: 10.1029/2003gl019396

[40] Hansen PC, O’Leary DP. The use of the L—curve in the regularization of discrete ill—posed problems. SIAM J Sci Comput. 1993; 14(6): 1487-1503. doi: 10.1137/0914086

[41] Liu Z, Liang C, Hua Q, et al. The seismic potential in the seismic gap between the Wenchuan and Lushan earthquakes revealed by the joint inversion of receiver functions and ambient noise data. Tectonics. 2018; 37(11): 4226-4238. doi: 10.1029/2018TC005151

[42] Wu Z, Zou K, Su J, Hua Y, Li P, Xu T. Crustal velocity structure beneath the eastern Tibetan Plateau derived from double—difference travel—time tomography. Chin J Geophys. 2024; 67(3): 871-888. [In Chinese]. doi: 10.6038/cjg2024R0746

[43] Sun H, Wang X, Chang L. S—wave velocity and radial anisotropy structure in Southeastern Tibetan Plateau and the deep seismogenic environment of Luding MS6.8 earthquake and Lushan MS7.0 earthquake. Chin J Geophys. 2024; 67(1): 108-127. [In Chinese]. doi: 10.6038/cjg2023Q0810

[44] Bao XW, Sun X, Xu M, et al. Two crustal low—velocity channels beneath SE Tibet revealed by joint inversion of Rayleigh wave dispersion and receiver functions. Earth Planet Sci Lett. 2015; 415: 16-24. doi: 10.1016/j.epsl.2015.01.020

[45] Fu YV, Jia R, Han F, Chen A. SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography. Phys Earth Planet Inter. 2018; 279: 15-20. doi: 10.1016/j.pepi.2018.04.002

[46] Zhang Z, Yao H, Yang Y. Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications. Sci China Earth Sci. 2020; 63(9): 1278-1293. doi: 10.1007/s11430—020—9625—3

[47] Clark MK, Royden LH. Topographic ooze: Building the eastern margin of Tibet by lower crustal flow. Geology. 2000; 28(8): 703-706. doi: 10.1130/0091—7613(2000)28<703:Tobtem>2.0.Co;2

[48] He X, Zhao LF, Xie XB, Tian X, Yao ZX. Weak crust in southeast Tibetan Plateau revealed by Lg—wave attenuation tomography: Implications for crustal material escape. J Geophys Res Solid Earth. 2021; 126(3): e2020JB020748. doi: 10.1029/2020JB020748

[49] Royden LH, Burchfiel BC, King RW, et al. Surface deformation and lower crustal flow in eastern Tibet. Science. 1997; 276(5313): 788-790. doi: 10.1126/science.276.5313.788

[50] Liu H, Pei S, Liu W, et al. Crustal and upper mantle attenuation structure beneath the southeastern Tibetan Plateau and its implications on plateau outgrowth. J Geophys Res Solid Earth. 2004; 129(2): e2023JB026977. doi: 10.1029/2023jb026977

[51] Wang W, Wu J, Fang L, Lai G, Cai Y. Crustal thickness and Poisson’s ratio in southwest China based on data from dense seismic arrays. J Geophys Res Solid Earth. 2017; 122(9): 7219-7235. doi: 10.1002/2017JB013978

[52] Rosenberg CL, Handy MR. Experimental deformation of partially melted granite revisited: implications for the continental crust. J Metamorph Geol. 2005; 23(1): 19-28. doi: 10.1111/j.1525—1314.2005.00555.x

[53] Kohlstedt DL, Evans B, Mackwell SJ. Strength of the lithosphere: constraints imposed by laboratory experiments. J Geophys Res Solid Earth. 1995; 100(B9): 17587-17602. doi: 10.1029/95jb01460

[54] He R, Shang X, Yu C, Zhang H, Van der Hilst RD. A unified map of Moho depth and VP/VS ratio of continental China by receiver function analysis. Geophys J Int. 2014; 199(3): 1910-1918. doi: 10.1093/gji/ggu365

[55] Shi Z, Wang G. Evaluation of the permeability properties of the Xiaojiang Fault Zone using hot springs and water wells. Geophys J Int. 2017; 209(3): 1526-1533. doi: 10.1093/gji/ggx113

[56] Cheng Y, Tang J, Chen X, Dong Z, Xiao Q, Wang L. Electrical structure and seismogenic environment along the border region of Yunnan, Sichuan and Guizhou in the south of the North—South seismic belt. Chin J Geophys. 2015; 58(11): 3965-3981. [In Chinese]. doi: 10.6038/cjg20151107

[57] Cui T, Chen X. Magnetotelluric imaging and tectonic movement characteristics of the central Yunnan sub—block and its adjacent areas. Sci China Earth Sci . 2023; 66(2): 377-392. doi: 10.1007/s11430—021—9997—y

[58] Cook KL, Royden LH. The role of crustal strength variations in shaping orogenic plateaus, with application to Tibet. J Geophys Res Solid Earth. 2008; 113(B8): B08407. doi: 10.1029/2007JB005457

[59] Royden LH, Burchfiel BC, van der Hilst RD. The geological evolution of the Tibetan Plateau. Science. 2008; 321(5892): 1054-1058. doi: 10.1126/science.1155371

[60] Burchfiel BC, Royden LH, Van der Hilst RD, et al. A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People’s Republic of China. GSA Today. 2008; 18(7): 4-11. doi: 10.1130/GSATG18A.1

[61] Zhang P, Wen X, Xu X, et al. Tectonic model of the great Wenchuan earthquake of May 12, 2008, Sichuan, China. Chin Sci Bull. 2009; 54(7): 944-953. [In Chinese]. doi: 10.1360/csb2009—54—7—944