Received: 14 March 2020 | Revised: 24 June 2020 | Accepted: 24 July 2020 DOI: 10.1111/ter.12492 RESEARCH ARTICLE Transfer of stress from the 2004 Mw9.2 Sumatra subduction earthquake promoted widespread seismicity and large strike- slip events in the Wharton Basin Laiyin Guo1,5 | Jian Lin2,3,4,5 | Fan Zhang2,3 | Zhiyuan Zhou2,3 1School of Ocean and Earth Science, Tongji University, Shanghai, China Abstract 2Key Laboratory of Ocean and Marginal The 2004 Mw9.2 Sumatra and 2012 Mw8.6 Wharton Basin (WB) earthquakes pro- Sea Geology, South China Sea Institute of vide the unprecedented opportunity to investigate stress transfer from a megathrust Oceanology, Innovation Academy of South China Sea Ecology and Environmental earthquake to the subducting plate. Comprehensive analyses of this study revealed Engineering, Chinese Academy of Sciences, that the 2004 earthquake excited widespread seismicity in the WB, especially in Guangzhou, China 3Southern Marine Science and Engineering regions of calculated stress increase greater than 0.3 bars. The 2004 earthquake Guangdong Laboratory (Guangzhou), stressed all three rupture planes of the 2012 Mw8.6 strike-slip mainshock and the Guangzhou, China largest Mw8.2 aftershock with mean values of Coulomb stress between 0.3 and 2.1 4Department of Geology and Geophysics, Woods Hole Oceanographic Institution, bars. For the 77 Mw ≥ 4 regional events since 2012, at least one nodal plane for Woods Hole, MA, USA 95% of the events, and both nodal planes for 72% of the events experienced stress 5Department of Ocean Science and increase due to the 2004 earthquake. Results of the analyses also revealed that the Engineering, Southern University of Science and Technology, Shenzhen, China regional stress directions in the WB may have controlled the sub-fault orientations of the 2012 Mw8.6 strike-slip earthquake. Correspondence Jian Lin, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA. Email: [email protected] Fan Zhang, Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China. Email: [email protected] Funding information Southern Marine Science and Engineering Guangdong Laboratory, Grant/Award Number: GML2019ZD0205; National Natural Science Foundation of China, Grant/ Award Number: 41890813, 91628301, 41976064 and 41976066; China–Pakistan Joint Research Center on Earth Sciences; Chinese Academy of Sciences, Grant/ Award Number: Y4SL021001, QYZDY-SSW- DQC005 and 133244KYSB20180029 Terra Nova. 2020;00:1–12. wileyonlinelibrary.com/journal/ter © 2020 John Wiley & Sons Ltd | 1 2 | GUO ET AL. 1 | INTRODUCTION is 40–80 Ma (Liu, Curray, & McDonald, 1983; Singh et al., 2017). Seafloor spreading created a distinctive set of high-angle, left-lat- The 26 December 2004 Mw9.2 earthquake ruptured 1,200– eral-offset fossil fracture zones with strikes of ~6° (Figure 2; Jacob, 1,600 km of the Sumatra subduction zone (Ammon et al., 2005; Lay Dyment, & Yatheesh, 2014). It has been suggested that the seis- et al., 2005). An increase in seismicity was reported in the Wharton micity in the WB could be the result of fracture zone reactivation Basin (WB) following the 2004 earthquake (Delescluse et al., 2012). (Deplus et al., 1998; Robinson, Henry, Das, & Woodhouse, 2001). Eight years later, the 11 April 2012 Mw8.6 earthquake struck the WB However, both the location and strike of the 2012 sub-faults differ and was ~310 km seaward of the epicentre of the 2004 earthquake significantly from the fossil fracture zones (Figure 2). This suggests (Lay, 2019). This largest instrument-recorded strike-slip event was that the 2012 Mw8.6 earthquake instead occurred on previously un- followed approximately 2 hr later with the Mw8.2 strike-slip after- identified conjugate faults. shock (Figure 1). These events provide the unprecedented opportu- Several studies indicate that the 2004 subduction earthquake nity to investigate stress transfer and the triggering of seismicity in has caused significant stress changes in the WB (Fan & Shearer, 2016; the subducting plate following a great subduction zone earthquake. Sevilgen, Stein, & Pollitz, 2012; Zhang, Chen, & Ge, 2012). For exam- The 2012 Mw8.6 mainshock consisted of multiple sub-faults, as ple, it has been suggested that a reduction in normal stress due to revealed by investigations using back-projection analysis (Ishii, Kiser, the 2004 earthquake resulted in the 2012 Mw8.6 earthquake (Ishii & Geist, 2013; Meng et al., 2012; Wang, Mori, & Uchide, 2012), et al., 2013). However, the magnitude and spatial variation of the W-phase inversion (Duputel et al., 2012), and finite fault model- large-scale stress change in the WB and specific effects on individ- ling (Hill et al., 2015; Wei, Helmberger, & Avouac, 2013; Yue, Lay, ual faults are still poorly quantified. & Koper, 2012). These analyses reveal that the conjugate strike-slip This study comprehensively investigated the stress transfer to faults are oriented NNE and NWW, respectively. The finite fault the WB from the 2004 Sumatra subduction earthquake. Our inves- model (FFM) from Wei et al. (2013) consists of three sub-faults with tigation focused on the stressing in different zones and for different fault lengths of ~260 (F1), 420 (F2), and 420 km (F3) (Figure 2). types of ruptures in the WB: (a) The three sub-faults of the 2012 The WB is on the eastern end of the Indian Plate and subduct- Mw8.6 mainshock; (b) the rupture plane of the largest 2012 Mw8.2 ing under the Eurasian Plate at an average convergence rate of aftershock; and (c) the 77 Mw ≥ 4 regional events after the 2012 4.7–5.7 cm/year (Carton et al., 2014). The crustal age in the WB events with focal mechanism solutions. 15° Mag. 654 Andaman Sumatra Trench 10° backarc system Indian Plate Eurasian Plate 2012 Mw8.6 5° FIGURE 1 Map view of the 2004 2004 Mw9.2 Mw9.2 Sumatra subduction earthquake (blue focal mechanism), the 2012 Mw8.6 strike-slip earthquake (red focal mechanism), and the largest 2012 Mw8.2 aftershock (green focal mechanism) in the Wharton Basin. The subduction zone 0° plate boundary (solid black lines) and the hypothesized Indo-Australian plate 2012 Mw8.2 boundary (dashed black lines) are from Bird (2003). Seismicity (solid white dots) Bathymetry (km) during 04/11/2012–11/01/2019 is from N Ninetyeast Ridge Australian Plate the USGS catalogue (https://earth quake. −4 −2 0 2 usgs.gov/earth quake s/search) –5° 85° 90° 95° 100° 105° GUO ET AL. | 3 optimal for modelling strike-slip faults (King, Stein, & Lin, 1994; Qiu & Chan, 2019; Toda, Stein, & Lin, 2011). The 2004 source fault con- 5˚ sists of a set of sub-patches; here, we adopted a nine-patch solution that was obtained using the observed seismological and geodetic constraints (Sevilgen et al., 2012). We also used a higher-resolution FFM with many more sub-patches for the 2004 source fault; the re- Fossil Fracture Zones F2 Mw9.2 sults were similar to the nine-patch solution because much of the study region is located sufficiently far from the 2004 source earth- Mw8.6 quake (Figure 3). In modelling the Coulomb stress transfer related to the 2012 Mw8.6 earthquake, we considered three sub-faults F1, F2, F5 F1 and F3, with 156, 260, and 260 sub-patches, respectively (Table 1). F3 The 2012 Mw8.2 aftershock was modelled using 299 sub-patches Mw8.2 (Table 1). 0˚ y F4 Boundar 3 | RESULTS ustralian Plate 3.1 | Overall pattern of stress change in the WB Indian-A Bathymetry (km) –4-4 –2-2 0 2 We first examined the overall pattern of stress change in the WB 90˚ 95˚ caused by the 2004 earthquake. Stress changes were calculated for three types of potential receiver faults in the WB. (a) Distributed FIGURE 2 Focal mechanisms of 77 Mw ≥ 4 regional events strike-slip receiver faults that are optimally oriented for Coulomb since 2012. The focal mechanism solutions are from global centroid moment tensor catalogue (https://www.globa lcmt.org/CMTfi les. html; Dziewoński, Chou, & Woodhouse, 1981; Ekström et al., 2012). Black lines indicate fossil fracture zones in the Wharton Basin from Jacob et al. (2014). Dashed line indicates assumed plate boundary between the Indian and Australia Plates. The red lines indicate the 0 three sub-faults of the Mw8.6 event, F1 (260 km), F2 (420 km) and Z (km) 100 F4 (420 km). The green line indicates the fault of the Mw8.2 event F4 (260 km), which was 2 hr later after the Mw8.6 event 2 | CALCULATION OF COULOMB STRESS 1200 CHANGES Changes in the Coulomb failure stress σ caused by a “source fault” Δ c 800 earthquake is defined as follows: ) (km Y Y F2 � Δc =Δs + Δn, (1) 400 where σ is the shear stress change on a given “receiver fault” plane F1 Δ s F3 (positive in the direction of receiver fault rake), Δσn is the normal stress change (positive for fault unclamping), and μʹ is the effective friction 0 coefficient, which includes the effects of pore pressure change (King & Cocco, 2001; Lin & Stein, 2004; Lin et al., 2011). In this study, changes 400 in the Coulomb stress were calculated using the Coulomb 3.3 model- Netslip (m) 0 ling software (Toda, Stein, Sevilgen, & Lin, 2011), which is suitable for ) –400 X (km modelling 3D stress and deformation in an elastic half-space. 019 8 –400 Using the FFM for the 2004 Mw9.2 Sumatra earthquake (Chlieh FIGURE 3 3D view of the 2004 Sumatra earthquake slip et al., 2007; Sevilgen et al., 2012), we calculated Δσ for various c model from Sevilgen et al. (2012). Shown is a solution of nine types of interested receiver faults. Previous studies have shown sub-patches.