CHEN Jie, GUO Laiyin, YANG Xiaodong, ZHANG Jinchang, ZHANG Zhiwen, SUN Mengyu, and LIN Jingxue

Large Active Faults and the Wharton Basin Intraplate Earthquakes in the Eastern Indian Ocean

CHEN Jie1), GUO Laiyin2), YANG Xiaodong3), 4), 5), *, ZHANG Jinchang3), 4), 5), *, ZHANG Zhiwen4), 6), SUN Mengyu4), 6), and LIN Jingxue4), 6)

1),,536007,2),,518055,3),511458,4),,,,511458,5),,45320,6),100049,

In recent years, great earthquakes occurred within the Wharton Basin in the eastern Indian Ocean, and they have been associa- ted with active faulting on the ancient oceanic crust. Large seismogenic faults were thought to be the fault reactivation on the ancient oceanic crust, but these phenomena are still unclear and require examination. This study used high-quality multibeam bathymetry and multichannel seismic data collected over the northern Ninetyeast Ridge to investigate detailed fault geometry, structure, and activity.We recognized 12 large linear active faults by integrating bathymetry maps and multichannel seismic reflection profiles. Our results showed that these faults have high angles, and they all displaced the basement and propagated to the seafloor with distinct fault scarps. They trended NWW-SEE with a spacing of 10–40km and were parallel to each other and the nearby subfault of the 2012 great intraplate earthquake, suggesting similar stress fields. These faults are also in agreement with the orientations of magnetic isochrons, implying their formation by seafloor spreading. Furthermore, regarding the strike-slip focal mechanism of 2012 earthquakes, we proposed that these faults were created early by a normal spreading process and then evolved into a strike-slip pattern since the ancient oceanic crust approached the subduction zones.

Ninetyeast Ridge; Wharton Basin; strike-slip faults; great earthquakes; seismogenic structure; earthquake mechanism

1 Introduction

The eastern Indian Ocean is geologically framed by mid-ocean ridges (southeast and central Indian ridges), subduction zones (Myanmar and Sumatra subductions), and hotspots (Kerguelen and Amsterdam-St. Paul hotspots) (Liu., 1983; Mutter and Cande, 1983; Royer and Sandwell,1989; McKenzie and Sclater, 2007). The southeast and cen- tral Indian ridges created most of the oceanic crust of the eastern Indian Ocean, which is documented by magnetic anomaly lineations (Krishna., 2012; Chen and Zhang, 2017). The Indo-Australia Plate subducts underneath the Eurasian Plate along the Myanmar and Sumatra trenches (Fig.1). The interaction between mid-ocean ridges and tren-ches deforms the intraplate structures in the eastern IndianOcean, as evidenced by observations of strong active fault- ing (Weissel., 1980; Bull and Scrutton, 1990, 1992;Krishna., 2009; Sager., 2010, 2013; Levchenko., 2014) and large earthquakes (Guo., 2021, 2022);this condition implies the formation of a diffusive plate boun- dary within the Indo-Australia Plate around the Wharton Basin (Wiens., 1985; Krishna., 1995, 1999; Royer and Gordon, 1997; Desa., 2009). Nearby, Ninetyeast Ridge is a remarkable feature formed by ridge-hotspot in- teraction (Kerguelen and Amsterdam-St. Paul hotspots and southeast Indian Ridge), and it extends N-S over 5000km near the 90?E meridian (Sclater and Fisher, 1974; Royer., 1991). All the above features are associated with to- pographic significance and tectonic or magmatic anomalies, providing insights into the formation and deformation of the eastern Indian Ocean and related geohazards.

The lithosphere in the Wharton Basin was created at the Wharton spreading center at least 84Myr (Fig.2), and the India Plate and Australia Plate were separated (Singh., 2017; Stevens., 2020). The India Plate collided with the Eurasia Plate at about 50Myr, and afterward, the spreading ceased at about 40Myr. Therefore, the India Plate and Australia Plate combined into a single plate, which subducted under the Eurasian Plate. From south to north along the subduction zone (Fig.1), the convergence rate decreased, and the subduction obliquity increased from theJava trench (normal subduction at 63mmyr?1) in the south to the Sumatra Trench east of the 2012 earthquake events (oblique subduction at 52mmyr?1) and to almost parallel subduction near the Andaman Islands (43mmyr?1)(Carton., 2014). Given the high obliquity of subduction, a sliver plate between the subduction zone and a complex right-lateral fault system was formed. In addition, the Andaman backarc rift-transform system, lying 200–400km from the subduction zone, linked the Sagaing fault to the north and the Great Sumatra Fault to the south (Sevilgen., 2012).

Fig.1 Tectonic map of the Wharton Basin and adjacent areasof the eastern Indian Ocean. Bathymetry data were obtained from the Global Multi-Resolution Topography Data Synthesis. Black lines represent the plate boundaries between the Indo-Australian Plate and Eurasian Plate. The dashed line denotes the diffusive boundary within the Indo-Aus- tralian Plate mentioned in the text. The red box is enlarged in Fig.2. The inset map shows the location of the study area in the Indian Ocean.

The December 26, 2004 Sumatra Mw9.2 earthquakeruptured more than 1300km along the Sumatra subduction zone (Fig.2) (Ammon., 2005; Lay., 2005). Published finite-slip models for the year 2004 showed relative differences between the detailed time/space distribution of the slip at each patch (Ammon., 2005; Gahalaut., 2006; Banerjee., 2007; Chlieh., 2007; Pietrzak., 2007; Rhie., 2007). This megathrust increased seismicity in the Wharton Basin and rifted but shut down transformations in the Andaman Sea (Sevilgen., 2012; Guo., 2021).

As the spreading center in the Wharton Basin is almost oriented E-W and the fracture zones were nearly oriented N-S (Fig.2) (Jacob., 2014), the NW-SE compression from the Indian-Eurasia collision and the slab pull forces from the Sumatra subduction zone resulted in the reactivation of fossil fracture zones in the Wharton Basin, and they were capable of hosting great seismicity (Singh., 2011; Hill., 2015). The April 11, 2012 Mw8.6 Whar- ton Basin earthquake sequence was among the most intri- guing ones (Fig.2). The results for the 2012 event were ob- tained using different seismological methods, including back-projection (Meng., 2012; Yue., 2012; Ishii., 2013),phase inversion (Duputel., 2012), andfinite fault models (Yue., 2012; Wei., 2013), anddemonstrated a great complex rupture pattern. For example, Wei. (2013) pointed out that the Mw8.6 earthquake consisted of three subfaults: Fa (89?/289? for dip/strike), Fb (74?/20?), and Fc (60?/310?). In addition, the strike of the Mw8.2 earthquake (Fd) was oriented at 16?, which nearly paralleled the Fb that occurred 2h later. Fb and Fd were in agreement with the strike-slip of fracture zones, trending NE-SW (Fig.2), which implied that they were likely the reactivation of fossil fracture zones. Meanwhile, Fa and Fcmay match the trends of fossil spreading ridges, where nor- mal faults would be expected. However, as mentioned above, the strike-slip faulting property revealed from the earthquakefocal mechanism that the normal faults formed by spreading ridges were reactivated and shifted to strike-slip faults. This complexity is poorly understood and needs examination.

Fig.2 Earthquakes and controlling faults (left) and gravity anomaly map with magnetic isochrons and fracture zones (right). The red box is the study area shown in Fig.3. The focal mechanism was selected from the Global Centroid Moment Tensor Project. Earthquake catalogs between Mw 4 and 5 were from the International Seismological Centre. Seismogenic faults (red lines) refer to the work of Guo et al. (2021, 2022). The free-air gravity anomaly data were from the global marine gravity database (Sandwell et al., 2013). The magnetic isochrones (green lines with numbers) and fracture zones (dashed white lines) were from the compiled datasets of Cande et al. (1988), Krishna et al. (1995, 1999, 2012), Desa et al. (2009), and Chen and Zhang (2017).

Many active faults occur over the northern Ninetyeast Ri- dge in the proximity of Fc and with a similar NW-SE trend (Sager., 2010, 2013).They may be reactivated faults that were formed by spreading centers and then transform- ed from normal to strike-slip faults. The structure of these active faults can be examined to reveal the relationship with the nearby fault Fc that hosted the large intraplate earthquake within the Wharton Basin. Practically, the interpretation of multibeam bathymetry and multichannel seismic re- flection profiles collected over the northern Ninetyeast Ri- dge allows us to observe the seafloor and subseafloor struc- tures of faults related to Fc. This study can test whether such faults were caused by reactivated and transformed faults from pre-existing basement normal faults and provideinsights into the fault tectonic process and great earthquake potential in the eastern Indian Ocean.

2 Data and Methods

2.1 Bathymetry and Earthquake Data

The high-resolution bathymetry data were downloaded from the Global Multi-Resolution Topography Data Synthesis (https://www.gmrt.org/). The earthquake catalogs be- tween Mw4 and 5 (from 1976-01-01 to 2021-12-31) werecollected from the International Seismological Centre (http:// www.isc.ac.uk/). The focal mechanism was selected from the Global Centroid Moment Tensor Project (https://www. globalcmt.org/).

2.2 Gravity and Magnetic Data

The free-air gravity anomaly data were obtained from the global marine gravity database (satellite altimetry fromthe Scripps Institute of Oceanography; Sandwell., 2013).The magnetic isochrones and fracture zones were from the compiled datasets of Cande. (1988), Krishna. (1995, 1999, 2012), Desa. (2009), and Chen and Zhang(2017). The bathymetry and gravity maps were plotted us- ing the Generic Mapping Tool (Wessel., 2019).

2.3 Multichannel Seismic Reflection Data

The two-dimensional multichannel seismic reflection data in this study were acquired from a survey cruise KNOX 06RR at several sites of the Ninetyeast Ridge aboard thein 2007 (Eisin, 2009; Sager., 2010,2013). Only the data from Site 216 (10 profiles) were usedbecause of the proximity to our study area (Fig.3). The sei- smic data were collected by a shooting source as an array of two GI air guns (2.46L) and a 48-channel streamer witha receiver group interval of 12.5m. This acquisition system can generate a maximum of 10 folds after stacking. Data processing was conducted with the ProMAX software, and processing steps included trace edit, bandpass filtering, velocity analysis, normal moveout correction, stacking, and time migration. The seismic profiles were interpreted by standard seismic stratigraphy principles (Mitchum., 1977). Three major horizons were interpreted (Figs.4–9), referring to the work of Eisin (2009), from bottom to top,., acoustic basement, intra-sediment boundary, and seafloor. These seismic horizons were also correlated with ocean drilling results, which were crossed with the seismic profiles (Davies., 1974; von der Borch., 1974; Peirce., 1989). Adobe Illustrator was applied to make graphic layers and annotations.

Fig.3 Integrated bathymetry and fault mapping on the Ninety-east Ridge. (a) Bathymetry overlain by seismic profiles avail-able for this study. All red lines are seismic profiles used for structural analysis; bold red lines are present in the paper, and thin ones are not presented. (b) Mapped faults in black lines using the bathymetric map. Fault Fc corresponds to the proposed fault F3 in the work of Guo et al. (2021, 2022), and faults F1–F12 are newly mapped faults by this study. Earthquake catalogs of Mw between 3 and 5 were from the International Seismological Centre.

3 Results

3.1 Fault Mapping on Bathymetry

Here, we integrated the high-resolution bathymetry data to map the seafloor structures on northern Ninetyeast Ridge (Fig.3). As shown in Fig.3a, the most distinguishing features on this map were the linear fault traces that trended NWW-SEE on the ridge and were generally parallel to eachother, suggesting that they were of similar origin or had ex-perienced similar stress fields. In total, 13 faults were recog-nized in this map, and they were categorized into two groups: 1) Fc on the northmost and corresponded to the F3 fault identified by Guo. (2021, 2022) using seismicity and focal mechanism; 2) F1–F12 were the remaining fault struc- tures. However, determining whether F1–F12 faults have extended westward and eastward away from the ridge wasdifficult due to the limited high-resolution bathymetry co- verage.

3.2 Fault Interpretation Using Multichannel Seismic Profiles

In addition to bathymetry data, ten multichannel seismic profiles were available for the imaging of the subsurface structures in this area, particularly the large faults (F1–F12) with distinct seafloor expressions (Fig.3, see also Sager., 2010, 2013). Here we selected the six best profiles in terms of image quality and locations in the main text for a detailed description, and the rest of the seismic profiles were not presented but used to assist in the overall structural interpretation.

As shown in Fig.4, seismic profile 1b presented three stratigraphic units from deep to shallow levels: basement, sediment unit 2, and sediment unit 1 (Table 1; Luyendyk, 1977; Eisin, 2009). The interpreted faults can be categorized into three types, namely, major active (bold red lines), smallactive (thin red lines), and inactive faults (black lines). The major active faults (., F1–F5 and F7) have all displaced the basement and propagated all the way to the seafloor with distinct fault scarps (Pilipenko, 1996; Sager., 2013). These faults were visible on the bathymetry map (Fig.3). Overall, the major active faults on this profile were exclusively extensional basement-involved faults, except for the observed thrust faults on the east side of the southern Ninetyeast Ridge near DSDP Site 214 (Pilipenko, 1996).On the cross-section, the lateral sense of slip was difficult to determine. The major fault space ranged between 10 and 15km. The small active faults showed minor displacement and very limited seafloor scarps. By contrast, the inactive faults were blind normal faults overlain by sediments and without seafloor expressions.

Fig.4 Structural interpretation of seismic profile 1b. Bold and thin red lines denote major and minor active faults, respectively. Thin black lines correspond to inactive faults. Bold black lines represent the three main seismic horizons (bottom to top, basement, and sediment units 2 and 1) interpreted by Eisin (2009). Green lines are other interpreted stratigraphic subunits.

Table 1 Three main stratigraphic units in the multichannel seismic reflection profiles from the work of Eisin (2009) and Luyendyk (1977)

Note: Tops of units are marked by the major seismic horizons.

Seismic profile 1c (Fig.5) is on the east slope of the Nine- tyeast Ridge. The topography dips to the Wharton Basin to- ward the east and southeast. On this profile, the major active faults were F8–F10, which displaced three stratigra- phic units to form evident scarps on the seafloor. By contrast, only two inactive faults were identified, and small active faults were abundant. Although small faults were active, they only formed small scarps on the seafloor. We, therefore, suggest that the majority of the strain was localized on the major normal faults. The fault space varied from 14km to 40km on this profile, suggesting that the fault toward the basin was less developed.

Fig.5 Structural interpretation of seismic profile 1c. Other plot conventions are the same as in Fig.4.

Profile 3 (Fig.6) obliquely traverses the Ninetyeast Ridge, and its topography is high in the middle and low on both sides. Three major faults (F9, F11, and F12) were mapped near the top of the ridge. F9 and F11 have displaced the three stratigraphic units with clear seafloor scarps. By contrast, F12 was at its east termination (Fig.3b) and has only displaced the shallowest unit (late Oligocene to Pleistocene). The remaining faults were either inactive overlain by sediments or small active faults with small seafloor expressions. All the faults had very high angles of approximately 90?.

Profile 5 (Fig.7) is the longest seismic profile (125km), and it trends W-E and traverses the Ninetyeast Ridge. On the eastmost of this profile, a small seamount was observed with 1s height in two-way travel time. The major faults onthis profile were F5 and F6. A number of small active faultswere interpreted with minor seafloor scarps. Similar to pre- vious observations, the inactive faults accounted for the minority, and they were mostly buried without seafloor ex- pressions. All the faults were high-angle ones,., 85?–90?.

Profile 8 (Fig.8) is a short profile with a 20km length, and it trends N-S,, approximately parallel to the Ninety- east Ridge. Three major faults (F5–F7) were interpreted onthis profile, among which F6 had the smallest offset on the seafloor and basement, suggesting the end of this locality.F5 and F7 formed distinct seafloor scarps and showed alarge displacement on the basement. In addition, four inactive faults were discerned without seafloor expression. Three small active faults were recognized, and they barely offset the basement, suggesting that they were of the sediment origin.

Profile 10 (Fig.9) is the second shortest profile, with a 32km length. It is parallel to the Ninetyeast Ridge,., it is N-S trending. Major faults (F5 and F7) were interpreted on this profile, and they displaced the three stratigraphic units, formed clear seafloor scarps, and showed a large dis- placement on the basement. In addition, several inactive faults were mapped without seafloor expressions. The small inactive faults were mostly basement-involved faults but showed relatively smaller displacement on the basement and seafloor. Similar to previous observations, these faults have high angles,, 85?–90?.

Fig.6 Same as in Fig.5 but for seismic profile 3.

Fig.7 Same as in Fig.5 but for seismic profile 5.

Fig.8 Same as in Fig.5 but for seismic profile 8.

4 Discussion

The overall subsurface fault interpretation suggests that major faults are basement-involved faults and have large displacement on the basement and relatively decreasing dis-placement toward the seafloor, implying that they are extre- mely active (Figs.4–9). By contrast, the small active faults have minor displacement on the basement and almost in- visible displacement on the seafloor. The small faults are active but have accumulated limited strain. The inactive faults are the minorities, and they are predominantly buriedor only form in the sedimentary sequence and have no recent activity. Therefore, the seismogenic faults are mostly likely to be the mapped 12 major active faults in addition to the Fc identified previously by Guo. (2021) (Fig.3).In the following section, we will combine the regional seis- micity and fault structure to discuss the earthquake mechanism in this region.

Fig.9 Same as in Fig.5 but for seismic profile 10.

4.1 Fault Formation Mechanism

By integrating bathymetry maps and multichannel seismic profiles, we mapped 12 major faults oblique to the Ninety- east Ridge. On the map view, they appeared to be parallel to each other with NWW strike (Fig.3). In addition, the fault strike was approximately parallel to magnetic isoch- rons (Fig.2). The fault space was less variable and ranged between 10 and 40km. We also noted that these faults are likely major structures because they traverse the entire Nine- tyeast Ridge and extend to the adjacent basins, with very distinct seafloor traces on the map (Fig.3). These first-order observations suggest that they were formed by a similar stress field. On the cross-section, these major faults showeddistinct extensional movement with clear scarps on the seafloor. At great depths, they are mostly basement structures and cut through the top of the basement and extend all the way to the seafloor (see Sager., 2013).

In terms of fault style, a normal sense of slip was evidentfrom our seismic interpretation and previous studies (Naini and Eittreim, 1974; Veevers, 1974; Curray and Munasinghe,1989; Pilipenko, 1996; Sager., 2010, 2013), and fault dip showed a very high angle,., 90?. Furthermore, previous studies (Guo., 2021) interpreted a major strike- slip fault (Fc in Figs.2 and 3) based on earthquakes and fo- cal mechanisms north of faults F1–F12. The Fc has a simi- lar orientation to F1–F12 (Fig.3). Altogether, we proposed that F1–F12 are strike-slip faults with a normal sense of slip component. Regarding the origin of these faults, previous authors noted that the basement offset is greater thanthe sediment offset and proposed that the recent faulting wasa reactivated original basement fault (Sager., 2010, 2013). A similar observation was made in the central Indian Basin, where the original spreading-ridge-generated normal faults were reactivated as active thrust faults (Bull and Scrutton, 1992), and the strike-slip faulting in the Whar-ton Basin is believed to form by the reactivation of theoriginal transform faults (Deplus, 2001; Delescluse., 2008). Furthermore, the overall parallel of fault strike withthe magnetic lineation implicates that they probably result- ed from seafloor spreading. The strike-slip sense of slip was interpreted to have partly transitioned from normal slip to lateral slip.

4.2 Relationship Between Faults and Earthquakes

As we interpreted, the Fc and F1–F12 have similar struc- tures in terms of origin and style of faulting, and the relationship between them was evident. The space between Fcand F1–F12 is approximately 40km, which is slightly widerthan that between F1–F12 faults. This finding was relatedto the timing, episodes, and rates of early seafloor spreading or to a normal feature in a tectonically active area. According to the seismicity map in Figs.2 and 3b, more seismicity was observed on the north and northwest corners of the F1–F12 area and near the Fc (Guo., 2021, 2022); this finding suggests that F1–F12 are less capable of producing major or large earthquakes but can produce small events (magnitude<4–5). By contrast, the Fc has ge- nerated a number of moderate to strong seismicity (magnitude>4–5) along its strand, particularly at its two ends (Guo., 2021, 2022). Thus, Fc is a powerful fault. Based on this information, we postulate that the length, dis- placement, and strength of Fc are greater than those of F1 –F12, which unfortunately cannot be corroborated by our current datasets. However, caution should be exercised as F1–F12 are all active structures, and their potential for generating hazardous earthquakes in the future still exists.

The shift from normal faults to dominant strike-slip faults is difficult to resolve. One possible reason is the transition from normal subduction to oblique subduction of the Indo- Australian Plate beneath the Eurasian Plate. Oblique sub- duction can lead to margin parallel slip, which in turn can cause the formation of large strike-slip faults such as the Great Sumatra fault (McCaffrey, 1989, 1992). F1–F12 and Fc are overall parallel to the trench and show similar orientations with the Great Sumatra Fault. Hence, a genetic link may exist between these strike-slip faults on the hanging wall and footwall of this margin.

5 Conclusions

This study aimed to examine the structure of active faults on the northern Ninetyeast Ridge and reveal the relationship with the nearby large strike-slip fault that caused the great intraplate earthquake within the Wharton Basin. The integrated interpretation of multibeam bathymetry maps and multichannel seismic reflection profiles showed the seafloor and subseafloor faulting structures. Although many small active faults were observed, the seismogenic faults were most likely the mapped 12 major active faults. These major faults have a high angle and are basement-involved. They have a large displacement on the basement, sediment, and seafloor and clear fault scarps on the seafloor. They transverse the ridge in the NWW-SEE direction and are parallel to each other with a spacing of 10–40km. The si- milarity of the structure indicates that these faults were form- ed by similar mechanisms. Their fault strikes are approximately parallel to the surrounding magnetic anomaly lineations, which implies their origin from seafloor spreading. Moreover, these faults are parallel to the strike-slip seismo- genic fault that was revealed by the 2012 great earthquake, suggesting a recent analogous stress field. Hence, we proposed that the pre-existing basement normal faults that were originally formed by seafloor spreading shifted to strike- slip faults after the oceanic crust of the Indo-Australian Plate ceased accretion and began subduction underneath the Eurasian Plate.

Acknowledgements

This research was supported by the Guangdong Basic and Applied Basic Research Foundation (No. 2021B1515 020098), the Project of Science and Technology Department of Guangxi Zhuang Autonomous Region to Chen J. (No. 2019AC17008), the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (No. GML 2019ZD0205), the National Natural Science Foundation of China (No. 41890813), the Chinese Academy of Sciences Project (Nos. 133244KYSB20180029, 131551KYSB20200 021,Y4SL021001, QYZDY-SSW-DQC005, ISEE2021PY03, and E1SL3C02), the Development Fund of South China Sea Institute of Oceanology of the Chinese Academy of Sciences (No. 202207), and the Guangdong Provincial Research and Development Program in Key Areas (No. 2020B1111520 001).

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(August 19, 2022;

October 25, 2022;

November 4, 2022)

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2023

E-mail: xdyang@scsio.ac.cn

E-mail: jzhang@scsio.ac.cn

(Edited by Chen Wenwen)