C H A P T E R

4

Reactivation of Oceanic FractureZones in Large Intraplate Earthquakes?

Thorne LayDepartment of Earth and Planetary Sciences, University of California Santa Cruz,

Santa Cruz, CA, United States

O U T L I N E

1 Introduction 89

2 The Largest Oceanic IntraplateEarthquakes Away From Boundaries 90

3 Large Events Outside the WhartonBasin 92

4 Wharton Basin Activity 93

5 Discussion and Conclusion 99

Acknowledgments 101

References 102

1 INTRODUCTION

Oceanic spreading centers impart the primary fabric in oceanic crust that may influencefaulting as the plate ages and experiences stresses on its boundaries. Spreading ridge trans-form faults are commonly seismogenic at shallow depths due to the relative plate motionsbetween offset ridge segments being resisted by friction in the near-surface brittle part ofthe evolving lithosphere (e.g., Sykes, 1967; Engeln et al., 1986; Bergman and Solomon,1988; Abercrombie and Ekstr€om, 2001). On the intraplate fracture zone extensions, contrastsin the lithospheric age, differential subsidence rates, thermal contrasts, and bathymetrysustain minor levels of faulting near to the ridge system that drop off rapidly with distance,leaving fracture zones largely aseismic. An exception is the July 15, 2003 MW 7.6 right-lateralstrike-slip rupture that extended about 210km along a fracture zone in young (<12Ma) In-dian plate lithosphere immediately adjacent to the Central Indian Ridge (e.g., Bohnenstiehl

89Transform Plate Boundaries and Fracture Zones # 2019 Elsevier Inc. All rights reserved.

https://doi.org/10.1016/B978-0-12-812064-4.00004-9

et al., 2004; Antolik et al., 2006). The corresponding active transform fault on the ridge has left-lateral strike-slip faulting. Strike-slip faulting on fracture zones usually has reversed senseof motion relative to the active transform fault region (i.e., the sense of fracture zone faultingis similar to the ridge segment offset) (e.g., Engeln et al., 1986; Robinson, 2011). Off-ridgenormal faulting and strike-slip faulting diminish progressively as lithospheric age increasesto about 20Ma in most plates, while reverse faulting extends across most lithospheric ages(e.g., Wiens and Stein, 1984). The seismogenic expression of fracture zones is thus mostlylocalized to the near-ridge environment or located far from the ridge. Activity on intraplatefracture zones may be induced by off-ridge volcanism (e.g., Okal and Stewart, 1982), butgenerally is attributed to large-scale plate stresses and deformation (e.g., Stein and Okal,1978; Robinson, 2011).

Oceanic lithosphere older than 40Ma exhibits increased strike-slip activity, including somevery large earthquakes and it is commonly assumed that fracture zones may be involved asthey represent lithosphere-traversing fault systems that may retain deviatoric stresses frombathymetry and thermal contrasts and may have disrupted mechanical strength due to thepresence of gouge and fluid accumulations. However, it actually does not appear that intra-plate fracture zones are particularly weak relative to surrounding oceanic lithosphere (e.g.,Sandwell and Schubert, 1982; Bergman and Solomon, 1992), so the influence of fracture zoneson large intraplate seismicity remains unclear. Here we consider large oceanic intraplateactivity at distances >100km from plate boundaries (including away from the outer-riseregion near subduction zones where large normal faulting associated with plate bendingoccurs), to evaluate the role played by preexisting fracture zones.

2 THE LARGEST OCEANIC INTRAPLATE EARTHQUAKESAWAY FROM BOUNDARIES

Information about global oceanic seismicity is limited to the seismological record, whichdates back to about 1900. Restricting our attention to intraplate seismicity more than 100kmfrom mid-ocean ridges and subduction zones, with long-period seismic magnitude (MW orMS)�7.8, the US Geological Survey National Earthquake Information Center (USGS-NEIC)database contains only 9 events (Table 1) dating back to 1900 (drawing upon the revisedmagnitudes in the ISC-GEM catalog). The seismic moment tensors and locations of thesenine events are shown in Fig. 1. The source mechanisms are all predominantly strike slip,although several of the events have significant non-double-couple components. These larg-est of oceanic intraplate earthquakes are few in number, but widespread, with eventslocated in the Atlantic, Pacific, Antarctic, and Indo-Australian plates. The May 26, 1975Azores and December 23, 2004 Macquarie events are relatively close to plate boundarytransform faults, but are clearly offset from the main active boundary. Events in the Gulfof Alaska and offshore of Sumatra are relatively close to subduction zone boundaries,but are seaward of the outer rise. We briefly discuss the relationship of these large eventsto the oceanic fracture zone structure.

90 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

TABLE 1 The Nine Largest Intraplate Strike Slip Events in Oceanic Lithosphere

Date Origin Time Region Lat. (degree) Lon. (degree) Mw

April 11, 2012 08:38:36 Wharton Basin 2.327 93.063 8.6

April 11, 2012 10:43:10 Wharton Basin 0.802 92.463 8.2

March 25, 1998 03:12:25 Balleny Islands �62.877 149.527 8.1

December 23, 2004 14:59:04 Macquarie �49.312 161.34 8.1

May 26, 1975 09:11:51 Azores 35.997 �17.649 7.9(Ms)

June 18, 2000 14:44:13 Wharton Basin �13.802 97.453 7.9

November 30, 1987 19:23:19 Gulf of Alaska 58.679 �142.786 7.9

March 6, 1988 22:35:38 Gulf of Alaska 56.953 �143.032 7.8

March 2, 2016 12:49:48 Wharton Basin �4.952 94.330 7.8

90° 120° 150° 180° −150° −120° −90° −60° −30° 0°

−60°

−30°

0°

30°

60°

11 April 2012 Mw 8.2, 8.6

25 March 1998 Mw 8.1

23 December 2004 Mw 8.1

26 May 1975 Ms 7.9

30 November 1987 Mw 7.9

2 March 2016 Mw 7.8

6 March 1988 Mw 7.8

18 June 2000 Mw 7.9

FIG. 1 Moment-tensor solutions for the nine largest seismologically modeled intraplate ruptures in oceanic lith-ospherewell-removed from active plate boundaries. The focalmechanisms for all events are from the global centroid-moment tensor catalog with the exception of the Azores earthquake of May 26, 1975, for which the surface wavesolution from Lynnes and Ruff (1985) is shown. Each solution is centered at the event hypocenter other than forthe 1988 and 2012 events with offset positions. Faulting directions and relationship to nearby fracture zones for eachcase are discussed in the text.

912 THE LARGEST OCEANIC INTRAPLATE EARTHQUAKES AWAY FROM BOUNDARIES

3 LARGE EVENTS OUTSIDE THE WHARTON BASIN

The May 26, 1975MS 7.9 Azores earthquake (Fig. 1) occurred south of the seismicity trendextending from theAzores Islands toGibraltar that is thought to be theAfrican-Eurasian plateboundary in the eastern Atlantic. The November 25, 1941 MS�8.0 earthquake lies on thattrend along the Gloria transform fault 200km NNW of the 1975 hypocenter. The easterlytrending fault plane of the 1975 event is quasi-parallel to the plate boundary, but backgroundseismicity in the source region is localized, so the event is best characterized as intraplate.Lynnes and Ruff (1985) analyzed body and surface waves for the event, preferring a right-lateral mechanism with strike 288 degree, dip 65 degree, and rake 205 degree, with a seismicmoment of 7�1020Nm. The centroid depth is less than 25km and the fault length is only50–100km. Aftershock relocations suggest eastward propagation of the rupture. There isno clear relationship to local structural features within the African plate.

Two of the largest oceanic strike-slip events struck in the Gulf of Alaska, southeast ofthe 1964 MW (9.2) subduction zone earthquake, and southwest of the Yakutat terrane. Thesequence began with an MW 7.2 rupture on an east-west trending fault on November 17,1987, followed by November 30, 1987 MW 7.9 and March 6, 1988 MW 7.8 ruptures along anorth-south trend (Fig. 1) (Lahr et al., 1988; Pegler andDas, 1996; Quintanar et al., 1995). Therehad been no prior large seismicity in the region. Hwang and Kanamori (1992) inverted bodywaves finding that the centroid depths of the largest two events were 20 and 15km, withfaulting extending to at least 25km. The large-slip rupture lengths of 110 and 40km, respec-tively, are shorter than for comparable size continental strike-slip events, suggesting rela-tively high stress drop. The north-south trend is parallel to a magnetic lineation, so afracture zone was not involved, and the horizontal orientations of the principle stressesare attributed to pull toward the northwest from the 1964 underthrusting event and compres-sion from the northeast due to the collision of the Yakutat block with the arc. Quintanar et al.(1995) suggest that these events represent a tear in the Pacific plate.

The December 23, 2004 MW 8.1 Macquarie event occurred about 150km from theMacquarie Ridge Complex plate boundary between the Australian and Pacific plates(Fig. 1). The event is located within a deformation zone called the Puysegur (or Macquarie)Block, which hasmultiple curved fracture zones within it, now striking at oblique angles nextto the plate boundary due to progressive increase in translational motion across the formerdivergent ridge (e.g., Massell et al., 2000; Mosher and Symons, 2008; Hayes et al., 2009).Historic intraplate activity in the southern Tasman Sea indicates broad intraplate deforma-tion, with large events (up to M�7.8) dating back to 1924 also having locations off the plateboundary (e.g., Valenzuela and Wysession, 1993). The great May 23, 1989 MW 8.0 strike-slipevent (e.g., Ruff, 1990) occurred directly on the Macquarie Ridge. Das (1992, 1993) suggeststhat some of the much smaller aftershocks of that event are located on a fracture zone obliqueto the ridge, southwest of the 2004 event. Kennett et al. (2014) apply back projection ofteleseismic signals for the 2004 event and infer an initial bilateral rupture expansion followedby activation of a parallel rupture to the west. Such back projections are, however, unstabledue to the near nodal position of downgoing P waves. The relocated 2004 mainshock andaftershocks generally lie along one of the curved fracture zones (Robinson, 2011). Robinson(2011) models broadband P and SH, finding an along rupture change in strike of about 18

92 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

degreewith right-lateral strike-slip faulting in the north and oblique thrusting in the south, pos-sibly accommodating motion on the curved fracture zone. For slip models restricted to 18kmdepth, and a fault length of 150km, the estimated stress drop is 20MPa. As for the previouslydiscussed large events, this stress drop is larger than typical continental strike-slip faulting,mainlydue to the shorter rupture length for its size.However, the depth constraint is not strong,and allowing rupture to extend to 28kmdepth, at least overlapping the global centroid-momenttensor (GCMT) centroid depth of 27.5km, lowers the stress drop estimate to 8MPa.

The largest intraplate earthquake to rupture the Antarctic plate is the March 25, 1998Balleny IslandsMW 8.1 earthquake (Fig. 1). It struck a region with no prior large events about250km west of the Antarctic-Australian plate boundary, with aftershocks and body-waveform inversions indicating predominantly left-lateral strike-slip rupture toward thewest, transverse to the regional north-south trending fracture zones (e.g., Kuge et al., 1999;Nettles et al., 1999; Antolik et al., 2000; Henry et al., 2000). The rupture traversed lithosphereof 33–40Ma age along a nearly 300km long trend, with the largest aftershock locating 100kmsouth of the mainshock faulting. The GCMT solution has a significant non-double couple,which Kuge et al. (1999) and Antolik et al. (2000) attribute to secondary oblique normalfaulting either between en echelon segments or to the east of the hypocenter, respectively.

Henry et al. (2000) invert broadband body waves, favoring an initial 140-km long west-ward rupture with 45s duration with slip bracketed between two fracture zones, and a sec-ond, dynamically triggered, slip event initiating 40s later about 100km further to thewest thatextends westward for about 60km. Themechanisms of the subevents have dips of�69 degreeand rakes of ��18 degree; this oblique normal component combines with the space-timeextent of the rupture to account for the GCMT non-double-couple moment tensor. Henryet al. (2000) estimated large stress drops of 24 and 21MPa for the first and second subevents,respectively, assuming a 15-km maximum depth of rupture. Hj€orleifsdottir et al. (2009)analyze long-period signals and demonstrate that observed amplitudes and directivity favorcontinuous rupture along the westward trend with the long-period slip occurring betweenthe slip patches in the model of Henry et al. (2000), rather than separated dynamic triggering.This complicates the estimation of stress drop, as the slip area may be much larger and mayextend deeper. The fracture zones appear to play little, if any, role in the rupture. Tsuboi et al.(2000) propose that this event was driven by postglacial rebound, as the orientation of theprinciple stress axis is consistent with predictions for Antarctic unloading.

Of these large strike-slip events, only the 2004 Macquarie event is convincingly associatedwith fossil fracture zone reactivation, and for that event the strong intraplate deformationzone caused by the change of relative plate motion appears to be a controlling factor. Thusfracture zones do not necessarily play a major role in the largest intraplate faulting unlessthe plate is pervasively deforming. Fig. 1 indicates that the Indo-Australian plate experiencesthe greatest concentration of intraplate faulting in theWharton Basin south of Sumatra wherethere is a broad deformation zone, and we focus on that activity for the rest of this review.

4 WHARTON BASIN ACTIVITY

Fig. 2 displays the intraplate seismicity within the broad deformation zone in the Indo-Australian plate around the Wharton Basin. Locations of all events with M�6.0 from the

934 WHARTON BASIN ACTIVITY

USGS-NEIC database from 1900 to 1975 are shown, along with focal mechanisms for allevents with MW�5.0 from the GCMT catalog from 1976 to 2017. Four of the large eventsin Fig. 1 are labeled on themap. The positions of fracture zones are indicated bywhite dashedcurves (Matthews et al., 2011) and the north-south trending Ninetyeast Ridge is evident fromthe bathymetry. This is the most intensively deforming large-scale oceanic lithosphere on theplanet (small deforming regions like the Gorda plate, wedged between the Mendocino andBlanco faults, also have extensive intraplate strike-slip activity, but the scale is vastlygreater here).

The diffuse deformation region between the Indian and Australian plates accommodates�11mm/year of internal deformation based on global plate motionmodels (e.g., Wiens et al.,1985; Gordon et al., 1990; Royer et al., 1997; Delescluse and Chamot-Rooke, 2007; DeMetset al., 2010). The largest earthquakes in this region involve strike-slip faulting; among themis the largest known intraplate rupture of April 11, 2012 (MW 8.6). Lower magnitude thrustfaulting is observed in the southern andwestern portions of the region, extending off of Fig. 2.

The north-south striking fracture zones east of the Ninetyeast Ridge are associated withleft-lateral offsets of the Wharton fossil ridge (dark red in Fig. 2), which was active between38 and 85Ma and had very rapid spreading (e.g., Deplus et al., 1998; Matthews et al., 2011;Jacob et al., 2014). Strike-slip faulting near these structures is primarily left lateral, assumingrupture of the north-south striking planes, again displaying the typical reversal of motion rel-ative to faulting that would have occurred on the transform fault segments. It is commonly

90° 100°−15°

−10°

–5°

0°

5°

km

M5 M6 M7 M8

Wharton Basin

11 April 2012 Mw 8.6

18 June 2000 Mw 7.9

2 March 2016 Mw 7.8

11 April 2012 Mw 8.2

Indian

Plate

IFZ

Sunda Megathrust

0 10 20 30 40 50 60 70 80 90

Source depthN

inet

yeas

t Rid

ge

AustralianPate

FIG. 2 Map of large seismically recordedintraplate earthquakes in the Indo-Australian plate. Solid colored circles indi-cate magnitude �6.0 earthquake epicentersfrom the USGS-NEIC catalog from 1900 to1975, color coded for hypocentral depth,with radius scaled proportional to magni-tude. The GCMT catalog centroid-momenttensor solutions with MW�5.0 for eventsoutside the Sunda Megathrust from 1976topresent are also shown,with radius scaledproportional to MW. The largest four intra-plate ruptures are labeled. White dashedcurves are tracks of fracture zones fromMatthews et al. (2011). The investigator frac-ture zone is labeled IFZ. The red curve isthe fossil Wharton ridge location with theyoungest active age of �38Ma.

94 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

asserted that the regional earthquake faulting geometry is controlled by the reactivation offossil fracture zone by present day deformation within the intraplate oceanic basins andsubducting slab of the Indo-Australian plate (e.g., Bull and Scrutton, 1990; Deplus et al.,1998; Abercrombie et al., 2003; Delescluse et al., 2008; Rajendran et al., 2011; Geersen et al.,2015; Aderhold and Abercrombie, 2016). There is also extensive activity along the NinetyeastRidge, but the actual fault planes are not known for most events (e.g., Stein and Okal, 1978;Sager et al., 2013). There is indeed a general tendency for spatial concentrations of seismicdeformation to be aligned with the fracture zone fabric, as are the focal mechanisms, butthe regional stress state must also be considered.

The predominance of intraplate strike-slip deformation across the Wharton Basin is likelydue to horizontal principle compressional and tensional stresses in the Indian and Australianplates. This is the expected result from the combination of Himalayan collision along thenorthern Indian plate and slab pull due to subduction of the Australian plate along the Sundamegathrust (e.g., Cloetingh and Wortel, 1985; Coblentz et al., 1998; Delescluse et al., 2012;Wiseman and B€urgmann, 2012). Close to the trench, there is some outer-rise extensionalactivity (Fig. 2) typical of slab bending, but strike-slip faulting continues all the way to thetrench from 2° to 5°N along the southern portion of the great 2004 Sumatra-Andaman rupturezone. This is a rare predominance of strike-slip deformation over normal faulting seaward ofa major underthrusting earthquake. The Gulf of Alaska activity discussed above has somesimilarity in having an along plate boundary variation from great underthrusting eventsto terrane collision, but again the scale of continental collision of the Indian plate dwarfs ac-tivity elsewhere.

The diffuse Indo-Australian deformation zone may progressively localize faulting, even-tually resulting in a well-defined plate boundary along the lines of what has already occurredto segment the Capricorn plate from the Indian plate (e.g., Royer and Chang, 1991; Royer andGordon, 1997; DeMets and Royer, 2003; DeMets et al., 2005), but this process is greatly com-plicated by the lateral compression in the plate due to the India-Eurasia collision inhibitingprogress in the development of a discrete boundary. At present, a nascent boundary betweenIndian andAustralian plates in the northwesternWharton Basinwould have to primarily be aconvergent one, and onset of subduction within old oceanic lithosphere presents a difficultchallenge. While the long-term evolution is unclear, the present state of stress in the plateand the influence of fracture zones can be considered relative to the large intraplate earth-quakes in the region.

The June 18, 2000 MW 7.9 (Fig. 2) earthquake struck west of the Investigator fracture zone(IFZ) near a parallel fracture zone (F) (Robinson et al., 2001; Abercrombie et al., 2003). TheGCMT has a large non-double-couple component indicative of complexity in the rupture,and larger aftershocks included strike-slip and thrust faulting, with an earlier thrust faulthaving occurred nearby in 2001 (Fig. 3). Robinson et al. (2001) model the source as twostrike-slip events with similar mechanisms but orthogonal fault planes, one of which parallelsF and the other is orthogonal to IFZ. Abercrombie et al. (2003) also model the event with twosubevents, the first of which has 75% of the total moment and is a left-lateral strike-slip eventand the second being a less well-resolved oblique reverse slip event (Fig. 3), significantly dif-ferent from the second event in the model of Robinson et al. (2001). The Abercrombie et al.(2003) model accounts for the non-double-couple moment tensor much better and also pre-dicts the observed P and SH waves much better. This model involves faulting off of both

954 WHARTON BASIN ACTIVITY

fracture zones. The stress drop for the primary strike-slip rupture was estimated as 5–10MPaby Abercrombie et al. (2003) and 20MPa by Robinson et al. (2001). While fabric of the platemay have influenced the strike-slip geometry, it appears that ruptures did not occur directlyon a fracture zone, indicating that F and IFZ are not necessarily weak portions of the plate thatlocalize deformation.

The MW 7.8 earthquake on March 2, 2016 ruptured a region in between well-defined frac-ture zones in close proximity to the fossil Wharton Ridge (Fig. 2). The failure process of thisevent is less complicated than for the June 18, 2000 event, with almost pure strike-slip faultingand a simple GCMT with a centroid depth of 34.6km. Re-located aftershocks define a north-south trend, indicating a left-lateral strike-slip faulting with a strike of about 5 degree (Layet al., 2016). Analysis of broadband body and surface waves reveals minor overall directivity,indicative of bilateral rupture over limited spatial extent, and Lay et al. (2016) favor a faultvery steeply dipping to the east with a rupture expansion speed of 2km/s with slip of upto 14 m concentrated at depths less than 30km.Models with a deeper centroid depth are hardto reconcile with the body waves, but the resolution of slip distribution from seismic waveanalysis is limited due to the lack of clear directivity.

FIG. 3 The 18 June 2000 MW 7.9 Wharton Basin rupture sequence. NEIC locations of the sequence are shown bycrosses with relocations given by stars. The global centroid-moment tensor solutions for a nearby 2001 event, the 2000mainshock and two aftershocks are shown by the light gray mechanisms. The dark gray mechanisms are twosubevents of the mainshock determined by teleseismic broadband body-wave modeling indicating rupture on sep-arate faults; the first subevent likely ruptured the northwest striking fault, the fault for the second subevent is notresolved. The events are located west of two fracture zones (F and IFZ: investigator fracture zone). Modified from

Abercrombie, R.E., Antolik, M., Ekstr€om, G., 2003. The June 2000 MW 7.9 earthquakes south of Sumatra: deformation in

the India-Australia Plate. J. Geophys. Res. 108(B1), 2018, https://doi.org/10.1029/2001JB000674.

96 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

Although the 2016 event strike-slip faulting generated limited tsunami, there are clear re-cordings of tsunami signals from this event and Gusman et al. (2017) take advantage of thesensitivity of slow tsunami waves to spatial extent and geometry of faulting to improve con-straints on the rupture process. They obtain a model from joint inversion of seismic and tsu-nami waveforms that has a bilateral rupture with speed of 2km/s and a steeply dipping faultto the west, with the slip being spread more along strike than in the seismic wave solution ofLay et al. (2016). The tsunami data appear to prefer slip at greater depth than the seismic data,but still within the upper 30km of the lithosphere. The rupture clearly parallels the nearbyfracture zone, but is only directly related to one if there is a very subtle fracture zone not ev-ident as a ridge offset or in the bathymetric structure. Errors in event locations certainly exist,but as in the other cases discussed, theywould have to be unexpectedly large to shift the eventto the nearest fracture zone.

The two largest intraplate events shown in Fig. 1 ruptured in theCocos Basin (northWhartonBasin) on April 11, 2012, in a region of intense intraplate activity that extends all the way to thetrench (Fig. 2). The great earthquakes were preceded by the January 10, 2012MW 7.2 strike-slipearthquake to the north, which was located near, but not on a fracture zone. The focal mech-anism for this event has a north-south plane paralleling the fracture zone, but the aftershocks donot clearly define the fault plane and body-wave inversion slightly favors rupture on an east-west plane (Aderhold andAbercrombie, 2016). TheMW 8.6 and 8.2 great strike-slip earthquakesalso have moment tensors and locations possibly consistent with rupture of either a well-defined fracture zone (Fig. 2), or on a more subtle parallel structure to the west (Singh et al.,2011). Numerous investigations have examined the faulting complexity for these events, al-though the MW 8.2 aftershock signals are partially obscured by signals from the mainshock(e.g., Duputel et al., 2012; Satriano et al., 2012; Meng et al., 2012; Yue et al., 2012; Ishii et al.,2013; Wei et al., 2013; Hill et al., 2015; Wang et al., 2012; Zhang et al., 2012; Yadav et al., 2017).

The MW 8.6 earthquake is the largest recorded intraplate event and also the largestrecorded strike-slip event. It is also one of the most complex events ever, in that it involvedfailure of at least four near-orthogonal fault segments spread over a region �350km across.The faulting complexity is apparent in the spatial distribution of aftershocks, in the loci oflocations of coherent short-period and long-period seismic radiation imaged by P-wave backprojection (e.g.,Meng et al., 2012; Satriano et al., 2012;Wang et al., 2012; Yue et al., 2012; Zhanget al., 2012), and in finite-source modeling based on regional and teleseismic body waves andregional geodetic data (Yue et al., 2012; Wei et al., 2013; Hill et al., 2015). Satriano et al. (2012)interpret back-projection results as favoring a westward migration of rupture of three en ech-elon parallel NNE trending fracture zones. In contrast, the back-projection interpretationsfrom Meng et al. (2012), Wang et al. (2012), Yue et al. (2012), and Zhang et al. (2012) favorinitial rupture on aWNW-ESE plane, followed by rupture on an orthogonal NNE-SSW planethat crosses the initial fault, and then rupture on another WNW-ESE plane to the south, withfurther faulting on a separate plane either parallel to or perpendicular to the Ninetyeast Ridge(Zhang et al., 2012 only image the first three faults). Satriano et al. (2012) recognize the pos-sibility of faulting in theWNW-ESE direction, but invoke the common, but casual assumptionthat most strike-slip faulting in theWharton basin is along fracture zones. As described beforethis is actually not the case for most large events globally.

The slip distribution of the multiple faults for theMW 8.6 event was obtained by finite-faultinversions constrained by back projections and aftershock patterns. The solution from Hillet al. (2015) shown in Fig. 4 is based on joint analysis of teleseismic bodywaves, surfacewaves

974 WHARTON BASIN ACTIVITY

and regional high-rate and campaign GPS measurements along Indonesia and the NicobarIslands. The inclusion of the geodetic observations improves resolution of the fault geome-tries and orientations relative to earlier seismic wave only inversions. Six faults rupture inthe numbered sequence in Fig. 4, with time lags indicated by the superimposed moment-ratefunctions andwith relative position indicated by the hypocenters (stars). Slip is largest at shal-low depths (the fault models have four 20-km wide segments on steeply dipping planes), butslip of about 5m does extend down to 60km in limited portions of some of the faults. This isalso found in the models of Yue et al. (2012) and Wei et al. (2013). This is of particularrelevance given that the GCMT centroid depth is 45.6km, while it is 54.7km for the MW

8.2 aftershock. Centroid depths of 30–40kmwere estimated using long-period signals for bothevents by Duputel et al. (2012), with a two point-source model being used for the MW 8.6event. There is limited resolution of the spatial extent and magnitude of slip in the finite-faultmodels, but Hill et al. (2015) estimate an average stress drop of 17MPa for the MW 8.6 eventoverall, and �25MPa for the first fault, which has the largest slip. There is uncertainty in thewesternmost fault orientation. Meng et al. (2012) discuss multibeam bathymetry showing nu-merousWNW-ESE fault scarps along the ridge, butHill et al. (2015) present evidence from thegeodetic fitting for the fault plane being parallel to the Ninetyeast Ridge. It is possible that

Mo = 3.9 x 10 Nm

Mo = 2.4 x 10 Nm

Mo = 1.2 x 10 Nm

Mo = 1.2 x 10 Nm

Mo = 1.5 x 10 Nm

Mo = 2.0 x 10 Nm

1

2

3

4

5

6

1

2

3

4

5

6

40 80 120 160 200 s0

40 80 120 160 200 s0

40 80 120 160 200 s0

40 80 120 16 200s0

40 80 120 160 200 s0

40 80 120 160 200 s0

Slip (m)0 5 10 15+

40 80 120 160 200 s0

21

21

21

21

21

21

88° 90° 92° 94°4°

2°

FIG. 4 Six-fault rupture model for the April 11, 2012MW 8.6 earthquake, showing the configuration of faults, num-bered chronologically, along with the individual fault moment-rate functions and slip distributions determined fromthe joint analysis of seismic and geodetic observations. The moment-rate functions are overlapped at lower left withtrue relative timing andpeak amplitudes. The fault grids all extend from the surface down to 60km andhave commonlength scales corresponding to the red line segments for each fault.Modified from Hill, E.M., Yue, H., Barbot, S., Lay, T.,

Tapponnier, P., Hermawan, I., Hubbard, J., Banerjee, P., Feng, L., Natawidjaja, D., Sieh, K., 2015. The 2012 Mw 8.6 Wharton

Basin sequence: a cascade of great earthquakes generated by near-orthogonal, young, oceanic-mantle faults. J. Geophys. Res. 120,https://doi.org/10.1002/2014JB011703.

98 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

faults 3 and 4 in Fig. 4 locate along fracture zone F7b (Singh et al., 2017), with some lateraloffset accounting for an apparent oblique trend relative to the fracture zone.

Low-resolution bathymetry does not reveal clear WNW-ESE structures along fault planes1, 2, and 5 in Fig. 4, and there is substantial sediment cover on the seafloor. However, Singhet al. (2017) report evidence from high-resolution bathymetry and seismic reflection imagesfor multiple parallel shear zones striking N294°E at high angle to the fracture zone fabric inthe plate from 0.5° to 1°N both west and east of fracture zones F6ab, F7ab. These shear zonesare defined by sets of normal faults striking N335°E, along the principal compressional stressdirection in the lithosphere. The trend of the shear zones is consistent with the major WNW-ESE fault 5 (Fig. 4), supporting the faulting model, but also indicating that the fault is juvenilein that it has not localized into a single through-going structure.

Ishii et al. (2013) argue that theWNW-ESE ruptures are the result of slip partitioning of theoblique convergence of the Indian plate relative to Sumatra. Typically, slip partitioning occurson through-going strike-slip faults in the upper plate for highly oblique convergent zones.The fore-arc sliver block separated from the Sunda plate by the Sumatran Fault rigidly accom-modates an average of 15–16mm/year of the slip partitioning, with little internal deformation(Bradley et al., 2017). Together with obliquity of thrusting on the megathrust (accounting forabout 40% of the trench-parallel component), nearly the full budget of oblique plate conver-gence is accommodated, leaving little role for the intraplate deformation in the Indian plate inthe overall slip partitioning. Thus, the presence of the partitioned Sumatran fault in the upperplate and the lack of a single continuous fault in the Indian plate make it more appropriate toattribute the strike-slip faulting to intraplate deformation just as for other strike-slip events inthe deformation zone far from the subduction zone which are not directly manifestations ofslip partitioning.

The MW 8.2 event appears to have ruptured along the NNE-SSW plane, in the vicinity offracture zone F7b (Yue et al., 2012;Wei et al., 2013).Wei et al. (2013) use regional waveforms toimage slip extending down to at least 50km with a rupture length of about 150km. The largerupture depth is consistent with the centroid depth estimate of Duputel et al. (2012). Preciselocation of the rupture is not known due to high noise levels produced by the mainshock,which occurred 2h earlier. This may be the largest earthquake to rupture through the litho-sphere on a fossil fracture zone.

5 DISCUSSION AND CONCLUSION

The broad deformation zone in the Indo-Australian plate involves numerous events withruptures that extend to large depths into the lithosphere. One of the deepest of these is theMay 21, 2014 MW 6.0 Bay of Bengal strike-slip earthquake located below the edge of thethickest part of the Bengal Fan (at latitude 18.2°N, longitude 88.0°E). The NEIC reports a hy-pocentral depth of 47km and aW-phase centroid depth of 60.5km. Rao et al. (2015) estimate asource depth of 50km by modeling broadband recordings in India, and Aderhold andAbercrombie (2016) estimate a centroid depth of 53km from teleseismic P-wave modeling.The lithospheric age in this region is 110Ma, placing the source depth near the 600°C isothermfor half-space and plate-cooling models (Aderhold and Abercrombie, 2016).

995 DISCUSSION AND CONCLUSION

In general, the moderate size intraplate seismicity in the Wharton Basin also lies above the600°C isotherm as shown in Fig. 5. Estimates of earthquake centroid depth and vertical rup-ture extent for events in the Basin are summarized, extending slightly the compilation ofAderhold and Abercrombie (2016). The Bay of Bengal event plots on the far right. The greatruptures in 2012 have GCMT centroid depths that locate deeper than the 600°C isotherm, butthe 30–40km centroid depth estimates of Duputel et al. (2012) indicate that the GCMT esti-mates are too deep for these events. Allowing for that upward shift, the 600 degree isothermbounds most of the activity, although it does appear that for the 2012 events rupture may ex-tend to greater depth. This is a strong indication of lithosphere transecting ruptures in thelargest strike-slip events.

The depth extent of the intraplate strike-slip faulting is substantially greater than found incontinental strike-slip systems where coseismic slip is limited to the crust, and comparablemoment earthquakes tend to have very different rupture lengths in the two environments.In addition, the stress drop estimates for the large intraplate events discussed above, whilevery uncertain, tend to be in the 15–25MPa range, higher than for continental strike-slipevents. Thus, slip can be large in the intraplate events, with estimates of localized slip greaterthan 25m for the first faults to rupture in the MW 8.6 event in 2012 (e.g., Yue et al., 2012; Weiet al., 2013; Hill et al., 2015). The oceanic intraplate events clearly involve rupture in themantleas well as the crust, and apparently the high strength of oceanic mantle plays a major role inproducing the high stress-drop fractures.

Considering all of the large events discussed here, the commonly invoked notion that frac-ture zones intrinsically define weak regions within oceanic plates prone to reactivation byintraplate stresses is not very well supported. The strongly deforming regions near the

Lithospheric age (Ma)

0 10 20 30 40 50 60 70 80 90 100 110 120

400°

800°

700°

500°

600°

0

10

20

30

40

70

60

50D

epth

(km

)

11 April 2012 Mw 8.2

11 April 2012 Mw 8.6

2 March 2016 Mw 7.8

18 June 2000 Mw 7.9

FIG. 5 Centroid depths (symbols) and rupture extents (ellipses) of earthquakes in the Indo-Australian lithosphereversus lithospheric age. Labeled isotherms with dotted and dashed lines are for a half-space cooling model (McKenzieet al., 2005). Centroid depth estimates are from Buchanan (1998) (stars), Aderhold andAbercrombie (2016) (circles), andglobal centroid-moment tensor (hexagons). Rupture depth extents forMW�7.7 events are based on the seismicmomentof the point source model assuming circular rupture and constant stress drop of 5MPa. Rupture extents for the 2012great earthquakes are from Duputel et al. (2012), and for the 2016 event from Lay et al. (2016). Based on Aderhold, K.,

Abercrombie, R. E., 2016. Seismotectonics of a diffuse plate boundary: observations off the Sumatra-Andaman trench.

J. Geophys. Res. Solid Earth 121, 3462–3478, https://doi.org/10.1002/2015JB012721.

100 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

Macquarie ridge and in the northern Wharton Basin do provide a few cases where large rup-tures appear to locate on fracture zones, but there are many events that rupture on faults lat-erally offset from fracture zones or on orthogonal structures, even though the focalmechanism is generically compatible with reactivation of a fracture zone. Event location un-certainties in the remote areas are significant, but probably cannot place the major events in2003 and 2016 on the prominent nearby fracture zones. As mentioned before, it could be thatthere are very subtle fracture zone features that are not identified in the ridge offsets or ba-thymetry, or it may be that some form of fracture zone parallel fabric, perhaps associatedwithlateral thermal gradients, plays a role where these ruptures occur. However, if the moreprominent nearby fracture zones are truly very weak structures, it is unclear why they werenot preferentially activated.

The NW-SE principle stress direction in the Indian plate arising from the configuration ofthe India-Eurasia continental collision combines with the oblique orientation of underthrust-ing and NE-SW slab pull in the Sunda trench to produce intraplate stresses favoring strike-slip faulting that happens to align generally with fossil fracture zones and the NinetyeastRidge. It is not clear that the fracture zones or the ridge serve as weak zones that activate pref-erentially in this stress environment, although some activity does appear to occur on fracturezones (as for the 2012 MW 8.2 event and possibly the N-S fault activated in the MW 8.6mainshock). With the primary moment release being on the orthogonal planes for the largestevent in the plate, and with other large events locating off of nearby well-defined fracturezones, there is not compelling evidence that either the preexisting transform fault or ridgeparallel fabric play controlling roles in the intraplate faulting, despite the intuitive appealof this idea derived from the general consistency of focal mechanisms (Fig. 2).

The fast-spreadingWharton Ridge produced thin crust and the associated transform faultslikely had only shallow activity when the system was active. In the 38 million years sincespreading ended, cooling has thickened the lithosphere progressively across the region, pos-sibly with relatively small “memory” of the fracture zone genesis. In that perspective, theoccurrence of large strike-slip events offset from fracture zones or rupturing lithosphere inorthogonal directions is most likely a manifestation of the plate failure being primarily con-trolled by the stress state and boundary conditions on the plate rather than by any internalfabric. This argument can also be applied to the large earthquakes in the Antarctic, Pacific,and Atlantic plates, which are also not results of fracture zones serving as zones of weakness.That intuitive ‘expectation’ should perhaps be downgraded.

Note Added in Proof

An Mw 7.9 strike-slip earthquake struck on January 23, 2018 under the Gulf of Alaska seaward of Kodiak Islands.Preliminary analysis indicates rupture of multiple, quasi-orthogonal faults, with primary slip on planes parallel tomagnetic lineations rather than on fossil transforms.

Acknowledgments

This work made use of GMT software. We thank an anonymous reviewer and the editor for helpful comments on themanuscript. The IRIS DMS data center was used to access the seismic data from Global seismic network and Feder-ation of Digital Seismic Network stations. This work was supported by NSF grant EAR1245717 (T.L.).

101ACKNOWLEDGMENTS

References

Abercrombie, R.E., Ekstr€om, G., 2001. Earthquake slip on oceanic transform faults. Nature 410, 74–77.Abercrombie, R.E., Antolik, M., Ekstr€om, G., 2003. The June 2000MW 7.9 earthquakes south of Sumatra: deformation

in the India-Australia Plate. J. Geophys. Res. 108 (B1), 2018. https://doi.org/10.1029/2001JB000674.Aderhold, K., Abercrombie, R.E., 2016. Seismotectonics of a diffuse plate boundary: observations off the Sumatra-

Andaman trench. J. Geophys. Res. Solid Earth 121, 3462–3478. https://doi.org/10.1002/2015JB012721.Antolik, M., A. Kaverina, and D. Dreger (2000), Compound rupture of the great 1998 Antarctic Plate

earthquake, J. Geophys. Res., 105, 23,825–23,838.Antolik, M., Abercrombie, R.E., Pan, J., Ekstr€om, G., 2006. Rupture characteristics of the 2003 MW 7.6 Mid-Indian

ocean earthquake. J. Geophys. Res. 111B04302. https://doi.org/10.1029/2005JB003785.Bergman, E.A., Solomon, S.C., 1988. Transform fault earthquakes in theNorthAtlantic: sourcemechanisms and depth

of faulting. J. Geophys. Res. 93, 9027–9057. https://doi.org/10.1029/JB093iB08p09027.Bergman, E.A., Solomon, S.C., 1992. On the strength of oceanic fracture zones and their influence on the intraplate

stress field. J. Geophys. Res. 97 (15), 365–15,377.Bohnenstiehl, D.R., Tolstoy, M., Chapp, E., 2004. Breaking into the plate: a 7.6 MW fracture-zone earthquake adjacent

to the Central Indian Ridge. Geophys. Res. Lett. 31L02615. https://doi.org/10.1029/2003GL018981.Bradley, K.E., Feng, L., Hill, E.M., Natawidjaja, D.H., Sieh, K., 2017. Implications of the diffuse deformation of the

Indian Ocean lithosphere for slip partitioning of oblique plate convergence in Sumatra. J. Geophys. Res. SolidEarth 122, 572–591. https://doi.org/10.1002/2016JB013549.

Buchanan, S., 1998. Seismicity and Tectonics of the Central Indian Ocean Basin, Ph.D. Thesis. 230 p., Univ. ofEdinburg, Edinburg.

Bull, J.M., Scrutton, R.A., 1990. Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere.Nature 344, 855–858.

Cloetingh, S., Wortel, R., 1985. Regional stress field of the Indian plate. Geophys. Res. Lett. 12, 77–80.Coblentz, D.D., Zhou, S., Hillis, R.R., Richardson, R.M., Sandiford, M., 1998. Topography, boundary forces, and the

Indo-Australian intraplate stress field. J. Geophys. Res. 103, 919–931.Das, S., 1992. Reactivation of an oceanic fracture by the Macquarie Ridge earthquake of 1989. Nature 357, 150.Das, S., 1993. The Macquarie Ridge earthquake of 1989. Geophys. J. Int. 115, 778–798.Delescluse, M., Chamot-Rooke, N., 2007. Instantaneous deformation and kinematics of the India–Australia Plate.

Geophys. J. Int. 168, 818–842. https://doi.org/10.1111/j.1365-246X.2006.03181.x.Delescluse, M., Mont�esi, L.G., Chamot-Rooke, N., 2008. Fault reactivation and selective abandonment in the oceanic

lithosphere. Geophys. Res. Lett. 35L16312. https://doi.org/10.1029/2008GL035066.Delescluse, M.N., Chamot-Rooke, R., Cattin, L., Fleitout, O.T., Vigny, C., 2012. April 2012 intra-oceanic seismicity off

Sumatra boosted by the Banda-Aceh megathrust. Nature 490, 240–245. https://doi.org/10.1038/nature11520.DeMets, C., Royer, J.-Y., 2003. A new high-resolution model for India-Capricorn motion since 20 Ma: implications for

the chronology and magnitude of distributed crustal deformation in the Central Indian Basin. Curr. Sci. 85 (3),339–345.

DeMets, C., Gordon, R.G., Royer, J.-Y., 2005. Motion between the Indian, Capricorn and Somalian plates since 20 Ma:implications for the timing andmagnitude of distributed lithospheric deformation in the equatorial Indian ocean.Geophys. J. Int. 161, 445–468. https://doi.org/10.1111/j.1365-246X.2005.02598.x.

DeMets, C., Gordon, R.G., Argus, D.F., 2010. Geologically current plate motions. Geophys. J. Int. 181, 1–80. https://doi.org/10.1111/j.1365-246X.2009.04491.x.

Deplus, C., et al., 1998. Direct evidence of active deformation in the eastern Indian oceanic plate. Geology 26 (2),131–134.

Duputel, Z., Kanamori, H., Tsai, V.C., Rivera, L., Meng, L., Ampuero, J.-P., Stock, J.M., 2012. The 2012 Sumatra greatearthquake sequence. Earth Planet. Sci. Lett. 351–352, 247–257. https://doi.org/10.1016/j.epsl.2012.07.017.

Engeln, J.F., Wiens, D.A., Stein, S., 1986. Mechanisms and depths of Atlantic transform earthquakes. J. Geophys. Res.91, 548–577.

Geersen, J., Bull, J.M., McNeill, L.C., Henstock, T.J., Gaedicke, C., Chamot-Rooke, N., Delescluse, M., 2015. Pervasivedeformation of an oceanic plate and relationship to large > MW8 intraplate earthquakes: the northern WhartonBasin, Indian Ocean. Geology 43, 359–362.

Gordon, R.G., DeMets, C., Argus, D.F., 1990. Kinematic constraints on distributed lithospheric deformation in theequatorial Indian Ocean from present motion between the Australian and India plates. Tectonics 9, 409–422.

102 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?

Gusman, A.R., Satake, K., Harada, T., 2017. Rupture process of the 2016Wharton Basin strike-slip faulting earthquakeestimated from joint inversion of teleseismic and tsunami waveforms. Geophys. Res. Lett. 44, 4082–4089. https://doi.org/10.1002/2017GL073611.

Hayes, G.P., Furlong, K.P., Ammon, C.J., 2009. Intraplate deformation adjacent to the Macquarie Ridge south of NewZealand: the tectonic evolution of a complex plate boundary. Tectonophysics 463, 1–14.

Henry, C., Das, S., Woodhouse, J.H., 2000. The great March 25, 1998, Antarctic Plate earthquake: moment tensor andrupture history. J. Geophys. Res. 105, 16,097–16,118.

Hill, E.M., Yue, H., Barbot, S., Lay, T., Tapponnier, P., Hermawan, I., Hubbard, J., Banerjee, P., Feng, L.,Natawidjaja, D., Sieh, K., 2015. The 2012Mw 8.6Wharton Basin sequence: a cascade of great earthquakes generatedby near-orthogonal, young, oceanic-mantle faults. J. Geophys. Res. 120. https://doi.org/10.1002/2014JB011703.

Hj€orleifsdottir, V., Kanamori, H., Tromp, J., 2009. Modeling 3D wave propagation and finite slip for the 1998 BallenyIslands earthquake. J. Geophys. Res. 114, B03301. https://doi.org/10.1029/2008JB005975.

Hwang, L.J., Kanamori, H., 1992. Rupture processes of the 1987-1988Gulf of Alaska earthquake sequence. J. Geophys.Res. 97, 19,881–19,908.

Ishii, M., Kiser, E., Geist, E.L., 2013.MW 8.6 Sumatran earthquake of 11April 2012: Rare seaward expression of obliquesubduction. Geology 41, 319–322.

Jacob, J., Dyment, J., Yatheesh, V., 2014. Revisiting the structure, age, and evolution of the Wharton Basin to betterunderstand subduction under Indonesia. J. Geophys. Res. Solid Earth 119, 169–190. https://doi.org/10.1002/2013JB010285.

Kennett, B.L.N., Gorbatov, A., Spiliopoulos, S., 2014. Tracking high-frequency seismic source evolution: 2004MW 8.1Macquarie event. Geophys. Res. Lett. 41, 1187–1193. https://doi.org/10.1002/2013GL058935.

Kuge, K., Kikuchi, M., Yamanaka, Y., 1999. Non-double-couplemoment tensor of theMarch 25, 1998, Antarctic earth-quake: composite rupture for strike-slip and normal faults. Geophys. Res. Lett. 26 (22), 3401–3404.

Lahr, J.C., Page, R.A., Stephens, C.D., Christensen, D.H., 1988. Unusual earthquakes in the Gulf of Alaska and frag-mentation of the Pacific plate. Geophys. Res. Lett. 18, 1483–1486.

Lay, T., Ye, L., Ammon, C.J., Dunham, A., Koper, K.D., 2016. The 2 March 2016 Wharton Basin MW 7.8 earthquake:high stress drop north-south strike-slip rupture in the diffuse oceanic deformation zone between the Indian andAustralian Plates. Geophys. Res. Lett. 43, 7937–7945. https://doi.org/10.1002/2016GL069931.

Lynnes, C.S., Ruff, L.J., 1985. Source process and tectonic implications of the great 1975 North Atlantic earthquake.Geophys. J. R. Astron. Soc. 82, 497–510.

Massell, C., Coffin, M.F., Mann, P., Mosher, S., Frohlich, C., Duncan, C.S., Karner, G., Ramsey, D., Lebrun, J.-F., 2000.Neotectonics of the Macquarie Ridge complex, Australia-Pacific plate boundary. J. Geophys. Res. 105,13457–13480.

Matthews, K.J., M€uller, R.D.,Wessel, P., Whittaker, J.M., 2011. The tectonic fabric of the ocean basins. J. Geophys. Res.116, B12109. https://doi.org/10.1029/2011JB008413.

McKenzie, D., Jackson, J., Priestley, K., 2005. Thermal structure of oceanic and continental lithosphere. Earth Planet.Sci. Lett. 233, 337–349.

Meng, L., Ampuero, J.-P., Stock, J., Duputel, Z., Luo, Y., Tsai, V.C., 2012. Earthquake in amaze: compressional rupturebranching during the 2012 MW 8.6 Sumatra earthquake. Science 337, 724–726.

Mosher, S., Symons, C.M., 2008. Ridge reorientation mechanisms: Macquarie Ridge Complex, Australia-Pacific plateboundary. Geology 36 (2), 119–122. https://doi.org/10.1130/G24236A.1.

Nettles, M., Wallace, T.C., Beck, S.L., 1999. TheMarch 25, 1998 Antarctic plate earthquake. Geophys. Res. Lett. 26 (14),2097–2100. https://doi.org/10.1029/1999GL900387.

Okal, E.A., Stewart, L.M., 1982. Slow earthquakes along oceanic fracture zones: evidence for asthenospheric flowaway from hotspots? Earth Planet. Sci. Lett. 57, 75–87.

Pegler, G., Das, S., 1996. The 1987-1992 Gulf of Alaska earthquakes. Tectonophysics 257, 111–136.Quintanar, L., Madariaga, R., Deschamps, A., 1995. The earthquake sequence of November 1987 and March 1988 in

the Gulf of Alaska, A new insight. Geophys. Res. Lett. 22, 1029–1032.Rajendran, K., Andrade, V., Rajendran, C.P., 2011. The June 2010 Nicobar earthquake: fault reactivation on the sub-

duction oceanic plate. Bull. Seismol. Soc. Am. 101, 2568–2577.Rao, C.N., Rao, N.P., Kumar, M.R., Prasanna, S., Srinagesh, D., 2015. Structure and tectonics of the Bay of Bengal

through waveform modeling of the 21 May 2014 earthquake of magnitude 6.0. Seismol. Res. Lett. 86, 378–384.Robinson, D.P., 2011. A rare great earthquake on an oceanic fossil fracture zone. Geophys. J. Int. 186, 1121–1134.

https://doi.org/10.1111/j.1365-246X.2011.05092.x.

103REFERENCES

Robinson, D.P., Henry, C., Das, S., Woodhouse, J.H., 2001. Simultaneous rupture along two conjugate planes of theWharton Basin earthquake. Science 292, 1145–1148.

Royer, J.-Y., Chang, T., 1991. Evidence for relative motions between the Indian and Australian plates during the last20 Myr from plate tectonic reconstructions: implications for the deformation of the Indo-Australian plate.J. Geophys. Res. 96, 11779–11802.

Royer, J.-Y., Gordon, R.G., 1997. The motion and boundary between the Capricorn and Australian plates. Science277, 1268–1274.

Royer, J.-Y., Gordon, R.G., DeMets, C., Vogt, P.R., 1997. New limits on the motion between India and Australia sinceChron 5 (11 Ma) and implications for lithospheric deformation in the equatorial Indian Ocean. Geophys. J. Int.129, 41–74.

Ruff, L.J., 1990. The great Macquarie Ridge earthquake of 1989: Introduction. Geophys. Res. Lett. 17, 989–991.Sager, W.W., Bull, J.M., Krishna, K.S., 2013. Active faulting on the Ninetyeast Ridge and its relation to deformation of

the Indo-Australian plate. J. Geophys. Res. Solid Earth 118, 1–21. https://doi.org/10.1002/jgrb.50319.Sandwell, D., Schubert, G., 1982. Lithospheric flexure at fracture-zones. J. Geophys. Res. 87, 4657–4667.Satriano, C., Kiraly, E., Bernard, P., Vilotte, J.-P., 2012. The 2012 MW 8.6 Sumatra earthquake: evidence of westward

sequential seismic ruptures associated to the reactivation of a N-S ocean fabric. Geophys. Res. Lett. 39, L15302.https://doi.org/10.1029/2012GL052387.

Singh, S.C., et al., 2011. Extremely thin crust in the Indian Ocean possibly resulting from Plume-Ridge interaction.Geophys. J. Int. 184, 29–42.

Singh, S.C., et al., 2017. The discovery of a conjugate system of faults in the Wharton Basin intraplate deformationzone. Sci. Adv. 3, e1601689.

Stein, S., Okal, E.O., 1978. Seismicity and tectonics of the Ninetyeast Ridge area: evidence for internal deformation ofthe Indian plate. J. Geophys. Res. 83, 2233–2246.

Sykes, L.R., 1967. Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges. J. Geophys. Res.72, 2131–2153.

Tsuboi, S., Kikuchi, M., Yamanaka, Y., Kanao, M., 2000. The March 25, 1998 Antarctic earthquake: great earthquakecaused by postglacial rebound. Earth Planet. Sp. 52, 133–136.

Valenzuela, R.W., Wysession, E.M., 1993. Intraplate earthquakes in the southwest Pacific Ocean Basin and theseismotectonics of the southern Tasman Sea. Geophys. Res. Lett. 20, 2467–2470.

Wang, D., Mori, J., Uchide, T., 2012. Supershear rupture on multiple faults for the MW 8.6 Off Northern Sumatra, In-donesia earthquake of April 11, 2012. Geophys. Res. Lett. 39, L21307. https://doi.org/10.1029/2012GL053622.

Wei, S., Helmberger, D., Avouac, J.-P., 2013. Modeling the 2012 Wharton basin earthquakes off-Sumatra: completelithospheric failure. J. Geophys. Res. Solid Earth 118, 3592–3609. https://doi.org/10.1002/jgrb.50267.

Wiens, D.A., Stein, S., 1984. Intraplate seismicity and stresses in young oceanic lithosphere. J. Geophys. Res. 89 (11),442–11,464.

Wiens, D.A., et al., 1985. A diffuse plate boundary model for Indian Ocean tectonics. Geophys. Res. Lett. 12, 429–432.Wiseman, K., B€urgmann, R., 2012. Stress triggering of the great Indian Ocean strike-slip earthquakes in a diffuse plate

boundary zone. Geophys. Res. Lett. 39(22). https://doi.org/10.1029/2012GL053954.Yadav, R.K., Kundu, B., Gahalaut, K., Catherine, J., Gahalaut, V.K., Ambikapthy, A., Naidu, M.S., 2017. Coseismic

offsets due to the April 11, 2012 Indian Ocean earthquakes (MW 8.6 and 8.2) derived from GPS measurements.Geophys. Res. Lett. 40, 3389–3393. https://doi.org/10.1002/grl.50601.

Yue, H., Lay, T., Koper, K.D., 2012. En �echelon and orthogonal fault ruptures of the 11April 2012 great intraplate earth-quakes. Nature 490, 245–249. https://doi.org/10.038/nature11492.

Zhang, H., Chen, J., Ge, Z., 2012. Multi-fault rupture and successive triggering during the 2012 MW 8.6 Sumatraoffshore earthquake. Geophys. Res. Lett. 39, L22305. https://doi.org/10.1029/2012GL053805.

104 4. REACTIVATION OF OCEANIC FRACTURE ZONES IN LARGE INTRAPLATE EARTHQUAKES?