GEOSCIENCE FRONTIERS 3(4) (2012) 473e481

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China University of Geosciences (Beijing)

GEOSCIENCE FRONTIERS

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RESEARCH PAPER

Testing the intraplate origin of mega-earthquakesat subduction margins

Prosanta K. Khan a,*, Partha Pratim Chakraborty b, G. Tarafder c, S. Mohanty d

aDepartment of Applied Geophysics, Indian School of Mines, Dhanbad, IndiabDepartment of Geology, University of Delhi, Delhi, IndiacNational Geophysical Research Institute, Hyderabad, IndiadDepartment of Applied Geology, Indian School of Mines, Dhanbad, India

Received 21 March 2011; accepted 1 December 2011Available online 13 December 2011

KEYWORDSFlexing zone;Non-coaxial deformation;Shear crack;Neutral surface;Rheology;Compressive stress

* Corresponding author. Tel.: þ91 9431

E-mail addresses: pkkhan@indiatim

com (P.K. Khan).

1674-9871 ª 2011, China University of G

University. Production and hosting by Els

Peer-review under responsibility of Ch

(Beijing).

doi:10.1016/j.gsf.2011.11.012

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Abstract The disastrous Mw 9.3 (seismic moment 1.0� 1030 dyn/cm) earthquake that struck northwest

Sumatra on 26 December 2004 and triggered w30 m high tsunami has rejuvenated the quest for identi-

fying the forcing behind subduction related earthquakes around the world. Studies reveal that the stron-

gest part (elastic core) of the oceanic lithosphere lie between 20 and 60 km depth beneath the upper

(w7 km thick) crustal layer, and compressive stress of GPa order is required to fail the rock-layers within

the core zone. Here we present evidences in favor of an intraplate origin of mega-earthquakes right within

the strong core part (at the interface of semi-brittle and brittle zone), and propose an alternate model

exploring the flexing zone of the descending lithosphere as the nodal area for major stress accumulation.

We believe that at high confining pressure and elevated temperature, unidirectional cyclic compressive

stress loading in the flexing zone results in an increase of material yield strength through strain hardening,

which transforms the rheology of the layer from semi-brittle to near-brittle state. The increased compres-

sive stress field coupled with upward migration of the neutral surface (of zero stress fields) under non-

coaxial deformation triggers shear crack. The growth of the shear crack is initially confined in the

711020.

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P.K. Khan et al. / Geoscience Frontiers 3(4) (2012) 473e481474

near-brittle domain, and propagates later through the more brittle crustal part of the descending oceanic

lithosphere in the form of cataclastic failure.

ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by

Elsevier B.V. All rights reserved.

1. Introduction

The December, 2004 earthquake (MwZ 9.3) off the western coastof Sumatra (latitude 3.30�N, longitude 96.00�E) in Southeast Asiashares the same rank with 1960 Southern Chile (MwZ 9.5), 1964Alaska (MwZ 9.2), 1957 Aleutian (MwZ 9.1), 1952 Kamchatka(MwZ 9.0) and 1965 Rats Islands (MwZ 8.7) events (Fig. 1).This has spurt the quest for understanding the significance of thehighly dynamic physical and chemical response of the Earth’suppermost rigid thin moving shell (lithosphere) at the subductionmargin (Stern, 2011). Since the introduction of modern space-based geodesy and broadband seismograph, the giant 2004earthquake offered the first scope for in-depth investigationregarding the source characteristics of shallow-focus mega-thrustearthquakes (cf. Khan and Chakraborty, 2009). The off Sumatraevent rupture was initiated at a depth of 30 km, expanded veryslowly over 50e60 s towards northwest through bilateral growthof fresh rupture surface and followed by its very rapid propagationalong a shallow-dipping (w8�) plane towards east-northeast in theform of cataclastic thrust-type failure (Gupta, 2005; Ishii et al.,2005; Kennett and Cummins, 2005; Kr€uger and Ohrnberger,2005; Lay et al., 2005; Subarya et al., 2006; Tolstoy andBohnenstiehl, 2006). Similar two-stage fracture criteria was alsonoted during rupture propagation in 1960 Chilean earthquake(Cifuentes, 1989), 1964 Alaska and 1965 Rats Islands earthquakes(Ruff and Kanamori, 1983a). The self-similar character of mega-earthquake events in terms of their source depth, rupture propa-gation and energy release (McCaffrey, 2008, and referencestherein) prompted us to explore the causative source zone of theseevents in the descending oceanic lithosphere.

Journey of the oceanic lithosphere from mid-oceanic ridge tothe subduction margin is governed by three major forces viz.ridge-push (FRP), slab-pull (FSP), and slab-resistive (FSR) (Forsythand Uyeda, 1975). At subduction margin lithosphere penetratesthrough the asthenosphere with varying trajectories under theguidance of two oppositely acting torques caused by slab-pull andhydrodynamic lifting forces (Scholz and Campos, 1995). Esti-mation of relative strength of various plate driving forces fromobserved relative motions and geometries of the lithosphericplates using linear inverse theory advocated the trade-off betweenFSP and FSR that leads to either extensional (FSR< FSP) orcompressional (FSR> FSP) rock-failures at the shallower parts ofthe subducting lithosphere (Forsyth and Uyeda, 1975). Interplatemodel explains the shallow-focus mega-thrust earthquakes as theresult of sudden stress-relieving displacement between thedescending oceanic lithosphere and the overriding continentalplate at their contact zones locked by penetration of asperities(topographic highs/seamounts) (Dmowska et al., 1988; Cloos,1992), correlated with strong seismic coupling (coefficientcZw1.0) (Peterson and Seno, 1984; Christensen and Ruff,1988; Pacheco et al., 1993; Scholz and Campos, 1995). Therelationship between interplate coupling and seamount subduc-tion, however, is ambiguous as estimated c values vary widely

irrespective of asperity subduction (Peterson and Seno, 1984;Pacheco et al., 1993; Scholz and Campos, 1995; Prawirodirdjoet al., 1997; Khan and Chakraborty, 2009). Statistical signifi-cance test on randomness of observed c values for earthquakemoments (at best 90 year and at worst 30 year time period)allowed drawing of inference that subduction zones with knownsignificant earthquakes may or may not have a large couplingcoefficient (McCaffrey, 1997). In fact, deep-sea trenches withsmall coupling coefficients are ubiquitous worldwide and not anexception, which suggest that interplate thrust earthquakes have anaspect invisible to conventional seismotectonic observations (cf.Heki et al., 1997).

On seamount subduction model there are two opposite schoolsof thought: one in favor (Cloos and Shreve, 1996) and the otheropposing (Kelleher and McCann, 1976) its possible role behindgeneration of mega-thrust earthquakes. Simulation of normalstress across the subduction interface (sc) and assessment inregard to both coupled and decoupled seismic zones have dis-carded seamount subduction as the necessary condition for largesubduction zone earthquakes (Scholz and Small, 1997). Also thesea floor bathymetry (Kelleher and McCann, 1976) in six mega-shock locales (Table 1) does not comply with the magnitudes ofrecorded earthquake events. None of these subduction marginsfeature presence of seamount as high as 8.0 km, required fortriggering seismic shock of �9.0 magnitudes. In this backdrop it isqueer why forbearers of interplate hypothesis (Ruff and Kanamori,1983b; Cloos and Shreve, 1996; Dmowska et al., 1996) continuedputting stress on the asperity subduction model despite noticingdistinct bend in reconstructed Benioff zone trajectory. Flexing inthe geometry is noticed in many subduction margins around30e40 km depth in coincidence with the most active zone ofseismic slip accumulation (seismogenic zone; Stern, 2002). Also,variations of dip angle in the subducting plate through depththough noticed and used for constraining parameters of subductionzones (Jarrard, 1986); were always underplayed when sources oflarge thrust earthquakes triggered from the shallower parts of thedescending slabs were characterized (Moores and Twiss, 1996;Mahesh et al., 2011). From reconstruction of depth-dip angletrajectories and estimation of seismic moment release, here wepropose intraplate origin (Khan, 2011a) for the shallow-focusmega-earthquake events around the World.

2. Seismic data and analysis

Earthquake data (magnitude mb� 4.0 and recorded at 12 or morestations) for the Aleutian, Alaska, Chile, Sumatra, Kamchatka,and Rats Island were taken from the International SeismologicalCenter (ISC) catalog covering the period between 1964 and 2004(41 years). Seismicity maps (Fig. 1) and Benioff Zone profiles(Fig. 2) were reconstructed using this data set. Variations in-depth-dip angle profiles allowed classification of descending slabtrajectories into three categories viz. shallow (<30 km) withmonotonic gradual increase in dip-angle, intermediate (30 to

Figure 1 Maps (after Lay et al., 1989; Johnson et al., 1996; Lu and Wyss, 1996; Curray, 2005) represent historical seismicity distributions with

projection of mega-shock events in six major subduction margins around the Globe. CHL: Chile Mw 9.5, SMT: Sumatra Mw 9.3, ALS: Alaska Mw

9.2, ALT: Aleutian Mw 9.1, KMC: Kamchatka Mw 9.0, RTI: Rats Island Mw 8.7. Mega-earthquake epicenters are shown by white star. AA0, BB0,CC0, DD0, EE0 and FF0 represent depth-section profiles.

P.K. Khan et al. / Geoscience Frontiers 3(4) (2012) 473e481 475

Table 1 Variation in dip-angle (d) against depth (d) of the descending oceanic lithosphere and coupling coefficient (c) between overriding

and descending lithospheres in six major plate margins along with records of mega-earthquake events (modified after Khan and Chakraborty,

2005).

Date Chile Sumatra Alaska Aleutian Kamchatka Rats Islands

22/05/1960 26/12/2004 28/03/1964 09/03/1957 04/11/1952 04/02/1965

Latitude 39.50�Sa 3.30�Nc 61.04�Nd 51.63�Nf 52.76�Ng 51.21�Ng

Longitude 74.50�Wa 96.00�Ec 147.73�Wd 175.41�Wf 160.06�Eg 178.50�Eg

Focal depth (km) 35.0b 30.0c 30.0e 37.0b 46.0b 31.0b

Magnitude (Mw) 9.5 9.3 9.2 9.1 9.0 8.7

Focal depth (km)? 39 25 20 34 45 30

d at shallow depth 3e7�

(d� 20 km)

2e7�

(d� 24 km)

1e10�

(d� 25 km)

2e8�

(d� 24 km)

4e10�

(d� 24 km)

1e9�

(d� 27 km)

d at intermediate

depth

7e43�

(20< d� 56 km)

7e47�

(24< d� 60 km)

10e29�

(25< d� 55 km)

8e42�

(24< d� 64 km)

10e43�

(24< d� 65 km)

9e54�

(27< d� 63 km)

d at deeper depth 43e56�

(d> 56 km)

47e57�

(d> 60 km)

29e40�

(d> 60 km)

42e55�

(d> 64 km)

43e59�

(d> 65 km)

54e57�

(d> 63 km)

c 0.86e1.57 0.007e0.6 0.77 0.33e0.84 0.67e1.34 0.31e0.82

Bathymetry (km) 2.0 5.0 2.0 3.0 3.4 3.0

Dip-angles were computed using the best-fit lines (solid lines, Fig. 2) passing through the distribution of hypocenters. Compiled bathymetry data after

Kelleher and McCann (1976).a Mogi (1969).b Lay et al. (1989).c Lay et al. (2005).d Johnson et al. (1996).e Kanamori (1970).f Ruff et al. (1985).g Kanamori (1977).

P.K. Khan et al. / Geoscience Frontiers 3(4) (2012) 473e481476

<65 km) with sharp increase in dip-angle, and deep (�65 km)with monotonic gradual increase in dip-angle (Table 1). Inter-mediate segment coincides with the zone of flexing (cf. Khan andChakraborty, 2005) where the descending lithosphere assumesa sharp deviation in inclination to reach the deeper level.Projections of mega-shock hypocenters on the respective depth-sections show their invariable incidences within the flexing zonethose mark sharp changes in inclination of respective Beniofftrajectory from gentle, monotonic to steep, high-gradient one(Fig. 2). Sharp changes in released cumulative seismic moment(MoZ10ð1:53mb þ16:17Þ) zones of all the six reconstructed subduc-tion profiles (Table 1 and Fig. 3). This corroborates well with theestimation of Conrad and Hager (1999) that w60% of energydissipation occurs through the bending (flexing zone) of the sub-ducting slab. It is also postulated that the maximum bendingstresses of descending oceanic lithosphere at the subductionmargins are about an order of magnitude larger than the maximumstrength of the oceanic lithosphere (Kohlstedt et al., 1996; Conradand Hager, 1999); only 10% of the elastic bending stress is sup-ported without deformation, and the remaining stress is relieved inthe form of seismicity by fracturing of rocks (e.g., Turcotte andSchubert, 1982). Flexure studies for oceanic lithosphere(McAdoo et al., 1978; Goetze and Evans, 1979; Mueller et al.,1996) involving the rheological strength profile reveals that itsstrongest part (elastic core; Molnar, 1988; Watts and Burov, 2003)lies in the uppermost mantle, and this depth-window in the mantlecoincides with the zones of occurrence of mega-events alongsubduction margins around the world. Global compilation ofearthquake data for the flexed oceanic lithosphere also suggeststhat the seismogenic layer thickness (TS) exceeds the averagethickness of the oceanic crust, and continued well in the uppermantle (Watts and Burov, 2003). A natural coincidence of these

dynamic parameters provoked us to explore an alternativehypothesis/model for shallow-focus mega-thrust earthquakes ina subducting oceanic lithosphere and propose a process-responsemodel involving plate flexing, plate rheology, built-up intraplatestress field and rock-layer failure.

3. Modeling

Available models on plate-flexure, elastic plate thickness andintraplate stress balance consider hanging oceanic lithosphere thatoverlies fluid-like asthenosphere and flexed (flexural rigidityw1021e1023 Nm) by plate driving forces (Isacks and Molnar,1969, 1971; Sykes and Sbar, 1973; Forsyth and Uyeda, 1975;Chapple and Forsyth, 1979; Ward, 1984; Conrad and Hager, 1999)(Fig. 4a). Two major forces experienced by such subductinglithosphere are (1) the unbalanced slab-pull force, which impartsthe vertical load, and (2) the slab viscous force that resists thelithosphere against penetration into the asthenosphere. Estima-tions of stress field and strain rate from the shallow-focus mega-thrust earthquakes provide support for periodic build-up stressfield. Superposing of this periodic stress on the time-averagebending stresses generated around the flexing zones of subduct-ing oceanic plate can turn on or off failure controlled dominantlyby the time-averaged stress state (e.g., Chapple and Forsyth, 1979;Scholz, 1990; Dmowska et al., 1996). Considering a temporallyincreasing average stress field, the stress may vary and becomemuch higher than the average stress value in the presence of anygeometrical discontinuity in the stress-loading domain(Timoshenko, 1957; Anderson, 1995; Burr and Cheathan, 1997).We thus propose that the flexing zones of subducting oceanicplates as possible nodal areas of stress concentration where, at

Figure 3 Plots showing the depth-wise variation of normalized

cumulative seismic moment (Mo) release in the six subduction

margins. Zone of flexing as the nodal area of compressive stress

accumulation is marked by cross-hatched area (AB).

Figure 2 Plots showing the reconstructed depth-sections along six profiles (AA0, BB0, CC0, DD0, EE0, FF0, Fig. 1) in the six subduction margins.

Relative positions between upper envelopes (dashed lines) of Benioff zones and mega-earthquake hypocenter locations (open stars) are also

shown. Best-fit thin solid lines drawn through the distribution of hypocenters in each subduction zones were used for computing the depth-wise

variation of dip-angle of the descending oceanic lithosphere.

P.K. Khan et al. / Geoscience Frontiers 3(4) (2012) 473e481 477

high confining pressure and elevated temperature, cyclic stress(Khan, 2011b) loading results in the increase of material yieldstrength to higher values (i.e., strain-hardening in a semi-brittleregime, Kirby, 1980; Fig. 4). Such increase in strength producedby strain-hardening is accompanied by decrease in toughness andductility (Timoshenko, 1957; Marin, 1966; Gupta, 1997). Thisprocess further transforms the semi-brittle rheology of thedescending oceanic lithosphere, below its crustal sub-layer, toa near-brittle layer and continues to do so until failure occurs atthe maturity of the earthquake cycle.

For a hydrostatically supported lithosphere; subducting ata constant velocity, the flexure vis-�a-vis its related zone of strain-hardening is likely to roll back resulting in the development ofa fresh zone or plate segment that would be, in general, strain-hardened and rheologically less tough and ductile (i.e., trans-formation from semi-brittle to brittle state). We assume that theregional stress field operates periodically on the flexing zone ofthe subducting plate (Taylor et al., 1996), and the oscillations areconfined within the compressional domain (McNutt and Menard,1982; Kirby, 1983; Wessel, 1992). The long-term compressivestress accumulation possibly enhances the yield strength of theoceanic lithosphere. The idea gets support from the dominance ofthrust-type earthquake events located in all six subduction marginsunder study (Sykes and Sbar, 1973). As such, thrust-fault

Figure 4 Diagram showing the profile of trench-normal oceanic lithospheric penetrating into the asthenosphere. Time-average pulsating tectonic

stress (sav) is superimposed on the plate bending stresses. aec: yield strength envelopes (in compression) with variation of rock-fiber stresses (PQ)with

depth;d: initial bendingwith development of extension and compression domains; e: upwardmigration of neutral surface (N.S.) at intermediate stage of

the earthquake cycle; f: upward migration of neutral surface results the initiation of failure at the interface between semi-brittle and near-brittle layers

with rheological transformations under long-term pulsating stress superimposed on time-averaged bending stress of oceanic lithosphere. Open circles

and ellipses in the lithosphere represent the pattern of strain variation with respect to the migration of the neutral surface. Increase in ellipticity of the

strain ellipse across the neutral surface and the lateral variation in the axial orientations of the strain ellipses along the neutral surface indicating non-

coaxial deformation in the bending lithosphere may be noted; g: details of rupture in f and arrows indicate the directions of compression and extension.

Two possible shear failure planes (F1 and F2) at an angle ofw30�e35� from the compression/shortening axis are also shown.

P.K. Khan et al. / Geoscience Frontiers 3(4) (2012) 473e481 479

earthquakes are reported (Forsyth, 1982) from the compressionzone (i.e., below the neutral surface) of flexed subducting litho-spheres, where the neutral surface migrates upward few kilometersdue to superposition of large compressive stress on the bendingstress of the lithosphere (Hanks, 1971; McAdoo et al., 1978).A natural corollary is that a significant horizontal tensionalcomponent resulted from gravitational sinking of the lithosphere isnot applied to the stress field. Instead, a large horizontalcompressive stress of tectonic origin dominates the ambient stressfield in the regions which in turn modulates those average stresses,moving the total stress toward or away from failure and thuscontrols the timing of seismicity. Such unidirectional recurrentloading of stress would cause very less loss in strain energy byheat and internal friction and induce further loss in ductility. Thenet compressional lithospheric strength is estimated by integratingthe negative portion of the yield envelops over the thickness of thelithosphere (Khan and Chakraborty, 2009), and a similar integra-tion for the positive side of the yield envelope provides the nettensional strength. The net compressional strength associated witha standard lithospheric yield envelope is �3.6� 1013 Nm�1,where as the net tensional strength is 1.3� 1013 Nm�1. Therefore,lithosphere behaves as considerably stronger under compressionthan in tension, and is supported by the observation thatcompressional great earthquakes are normally preceded bytensional earthquakes (Dmowska et al., 1988; Taylor et al., 1996),and mark the maturity of the earthquake cycle.

4. Discussion and conclusions

For an upward-convex lithosphere with fixed geometrical config-uration the position of neutral surface (of zero stress fields) overa constrained time frame will be a function of material strength andincreasing bending moment (Goetze and Evans, 1979; Burr andCheathan, 1997; Raghavan, 1998). With strain-hardening andincreasing yield strength, the plate sector around the flexure domainwill experience rise in neutral surface towards more brittle shal-lower part, thereby increase the compressive stress field (Fig. 4) andwill prevent any failure until the stress concentration reaches theyield limit. For a vertical line load on such mechanically stronggravitating lithosphere, floating on a much weaker asthenosphere,upward transport of neutral surface through 10e20 km ina compressive stress field without significantly deforming the plateprofile or increasing the maximum internal stress has been appre-ciated theoretically (Ward, 1984) and interpreted through contrastsin interplate coupling. The enhanced stress is accommodated eitherwithin the flexural rigidity of the upper brittle part or through time-dependent progressive deformation within semi-brittle/ductile part.We agree with this theoretical contention; however, we differ withthe interpretation part. We believe that rheological transformationwithin the subducting lithosphere in response to external cyclicstress loading is the major causative reason behind rise in neutralsurface and lithospheric behavior over long time span (Khan,2011b). In presence of increasing residual stress within the strain-hardened sub-oceanic mantle the enhanced compressive stresswill cause the material to reach yield envelope at a stress concen-tration location, which may be facilitated by an abrupt geometricaldiscontinuity in the plate cross section i.e., plate-flexure. Ina compressive stress field and transforming (i.e., lowering ofductility and toughness) rheological condition the material anisot-ropy will provoke non-coaxial deformation whereby the principalstrain axes will continuously diverge from the orientation of the

instantaneous stretching axes and the deformation path (Lister andWilliams, 1983). It may be noted that two types of non-coaxialdeformations act in the region under discussion: one of these isrelated to the plate bending (shown as dextrally rotated strainellipses in Fig. 4) and the other is related to the differential motionbetween the subducting and overriding lithospheric plates. Thelatter deformation acts opposite to the former. As a result, the finitestrain ellipses will lie in between the two competing patterns andwill be less steep than what is shown in Fig. 4. The direction ofprincipal stress in such non-coaxial system will assume an acuterelation (i.e., w30�e35�) with the plane of ‘no strain’ and mayproduce bilateral shear fracture (Ramsay and Huber, 1987). It maybe reasonable to assume that the micro-fracture/crack will initiatepreferably within the sub-oceanic uppermost mantle coincidingwith the interface between its strain-hardened near-brittle part andthe underlying semi-brittle part (Fig. 4). With concentration ofavailable stress at the tip the fracture will propagate through therelatively weaker part of the lithosphere. Coupled with this, theturning effect that may arise because of rise in neutral surface willfurther enhance shortening and may force sudden movement of thehanging wall up the detachment surface resulting the initiation ofthrust movement.

Reconstruction of flexure domains on the plate trajectories insix major subduction margins of the World and appreciation ofincidences of mega-shock hypocenters vis-�a-vis maximumnormalized cumulative seismic moment release from the inferreddepth of flexing suggest intraplate origin for these events triggeredby inhomogeneous, unusual stress concentration at these zones.The sudden change in dip-angle at the shallow depths of thedescending lithosphere generate environment favorable for thestress accumulation under cyclic unidirectional (compressive)axial stress-loading regime. It is thus proposed that the uncom-pensated portion of slab-resistive force offer optimum stressconcentration with the best rheological boundary condition, whichallow mega-thrust shear failure within the flexing domain alongthe subduction plate margins globally.

Acknowledgements

The first author is thankful to the Ministry of Earth Sciences,Govt. of India for the financial support. P.K. Khan expresses hisgratitude to Director, Indian School of Mines for providingnecessary infrastructure facilities.

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