Rendiconti Accademia Nazionale delle Scienze detta dei XLMemorie di Scienze Fisiche e NaturaliISBN 987-88-548-7171-7DOI 10.4399/97888548717171pag. 9–27

Asymmetric Earth: mechanisms of platetectonics and earthquakes∗

C D

. Abstract

The Earth is subject to the secular cooling of a heterogeneous andstratified mantle, and to astronomical tuning. The combination ofthese two parameters is here inferred as the main controlling fac-tors of plate tectonics. The planet is still hot enough to maintainat about km depth a layer where partial melting determines alow–velocity and low–viscosity layer at the top of the asthenosphere,allowing partial decoupling of the lithosphere with respect to theunderlying mantle. The Earth’s rotation and the tidal despinninggenerate a torque acting on the lithosphere, and producing a netwesterly directed rotation of the lithosphere with respect to the un-derlying mantle, being this rotation decoupled in the low–velocityasthenospheric layer. This polarization controls a diffuse asymmetryalong plate boundaries, which are shaped by the “eastward” relativemantle flow with respect to the overlying lithosphere. Velocity gra-dients are the by–product of the lateral viscosity variations in thelow–velocity layer, i.e., the decollement plane. The lower the viscos-ity, the faster westward motion of the overlying lithosphere (e.g., thePacific plate). Velocity gradients among plates determine tectonicsat plate margins and related seismicity. The horizontal component ofthe solid Earth’s tide pushes plates to the “west”. When faults arriveat the critical state, the vertical component of tides may trigger theearthquake due to variations of g. The brittle–ductile transition isinferred to act as a switch for earthquakes. During the interseismicstage, a dilatational band forms in the brittle upper crust. The stretch-

∗ Prolusione tenuta in occasione dell’Inaugurazione del ° anno accademico. Roma, aprile presso la Biblioteca dell’ Accademia.

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ing is partially recovered during the coseismic stage when the faulthangingwall falls down releasing its gravitational energy along anormal fault. On the contrary, for a thrust fault, during the interseis-mic stage an over compressed band forms above the brittle–ductiletransition, which is opposingly dilated during the earthquake, deliv-ering the elastic energy accumulated in the hangingwall. Therefore,energy storage is different and gravity acts with opposed versus as afunction of the tectonic style. Fluids react accordingly, pre, syn andpost the earthquake.

. Introduction

The origin of plate tectonics and the mechanisms governing theEarth’s geodynamics are still under debate. The most acceptedmodel for the dynamics of the planet is that tectonic plates are thesurface expression of a convection system driven by the thermalgradient from the hot inner core (– °C) and the surfaceof the Earth, being the shallowest about km thick the upperthermal boundary layer, a < °C internally not convecting layercalled lithosphere. However, during the last decades it has beenshown that ) mantle convection driven from below cannot explainthe surface kinematics; ) plates do not move randomly as requiredby a simple Rayleigh–Bénard convective system, but rather follow amainstream of motion; ) plate boundaries rather show asymmetriccharacters. Moreover, convection models are generally modelled asdeforming a compositionally homogeneous mantle, whereas theEarth is chemically stratified. Based on these assumptions, it is herereviewed an alternative model of plate tectonics, in which the inter-nal heat still maintains possible mantle convection, but this is mostlydominated from above (Anderson, ), being the lithosphere activein the process, but sheared by astronomically forces. Moreover, themantle is assumed not chemically homogeneous, and tomographicimages are not indication of cold and hot volumes, but also the resultof chemical heterogeneity (Anderson, ; Thybo, ; Tackley,; Trampert et al., ).

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

. Tectonic equator and westward drift

The analysis of past and present motion of plates point for a non ran-dom distribution, but rather a coherent flow (Fig. ). Past motion canbe reconstructed based on magnetic anomalies in the oceanic crustand paleo–direction of shortening in the orogens associated to sub-duction zones. Present motions can be described by the space geodesydata, i.e., the GPS, VLBI (Very Long Baseline Interferometry) and SLR(Satellite Laser Ranging). The mainstream can be summarized by thegreat circle describing plate motions, defined as the “tectonic equa-tor” (Doglioni, ; Crespi et al., ). It is worth noting that pastmovements coincide with present day directions of motion (DeMetset al., ; Argus et al., ). During their journey, some plates mayexperience a further internal rotation. Since the motion of a plateon a sphere is defined by a rotation, this extra movement has beendefined as plate sub–rotation (Cuffaro et al., ), which disturbs thefirst mainstream of plates. Sub–rotations imply that plates have a first

Figure . The direction of plate motions across the six major plate boundaries ofthe Earth (, Indian ridge; , Alpine–Himalayan–Indonesia subduction zones; ,Western Pacific subduction zone; , East Pacific Rise; , Andean subduction zone;, Mid–Atlantic ridge) shows that lithospheric plates moved along a mainstreamwhich can be summarized by its “tectonic equator”. This flow appears persistentboth in the geologic past (left) and in the present day motions visible on the GPSNasa data base (right).

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order pole of rotation, plus a second pole of the sub–rotation that isthe only point of the plate maintaining the same distance with respectto the first pole. The tectonic equator appears stable through time(at least Myr), and makes an angle of about ° relative to thegeographic equator. The angle of the ecliptic plane plus the angle ofthe Moon’s revolution sum to a very close value. Moreover, platesmove faster at low latitudes, as recorded by kinematic data, and byseismicity, which decreases toward the polar areas (Riguzzi et al., ;Varga et al., ). Therefore, the shape of the tectonic equator andangle suggest an astronomical tuning of plate motions. The tectonicequator could be interpreted as the plane located at interception ofthe cone described by the precession of the Earth’s axis. In fact, theMaxwell time (the time requested for a solid material to flow) of thelithosphere has the same order of magnitude of the precession cycles(– kyr), as proposed in Fig. .

Figure . The tectonic equator describes the mean faster motion of the lithosphericplates. The tectonic equator has an angle of about –°, which is here consideredas the angle of the ecliptic plane (°) plus the Moon’s revolution plane (°) withrespect to the geographic equator. The location of the tectonic equator is inferredas the mean direction of the Earth’s precession axis. The Maxwell relaxation timeof the lithosphere (viscosity/rigidity ratio, Pa s/Pa=s) is of the sameorder of magnitude of the kyr precession. This could explain the stability of thetectonic equator during the geologic past (Nesi et al., in prep.).

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

Besides this first simple kinematic description, plates move relativeto the mantle. They are decoupled at a depth varying between and km (average km), in the low–velocity layer, where seismicwaves slow down because of the presence of some percentage of meltin the mantle peridotites (Green et al., ; Hirschmann, ; Ander-son, ; Schmerr, ). This layer, named LVZ (Low Velocity Zone),is at the base of the lithosphere, and at the top of the asthenosphere. Itis the basic decollement for plate tectonics. The overlying lithosphereacts as an insulator for the heat dissipated by the underlying mantledue to the pristine heat from the Earth’s early stage of magma ocean,plus the heat delivered by radiogenic decay from the internal mantleitself. Moreover, in the LVZ has been inferred a larger amount of HOdelivered by the pargasite mineral when located at pressure greaterthan GPa, > km (Green et al., ). Therefore, the LVZ is a layerin which the viscosity is extremely depressed and could reach valuesas low as Pa s ( Jin et al., ). Moreover, the top of the astheno-sphere is in superadiabatic condition, having a potential temperaturepossibly higher than the underlying mantle, which has rather beeninferred to be subadiabatic and less convecting (Anderson, ).

When measuring plate motions relative to the mantle, it results thatplates have a mean “westerly” directed component, also defined aswestward drift of the lithosphere or net rotation. The resulting velocityof this rotation depends on the used reference frame. If the sourceof the magmatism describing linear features at the Earth’s surfacethat allow to compute the relative motion of the lithosphere relativeto the mantle are located below the decoupling layer, then the netrotation amounts to .–.° Myr (Gripp and Gordon, ; Conradand Behn, ; Torsvik et al., ). However, volcanoes may besourced from within the decoupling LVZ layer. Therefore, part of thedecoupling recorded by the volcanic tracks may be undetected. Sincemore and more petrologic and geophysical data support a shallowintra–asthenospheric origin of volcanic plumes (e.g., Bonatti, ;Foulger et al., ; Anderson, ; Presnall and Gudfinnsson, ),the net rotation of the lithosphere can be much faster, in the order of>°Myr (Crespi et al., , Cuffaro and Doglioni, ) as shown inFig. . In this plate motions reconstruction, all plates move “westerly”,along an undulate flow, and the fastest plates are those having at theirbase a LVZ with lower viscosity (e.g., the Pacific plate, Pollitz et al.,

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Figure . Model of plate motions relative to the mantle assuming an intra–asthenospheric source of volcanic trails (after Cuffaro and Doglioni, ). Thelithosphere has a net rotation toward the “west”, with an equator of this rotationthat closely overlaps the tectonic equator.

). Therefore, plate motions are polarized toward the west, andthe equator of the westward drift (Fig. ), largely overlaps the shape ofthe tectonic equator (Fig. ).

During the last years it was proposed that the misalignment of thetidal bulge relative to the gravitational Earth–Moon alignment due tothe anelastic reaction of the Earth with respect to the Moon gravityfield (Fig. ) could be responsible for the westerly directed torqueacting on the lithosphere (Scoppola et al., ; Riguzzi et al., ). Apermament oscillation acting on two layers with a difference in vis-cosity of about – orders of magnitude would allow the decouplingbetween the lithosphere and the asthenosphere (Doglioni et al., ).In the decoupling layer, the mantle is undergoing shearing and crys-tals may roll due to the intra–crystalline melt, lowering the viscositywhen measured parallel to the horizontal shear (Fig. ). If a segment oflithosphere is moving westward faster relative to another segment tothe east, a lower viscosity can be inferred within the LVZ beneath that

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

Figure . The westward drift of the lithosphere relative to the mantle can beinferred as generated by the torque exerted by the misalignment of the excess ofmass of the Earth’s bulge with respect to the Earth–Moon gravitational alignment.This momentum should be allowed by the presence of a hot, low–viscosity layerat the lithosphere base, which is detected as the low–velocity layer where seismicwaves slow down in the asthenosphere. The tidal oscillation of the lithospherecontributes to activate the decoupling.

plate, generating a rifting in the overlying lithosphere. The spreadingis transferred up toward the surface into the brittle regime wheremost of the faults and related earthquake develop. In case of fasterplate to the east relative to a plate to the west, a subduction and relatedorogeny will occur in between.

The westward drift of the lithosphere (Bostrom, ; Knopoff andLeeds, ; Moore, ) generated a number of asymmetries at plateboundaries.

W–directed subduction zones are steeper (average °) than thosedirected to E or NE (average °), and the associated orogens arerespectively characterized by lower structural and topographic eleva-tion, occurrence of backarc basins, whereas E or NE–directed subduc-tions show higher structural and morphological elevation and absentbackarc basin (Fig. ). This asymmetry is also marked by the opposite

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Figure . Lateral variations of viscosity in the asthenosphere determine differentvelocity among plates, hence generating plate boundaries where the lithosphereis either spread apart producing a rift, or converging along subduction zones.Differential velocity is transferred upward generating surface tectonics like thegraben of the picture and seismicity.

state of stress of the slabs: down–dip compression and down–dip ex-tension for W and E or NE–directed slabs respectively (Doglioni et al.,; Carminati and Petricca, ; Riguzzi et al., ). These asym-metries are striking when western and eastern Pacific subductionzones are compared. Such differences have usually been interpretedas related to the age of the downgoing oceanic lithosphere (usuallyolder, cooler and denser in the western side). However these differ-ences persist elsewhere, regardless of the age and composition of thedowngoing lithosphere, e.g., in the Mediterranean Apennines andCarpathians Vs. Alps and Dinarides, or in the Banda and Sandwicharcs, where even continental or zero–age oceanic (mid–oceanic ridge)lithosphere is almost vertical along W–directed subduction zones(Cruciani et al., ; Doglioni et al., ). W–directed subductions

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

Figure . The “westward” drift of the lithosphere relative to the mantle produces anumber of asymmetries at plate boundaries, both along subduction and rift zones.For example, slabs are steeper along W–directed subduction zones and are alwaysassociated to backarc spreading. E– or NE–directed subduction zones are rathershallower and the overlying orogens show much higher topography and structuralelevation with respect to the opposite slabs. Moreover they are double vergingorogens, they have shallow foredeeps and wider outcrops of metamorphic rocks.The subduction hinge generally diverges relative to the upper plates along theW–directed slabs, whereas it converges along the opposite E– or NE–directed slabs.Grey layer, oceanic lithosphere; green, continental lithosphere. Legend: η, viscosityvalues. α and β, dip of the orogens envelope and regional foreland monoclinerespectively (after Carminati and Doglioni, ).

and E or NE–directed have subduction hinges respectively divergingand converging relative to the upper plate, fast versus low subductionrates, low versus high topographic envelopes (α) and high versus lowforeland monoclines (β) (Doglioni et al., ). Finally the lithosphericvolume subducted is three times bigger in W–directed subductions(Doglioni et al., , ). The archetypes of the opposite subduc-tion zones can be recognized when comparing Alps (E–SE–directedslab) and Apennines (W–directed), (see Panza et al., ; Carminatiand Doglioni, ; Carminati et al. ).

Oceanic rifts are also asymmetric, with east flanks more elevated ofabout – m worldwide (Doglioni et al., ) (Fig. ). Moreover,along a cross section across the Mid Atlantic Ridge (MAR), Panza et al.

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() (Fig. ) showed that shear waves velocities (Vs) suggest a thickerwestern limb of the MAR and a thinner eastern limb. These authorsalso emphasized the occurrence of a low velocity zone (LVZ) betweenthe lithospheric mantle (LID) and the upper asthenosphere of about km depth. The asymmetry of the limb thickness at oceanic riftzones was also recently reproduced by Cuffaro and Miglio (),modelling mantle upwelling beneath a migrating Mid Atlantic ridge,using absolute velocities, as boundary conditions.

All these asymmetries are compatible with the westward drift of thelithosphere >°Myr and can be envisaged moving along the flow linesof plate motions described by the tectonic equator. Therefore we needa dynamic model to explain this polarized system. Solid Earth’s tides(or body tides) generate significant and often disregarded oscillationsin the lithosphere. They uplift the ground of – cm, swinging backand forth horizontally – cm at every passage of the Moon and Sun(. of the Moon tide) gravitational waves. Therefore the lithosphereis constantly subject to a vibration, which is westerly oriented due tothe misalignment of the Earth’s bulge. We infer that after the passageof the body tide, a residual permanent shear remains, like a shiftingplastic bottle subject to the wind at sea. The lithosphere appears tobehave like a worm, i.e., it is uplifted and sheared horizontally underthe effects of the solid Earth’s tide (Fig. ). A hysteresis of about .mm every Moon passage ( hours and ’) could explain a lithosphericdrift of . cm/yr relative to the mantle.

. Seismicity

Most of the energy delivered by the Earth’s seismicity (>%) is dis-sipated in the first km and by earthquakes with magnitude >..Seismicity is the result of plate tectonics, and the understanding ofthe forces governing geodynamics may provide fundamental clues forunravelling earthquakes. Therefore, the mechanisms that move platesare of paramount importance for tackling the issue of the seismichazard. The most energetic earthquakes form along subduction zonesbecause they are cold zones, and breaking rocks under contractionneeds more energy than under extension. Recently it has been ob-served some correlation between seismicity and tides (Wilcock, ;

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

Figure . The solid Earth’s tide generates a wave, which uplifts and shifts hor-izontally the lithosphere. A hysteresis of about . mm (variable with latitude)should accumulate after every solid Earth’s passage, due to the westerly–directedtorque acting on the lithosphere. At the end of the year, this sums up to the fewcentimetres of the lithosphere relative to the mantle. The horizontal componentof tides is inferred as the pumping system at plate boundaries where faults aregradually loaded of energy over the centuries. The vertical oscillation rather modi-fies the acceleration of gravity, and is interpreted as the triggering mechanism ofearthquakes when a fault has reached its critical state. The model proposes thatthe westward drift deforms the lithosphere as a worm, which is crawling.

Cochran et al., ; Métivier et al., ). For example, along theCascadia subduction zone in the western North America, it has beennoted that tremors are correlated during the high tide. Conversely,normal fault–related earthquakes seem more frequent during the lowtide (Fig. ). The tidal forces are too small to generate earthquakes,and for this reason they have been disregarded for long time. However,we have seen that the tide has a horizontal and a vertical component.The horizontal component can pump the system and load of energya given fault which is locked. When the critical state is reached, evena small change in the lithostatic load can trigger the earthquake. Forexample, during the high tide, the gravity is at minimum, whereasduring the low tide, the Earth’s gravity is at its maximum. Sincethe lithostatic load %gz (being % the density of rocks, g, the acceler-

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Figure . The passage of the solid Earth’s tides determines oscillations of theacceleration of gravity (g). High tide corresponds to low gravity, whereas a lowtide is associated to a high gravity field. Gravity oscillations generate variations ofthe lithostatic load (given by %gz, i.e., density of rocks, acceleration of gravity andthickness of rocks). In extensional tectonic settings, the increase of g (low tide)increases the differential stress and facilitates failure of the rocks. In compressionaltectonic settings instead, the decrease of g (high tide) increases the differentialstress, approaching the failure of rocks (modified after Riguzzi et al., ).

ation of gravity and z the thickness of the rocks column) acts as theminimum stress tensor in compressional tectonic settings and themaximum stress tensor in extensional tectonic settings, their variationdetermines opposite effects on the activation of faults. The differentialstress increases during the high tide in compressional regimes (i.e., thelithostatic load decreases for a smaller g), thus favouring earthquakenucleation. Alternatively, the differential stress increases during thelow tide in extensional regimes (i.e., the lithostatic load increases fora higher g). Therefore, high tide favours fault slip in compressionalenvironment, whereas the low tide is rather positive for normal fault-ing, which is exactly what observed (Fig. ). For these reasons, the

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

two tidal components could be considered as potentially relevantfor earthquakes, being the horizontal one providing the energy dur-ing the centuries, and the vertical one acting as the final drop thatgenerates the instability of the fault at the coseismic stage.

Within the crust, pressure and temperature increase with depth.However, pressure increases the stability of rocks, whereas tempera-ture has the opposite effect. In a crust of about km thickness, thefirst km are dominated by the effect of pressure and rocks behavein a brittle way. The deeper – km are rather controlled by thetemperature effect, which is increasing with depth, decreasing theenergy required for deforming plastically the rocks. Therefore, thehighest energy release by earthquakes occurs near the brittle–ductiletransition (BDT), and the main seismic swarm and aftershocks areaffecting the brittle upper crust (Fig. ). In the lower crust, faults (ormore exactly shear zones) may be active in a steady state regime,whereas in the upper crust, faults are locked and they move onlyepisodically (stick–slip behaviour).

These basic observations allow us to infer that the BDT plays animportant role in earthquakes. In fact, during the interseismic period,

Figure . The crust is generally divided into at least two layers, due to the inter-ference of pressure and temperature stability field of rocks. In the upper crust,rocks behave in a brittle regime where pressure dominates, and the deformation isepisodic; in the lower crust, the temperature is instead controlling the rheologicalbehaviour of rocks, which rather deform in a steady state plastic–ductile regime.The transition between the upper brittle and lower ductile regimes represents themain basal boundary of seismicity. The stars indicate the hypocentres of the Izmit earthquake sequence. The black dots are the aftershocks mostly confined inthe upper brittle crust.

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at the BDT should form a dilated band along extensional fault, andan overcompressed band along a contractional fault (thrust or reversefault). During the coseismic stage, the bands reverse their behaviour,being the dilated band recompressed, and the overcompressed bandrather stretched at the seismic event (Fig. ). Fluids should reactdifferently as a function of the opposite tectonic styles and in thefuture they may provide possible seismic precursors.

Figure . Assuming a simplified two–layer crustal rheology, the ductile lowercrust is characterized by a constant strain rate, whereas the brittle upper crustdisplays episodic locking–unlocking behaviour. Tensional and compressional faultsgenerate opposite kinematics and mechanical evolutions. In the tensional tec-tonic environment, the triangle of crust above the BDT remains “suspended”while a dilated area forms during the interseismic period. Fluids may enter thefractured volume. Once shear stress along the locked part of the fault becomeslarger than fault strength; the hangingwall will begin to collapse, increasing thefluid pressure into the fault gouge. This mechanism is self–supporting because itdecreases fault strength, facilitating the final fall of the hangingwall generating themainshock. Conversely, during the interseismic period, along a thrust plane anover–compressed band separates the ductile shear from the overlying locked faultsegment. The hangingwall is eventually expelled during the coseismic period. Flu-ids discharge behaves differently as a function of the tectonic field (after Doglioniet al., ).

Asymmetric Earth: mechanisms of plate tectonics and earthquakes

. Conclusions

The Earth’s models are generally performed assuming internal dy-namics controlled by lateral density variations due to thermal gradi-ents, which should generate both vertical and horizontal movementsin the mantle and in the overlying lithospheric plates. The analysisof the geometry at plate boundaries, and the kinematic constraintsrather point for a “westerly” polarized flow of plates, which implies arelative opposed movement of the underlying Earth’s mantle. Thisflow determines an asymmetric pattern along subduction zones andtwo end members of orogens associated to the downgoing slabs, suchas the differences between the low topography in the hangingwall ofthe western Pacific subduction zones, versus the high topography anddeep rocks exhumation along the eastern Pacific subduction zones.Rift zones are also asymmetric, having in average the western flankfew hundreds meters deeper, and a thicker and faster lithospherewith respect to the eastern flank. All plate boundaries move “west”.The decoupling of the lithosphere in the low–velocity zone (LVZ) atabout – km depth allows the “westerly” directed rotation of thelithosphere with respect to the underlying mantle (Fig. ). This asym-metric movement determines larger volumes of lithosphere returnedto the mantle with respect to the opposed subduction zones, allowingsome constraints for the reconstruction of the convection pattern inthe upper mantle (Fig. ). The motion of the lithosphere is inferredas the combination of an astronomically driven “westerly” directedhorizontal shear acting on plates, combined with the heat dissipationof the planet that maintains the temperature high enough in the LVZto have the low viscosity required for the lithospheric decoupling. Thevariations in viscosity within the LVZ control variations in velocityamong plates, and eventually seismicity at plate boundaries.

The solid Earth’s tide (Fig. ) may be crucial in understandinggeodynamics and earthquakes. The horizontal component, which iswesterly polarized due to the misalignment of the Earth’s bulge, mayprovide the source for shearing the lithosphere and loading the faults.The vertical component generates oscillations of the lithostatic load,which may locally trigger earthquakes along fault planes at the criticalstate (Fig. ). However, the effect of tides on faults is opposed as afunction of the tectonic style. When a fault moves from the interseis-

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mic to the coseismic activation, the stress field that characterized thelong lasting inactive period, gradually or instantaneously inverts theporosity (either increasing or decreasing it). Therefore, fluids react ac-cordingly, and they might become in the future important precursorsof imminent earthquakes (Fig. ).

. Acknowledgments

This article summarizes past and on–going researches performed witha number of colleagues which are deeply thanked: Salvatore Barba,Eugenio Carminati, Françoise Chalot-Prat, Mattia Crespi, Marco Cuf-faro, Adriana Garroni, Corrado Mascia, Enzo Nesi, Giuliano Panza,Federica Riguzzi, Davide Scrocca. Discussions with Don Andersonand Enrico Bonatti have always been very stimulating.

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Carlo DoglioniDipartimento di Scienze della Terra

Sapienza Università di Romacarlo.doglioni@uniroma.it